Lasik and Beyond Lasik Wavefront Analysis and Customized Ablations

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Contents

Editor-in-Chief: BENJAMIN F. BOYD, M.D., F.A.C.S.

Editors: SUNITA AGARWAL, M.S.;D.O.;F.S.V.H. ATHIYA AGARWAL, M.D.;D.O.;F.R.S.H. AMAR AGARWAL, M.S.;F.R.C.S.;F.R.C.Ophth

i

Project Director: Production Manager: Page Design and Typesetting: Art Design: Spanish Translation: Sales Manager: Marketing Manager: Customer Service Manager: International Communications:

Andres Caballero, Ph.D Kayra Mejia Kayra Mejia Laura Duran Eduardo Chandeck Prof. Juan Murube, M.D. Tomas Martinez Eric Pinzon Miroslava Bonilla Joyce Ortega

©Copyright, English Edition, 2002 by HIGHLIGHTS OF OPHTHALMOLOGY All rights reserved and protected by Copyright. No part of this publication may be reproduced, stored in retrieval system or transmitted in any form by any means, photocopying, mechanical, recording or otherwise, nor the illustrations copied, modified or utilized for projection without the prior, written permission of the copyright owner. Due to the fact that this book will reach ophthalmologists from different countries with different training, cultures and backgrounds, the procedures and practices described in this book should be implemented in a manner consistent with the professional standards set for the circumstances that apply in each specific situation. Every effort has been made to confirm the accuracy of the information presented and to correctly relate generally accepted practices. The authors, editors, and publisher cannot accept responsibility for errors or exclusions or for the outcome of the application of the material presented herein. There is no expressed or implied warranty for this book or information imparted by it. Any review or mention of specific companies or products is not intended as an endorsement by the authors or the publisher. Boyd, Benjamin F., M.D.; Agarwal, Sunita, M.S.; Agarwal, Athiya, M.D.; Agarwal, Amar, M.D. "LASIK and Beyond LASIK - Wavefront Analysis and Customized Ablation" ISBN Nº 9962-613-04-3 Published by: Highlights of Ophthalmology Int'l P.O. Box 6-3299, El Dorado City of Knowledge Clayton, Bldg. 207 Panama, Rep. of Panama Tel: (507)-317-0160 FAX: (507)-317-0155 E-mail: [email protected] Worldwide Web:www.thehighlights.com Printed in Bogota, Colombia. You may contact HIGHLIGHTS OF OPHTHALMOLOGY INC., for additional information about other books in this field or about the availability of our books.

ii

Contents

EDITOR-IN-CHIEF BENJAMIN F. BOYD, M.D., D.Sc. (Hon), F.A.C.S. Doctor Honoris Causa Immediate Past President, Academia Ophthalmologica Internationalis Honorary Life Member, International Council of Ophthalmology Editor-in-Chief and Author, HIGHLIGHTS OF OPHTHALMOLOGY, 25 Hard Cover Volumes and the 15 million copies of HIGHLIGHTS OF OPHTHALMOLOGY Bi-Monthly Journal. Recipient of the Duke-Elder International Gold Medal Award (International Council of Ophthalmology), the Barraquer Gold Medal (Barcelona), the First Benjamin F. Boyd Humanitarian Award and Gold Medal for the Americas (Pan American), the Leslie Dana Gold Medal and the National Society for Prevention of Blindness Gold Medal (United States), Moacyr Alvaro Gold Medal (Brazil), the Jorge Malbran Gold Medal (Argentina), the Favaloro Gold Medal (Italy). Recipient of The Great Cross Vasco Nuñez de Balboa Panama's Highest National Award.

EDITORS DR. SUNITA AGARWAL, M.S.;D.O.;F.S.V.H. (Germany) Pioneer in the world in the field of Laser Cataract Surgery. She heads Dr.Agarwal’s Eye Hospital at Bangalore, India and Dr.Agarwal’s Eye Clinic at Dubai (UAE). She is an expert in the field of Lasik Laser as she is in the field of cataract surgery under No Anesthesia. She has brought out for the first time in the world the use of the Air Pump as a new method to prevent surge and also found out that the internal tube of the phaco machine can lead to endophthalmitis . She practices at Dr.Agarwal’s Eye Hospital, 15 Eagle Street, Langford town, Bangalore-560 025, India and at Dr.Agarwal’s Eye Clinic,Villa No.2, Roundhouse 3, Al Wasl Road, Dubai Post box 9168, UAE.

DR. ATHIYA AGARWAL, D.O.;F.R.S.H. (London) Excellent surgeon who trains ophthalmologists from all over the world in Lasik and Phaco. She performs No anesthesia cataract surgery, Phakonit and Lasik Laser quite easily. Dr.Athiya is a very good speaker and teaches routinely in various conferences nationally and internationally. She practices at Dr.Agarwal’s Eye Hospital, 19 Cathedral Road, Chennai (Madras) – 600 086, India.

DR. AMAR AGARWAL, M.S.;F.R.C.S.;F.R.C.Ophth (London) Started for the first time in the world "No anesthesia cataract surgery", "Phakonit (cataract removal through a 0.9 mm incision)" and "Favit" a new technique to remove dropped nuclei. He is a very dynamic speaker. He has a double FRCS to his credit. His parents Dr. J. Agarwal and Dr. Mrs.T.Agarwal, Sister Dr.Sunita Agarwal, wife Dr.Athiya Agarwal and Brother-in-law Mr.Pankaj Sondhi help him in his aim to perfection. He practices at Dr.Agarwal’s Eye Hospital, 19 Cathedral Road, Chennai (Madras) – 600 086, India. Dr.Agarwal’s Eye hospital at Chennai is the only eye hospital in the world built in the shape of an eye and has been included in the Ripley’s Believe it or not series. Dr.Agarwal’s Eye Hospital is at Chennai (India), Bangalore (India) and Dubai. The web site of the hospital is: http://www.dragarwal.com

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Contents

Contributing Authors

Agarwal, Amar, M.S.,FRCS;FRCOphth Medical Director, Dr. Agarwal's Group of Eye Hospital Chennai, India Agarwal, J., FORCE;DO;FICS Chairman, Dr. Agarwal's Group of Eye Hospital, Chennai, India; Bangalore, India; Dubai (UAE) Agarwal, Sunita, MS;DO;FSVH Medical Director; Dr. Agarwal's Group of Eye Hospital Bangalore, India Agarwal, T.; FORCE;DO;FICS Managing Director, Dr. Agarwal's Group of Eye Hospital, Chennai, India; Bangalore, India; Dubai (UAE) Alio, Jorge L., M.D. Director, Instituto Oftalmologico de Alicante Alicante, Spain Attia, Walid, M.D. Instituto Oftalmologico de Alicante Alicante, Spain

Avalos U., Guillermo, M.D. Chief, Department of Ophthalmology Hospital Sagrado Corazon; Medical Director, Clinica Laser Oftalmico Guadalajara, Mexico Belda, Jose I., M.D. Instituto Oftalmologico de Alicante Alicante, Spain Benelli, Umberto, M.D. Department of Neurosciences Section of Ophthalmology University of Pisa Pisa, Italy Border, Andrea D., O.D. Discover Vision Centers Kansas City, Missouri

Boyd, Benjamin F., M.D., F.A.C.S. Editor-in-Chief, Highlights of Ophthalmology Int., Panama, Rep. of Panama Butler, Jason, M.D. Long Beach Laser Center Long Beach, California Carriazo E., Cesar, M.D. Medical and Scientific Director Carriazo Ophthalmological Center, Colombia

iv

Carvalho, Luis A., PhD. Institute of Physics University of Sao Paulo, Brazil Castro, Jarbas C., PhD. Professor, Institute of Physics University of Sao Paulo, Brazil Cigales, Melania, M.D. Instituto Oftalmologico de Sabadell Sabadell, Spain Coelho, Etelvino, M.D. Director, Centro de Microcirugia Refrativa & Excimer Laser de Minas Gerais Belo Horizonte, MG Brazil Coret, Andreu, M.D. Medical Director, Instituto Oftalmologico de Barcelona Barcelona, Spain Cummings, Arthur, MB, ChB Mmed (Ophth) FCS(SA), FRCS (Edin) Wellington Ophthalmic Laser Clinic Dublin, Ireland Charles, Steve, M.D. Clinical Professor Department of Ophthalmology University of Tennessee Center of Health Science Memphis, Tennessee

Contents

CONTRIBUTING AUTHORS Chamon, Wallace, PhD. Refractive Surgery Division Escola Paulista de Medicina University of Sao Paulo, Brazil Choudhry, Saurabh, D.O., FERC. Fellow, Dr. Agarwal's Eye Hospital, Chennai, India Choudhry, Reena, M., DO.;FERC. Fellow, Dr. Agarwal's Eye Hospital, Chennai, India Davis, Elizabeth, A., M.D. Associate, Minnesota Eye Consultants, PA; Assistant Clinical Professor University of Minnesota Minneapolis, Minnesota Denning, James A., B.A., B.S. Discover Vision Centers Kansas City, Missouri

Hardten, David R., M.D. Director of Refractive Surgery Minnesota Eye Consultants; Clinical Associate Professor of Ophthalmology University of Minnesota Minneapolis, Minnesota

Lindstrom, Richard, M.D. Medical Director, Phillips Eye Center for Teaching and Research; Clinical Professor, University of Minnesota, Minnesota, Minneapolis

Haw, Weldon, M.D. Cornea & Refractive Surgery, Department of Ophthalmology, Stanford University School of Medicine Stanford, California

Mahmoud M. Ismail, M.D., Ph.D. University of Al-Azhar, Cairo, Egypt

Hoyos, Jairo E., M.D. Medical Director Instituto Oftalmologico de Sabadell Sabadell, Spain Hoyos-Chacon, Jairo, M.D. Instituto Oftalmologico de Sabadell Sabadell, Spain

Doane, John F., M.D. Discover Vision Centers Kansas City, Missouri

Katsanevaki, VJ, M.D. Department of Ophthalmology University of Crete-Medical School Crete, Greece

EuDaly, Lon S., O.D. Discover Vision Centers Kansas City, Missouri

Knorz, Michael C., M.D. Klinikum Mannheim Mannheim, Germany

Feinerman, Gregg, M.D. Medical Director, Feinerman Vision Institute, Long Beach Laser Center, Long Beach, California; Assistant Clinical Professor, University of California, Irvine, California

Koch, Paul S., M.D. Koch Eye Associates Warwick, Rhode Island

Gatell, Jordi, M.D. Instituto Oftalmologico de Barcelona Barcelona, Spain Ginis, HS., BSc Department of Ophthalmology University of Crete-Medical School Crete, Greece Gomez, Javier J., M.D. Instituto Oftalmologico de Alicante Alicante, Spain

Krueger, Ronald, M.D. Medical Director, Department of Refractive Surgery, The Cleveland Clinic Foundation Cole Eye Institute Cleveland, Ohio Lara, Elvira, O.D. Instituto Oftalmologico de Barcelona Barcelona, Spain Lavery, Frank, MCh FRCSI FRCS (Edin) DOMS Wellington Ophthalmic Laser Clinic Dublin, Ireland

v

Manche, Edward E., M.D. Assistant Professor of Ophthalmology Stanford University School of Medicine Stanford, California Martiz, Jaime R., M.D. Refractive Surgery Consultant; The Laser Center, Houston, Texas; Course Director & President International Lasik Course Houston, Texas McDonald, Marguerite, M.D. Director, Southern Vision Institute New Orleans, Louisiana Morris, Scot, O.D. Discover Vision Centers Kansas City, Missouri Murube, Juan, M.D. Professor of Ophthalmology, University of Alcala; Chairman, Dept. of Ophthalmology, Hospital Ramon y Cajal Madrid, Spain Narang, Sameer, M.S. Director, Narang Eye Clinic Ahmedabad, Gujarat, India Narang, Priya, M.S. Director, Narang Eye Clinic Ahmedabad, Gujarat, India Narasimhan, Smita, M.B.B.S.,FERC Consultant, Dr. Agarwal's Eye Hospital, Chennai, India

Contents

CONTRIBUTING AUTHORS Nardi, Marco, M.D. Associate Professor Department of Neurosciences Section of Ophthalmology University of Pisa Pisa, Italy Nguyen, Kim, M.D. Long Beach Laser Center Long Beach, California Oliveira, Canrobert, M.D. Director, Hospital de Olhos de Brasilia Brasilia, DF Brazil Pallikaris, Ioannis G., M.D. Department of Ophthalmology University of Crete-Medical School Crete, Greece

Preetha R., M.B.B.S; FERC Fellow, Dr. Agarwal's Eye Hospital, Chennai, India

Simon-Castellvi, Jose Ma., M.D. Clinica Oftalmologica Simon, Barcelona, Spain

Probst, Louis E., M.D. Medical Director, TLC The Windsor Laser Eye Center, Windsor, Ontario, Canada

Simon-Castellvi, Sarabel, M.D. Clinica Oftalmologica Simon, Barcelona, Spain

Sasikanth, RR., MD Dr. Agarwal's Eye Hospital Chennai, India Schor, Paulo, PhD, M.D. Bioengineering Division Escola Paulista de Medicina University of Sao Paulo Sao Paulo, Brazil

Parul, Goel, M.S., F.E.R.C. Consultant, Dr. Agarwal's Eye Hospital, Chennai, India

Shalaby, Ahmad, M., M.D. Instituto Oftalmologico de Alicante Alicante, Spain

Perez-Santoja, Juan J., M.D. Refractive Surgery and Cornea Unit Alicante Institute of Ophthalmology Miguel Hernandez University School of Medicine Alicante, Spain

Simon-Castellvi, Cristina, M.D. Clinica Oftalmologica Simon, Barcelona, Spain

Peters, Tim, M.D. Nationwide Vision Laser & Eye Center, Clinical Lecturer, University of Arizona Phoenix, Arizona

Simon-Castellvi, Guillermo L., M.D. University of Barcelona, Faculty of Medicine, Dept. of Ophthalmology; Chief Anterior Segment Surgeon, Clinica Oftalmologica Simon, Barcelona, Spain

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Slade, Stephen G., M.D. The Laser Center, Houston, Texas Viera de Carvalho, Luis A., PhD. Professor, University of Sao Paulo Brazil Waring, George, M.D. Professor of Ophthalmology Emory University; Co-Founder Emory Vision Correction Center Atlanta, Georgia Werner, Leonardo P., M.D. Department of Ophthalmology, São Geraldo Eye Hospital, Federal University of Minas Gerais and the "Instituto Vizibelli" Belo Horizonte, Minas Gerais, Brazil Wilson, Steven E., M.D. Chair, Department of Ophthalmology, and Professor of Vision Research University of Washington Seattle, Washington

Contents

Contents

SECTION I - LASIK FUNDAMENTAL PRINCIPLES OF DIAGNOSIS, CORNEAL MAPPING, MECHANISM OF ACTION OF EXCIMER LASERS

CHAPTER 1

CHAPTER 3

UNDERSTANDING REFRACTIVE LASERS

EVALUATION OF THE LASIK FLAP BY CONFOCAL MICROSCOPY

Therapeutic Principles of Excimer Lasers Advances in Excimer Laser Technology Scanning Lasers Eye Tracking Systems How the Corneal Tissues are Affected in LASIK vs Incisional Keratotomy

1 2 2 6

Contents

Section 1 Frequent Problems With the Flap What is Confocal Microscopy? Confocal Microscopy Procedure Results The Importance of Confocal Microscopy to the Sands of Sahara’s Syndrome How to Prevent Sands of Sahara Syndrome

6

CHAPTER 2

Section 2

61 61 62 62

Section 3

63 63

Section 5

Section 4

Section 6

FUNDAMENTALS ON CORNEAL TOPOGRAPHY

CHAPTER 4 Section 7

PREDICTIVE FORMULAS FOR LASIK

Introduction: Human Optics and the Normal Cornea 9 Instruments to Measure the Corneal Surface 10 Causes of Artefacts of the Corneal Topography Map 14 Understanding and Reading Corneal Topography 15 Topographic Scales 16 Computer Displays: Presentation of Topographic Information 16 Special Software Applications and Displays 23 ATLAS OF CORNEAL TOPOGRAPHY 31-42 TOPOGRAPHERS NOW AVAILABLE 43

The Predictive Formulas Main Components Developing Individualized Predictive Formulas The Healing Pattern of the Cornea Excimer Laser Ablation Nomograms for: Photorefractive Keratectomy LASIK Predictive Formulas for LASIK with: VISX S2 SmoothScan Chiron Technolas 116 Technolas 217 Excimer Laser

vii

65 65 65 66 66 67 68 69 70

Subjects Index Help ?

CONTENTS

SECTION II - LASIK HIGHLIGHTS OF SURGICAL INSTRUMENTATION (MICROKERATOMES) AND SURGICAL TECHNIQUES CHAPTER 5

CHAPTER 9

MICROKERATOMES

LIMITATIONS AND CONTRAINDICATIONS OF LASIK

Different Mechanical Components Applanation Lenses Tonometer Guidelines for Microkeratome Use Astigmatism Inductions by Hinge Ablation Free Cap The Main Microkeratomes: Outline Description How to Use Them

78-80 80 80 80 86 87 87-99

Preoperative Evaluation Special Cases LASIK after IOL Implantation Bilateral Simultaneous vs. Sequential Surgery LASIK after RK Alternatives to LASIK PRK Refractive Lensectomy Phakic Intraocular Lens Thermokeratoplasty (LTK)

127 134 134 134 134 135 135 135 136 136

CHAPTER 6 AUTOMATIC CORNEAL SHAPER

Contents

CHAPTER 10 Note from the Editor-in-Chief Different Components & Instrumentation Surgical Technique Step by Step Troubleshooting Care, Handling & Sterilization

101 101-3 104-5 105 107

Section 2 Patient Selection PREOPERATIVE PREPARATION The Patient The Instruments The Laser The Keratome The Surgeon PREPARATION IN OPERATING ROOM Draping Speculum Positioning the Patient THE LASIK PROCEDURE Marking Placement of the Suction Ring The Microkeratome Cut Laser Ablation Replacing the Flap Intraoperative Bleeding in LASIK Postoperative Care Home Care Instructions

CHAPTER 7 DOWN UP LASIK Setting up of the Hansatome Care, Maintenance & Sterilization Troubleshooting Surgical Technique Step by Step Advantages & Disadvantages

109 112 114 114 114

CHAPTER 8 ALL LASER LASIK With the Pulsion FS Laser Preoperative Evaluation Surgical Logistics Surgical Technique Step by Step Postoperative Care

Section 1

LASIK SURGICAL TECHNIQUE

119 120 120-4 125

viii

139 140 140 140 140 141 142 142 142 142 143 143 143 143 144 145 146 148 150 150

Section 3 Section 4

Section 5

Section 6 Section 7 Subjects Index Help ?

CONTENTS Maintain Consistent Hydration Perform the Appropriate Ablation Prevent and Remove Debris from Beneath the Flap Properly Align the Flap Achieve Good Flap Adhesion Avoid and Treat Loose Epithelium Conclusion

CHAPTER 11 PEARLS IN LASIK TECHNIQUE Patient Counseling Achieving Adequate Exposure Achieve and Confirm Adequate Suction Create a Complete Flap

151 151 152 153

153 154 155 156 157 157 158

SECTION III LASIK IN COMPLEX CASES

Compound Myopic Astigmatism 188 Simple Hyperopic Astigmatism 188 Compound Hyperopic Astigmatism 188 Mixed Astigmatism 189 Negative Cylinder Ablation to Treat Mixed Astigmatism 189 Positive Cylinder Ablation to Treat Mixed Astigmatism 190 Bitoric Ablation to Treat Mixed Astigmatism 190 Results of Lasik in Mixed Astigmatism 192 Conclusion 192

CHAPTER 12 LASIK FOR HYPEROPIA Technique, Safety and Efficacy 161 Hyperopic Correction using the Excimer Laser 162 Patient Selection and Preoperative Considerations163 Technique 164 Clinical Results 164 Secondary Hyperopia 165 Hyperopia with Astigmatism 165

CHAPTER 15

CHAPTER 13

Section 1

RELASIK LASIK FOR IRREGULAR ASTIGMATISM Etiology of Irregular Astigmatism Diagnosis Clinical Classification Corneal Topography Patterns 1. Irregular astigmatism with defined pattern 2. Irregular astigmatism with undefined pattern Preoperative Evaluation Treatment of Irregular Astigmatism Surgical Techniques with Excimer Laser Automated Anterior Lamellar Keratoplasty Intracorneal Ring Segments (INTACS) Other Non-Surgical Procedures Contact Lens Management

Procedure Results Discussion

169 170 170 170 170

CHAPTER 16

171 171 175 175 184 184 184 184

187 187

ix

Section 7 Subjects Index Help ?

LASIK AFTER PENETRATING KERATOPLASTY

LASIK IN MIXED ASTIGMATISM

Section 4

Section 6 201 202 203 204

CHAPTER 17

Eligible Patients Timing of Surgery Surgical Technique Postoperative Treatment Risks and Possible Complications Results Conclusions

Section 3

Section 5

LASIK AFTER RK AND PRK

CHAPTER 14

Classification Simple Myopic Astigmatism

Section 2 195 196 198

Results of LASIK After RK and PRK RK Group PRK Group Discussion

Contents

208 208 209 209 210 210 212

CONTENTS

CHAPTER 18 LASIK AFTER PREVIOUS CORNEAL SURGERY General Considerations After RK Residual Myopia After RK Hyperopia After RK The Cornea After RK LASIK AFTER RK Preoperative Considerations Contraindications lntraoperative Considerations Results (Pilot Study) LASIK AFTER AK The Cornea After AK Performing LASIK After AK Preoperative Considerations lntraoperative Considerations Results (Pilot Study) LASIK AFTER PRK The Cornea After PRK Performing LASIK After PRK Preoperative Considerations Intraoperative Considerations Postoperative Treatment Results (Pilot Study) LASIK AFTER LTK The Cornea After LTK Performing LASIK After LTK Preoperative Considerations

215 215 216 216 216 217 217 217 218 218 219 219 220 220 220 220 220 221 221 221 222 222 223 223 223 223

lntraoperative Considerations Results (Pilot Study) LASIK AFTER PKP Performing Excimer Laser After PKP Preoperative Considerations Indications High Risk Cases & LASIK Contraindications Preoperative Medications When to Operate Intraoperative Considerations Conclusions LASIK AFTER ALK The Cornea After ALK Performing LASIK After ALK Preoperative Considerations Intraoperative Considerations Conclusions LASIK AFTER EPIKERATOPHAKIA

224 224 225 226 226 227 227 227 228 229 229 229 229 229 229 230 230 230

LASIK AFTER CORNEAL TRAUMA

230

FUTURE OF LASIK AFTER OTHER CORNEAL SURGERIES

230

CHAPTER 19 PEDIATRIC LASIK Patient Selection Surgical Technique Ablation Parameters Results

Contents

234 234 234 235

Section 1

Section 2 Section 3 Section 4

SECTION IV

Section 5

LASIK COMPLICATIONS

Section 6 Section 7 Subjects Index

CHAPTER 20

CHAPTER 21

FIRST NON-INVASIVE TREATMENT FOR SUBLAMELLAR EPITHELIAL INGROWTH AFTER LASIK BY LOCAL FREEZING

PREVENTION AND MANAGEMENT OF LASIK COMPLICATIONS

Sequence of Events Management Techniques Presently Available The New Non-Invasive Method Technique Step by Step Results

Incidence - Relation to Multiple Variables Classification Intraoperative Complications Early Postoperative Complications Late Postoperative Complications

243 243 243 244 245

x

247 247 247 250 253

Help ?

CONTENTS

CHAPTER 26

CHAPTER 22 FLAP COMPLICATIONS Diminishing Complications with New Microkeratomes Classification of Complications I) Intraoperative II) Early postoperative period III) Late postoperative period Management of Variety of Flap Complications Sands of Sahara Syndrome Dry Eye Syndrome Epithelial Ingrowth The Hansatome (“Down-Up”) Microkeratome Main Advantages and Disadvantages Cleaning of the Instrument PEARLS TO ASSIST WITH THE MAKING OF A GOOD FLAP

INFLAMMATORY AND INFECTIOUS COMPLICATIONS AFTER LASIK 267 267 267 267 267 267-73 272 273 273 274 274 275 275

CHAPTER 23 FOLDS AND STRIAE OF THE DISC POST LASIK Definition Treatment of: Folds Striae Surgical Technique

INFECTIOUS KERATITIS FOLLOWING LASIK Clinical Findings Causative Organisms Diagnosis & Differential Diagnosis Treatment Prognosis Prevention

297 297 299 300 301 302 303

PREVENTION AND MANAGEMENT OF LASIK COMPLICATIONS

277 280

INTRAOPERATIVE COMPLICATIONS Flap Complications Ablation Complications POSTOPERATIVE COMPLICATIONS

280

TREATMENT OF FLAP STRIAE 284 284 284 285 285 286

307 307 309 311

Contents

Section 1


CHAPTER 28

Section 5

VITREORETINAL COMPLICATIONS OF REFRACTIVE SURGERY


Preoperative Evaluation Indications for Prophylaxis of Retinal Breaks and Degenerations Theoretical Mechanisms Resulting in Retinal Breaks and Detachment Anterior Chamber Shallowing Vitreoretinal Complications of PRK & LASIK Retinal Detachment After PRK Retinal Detachment After LASIK Macular Hemorrhage Nerve Fiber Layer Damage Endophthalmitis Dislocated Intraocular Lenses

CHAPTER 25 KERATECTASIA INDUCED BY MYOPIC LASIK Corneal Stromal Changes Induced by LASIK Corneal Evaluation Using the Orbscan Topography System How the Orbscan Helps Evaluating High Risk Cases for LASIK and FFK

293 293 294 294 295-96 296 296 297

CHAPTER 27

CHAPTER 24

SURGICAL TREATMENT Massaging the Flap Using: a) A Spatula over a Contact Lens b) Direct Massaging Appearance of the Cornea After Treatment Outcome

DIFFUSE LAMELLAR KERATITIS (DLK) SYNDROME (SANDS OF SAHARA) Causative Agents Clinical Findings DLK Staging Diagnosis &Differential Diagnosis Treatment Prevention Conclusions

287 288

291

xi

317 Help ?

317 318 318 319 319 319 320 321 321 321

CONTENTS

SECTION V BEYOND CONVENTIONAL LASIK Corneal Custom Ablation Guided by Wavefront Mapping

CHAPTER 29

CHAPTER 32

REFINING CUSTOM ABLATION THROUGH WAVEFRONT MAPPING

WAVEFRONT ANALYSIS AND CUSTOM ABLATION

Wavefront Analysis Mapping a Profile of the Whole Eye Development of Wavefront Technology The Mechanisms of Wavefront Devices Benefits of Wavefront Analysis Linking Diagnostic Information from Wavefront Mapping to Laser Treatment Wavefront Analysis in Conjunction with Corneal Topography Personalized LASIK Nomograms

Promising Achievements Principle of Wavefront Analysis Availability of Technology Custom Intraocular Lens Goal in Mind

325 325 325 328 329 329

CHAPTER 33 331

THE ROLE OF DIFFERENT ABERRATIONS IN WAVEFRONT ANALYSIS

331

What Do We Mean by Wavefront Sensing Analysis? What do we Understand as an Aberration of the Optical System? How Do Different Aberrations Affect Vision in Humans? Do Aberrations Contribute to Sight in Any Positive Way? Principles for the Study and Diagnosis of Aberrations

CHAPTER 30 COMPUTERIZED CORNEAL TOPOGRAPHY AND ITS IMPORTANCE TO WAVEFRONT TECHNOLOGY Corneal Topography and Wavefront Analysis Current Status of Custom Ablation

339 339 340 340 340

333 334

341 341 343 343

Contents

Section 1

Section 2 Section 3

344 Section 4

Section 5

CHAPTER 34

Section 6

REFRACTION EVALUATION SYSTEMS FOR WAVEFRONT ANALYSIS

CHAPTER 31 CUSTOMIZED CORNEAL ABLATION THROUGH WAVEFRONT MAPPING The Quest for Bionic or Super Vision Promising New Technology Attaining Bionic or Super Vision Generating the Wavefront Map Wavefront Analysis & Corneal Topography

What is Wavefront Technology? Current Ocular Refraction Evaluation Systems Phoroptor and Autorefractors Corneal Topography 20/10 Perfect Vision Wavefront System Other Wavefront Sensing Devices How the Visx 20/10 Wavefront System Works How to Read a Wavefront Map The Shortcomings of Shack-Hartmann Wavefront Analysis Clinical Examples

337 337 337 338

xii

347 349 349 349 349 349 351 353 355 357-69

Section 7 Subjects Index Help ?

CONTENTS Technical Development of PALM Technique

CHAPTER 35 ZYOPTIX PERSONALIZED LASER VISION CORRECTION Performing Zyoptix Treatment Orbscan II (Elevation Topography) The Zywave Aberrometer Bausch & Lomb Technolas 217z Excimer Laser Zyoptix Patient Case

CHAPTER 38 CUSTOMIZED ABLATIONS IN LASIK 373 374 374 375

Present Role of Customized Ablations Technique of TopoLink Examples of Uses of TopoLink Results of TopoLink in Repair Procedures The Bausch & Lomb Aberrometer Wavefront-Deviation Guided LASIK

377

CHAPTER 36 ZYOPTIX Preoperative Procedure Zywave Aberrometer Elevation Topography (Orbscan) Zylink Preparing the Laser Treatment Advantages & Disadvantages Clinical Cases

401 402 402 407 409 411

CHAPTER 39 379 380 386 386 389 391 391 391-3

WAVEFRONT MEASUREMENTS OF THE HUMAN EYE WITH HARTMANN-SHACK SENSOR

Principles of Eye Aberration Measurements with the Hartmann-Shack Sensor Present Technologies for Optimizing Visual Acuity through Refractive Surgery

CHAPTER 37 LASIK – PALM The PALM Gel The PALM Procedure

399

A Look into the Future of Refractive Surgery

396 398

413 Contents

417

Section 1 417


SECTION VI

Section 5

LASIK IN PRESBYOPIA

Section 6 Section 7

CHAPTER 41

CHAPTER 40

PRESBYOPIA

PRESBYOPIA

Theories of Accommodation Treatment with Optical Devices Surgical Methods SCLERAL TECHNIQUES Anterior Ciliary Sclerotomy (ACS) (Thornton’s Technique) Scleral Expansion Band - Schachar´s Technique INTRACORNEAL TECHNIQUE Intracorneal Implants INTRAOCULAR TECHNIQUES LASER TECHNOLOGY TECHNIQUES

Surgical Correction - Current Trends Surgery for Management of Presbyopia through MONOVISION The LADARVision Laser for Myopia and Presbyopia Hyperopia and Presbyopia Emmetropia with Presbyopia Description of Operations on the Sclera to Improve Presbyopia

Subjects Index

427 427 427 428 428

xiii

Help ?

436 437 438 439 439 439 441 441 441 444

CONTENTS

SECTION VI I ALTERNATIVES TO LASIK

CHAPTER 42 NO ANESTHESIA CATARACT / CLEAR LENS EXTRACTION NUCLEUS REMOVAL TECHNIQUES Karate Chop Soft Cataracts Agarwal Chopper Step by Step Technique NO ANESTHESIA CLEAR LENS EXTRACTION Step by Step Technique

451 451 452 452 452-57 458 458-62

Three Basic Styles of Phakic IOL'S ANTERIOR CHAMBER PHAKIC IOL'S THE ARTISAN LENS Step by Step Technique THE NU-VITA ANTERIOR CHAMBER LENS Step by Step Technique POSTERIOR CHAMBER PLATE LENSES THE BARRAQUER PRE-CRYSTALLINE LENS Step by Step Technique THE POSTERIOR CHAMBER FOLDABLE PLATE PHAKIC LENS (The Implantable Contact Lens) Step by Step Technique

471 472 472 473-80 481 481-82 485 485 485-91 492 492 492-97

CHAPTER 43 CHAPTER 45

PHAKONIT AND LASER PHAKONIT Phakonit to Correct Refractive Errors TECHNIQUE OF PHAKONIT FOR CATARACTS Surgical Technique Step by Step PHAKONIT IN CLEAR LENS EXTRACTION Surgical Technique Step by Step

LASIK vs PHAKIC LENS IMPLANTATION TO CORRECT MYOPIA

463 454

Section 1

464-66 467 468

CHAPTER 44 PHAKIC IOL's - SURGICAL MANAGEMENT OF HIGH MYOPIA Limitations of LASIK in Very High Myopia The Important Role of Phakic Intraocular Lenses Contributions of Phakic IOL's Advantages Over Corneal Refractive Surgery Limitations of Phakic IOL's

Contents

Surgical Technique: Ophtec Artisan Myopia Implant Surgical Technique: LASIK The Study: Ophtec Artisan Myopia Implant vs. LASIK Results

499 503 507 508


Section 5

Section 6

CHAPTER 46

469

Section 7

INTACS TM REFRACTIVE CORRECTION WITH AN INTRACORNEAL DEVICE

469 469 470

Surgical Procedure Clinical Outcomes Safety Assurance and Further Indications

470

xiv

514 514 519

Subjects Index Help ?

Contents

Section 1 Section 2

Section 3

Section 4

Section 5


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Zyoptics, Personalized Laser Visual Correction UNDERSTANDING REFRACTIVE LASERS

Chapter 1 UNDERSTANDING REFRACTIVE LASERS Benjamin F. Boyd, M.D., F.A.C.S.

Therapeutic Principles of Excimer Lasers The most significant advance in the past three years has been the emergence of the excimer laser and its rapid rise to dominate refractive corneal surgery. The excimer laser is a source of energy that is very difficult to control and apply to the human eye with the assurance of safety.

Harnessing this laser to safely perform corneal surgery has been a major technical achievement. The argon fluoride (ArF) 193 nanometer excimer laser is a pulsed laser that has wide potential because it can create accurate and very precise excisions of corneal tissue to an exact depth with minimal disruption of the remaining tissue. Fig. 1-1 presents the comparative mechanisms of the excimer laser vs various other lasers commonly used in ophthalmology.

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Section 6 Figure 1-1 Comparative Mechanisms of the Various Lasers Used in Ophthalmology

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(1) The argon and krypton lasers em- Subjects Index ploy a thermal mechanism whereby the laser (L) heats the chorioretinal photocoagulated tissue and produces scarring (arrow). Retina (R), choroid (H) and pigment epithelium (E). (2) The YAG laser works by photodisruption of tissues, creating small acoustical explosions that produce openings (arrow) such as we make in posterior capsulotomy (P). A plasma screen Help ? of ions (+ and -) is created. (3) Excimer ultraviolet laser works by photoablation. Small amounts of tissue (T), essentially the stroma in cases of LASIK, are removed from the cornea (C - arrow) with each pulse. (After Boyd´s "Atlas of Refractive Surgery").

LASIK AND BEYOND LASIK

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Figure 1-2: Excimer Laser - Effects on the Tissue The high intensity energy of ultraviolet light from an excimer laser during tissue ablation breaks inter and intramolecular bonds, causing the molecules of the area of ablation to explode away from the surface. Please observe that there is minimal disruption of the remaining surrounding tissue. (After Boyd´s "Atlas of Refractive Surgery").

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Ophthalmic excimer lasers use ultraviolet radiation at a wavelength of 193 nanometers. It is a wavelength that practically does not heat the tissue but actually breaks inter- and intramolecular bonds. The molecules in the area of ablation explode away from the surface (Fig. 1-2). The concept of ablative surgery is that by removing small amounts of tissue from the anterior surface of the cornea (Fig. 1-3) a significant change of refraction can be attained. The effect in myopes is achieved by flattening the anterior dome of the central cornea over a 5 mm disc shaped area.

ADVANCES IN EXCIMER LASER TECHNOLOGY Scanning Lasers As pointed out by Peter McDonnell, M.D., Professor and Chair, Department of Ophthal2

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mology, University of California, Irvine, and Professor of Ophthalmology and Director of Refractive Surgery at the Doheny Eye Institute, University of Southern California at Los Angeles, in most parts of the world broad beam lasers still predominate in the laser market (Fig. 1-3). Recently, however, scanning or flying spot lasers have gained attention. Instead of using an iris diaphragm to control a broad beam, some scanning lasers use a small slit that is scanned across the corneal surface (Fig. 1-4). Flying spot is another type of scanning laser. Instead of a slit that scans the surface , flying spot lasers (Fig. 1-5) have a small circular or elliptical spot perhaps 1 mm to 2 mm in diameter that is moved across the surface of the cornea by computercontrolled galvanometric mirrors. Advantages of Scanning Lasers Scanning lasers (Figs. 1-4 and 1-5) have several potential advantages over broad beam

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Figure 1-4 (below): Concept of the Scanning Type Laser for Refractive Surgery Another type of excimer laser uses a scanning laser beam. The laser beam (L1) strikes a moving mask (M-arrow) which has a slit through which the beam passes (L2) in a predetermined fashion. More ablation occurs centrally (C) and less peripherally (P) to achieve the desired corneal reshaping. (After Boyd´s "Atlas of Refractive Surgery").

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Figure 1-3 (above): Concept of Broad Beam Laser Application for Refractive Surgery

Section 2 The most common type of excimer laser is the broad beam laser (L1). The method of application uses a widening diaphragm or pre-shaped ablatable mask (M) through which the laser beam (L2) passes. To produce more ablation of the cornea in the center (C) than in midperiphery (P), the thinner central portion of the mask allows the laser to ablate the central cornea longer. As the disk is ablated in a peripheral direction (arrows), the cornea is shaped accordingly in a desired gradient fashion.(After Boyd´s "Atlas of Refractive Surgery").

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Figure 1-5 (left): Concept of the "Flying Spot" Scanning Laser Application for Refractive Surgery

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A third type of excimer laser application is known as the "flying spot". A small laser beam (L) moves across the cornea (arrow) in a predetermined, computer driven pattern to ablate more tissue centrally (C) than in the mid-periphery (P). This type of laser application is very flexible with regard to the type of ablation pattern that can be applied. (After Boyd´s "Atlas of Refractive Surgery").


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Figure 1-6: Flexibility of the "Flying Spot" Scanning Laser May Provide Customized Ablation The "Flying Spot" type excimer laser has an advantage over other broad beam and slit scanning lasers by providing increased flexibility in the ablation profile. The profile can be altered to provide aspheric as well as spherical ablations. The mid-peripheral cornea (red shaded area-P) can be treated with the laser (L) to produce a different curvature than that of the central cornea (D - blue line shaded area). This allows the possibility of a customized ablation unique for every cornea. A lamellar corneal flap (B) is retracted. (After Boyd´s "Atlas of Refractive Surgery").

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lasers (Fig. 1-3). The postoperative corneal surface may be smoother, resulting less often in a healing response which can progress to corneal haze or opacity. A higher quality of vision and improved visual acuity may also result from the smoother and more uniform corneal surface when using scanning lasers. McDonnell emphasizes that another possible advantage of scanning technology is increased flexibility in the ablation profile or algorithm. The profile can produce aspheric rather than only spherical ablations (Figs. 1-6 and 1-7). Larger diameters of ablation can be made. The possibility of using topographical maps of the cornea to guide the ablation is a distinct advantage, which will allow for more flexibility in treating astigmatism. Some patients do not have perfect symmetry of the cornea, particularly those with surgically induced astigmatism after penetrating keratoplasty or cataract surgery, or those with keratoconus. Broad beam lasers do not take the asymmetry of irregular astigmatism into account; they treat all corneas the same. The scanning technology allows the possibility of a customized ablation that is unique for every cornea (Fig. 1-7). 4

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(Note from the Editor in Chief: this flexibility of ablating different profiles in the same cornea is being utilized by some surgeons to create or "sculpt" the so-called "multifocal cornea" which is a significant step forward when it works but of increased risk to the patient's quality of vision when even a minor error in the sculpting occurs. This procedure is experimental). It may even be possible to improve upon the naturally occurring corneal surface, with improvement in best corrected visual acuity, bringing patients who are 20/20 with correction preoperatively to 20/15 uncorrected postoperatively. We still need more experience to know more definitively whether scanning lasers can actually fulfill their early promise. Currently Available Scanning Lasers Several companies are now working on developing scanning lasers. Chiron (now a division of Bausch & Lomb) has the Technolas laser. Autonomous Technologies, recently purchased by Summit, the company that manufactured one of the first broad beam lasers, also manufactures a superior quality scanning laser. This indicates

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Section 4 Figure 1-7: Concept of Spherical vs. Aspherical Ablation Profile as Obtained by the "Flying Spot" Type Laser This cross section of the anterior globe compares a spherical ablation profile (S) to an aspherical profile (A). A spherical treatment results in a corneal surface (1) which has the same radius (R1) throughout its curvature. The common center of the spherical curvature is shown at (C1). By comparison an aspherical ablation profile, made possible by the "flying spot" type excimer laser, is defined as one which has varying curvatures across the treatment zone. In the aspherical example (A), the central curvature (2) has longer radius (R2) than the mid-peripheral curvature (3), which has shorter radius (R3). The centers of curvature for the two areas of the cornea are different (C2 and C3). The curvature change between these two areas is gradual. Thus, the central cornea has a "flatter" curvature than the mid-peripheral cornea in this case. The dotted line represents the pre-op corneal curvature. (After Boyd´s "Atlas of Refractive Surgery").

that they believe the future of lasers is in scanning technology. The Japanese company Nidek and the U.S. company LaserSight also manufacture scanning lasers. The Nidek laser involves a slit that can be moved across the surface like the rectangular beam of a slit lamp . The Meditec is similar. The Visx laser has recently been modified ("Smooth Scan") to achieve a scanning effect. Although it is a broad beam laser, the smooth scan modification allows the broad beam to be broken up into individual beams that scan

the surface. It is predicted that the smoother ablation that results will improve results of surgery. McDonnell explicitly adds, however, that improved surface smoothness has yet to be proven in a prospective, randomized trial to translate into improved visual acuities. The Nidek and Autonomous lasers are now commercially available, with recent approval by the U.S. Food and Drug Administration (FDA). Other scanning lasers, such as the Technolas and LaserSight are now approved in USA by FDA. LASIK AND BEYOND LASIK

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Figure 1-8: Concept of Eye Tracking For More Accurate Corneal Ablations During Movements of the Eye New eye tracking technology can trace eye movements by detecting displacement of the pupil. In microseconds the eye tracking computer can move the treatment spot of an Excimer laser beam appropriately to compensate for these eye movements. For example, laser beam (LA) is treating an area of the cornea when the eye is in position (A). Suddenly, during treatment, the eye moves slightly to the left to position (B). The eye tracking computer detects the movement of the pupil to the left (dotted circle) and commands the laser to track left (LB) the same amount, within microseconds. Thus the laser continues treating the same area of the cornea as desired before the eye movement took place. Such technology aims to increase the accuracy of the desired ablation and resulting correction. (After Boyd´s "Atlas of Refractive Surgery").

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Eye Tracking Systems Another advantage of the scanning lasers is that they can be used in combination with eye tracking technology and computer controlled mirrors (Fig. 1-8) to move the spot automatically in microseconds to compensate for eye movements. At least theoretically, such a laser is not dependent on absolute fixation and can thereby improve the quality of the surface. As described by McDonnell, landmarks are identified at the beginning of the procedure. Without eye-tracking systems, if the patient looks slightly away from the fixation target while a broad beam laser is being used, the surgeon must quickly release the foot pedal and stop the ablation. With eye tracking technology, however, the laser immediately registers the movement of the eye and moves the spot accordingly without interrupting the surgery. Technologically, some of these eye tracking devices are quite impres6

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sive. Even if a patient moves considerably, the ablation spot can be placed perfectly. Autonomous Technologies, Nidek, LaserSight and several other companies now manufacture eye tracking systems. Proof that these trackers improve surgical outcomes is still to be established, according to McDonnell. Data have not yet shown that the eye tracker prevents decentration, or results in improved vision compared to results from a broad beam laser without eye tracking capacity.

How the Corneal Tissues are Affected in LASIK vs Incisional Keratotomy With excimer ablation in LASIK (laser in situ keratomileusis) and PRK (Photorefractive Keratectomy) most of the tissue is removed from the central part of the cornea. The ultraviolet light has so much energy that it smashes the inter- and intra-molecular bonds, ejecting the

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Figure 1-9: How Corneal Tissues are Affected with Different Refractive Techniques In this figure, you will clearly observe the differences in corneal tissue invasion comparing incisional keratotomy to laser in situ keratectomy (LASIK). (A) This cross section view of the cornea shows the tissue penetration of the diamond knife in RK with a deep incision of 500 microns reaching down close to Descemet's. The space shown between the arrows demonstrates the thin, untouched and intact corneal area. The corneal tissue strength is significantly weak and unstable because of the radial cuts. (B) This represents the corneal depth reached in LASIK by the excimer laser in a patient with -8.00 (myopic) diopters. The higher the myopia, the larger the ablation but limited by Jose Barraquer's thickness law. In this case the ablation depth reaches 240 microns (160 microns: corneal flap + 80 microns: stromal laser ablation). The rest of the corneal stroma is untouched (between arrows). (After Boyd´s "Atlas of Refractive Surgery").

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molecules at high speed (Fig. 1-2). Tissue ablation in LASIK reaches an average of 250 microns (Fig.1-9 B) from the original surface of the cornea. On the other hand, during incisional keratotomy (radial keratotomy for myopia and astigmatic keratotomy for astigmatism) the depth of incision into the corneal stroma reaches down to 500 microns, close to Descemet's membrane and almost 90% of the corneal thickness (Fig. 1-9-A). This major difference between the two techniques reveals how the stroma is significantly weakened in incisional keratotomy thereby affecting the strength and stability of the globe. LASIK involves no heat damage, no permanent scarring, not even a thermal effect.

In the long run, RK patients carry two swords of Damocles over their heads. One is the threat of a blow to the eye severe enough to cause a rupture. The RK patient is always more susceptible to rupture because the corneal scars will never be as strong as the original cornea. The second threat is that these scars apparently stretch or relax with time, which may give the patient more effect than the original result. An undercorrected patient moves toward a better result, but a properly corrected or overcorrected patient moves into hyperopia, and can become quite farsighted.


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Chapter 2 FUNDAMENTALS ON CORNEAL TOPOGRAPHY Guillermo L. Simón.M.D. , Sarabel Simón, M.D., José Mª Simón, M.D., Cristina Simón, M.D.,

Introduction: Human Optics and the Normal Cornea The cornea is the highest diopter of human eye, accounting alone for about 43-44 diopters at corneal apex (about two thirds of the total dioptric power of the eye). It has an average radius of curvature of 7,8 mm. A healthy cornea is not absolutely transparent: it scatters almost 10 % of the incident light, primarily due to the scattering at the stroma. The corneal geography can be divided into four geographical zones from apex to limbus, which can be easily differentiated in colour corneal videokeratoscopy : 1- The central zone (4 central millimeters): it overlies the pupil and is responsible for the high definition vision. The central part is almost spherical and called apex. 2- The paracentral zone: where the cornea begins to flatten 3- The peripheral zone 4- The limbal zone Refractive surgery refers to a surgical or laser procedure performed on the cornea, to alter its refractive power. The major refractive component of the cornea being its front surface, it is not difficult to understand that most refractive techniques have involved this frontal surface (PRK, radial keratotomies, …). Nevertheless, posterior surface of the cornea also accounts, and that is the reason why a “posterior surface corneal topographer” like the Orbscan™ - Bausch & Lomb® was developed by Orbtek®, in the race for a more precise refractive surgery.

The cornea of an eagle is almost as transparent as glass: there is almost no scattering of incident light. That alone explains the resolution of an eagle eye being much better than ours. As we are never satisfied, we are now developing new tools and exContents tremely promising laser surgical techniques that have proven to increase human being visual acuity by re- Section 1 ducing corneal aberrations: we reduce diopters and Section 2 also improve visual acuity. The new dream is “super-vision”. Topographic and aberrometer-linked Section 3 LASIK are on the way to achieve this goal of betterSection 4 than-normal vision. Bausch & Lomb®’s Zywave™ combines topography and wavefront measurements Section 5 to achieve customized computer controlled flying spot excimer laser ablation, which appears to be fun- Section 6 damental in treating irregular astigmatisms or retreatSection 7 ing unsatisfied LASIK patients to regularize the corneal shape. Regularizing the corneal shape has the Subjects Index theoretical advantage of improving the quality of vision by means of reduction of halos, glare and any other optical aberrations. We are on the way to achieve an aberration-free visual system, though the influence of all other dioptric surfaces (vitreous, lens, …) and interfaces still has to be ascertained. In this chapter we will try to introduce the Help ? novice to this interesting new world of instruments recently developed due to the advent of refractive corneal surgery. We have tried to show different maps from different systems, trying to make an interesting basic atlas of corneal topography. Corneal maps of rare cases and complications can be found in the different chapters of this book. Please refer to them for better knowledge. There is no perfect system to assess true corneal surface shape, but we still have to rely on the instruments we have, waiting for new


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instruments and methods being developed for better accuracy. With that goal in mind BioShape AG® has developed the EyeShape™ system, based on a principle called fringe projection. Patterns of parallel lines are first imaged onto a reference and then onto the surface to be measured. Detection of the lines with a digital camera under a tilted angle yields distorted line patterns. The deviation of the detected lines from the original lines together with the tilt make it possible to calculate the absolute height at any point on the surface of the cornea (or not).

Instruments to Measure the Corneal Surface The normal corneal surface is smooth: a healthy tear film neutralizes corneal irregularities. The cornea, acting as a convex “almost transparent” mirror, reflects part of the incident light. Different instruments have been developed to assess and measure this corneal reflex. These non contact instruments use a light target (lamp, mires, Placido disks, …) and a microscope or another optic system to measure corneal reflex of these light targets.

1- Keratometry A keratometer quantitatively measures the radius of curvature of different corneal zones of 3 mm (diameter). The present day keratometer allows the operator to precisely measure the size of the reflected image, converting the image size to corneal radius using a mathematical relation r= 2 a Y/y where r : anterior corneal radius a: distance from mire to cornea (75 mm in keratometer) Y. image size y: mire size (64 mm in keratometer)

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The keratometer can convert from corneal radius r (measured in meters) into refracting power RP (in Diopters) using the relationship: RP = 337,5 / r Modern -automated or not- keratometers also known as ophthalmometers directly convert from radius to diopters and inversely. They are mainly used to calculate the power of intraocular lenses through different formulas (Hoffer, SRK-T, SRK-II, Holladay, Enrique del Rio & S. Simón, …). Although the theory of measuring corneal reflex may appear to be simple, it is not, since eye movement, decentration or any tear film deficiency may difficult the measure creating errors. Modern video methods (topographers) can freeze the reflected cornea image, and perform the measurements once the image is captured on the video or computer screen, allowing greater precision. Notice that most traditional keratometers perform measurements of the central 3 mm, while computerised topographers can cover almost the whole corneal surface.

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2- Keratoscopy or Photokeratoscopy

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It is a method to evaluate qualitatively the Section 6 reflected light on the corneal surface. The projected Section 7 light may be a simple flash lamp or a Placido disc target, which is a series of concentric rings (10 or 12 Subjects Index rings) or a tube (cone) with illuminated rings lining the inside surface. When we look at the keratoscope, an elliptical distortion of mires suggest astigmatism, and small, narrow and closely spaced mires suggest corneas that have high power (steep regions or short radius of curvature). The use of keratoscopes is being abandoned Help ? in favour of computerised modern topographers which allow qualitative and quantitative measurements of the corneal surface, with higher definition and accuracy (more than 20 rings), and more sensitivity in the peripheral cornea.


Contents Figure 2-1 : The “ring verification display” in modern videokeratoscopes is a static picture of what the explorer viewed at the keratoscope. Looking at the keratoscope, the explorer is able to evaluate qualitatively the corneal surface. In this case, notice the huge distortion of the mires on the temporal side of a right eye of a patient who underwent a keratoplasty for a keratoconus, and is wearing a soft plano-T therapeutic contact lens. The distortion of the mires is due to an irregularity at contact lens surface: air is in between the cornea and the lens.

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Some of the known deficiencies of the Placido method are: • It requires assumptions about the corneal shape • It misses data on the central cornea (not all topographers) • It is only able to acquire limited data points • It measures slope not height Some more subjective complaints include: • It is difficult to focus and align • In most topographers, the patient is exposed to high light Large Placido disk systems work far away from the eye, while small Placido cones get much closer to the eye. While Placido disk systems easily create shadows caused by the nose and brow blocking the light of the rings, small cone systems fit un-

der the brow and beside the nose, avoiding shadows, Section 7 but can get in contact with large noses and make the patient blink and be afraid. Most small cones have a Subjects Index reputation for difficult focusing: some manufacturers -like Optikon 2000®- have worked out worthwhile automatic capture devices for improved accuracy, precision, and repeatability of measurements.

3- Computerized Videokeratoscopy: Modern Corneal Topographers

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Corneal topography has gained wide acceptance as a clinical examination procedure with the advent of modern laser refractive surgery. It has many advantages over traditional keratometers or keratoscopes: they measure a grater area of the cornea with a much higher number of points and produce permanent records that can be used for followup.


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Figure 2-2: The Placido cone consists of a series of concentric dark and light rings in the configuration of a cone of different sizes depending on the number of rings and the manufacturer. Usually, it is better to have a large number of rings, since more corneal radius values can be measured: notice that while describing the technical characteristics of videokeratographers some manufactures count both clear and dark rings, while others only count light ones. The mires of most systems exclude the very central cornea (where the video camera or CCD is located) and the paralimbal area. Picture shows a large cone of the Haag-Streit® Keratograph CTK 922™ with 22 rings (dark and light rings). (Published with permission from Haag-Streit® AG International).

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Basically, a projection corneal topographer consists of a Placido disk or cone (large or small) that illuminates the cornea by sending a mire of concentric rings, a video camera that captures the corneal reflex from the tear layer and a computer and software that perform the analysis of the data trough different computer algorithms. The computer evaluates the distance between a series of concentric rings of light and darkness in a variable number of points. The shorter the distance, the higher the corneal power, and inversely. Final results can be printed in colours or black-and-white. The Placido disk (Figure 2-2) consists of a series of concentric dark and light rings in the configuration of a disk or a cone, of different sizes, depending on the number of rings and the manufacturer. Usually, it is better to have a large number of rings, since more corneal radius values can be measured. The mires of most systems exclude the very central cornea and the paralimbal area. The reproducibility of videokeratography measurements is mainly dependent on the accuracy of manual adjustment in the focal plane. Videokeratoscopes having small Placido cones show a considerable amount of error when the required working distance between cornea and keratoscope is not maintained. The advantages of small cones (optimal illumination and the reduction of anatomi-

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cally caused shadows) are in no proportion to the Section 2 disadvantage—poor depth of focus, resulting in poor reproducibility. Which one should you choose, a Section 3 small Placido cone or large Placido disk ? Not easy to answer: each family of topographers has advan- Section 4 tages and disadvantages. Being no ideal instrument, Section 5 topographer potential buyers will have to decide upon other important factors, like software ability to ex- Section 6 actly reproduce real corneal height, number of rings, Section 7 price, …. There are two main groups of corneal topog- Subjects Index raphers: those which use the principle of reflection (most), and those which use the principle of projection. Let’s notice that the image captured by most topographers is produced by the thin tear layer covering the cornea that almost reproduces the shape or contour of the corneal surface. Most instruHelp ? ments perform indirect measurements of the corneal surface (reflection technique) and extrapolate to know the height of each point of the cornea. Reflection techniques amplify the corneal topographic distortions. Euclid Systems Corporation® ET-800 uses a completely different method of topography called Fourier profilometry using filtered blue light that induces fluorescence of a liquid that has been applied to the tear film before the examination. This


projection technique visualizes the surface directly while a reflection technique amplifies the corneal topographic distortions.

Table 1: Advantages and Disadvantages of Projection-Based Systems over Reflection -Based Ones. Advantages: Measurement of direct corneal height Ability to measure: irregular corneal surfaces non-reflective surfaces Higher resolution (theoretical) Uniform accuracy across the whole cornea Less operator dependent Do not suffer from spherical bias Disadvantages: Not standard instruments (most are still prototypes): complex to use need clinical experience validation non standard presentation maps (more difficult to learn)

Figure 2-3: There are different methods of following the clinical course of a corneal ulceration or corneal abscess. While daily slit-lamp examination and daily photographs are invaluable, corneal topographic maps, being less “explorer dependant”, can also be very useful in the follow-up.

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Longer examination time: longer image acquisition time longer image analysis

Section 3 Fluorescein instillation needed (in some, like the Euclid Systems Corporation® ET-800™)

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Table 2: Indications and Uses of Corneal Topographers: The use of computerised Corneal Topography is indicated in the following conditions: 1- Preoperative and postoperative assessment of the refractive patient 2- Preoperative and postoperative assessment of penetrating keratoplasty 3- Irregular astigmatism 4- Corneal dystrophies, bullous keratopathy 5- Keratoconus (diagnostic and follow-up) 6- Follow-up of corneal ulceration or abscess (Figure 2-2). 7- Post-traumatic corneal scarring 8- Contact lens fitting 9- Evaluation of tear film quality 10- Reference instrument for IOL-implants to see the corneal difference before and after surgery 11- To study unexplained low visual acuity after any surgical procedure (trabeculectomy, extracapsular lens extraction, …). 12- Preoperative and postoperative assessment of Intacs™ corneal rings (intrastromal corneal rings)

Table 3: Different Methods of Measuring Corneal Surface Used by Modern Corneal Topographers Placido systems (small cone or large disk) are the most popular


Placido cone with arc-step mapping (Keratron™ from Optikon 2000®) Placido disk with arc-step mapping (Zeiss Humphrey® Atlas™) Slit-lamp topo-pachimetry (Orbscan™ - Bausch & Lomb®) Fourier profilometry (Euclid Systems Corporation® ET-800™)

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Fringe projection or Moiré interference fringes (EyeShape® from BioShape AG™) Triangulation ellipsoid topometry (Technomed™ colour ellipsoid topometer) Laser interferometry (experimental method, it records the interference pattern generated on the corneal surface by the interference of two lasers or coherent wave fronts)


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Figure 2-4: Trichiasic cilia projects a shadow that may interfere with the mapping. This situation should be addressed prior to corneal topography.

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Section 4 Figure 2-5: Ptosis or non-sufficient eye opening because of induced photophobia or patient anxiety limits and distorts the mapping of the cornea. Notice that the map is not round but oval.

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Causes of Artefacts of the Corneal Topography Map: a- shadows on the cornea from large eyelashes or trichiasis (Figure 2-4). b- ptosis or non-sufficient eye opening (Figure 2-5)

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c- irregularities of the tear film layer (dry eye, mucinous film, greasy film) d- too short working distance of the small Placido disk cone e- incomplete or distorted image (corneal pathology) (Figure 2-6)


Figure 2-6: An advanced corneal herpetic keratopathy produces an irregular completely distorted corneal map in which no regular pattern can be identified. Notice that the low-vision patient is unable fixate the fixation light.

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Understanding and Reading Corneal Topography The meaningful interpretation of topographic maps requires the examiner to have detailed knowledge and clinical experience on the patterns detailed in them. At first, one must understand how to read the colour scales. The untrained eye may find some confusion and sometimes misinterpretation in evaluating corneal maps. Modern topographers

(videokeratographers) use the Louisiana State University Color-Coded Map to display corneal superficial powers. The power values (measured in diopters) are preferred by clinicians over the radius values (measured in millimeters), although all topographers can map the corneas using both values. Projection-based topography systems, adopted a similar colour scale to represent their height maps. High areas are depicted by warm colours, while low areas are depicted by cool colours.

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The Louisiana State University Color-Coded Map Colours correspond to the following:

Cool colours (violets and blues): low powers. They correspond to flat curvatures (low diopter)

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Greens and yellows: colours found in the normal corneas

Warm or hot colours (oranges and reds): higher powers. They correspond to steep curvatures (high diopter).


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Facing a corneal topography, care has to be taken to interpret coloured maps, since scales (and sometimes colour coding) can be modified in most topographers’ software. For patient examination manufacturer sets default values which are operator adjustable (diopter interval, radius interval). When operator adjusts the values to new parameters, colour scales are modified. Rare are the topographers that directly measure the corneal elevation: most act by extrapolation from corneal curvature and power at each measured point. The Optikon 2000® Keratron™ is one of those systems that accurately maps aspheric surfaces by means of its own method of arc-step mapping. The range of powers found in the normal cornea range from 39 D found at peripheral cornea, close to the limbus, to 48 D found at corneal apex. The colours do not always represent an elevation map, they correspond to curvature values. Therefore, the cornea is most curved towards the centre (green) and flattens out towards the periphery (blue). The nasal side becomes blue more quickly, indicating that the nasal cornea is flatter than the temporal. Some advanced instruments like the Optikon2000® Keratron™, are able to directly represent a coloured elevation map. Apart from colour maps, most topographers also display values of simulated keratometry, that should be equivalent to those obtained by a keratometer. Simulated keratometry values are obtained form the radius values at the corneal position (3 central millimeters) where the reflection from the keratometer mires would take place.

Topographic Scales: Two basic scales are commonly used: absolute and relative.

Absolute, Standardized or International Standard Scale: same scale for every map produced. Good for direct comparisons between different maps, for screening and for gross pathologies. It was designed to make only clinically relevant information obvious, by setting the interval between the contours of the power plot (i.e. in practice, the contours of colours) at 1,5 diopters (which means it has low resolution).

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Relative, Normalized or Adaptative Colour Scale: different scale for each map. The computer determines maximum and minimum curvatures for the map and automatically distributes the range of colours. The computer contracts or expands its colour range according to the range of colours present in a given cornea. It is best suited for looking at variations for a particular cornea. It has the advantage of offering great topographic detail since incremental steps are smaller (around 0.8 diopters) giving high resolution, but suffers from some inconveniences: the meanings of colours are lost (explorer and clinician have to carefully check the meaning of the colours, according to the new scale), a normal cornea may look abnormal while abnormal corneas may appear closer to normal. With this scale, subtle features are made apparent, being good for detail.

Computer Displays: Presentation of Topographic Information

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When confronted to a topography display, ei- Section 3 ther a printed report or on screen, one should study it Section 4 in a structured way to avoid mistakes in interpretation, and get the most of it. Section 5 Proceed as follows: • Check the name of the patient, date of exam Section 6 and examined eye. Section 7 • Check the scale: • type of measurement (height in microns, cur- Subjects Index vature in mm, power in diopters) • step interval • Study the map (type of map, form of abnormalities, …) • Evaluate statistical information (cursor box, statistical indices when given …) Help ? • Compare with topography of the other eye (always perform bilateral exams, when possible) • Compare with the previous maps first verifying they are in the same scale) • Apply statistical analysis or other needed software application (contact lens fit, surgical modules, 3-D colour maps, neural networks, …) • Explain the exam’s results to the patient


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Figure 2-7: Absolute Scale Section 1 Section 2

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Figure 2-8: Normalised Scale

Figures 2-7 and 2-8: These two maps may look different but are the same axial diopter map of the left eye of the same patient (keratoconus) measured in different scales, absolute on the left and relative on the right. Notice very high diopter values under corneal vertex, where corneal surface is most elevated.


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To present a corneal topography, each software application (i.e. each instrument) has a large number of computer displays. Most are produced form data of a single application, and are software dependent. Most instruments are able to show: a ring verification, a numerical display, a large number of corneal maps, a simulated keratometry, a meridional plot, and some can display a 3-D reconstruction of the corneal surface. a) Ring Verification (keratoscopic raw image): (Figure 2-1, in this chapter): displays a

keratoscopic image of the Placido rings reflex on the examined cornea. It is a raw image, that allows qualitative evaluation of the image taken (irregularity of tear film layer, lids aperture, …), helping the examiner to either accept or reject the taken image. It is very useful when there is a question regarding the accuracy of the displayed data. b) Numeric Display: of a number of corneal power values along several meridians shown in a radial display. Helpful to make the data amenable to statistical methods.

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Section 3

Section 4 Figures 2-9 and 2-10: The numeric display shows a number of corneal power values along several meridians in a radial display. It is a very helpful presentation to make the data amenable to statistical methods. Notice that picture on the left (Axial Diopter) displays corneal powers in diopters and that on the right (Axial Radius) shows the same values in millimetres (corneal radius). Most topographers allow you to choose the way you want the results to be shown.

Section 5


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a) Corneal Maps: details of the most common (axial, tangential, 3-D, …) will be discussed later in this chapter. Each topographer offers different maps or ways of presenting the results. Please refer to your topographer’s manual for more details. b) Simulated Keratometry Readings (SimK): obtained form the radius values at the corneal position (3 millimeters central zone) where the reflection from the keratometer mires would take place. The major axis is that is that with the greatest power, and the minor axis is at 90º to it (per-

pendicular axis). The cylinder is the difference between the major and minor axis. The meridian with the lowest mean power can also be displayed. c) Meridional Plot: shows the minimum and the maximum corneal power values, displaying a cross sectional profile of the cornea along the chosen meridian. It is used to show the general shape of the cornea to the patient, and assessing the toricity for contact lens adaptation. The helps identifying the ablation zone limits following LASIK or PRK.

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Figure 2-11: Shows a “multiple exams” of both eyes of the same patient, a 38 year old man who underwent LASIK in both eyes at a time for high myopia. Corneal map is overlaid upon the keratoscope eye image to aid interpretation. The overlay shows the spatial relationship between the pupil, the ablation zone and the cornea. Notice that immediately after surgery (the day after), ablation zones differ from each other: it is due to the fact that a different excimer laser was used for each eye. Schwind® Keratom™ was used on right eye, while left eye was operated using the Bausch & Lomb® -Chiron Technolas 217™. Although ablation zone seems more perfect and regular on right eye (tangential diopter map), this does not mean that visual result is better. The meridional plots shown under tangential diopter maps help the surgeon to evaluate the effectiveness and ablation pattern of the excimer laser he or she uses.


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or the depth of a corneal defect (ulceration, laser ablation zone, keratoconus, …). Some topographers display the spherical height map relative to a reference spherical surface, by comparing to a best fit calculated reference sphere.

Common Corneal Maps 1)

2)

Axial Map: it is the original and most commonly used map. It provides measurements based on the keratometer formula. It is helpful is evaluating the overall characteristics of the cornea and classify the corneal map (normal or abnormal). It can differentiate between spherical, astigmatic or irregular corneas. It is the most stable type of map, but may confuse the explorer when evaluating the peripheral cornea. (see Figure 2-18 in this chapter). Height Map: true height data (in microns) is immediately available from systems using the principle of projection, although a reflection system like the Optikon 2000® Keratron™ does a good job with its own arc-step method of representing corneal height. Very useful in numeric or cross-sectional format to quantify the elevation

3)

Tangential Map (see Figure 2-11 in this chapter): this very useful display provides a measurement of corneal power over a large portion of the cornea, based on a mathematical radius formula. It is more accurate than axial map in the corneal periphery, but is subject to greater variation when comparing several exams that are repeated. It may help detecting mild corneal changes that might not be detected by standard axial map. It is used for locating corneal distances on the map, and to locate a cone or peak position in keratoconus, as well as to locate the ablation diameter and position after laser refractive surgical ablation.

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Figure 2-12: Shows a “multiple exams view” of left eye of the same patient, a 58 year old women who underwent (a couple of years before consultation) complicated phaco-emulsification converted to extracapsular surgery. In the hurry, surgeon sutured the cornea too loose, thus creating a peripheral superior corneal wound defect. High against-the-rule astigmatism is well represented by the axial diopter display (superior right), and well measured by the keratometer display (5.25 D at 87 º). But only tangential diopter map (down-right) accurately represented the corneal wound suture defect: notice the red superior area where the sutures used to be.

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4) Refractive Map: it is a map based on an axial map, using Snell’s law to calculate the refractive power of the cornea. It is mainly used in pre and post corneal surgery. 5) Elliptical Elevation Map: it represents the height of the cornea in microns, at different cor-

neal positions, relative to a reference elliptical surface. It is useful to visualize corneal shape. In contrast to the spherical height map -which uses a simple spherical reference- the elliptical elevation map matches better to the inherently elliptical shape of the healthy cornea.

Figure 2-13: Shows a “multiple exams view” of both eyes of the same patient, a young man referred for refractive surgery who -to our surprise- was never diagnosed astigmatic. Axial diopter maps are displayed, in normalised (right eye) and absolute scales (left eye). Elliptical elevation with keratometer overlay maps help better assess true corneal shape and direction or axis of astigmatism. Radius of the reference ellipse are displayed and can be modified by operator: BaseR refers to central radius value, and BaseR (2.5 mm) refers to the radius value at 2.5 mm.

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6) 3-D reconstruction Map: is used to visualize the overall shape of the cornea in a more realistic way. Understandable for the patient, it can be rotated and tilted as desired. Some instruments like OCULUS® Keratograph and Haag-Streit® Keratograph CTK 922 offer excellent comprehensive kinetic three-dimensional (3-D) analysis of corneal topography for simple explanation to the patient. (see Figures 2-26, 2-27, 2-32b and 2-38 in this chapter).

7) Irregularity Map: It calculates a best sphere/cylinder correction for the cornea, subtracting the correction from either axial or tangential data and presents the remaining irregularities. Used after refractive surgery to detect irregularities that may explain a low visual acuity. It reports an index that measures eccentricity (a measure of asphericity) and the amount of astigmatism that has been subtracted form the original corneal data (Fig. 2-14).


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Contents Figure 2-14: Shows an axial irregularity map in diopters of the right eye of a 55 year old man suffering form a paracentral progressive corneal ectasia (central keratoconus). Notice the Q index with a value of -1,25 (measuring eccentricity) and an astigmatism of 4.5 D, resulting form the subtraction of the original corneal data and the best sphere/cylinder for that cornea. An overlay option adds an irregularity index to the map for increasing circles of 1 mm radius, best visualised thanks to the overlay circular grid option. Normal values would be 0.2 or 0.4, but this exceptional case shows 3.5 and 4.0 zonal indices.

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Section 5 Table 4: Common overlays that can be added to a topography map to help interpretation (See Fig. 2-15) Pupil margin: displays the visually important region. Helps evaluating photopic pupillary size, and the centration or refractive surgery. Grids Square: helps defining size and location of abnormalities. Circular: helps defining size and location of abnormalities. Polar: helps defining axis of abnormalities and the assessment of radial keratotomies. Optical zone: useful in refractive surgery for planning procedures or assessing results. Angular Scale: useful in refractive surgery of astigmatism for planning procedures or assessing results. It is similar in use to polar grid. Eye image: more realistic than a simple map, it eases patient’s interpretation of the map. Keratoconus : a peak or keratoconus overlay can be applied by Dicon’s CT-200. It is called Bull’s Eye target: if one peak area exists with an index of 10 or greater, the system automatically marks it with a target, indicating the location of this elevation to some but not all maps (see Figures 2-12 and 2-15 in this chapter). Keratometer mires: it is a graphic reference showing a 3mm circle with both major and minor meridians, representing the calculated keratometry readings, 90 degrees apart (perpendicular). It also shows a 5 mm with the steepest and flattest meridians. (see Figures 2-13 in this chapter).

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Section 1 Figure 2-15: Different overlays can be added to a topography to help interpretation. The picture shows a quadruple view of an almost normal cornea of a young contact lens user with mild corneal warpage only diagnosed by means of the tangential maps (c and d). Notice that b) is displayed in radius (mm) while the rest of maps are displayed in diopters (see the colour scale). Map a) displays a centre overlay (small red cross) that indicates where the true centre of the cornea is, and a pupil outline overlay that reproduces pupil margin, the visually important region. Map b) shows a “verify rings” overlay, to better asses the quality of the taken image. Red and green concentric rings should alternate and not cross. The red rings should be located on the outer edge of the white rings, and the green rings should be located on the outer edge of the black rings. Map c) shows an angular scale that helps to locate the axis of astigmatism. Map d) shows “eye image” overlay, the image of the patient’s eye is displayed to ease patient’s interpretation of the map. Notice that a paracentral target marks an elevation zone that has to be carefully inspected. Angular scale is also displayed in map d).

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Special Software Applications and Displays Each available instrument is sold with standard software package and most offer optional packages at additional price. The most common are: Multiple Display Option: a customisable multiple display allows simultaneous screen display

for rapid analysis and ease of use. Depending on the software of your topographer, you can simultaneously view either one, two or four maps. Extremely practice in daily use to ease work and interpretation. Surgical Applications: used to predict the results of refractive surgery, and for postoperative evaluation. Some -but not all- allow refractive surgery simulations and topography linked laser refractive surgery with special excimer laser brand names.


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A

B Contents

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Figures 2-16 a and b: Dicon’s CT200™ trend analysis displays a series of exam maps (pre-operative exam, first post-operative exam, most recent exam and a choice of a K-trend graph, a pre/post operative difference map or a post-last difference map.) Shown are trend analysis of both eyes of a patient who underwent myopic LASIK with two different excimer lasers. Shown are axial diopter pre-operative, tangential diopter immediate postoperative and K-trend graph. Notice that immediately after surgery (the day after), ablation zones differ form each other: it is due to the fact that a different excimer laser was used for each eye. Schwind® Keratom™ was used on right eye, while left eye was operated using the Bausch & Lomb® -Chiron Technolas 217™. K-trend graph shows the major (green) and minor (blue) K values for all exams in the series. The Y axis is power in diopters, and the X axis is the exams’ number spaced out over time. The vertical line marks the date of surgery. Trend analysis eases a rapid overview of healing trend over time.

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Contact Lens Fitting Application: they are used for contact lens fitting, and help choosing the best suggested lens for each case, by simulating the fluorescein pattern and contact lens position of rigid contacts. Not all topographers offer this feature: in some cases this software module is sold as an option. For instance, Dicon’s CT200™ (Fig. 2-16 A-B) offers as standard the Mandell Contact Lens Module “Easy-Fit™”, and as an option the Mandell Contact Lens Module “Advanced-Fit”™ for toric, bi-toric, keratoconic fitting and post-surgical fitting with Labtalk™. Contact your dealer for more precise information. The simulated fluorescein feature is intended to reduce fitting time by viewing the effect of changing lens parameters on a personalized basis, depending on the patient’s corneal exam. Let’s notice that

the true “in vivo” result of any computerised fluorescein test may vary due to differences caused by lid action on the lens (aperture and weight). Ask the manufacturer of your topographer for special software applications, and for the possibility to link your topographer and your excimer laser for better results.

Topography Maps of the Normal Cornea When considering the topography of a normal cornea, we feel the need to remember that there is a wide spectrum of normality. No human cornea demonstrates the kind of regularity found in the calibration spheres of a topographer: the eye is not moulded glass-made. Normal corneal topography can take on many topographic patterns (see table 5):

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Table 5: normal topographic patterns:

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Spherical (Round) (Figure 17) 20% With-the-rule (Oval) (Figure 18) 20% With-the-rule (Symmetric bow tie) 17% With-the-rule (Asymmetric bow tie) 30% Against-the-rule Displaced apex: Inferiorly Nasally Irregular 7% causes of irregularity: dry eye corneal scar or ulceration trauma corneal degeneration corneal edema pterygium contact lens overuse (corneal warpage) surgery (cataract, keratoplasty, …)

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Contents Figure 2-17: Shows a “multiple exams view” of left both eyes of the same patient, a 38 year old woman prior to LASIK surgery. Corneal topography remains a routine exam for preoperative and postoperative assessment of the refractive patient. This report shows normal, spherical (round), corneas in both eyes (44 D at vertex, and mostly green colour in the map). The colour zones are approximately circular in shape. Notice that lid aperture is not the same in both eyes, thus making it more difficult to map superior corneal periphery in left eye.

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Section 6 Section 7

Regular astigmatism (with-the-rule) gives an oval axial corneal map, being the most common deviation from optically perfect spherical (round) cornea. If the bow tie is vertical (the long axis is near the vertical meridian) in an axial map, it represents a cornea having with-the-rule-astigmatism. If the bow tie is horizontal, it represents an “against-the-rule” astigmatism, ninety degrees rotated when compared to a with-the-rule astigmatism. When the bow tie is diagonal, it represents a cornea having an oblique astigmatism. The shape and colours of the bow tie are influenced by the rate of peripheral corneal flattening, and the appearance is influenced by the scale interval chosen by the explorer. The bow tie may be symmetrical or asymmetrical along the perpendicular meridian: one half of the bow tie is significantly larger than the other,

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the corneal apex being located in the direction of the Subjects Index larger bow half, slightly decentered form the visual axis. In the normal eye, nasal cornea is flatter than temporal. The nasal side of a healthy corneal map becomes blue more quickly, indicating that the nasal cornea is flatter than the temporal. There is a physiological astigmatism of around 0,75 diopter. Help ? Physiologically, the axis may not be the same superiorly than inferiorly. In an axial map, the rate of flattening is greater when the colour scale interval is larger, and there are many colour zones. A focal steepening inferiorly may exist due to the lower tear meniscus. Generally, the two eyes of the same subject are very similar, and present a mirror image of each other (Figures 2-18 and 2-19). This


Figure 2-18: Axial diopter displays are showed for both right and left eyes. The patient suffered from regular astigmatism (with-the-rule), that gives an oval corneal map, being the most common deviation from optically perfect spherical (round) cornea. The long axis is near the vertical meridian. The shape and colours of the bow tie are influenced by the rate of peripheral corneal flattening: notice the nasal peripheral flattening in left eye (purple colour). This binocular report form Dicon’s CT-200 topographer shows pupil size and simulated keratometry of both eyes. RE size pupil is 4.03 mm, and astigmatism 3.12 D at 8 º. Notice that the two eyes present a mirror image of each other: this phenomenon is called enantiomorphism.

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Figure 2-19: Enantiomorphism is the phenomenon wherein an individual’s topographies are non-superimposable almost mirror images of each-other. The knowledge of this fact is useful to decide whether a cornea is normal or not, by comparing to the map of contralateral eye. Notice that even pachimetry maps reflect this phenomenon (Corneal thickness was mapped with Bausch & Lomb® Orbscan™ topo-pachimeter).


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phenomenon is called enantiomorphism. The knowledge of this fact is useful to decide whether a cornea is normal or not, by comparing to the map of contralateral eye. Small changes in corneal shape do occur throughout life: • in infancy the cornea is fairly spherical, • in childhood and adolescence, probably due to eyelid pressure on a young tissue, cornea becomes slightly astigmatic with-the-rule • in the middle age, cornea tends to recover its sphericity • late in life, against-the-rule astigmatism tends to develop Short-term fluctuation and diurnal variations are not rare, and usually remain unnoticed by individuals with normal corneas. Some conditions like corneal dystrophies, ocular hypotony, radial keratotomies or contact lens use can make them apparent.

Table 7: Uses of Substraction or Difference Maps: validation of various exams taken in a same session ascertain the existence of progressive corneal astigmatism comparison of preoperative and postoperative corneal maps (LASIK and PRK) follow-up of myopic regression (LASIK and PRK) establishing ablation zone centration (LASIK and PRK) assessing resolution of corneal warpage in rigid contact lens users assessing evolution of a corneal ulcer or abscess

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Table 6: Factors that Slightly Affect the Normal Curvature of the Cornea

Section 7 Subjects Index

Lid closure during sleep time Tear film quality Lid pressure on the cornea (weight, exoftalmos) intraocular pressure Menstruation Pregnancy

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Comparing Displays: Maps can be compared directly only on the same scale, when taken with the same instrument, and preferably by the same explorer. It is not a good idea to compare maps taken with different instruments: every instrument uses a different measuring

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algorithm that may confuse you, specially when comparing subtle details. Most software applications allow the comparison of different maps over time, and even subtract values form two different exams (substraction or difference maps) (Figure 2-20). They are invaluable to the refractive surgeon.


Figure 2-20: A tangential diopter difference map o the left eye of a 21 year-old patient is shown. The subtraction has been performed between two different eye fixations to determine the existence of any irregularity in the ablation zone. The patient underwent a successful bilateral LASIK surgery to correct a high myopic astigmatism in both eyes a year before.

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Figure 2-21: A diopter difference map is useful to assess the validity of the different exams with the same fixation performed in the same session. Low differences due to tear film irregularities, lid aperture and blinking is acceptable. In case of difference between maps taken at the same moment, they need to be repeated, after a few blinks form the patient. If significant difference persists, try instillating a tear substitute in both eyes and wait a few minutes. Should differences persist, repeat the exams in a few days. Image shows a left eye with regular (wit-the-rule) high astigmatism : both axial diopter maps were taken in the same session: differences exist between the exams. Eye fixation is the same (center): differences a attributable to different lid aperture and form blinking. Axial diopter difference (down, with a square grid overlay) shows that differences are almost non significant (around 0.25 - 0.50 diopters), but exist. Such differences are physiological: difference maps allow validation of various exams taken in a same session.


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Section 1 Section 2 Figure 2-22: Difference maps ease the astigmatism progression follow-up . Tangential diopter displays show right eye maps of a 22 year old myopic patient referred for refractive surgery. To our surprise, neither glasses nor contacts had astigmatism. The existence of astigmatism was ascertained with the keratometer, subjective refraction and skiascopy. Corneal topography was performed and helped the demonstration of its existence. Picture shows a difference map between two exams taken with a 3 months delay (see the dates of the exams). Tangential diopter difference is 0 (green), meaning that no changes have occurred in that period of time. The first impression is that the guy never had good refraction, but new topographic exams will be performed 6 months and one year later, before refractive surgery is decided, so as to make sure that no keratoconic formation is on the way.

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BRIEF ATLAS OF CORNEAL TOPOGRAPHY SPECIAL TOPOGRAPHIC CONDITIONS Figures a1 to a20: All maps have been taken with a KERATRON™ Corneal Topographer (Optikon 2000® S.p.A, Italy - Europe). The corneal maps are courtesy of: Istituto Scientifico Ospedale San Raffaele - Milano (Prof. Brancato - Dr. Carones) Ospedale Fatebenefratelli - Roma (Prof. Neuschüller - D.ssa Cantera) Centro Oculistico - Rovigo (Prof. Merlin - Dr. Camellin) Clinica Oculistica Universitaria - Padova (Prof. Bisantis) University of North Carolina - Chapel Hill (Prof. Cohen - D.ssa Tripoli) University of California - Jules Stein Institute - Los Angeles (Dr. Maloney) We want to specially thank them as well as the manufacturer of the Keratron™ videokeratoscope, Opticon 2000® S.p.A., for the permission to reproduce them.

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Figure a1: Map of a normal round cornea There is a wide spectrum of normality. No human cornea demonstrates the kind of regularity found in the calibration spheres of a topographer: the eye is not polished glass-made. Normal corneal topography can take on many topographic patterns: picture shows the axial map of a right eye normal round cornea, with concentric green rings in an absolute scale. Notice that the nasal side of this healthy corneal map becomes blue more quickly than temporal side, indicating that the nasal cornea is flatter than the temporal. In the central 3 mm zone, there is a small amount of astigmatism (1 D displayed), which is within normal limits, and does not mean that the patient needs to be corrected with this astigmatism.


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Figures a2 and a3: Normal cornea with astigmatism according to the rule Help ? Regular astigmatism (with-the-rule) gives an oval axial corneal map, being the most common deviation from optically perfect spherical (round) cornea. Observe that the bow tie is vertical (the long axis is near the vertical meridian) in an axial map, representing a cornea having with-the-rule-astigmatism. Picture displays an axial curvature map of a -3.7 D regular astigmatism in an adjustable scale. Always check the scale in which the map is offered: colour differences do not always mean a difference in dioptric or radial values, but can mean a difference in the scale used by the explorer. Notice that a simulated keratometric overlay is displayed at the centre of the bow tie. Modern topographers run under Windows™ operating system, and are easy to use. Most software enables to enlarge desired areas for better explanation to the patient and to better view the details. Picture shows an enlarged area of a with-the-rule astigmatism with an absolute scale.

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Section 5 Figure a4: Topographic map of astigmatism expressed in heights

Section 6

Representation of the topographic map of an astigmatism (-3.75 D at 176º) expressed in height (in microns). The yellow area corresponds to a sphere with a defined radius, while orange-red and green-blue areas correspond to either elevation or flattening of the cornea. Notice that colour scale may confuse the explorer.


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Figures a5 and a6: Keratoconus An important indication of corneal topography is the screening of candidates for refractive surgery. It is very important to identify patients with corneal ectasia, since surgical outcomes are uncertain in most cases. Early detection of a subclinical keratoconus can save the patient of a refractive procedure (incisional or photoablative) that likely will not result in the desired visual outcome, and may result in dangerous corneal thinning. The most frequent ectatic corneal disorder is keratoconus. This condition is characterised by a corneal stromal thinning. It typically presents in early adulthood, is almost always bilateral (although can be very asymmetric), and progresses slowly over the years. Mild keratoconus cannot be detected easily at the slit-lamp, and only corneal topography can help detecting them. Some other conditions, like corneal warpage of RGP contact lenses may mimic mild keratoconus corneal maps. In most cases, the corneal thinning occurs just inferior to the corneal centre. Protrusion of this region gives the cornea an exaggerated prolapsing shape. The point of maximum protrusion is called the apex of the cone. Picture displays a typical map of a moderate keratoconus (- 5.6 D), showing a corneal steepening inferior to corneal vertex (orange-red, in absolute scale, in the shape of a pear fruit). Notice the high corneal central power (around 50 D), the inferior cornea (orange) steeper than superior cornea (green), and the large difference between the power of the corneal apex and that of the periphery. (Cont. in page 35)

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(Cont. from page 34) A topographic classification of keratoconus can been established: Severity

Site of the cone

Shape of the cone

Slit-lamp detectable

Subclinical

Inferior

like a pear fruit

No

Clinical: Mild

Inferior

Typical, oval like a pear fruit

Moderate

Central +/- Inferior

Globus

Sometimes needs a trained explorer Yes

Severe

Superior

Nipple

Yes, visible without slit-lamp

The comparison of representation of dioptric powers, axial (left) and local (right), of the same eye with an inferotemporal keratoconus is surprising: notice the minimal extension of the corneal surface involved in the pathology, and the flattening of the adjacent area.

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Figure a7: Corneal ulcer By quantifying the irregularity of the cornea, topography helps to determine the proportion of the visual loss of a patient suffering from a corneal ulceration or epithelial disruption close to the visual axis. It also helps to follow-up a corneal abscess or ulceration. Picture shows the true curvature map of a corneal inferior ulceration. Notice the local flattening of the corneal surface (in blue), resulting form the localised depression of the ulcer, surrounded by a ring of oedematous elevated tissue (in red).


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Figure a8: Marginal pellucid degeneration Stromal corneal disease include a variety of inflammatory and non-inflammatory disorders, like Terrien’s marginal degeneration, Mooren’s ulceration, pellucid marginal degeneration an others. Picture displays true elevation map (left) and axial map (right) of a pellucid marginal degeneration, a narrow band of corneal thinning located 1-2 mm from the inferior limbus. Observe the flattening of the central cornea (true elevation and axial maps) along the vertical axis. Extensive peripheral guttering leads to irregular against-the-rule astigmatism, such as this arching inferior bow tie visible in the axial map. These topographic findings are characteristic: they help establishing diagnosis even in patients without slit-lamp typical findings.

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Section 3 Figure a9: Contact lens overuse (warpage) Different types of contact lenses have different impact on corneal surface and different indications. We can classify them into three main groups: soft, rigid gas permeable (RGP) and hard (PMMA). The last are no longer considered suitable for making contacts, and are only prescribed in special cases. Rigid gas permeable contact lenses a relatively popular: they offer good visual performance, they can be polished, they tolerate most known cleaning solutions, and custom designs are possible. The bad side also exists, since they require individualised fitting (by means of k readings, topographic maps, …), they are not easily tolerated at first, and induce with relative ease changes of the shape of the cornea: the process of changes is termed warpage. It is thought due to mechanical pressure on the cornea, although other factors like oxygen deficiency have not been excluded. Many topographic patterns may result, like the one in the picture, depending upon the fit (size, curvature, …) and position of the lens. In this case, observe the inferior steepening in the axial map causing meridian asymmetry as a result of superior riding contact lens. The true elevation map shows corneal surface irregularity (orange). Cessation of the lens wear and good ocular lubrication result in return to corneal former shape.

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Contents Figure a10: Curved arcuate keratotomies (astigmatic keratotomies)

Section 1

Most refractive efforts have concentrated on altering the shape of the cornea, which is the main diopter of the eye. Topography is valuable in the preoperative assessment and planning of the surgery. Picture displays both true elevation (left) and axial maps (right) of an astigmatic patient who underwent astigmatic keratotomies. Two paired circumpherential relaxing incisions centered on the steep axis result in focal steepening (orange-red in true elevation map) and central flattening in that meridian (blue). The final result is 0.13D of astigmatism in the 3mm central cornea.

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Figure a11: Keratotomy with a resulting ectasia Every surgical procedure has some risks the patient must be aware of. Any kind of keratotomy (radial, astigmatic or other) may perforate the globe or result in an ectasia like the one shown in the picture. The inferior ectasia simulates an irregular keratoconus, in both true elevation (left map) and axial (on the right) maps.


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Figure a12: Photorefractive keratectomy PRK To correct myopia, the excimer laser removes more tissue from the centre than the periphery of the treatment zone (ablation zone). The ablation profile is different for every model of laser. The map on the left represents the spherical approach (axial curvature) of a patient who suffered myopic photorefractive keratectomy. Only the “true elevation” map (on the right) shows the transition area, where dioptric powers are very high (ring in red).

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Help ? Figure a13: Subtraction map in a PRK The most effective way of displaying the changes in a cornea that undergoes a refractive procedure are difference maps. The change induced by surgery is obtained by subtracting the preoperative map (upper small axial map) from the postoperative map (lower small axial map). The image on the right shows the result (in terms of dioptric variation, axial curvature) of a myopic PRK. In red, the ablation zone. In orange, the transition zone, which is easily delineated in the postoperative axial map which shows that the central cornea has been flattened (lower small axial map on the left).

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Figure a14: True curvature analysis in pre/post PRK

Contents The comparison between preoperative and postoperative true curvature analysis of the same PRK patient shows no variations of the peripheral cornea after surgery.

Section 1 Section 2

Section 3

Section 4

Section 5


Help ? Figure a15: Paracentral island Many are the potential complications of laser refractive surgery. Some may be attributable to the ablative pattern of each model of excimer laser, like central or paracentral islands, although the origin is uncertain. They are defined as any area within the ablation zone surrounded by areas of lesser curvature on more than 50% of its boundaries. They are a topographic pattern in PRK and LASIK patients, not always obvious. Picture displays a paracentral island after myopic PRK: it can be identified down inside the ablation red ring as a yellow-orange spot. Notice that only with the calculation method of local powers (true curvature map on the right), this small abnormality is made visible, remaining invisible in the axial map (on the left).


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Figure a16: Effect of suture removal after keratoplasty Serial topographic exams after a penetrating keratoplasty reveal large configurational changes the first two months, which remain stable until suture removal. Topography is then used to determine the suture to be removed in order to lower suture induced astigmatism and enhance visual recovery. Picture shows a test comparison: left map displays high astigmatism after a penetrating keratoplasty (-6.18 at 173º), right map displays the reduction to 1.33D after suture removal. Notice the asymmetry of power between the two hemi-meridians, that improves after suture removal. Observe the red areas of high power (and elevation) near the wound. Topographer is preferred over keratometer as most changes do occur outside the 3mm area measured by the keratometer.

Contents

Section 1 Section 2

Section 3

Section 4

Section 5


Help ? Figure a17: Software adjustment of a decentred axis The Keratron™ Corneal Topographer (Optikon 2000® S.p.A, Italy - Europe) offers some interesting features like the possibility of replacing the optical axis when the patient’s fixation is not as desired or corneal centration is not perfect. The system is able to recalculate the optical power values for the whole cornea. Notice that values at the optical axis differ from the original map with geometric axis calculations (on the left) and the recalculated map with the new visual axis position (map on the right).

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Contents

Section 1 Section 2 Figure a18: Myopic and hyperopic keratomileusis (LASIK)

Section 3

Shown are two “true curvature” maps of both myopic (left) and hyperopic (right) keratomileusis. To correct myopia, excimer laser removes a central disc of corneal stroma, resulting in central flattening (blue) and the presence of a relative peripheral steepening ring (red). Corneal topographic changes similar to those seen after photorefractive keratectomy (PRK) occur after LASIK for myopia. To correct hyperopia, the excimer laser does just the opposite: it removes an annulus or ring of tissue from the mid-periphery (blue) to steepen the central cornea (red).

Section 4

Section 5


Help ?


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Chapter 2

Figure a19: Intrastromal segmented graft for the correction of high myopias Picture shows a “true curvature” map of a left eye cornea that received an intrastromal segmented graft for the correction of high myopia. The map is similar to that of a myopic LASIK, but less regular.

Contents

Section 1 Section 2

Section 3

Section 4

Section 5


Help ? Figure a20: Fluorescein simulation in RGP contact lens Contact lens fitting applications are used to help choosing the best lens for every case, by simulating the fluorescein film pattern and contact lens position of rigid contact lenses (RGP and PMMA). The simulated fluorescein feature is intended to reduce fitting time by viewing the effect of changing lens parameters on a personalised basis, depending on the patient’s corneal exam. Let’s notice that the true “in vivo” result of any computerised fluorescein test may vary due to differences caused by lid action on the lens (aperture and weight).

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Figure 2-24: Elevation Map Figure 2-23: Zeiss Humphrey Systems® ATLAS™ Corneal Topography System Models 993 and Eclipse 995

Contents

Section 1 ®

TOPOGRAPHERS CURRENTLY Technomed Color Ellipsoid Topometer AVAILABLE The reproducibility of videokeratography Zeiss Humphrey Systems® ATLAS™ Corneal Topography System Models 993 and Eclipse 995 (Figure 2-23, with permission) Zeiss Humphrey Systems® ATLAS™ Corneal Topography System Models 993 and Eclipse 995 are best sellers in the USA. They measure true elevation data (Figure 2-24, with permission) through an advanced arc-step algorithm (similar to Optikon 2000® Keratron™), by means of 20-22 ring conical Placido disk. The Atlas Eclipse 995 offers ultra-low illumination and increased peripheral coverage (limbus to limbus). They also offer automatic pupil measurement. Software displays are viewed in a 10,4 “ TFT 640x480 pixel resolution in 18 bit colour; they include: photokeratoscope view, axial map, tangential map, numeric view, and profile view. Very interesting optional software packages are available at a price: MasterFit™ contact lens module, corneal elevation map, corneal irregularity map, refractive power map, keratoconus detection map, VisioPro™ ablation planing software and Healing Trend/ STARS™ display.

Section 2

Section 3

measurements is mainly dependent on the accuracy Section 4 of manual adjustment in the focal plane. Videokeratoscopes having small Placido cones show Section 5 a considerable amount of error when the required working distance between cornea and keratoscope Section 6 is not maintained. The advantages of small cones Section 7 (optimal illumination and the reduction of anatomically caused shadows) are in no proportion to the Subjects Index disadvantage, poor depth of focus, resulting in poor reproducibility. The Color Ellipsoid Topometer compensates defocusing errors with software and hardware, by means of a triangulation measurement., enhancing precision and theoretically avoiding measuring Help ? artefacts. It is the only Placido (30 ring) system with colour coded rings (three coloured rings). By means of a laser, it measures 10800 points, providing real height values and has ray tracing software. A new module enables topography-driven laser ablation. This unit is specially useful in diagnosing postoperative problems in a refractive practice, specially in those cases with a loss of vision that cannot be explained. The Color Ellipsoid Topometer can predict the quality of vision based on the shape of the cornea and pupil.


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Chapter 2

Figure 2-25: DICON ® CT200

®

DICON CT200 (Figure 2-25, with permission) The reproducibility of videokeratography measurements is mainly dependent on the accuracy of manual adjustment in the focal plane. The

DICON® CT200 is a cheap easy to use instrument with autofocus and autoalignment that eliminate joystick and explorer subjectivity, thus improving repeatability. The big Placido disk cone in managed from the computer by means of the mouse. Final alignment (up and down) and focusing (forwards and backwards) are automatically performed by the motorized instrument head. It can explore the whole cornea (apex and limbus to limbus) thanks to an offset fixation. The patient can fixate different green lights, to allow complete cornea coverage. Offset-fixation mapping allows for more precise mapping of the central 3mm of the cornea. More true data points from the apex and true limbus-to-limbus measurements over a large corneal area provide for better coverage without extrapolation. Nevertheless, we miss a different chin rest to allow faster exams by eliminating the need for patient’s head re-centration from one eye to the other. The system generates maps in seconds and detailed customized reports can be printed in less than a minute with any colour printer running under MS Windows ’95 ™ operating system.

Contents

Section 1 Section 2

Section 3

Section 4

Section 5

Section 6 Section 7 Figure 2-26: Dicon’s CT-200™ can explore the whole cornea (apex, and limbus to limbus) thanks to an offset fixation. Patient fixates different green lights: shown is a quadruple view of right eye corneal maps display a nasal fixation, including 3-D reconstruction with a 45º tilt (left and down). Optional software (Multiview™) provides total cornea coverage using the mentioned multiple fixation targets. Limbal measurements aren’t always reliable, being subject to many artefacts.

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Subjects Index

Help ?


Figure 2-27: Picture shows a quadruple display map of the right eye of a 55 year-old man suffering form progressive bilateral corneal central ectasia. Notice the distortion of the mires in the ring verification map (up and left), the enormous “red” central and paracentral elevation in the axial diopter map (up and right). Statistical information is displayed following the peak detection, identifying the location, size, maximum power, peak index and probability statement (“very high suspect peak area detected”). One such high index (index = 9370) always means that we face a keratoconus or another kind of corneal ectasia. The ectasia was clearly visible at the slit-lamp.

Contents

Section 1 Section 2

Section 3

Section 4

Section 5

A very interesting feature of this instrument is the Bull’s Eye Targetting™: the system automatically targets the apex position of a cone (keratoconus or other), providing a numerical index for that cone. An auto-alarm is activated so that any suspicious case of keratoconus (or excessive corneal elevation with an index higher than 10) is automatically detected and acoustically signalled as a peak detection warning window appears in the display after the image capture is complete. New users will appreciate this feature: a low index is not uncommon, and does not always mean that we face a pathologic cornea. High indices in a tangential map almost always mean that we face a keratoconus or another kind of corneal ectasia (Figure 2-27).

Peak detection can be triggered by any sus- Section 6 pect peak, including mucous in the tear film, or localized areas of film break-up. In one such case, al- Section 7 ways have the patient close the eyes for a while and Subjects Index blink a few extra times before retaking the picture. In case of doubt, it is advisable to retake the picture again. The determination of the condition producing the corneal elevation needs to be confirmed by other clinical tests, like slit lamp examination or others. The DICON ® CT-200™ software includes an optional refractive module that allows single Help ? analysis, trend analysis of multiple displays and a special package called VISX ® STAR S2™ Ablation Planner (Fig. 2-28).


45

Chapter 2 Figure 2-28: The “Single Analysis” menu option of the DICON ® CT200™ displays a single exam with four customisable map views a) axial diopter, b) refractive diopter (shown with a square grid overlay), c) spherical height and d) irregularity (shown without the eye overlay). The irregularity map d) reports an index (Q = - 0.10) that measures eccentricity (a measure of asphericity) and the amount of astigmatism that has been subtracted form the original ideal spherical corneal data (in this case, 1.12 D).

Contents ®

The VISX STAR S2™ Ablation Planner is offered as an option and is intended to learn the control system for the Visx® laser. It offers a custom display of the CT 200 Elliptical Elevation Map, and access to the VISX® STAR S2™ control panel. It allows a simulated (not real) image of the before/ after laser ablation for better comprehension of the procedure. Developed by Dr. Robert B. Mandell is a simplified contact lens fitting software, with fluorescein simulation. You can design unique lenses for each cornea (personalized designs) and send the data directly to the manufacturer (via modem) or print the order sheet for faxing or mailing.

EYE SYS® 2000 Topographers from Premier Laser Systems, EyeSys Corneal Analysis System 2000 and EyeSys Vista Hand-held corneal topographer, have been the leading topographers in the USA for years but might have been discontinued at the moment you may read this chapter due to Premier Laser Systems’ bankruptcy. We have included them to honour the topographers we learned with, as most topographic texts still refer to them. We hope that new partners in early future or potential buyers help to guarantee the survival of EyeSys topographers in this hard marketplace.

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KERATRON™ Corneal Topographer (Optikon 2000® S.p.A, Italy -Europe) (Figure 2-29, with permission)

Section 1 Section 2

Section 3

The Keratron™ topographer is one of our preferred systems: it is a must if you are in refractive Section 4 surgery. The Keratron Topographers offer automatic Section 5 image capture. A patented corneal vertex detector system is housed inside a slight protrusion on either Section 6 side of the cone. If you position the Keratron™ too Section 7 close or too far, image capture just will not happen. Only when the system detects the vertex in the exact Subjects Index right position, image is automatically captured, thus obtaining more reproducible maps. Introduced in 1994, the Keratron™ was the first hardware platform designed to get the most of an ARC STEP surface reconstruction, achieving accuracy and sensitivity, without smoothing of data or extrapolating to fill in topographic shadows. The Help ? Keratron’s own method of arc-step mapping accurately maps aspheric surfaces. It uses a small Placido cone of rings. It’s patented infra-red vertex detector sensor determines the exact position of the corneal vertex and begins constructing a web of “Arcs” between the intersections of 26 rings and 256 meridians, from the vertex to the periphery. Defining corneal vertex position and starting measurements from it provide


Figure 2-29: KERATRON™ Corneal Topographer (Optikon 2000® S.p.A, Italy -Europe)

this topographer with high accuracy. Curvature and height are simultaneously derived from the length and shape of each arc. Mapping beginning at the corneal vertex, this instrument easily detects up central islands or minor defects. Each data point of the “web” is related to another one, thus eliminating inaccuracies of traditional Placido “concentric rings method” which take measurement of each point independently from one another, resulting in possible errors. While most topographers first create an axial map and then convert the axial data into different maps, every Keratron’s map is calculated separately without conversions, thus decreasing probability of errors. Since the Keratron does not convert data, map error is minimal in all maps. True corneal elevation (height) in microns as well as the traditional curvature maps are created. This system enables to map the image of a patient with bad fixation-through mathematics reconstruction. The system is fast and easy to use, working under MS Windows™ environment. The powerful software is the gem of the system: novice will find some difficulty but once you master it you will not want to get rid of this topographer. You can design unique lenses for each cornea (personalized designs) and send the data directly to the manufacturer (via modem). A recently developed software by Jim Edwards, OD (patents pending) called WAVE uses a unique but logical approach to contact lens design by effectively creating a mir-

ror image of the peripheral cornea in the lens design process. Contact lenses designed with Wave drape the cornea in a manner similar to a soft lens. As the lens periphery matches the peripheral cornea, lens centration should be unsurpassed, even with reverse geometry lenses. Optikon 2000® has made a small portable topographer called Scout Portable Topographer with the same features as the full size device: at the moment these lines are written it suffers from some youth design defects that will be soon addressed by Opticon 2000®. It is available as slit-lamp model, hand-held model, table top model or surgical microscope model.

ET-800 Corneal Topography System Contents

Euclid Systems Corporation® ET-800 CTS is another interesting product in this round-up, since Section 1 it uses a completely different method of topography Section 2 called Fourier profilometry. The technique uses the projection of 2 iden- Section 3 tical sine wave patterns onto the surface of the eye. The projection is done using filtered blue light that Section 4 induces fluorescence of a liquid (fluorescein) that Section 5 has been applied to the tear film before the examination. The resulting image is captured by a CCD cam- Section 6 era. Two dimensional Fourier transform mathematics are used to calculate the phase shift of the pro- Section 7 jected wave pattern. The phase shift is directly re- Subjects Index lated to the height information. This method analyses over 300,000 data points to achieve true elevation co-ordinates, with each point accurate to approximately the thickness of the tear film (about one micron). The problem is that thickness of the tear film varies with daytime, and is not the same for each patient. Help ? The system uses no rings or Placido disk. It is quite fast (processing time : 4 seconds). The focusing mechanism is a live TV camera. It provides full scleral and corneal coverage up to 22 x 17mm (useful to assess pterygium evolution). It is sold as the “only” topographer to measure true corneal elevation. Let’s observe again that most topographers measure corneal elevation by extrapolating from corneal reflex (thus interfered by tear film layer quality). It might well be the most precise method, each


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Chapter 2

of the 300,000 data points being accurate to about 1 micron, but unfortunately it is not widespread enough to become a reference system. It still needs clinical validation. This projection technique visualizes the surface directly while a reflection technique amplifies the corneal topographic distortions. It measures with low light level for patient, offering full K analysis, “e” value analysis, cross sections, ellipsoidal difference map, full patient and radiological histories, and a easy to use four click exam wizard.

Oculus® Keratograph™ and HaagStreit® Keratograph CTK 922 OCULUS ® Keratograph (Figure 2-30) and Haag-Streit ® Keratograph CTK 922 (Figure 2-31) are very similar instruments sold under different brand names and different packaging. They are com-

Eye Map EH-290 Alcon ® Corneal Topography System Alcon® EH-290 Eye Map corneal Topography System is a large 23 narrow modified Placido disk system. The modified patented Placido cone design is supposed to be very accurate and sensitive. Easy and intuitive to use (software runs under Windows™), it offers advanced contact lens software, keratoconus detection, corneal statistics information and advanced communication software.

Contents

Section 1 Section 2

Section 3

Section 4

TOMEY® Autotopographer Tomey® auto-topographer is a cheap, small and portable fully automatic self-topographer that requires no operator alignment. The patient places his or her face on an ergonomically designed face rest and the automated topographer is activated by proximity sensors, automatically taking the measurements. The software, that can be installed in a preowned PC, runs under Windows™ operating system. The software is very complete and comprehensive, and includes a contact lens wizard with interactive fluorescein displays. Optional software packages include : Height and Height Change Maps, Klyce Corneal Statistics, Keratoconus Screening and the Contact Lens Wizard. The low level lights cone is intended to produce minimal glare and disturbance for the patient.

Section 5

Section 6 Figure 2-30: OCULUS

®

Keratograph

Subjects Index

Help ?

Figure 2-31: Haag-Streit ® Keratograph CTK 922

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Section 7


pact systems that can fit any refractive unit and include built-in keratometer in connection with the topography system. The software runs under Windows™ operating system and is easy to use, with automatic measurement. The Oculus® can be an integrated computerised system (Keratograph C, in the picture) or an independent system linked to a preowned computer. A non-contact measurement large Placido system with 22 rings in a hemisphere and 22.000 measuring points try to guarantee a high resolution.

The working distance of 80 mm is enough to make the patient feel comfortable. The light system (warm coloured) is intended to produce minimal glare and disturbance for the patient. They have an interesting software that allows contact lens-fitting in three simple steps: automated contact lens recommendation with a database that includes 20.000 lens geometries from all major contact lens manufacturers, and can be easily enlarged, and realistic fluo-image simulation of contact lens adaptation (Figure 2-32). There is a possibility of

Contents

Section 1 Section 2

Section 3

a

b

Section 4

Section 5


c

d Figure 2-32 A-E: Haag-Streit® KERATOGRAPH CTK 922™ output modalities include a) Overview image with simulated keratometer (right and down), b) comprehensive kinetic threedimensional (3-D) analysis of corneal topography for simple explanation to the patient, c) zoom-up image of a map d) fluorescein image simulation for contact lens fitting, and e) Fourier expressive analysis (Published with permission from HAAG-STREIT® AG International).

Help ?

e LASIK AND BEYOND LASIK

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Chapter 2

Figures 2-33 and 2-34: Oculus® Keratograph™ screen shots with elevation (height) map and refractive map that will be included in 2001 software version (latest review). A new algorithm method for increased precision (Published with permission from OCULUS Optikgeraete GmbH).

Contents

Section 1 Section 2

Section 3

Section 4

Section 5


Help ?

measuring the back surface of rigid gas permeable contact lens through optional Lens Check software. There is also an optional statistics software package called Datagraph, intended for refractive surgeons. This systems allows wonderful comprehensive kinetic three-dimensional analysis of corneal topography for simple explanation to the 50

SECTION I

patient (Figure 2-31). Fourier surface analysis (Figure 2-31) is available and new software is under development for refractive surgery and contact lens fitters. Also optional is the Topolink software, that integrates the corneal topography data and some but not all excimer laser software .


Figure 2-35: ORBSCAN IIz™- Bausch & Lomb® Surgical, Inc.

ORBSCAN IIz™ - Bausch & Lomb® Surgical, Inc. (USA) (Figure 2-35, with permission) This is a truly revolutionary instrument for the study of the cornea. It combines a slit scanning system and a Placido disk (with 40 rings) to measure the anterior elevation and curvature of the cornea and the posterior elevation and curvature of the cornea. It offers a full corneal pachimetry map with white to white measurements.

ORBSCAN IIz™ takes a series of slit-beam images of two scanning slitlamps projected beams at 45 degrees, to the right or left of the instrument axis. During the exam, the patient fixates on a blinking red light coaxial with the imaging system. Forty images are taken by the system, 20 with slit beams projected from the right and 20 from the left. The 20 images are acquired in 0.7 seconds each. Simultaneously, a tracking system measures the non voluntary movements of the eye during the exam. Orbscan IIz™ is able to measure anterior chamber depth, angle kappa, pupil diameter, simulated keratometry readings (3 and 5 central mm of the cornea), Contents and the thinnest corneal pachimetry reading. It offers every traditional map apart Section 1 form those of posterior corneal surface. Elevation topography of the anterior cor- Section 2 nea enables clinicians to more accurately Section 3 visualize the shape of abnormal corneas, which should lead to more accurate diag- Section 4 noses and better surgical results. It has proven to be and extraordinary tool for re- Section 5 search and for the refractive surgeon. Section 6 The system is able to acquire over 9000 data points in 1.5 seconds, which is fast, but not enough Section 7 for the patient to feel comfortable. Not every patient can avoid blinking, and in some cases measurements Subjects Index have to be repeated. A faster processing speed would be desirable, although we feel very comfortable with the system. Easy to use and running under Microsoft® Windows™ NT 4.0 operating system, the major disadvantage is the high price, that makes it not affordHelp ? able for most ophthalmologists. Any colour printer running under NT 4.0 can be used. Three dimensional views of the different maps are available (see Figure 2-38 in this chapter).


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Figure 2-36:

MYOPIC LASIK PRE/POST OPERATIVE with ZYOPTICS™ Excimer laser.

Preoperative Orbscan™ Imaging Anterior Float BFS Keratometric

Posterior Float BFS Thickness

Postoperative (Myopic Zyoptics™ Lasik) Orbscan™ Imaging Anterior Float BFS Keratometric

Picture displays different preoperative and postoperative maps of the right eye of a patient who underwent a refractive myopic Zyoptics™ Lasik procedure. Images were taken with ORBSCAN IIz™ - Bausch & Lomb® Surgical, Inc. (USA) topographer. The Anterior Best Fit Sphere (BFS) is calculated to best match the anterior corneal surface. The Elevation BFS map subtracts the calculated best fit sphere size against the eye surface in millimeters (mm). The difference between the sphere and the eye surface is expressed in distance, in a radial way, from the centre of the sphere as shown in the figure (map Anterior Float BFS). The shape of a sphere being easily imagined by the explorer, deviation from that spherical surface in a special case helps to appreciate the true shape of the eye and its deviation from symmetry (asymmetry). The map has 35 default colour steps, the size of each step being measured at the bottom of each colour. (Five microns is the default for the BFS map). The best fit between eye surface and sphere is represented in green. Areas under this spherical ideal surface are represented in blue, while warmer colours (orange-red) identify areas above this ideal sphere.. The box in the middle of the displays shows patient information of interest like patient’s name,

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examination date, diameter (mm) and power (D) of Contents the ideal sphere, diagnosis, simulated keratometry readings, white to white distance, pupil diameter, Section 1 thinnest measurement for that cornea, anterior chamber depth (either from epithelium or endothelium), Section 2 angle Kappa, and Kappa intercept. The Posterior Best Fit Sphere (BFS) is cal- Section 3 culated to best match the posterior corneal surface. Section 4 The Keratometric simulates keratometric Section 5 values at special areas. The Thickness Map (Pachymetry map) Section 6 shows the differences in elevation between the anterior and posterior surfaces of the cornea. By moving Section 7 the mouse over the map, explorer can obtain measurements of the thickness at each point. This map Subjects Index can be overlaid by the average measurements that would be taken with a traditional ultrasound pachymeter (encircled values). This map is invaluable for preoperative assessment of the refractive patient, and to determine the true ablated tissue depth in the postoperative period of PRK and refractive Help ? patients. Thickness maps clearly demonstrate that ablation zone (arrow) has decreased in thickness form 544 to 405 microns. Notice that corneal thickness increases as we get closer to the limbus. (Courtesy of Dr. Andreu Coret, Institut Oftalmològic de Barcelona, Barcelona - Spain)


Contents

Section 1 Section 2

Section 3

Section 4

Section 5


Help ?

Figure 2-36:

MYOPIC LASIK PRE/POST OPERATIVE with ZYOPTICS™ Excimer laser.


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Chapter 2

Contents

Section 1 Section 2

Section 3

Section 4

Section 5


Help ?

Figure 2-37: KERATOCONUS Anterior Float BFS Keratometric


Anterior Float BFS Keratometric


Picture displays different maps of the left (OS) eye of a patient with a keratoconus. Images were taken with ORBSCAN IIz™ - Bausch &

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Lomb® Surgical, Inc. (USA) topographer. Notice the central elevation in both anterior and posterior surfaces of the cornea ,with reduced corneal thickness (comparing to a normal eye) and high astigmatism. The four inferior maps display different cross section along the 0º180º meridian that demonstrate how the cornea is higher than the best fit sphere centrally (reddish central mountain overlaid on the corneal display) and lower in the mid-periphery (bluish depression at both sides of the mountain). (Courtesy of Dr. Andreu Coret, Institut Oftalmològic de Barcelona, Barcelona - Spain).


Contents

Section 1 Section 2

Section 3 Figure 2-38: 3-D imaging of both surfaces of the cornea with ORBSCAN IIz™ software is really meaningful for the patient. Notice that central protrusion is higher in posterior than in anterior surface of the cornea: in between, corneal thickness is reduced. (Courtesy of Dr. Andreu Coret, Institut Oftalmològic de Barcelona, Barcelona - Spain).

Section 4

Section 5


Help ? SPECIAL NOTICE FOR TOPOGRAPHER USERS: Always Follow Manufacturer’s Instructions ALWAYS RECALIBRATE THE SYSTEM: AT LEAST ONCE WEEKLY BEFORE ANY DELICATE EXAM AFTER CLEANING THE CONE. VERIFY CALLIBRATION EACH DAY BEFORE PATIENT TESTING


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REFERENCES 1.

Applegate RA, Nunez R, Buettner J, Howland HC. How accurately can videokeratographic systems measure surface elevation? Optom Vis Sci 1995; 72:785-92.

2.

Arffa RC, Warnicki JW, Rehkopf PG. Corneal topography using rasterstereography. Refract Corneal Surg 1989; 5: 414-17.

3.

Belin MW, Litoff FK, Strods SJ, Winn SS, Smith RS. The PAR technology corneal topography system. Refract Corneal Surg 1992;8: 88–96.

4.

Belin MW, Zloty P. Accuracy of the PAR corneal topography system with spatial misalignment. CLAO J 1993; 19: 64-8.

5.

Belin MW, Ratliff CD. Evaluating data acquisition and smoothing functions of currently available videokeratoscopes. J Cataract Refract Surg 1996; 22: 4216.

6.

Borderie VM, Laroche L. Measurement of irregular astigmatism using semimeridian data from video-keratographs. J Refract Surg 1996;12: 595–600.

7.

Brancato R, Carones F. Topografia corneale computerizzata. Milano, Italy: Fogliazza, ed. 1994.

8.

Cantera E, Carones F, Brancato R, Cantera I, Neuschuler R. Evaluation of a new autofocus device for computer-assisted corneal topography. Invest Ophthalmol Vis Sci 1994; 35 (Suppl): 2063.

9.

Cohen KL, Tripoli NK, Holmgren DE, Coggins JM: Assessment of the height of radial aspheres reported by a computer-assisted keratoscope. Invest Ophthalmol and Vis Sci 1993;34 (suppl): 1217.

10. Cohen KL, Tripoli NK, Holmgren DE, Coggins JM. Assessment of the power and height of radial aspheres reported by a computer-assisted keratoscope. Am J Ophthalmol 1995; l l9: 723-32. 11. Corbett MC, O’Brart DPS, Stultiens Bath, Jongsma FHM, Marshall J. Corneal topography using a new moiré image-based system. Eur J Implant Ref Surg 1995;7: 353 – 70. 12. Corbett MC, Rosen ES, O’Brart D.P.S.. Corneal topography: principles and applications. BMJ books, Great Britain, 1999. 13. Chan WK, Carones F, Maloney RK. Corneal topographic maps: a clinical comparison. International Society of Re-

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fractive Keratoplasty 1994 - Abstract book. 14. Dekking HM. Zur Photographie der Hornhautoberfl-Eche. Graefes Arch Ophtalmol 1930; 124:708-30. 15. Dingeldein SA, Klyce SD, Wilson SE. Quantitative descriptors of corneal shape derived from the computer- assisted analysis of photokeratographs. Refract Corneal Surg 1989;5:372–8. 16. Doss JD, Hutson RL, Rowsey JJ, Brown DR. Method for calculation of corneal profile and power distribution. Arch Ophthalmol 1981; 99: 1261-5. 17. Duke Elder S. – System of Ophthalmology, St Louis, Mo : CV Mosby Co, 1970, V, 96-101. 18. Ediger MN, Pettit GH, Weiblinger RP. Noninvasive monitoring of excimer laser ablation by time-resolved reflectometry. Refract Corneal Surg 1993;9: 268–75. 19. el-Hage SG: The computerized corneal topographer EH270. In: Shanzlin DJ, Robin JB, eds. Corneal topography: measuring and modifying the cornea. New York: SpringerVerlag 1991:l 1-24. 20. el-Hage SG: Suggested new methods for photokeratoscopy: a comparison of their validities. I. Am J Optom Arch Am Acad Optom 1971; 48 :897-912.

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Section 3

Section 4

21. Eghbali F, Yeung KK, Maloney RK. Topographic determination of corneal asphericity and its lack of effect on the outcome of radial keratotomy. Am J Ophtha1mol 1995;119: 275–80.

Section 5

22. Fleming JF. Should refractive surgeons worry about corneal asphericity? Refract Corneal Surg 1990; 6: 455–7.

Section 7

23. Friedman NE, Zadnik K, Mutti DO, Fusaro RE. Quantifying corneal toricity from videokeratography with fourier analysis. J Refract Surg 1996;12: 108–13.

Section 6

Subjects Index

24. Gardner B.P., Klyce S.D., Thompson H.W., et al. Centration of photorefractive keratectomy : topographic assessment. Invest Ophthalmol Vis Sci, 1993, 35, 803. 25. Greivenkamp JE, Mellinger MD, Snyder RW, Schwiegerling JT, Lowman AE, Miller JM. Comparison of three videokeratoscopes in measurement of toric test surfaces. J Refract Surg 1996; 12: 229-39. 26. Grimm BB. Communicating with keratography. J Refract Surg 1996;12: 156–9. 27. Hannush SB, Crawford SL, Waring GO III, Gemmill MC, Lynn MJ, Nizam A. Accuracy and precision of keratometry, photokeratoscopy and corneal modeling on calibrated steel balls. Arch Ophtalmol 1989; 107:1235-9.

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28. Holladay J., Warring G.O. Optics and topography in radial keratotomy. In : Warring GO, ed. Refractive keratectomy for myopia and Astigmatism. Mosby- Year book, Inc. 1992, 37- 144.

42. Le Geais J.M., Ren Q., Simon G., Parel J.M. Computer Assisted corneal topography: accuracy and reproducibility of the topographic modeling system. Refract Corneal Surgery, 1993, 9, 347-357.

29. Holladay JT, Cravy TV, Koch DD. Calculation of surgically induced refractive change following ocular surgery. J Cat Refract Surg 1992;18: 429–43.

43. Leroux Les Jardins., Pasquier N., Bertrand I. Topographie cornéenne computérisée : Résultats apres kératotomie Radiaire et « T-Cuts ». Bull Soc. Opht. France, 1991, 8-9, XCL, 729-734.

30. Holladay JT. Corneal topography using the Holladay diagnostic summary. J Cat Refract Surg 1997; 23: 209–21. 31. Holladay J.T. – The Holladay diagnostic summary. In : Corneal topography : the state of art, James P. Gills editor, Slack Inc., 1995, 309-323. 32. Huber C, Huber A, Gruber H. Three-dimensional representations of corneal deformations from kerato- topographic data. J Cat Refract Surg 1997; 23: 202–8. 33. Johnson DA, Haight DH, Kelly SE et al. Reproducibility of videokeratographic digital subtraction maps after excimer laser photorefractive keratectomy. Ophthalmology 1996;103: 1392–8. 34. Jongsma FHM, Laan FC, Stultiens BATh. A moiré based corneal topographer suitable for discrete Fourier analysis, Proc Ophthal Tech 1994;2126: 185 – 92. 35. Kawara T. Corneal topography using moiré contour fringes. Appl Optics 1979; 18: 3675 – 8. 36. Kelman SE. Introduction of neural networks with applications to ophthalmology. In: Masters BR (ed) Non-invasive diagnostic techniques in ophthalmology. SpringerVerlag, New York, 1990. 37. Klein SA, Mandell RB. Axial and instantaneous power conversion in corneal topography. Invest Ophthalmol Vis Sci 1995; 36: 2155-9.

44. Leroux Les Jardins., Pasquier N., Bertrand I. Modification de la chirurgie de l’astigmatisme en fonction des résultats de la topographie cornéenne computérisée. Bull Soc. Opht. France, 1991, 12, XCLS, 1097-1104. 45. Koch DD, Foulks GN, Moran CT, Wakil JS. The corneal EyeSys System: accuracy analysis and reproducibility of first-generation prototype. J Refract Corneal Surg 1989; 5: 424-9. 46. Lundergan MK, The Orbscan corneal topography system: verification of accuracy. International Society of Refractive Keratoplasty 1994 - Abstract book. 47. Maeda N, Klyce SD, Smolek MK, Thompson HW. Automated keratoconus screening with corneal topography analysis. Invest Ophthalmol Vis Sci 1994; 35: 2749–57. 48. Maeda M, Klyce SD, Smolek MK. Neural network classification of corneal topography. Invest Ophthalmol Vis Sci 1995;36: 1327-35. 49. Maguire LJ, Singer DE, Klyce SD. Graphic presentation of computer analysed keratoscope photographs. Arch Ophthalmol 1987;105: 223 – 30. 50. Maguire LJ, Wilson SE, Camp JJ, Verity S. Evaluating the reproducibility of topography systems on spherical surfaces. Arch Ophthalmol 1993; 111: 259-62.

38. Klein SA. A corneal topography algorithm that produces continuous curvature. Optom Vis Sci 1992; 69: 829-34.

51. Maloney RK, Bogan SJ, Waring GO III. Determination of corneal image- forming properties from corneal topography. Am J Ophthalmol 1993; l l 5: 31-41.

39. Klyce SD. Computer-assisted corneal topography: high resolution graphic presentation and analysis of keratoscopy. Invest Ophthalmol Vis Sci 1984;25: 1426 – 35.

52. Mandell RB, Horner D. Alignment of videokeratoscopes. In: Sanders DR, Koch DD, eds. An Atlas of Corneal Topography. Thorofare NJ: Slack, 1993: pp 197-206.

40. Klyce SD, Wang JY. Considerations in corneal surface reconstruction from keratoscope images. In: Masters BR, ed. Noninvasive diagnostic techniques in ophthalmology. New York: Springer-Verlag, New York, 1990: 76.

53. Mandell RB. Contact lens practice, 4th ed. Springfield, IL: Charles C.Thomas, 1988: pp 107-35.

41. Klyce SD, Dingeldein SA. Corneal topography. In: Masters BR, ed. Noninvasive diagnostic techniques in ophthalmology. New York: Springer-Verlag, 1990: pp 78-91.

Contents

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Section 3

Section 4

Section 5


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54. Mandell RB. Keratometry and contact lens practice. Optometric Wkly, May 6, 1965: 69-75. 55. Munger R, Priest D, Jackson WB, Casson EJ. Reliability of corneal surface maps using the PAR CTS. Invest Ophthalmol Vis Sci 1996; 37: s562.


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56. Mattioli R, Carones F. How accurately can corneal profiles heights be measured by Placido-based videokeratography? Invest Ophthalmol Vis Sci 1996; 37: s932. 57. Mattioli R, Carones F, Cantera E. New algorithms to improve the reconstruction of corneal geometry on the Keratron™ videokeratographer. Invest Ophthalmol Vis Sci 1995; 36:s302. 58. Mattioli R, Tripoli NA. Corneal geometry reconstruction with the Keratron Videokeratographer. Optom Vis Sci, 1997; 74:881-894 59. Merlin U. I cheratoscopi: caratteristiche e attendibilita. In: Buratto L, Cantera E, Dal Fiume E, Genisi C, Merlin U, eds. Topografia Corneale. Milano Italy: CAMO, 1995: 43-56. 60. Mishima S. Some physiological aspects of the precorneal tearfilm. Arch Ophthalmol 1965;73: 233. 61. Naufal SC, Hess JS, Friedlander MH, Granet NS. Rasterstereography-based classification of normal corneas. J Cat Refract Surg 1997;23: 222–30. 62. O’Bart D.P.S., Corbett M.C., Rosen E.S. The topography of corneal disease. Eur J Implant Ref Surg, 1995, 7, 173183. 63. Olsen T, Dam-Johansen M, Beke T, Hjortdal JO. Evaluating surgically induced astigmatism by Fourier analysis of corneal topography data. J Cat Refract Surg, 1996;22: 318– 23. 64. Parker P.J., KLYCE S. D., Ryan B. L. et al. Central topographic islands following photorefractive keratectomy. Invest Ophthalmol Vis Sci., 1993, 34, 803. 65. Prydal JI, Campbell FW. Study of precorneal tear film thickness and structure by interferometry and confocal microscopy. Invest Ophthalmol Vis Sci 1992;33: 1996–2005. 66. Rabinowitz YS, McDonnell PJ. Computer-assisted corneal topography in keratoconus. Refract Corneal Surg 1989;5:400-8. 67. Rabinowitz YS, Garbus JJ, Garbus c, McDonnell PJ. Contact lens selection for keratoconus using a computer assisted videokeratoscope. CLAO J 1991; 17:88-93. 68. Roberts C. The Accuracy of power maps to display curvature data in corneal topography systems. Invest Ophthalmol Vis Sci 1994; 35: 3524- 3532.

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69. Roberts C. Characterization of the inherent error in a spherically-biased corneal topography system in mapping a radially aspheric surface. J Refract Corneal Surg 1994; 10: 103-116. 70. Rowsey JJ, Reynolds AE, Brown DR. Corneal topography. Corneascope. Arch Ophthalmo1 1981;99: 1093–100. 71. Ruiz-Montenegro J., Mafra C.H., Wilson S.E. et al. Corneal topography alterations in normal contact lens wearers. Ophthalmology. 1993, 100, 128-134. 72. Salabert D., Cochener B,, Mage F., Collin J. Kératocone et anomalies topographiques cornéennes familiales. J. Fr. Ophtalmol., 1994, 17, lI, 646-656. 73. Sanders RD, Gills JP, Martin RG. When keratometric measurements do not accurately reflect corneal topography. J Cat Refract Surg 1993;19 (Suppl): 131–5. 74. Seiler T, Reckmann W, Maloney RK. Effective spherical aberration of the cornea as a quantitative descriptor in corneal topography. J Cat Refract Surg 1993;19 (Suppl): 155 – 65. 75. Takeda M, Ina H, Kobayashi S. Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry. J Optical Soc Am 1982;72: 156–60. 76. Taylor CT, Sutphin JE. Accuracy and precision of the Orbscan topography unit in measuring standardized radially aspheric surfaces. Invest Ophthalmol Vis Sci 1996; 37: s561.

Contents

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Section 3

Section 4

Section 5

Section 6

77. Thall EH, Lange SR. Preliminary results of a new intraSection 7 operative corneal topography technique. J Cat Refract Surg 1993;19 (Suppl): 193-7. Subjects Index 78. Tripoli NK, Cohen KL, Holmgren DE, Coggins JM. Assessment of radial aspheres by the arc-step algorithm as implemented by the Keratron keratoscope. Am J Ophthalmol 1995; 120: 658-64. 79. Tripoli NK, Cohen KL, Obla P, Coggins JM, Holmgren DE. Height measurement of astigmatic test surfaces by a keratoscope that uses plane geometry reconstruction, Am J Ophthalmol 1996; 121; 668-76. 80. Vass C, Menapace R. Computerised statistical analysis of corneal topography for the evaluation of changes in corneal shape after surgery. Am J Ophthalmol 1994;118:177– 84. 81. Vass C, Menapace R, Rainer G, Schulz H. Improved algorithm for statistical batch-by-batch analysis of corneal topographic data. J Cat Refract Surg 1997;23:903–12.

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82. Vass C, Menapace R, Amon M, Hirsch U, Yousef A. Batchby-batch analysis of topographic changes induced by sutured and sutureless clear corneal incisions. J Cat Refract Surg 1996; 22: 324–30. 83. Wang J, Rice DA, Klyce SD. A new reconstruction algorithm for improvement of corneal topographical analysis. J Refract Corneal Surg 1989; 5:379-87 84. Warnicki JW, Rehkopf PG, Arrra RC, Stuart JC. Corneal topography using a projected grid. In: Schanzlin DJ, Robin JB (eds) Corneal topography. Measuring and modifying the cornea. Springer-Verlag, New York, 1992. 85. Warnicki JW, Rehkopf PG, Curtin DY, Burns SA, Arffa RC, Stuart JC. Corneal topography using computer analyzed rasterstereographic images. Appl Optics 1988;27: 1135–40. 86. Warning G.O., Hannush S.B., Bogan S.J., Maloney R.K. – Classification of corneal topography with videotopography. In : Shanzlin D.J., Robin J.B., eds. Corneal topography : measuring and modifying the cornea. New York, NY, Springer-Verlag, 1992, 47-73.

Contents

Section 1 Section 2

87. Wilson SE, Klyce SD, Husseini ZM. Standardized colorcoded maps for corneal topography. Ophthalmology 1993;100: 1723-7.

Section 3

Section 4

88. Wilson SE, Wang JY, Klyce SD. Quantification and mathematical analysis of photokeratoscopic images. In: Shanzlin DJ, Robin JB eds. Corneal topography: measuring and modifying the cornea. New York, Springer-Verlag, 1991: 1-81.

Section 5

89. Wilson SE, Klyce SD. Quantitative descriptors of corneal topography. A clinical study. Arch Ophthalmol 1991;109:349-53.

Subjects Index

Section 6 Section 7

90. Wilson SE, Verity SM, Conger DL. Accuracy and precision of the Corneal Analysis System and the Topographic Modeling System. Cornea 1992; 11: 28-35. 91. Young JA, Siegel IM. Isomorphic corneal topography: a clinical approach to 3-D representation of the corneal surface. Refract Corneal Surg 1993;9: 74–8.

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92. Young JA, Siegel IM. Three-dimensional digital subtraction modeling of corneal topography. J Refract Surg 1995; 11: 188–93.

Dr. Guillermo L. SIMÓN University of Barcelona - Faculty of Medicine Dept. of Ophthalmology Chief Anterior Segment Surgeon Simon Eye Clinic, Barcelona (Spain) E-mail: [email protected]


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Chapter 3 EVALUATION OF THE LASIK FLAP BY CONFOCAL MICROSCOPY Mahmoud M. Ismail, M.D., Ph.D.

Contents

(Note from the Editor in Chief: this chapter is important in describing a new diagnostic technique for detecting flap problems in LASIK and an important and easy method to prevent the Sands of the Sahara Syndrome.)

edema or wrinkling of the flap. In other occasions, fluctuation of the patient’s refraction is commonly seen during the early postoperative period. The evaluation with the slit lamp is not always decisive in such situations.

Frequent Problems With the Flap

What is Confocal Microscopy?

Section 1 Section 2

Section 3

Section 4

Section 5

The LASIK procedure is a continuous challenge towards perfection. In spite of all the recent advances in the technology of excimer lasers and the updated modifications in the microkeratome industry, we still experience some complications. The major problems that can appear with LASIK are always related to the corneal flap architecture (1)(2)(3). Buttonholes flaps, free cuts, intrastromal keratitis, and superficial flaps among others, are considered to be the most important technical complications (3)(4)(5). This might lead to further and even more serious consequences such as loss of one or more lines of preoperative best-corrected visual acuity (BCVA), epithelial ingrowth and the subsequent flap melting. In order to achieve the desired outcome, calibration of the microkeratome and use of the adequate nomogram are essential for obtaining the correct diameter and thickness of the flap and an adequate result. Also, delayed recovery of the BCVA following LASIK can occasionally occur due to

Confocal microscopy is a revolutionary new Section 6 diagnostic technique offering a high magnification view in living cornea. It is able to visualize Section 7 structures posterior to haze, scars, edema or opacities within the cornea. With the incorporation of the Subjects Index scanning mechanism, a complete and panel controlled automated scan to the corneal layers can be done in 2 seconds. This can provide an accurate measurement of each layer of the cornea, as well as total corneal thickness measurement (6)(7). It also provides understanding of clinical findings such as inHelp ? terface debris deposition and inflammation i.e. “Sands of the Sahara’s Syndrome”. Another use of the confocal microscopy is early detection of epithelial ingrowth, a good follow-up and prompt treatment. The Confocal microscopy post LASIK surgery is used to evaluate the following: 1- The whole corneal thickness (in microns). 2- The flap thickness (in microns). 3- Amount of stromal edema.


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Figure 3-1: Tandem Scanning Confocal Microscopy ASL 1000

Figure 3-2: LASIK interface a seen by confocal microscopy with bits of debris and inflammatory cells.

Confocal Microscopy Procedure

interface were imaged with debris and inflammatory cells (Figure 3-2). The measurement in microns is read followed on the monitor screen or from the micrometer. The stromal edema can be evaluated by the appearance of lacunas adjacent to the flap interface.

We use in our studies a Tandem Scanning Confocal Microscopy ASL 1000 (Advanced Scanning, New Orleans). The confocal microscopic examination is done under topical anesthesia (Figure 3-1) 1, 3 and 7 days post LASIK. We applied one drop of methyl cellulose on the tip of the confocal microscope objective and gently approached the cornea to be examined. The corneal thickness and the LASIK flap thickness were measured by focusing the confocal image on the superficial layer of the epithelium and subsequently focusing the scanning system until the endothelium or the stromal LASIK

Contents

Section 1 Section 2

Section 3

Results

Section 4

The measurements from the flap and whole corneal thickness are plotted in (Table 1). The identification of the flap thickness was determined in all eyes at the 1st day, 3rd and 7th day visits. The ultrasonic flap thickness is done intraoperatively. There

Section 5


Table 1 Preoperative

1st day

3rd day

7th day

Confocal

545 ± 33µ

___

___

494 ± 45µ

cornea Ultrasonic

534 ± 28µ

___

___

486 ± 65µ

cornea Confocal flap

___

129.85 ±8 µ

120.25 ±3 µ

119.25 ±3µ

thickness Ultrasonic flap

12.6 ±5 µ*

___

___

___

thickness Edema

___

Present

Present

Absent

Mean BCVA

20/25

20/63

20/32

20/25

*Intraoperative measurement

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is no statistically significant difference when comparing ultrasonic pachymetry and confocal measurements.

The Importance of Confocal Microscopy to Sands of the Sahara’s Syndrome Interface debris deposition and subsequent inflammation commonly named Sands of the Sahara’s Syndrome is one sight threatening complication following LASIK . (Important but fortunately infrequent - Note from the Editor in Chief). It typically present 1 to 4 days following LASIK. Patients usually complain of decreased or cloudy vision, foreign body sensation and significant photophobia. Slitlamp examination reveals fine granular infiltrates with very mild ciliary injection. This condition was spontaneously appearing in sporadic cases in various refractive surgery centers. It was not properly identified until the introduction of the confocal microscope in the field of refractive surgery. Interface debris deposition and consequently inflammation was found to be due to accumulation of greasy material from the microkeratome blades. In rabbits, we experimented cleaning the blades with acetone and rinsing them with distilled water a dramatic improvement of such condition was notable Confocal microscopy imaging in cases of SOS revealed, besides abundant polymorph nuclear leukocytes, significant deposition of greasy material from the microkeratome blades (9)(10)(11). We performed a prospective study to verify the effect of blades cleaning by acetone and absolute alcohol in order to reduce debris deposition in LASIK interface. By such means we can eliminate an important predisposing factor for Sands of the Sahara’s Syndrome. We included in this study 40 patients undergoing bilateral LASIK randomly and equally divided into 2 groups (A and B). The patients were operated simultaneously on both eyes using the Nidek 5000 Excimer Laser and the Carriazo Barrraquer microkeratome (Moria). The mean age was 28.1 years (range from 19 to 52) and the mean spherical correction was -5.75 ± 1.63 D (range from -2.25 to 11.5). In the right eye of all patients in both groups, the microkeratome blade was taken directly from its

Figure 3-3: LASIK interface as seen by confocal microscopy with lot of debris and inflammatory cells suggesting SOS.

package without cleaning. However in the left eye, in group A, the same blade was cleaned with absolute alcohol and rinsed with distilled water prior to use. In group B, the same blade used for their right eye was soaked in acetone and rinsed with distilled water prior to use. Meticulous washing of the interface was performed and flap Reposition was done without contact lens. After 5 days of surgery, slitlamp and confocal microscopy examination were used to record any interface debris.

Contents

Section 1 Section 2

Section 3

Section 4

Section 5

Section 6

How to Prevent Sands of Sahara Syndrome


All patients had uneventful postoperative period with the LASIK flaps well-reposted and mean follow-up of 9.2 months (8 to 15 months). Clinical examination by slit-lamp showed only 4 eyes with significant interface debris deposition scattered all over the flap area (Figure 3-2). Such 4 eyes corresponded all to the right eye of patients with flap cut using uncleaned microkeratome blades i.e. taken directly from its package. In vivo examination by the confocal microscope of the LASIK interface to the right eye of patients revealed microscopic objects of approximately 10 to 20µ in diameter. These objects correspond to bits of debris scattered throughout the flap interface. Associated with the interface debris, numerous inflammatory cells were seen, mainly polymorph nuclear leukocytes (Figure 3-3). LASIK AND BEYOND LASIK

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On the other hand, no debris deposition was seen by slit-lamp examination of the left eye of the same patients (previously cleaned blades). Also, confocal microscopic examination of the LASIK interfaces of such eyes showed very scanty debris deposition. This debris was seen in 3 to 4 focal pockets surrounded by very few clusters of inflammatory cells. The rest of the interface of the LASIK flap created by the cleaned blade showed no debris or inflammatory cells. Comparing between alcohol and acetone for blade cleaning, no significant difference was seen regarding the confocal microscopic examination.

Other Contributions of Confocal Microscopy The confocal microscope offers the ability to examine objects at high magnification and literally can identify the cellular structure of the cornea. This revolutionary new tool permits real-time observation of living cornea in patients at magnifications ranging from 20 x to 500 x. And as a great advantage, it offers the possibility to visualize structures posterior to haze, scars or edema within the cornea. These advantages makes the confocal microscope the most suitable method to examine LASIK interface. Confocal microscopy can be employed in refractive surgery in general, and specifically in LASIK procedures for the following purposes: 1- Evaluation of interface edema 2- Accurate measurement of the flap thickness 3- Evaluation of interface for the diagnosis of the Sands of Sahara’s syndrome 4- Early diagnosis of epithelial ingrowth.

REFERENCES 1- Knorz MC, Wiesinger B, Liermann A, Seiberth V, Liesenhoff H.: Laser in situ keratomileusis for moderate and high myopia and myopic astigmatism. Ophthalmology 1998; 105:932-940. 2- Arbalez MC, Pérez-Santonja JJ, Ismail MM, Alio JL et al.: Automated Lamellar Keratoplasty (ALK) and Laser In Situ Keratomileusis (LASIK). Chapter 9:131-150 in: Refractive Surgery: Current Techniques and Manage-

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ment. Olivia Serdarevic, IGAKU-SHOIN Medical pubishers, New York-Tokyo, October 1996. 3- Gimbel HV, Penno EE, Van Westenbrugge JA, Ferensowicz, Furlong MT.: Incidence and management of intraoperative and early postoperative complications in 1000 consecutive laser in situ keratomileusis cases. Ophthalmology 1998; 105:1839-1847. 4- Wilson SE.: LASIK: management of common complications. Laser in situ keratomileusis. Cornea 1998; 17:45967. 5- Smith RJ, Maloney RK.: Diffuse lamellar keratitis. A new syndrome in lamellar refractive surgery. Ophthalmology 1998; 105:1721-6. 6- Beuerman RW, Larid JA, Kaufman SC, Kaufman HE.: Quantification of real-time images of the human cornea. J Neurosci Methods 1994; 54:197-203.

Contents

Section 1

7- Ismail MM.: Corneal Imaging Using white-light Confocal microscopy. Bull Ophthalmol Soc Egypt, 1999. Vol 92, 2:1113-1116

Section 2

Section 3

8- Ismail MM, Kaufman S, Alio JL, Beurman R.: Evaluation of the LASIK flap by confocal microscopy. Cornea 2001 , In Press. 9- Kaufman SC, Maitchouk DY, Chiou AG, Beuerman RW.: Interface inflammation after laser insitu keratomileusis Sands of the Sahara syndrome. J cataract Refact Surg, 1998; 21:1589-1593.

Section 4

Section 5


10- Kaufman SC, Ismail MM, Beuerman RW, Maitchouk D, Ohta T,Palkama A, Mustonen R, Chiou AGY.: PostLASIK interface debrisand interface inflammation (Sands of the Sahara). ISRS 1998 Pre-American Academy Conference. November 6-7, 1998. New Orleans-USA. 11- Kaufman SC, Ismail MM, Beuerman RW, Ohta T, Palkama A,Mustonen R.: Post-LASIK interface debris and keratitis: Doesforeign material on the microkeratome blade Cause “Sands of theSahara” Syndrome? Abstract Book page 899, 1999 ARVO meetingFlorida-USA. Mahmoud M. Ismail, M.D., Ph.D. University of Al-Azhar, Cairo - Egypt 21-A El Obour Buildings Salah Salem, 11371 Cairo, EGYPT. E-mail: [email protected]

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Chapter 4 PREDICTIVE FORMULAS FOR LASIK Louis E. Probst V MD., Jonathan Woolfson MD., Michiel Kritzinger MB

The Predictive Formulas Main Components The predictive formulas for laser in situ keratomileusis (LASIK) have two components, the excimer laser ablation nomogram and the adjustment factors. The excimer laser ablation nomogram controls the relative distribution of the refraction correction into one or more zones. In some of the newer excimer lasers, such as the VISX Star, the excimer ablation nomogram is controlled by the lasers computer, while in other excimer lasers, such as the Chiron Technolas 116, the ablation nomogram is fully programmable by the surgeon. The adjustment factors allow surgeons to refine the treatment protocol to reflect their particular refractive situation. In order for these formulas to be predictive, a high level of consistency must be achieved in the application of both the ablation nomograms and adjustment factors. Other extraneous variables such as the methods of preoperative refraction, the room temperature and humidity, and room air quality and flow, surgical technique and time, and the postoperative medications must be tightly controlled to avoid deviations from the intended correction.

Developing Individualized Predictive Formulas It is crucial to remember that the predictive formulas including both the excimer laser ablation nomogram and the adjustment factors must be individualized for each surgeon. Direct extrapolations

from the experience of one surgeon or one center will likely lead to an unexpected deviation of the surgical results from emmetropia. Since it is impossible to control every aspect of surgery, each surgeon must develop their own predictive formulas once their technique has become standardized and their postoperative results can be analyzed. For the beginning LASIK surgeon, conservative corrections are preferable as enhancements are easy to perform while overcorrections can be much more challenging.

Contents

Section 1 Section 2

Section 3

Section 4

Section 5

The Healing Pattern of the Cornea

Section 6

Once the excimer ablation nomogram and the Section 7 adjustment factors have been standardized and indiSubjects Index vidualized for each excimer laser surgeon, the final uncontrolled variable with LASIK is the healing pattern of the cornea. While there is a clear tendency for greater amounts of regression after LASIK for higher levels of myopia, often the degree of regression after LASIK is unpredictable. Younger patients (< 25 years) often demonstrate significant regression Help ? while older patients (> 50 years) may not regress at all. We have often observed regression of 1.0 – 2.0 D in one eye and no regression in the other eye after bilateral simultaneous LASIK in which the excimer nomogram, the adjustment factors, surgical technique, and extraneous variables were all exactly the same for the correction of both eyes. This unpredictable healing pattern of the cornea represents the limitation of corneal refractive surgery. In order to avoid LASIK overcorrections, it is best to plan for a 10 – 20 % enhancement rate for lower myopes and LASIK AND BEYOND LASIK

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even higher rates with high myopes, which will allow retreatment for those patients that have regressed.

Excimer Laser Ablation Nomograms for Photorefractive Keratectomy The excimer laser nomograms for LASIK have been developed from the excimer LASIK experience with photorefractive keratectomy (PRK). The concepts of laser pretreatment to prevent central islands and multizone ablations to decrease ablation depth and smooth the laser contour evolved as the worldwide PRK experience as well as the technological capabilities of the excimer lasers increased.

Pretreatment Protocols Pretreatment protocols have been added to the ablation profiles of the broad beam excimer lasers such as the Visx Star, Summit Omnimed, and Chiron Technolas Keracor 116 to reduce the incidence of postoperative central islands.1 The Visx Star pretreatment is automatically calculated by the central island factor (CIF) 4.01 software and incorporated into the excimer ablation protocol. Approximately 1 micron per diopter of spherical correction plus an additional 2 microns is added to each ablation protocol and is performed at 2.5 mm. The Chiron Technolas Keracor 116 pretreatment is surgeon programmable. Generally, 1 micron per diopter plus 2 - 4 microns is added to each ablation protocol and is performed at 3.0 mm.2 The Summit Omnimed excimer laser has a gaussian beam distribution for which a relatively greater amount of laser energy is produced in the center of the ablation circle, so less pretreatment is required. A pretreatment of 1 - 2 microns per diopter is generally performed using the patient training “A” mode with optical zone of 2.6 to 2.8 mm. The newer scanning excimer laser systems such as the Chiron Technolas 217 excimer laser do not need pretreatment protocols as this phenomena of undertreatment of the central cornea is avoided with these scanning laser systems

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Single and Multizone Ablations Protocols All excimer laser refractive procedures modify the refracting power of the cornea by altering the anterior corneal curvature by the process of photoablation. The correction of myopia involves the relative flattening of the central cornea compared to the peripheral cornea, which reduces the anterior corneal curvature and hence reduces the refractive power of the treated area. Because the maximal corneal stromal tissue will be photoablated from the central cornea, the thickness of the central cornea becomes important when LASIK is performed for high refractive errors with large ablation depths. The excimer ablation techniques have evolved. The initial single zone techniques increased from 4.0 to 6.0 mm,3,4 to improve the quality of the postoperative vision and reduce the incidence of halos and regression. The multipass multizone technique was developed by Mihai Pop, MD for the Visx excimer laser5,6 and the multi-multizone technique was developed by Jeffery J. Machat, MD for the Chiron Technolas excimer laser.1 These multizone techniques divide the myopic treatment into multiple zones, which decreases the ablation depth and creates a smoother ablation surface. This blending and smoothing effect of the multizone protocols has helped to reduce the incidence of post-PRK regression and haze particularly for the treatment of high myopia.6

Contents

Section 1 Section 2

Section 3

Section 4

Section 5


Excimer Laser Ablation Nomograms for LASIK The creation of the corneal flap and the routine correction of higher levels of myopia with LASIK introduced new considerations into the excimer nomograms. The depth of the ablation and the size of the ablations zones have become recognized as crucial consideration to achieve a good quality and quantity of correction while maintaining the safety of the procedure and the stability of the cornea. If all of these factors are considered, LASIK has correction limits of 10 to 15 D of myopia depending on which ablation nomogram is used.

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Basic Tenets of LASIK There are four critical values or dimensions that must be considered when performing LASIK which are the flap thickness, the amount of the residual corneal stroma, the diameter of the excimer ablation, and the depth of the excimer ablation. The flap thickness must be sufficient to prevent irregular astigmatism while not so excessive to remove stroma potentially available for ablation. We generally use a 160 µm flap for thin corneas or large refractive corrections (> 10.0 D), a 180 µm flap for average corneas and moderate corrections (> 6.0 D), and a 200 µm flap for thick corneas and small corrections (< 6.0 D). Sufficient residual posterior stroma must be left after the LASIK procedure to avoid a decrease of the corneal integrity and the subsequent development of corneal ectasia. Since iatrogenic keratoconus has been observed after automated lamellar keratoplasty (ALK) with 200 µm of remaining posterior stromal tissue, we generally elect to leave at least 250 µm of posterior stromal tissue. The diameter of the excimer ablation should be at least 6.0 mm create a functional postoperative optical zone of at least 4.0 mm which will allow for sufficient quality of vision. Finally, the depth of the excimer ablation determines the quantity of myopia the can be safety treated while preserving adequate residual corneal stroma.

Ablation Depth Per Diopter Each excimer laser ablates a different amount of stromal tissue per diopter of refractive correction because of the differences in the ablation zone diameters, amount of pretreatment, and the ablation protocols. The Munnerlyn formula8 (depth of ablation = diopters of correction X ablation diameter2 : 3) indicates that each spherical equivalent (SE) diopter of myopic correction performed at a 6.0 mm single zone will ablate 12 microns of tissue. Pretreatment protocols added to the ablation profile of broad beam excimer lasers such as the VISX Star and the Chiron Technolas Keracor 116 will increase the depth per SE diopter to 17-20 microns for low corrections (1 - 2 diopters) and 15-17 microns for higher corrections (3 or more diopters). The Summit Omnimed will ablate 13.0 to 14.0 microns per SE diopter for all levels of myopia with pretreatment.

The multipass multizone ablation technique used with Visx Star “international cards” has an average stromal ablation 12.5 microns per SE diopter. The full multi-multizone ablation technique with the ablation pattern distributed between 3.6 and 6.2 mm reduces the average stromal ablation to approximately 10 microns per SE diopter.1 While the full multi-multizone protocol significantly reduces ablation depth, it should be only be used for LASIK when necessitated by a thin cornea associated with high myopia because of the compromised quality of postoperative night vision.

Correction Limits for Primary LASIK The average central cornea thickness is approximately 550 + 100 µm.9 Since the flap thickness during the LASIK procedure is generally 160 µm, the average cornea will have 390 µm of posterior stromal bed left after the flap creation. Therefore, the maximal myopic correction that should be performed on a patient with a 550 µm cornea using a full multi-multizone technique is generally less than 14 D while leaving a residual posterior stromal bed of 250 microns. A partial multizone ablation as performed with the Chiron Technolas 116 and the VISX Star would allow a maximal correction of approximately 12.0 D, and a single zone ablation would allow a maximal myopic correction of 10.0 D.10

VISX Star: Predictive Formulas for LASIK

Contents

Section 1 Section 2

Section 3

Section 4

Section 5


The VISX Star utilizes a propriety multizone nomogram developed from the PRK multizone experience with the VISX 20/20 excimer laser. A pretreatment is combined with a multizone ablation that is not surgeon programmable. While this limits the flexibility of the laser for adjusting ablation zone size, which would be beneficial for patients with large pupils, it does reduce the variation in the techniques of surgeons and therefore allows for good comparisons between centers. One number of factors may contribute to the increased effectiveness of the VISX Star excimer laser when used for LASIK. The vacuum nozzle of this machine may decrease stromal hydration, in-


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creasing the effect of the excimer ablation. The slower pulse frequency may allow the corneal to dehydrate. Surgeon factors such as the time taken to perform the procedure and the methods of drying the cornea have also been considered. Once several centers gained more experience with the VISX Star laser, a pattern began to emerge regarding the “adjustment factors”. The nomograms for 15 surgeons that have performed more than 500 LASIK procedures each on the VISX Star laser were compared (Tab. 1). The range of the myopia treated and the enhancement rates were recorded to ensure that most surgeons were achieving a similar level of predictability with their procedures. The temperature room and the humidity range of the procedure rooms did not correlate with the adjustment factors used at each center. Most surgeons did not use drying techniques. The altitude emerged however as a consistent factor that seemed to be related to the adjustment factor. When the two factors were correlated it was found that there was a statistically significant correlation between the altitude of the refractive center and the surgeon spherical adjustment factor.

This variation in the effectiveness of the excimer laser at different altitudes is probably a function of the changes in air density. Increased humidity will actually decrease the air density. However, its effect on the air density is minimal and most refractive centers have humidity control systems installed. Temperature is usually well controlled so it is unlikely that this would introduce significant variation in results. The station pressure is the direct barometer reading. While we currently adjust the spherical component of the VISX Star ablation by a constant value that seems to be related to altitude, perhaps our adjustments in the future will be based on the daily barometric reading and the corresponding calculations of air density.

VISX S2 SmoothScan: Predictive Formulas for LASIK

Contents

Section 1

The adjustment factors utilized for the VISX S2 Smoothscan excimer laser upgrade are very similar to those used for the VISX Star laser. Most surgeons have made only minor adjustments to their

Section 2

Section 3

Section 4

Section 5

Table 1 Nomogram Comparison Table


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VISX Star reduction factor. Increasing the frequency of the excimer pulse to 10 Hz reduces the laser time by almost 50% but does not effect the degree of refractive correction. The spherical reduction factor that we currently utilize at TLC Chicago is outlined (Table 2). Table 2 Probst Nomogram for Visx S2 SmoothScan < 25 years

25 – 45 years > 45 years

< 2.0 D myopia

100%

100%

100%

> 2.0 D myopia

90%

86%

84%

For hyperopic corrections, we utilize the 5.0-mm optical zone with a 9.0-mm blend zone for hyperopic LASIK with the Hansatome. For hyperopes, we add between 10 and 20% to the spherical refractive error to account for the greater amount of regression that occurs with the treatment of hyperopia.

Chiron Technolas 116: Predictive Formulas for LASIK With the Chiron Technolas 116, the ideal zone depth for each step of the multizone LASIK ablation was initially felt to be 15-20 µm to create the smoothest blended multizone ablation2 (Tab. 3). This algorithm was similar to those used for PRK and provided a smooth blend of the myopic ablation with zones that extended from 3.6 mm to 6.2 mm for maximum seven zones. This full multizone ablation allowed the treatment of high levels of myopia as the total ablation depth was minimized with the smaller zone size. Unfortunately, the smaller zone sizes resulted in a significant reduction in the effective optical zone observed on corneal topography following LASIK. High myopes treated with the full multizone algorithm demonstrated effective an optical zone that was often less than 4.0 mm. Clinically, this resulted in complaints of visual distortion and halos at night in the same manner the early small zone PRK ablations were associated with these difficulties. Decentrations are poorly tolerated with the small zone ablations as the refracted light for the edge

Table 3 Previous Machat Chiron 116: LASIK nomogram for high myopia Important points • No vertex distance correction. • Pretreatment 1 micron per diopter plus 2 to 4 microns total • Depth at 3mm for Chiron Technolas Keracor 116 to compensate for central island information • All treatment zones of equal depth (not including pretreatment step) • All zones ideally between 15 and 20 microns • Goal is to achieve at least 6-mm effective optical zone • Compressed air is used intraoperatively to control hydration.

Contents

Lasik nomogram for -13 D attempted correction Optical zone

Dioptric distribution

Percentage of treatment

Micron depth

Petreatment (1 µm/D + 3) 27.9 18.9 14.6 11.2 9.5 8.8 8.15

16

Section 1 Section 2

Section 3 3mm

-5.4D

3.6 mm 4.2 mm 4.8 mm 5.4 mm 5.8 mm 6 mm 6.2 mm

-3.63D -.258D -1.90D -1.46D -1.23D -1.14D -1.06D

Section 4

17 17 17 17 17 17 17

Eight zones of equal micron depth. Each zone 0.6 mm larger than the previous zone, with two additional zones within 0.2 mm of 6-mm optical zone to ensure adequate peripheral blend for reduced spherical aberration and night visual disturbances.

Section 5


of the ablation could be in close proximity to the visual axis. Machat has found that the size of the ablation zones should be increased so that most of the treatment is performed at a 5.5 mm or larger ablation zone.11 This increases the depth of ablation to 30 - 40 µm per zone in the partial multi-multizone protocol which accounts for the hydration effects of the deeper stroma and the masking effect of the corneal flap. By increasing the size and depth of ablation of each zone to 30 - 40 µm and decreasing the number of zones, the effective postoperative optical


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zone on videokeratography can be increased, and night vision difficulties minimized (Tab. 4). This partial multi-multizone ablation that we now use for LASIK ablations removes 12.5 microns of stroma per SE diopter. While the quality of the night vision is improved with these new algorithms, the depth of ablation per diopter must be increase with the corresponding increase in zone size. This limits the quantity of myopia that can be safely corrected to approximately 15.0 D in an eye with an average corneal thickness.

KRITZINGER NOMOGRAM FOR TECHNOLAS 217 EXCIMER LASER The success of excellent postoperative visual results does not only depend on the nomogram, but also comes into play: • Environmental factors in the operating room; temperature, humidity and drafts in the air. • surgical technique of the surgeon. • preoperative refraction of the patient. • postoperative medication to the patient.

Table 4 TLC the laser center: Chiron 116 nomogram Contents

Section 1 Section 2

Section 3

Section 4

Section 5


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•

type and make of the laser in use, broadbeam/ scanning.

• •

A) GENERAL RULES • • • • •

•

•

•

•

Room temperature: 16º - 18º C. Room humidity: 45 - 50%. Exact super-imposition of red and green HeNe beam critical - or under corrections will occur. Use 6 x magnification - not larger, because you can loose your orientation to the visual axis. Correctly align patient prior to lifting the flap, to limit exposure time of stroma before the treatment starts: Thus it will give you more accurate and consistent visual results. Lift flap with a Colibri - do not use BSS cannula as this may introduce moisture to the bed. Do not use a spatula, since foreign material e.g. epithelium may be introduced into the interface. Commence laser treatment, and let the assistant press the “enter” key to give continuous treatment without breaks, so that you make the treatment time and the stromal exposure time as short as possible. Avoid contact with the stromal bed - do not wipe whilst lasering the stroma - this is totally contraindicated, because it will give overcorrections with the 217 laser.

Minimum residual cornea after ablation 250 micron (excluding flap thickness). Ideal treatment zone 4mm - 6mm. It is advisable not to use a smaller zone diameter than 4.0mm (night vision glaze) and a maximum zone diameter of 6mm (unnecessary vertex ablation and over - corrections will results).

B) KRITZINGER NOMOGRAMS 1. Myopia • • • •

For treatment of -1.0 to - 13.0 spherical equivalent diopters. Use subjective spectacle correction for minus spheres. Add 10% to sphere and cylinder. Subtract / or add the calculated cylindrical correction from / to the calculated spherical correction, because:

Contents

Section 1 Section 2

Section 3

- 20% Hyperopic coupling shift with negative cylinders, on the spherical diopters. - 10% Myopic coupling shift with plus cylinders, on the spherical diopters.

Section 4

Section 5


Probst Nomograms Chiron Technolas 217 Planoscan version 2.422

Zone

Sphere

Cylinder

Comment

5.5 6.0

add 10% add 10%

add 10% no change

add 20% -cyl to -sph

Myopia (minus cyl)

Kritzinger Probst

Hyperopia (plus cyl)

Argento4.2-5.5 add 50-75% add 25% add +sph >40 yrs Kritzinger 5.5 add 15% add 10% subtract 10% of cyl from sph Probst 6.0 add 20% no change

Mixed (plus cyl)

Kritzinger Probst

5.5 6.0

add 10% no change

add 10% no change

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add 1/3 +cyl to -sph add 1/3 +cyl to -sph


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2. Hyperopia • •

• • •

For treatment of + 1.0 to + 3.0, (rarely up to +4.0 D) spherical equivalent diopters. Selection of treatment program, of the (217) laser (Hyperopia/myopia) is dependent on the sphere, and not the cylinder. Use subjective spectacle correction. Add 15% to sphere and 10% to the cylinder. Subtract / or add the calculated cylindrical correction from / to the calculated spherical correction, because:

correction, unless monovision is planned in the nondominant eye. Older patients (50 years old >-7 diopters sphere or > 3 diopters of cylinder Patient with nystagmus have the option for Laser with an eye tracker system.

PREOPERATIVE PREPARATION The Patient Patient’s preoperative preparation includes an oral sedative such as Valium (5 to 10 mg) prior to the procedure. Immediately before prepping, one drop of a topical anesthetic (Proparacaine) should be instilled and then one more drop before the keratectomy. No preoperative miotic is used. The pa140

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-Looked lid speculum -Smooth-angle forceps -Merocel sponges -Curved Irrigation cannula with BSS -Clear Eye shield -3M Sterile Drape 1020 -Gauze 4 x 4 -Assembled microkeratome -Excimer Laser

Contents

Section 1 Section 2

Section 3

The Laser

Section 4

The proper laser room environment is critical Section 5 for optimal laser performance. The laser is set up per the manufacturer’s recommendations, but we use Section 6 the following nomogram: Section 7 • If Plano result is desired back up –0.25 D on Subjects Index laser set up • Patient 35 to 45 years old back up –0.50 D for non-dominant eye • Patient >45 years old back up –0.75 D or more for non-dominant eye • To determine the dominant eye the patient is asked which is his or her “camera” eye or “shooting Help ? eye”. The laser room environment should be maintained for two endpoints, standard treatment and longevity of laser optics. The best environment for laser optics is in a room that is cool, dry and as low particulate matter count as possible. Ideally the temperature should be maintained between 600 to 700 F (180 to 240 C), and the humidity should be kept at a stable level between 30 to 40%. In addition, several air filtration units should be used continuously in the laser room to keep the atmosphere surgically clean


Figure 10-5. Ablation depth should leave more than 250 microns residual bed.

Figure 10-4. A sample of color change in the fluence test from white to red.

and to achieve standard laser ablation rates. It is unacceptable for the environment to be turned off for night or weekends. Laser beam calibration, homogeneity and alignment of the beam are achieved by fluence testing. In the case of the Bausch & Lomb Technolas 217 laser appropriate fluence is 65 pulses. The test is perform on polymethacrylate (PMMA) plate on which a subtle silver-plated foil is placed with the interposition of a layer of glue. The total number of spot necessary to obtain a complete exposure of the PMMA foil must be equal to 65 ± 2; in normal conditions, there is a color change from white to red in an interval of five to seven spots. A fine and dispersed white granularity can remain. (Figure 10-4) The surgeon should also verify patient data inserted in the computer is appropriate for surgical operation and that the axis of astigmatism is correct and corresponds with that found topographically and by refraction. The minimum diameter of the ablation should be appropriate to the patient’s pupil diameter and the ablation depth should leave more than 250 microns residual stroma bed (Figure 10-5).

The Keratome After inserting the blade into the microkeratome head, examine it very carefully under the operating microscope at maximum magnification to check the condition of the blade edge (Figure 10-6). All the parts of the keratome should be inspected

Contents

Section 1 Section 2

Section 3 Figure 10-6. Always check the condition of the blade edge.

Section 4

Section 5

Section 6

before proceeding with the cut. Discard a blade with Section 7 notches or stains. Very small irregularities on the Subjects Index blade margin can be also detected by observing the reflection of the microscope’s light on the edge of the blade. By depressing the foot pedal, the head advances along the track for a test run on the suction ring. Although an alarm indicates when the microkeratome has reached the end of its pass, the surgeon should also understand and visually memorize Help ? the end point of the forward pass, so as not to rely solely on hearing the “beep-beep”. If during advancement, the speculum obstructs the microkeratome and interrupts the run, the computer interprets this interruption as the end of the pass. Once the head reaches the end of the pass, push the foot pedal once again to return the microkeratome to its starting point. At this point, rotating the head through half turn in the opposite direction to insertion raises the keratome head along the pin until it detaches from the suction


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Figure 10-7. During advancement, the current on the display should not exceed 80 milliamps. Figure 10-8. Make sure to include the lid margins in the adhesive backing of the drape so they will not be in the way of the microkeratome on course over the suction ring.

Contents

ring. During advancement, the current on the display should not exceed 80 milliamps (Figure 10-7). The instrument is now ready for surgery.

Section 1

The Surgeon

Section 3

Section 2

Section 4

Lasik should be performed in a sterile environment wearing cap, mask and boots. We prefer no-glove technique with a Betadine hand scrub between patients, drying with a lint free cloth.

Section 5

Section 6 Section 7

SURGERY PREPARATION

Subjects Index

Draping Apply a disposable self-adhesive drape (fenestrated is easier to apply). Ask the patient to open both eyes as much as possible. To exclude the eyelashes from the operating field, have your assistant hold the drapes’ opposite corners as you apply the drape at the edge of the superior eyelid first and then do the same with the inferior eyelid. Make sure to include the lid margins in the adhesive backing of the drape so they will not be in the way of the microkeratome on course over the suction ring (Figure 10-8).

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Figure 10-9. Accommodate suction ring within the intrapalpebral opening.

Speculum A locking eyelid speculum is recommended, but either locking or non-locking speculum can be used. The ideal speculum should provide maximal patient comfort when fully opened, allow for temporal and superior approaches, accommodate suction ring within the intrapalpebral opening and maximize exposure to enable clear passage of microkeratome (Figure 10-9).

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Figure 10-10. Prior to placing the LASIK suction ring, the head should be positioned so the chin and forehead are in the same frontal plane.

Figure 10-11. The LASIK suction ring is placed slightly decentered 1 mm superiorly.

Contents

Positioning the Patient

Section 1 Section 2

Prior to placing the LASIK suction ring, the head should be positioned so the chin and forehead are in the same frontal plane (Figure 10-10). Make sure the amount of inferior and superior sclera’s are the same therefore the cornea is centered between the lids (Figure 10-9).

Section 3

Section 4

Section 5

Section 6

THE LASIK PROCEDURE Marking

Section 7 Figure 10-12. Confirming that the area of applanated cornea is Subjects Index the same side or smaller than the circular mark on the applanating surface of the tonometer.

The cornea should be marked with a pararadial line that facilitates exact repositioning of the flap in case of a free cap. A minimum amount of gentian violet should be used to avoid epithelial toxicity. Help ?

Placement of the Suction Ring The LASIK suction ring is placed slightly decentered 1 mm superiorly (Figure 10-11). Suction ring should be firmly placed on the globe with one hand and at the same time apply downward pressure on speculum to proptose the eye. The vacuum pump is activated and the intraocular pressure is checked with a Barraquer tonometer lens to assure an intraocular pressure

greater than 65 mm Hg., confirming that the area of applanated cornea is the same side or smaller than the circular mark on the applanating surface of the tonometer (Figure 10-12). The tonometer and corneal surfaces should be dried to avoid a false reading. Many expert surgeons no longer perform tonometry, relying in digital touch, small displacements and observing the slight mydriasis induced by the suction itself. The surgeon should remember that reLASIK AND BEYOND LASIK

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Figure 10-13. Loading the microkeratome.

Figure 10-14. After loading the keratome onto the post of the suction ring press down on the motor to slightly compress the cornea and thereby fully seat the eye adapter before engagement of gear to the rack.

dundant conjunctiva could produce the false sensation that the suction ring has adhered to the globe by obstructing the suction. It is very important to remember to keep the cornea wet at all times, and just dried prior to the tonometer reading, as it could result in a complication.

Contents

Section 1 Section 2

Section 3

Section 4

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The Microkeratome Cut

Section 5

When the above is satisfied the surgeon is ready to progress with the keratectomy. After loading the keratome onto the post of the suction ring press down on the motor to slightly compress the cornea and thereby fully seat the eye adapter before engagement of gear to the rack (Figures 10-13 and 10-14). Immediately prior to passage of the microkeratome, one or two drops of glycerin-based anesthetic (Proparacaine) are instilled over the surface of the cornea to allow the microkeratome to advance more smoothly. Make sure to apply the drops directly from the bottle and not through a cannula since the drop size vary and sometimes is not enough fluid to lubricate the cornea. Excess fluid should be removed using a Merocel sponge to prevent it from going into the gears of the microkeratome and then onto the mirrors of the laser consequently disturbing beam quality. Proparacaine is used rather than BSS in order to keep salts away from the microkeratome.

Section 6

SECTION II


Figure 10-15. The microkeratome and suction ring can be removed at the same time as a one unit.

The keratome is placed into the suction ring and advanced by depressing the pedal. The microkeratome is reversed and the vacuum is stopped. The microkeratome and suction ring can be removed at the same time as a one unit (Figure 10-15).

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Figure 10-16. The corneal flap is lifted superiorly with curved forceps

Figure 10-17. The corneal flap is lifted superiorly with curved cannula.

Contents

Section 1 Section 2

Section 3

Section 4

Section 5

Section 6 Section 7 Figure 10-19. The head is rotated to the right and the body to the left; as a result the ablation will be decentered. Subjects Index Figure 10-18. The laser focus is achieved over the pupillary center

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Laser Ablation The corneal flap is lifted superiorly with curved forceps (Figures 10-16, 10-17), the laser focus is achieved over the pupillary center (Figure 10-18), and patient’s head is again aligned so the chin and forehead are in the same frontal plane; a straight imaginary line passes through the feet, umbilicus and nose (Figures 10-19 and 10-20). At this point, the surgeon can proceed with the ablation of stromal bed.

Figure 10-20. A straight imaginary line passes through the feet, umbilicus and nose


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Figures 10-21 & 10-22. If ablation or astigmatism are being completed the surgeon must protect the hinge from ablation by holding a Merocel sponge over this area.

Contents

Section 1 Section 2

Section 3

Section 4

Section 5

Section 6 Section 7 Figure 10-23. Couples of drops of BSS are added onto the stromal bed.

Figure 10-24. The corneal flap is replaced using the cannula starting superiorly. Subjects Index

When the larger zones of ablation or astigmatism are being completed the surgeon must protect the hinge from ablation by holding a Merocel sponge over this area (Figures 10-21 and 10-22). Help ?

Replacing the Flap When the ablation is complete, couple of drops of BSS is added onto the stromal bed and then the corneal flap is replaced using the cannula starting superiorly (Figures 10-23 and 10-24). Make sure the tip of the cannula is outside the flap before flap is positioned back because surgeon can either place a hole on the flap or scratch the stromal bed, especially when using sharp tips cannulas. (Figure 10-25 and 10-26) The cannula is placed underneath the 146

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Figure 10-25. Make sure the tip of the cannula is outside and parallel to the flap before flap is positioned back


Figure 10-26. Canulla is not parallel; surgeon can accidentally scratch the stromal bed

Figure 10-27. The cannula is placed underneath the flap and irrigation is completed to clear any remaining debris

Contents

Section 1 Section 2

Section 3

Section 4

Section 5


Figure 10-28. BSS is use under the flap to facilitate “floating” back into its original position.

Figure 10-29. Merocel sponge is moistened and squeezed dry and then used to “paint the flap” in the direction of the hinge. Help ?

flap and irrigation is completed to clear any remaining debris from the interface as well as allowing BSS under the flap to facilitate “floating” back into its original position. (Figures 10-27 and 10-28)

The Merocel sponge is moistened and squeezed dry and then used to “paint the flap” in the direction of the hinge (Figure 10-29). The flap is inspected to reassure that there are no wrinkles and for proper position by making sure


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Figure 10-30. The flap is inspected to reassure that there are no wrinkles and identical distance between the gutter and keratectomy edge is present.

Figure 10-31. When striae test is positive around the flap edge appropriate apposition has been achieved.

Contents

an identical distance between the gutter and keratectomy edge is present all over the flap circumference (Figure 10-30). Depressing the peripheral “non flap” cornea with closed blunt 0.12 forceps completes a Slade’s striae test (Figure 10-31). When striae test is positive around the flap edge appropriate apposition has been achieved. During this phase it is recommended to keep a BSS drop over the central corneal epithelium to keep it wet. There is no specific waiting time with this technique, but we recommend waiting 3-5 minutes before removing the speculum. The case is completed by carefully removing the speculum. When doing this step, make sure to lift and close the speculum at the same time to avoid displacement of the flap. The patient is then instructed to blink normally, and is observed through the microscope. The flap should remain in the same position and appear adhered to the cornea bed (Figure 10-32).

Section 1 Section 2

Section 3

Section 4

Section 5

Section 6 Section 7 Subjects Index Figure 10-32. The patient is instructed to blink normally, and is observed through the microscope.

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Intraoperative Bleeding in LASIK Bleeding of peripheral corneal vessels usually occur in long-term contact lens wear patients. The occurrence is higher when we use the 9.5 Hansatome suction ring. We prevent this by using an 8.5 suction ring if we notice any limbal pannus in the slit lamp examination. We don’t use any drug to stop intraoperative bleeding, because some of them could

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interfere with the iris, causing irregular dilation of the pupil. We use a Merocel sponge and apply some pressure on the peripheral vessels to stop the bleeding; generally is over by the time we finish the ablation treatment and reposition the flap (Figures 10-33 to 10-36)


Figure 10-33. The corneal flap is lifted superiorly with curved forceps

Figure 10-34. Merocel sponge is use to clean the corneal stromal bed

Contents

Section 1 Section 2

Section 3

Section 4

Section 5

Section 6 Section 7 Figure 10-35. The laser focus is achieved over the pupillary center and the ablation start.

Figure 10-36. Cornea still looks with some blood cell.

Subjects Index

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Inferior eyelashes bleeding is usually due a poor technique when loading the keratome; the surgeon is either applying to much pressure onto the suction ring and it end deeper than the speculum or not applying downward pressure on speculum to proptose the eye. (Figure 10-37)

Figure 10-37. The surgeon is applying too much pressure onto the suction ring and it end deeper than the speculum.


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Postoperative Care Immediately postoperatively, several drops of an antibiotic are instilled. The eye is not taped or shielded. The patient is asked to follow the home care instructions.

HOME CARE INSTRUCTIONS -Wear a clear eye shield to sleep for the first five days. -Wear protective sunglasses anytime patient is outside for the first five days. -Use Acular eye drops only on the first day post-op and only for discomfort. -Two hours after surgery start 1 drop of Ocuflox and Lotemax every 3 hours while awake -Patient should wait about one minute between drops. -Make sure to shake the Lotemax before using. -Next four days use Ocuflox and Lotemax four times a day. -Patients may need Lubricating drops for dry eyes. -Do not to rub the eyes for 5 days after the surgery, avoid any trauma to the eyes. -Patient may wash face, but avoid getting anything into the eyes. -Use good hand washing technique and cleanliness. -No eye makeup for 3 days -No swimming for 10 days -Stay out of hot tubs for 4 weeks -Patient may shower; however keep the force of the water away from the eyes -We advice against driving

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Section 1 Section 2

Section 3

Section 4

Section 5


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PEARLS IN LASIK TECHNIQUE

Chapter 11 PEARLS IN LASIK TECHNIQUE Elizabeth A. Davis, M.D., David R. Hardten, M.D., Richard L. Lindstrom, M.D.

LASIK surgery has become a quick, automated procedure. However, a good outcome still depends largely on the surgeon’s knowledge, skill, and experience. A successful procedure begins with appropriate patient selection and counseling. Intraoperatively, there must be great attention to detail. This chapter will describe surgical techniques designed to achieve the best outcomes and lowest risk of complications in LASIK surgery.

Patient Counseling A successful LASIK surgery depends as much on a technically good operation as it does on an appropriately counseled patient. The patient must not only be informed of the risks and benefits of the procedures, but also its limitations. Thus, the patient must have realistic expectations of what the outcomes could be. In counseling the patient, it is far better to counsel for lesser outcomes and have the patient pleasantly surprised than the converse. The goal of the surgery is functional vision without glasses or contact lenses. There can be no guarantee of 20/20 vision. The vast majority of the time, results will be excellent and the patient will be pleased. Presbyopic patients should understand that reading glasses will still be needed after LASIK. For surgeons who aim for some initial overcorrection, the patient should also be forewarned about some difficulty with their intermediate range of vision as well. Myopic patients need to understand that their faces may be blurry in the mirror postoperatively. Hyperopic patients should be informed that they may be temporarily myopic.

The surgeon should explain that, particularly for the higher levels of correction, visual recovery may take several weeks to months. Although a big improvement in their uncorrected visual acuity will occur in the first 24 hours, continued improvement can occur after this. Additionally, patients should understand that 5-10% will require an enhancement to achieve the desired results. They should be given an estimate of the time at which this might occur, if desired, based upon their preoperative refractive error. We prefer to wait one month per diopter of myopia and three months per diopter of hyperopia prior to performing an enhancement.

Achieve Adequate Exposure

Contents

Section 1 Section 2

Section 3

Section 4

Section 5

Section 6 Section 7

Achieving adequate exposure is the first and one of the most important steps of the LASIK proce- Subjects Index dure. All of the subsequent maneuvers in the surgery rely on this step. Adequate exposure is critical to visibility, achieving adequate suction, ability to place the microkeratome properly, unobstructed passage of the microkeratome, and a well-exposed stromal bed. There are certain orbital anatomies that Help ? predispose to difficult exposure and these should be noted preoperatively. Deep set orbits, prominent brows, or small palpebral fissures may all interfere with placement of instruments on the globe. In these instances, as well as others, it is often helpful to have the patient maintain a chin-up position for adequate visibility and instrument placement. If the patient has a prominent lower cheek that overhangs the lower blade of the speculum, the surgeon may use his/her 4th and 5th fingers to retract this tissue inferiorly and out of the surgical field. Similarly, a technician can LASIK AND BEYOND LASIK

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Figure 11-1. Isolation of lashes: Tegaderm over upper and lower lids.

Figure 11-2. Placement of suction ring.

assist with this maneuver. Some speculae have been designed to help retract any overhanging skin. The lashes should be isolated from the surgical field. This is not only important for sterility purposes, but to prevent cilia from becoming engaged in the keratome and prematurely stopping the pass. Isolation of the lashes can be done by simply placing tape or steri-strips over the lashes. A Tegaderm adhesive plastic also works well. If cut in half, one half can be used for the upper lid and the other half for the lower lid (Figure 11-1). Another option is to use a surgical drape. Or, one may simply use a closed-bladed speculum. Placement of the suction ring may be facilitated by pressing down on the speculum gently (Figure 11-2). This causes the globe to proptose and results in greater exposure. Additionally, once adequate suction has been achieved, one can carefully and gently lift the eye up and out of the orbit with the suction ring to allow unobstructed passage of the microkeratome. Care must be taken during this maneuver not to break suction. In cases of small palpebral fissures, it is often useful to have several lid specula available. Often, switching from a closed bladed speculum to an openbladed one may provide the few millimeters needed to insert the suction ring. Exposure is sometimes limited by eyelid squeezing. The vast majority of these situations are easily circumvented by proper instruction to the patient and a calm, reassuring demeanor. Also, specula with a screw or locking mechanism counteract squeezing much better than self-retaining specula.

Nevertheless, in a few cases, it may not be possible to get the patient to relax the eyelids. Here, one might consider a facial nerve block. Less frequently, and only with the patient’s consent, one might perform a lateral canthotomy. Rarely, some surgeons have reported doing a retrobulbar injection in order to proptose the globe.

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Achieve and Confirm Adequate Suction

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Adequate suction is necessary to create a proper flap. Loss of suction or inadequate suction Section 6 can have serious consequences such as a short flap, Section 7 a small flap, a free cap, a buttonhole, or an irregular edge. As mentioned above, good exposure is criti- Subjects Index cal. If it is not possible to seat the suction ring on the globe unobstructed, vacuum cannot be obtained. Additionally, a prolonged struggle to fit the instruments onto the eye can lead to conjunctival chemosis and either inability to obtain suction or pseudosuction. Pseudosuction is when the vacuum registers high because the conjunctiva or drapes are Help ? occluding the suction holes. In this case, the intraocular pressure will not be sufficiently elevated to pass the microkeratome. In cases of conjunctival chemosis, a Merocel sponge can be used in an attempt to “milk” the excess fluid away from the limbus. If chemosis has occurred because of some intraoperative manipulations, then waiting 30 to 40 minutes for the edema to subside may result in success. If not, then the procedure should be postponed a day or two to allow reabsorption of the fluid.


It is important to seat the suction ring properly on the globe and then apply firm, even pressure. Once this is done, suction can be applied. There are several indicators of sufficient suction. No single indicator should be relied upon alone. Rather, the surgeon should be observant for several of them. Firstly, when adequate suction is achieved, the sound of the pump changes. Secondly, the vacuum pressure should register as greater than 25 mm Hg and this should be verbally announced by the assistant. Additionally, the pupil will dilate slightly. If asked, the patient will report that the vision has dimmed or blacked out. The surgeon may use tonometry to confirm that the intraocular pressure has risen. A commonly used device is the Barraquer tonometer. And lastly, one can confirm good suction if the globe can be elevated out of the orbit by lifting up gently on the suction ring. After adequate suction has been achieved and confirmed, it is important not to torque or pull on the suction ring to any great extent. Such maneuvers can result in immediate loss of suction.

Create a Complete Flap Flap complications should occur in less than 0.1% of cases with the newer microkeratomes in experienced hands. In some instances, unusual corneal anatomy may predispose to a flap complication. For example, corneas steeper than 46.00 D may buckle during the keratome pass, leading to buttonholed or centrally thinned flaps. Likewise, corneas flatter than 41.00 D are more prone to developing free caps. Keratometry readings should be noted preoperatively and used to select the appropriate ring size. A smaller deeper flap should be made when corneas are unusually steep and a larger flap should be created for flat corneas. The surgeon should carefully inspect all parts of the microkeratome including the blade, gears, and flap receptacle. The equipment should be meticulously cleaned and assembled. The motor should be tested prior to each pass to insure it runs with minimal resistance. As mentioned above, adequate exposure is necessary to allow unimpeded passage of the microkeratome. Both the surgeon and assistant should

check to make sure that the lids, lashes, and drapes are clear of the surgical field prior to the forward pass of the keratome. If resistance is met during the forward passage of the keratome or the keratome comes to a stop, the surgeon should stop and examine the field. Are the drapes caught in the speculum? Is the patient squeezing or is the speculum in the way? Any obstruction should be gently removed with care taken not to torque the suction ring off the eye. If no obvious obstruction is found, then the surgeon can tap on the forward foot pedal of the microkeratome. Sometimes this allows the motor to overcome some temporary resistance. If this is not successful then the microkeratome should be reversed and removed from the eye even if an incomplete pass has been made. One should never reverse the microkeratome and then Contents go forward. This can result in the blade penetrating to a deeper level than the initial pass. A case of ante- Section 1 rior segment perforation has occurred when the surgeon reversed the microkeratome part way and then Section 2 proceeded forward again. Section 3 In the case of an incomplete pass, if there is not enough room beneath the flap to perform the Section 4 ablation, then the surgeon should reposition the flap and conclude the surgery. Should a buttonholed flap Section 5 occur, ablation should not be performed through the Section 6 remaining epithelium. The flap should be repositioned and smoothed into place. In neither case Section 7 should the surgeon move on to the second eye. A waiting period of 3 to 6 months should ensue prior Subjects Index to attempting to create a new flap. Close observation for epithelial ingrowth is necessary during this time period in the case of a buttonholed flap.

Maintain Consistent Hydration Hydration of the stromal bed needs to be adjusted evenly and consistently in all cases. Too much moisture results in overhydration, less tissue removed per pulse of the laser, and undercorrection. Too little moisture results in an excessively dry bed, more tissue ablation per pulse of laser, and overcorrection. Uneven hydration can lead to central islands and/or irregular astigmatism. After the flap has been created and prior to turning it back, the top of the cap should be wiped off. This prevents any excess fluid LASIK AND BEYOND LASIK

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Figure 11-3. Adjusting hydration by drying bed.

Figure 11-4. Uneven hydration: with and without ring light.

Contents

from getting onto the stromal bed as the flap is lifted. Some surgeons prefer forceps to lift the flap rather than a cannula, which can drip fluid. Some surgeons prefer to treat with the bed moist, while others like it drier (Figure 11-3). Either method is acceptable, but it is critical to be consistent so that a nomogram may be developed that is based on that particular technique. In very humid or dry climates, it may be necessary to intermittently pause during the ablation to adjust the hydration of the stroma if there is excessive moisture or drying as the ablation proceeds. Using the ring light and high magnification will best allow the surgeon to determine the hydration status. Without the ring light it is much more difficult to follow fluid patterns (Figures 11-4 and 11-5). Excess pooling of fluid can often be found on the stromal bed near the hinge after folding back the flap. This should be wicked away with a soft surgical sponge. Likewise, any bleeding from the vessels of a peripheral pannus should be wiped away. If a soft surgical sponge is touched near the edge of the area of bleeding during the treatment, it may be possible to wick away the blood without having to stop the laser periodically. A smaller flap size may also be used to avoid cutting through these vessels. Some surgeons have reported success in using a drop of phenylephrine, iopidine, or brimonidine just prior to the procedure to constrict these vessels. However, this should be done with caution. If too much time 154

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Section 6 Figure 11-5. Even hydration: with and without ring light.


passes, pupil dilation can occur with phenylephrine. Additionally, some cases of slipped flaps have been reported when iopidine or brimonidine was used. In all situations the surgeon should try to minimize the amount of time between turning the flap and ablating. Focus and centration should be adjusted and treatment numbers checked prior to lifting back the flap. Once the flap is lifted, the stromal hydration adjusted, and patient fixation achieved, the ablation should proceed without delay.

Perform the Appropriate Ablation Performing the correct ablation is the surgeon’s responsibility. The original clinical work-up with refraction and topography should be brought to the operating room. The cylinder orientation should be

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checked against the topography. If the axes are vastly different, particularly if the power of the cylinder is significant, this should be double checked in a second refraction. If it is necessary to transpose the cylinder format before entering the refraction into the laser or to perform a nomogram adjustment, then special care must be made to ensure that the axis is shifted appropriately. Multiple checks by both the surgeon and technician can reduce the incidence of errors. The surgeon and technician should check the numbers prior to entering the laser suite and then again after the nomogram adjustment has been performed. One final check can then be performed by having the technician hold the chart next to the laser screen (Figure 11-6). The technician and surgeon then together check that the numbers entered into the laser are correct. This is done for the first eye before gloving and for the second eye before the flap is created. It is also helpful to read the patient’s name and eye to be treated aloud. In this way, no errors are made by using the wrong chart or switching the treatments for the two eyes.

Contents Figure 11-6. Surgeon and technician check entered refraction against patient’s chart.

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Prevent and Remove Debris from Beneath the Flap Debris can originate from multiple sources. Metal particles from the microkeratome or blade, meibomian oil and makeup from the lids, lint from surgical sponges, and debris from the cannula or irrigating fluid can all accumulate beneath the flap. In order to prevent these particles from gaining access to the surgical field, certain measures can be taken. Firstly, the surgeon should examine the lids preoperatively and treat ocular surface disease aggressively. Warm soaks, a topical antibiotic, or even systemic doxycycline should be considered to treat meibomian gland disease. Patients should be instructed to carefully remove makeup prior to surgery and clean the lids and lashes the night before and morning of surgery. One may consider irrigating the ocular surface prior to surgery, but sometimes this can just stir up more meibomian particles. However, this may useful to wash away other debris. A Chayet sponge may

Section 1


Figure 11-7. Irrigation beneath the flap to remove debris.

be placed around the cornea after the flap is created to isolate the stromal bed from the rest of the surgical field. If this is done, it is important to carefully monitor hydration to ensure no pooling of fluid is induced with the presence of a sponge. Once the ablation is performed and the flap replaced, a cannula should be used to irrigate beneath the flap (Figure 11-7). The cannula should be moved back and forth beneath the flap to loosen any particles adherent to the stromal bed or back of the flap. Excess irrigation should be collected with suction, a LASIK AND BEYOND LASIK

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Figure 11-9. Stroking flap into place with Merocel sponge. Figure 11-8. Marking the cornea.

Contents

Section 1 Section 2

suction speculum, a Merocel sponge, or a drain. Overly aggressive irrigation should be avoided, however, to prevent cap edema and retraction of the flap edges. Additionally, excess irrigation may lead to the reflux of more meibomian particles from the conjunctival fornices. Using high magnification and side illumination, the surgeon should carefully inspect the flap interface to ensure all debris has been removed. Some surgeons prefer to examine the patient once more at the slit lamp prior to discharging the patient. If any significant debris is noted, the flap can be relifted and irrigated.

Properly Align the Flap Preventing striae begins with proper flap alignment. Striae can be problematic if they are prominent and centrally located. In these cases, striae can result in irregular astigmatism and poor uncorrected visual acuity as well as a loss of best spectacle corrected acuity. Marking the cornea prior to creating the flap can help in aligning the flap after the ablation (Figure 11-8). However, excessive marking should be avoided to prevent epithelial toxicity. Also, the sur156

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geon should be aware that if the epithelium shifts Section 3 during passage of the microkeratome, the marks may Section 4 be misleading. A more important indicator of proper flap alignment is gutter symmetry. Once the flap is Section 5 turned over, care should be taken to make sure it is neatly positioned in the fornix or on the lid specu- Section 6 lum. It should not be folded over on itself or Section 7 wrinkled. If this occurs, a significant crease can develop across the flap that may be difficult to smooth Subjects Index out. Various instruments are available to iron the flap, but simply stroking or painting the flap back into position with a moistened surgical sponge can achieve an excellent result. Initially this is done by stroking the flap from the hinge toward the opposite Help ? direction (Figure 11-9). Once good alignment is achieved and the flap begins to adhere, gentle stroking may be performed in the oblique directions in a radial fashion. More aggressive surgeons may use a drier sponge to perform stretching by placing the sponge on the flap edge and pulling in a radial direction. The ring light is very useful in identifying striae. One can see discontinuities in the reflection rather than a smooth continuous circle of light (Figure 11-10).


Figure 11-10. Follow ring light to identify striae. Figure 11-11. Lubrication prior to microkeratome pass.

Lastly, as mentioned above, excessive irrigation should be avoided as this can lead to cap swelling, retraction from the edges of the bed, and gutter asymmetry. These flaps can be difficult to align and are more prone to striae postoperatively.

Achieve Good Flap Adhesion The surgeon should ensure that the flap adheres well to the underlying stromal bed before removing the lid speculum. There are several options available to drying the cornea. The flap may be allowed to simply air dry. This should be done for 3-5 minutes. Alternatively, with some lasers (such as the VISX laser) the surgeon may turn on the aspiration for 30-60 seconds to dehydrate the flap. Or, filtered low-power compressed air may be directed onto the epithelium for 10-20 seconds. Over-drying should be avoided as this can result in cap retraction and striae. Good adhesion can be confirmed with the striae test. Using a dry Merocel sponge or other blunt instrument, gentle pressure is applied downward on the epithelium just beyond the keratectomy. If good adhesion is present, fine folds will radiate into the flap. Lubrication should be applied to the cornea prior to removing the lid speculum. This will reduce friction from the lid postoperatively as the patient blinks and decreases the risk of flap displacement.

Care should be taken in removing the speculum so as not to displace the flap. As the speculum is closed it should be lifted upward off of the globe. The patient should be instructed not to squeeze on speculum removal. Once the speculum is out, the surgeon should have the patient open their eyes once more to inspect the flap position.

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Avoid and Treat Loose Epithelium

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Loose epithelium may be encountered intra- Section 7 operatively after the microkeratome makes its pass. Most commonly this is found near the flap hinge but Subjects Index occasionally it can involve a more diffuse area of the flap surface. Patients may be predisposed to this complication if they have evidence of anterior basement membrane dystrophy and/or recurrent erosions. A careful history depicting any prior episodes of spontaneous eye pain should be sought. A thorough Help ? slit lamp examination should be performed looking for epithelial abnormalities. In severe cases, topography will show irregular astigmatism. Epithelial toxicity should be avoided. The surgeon should minimize the use of preoperative anesthetic drops. No more than two drops per eye should be applied. Additionally, marking of the cornea should be done sparingly since the ink is toxic to the epithelium. A lubricating drop should be placed on the corneal surface just prior to the microkeratome pass (Figure 11-11). LASIK AND BEYOND LASIK

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One method, which appears to decrease the incidence of epithelial defects with the Hansatome microkeratome, is to release suction on the reverse pass. This allows the flap to unroll under no tension. Gentle downward pressure with the suction ring prevents the patient from moving their eye during these few seconds. If loose epithelium is encountered, after the flap is repositioned appropriately, it should be manipulated back into place with a Merocel sponge. If this is not possible, or if loose epithelium is an impediment to stroking the flap, then it should be removed with forceps or a dry Merocel sponge. For central or large epithelial defects, a bandage contact lens should be placed. This is typically left in position for a week until the surface is completely reepithelialized. Patients should be watched carefully for signs of infection or diffuse lamellar keratitis. The latter frequently occurs in association with an epithelial defect and should be treated appropriately with frequent topical steroid eye drops. Longer-term follow-up is necessary to monitor for epithelial ingrowth, which occurs in a greater percentage of these cases than usual.

Conclusion

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Although not exhaustive, the above sections provide some helpful techniques in achieving a good LASIK outcome. Each procedure must be modified as appropriate according to the particular case. Knowledge of potential complications, their risk factors, and methods of prevention are crucial to success.

Elizabeth A. Davis,M.D. Associate, Minnesota Eye Consultants, P.A. Assistant Clinical Professor University of Minnesota E-mail: [email protected]

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LASIK (Laser In-Situ Keratomileusis) FOR HYPEROPIA

Chapter 12 LASIK (Laser In-Situ Keratomileusis) FOR HYPEROPIA Weldon W. Haw, M.D., Edward E. Manche, M.D.

(Note from the Editor in Chief: This chapter is an excellent presentation of how hyperopic LASIK can be safe, predictable and effective at reducing low to moderate levels of hyperopia in appropriate candidates. It merits that it be read by all ophthalmic surgeons interested in further advancing their expertise in refractive surgery.)

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TECHNIQUE, SAFETY AND EFFICACY Until recently, excimer laser keratorefractive surgery has focused on the treatment of myopia and myopic astigmatism. Recent advances in excimer laser technology have resulted in the expansion of options available to patients with hyperopia. Excimer laser technology can be used to correct hyperopia by creating a doughnut shaped annular photoablation in the peripheryof the cornea. The peripheral ablation results in central steepening of the cornea that corrects hyperopia (Fig. 12-1). Initial attempts at correcting hyperopic refractive error with photorefractive keratectomy (PRK) using this method resulted in unacceptable levels of regression, decentration, and loss of best spectacle corrected visual acuity. (1) The expansion of the optical zone and the development of laser in situ keratomileusis (LASIK) have significantly improved the results of hyperopic keratorefractive surgery. (2-16) This chapter will review issues in technique, safety, and efficacy of LASIK for hyperopia.

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Section 6 Figure 12-1: LASIK in Hyperopia - Steepening Effect on Cornea Leading to Correction of Hyperopia Section 7 The central cornea has become steepened (A) following the mid-peripheral and peripheral ablation performed with the excimer laser (red). The darkened center within the red beam Subjects Index corresponds to the optical zone from which the ablation into the periphery begins. Dotted lines shows previous curvature. (Courtesy of Boyd’s “Atlas of Refractive Surgery” of HIGHLIGHTS OF OPHTHALMOLOGY.)

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Figure 12-2: LASIK for Hyperopia - Ablation of Peripheral and Mid-Peripheral Cornea The corneal flap with superior hinge has a diameter of 9.5 to 10.0 mm, certainly much larger than the flap performed with LASIK for myopia. This is now possible through more sophisticated microkeratomes. The optical zone in the center identified by the red arrow is 6.0 mm in diameter. It always must be 6 mm or larger. The mid-peripheral and peripheral zone identified as "A" is the area of stromal ablation where tissue is removed with the excimer laser. The ablation of the cornea into this area "A" begins where the limiting diameter of the six millimeter optical zone is located (red arrow). (Courtesy of Boyd’s “Atlas of Refractive Surgery” of HIGHLIGHTS OF OPHTHALMOLOGY.)

Figure 12-3: In the hyperopic eye, light rays are focused beyond the retinal plane resulting in a blurred retinal image. This may be due to a flatter than normal cornea or a shorter diameter eye (VISX Inc., Santa Clara, CA, USA)

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Hyperopic Correction using the Excimer Laser


In order to correct hyperopia, the excimer laser ablation must be concentrated in an annular ablation zone at the corneal plane (Figs. 12-2 & 12-3). Several excimer laser manufacturers have successfully applied their technology to accommodate the hyperopic ablation profile. VISX (Santa Clara, CA, USA) uses an offset scanning beam to concentrate the excimer laser energy in an annular transition zone 5 to 9 mm in diameter (Figs. 12-4, 12-5). LADARVision (Summit-Autonomous Technology, Orlando, FL) uses a flying spot beam (0.8 to 0.9mm) that scans across the surface of the cornea (Fig. 6). The cumulative ablation is achieved by partial overlap of multiple laser shots. During hyperopic correction, the density of laser ablation is concentrated peripherally. Currently, a 6.0 mm optical zone with a 1.5mm blend zone (9.0 mm total) is currently

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Figure 12-4: VISX uses an offset scanning beam to concentrate the excimer laser energy in an annular transition zone. This steepens the central cornea resulting in the correction of hyperopia. (VISX Inc., Santa Clara, CA, USA)

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Figure 12-5: The annular hyperopic ablation profile. (VISX Inc., Santa Clara, CA, USA)

Figure 12-6: LADARVision uses a flying spot beam to scan an annular ablation zone in order to correct hyperopia. (SummitAutonomous Technology, Orlando, FL, USA)

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Section 1

under clinical investigation. Hyperopic astigmatism may also be corrected by differential application of excimer laser along the annular ablation zone. Summit Technology (Waltham, MA) corrected hyperopia by creating an annular ablation profile by using an erodible disc and axicon quartz lens system.(18) The erodible disc delivery system has the theoretical advantage of transferring any tridimensional shape on to the corneal surface by photoablation. The axicon quartz lens is used to gradually smooth the peripheral blend zone.

Patient Selection and Preoperative Considerations Typically, LASIK has been successfully used to correct low to moderate levels of hyperopia (+1.0 to +5.0 diopters). As with myopic LASIK, patients should be older than 21 years and demonstrate stability in the refractive error for at least 12 months. Absolute contraindications include eyes with active pathology in corneal shape, thickness, or inflammation. Eyes with systemic vasculitis, autoimmune diseases, collagen vascular disorders, unstable diabetes or other states with abnormal healing are also suboptimal candidates.

As in the case of LASIK for myopia,the patient should Section 2 undergo a complete history and examination. This Section 3 includes a manifest refraction, cycloplegic refraction, corneal topography, slit lamp examination, Section 4 pachymetry, and dilated fundus examination. PotenSection 5 tial risk factors for a complicated procedure can be identified with a careful preoperative examination. Section 6 The identification of corneal neovascularization is especially important since a large primary keratec- Section 7 tomy is required to accommodate the large hyperSubjects Index opic annular photoablation. Cycloplegic refraction is also an integral part of the preoperative examination as it may unmask latent hyperopia in patients with vigorous accommodation ability. The mean central keratometry should be carefully evaluated. Eyes with low preoperative mean keratometry readings are more likely to have smaller diameter flaps Help ? which cannot accommodate a large diameter hyperopic ablation profile. Eyes with a postoperative mean central keratometry of >51 to 52 diopters may suffer from poor quality of vision, monocular diplopia and loss of best spectacle corrected visual acuity similar to patients with keratoconus. These eyes may also suffer from chronic apical dryness that may exacerbate the patient’s symptoms during the postoperative course. Thus, keratorefractive surgery should be avoided in eyes with a steep preoperative mean cen-


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tral keratometry and a large hyperopic correction. In these eyes, other alternatives such as a hyperopic phakic intraocular lens may be a more viable option. Patients with secondary hyperopia resulting from previous keratorefractive surgery (RK, PRK, LASIK) require special attention. Since overcorrections may regress significantly, the refractive stability should be evaluated prior to the retreatment procedure. In our experience, this requires a minimum of 6 months. In addition, the location and number of radial keratotomy incisions should be noted. LASIK on eyes with over 8 incisions may result in an unstable flap and wound dehiscence. It is also important to evaluate the RK incisions for epithelial plugging and/or wound gape. In these eyes, a thicker plate (i.e. 180 microns) will result in a thicker, more stable flap. The diameter and centration of the primary keratectomy of eyes that had undergone previous LASIK should be carefully noted. Significantly decentered flaps and small diameter flaps will require a new flap in order to accommodate the large diameter hyperopic ablation. Eyes with hyperopia from unintended overcorrection of myopia may require an adjustment in the hyperopic LASIK nomogram.(4) Since these eyes achieve more effective correction than eyes with primary hyperopia, we typically reduce our attempted correction by 20 to 30 %.

Technique The fundamentals of performing a successful hyperopic LASIK are similar to performing a successful myopic LASIK with a few exceptions. Thus, a surgeon skilled in lamellar surgery should make the transition from myopic LASIK to hyperopic LASIK with little difficulty. After the instillation of a topical anesthetic and topical vasoconstictor, we isolate the eyelashes with a sterile drape and speculum. The corneal marking is performed and the pneumatic suction ring is positioned. A large suction ring >9.5 mm should be used in order to accommodate the large ablation profile of a hyperopic correction. Careful positioning of the pneumatic suction ring is important in hyperopic LASIK. While a slightly decentered primary keratectomy will accommodate even the largest myopic ablation profiles, it

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may not accommodate a 9.5 mm hyperopic ablation profile. Slight decentration of the pneumatic ring towards the hinge is useful in placing the hinge out of the field of the hyperopic ablation. Positioning the pneumatic ring to place the hinge in areas of corneal vascularization will also limit the degree of hemorrhaging that may occur when making a large diameter flap. A moist methylcellulose sponge may be expanded and used to protect the hinge during the photoablation. Centration of the photoablation is mandatory since the relatively small optical zone created by hyperopic LASIK is less forgiving than the larger optical zone of a comparative level of myopic treatment. After the photoablation, the stromal bed is irrigated, the flap is refloated and the epithelial markings are re-aligned. A topical antibiotic, nonsteroidal anti-inflammatory, and steroid is administered. The postoperative medication regimen is identical to that following LASIK for myopia. We administer fluometholone 0.3% and ciprofloxacin 0.3% four times a day for four days.

Clinical Results

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A general review of the current literature Section 5 demonstrates effective reduction of low to moderate levels of spherical hyperopia, simple hyperopic Section 6 astigmatism and compound hyperopic astigmatism.(4-16) In these studies, approximately 70 to 90% Section 7 of the hyperopia is corrected depending on the level Subjects Index of preoperative hyperopia and duration of follow-up. Predictability is also an important measure of refractive accuracy and is typically recorded as the percentage of eyes within +/- 1.0 diopter of attempted correction. In these studies, 60 to 100% of eyes were within +/- 1.0 diopter of attempted correction for low to moderate hyperopia. For higher levels of correcHelp ? tion, the predictability within +/- 1.0 diopter of attempted correction decreases to approximately 50 to 80%. (5,7,16) Significant regression can occur following LASIK for hyperopia. (4) In our experience, eyes may be initially slightly overcorrected in the early postoperative period. Stability often requires 3 to 6 months before complete stabilization of the refractive error. After 6 months, eyes may be safely retreated.


Uncorrected visual acuity of 20/40 or better was demonstrated in approximately 70 to 95% of eyes depending on the level of preoperative correction. (4-16) In these same studies, loss of best spectacle corrected visual acuity generally ranged from 0 to 7%. However, LASIK for hyperopia greater than +5.0 diopters is not recommended as it may result in a loss of best spectacle corrected visual acuity in a significant number of eyes (13 to 15%). (8) The type and frequency of complications following hyperopic LASIK are similar to myopic LASIK with few exceptions. As noted previously, intraoperative hemorrhages may occur following the microkeratome pass for large diameter flaps. Common avoidable causes of loss of best spectacle corrected visual acuity include decentration and steepening of the central cornea >51 to 52 diopters. The comparatively small optical zone following hyperopic LASIK mandates meticulous attention to centration during the photoablation. There are anecdotal reports of an increased rate of epithelial ingrowth following hyperopic LASIK. This may be related to spill-over ablation outside margin of the primary keratectomy.

Secondary Hyperopia Secondary hyperopia may be safely treated using hyperopic LASIK technology. (4,10) In these limited studies hyperopic LASIK for eyes previously treated with myopic LASIK or RK demonstrated effective reduction of hyperopia. 83% to 93% of these eyes demonstrated an uncorrected visual acuity of 20/40 or better and no eyes lost best spectacle corrected visual acuity. At Stanford University, we prospectively evaluated 19 eyes with secondary hyperopia resulting from PRK, LASIK, or RK. These eyes underwent hyperopic LASIK with theVISX S2 Smoothscan excimer laser for a mean spherical equivalent of +1.64+/-0.80 diopters (range, +1.5 to +2.75 D). In these eyes with secondary hyperopia, we reduced our nomogram by 20 to 30%. The procedure was performed as described in the technique section of this chapter. The Hansatome microkeratome (Bausch & Lomb, Rochester, NY) was used with the 9.5mm pneumatic suction ring in cases that required a new flap.

On the first postoperative day, mean spherical equivalent was –0.16 +/-0.63 D and 82% demonstrated an uncorrected visual acuity of 20/40 or better. At 6 months, mean spherical equivalent was +0.58+/-0.59 D, 78% were within +/-1.0 D of attempted correction, and 78 % of eyes demonstrated an uncorrected visual acuity of 20/40 or better. No eyeslost two or more lines of best spectacle corrected visual acuity and there were no significant decentrations. A hyperopic shift of +0.76 D occurred during the first 6 postoperative months. Stability within +/-0.50 diopters occurred between 3 and 6 months postoperatively.

Hyperopia with Astigmatism Hyperopic astigmatism can be corrected by Contents additional steepening along the flat meridian. Toric correction of hyperopia may result in significantly Section 1 less predictable results and higher loss of BSCVA than comparative levels of spherical treatments. (7-8) As Section 2 in myopic corrections, the lower predictability of toric Section 3 ablations is likely related to axis misalignment. (17) In an ongoing prospective study at Stanford Section 4 University, 119 eyes of 76 patients with compound Section 5 hyperopic astigmatism underwent LASIK with the VISX S2 Smoothscan excimer laser (VISX Inc., Section 6 Santa Clara, CA). Inclusion criteria included eyes with +1.0 to +6.0 diopters of spherical hyperopia and Section 7 +1.0 to +4.0 diopters of hyperopic astigmatism. Mean Subjects Index preoperative sphere was +1.91 +/- 1.50 D (range +1.0 to +6.0), mean preoperative cylinder was +1.58 +/-0.88 D (range, +1.0 to +4.0), and mean spherical equivalent was +2.74+/-1.51 (range, +1.5 to +7.0 D). Patients were prospectively evaluated at 1 day, 1 month, 3 months, 6 months, and 12 months. On the first postoperative day, the mean Help ? spherical equivalent was –0.39 +/- 0.61 diopters (range, -2.75 to +1.0 D). 91% of eyes were within +/ -1.0 diopters of attempted correction. 91% of eyes demonstrated an uncorrected visual acuity of 20/40 or better and 0% of eyes lost 2 or more lines of best spectacle corrected visual acuity. 1.7% of eyes experienced a displaced flap on the first day following surgery. These flaps were repositioned without visual deficits. 0.8% of eyes experienced diffuse lamellar keratitis that resolved uneventfully with topical steroids. LASIK AND BEYOND LASIK

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At 1 year, mean sphere was –0.12+/-0.67 diopters (range, -1.25 to +1.5 D), mean cylinder was +0.44+/-0.43 diopters (range, 0 to +2.0 D), and mean spherical equivalent was +0.13 +/-0.69 diopters (range, -1.0 to +1.75 diopters). 88% of eyes were within +/-1.0 diopter of attempted correction and 97% of eyes demonstrated an uncorrected visual acuity of 20/40 or better. Vector analysis demonstrated a mean magnitude of error of –0.20 D +/-0.67 D. The mean angle of error was 0.37 degrees +/- 18.9 degrees. 92% of eyes had a difference vector within +/-1.0 diopters. There were no intraoperative flap complications, no significant decentrations, and no eyes lost 2 or more lines of best spectacle corrected visual acuity. There was a mean regression in the spherical equivalent of +0.4 diopters between first postoperative day and 6 months. Stability within +/-0.25 diopters occurred between 3 and 6 postoperative months. The average regression between 6 months and 1 year was +0.13 diopters. (Note from the Editor in Chief: For the surgical management of hyperopia, Mahmoud M. Ismail, M.D., Ph.D, a distinguished ophthalmic surgeon from Egypt has reported highly positive results with the use of intracorneal lenses for the correction of hyperopia in albino rabbits. Dr. Ismail used a new hydrogel intracorneal contact lens (PermaVision, Anamed, Inc.), a product developed to address the limitations reported with the current relatively thick hydrogel lens implants. It is comprised of water content more than 70% and a refractive index that is substantially close to the refractive index of the corneal tissue (1.376). It is designed to correct hyperopia up to +6 diopters. All animals were followed up for at least 6 months by confocal microscopy. The PermaVision lens intracorneal implant shows excellent compatibility according to the author. The new generations of soft intracorneal lenses may present a new alternative for the correction of hyperopia in the future. The procedure seems to be reproducible and implant removal is possible. This thin PermaVision lens which allows the passage of nutrients and fluid through the implant to nourish the corneal layers may offer a new scope for the application of intracorneal implants in hyperopia. We will need to see the results in humans in later years.)

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REFERENCES 1.

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3.

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8.

9. 10.

11.

12.

13.

14.

15.

16.

Dausch D, Klein R, Schroder E. Excimer laser photorefractive krratectomy for hyperopia. Refract Corneal Surg 1993; 9:20-8. Dausch D, Smecka Z, Klein R, Schroder, Kirchner S. Excimer laser photorefractive keratectomy for hyperopia. J Cataract Refract Surg 1997; 13:504-10. Jackson WB, Mintsioulis G, Agapitos PJ, Casson EJ. Excimer laser photorefractive keratectomy for low hyperopia: safety and efficacy. J Cataract Refract Surg 1997; 23:480-7. Lindstrom RL, Hardten DR, Houtman DM, et. al. Sixmonth results of hyperopic and astigmatic LASIK in eyes with primary and secondary hyperopia. Trans Am Ophthalmol Soc 1999; 97:241-55. Zadok D, Maskaleris G, Montes M. et. al. Hyperopic laser in situ keratomileusis with the Nidek EC-5000 excimer laser. Ophthalmology 2000; 107:1132-7. Contents Esquenazi S, Mendoza A. Two-year follow-up of laSection 1 ser in situ keratomileusis for hyperopia. J Refract Surg 1999; 15:648-52. Section 2 Barraquer C, Gutierrez AM. Results of laser in situ keratomileusis in hyperopic compound astigmatism. Section 3 J Cataract Refract Surg 1999; 25:198-204. Arbelaez MC, Knorz MC. Laser in situ keratomileusis Section 4 for hyperopia and hyperopic astigmatism. J Refract Section 5 Surg 1999; 15:406-14. Rosa DS, Febbraro JL. Laser in situ keratomileusis Section 6 for hyperopia. J Refract Surg 1999; 15:S212-5. Buzard KA, Fundingstand BR. Excimer laser assisted Section 7 in situ keratomileusis for hyperopia. J Cataract Refract Surg 1999; 25:197-204. Subjects Index Goker S, Er H, Kahvecioglu C. Laser in situ keratomileusis to correct hyperopia from+4.25 diopters. J Refract Surg 1998; 14:26-30. Argento CJ, Consentino MJ. Laser in situ keratomileusis for hyperopia. J Cataract Refract Surg 1998; 24:1050-8. Ibrahim O. Laser in situ keratomileusis for hyperopia and hyperopic astigmatism. J Refract Surg 1998; 14 Help ? (2 Suppl): S179-82. Chayet AS, Magallanes R, Montes M, et. al. Laser in situ keratomileusis for myopic, mixed and simple hyperopic astigmatism. J Refract Surg 1998; 14(2Suppl)S175-6. Ditzen K, Huschka H, Pieger S. Laser in situ keratomileusis for hyperopia. J Cataract Refract Surg 1998; 24:42-7. Argento CJ, Consentino MJ, Biondini A. Treatment of hyperopic astigmatism. J Cataract Refract Surg; 1997; 23:1480-90.

LASIK (Laser In-Situ Keratomileusis) FOR HYPEROPIA 17. Snibson GR, Carson CA, Aldred GF, Taylor HR. Oneyear evaluation of excimer alser photorefractive keratectomy for myopia and myopic astigmatism. Melbourne Excimer Laser Group. Arch Ophthalmol 1995; 113:994-1000. 18. Haw W., Manche E. Prospective study of photorefractive keratectomy for hyperopia using an axicon lens and erodible mask. Journal of Refractive Surgery 2000; 16:724-730.

Weldon Haw, M.D. Cornea & Refractive Surgery Department of Ophthalmology Stanford University School of Medicine 300 Pasteur Drive, Suite A157 Stanford, CA 94305 Phone:(650)-723-5517; Fax: (650)-723-7918 E-Mail: [email protected]

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IRREGULAR ASTIGMATISM: LASIK AS A CORRECTING TOOL

Chapter 13 IRREGULAR ASTIGMATISM: LASIK AS A CORRECTING TOOL Prof. Jorge L Alió, MD, PhD, José I. Belda Sanchis, MD, PhD., Dr. Ahmad MM Shalaby, MD

Introduction Irregular astigmatism represents one of the problems that are very difficult to manage and frustrating in results to refractive surgeons. It is also one of the worst sequelae of corneal injuries. It can also complicate certain corneal diseases as keratoconus. With the recent evolution of refractive surgery techniques and diagnostic tools, new types of irregular astigmatism are being observed 1,2. The astigmatism is defined as irregular if the principal meridians are not 90 degrees apart, usually because of an irregularity of the corneal curvature. It cannot be completely corrected with a sphero-cylindrical lens 3. Duke –Elder defines irregular astigmatism as a refractive state in which the refraction in different meridians conforms to no geometric plan and the refracted rays have no planes of symmetry 4. The alternatives for correction of irregular astigmatism are very scarce and with very limited expectations. Spectacle correction is usually not useful in the correction of corneal irregular astigmatism as it is difficult to define principle meridians. Hard contact lenses represent a good alternative in which the tear fluid layer under the contact lens evens out the irregularity. We should consider that adaptation and stability of contact lenses is limited by irregularity corneal surface and the patient’s comfort. We also must remember that our patients consented to undergo refractive surgery because they did not want to use more the contact lens. Lamelar and full thickness corneal grafting are surgical alternatives. The limited availability of corneal donor as well as the biological and refractive

complications of allografic corneal graft limit the clinical applicability of these procedures. Many surgeons have made great efforts in finding a solution to this problem.5-7 To this date, we believe there should be safe, efficient and predictable methods to resolve this problem. Accordingly, the approach to new surgical methods for the correction of irregular astigmatism is one of the greatest expectations in today’s refractive surgery, especially when the very near future is supposed to bring generalization of corneal refractive surgical techniques.

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Etiology of Irregular Astigmatism a) Primary Idiopathic

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There is a general prevalence of low levels of Subjects Index irregular astigmatism of unknown cause within the population. This might explain the mildly reduced best corrected visual acuity (BCVA) in patients presenting for laser vision correction 1.

b) Secondary 1) Dystrophic In the cornea, keratoconus, which, in optical terms, is primarily an irregularity of the anterior corneal surface, is the best example. Pelucid degeneration and keratoglobus may also be associated with posterior corneal surface irregularity causing irregular astigmatism. In the lens, lenticonus may cause irregular astigmatism; and in the retina, posterior staphyloma 1.


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2) Traumatic Corneal irregularity is caused commonly by corneal wounds (incision or excision) or burns (chemical, thermal or electrical) 1. 3) Postinfective Postherpetic keratitits is the most common form of postkeratitic healing and scarring that may lead to an irregular surface 1. 4) Postsurgical Irregular corneal astigmatism can complicate any if the following refractive surgical procedures: keratoplasty, photorefractive keratectomy (PRK), laser in situ keratomileusis (LASIK), radial keratotomy (RK), arcuate keratotomy (AK), and cataract incisions. Scleral encirclement or external plombage may also contribute 1.

Diagnosis of Irregular Astigmatism Clinically, irregular astigmatism will present with one of those typical retinoscopy patterns, the most common being spinning and scissoring of the red reflex. On attempting keratometry the mires will appear distorted. Corneal topography shows certain patterns for irregular astigmatism that will be discussed in detail later. The most recent and sophisticated technique is the application of wavefront analysis (aberrometers) 8. This emerging method measures the refractive status of the whole internal ocular light path at selected corneal intercepts of incident light pencils. By comparing the wavefront of a pattern of several small beams of coherent light projected through to the retina with the emerging reflected light wavefront, it is possible to measure the refractive path taken by each beam and to infer the specific spatial correction required on each path.

Clinical Classification of Irregular Astigmatism Following Corneal Refractive Surgery In corneal refractive surgery using laser in situ keratomileusis (LASIK) the surgeon uses a microkeratome, whether automated or manual, to fashion a corneal flap and a stromal bed. Once the flap is fashioned and lifted, the excimer laser is used 170

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to ablate tissue from the bed for the planned correction, depending on the capabilities of the laser. In this clinical prespective, irregular astigmatism induced by LASIK can be classified according to its location as: 1. Superficial: due to flap irregularities. 2. Stromal: induced by bed irregularities. 3. Mixed: due to irregularities in both flap and stroma.

Corneal Topography Patterns of Irregular Astigmatism Topographic classification of irregular astigmatism patterns is very important in the following aspects: 1. To unify terms and concepts when we referring corneal topography images. 2. To determine the cause of the subjective symptoms referred by the patient (Halos, glare, monocular diplopia, etc.). 3. Reaching a topographic basis for retreatment. The topographic approach for treatment patients with a previous unsuccessful excimer laser surgery should allow reshaping the cornea in the pattern appropriate for the specific patient.

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Based on the topography, we proposed the following classification for irregular astigmatism 7: Subjects Index • Irregular astigmatism with defined pattern, and • Irregular astigmatism with undefined pattern

1. Irregular astigmatism with defined pattern We define irregular astigmatism with defined pattern when there is a steep or flat area of at least 2 mm of diameter, at any location of the corneal topography, which is the main cause of the irregular astigmatism. It is divided into five groups: A. Decentered Ablation: Shows a corneal topographic pattern with decentered myopic ablation in more than 1.5 mm in relation to the center of the cornea. The flattening area is not centered in the center of the cornea; the

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optical zone of the cornea has one flat and one steep area (Figure 13-1a). Decentered Steep: Shows a corneal hyperopic treatment decentered in more than 1.5 mm in relation to the center of the cornea (Figure 13-1b). Central island: Shows an image with an increase in the central power of the ablation zone for myopic treatment ablation at least 3.00D and 1.5mm in diameter, surrounded by areas of lesser curvature (Figure 13-1c). Central irregularity: Shows an irregular pattern with more than one area not larger than 1.0 mm and no more than 1.50D in relationship with the flattest radius, located into the area of the myopic ablation treatment (Figure 13-1d). Peripheral Irregularity: It is a corneal topographic pattern, similar to Central Island, extending to the periphery. The myopic ablation is not homogeneous, there is a central zone measuring 1.5 mm in diameter and 3.00 D in relation to the flattest radius, connected with the periphery of the ablation zone in one meridian (Figure 13-1e).

2. Irregular astigmatism with undefined pattern We consider irregular astigmatism with undefined pattern when the image shows a surface with multiples irregularities; big and small steep and flat areas, defined as more than one area measuring more than 3 mm in diameter in the central 6 mm (Figure 13-1f). The differential between flat and steep areas were not possible to calculate in the Profile Map and Dk showed an irregular line or a plane line. Normally, Dk is the difference between the steep k and the flat k, given in diopters at the cross of the profile map. A plane line is produced when the ∆k cannot recognize the difference between the steep k and the flat k in severe corneal surface irregularities.

Evaluation of Irregular Astigmatism In managing irregular astigmatism patients, a meticulous preoperative evaluation is necessary. We perform a complete preoperative ocular examination, including previous medical reports and complete ocular examination: uncorrected and best corrected visual acuity, pinhole visual acuity and cycloplegic refraction, keratometry, contact ultrasonic pachymetry (Ophthasonic Pachymeter Teknar Inc. St. Louis, USA) and computerized corneal topography. We perform the corneal topography with Eye Sys 2000 Corneal Analysis System (Eye Sys Co., Houston, Texas, USA). We also use the Ray Tracing mode of the C-SCAN Color-EllipsoidTopometer (Technomed GmbH, Germany) to deterContents mine the Superficial Corneal Surface Quality (SCSQ) and the Predicted Corneal Visual Acuity (PCVA), in Section 1 addition to the topography. Recently, we have incorporated the elevation topography of the Orbscan Section 2 System (Orbtek, Bausch & Lomb Surgical, Orbscan Section 3 II corneal topography, Salt Lake City, Utah, USA) in Section 4 our evaluation tools. Follow up examinations after surgery were Section 5 performed at 48 hours, and then at one, three and six months. Post-operative follow up included: uncor- Section 6 rected and best-corrected visual acuity, pinhole visual acuity and cycloplegic refraction, biomicros- Section 7 copy with slip-lamp and complete corneal topogra- Subjects Index phy screening with the previously mentioned instrumentation. During the pre-operative and post-operative period the surface quality of the cornea was studied using the Ray Tracing module of the C-SCAN 3.0 (Technomed GmbH, Germany). This device determines the Predicted Corneal Visual Acuity from the Help ? videokeratography map, by simulating the propagation of rays emanating from 2 light dots, which impinge on the best-fit image plane after projection via the maximum of 10,800 previously determined corneal surface power values. Refraction and reflec-


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Topographic Patterns of Irregular Astigmatism With Ray Tracing Study (Figs. 13-1 A-F) Figure 13-1 A (left): Decentered ablation (myopic treatment more than 1.5 mm in relation to the center of the cornea. Note that although the peak distortion is minimal in the rat tracing study, the corneal surface quality outside the 3 mm reference pupil is markedly reduced, meaning that the patient will suffer glare and night vision troubles when this pupil dilates under scotopic conditions)

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Figure 13-1 B (right): Decentered steep (hyperopic treatment decentered in more than 1.5 mm in relation to the center of the cornea. Note the reduction in PCVA in spite of a uniform peak, and the irregular base diameter which correlates with the spherical aberrations this patient would suffer).

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IRREGULAR ASTIGMATISM: LASIK AS A CORRECTING TOOL Figure 13-1 C (left): Central Island (increase in the central power of the ablation zone for myopic treatment ablation at least 3.00D and 1.5 mm in diameter, surrounded by areas of lesser curvature. Note again the reduction in the peripheral SCSQ, i.e. night vision problems).

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Figure 13-1 D (right): Central irregularity (irregular pattern with more than one area not larger than 1.0 mm and no more than 1.50D in relationship with the flattest radius, located into the area of the myopic ablation treatment. Note the distorted base diameter and the marked peak separation, and the irregular reduced SCSQ).


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Figure 13-1 F: Irregular astigmatism with undefined pattern (surface with multiple irregularities with big and small steep and flat areas. Note the extensive scatter at the base diameter which is extremely irregular, even worse than the previous examples).

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tion of the rays at the optical interfaces, the pupil diameter, and the anterior chamber depth are taken into account according to laws of geometric optics. The Ray Tracing module calculates the pupil size by the captured image of the pupil during videokeratography. This is measured under the luminance of the videokeratography rings (25.5 cd/m2) and is automatically integrated into the Ray Tracing Analysis with the videokeratography map. Hence, the projection of objects onto a detection plane can be determined. The Ray Tracing module calculates the optical function of the eye by means of optical Ray Tracing, using the cornea as the refractive element of the system. It measured and analyzed the interaction between the corneal shape, the functional optical zone, and the pupil diameter, providing valuable additional information by the resulting diagram. The image points on the detection plane are represented by two intensity peaks that must be spatially resolved to discriminate them separately and individually. The peak distance (distance between the functional maxima) and the distortion index (basic diameter of the point cloud in the detection plane) are parameters defined to help understanding when these two peaks are spatially resolved. They help to objectively quantify the individual retinal image in each subject. We found it very useful to evaluate the corneal surface and corneal healing. It is very useful also to explain visual phenomena referred by the patients, and that cannot be explained by older versions of corneal topographers. We don’t consider it a substitution of the Eye Sys 2000 Corneal Analysis System (Houston, Texas, USA), but it showed to be a very useful tool 9. Subjective symptoms from the pre and postoperative periods should be noted in the medical report such as halos, glare, dazzling, corneal and conjunctival dryness, dark-light adaptation and visual satisfaction reported by the patient.

laser is gaining priority with the advent of finely controlled corneal ablation. Before that, limited alteration of corneal topography was possible by, for instance, selective incision placement, placement and removal of sutures, or penetrating and lamellar keratoplasty. Other “treatment” options for irregular corneal astigmatism include optical correction with hard contact lenses in which the tear fluid layer under the contact lens “evens out”’ the irregularity 1, but the patient’s aim to get rid of glasses as well as contact lenses still limits their use. Intracorneal ring segments, originally used for myopia treatment 10, represent another option that is under investigation.

Treatment of Irregular Astigmatism

2. Excimer Laser Assisted by Sodium Hyaluronate (ELASHY). Designed mainly to improve

Treatment options for irregular astigmatism have expanded greatly during recent years. Excimer

Surgical Techniques with Excimer Laser Contents

These represent the main subject of discussion in this chapter. The ultimate goal excimer laser Section 1 treatment is to correct the refractive error while Section 2 reducing corneal astigmatism and topographic disparity but not increasing aberrations within the eye. Section 3 With the advent of the excimer laser, it may be possible to correct directly some forms of corneal Section 4 irregularity. Before considering any treatment op- Section 5 tion, the relationship between the topographical irregularity and the refraction must be considered; a Section 6 therapeutic balance between refractive and corneal Section 7 astigmatism must be reached so that overall visual function is optimal. In other words, an optimal treat- Subjects Index ment should include both topographic and refractive values, rather than excluding one 1. We have used different methods for the surgical correction of irregular astigmatism. At this moment we consider three surgical procedures with excimer laser for correction of the irregular astigmatism: Help ? 1. Selective Zonal Ablation (SELZA). Designed to improve the irregular astigmatism with defined pattern.7

the irregular astigmatism with undefined pattern.11


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3. Topographic Linked excimer laser ablation 1. Selective Zonal Ablation (SELZA) (TOPOLINK). Combines data of the topography and patient refraction in as software to improve the irregular astigmatism with defined pattern and the refractive error, with the same procedure.12 The three surgical procedures were performed under topical anaesthesia of Oxibuprocaine 0.2% (Prescaina 0,2%; Laboratorios Llorens, Barcelona, Spain) drops; no patient required sedation. The postoperative treatment consisted of instillation of topical tobramycin 0.3% and dexamethasone 0,1% drops (Tobradex, Alcon-Cusi, Barcelona, Spain) three times daily for the five days of the follow-up and then discontinued. When the ablation was performed onto the cornea (surface ELASHY, some patients of SELZA), a bandage contact lens (Actifresh 400, power +0.5, diameter 14.3mm, radius of curvature 8.8mm – Hydron Ltd., Hampshire, U.K.) was used during the first three days of the post-operative and the patient was examined daily. It was removed when complete reepithelialization was observed. Then treatment with topical fluorometholone (FML forte, AlconCusi, Barcelona, Spain) was used three times daily for the three months of follow-up and then stopped.13 Non-preserved artificial tears (Sodium Hyaluronate 0.18%, Vislube‚, CHEMEDICA, Ophthalmic line, München, Germany) were used up to three months in every case. Supplementation with oral pain management medications was also used as necessary. Statistical Analysis. Statistical Analysis was performed with the SPSS/Pc+4.0 for Windows (SPSS Inc, Madrid, 1996). Measurements typically are reported as the mean ± 1 standard deviation (using [ n - 1]1/2 in the denominator of the definition for standard deviation, where n is the number of observations for each measurement) and as the range of all measurements at each follow up visit. Patients’ data samples were fitting the normal distribution curves. Statistically significant differences between data sample means were determined by the “Student’s t test” ; P values less than 0.05 were considered significant. Data concerning the standards for reporting the outcome of refractive surgery procedures, as the safety, efficacy and predictability, was analyzed as previously defined.14

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In this study we report the results of a prospective clinically controlled study performed on 31 eyes of 26 patients with irregular astigmatism induced by refractive surgery. All cases were treated with SELZA using an excimer laser of broad circular beam (Visx Twenty/Tweenty, 4.02, Visx, Inc. Sunnyvale, California, USA). The surgical planning was applied using the Munnerlyn formula 15, modified by Buzard 16, to calculate the depth of the ablation depending on the amount of correction desired and the ablation zone. In this formula the resection depth is equal to the dioptric correction, divided by 3, and multiplied by the ablation zone (mm) squared. We used a correction factor of 1.5 times, to avoid undercorrection:

Ablation

(Dioptric correction) x 1.5 depth = x (ablation zone)2 3

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Methods

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In general, we use ablation zone of 2.5 to 3.0 mm, depending on the steep area of the corneal Section 6 topography to be modified. The ablation zone was Section 7 determined by observing the color map. The form of videokeratoscope provides additional information Subjects Index about the irregular zones, and the profile map gave the values for performed ablation. In cases of irregular corneal surface, treatment was performed on the center of irregularity, which was located using the color map of the corneal topography. First we located the center of the cornea, then we located the exact center of irregularity. Here we use the dotted boxes in Help ? the map (each dot represents 1 mm2) to detect the exact center of irregularity in relation to the center of the cornea. The amount of ablation is determined using the cross section of the profile map (vertical line corresponding to diopters and horizontal line corresponding to corneal diameter). When the patient had LASIK previously we lift the flap or we do a new LASIK cut and after we perform excimer laser using PTK mode.


The technique is based on subtraction of tissues to eliminate the induced irregular astigmatism and to achieve a uniform corneal surface using excimer laser; we center the effect of laser on zones where the corneal surface is steeper.

Results In patients with Irregular Astigmatism with a Defined Pattern, the visual acuity improved significantly, reaching in many cases near the BCVA before the initial refractive procedure. The difference between the BCVA before the therapeutic procedure was highly statistically significant (P < 0.001). The mean BCVA after 3 months of surgery it was 20/25 ± 20/100 (range 20/50 - 20/20), which was as good as the initial BCVA 20/29 ± 20/100 (range 20/50 - 20/20). The BCVA before selective ablation improved from 20/40 ± 20/100 (range 20/100 - 20/25) to 20/25 ± 20/100 (range 20/50 - 20/20). We did not have any patients with one or more lines lost of BCVA. The Corneal Uniformity Index (CUI) before versus after selective zonal ablation with excimer laser improved from 55.65 ± 15.90 % (range 20 - 80 %) to 87.83 ± 10.43 % (range 70 - 100), a change that was also statistically significant (P < 0.005). The safety index (the ratio of mean postoperative BCVA over mean preoperative BCVA) was equal to 1.55. The efficacy of the procedure in percent UCVA 20/40 was 85%. The predictability (astigmatic correction) using CUI was expressed as a percentage. The various relationships between the preoperative CUI and the surgically induced postoperative CUI provided the information about the magnitude of irregular astigmatism correction and the corneal surface uniformity. Correction index, which is the ratio of mean postoperative CUI (87.83 ± 10.43 %; range, 70-100%) over the mean preoperative CUI (55.65 ± 15.90 %; range, 20-80 %), was equal to 1.58. The results observed in all cases of irregular astigmatism without a defined pattern were poor. Efficaccy in percentage of eyes with UCVA of 20/40 was 6%, and predictability (astigmatic correction) was 0.58. In some cases, visual acuity became worse: the refraction error and corneal topography were considerably modified.

Discussion The results of the selective zonal ablations technique were satisfactory as regards the correction of irregular astigmatism with a defined topographic pattern. Visual Acuity improved in the postoperative period, achieving values near the initial BCVA of the patients (before the initial surgical procedure). The corneal uniformity index was used to evaluate the central 3 mm zone of the cornea. It started to improve in the early postoperative period and stabilized after 3 months, just as the issues of visual acuity (p -1 D. 6,7.Perez et al observed similar results .8 Other possible mechanisms are proliferation of keratocytes in corneal stroma, epithelial hyperplasia and central corneal thickness associated with central corneal steepening. 2,9,12 As LASIK surgery was performed intrastromally, it allows the surgeon to do a second ablation to correct any undercorrection or any defect, which have occurred during the first surgery. One can do RK or PRK as an enhancement procedure after LASIK, but it has several disadvantages like the possibility of overcorrection, irregular astigmatism, tissue melting and limitation in correcting high myopia.2 Postoperative refraction in high myopia is supposed to be settled after six months.2 In 10 eyes where we did RELASIK within 3 months of the primary procedure, we could reopen the corneal flap by blunt dissection using a spatula. In 40 eyes we did the secondary procedure after six months. At this time as the healing was almost complete, we could not reopen the flap by blunt dissection. So we used the second cut for RELASIK using the same algorithms but used the residual myopia as a subjective refraction . We limited our ablation zone between 4-6 mm to prevent night glare problems due to smaller zone and unnecessary vertex ablation and overcorrection due to larger zone ablations. 4 As a safety we excluded the patients having pachymetry less than 410 µm (250µm-stromal bed and 160 µm of corneal flap) from the study. 2,3 The Mean spherical equivalent improved from -4.3 ± 1.83D (SD) to –0.45± 0.68(SD) at the last follow-up. (p< 0.005). The UCVA improved from 0.22 ± 0.15 (SD) to 0.53 ± 0.22 (SD) at the last follow-up. (p - 1.25 D for at least 1 year, if they did not want to wear spectacles, and if did not tolerate contact lenses. A full informed consent was obtained from all the patients before the procedure. Exclusion criteria included any active corneal pathology or inflammation, corneal scars, monocular patients, keratoconus and thin corneas, raised Intraocular pressure (more than 21 mm Hg), the presLASIK AND BEYOND LASIK

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ence of active collagen vascular disease, pachymetry less than 420 mm, narrow palpebral fissure, or Schirmer ‘s test less than 5mm. Routine preoperative tests included UCVA and BCVA according to Snellen’s visual acuity chart, cycloplegic refraction, and slit-lamp biomicroscopic anterior segment examination that assessed the palpebral aperture. Measures were also taken of the corneal diameter, pupil diameter, corneal sensitivity, Schirmer’s test, and intraocular pressure. Corneal topography (Tomey TMS 2.1) and pachymetry were also done. A complete retinal evaluation ruled out any retinal tears or pathology. Patients were instructed to remove soft contact lenses at least 2 weeks before and hard and semi-soft contact lenses at least 4 weeks before evaluation. LASIK was performed using the Chiron Technolas Keracor 217 Excimer laser (Bausch & Lomb) with the Automated Corneal Shaper microkeratome (Bausch & Lomb). Fluency was 130 j/cm2, with a 10 Hz repetition rate, diameter of 7.8mm to 8.2mm, and multi-zone algorithm with optical zone from 4mm-6mm. All the surgeries were performed under topical anesthesia using 4 % lidocaine. Fluence was checked before every procedure by verifying the homogeneity and symmetry of the pulses according to optimal values of 65 ± 1 shots. Before the procedure was begun, all the instruments were checked. The ACS microkeratome was test-run on the base plate before each individual procedure. The autotracking mechanism was used .A speculum was inserted to keep the palpebral fissure wide open and eyelashes out of field. The entrance pupil was marked with a Gentian violet tip marker. A reference mark was made on the cornea. The suction ring was centered on the corneal marking and activated. The intraocular pressure was confirmed with the presurgical tonometer to be more than 65 mm Hg. The microkeratome was adjusted on the suction ring and moved forward with the forward footswitch till it stopped at the permanent stopper to prevent a free cap. The microkeratome was moved back with a reverse footswitch and removed.The lamellar corneal flap was fashioned with a nasal hinge and carefully lifted with a blunt instrument and reflected on its hinge. The surgeon carefully performed the laser ablation to ensure accurate centration. Once the ablation was completed, tissue and both the sides of 202

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the flap were cleaned with balanced salt solution and a Merocel sponge. After determining that no foreign particles remained in the interface, the surgeon repositioned the flap in the original position. The corneal surface markers were checked to ensure that they were in opposition. Care was taken to ensure there were no striae in the flap. The suction ring was then gently removed. After a few minutes the adhesion of the flap was checked. Then the speculum was removed carefully. Patients were examined with the slit lamp 1 hour after surgery and sent home. The following medications were prescribed: topical antibiotic ciprofloxacin 0.1%, topical steroid Dexamethasone 0.1% with a tapered dose, and artificial tears for 1 month. Patients were examined on the first postoperative day, after 1 week, 1 month, 6 months, 1 year after surgery, and afterwards if necessary. At each follow-up, the surgeon monitored UCVA and BCVA, cycloplegic refraction, anterior segment evaluation, intraocular pressure, corneal topography, and a detailed fundus examination.

Results of Lasik After RK and PRK

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Section 4

Section 5

Demographic data are provided in Table 1. All the patients were followed. The mean postop- Section 6 erative follow-up period was 14.23 ± 2.23 (SD) months in the RK group, and 16.43 ± 1.54 (SD) Section 7 months in the PRK group. LASIK was performed Subjects Index after a mean period of 24.3 ± 0.75 (SD) months in the RK group and 22 ± 1.07 (SD) months in the PRK group after the primary procedure.

RK Group At the last follow-up, the mean spherical equivalent was –1.19 ± 0.71 (SD), compared to 6.05 ± 1.98 (SD) (P

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