Corneal Endothelial Transplant: (DSAEK, DMEK & DLEK)

Corneal Endothelial Transplant (DSAEK, DMEK & DLEK)

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DVD Contents.................................. DSAEK Surgical Techniques 1. Animation ........................................................................................ M Hodgkin MD 2. Surgery ............................................................................................. M Gorovoy MD 3. Surgery ............................................................................................. M Gorovoy MD 4. Surgery ............................................................................................. M Busin MD 5. Surgery ............................................................................................. F Price MD 6. Surgery ............................................................................................. T John MD 7. Donor Cornea ................................................................................. T John MD 8. Instruments ..................................................................................... T John MD 9. Instruments ..................................................................................... T John MD 10. Instruments ..................................................................................... T John MD 11. Instruments ..................................................................................... T John MD 12. Instruments ..................................................................................... T John MD 13. Instruments ..................................................................................... T John MD 14. Surgery ............................................................................................. M Terry MD

Corneal Endothelial Transplant (DSAEK, DMEK & DLEK) Editor Thomas John

MD

Clinical Associate Professor Loyola University at Chicago, Maywood, Illinois, USA Visiting Professor, Department of Defense Military Medical Academy, Belgrade, Serbia Thomas John Vision Institute, Tinley Park and Oak Lawn, Illinois, USA Chicago Cornea Research Center, Tinley Park, Illinois, USA

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Corneal Endothelial Transplant (DSAEK, DMEK & DLEK) © 2010, Jaypee Brothers Medical Publishers All rights reserved. No part of this publication and DVD ROM should be reproduced, stored in a retrieval system, or transmitted in any form or by any means: electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the editor and the publisher. This book has been published in good faith that the material provided by contributors is original. Every effort is made to ensure accuracy of material, but the publisher, printer and editor will not be held responsible for any inadvertent error(s). In case of any dispute, all legal matters to be settled under Delhi jurisdiction only.

First Edition: 2010 ISBN 978-81-8448-792-3

Typeset at JPBMP typesetting unit Printed at Nutech

To God of all faiths and religions. I give all credit to God, Jesus Christ, without whom I am nobody. To my wife, Annita and my kids Michelle, Andrea and Olivia for all their loving support. To my parents for their love and guidance. To all the teachers in this world for their immense contribution to society.

Contributors .................................... Richard L Lindstrom MD

Wisam A Shihadeh MD

Founder and Attending Surgeon Minnesota Eye Consultants Adjunct Professor Emeritus Department of Ophthalmology University of Minnesota, Associate Director, Minnesota Lions Eye Bank, USA Foreword

Assistant Professor/Medical School Jordan University of Science & Technology Consultant in Glaucoma, Cornea & Refractive Surgery King Abdullah University Hospital Irbid, Jordan Chapter 2

Roger F Steinert MD

Almamoun Abdelkader MD

Professor of Ophthalmology, Professor of Biomedical Engineering, Director of Cornea, Refractive & Cataract Surgery, Vice Chair of Clinical Ophthalmology, Department of Ophthalmology University of California, Irvine, CA, USA Introduction, Chapter 5

Assistant Lecturer of Ophthalmology Faculty of Medicine, Al-Azhar University Hospitals, Cairo, Egypt Chapter 2

Kenneth R Kenyon MD

Herbert E Kaufman MD

Founder, Cornea Consultants International, Boston, USA, & Munich, Germany, Associate Clinical Professor of Ophthalmology, Harvard Medical School, Boston, MA, USA, Senior Scientist, Schepens Eye Research Institute, Boston, MA, USA, Eye Health Vision Centers, North Dartmouth, MA, USA Chapter 39

Byod Professor of Ophthalmology; Pharmacology & Experimental Therapeutics; Microbiology, Immunology & Parasitology Louisiana State University Health Sciences Center in New Orleans, LSU Eye Center New Orleans, LA, USA Chapter 2

Thomas John MD

Senior Fellow, Cornea and Refractive Surgery, Massachusetts Eye & Ear Infirmary and Department of Ophthalmology Harvard Medical School Boston, MA, USA Chapter 3

Clinical Associate Professor, Loyola University at Chicago, Maywood, Illinois, USA, Thomas John Vision Institute, Tinley Park and Oak Lawn, Illinois, USA, Chicago Cornea Research Center, Tinley Park Illinois, USA Editor and Chapters 10, 11, 12, 13, 14, 23, 27, 28, 29, 31, 32, 39

Jay S Pepose MD, PhD

Pepose Vision Institute, St. Louis, MO, USA; Department of Ophthalmology and Visual Sciences, Washington University School of Medicine, St. Louis, MO, USA Chapter 1

Mujtaba A Qazi MD

Pepose Vision Institute, St. Louis, MO, USA; Department of Ophthalmology and Visual Sciences, Washington University School of Medicine, St. Louis, MO, USA Chapter 1

Pedram Hamrah MD

Eric C Amesbury MD, FACS

Corneal Fellow Department of Ophthalmology & Visual Sciences, University of Louisville, Louisville, KY, USA Chapter 3

Richard A Eiferman MD, FACS

Clinical Professor of Ophthalmology Department of Ophthalmology & Visual Sciences, University of Louisville, Louisville, KY, USA Chapter 3

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Corneal Endothelial Transplant George Baikoff MD

Clinique Monticelli, Marseille, France Chapter 4

Leejee H Suh MD

Assistant Professor of Clinical Ophthalmology Cornea and External Diseases Bascom Palmer Eye Institute Miami, FL, USA Chapter 6

William W Culbertson MD

Professor of Ophthalmology The Lou Higgins Distinguished Chair in Ophthalmology, Bascom Palmer Eye Institute, University of Miami School of Medicine, Cornea and External Diseases, Miami, FL, USA Chapter 6

Harriet O Lloyd

Research Associate Department of Ophthalmology Weill Medical College of Cornell University, New York, NY, USA Chapter 7

D Dan Z Reinstein MD, MA(Cantab) FRCSC, FRCOphth

London Vision Clinic, London, UK; Department of Ophthalmology, St. Thomas’ Hospital - Kings College, London, UK; Department of Ophthalmology, Weill Medical College of Cornell University, NY, USA Chapter 7

D Jackson Coleman MD, FACS

The John Milton McLean Professor of Ophthalmology, Chairman Emeritus Department of Ophthalmology, Weill Cornell Medical College of Cornell University, NY, USA Chapter 7

Ronald H Silverman PhD

Professor of Computer Science in Ophthalmology Research Director, Bioacoustic Research Facility, Department of Ophthalmology Weill Medical College of Cornell University New York, NY, USA Member Research Staff Frederic L. Lizzi Center for Biomedical Engineering Riverside Research Institute New York, NY, USA Chapter 7

Monica Patel MD

Research Fellow, Bioacoustic Research Facility, Weill Medical College of Cornell University, New York, NY, USA Chapter 7

Jasmeet S Dhaliwal MD

Henry Ford Health System, Department of Ophthalmology, Cornea and Refractive Surgery, Troy, MI, USA Chapter 8

Auguste G-Y Chiou MD

Clinical Associate Professor LSU Eye Center, Av.de Montbenon 2 1003 Lausanne, Switzerland Chapter 8

Stephen C Kaufman MD, PhD

Omer Gal

Research Fellow, Bioacoustic Research Facility, Weill Medical College of Cornell University, New York, NY, USA Chapter 7

Professor and Lyon Endowed Chair of Ophthalmology Director of Cornea and Refractive Surgery University of Minnesota 420 Delaware St. SE, MMC-493 Minneapolis, MN 55455, USA Chapter 8

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Contributors Ramagopal Rao PhD

Executive Chairman 3D Vision Systems, LLC Irvine, CA, USA Chapter 9

David Miller MD

Associate Clinical Professor of Ophthalmology, Harvard Medical School Boston, MA, USA; Founder and Chief Medical Officer, 3D Vision Systems, LLC Irvine, CA, USA Chapter 9

Enrique S Malbran MD

Director of the Clínica Oftalmológica Malbran. President of the Fundación Oftalmológica Argentina Jorge Malbran. Chattered Member of the Academia Nacional de Medicina de Buenos Aires, Argentina Chapter 13

Mark A Terry MD

Director, Devers Eye Institute, Portland, OR, USA; Scientific Director, Lions Vision Research Laboratory of Oregon, Portland, OR, USA; Professor of Clinical Ophthalmology, Oregon Health Sciences University, Portland, OR, USA Chapters 14, 15, 16, 17, 20

Luiz F Regis-Pacheco MD Cornea and External Disease Service Department of Ophthalmology University of the State of Rio de Janeiro Rio de Janeiro, Brazil Chapter 14

Ashraf Amayem MD

Consultant Ophthalmologist Director of Cornea & Refractive Surgery Unit, Magrabi Eye Center, Jeddah Saudi Arabia Chapter 18

Magdi Helal MD

Consultant Ophthalmologist Director of Glaucoma Unit Magrabi Eye Center Jeddah, Saudi Arabia Chapter 18

Anastasios John Kanellopoulos MD

Associate Professor of Ophthalmology NYU Medical School, New York NY, USA, Director, Laser Vision Institute Athens, Greece Chapter 19

Massimo Busin MD

Head, Department of Ophthalmology Villa Serena Hospital Forli, Italy Professor of Ophthalmology, University of Bonn, Germany Clinical Professor of Ophthalmology University of Catanzaro, Italy Chapter 21, 37

Vincenzo Scorcia

MD

Villa Serena Hospital Department of Ophthalmology Forli, Italy Chapter 21

Marianne O Price PhD, MBA

José G Pecego MD

Executive Director, Cornea Research Foundation of America, Indianapolis, IN USA Chapter 22

Paula J Ousley MT

Francis W Price MD

Cornea Service Department of Ophthalmology Federal University of Rio de Janeiro Rio de Janeiro, Brazil Chapter 14

Past Research Director Lions Vision Research Laboratory of Oregon Portland, Oregon, USA Chapter 15

President, Price Vision Group Indianapolis, IN, USA Chapter 22

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Corneal Endothelial Transplant Mark S Gorovoy MD

Gorovoy MD Eye Specialists’ Office Fort Myers, FL, USA Chapters 24, 33

Anthony Kuo MD

Fellow, Cornea and Refractive Surgery Duke University Eye Center, Durham NC, USA Chapter 30

Keith A Walter MD

Associate Professor of Ophthalmology Wake Forest University Eye Center Winston-Salem, NC, USA Chapter 25

Marshall E Tyler

Terry Kim MD

Associate Professor of Ophthalmology Duke University School of Medicine Director of Fellowship Programs Associate Director Cornea and Refractive Surgery Duke University Eye Center Durham NC, USA Chapter 30

Wake Forest Univeristy Eye Center Winston-Salem NC, USA Chapter 25

Juan M Castro-Combs MD

Ciro Tamburrelli MD

Head, Ospedale Oftalmico di Roma, Rome, Italy Chapter 26

Agostino Salvatore Vaiano MD Ophthalmologist Ospedale Oftalmico di Roma Rome, Italy Chapter 26

Emilio Balestrazzi MD

Head of Ophthalmology Institute, Catholic University of Rome, Rome, Italy Chapter 26

Post-Doctoral Fellow, Cornea and Refractive Surgery Services, The Wilmer Ophthalmological Institute, The Johns Hopkins University School of Medicine The Johns Hopkins Hospital, Baltimore MD, USA Chapters 34, 35

Naima B Jacobs-El

The Wilmer Eye Institute The Johns Hopkins University School of Medicine Baltimore, MD, USA Chapters 34, 35

Ashley Behrens MD

Assistant Professor of Ophthalmology Cornea and Refractive Surgery Services The Wilmer Ophthalmological Institute The Johns Hopkins University School of Medicine, The Johns Hopkins Hospital Baltimore MD, USA Chapters 34, 35

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Contributors Panagiotis Georgoudis MRCOphth

Ophthalmology Resident, St Peter’s Hospital, Chertsey, Surrey, UK St Peter’s Hospital Guildford Road, Chertsey Surrey, KT16 0PZ, UK Chapter 36

Michael J Tappin FRCOphth

Ophthalmic Specialist, St Peter’s Hospital Chertsey, Surrey, UK Chapter 36

Jui-Yang Lai PhD

Assistant Professor, Institute of Biochemical and Biomedical Engineering Chang Gung University, Taoyuan Taiwan, Republic of China; Molecular Medicine Research Center, Chang Gung University, Taoyuan, Taiwan Republic of China Chapter 38

Ging-Ho Hsiue PhD

Department of Chemical Engineering National Tsing Hua University, Hsinchu Taiwan, Republic of China Chapter 38

Foreword .......................................... At the current moment, keratoplasty is undergoing an incredible paradigm shift in surgical technique. A field dominated by Penetrating Keratoplasty, where advances such as improved trephination systems, corneal preservation media, and suturing techniques while meaningful, have clearly been only incremental, is simultaneously going lamellar, minimally invasive, and sutureless. Lead by the extraordinary success of Deep Lamellar Endothelial Keratoplasty (DLEK), the concept of transplanting only the corneal layer which is diseased or damaged and needs replacing is gaining significant traction amongst corneal surgeons worldwide. Replacing the corneal endothelium only in a patient with Pseudophakic/ Aphakic Bullous Keratoplasty or Fuchs’ Dystrophy rather than replacing the entire cornea with a full-thickness Penetrating Keratoplasty has gone from the research interest of a few pioneering surgeons to mainstream in an amazingly short time. In addition to DLEK, we now have Descemet’s Stripping Automated Endothelial Keratoplasty (DSAEK) competing for our attention. At the same time, the Intralase femtosecond laser is being harnessed in an attempt to make Penetrating Keratoplasty more precise with the promise of more predictable refractive outcomes. It is truly not only an exciting time for the corneal surgeon, but also a demanding one requiring the rapid assimilation of new knowledge as well as the development of new surgical skills. Fortunately, Thomas John MD has produced for us a timely and outstanding educational offering in his new book “Corneal Endothelial Transplant (DSAEK, DMEK & DLEK)”. This very comprehensive book includes the history of lamellar transplantation; an in-depth discussion of the basic science of corneal structure, physiology, biomechanics and pathology; a primer on advanced corneal imaging; a review of the surgical instrumentation required; and of utmost importance, detailed instruction by the leaders in the field on the current best practices of surgical technique and complications management. A final section provides an enticing glimpse to the future. This book is complete enough to serve the corneal fellow well and advanced enough for even the most accomplished corneal surgeons to include in their personal library. Thank you, Dr John and colleagues, for providing we corneal surgeons with such an extraordinary educational resource. Richard L Lindstrom MD Founder and Attending Surgeon Minnesota Eye Consultants Adjunct Professor Emeritus Department of Ophthalmology, University of Minnesota Associate Director, Minnesota Lions Eye Bank, USA

Preface .............................................. This book entitled, Corneal Endothelial Transplant (DSAEK, DMEK & DLEK) is dedicated to the new way of performing corneal transplantation namely, without the use of corneal sutures and an absence of a full-thickness corneal wound. Such a move towards advanced corneal replacement surgery eliminates the induction of much disliked corneal astigmatism. Such a textbook provides the corneal surgeon with a variety of surgical techniques and instrumentation that will be a useful surgical resource for posterior lamellar keratoplasty procedures. For several decades, full-thickness penetrating keratoplasty (PKP) has dominated the field of corneal transplantation and has remained as the gold standard for corneal replacement surgery. However, the time has come when improved lamellar corneal techniques has re-appeared in the global horizon as a rapidly popular surgical technique and is beginning to challenge and possibly replace PKP as the gold standard in the times ahead. The editor has previously introduced a new term namely, Selective Tissue Corneal Transplantation (STCT) which may become the procedure of choice in many corneal disease processes. Why remove the whole cornea when the pathology may be limited regionally to either the front, middle or back part of the cornea? I have previously edited two books that covered both anterior and posterior lamellar keratoplasty, entitled, Surgical Techniques in Anterior and Posterior Lamellar Corneal Surgery, and Step by Step Anterior and Posterior Lamellar Keratoplasty. Due to the increasing popularity of posterior lamellar keratoplasty among corneal surgeons all over the world and the rapidly changing and evolving sutureless corneal transplantation techniques, this book collectively provides the reader with a wide assortment of surgical techniques from world leaders in the field of sutureless corneal transplantation as we know it today (at the time of writing this book). This book has numerous color photographs to assist in fully understanding the various surgical techniques described in the text. The editor and contributors have made it their priority to present the surgical techniques in a way that is easily understood by the readers of this textbook. This surgical text consists of 11 sections and 39 chapters. In Section 1, new areas of interest such as corneal hysteresis and biomechanical properties of the normal cornea are described. In addition, corneal physiology is covered. Also described in this section is the most important layer of the cornea that is responsible for corneal clarity, namely, corneal endothelium, both in health and in the disease state. In Section 2, new ways of imaging the human cornea are described. In vivo, real-time imaging of the cornea provides useful information both before and after surgery. This includes, Optical Coherence Tomography (OCT), Very High Frequency (VHF) ultrasound and confocal microscopy. The area of imaging covers both the cornea and the anterior segment. Section 3 presents the new generation operating microscope. This futuristic microscope provides a 3D perspective that is novel and may change the way we perform ophthalmic surgery in the future. Also included in this section is the use of intraoperative surgical slit-lamp microscope to assist in lamellar corneal surgery, namely, both anterior and posterior lamellar keratoplasty. The next section deals with the various new and useful surgical tools for the corneal surgeon to assist in performing sutureless corneal transplantation. Much like a paint brush is to an artist, so is the proper surgical instrument to the surgeon that will help in consistently performing high quality surgical work. It is not enough to have the best microscope and operating room setup. Equally or more important are the appropriate surgical instruments. This section describes the various surgical instruments that are commercially available to assist in performing posterior lamellar keratoplasty. Section 5 deals with an essential and important part of the instrumentation to perform posterior lamellar keratoplasty, namely, the artificial anterior chamber. A good understanding of the various types of artificial anterior chambers that are available will help the surgeon doing posterior lamellar keratoplasty. Both non-disposable and disposable types of artificial anterior chambers are described in this section of the book. This is especially important when the surgeon prefers to cut his or her own donor corneal tissue in the operating room rather than to use the corneal tissue that are pre-cut by eye bank technicians and supplied by the various eye banks in the United States for an additional fee. Newly introduced in the United States is a reimbursement code for surgeons preparing and cutting their own donor corneal disk for DSAEK, in addition to the DSAEK code for the DSAEK surgery.

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Section 6 deals with the definition, various terminologies, and classification of lamellar corneal surgery. This includes both the anterior and posterior lamellar keratoplasty. A good understanding of these terminologies that are currently in use will be beneficial to the lamellar surgeon and to all those interested in the field of lamellar corneal surgery. It is of great interest to go back in time and learn what the pioneers in the field of lamellar keratoplasty had to go through in order to arrive at the present-day surgical techniques that have simplified the posterior lamellar keratoplasty procedure. Such improved and simplified posterior lamellar techniques have fuelled the interest in lamellar surgery among corneal surgeons all over the world and their continued rapid conversion from the familiar, full-thickness penetrating keratoplasty to the not so familiar sutureless corneal transplantation. In this regard, Section 7 covers the history of lamellar and penetrating keratoplasty. Section 8 covers various aspects of Deep Lamellar Endothelial Keratoplasty (DLEK), including the large incision technique, small incision technique, combined phacoemulsification along with DLEK and there is a final chapter in this section that describes DLEK along with scleral-fixated posterior chamber intraocular lens implant. Section 9 is of great interest to all corneal surgeons looking at Descemet’s Stripping Automated Endothelial Keratoplasty (DSAEK). This section starts off with a chapter on eye banking issues and donor tissue preparation in DSAEK. This is an important chapter since there are many corneal surgeons who do not “cut” their own donor corneal tissue and instead elect to use eye bank technician cut donor corneal tissue for their patients. Also, in this section there is a chapter on the use of eye bank pre-cut donor tissue for DSAEK surgery. Also, in this section, various leading posterior lamellar corneal surgeons from both the United States and from other parts of the world describe their surgical techniques in performing posterior lamellar keratoplasty. Armed with this knowledge, the reader can elect to choose the surgical technique that appeals most to the individual surgeon. The techniques vary from folding the donor corneal disk into a taco-fold, to the burrito trifold, to no-fold (no taco fold). Varying amounts of donor corneal endothelial cell loss is inevitable at the present time with all exisiting surgical techniques, and such endothelial cell loss occurs during the handling of the donor corneal disk, insertion of the donor disk into the patient’s anterior chamber, and subsequent attachment of the disk to the inner surface of the patient’s cornea. There is a continued search for techniques that will allow for the least amount of endothelial cell loss in DSAEK surgery. Equally important, is the chapter on the visual and refractive outcomes following DSAEK. Also of great interest to the lamellar surgeon is the chapter on simplified technique and instrumentation in performing DSAEK. Also included in this section is the use of femtosecond laser (Intralase) for DSEK surgery. Once posterior lamellar keratoplasty is performed, the surgeon needs to learn various techniques to keep the donor disk attached to the inner surface of the patient’s cornea. In this regard, there is a chapter on surgical techniques to facilitate donor disk adherence to the patient’s cornea. The surgeon can use a single technique or a combination of techniques to decrease the disc detachment rate following DSAEK surgery. Also included in the same section is a chapter on the management of complications and a chapter on the unanswered questions in DSAEK. Section 10 compares the older form of sutureless corneal transplantation technique namely DLEK to the newer technique of DSAEK surgery. This section also describes the various staining techniques using the commercially available dyes that will allow better visualization of the donor corneal disk within the patient’s anterior chamber through a cloudy cornea. Staining in addition to providing better visualization of the donor disk also helps to identify the donor stromal surface from the donor endothelial surface. Included in the same section is a chapter on the comparative visual recovery in DSAEK, DLEK and PKP surgeries. The final section in this book, Section 11 gives a glimpse of what the future holds for posterior lamellar keratoplasty. In this section there are chapters on the use of tissue adhesive, to a novel approach for corneal endothelial cell transplantation using Descemet’s membrane as a carrier. There is also a chapter on true endothelial cell (Tencell) transplantation. Also included is a new technique of Descemet Membrane Endothelial Keratoplasty (DMEK). In addition, there is a chapter on corneal endothelial reconstruction with a bioengineered cell sheet. The final chapter in this book projects the possible future of posterior lamellar keratoplasty. This book is a comprehensive textbook in sutureless posterior lamellar corneal surgery that the reader would enjoy as he or she travels through this wide landscape of surgical techniques and instrumentation as it relates to the current status of posterior lamellar corneal surgery. Continued improvements and refinements of the surgical techniques by the ophthalmic surgeons can only incrementally benefit their patients all over the world. Thomas John MD

Acknowledgments ........................... I acknowledge all those who contributed to this book on “Corneal Endothelial Transplant,” by taking time from their busy schedules to write their chapter(s). A collective contribution and passion for their surgical pursuits makes this compilation valuable for readers all over the world. I wish to acknowledge all my teachers in the Cornea Service, Massachusetts Eye and Ear Infirmary (MEEI), Harvard Medical School, Boston, MA, USA, from whom I have learnt immensely both in the clinical and research aspects of “Cornea”. I am thankful to Drs Kenneth R Kenyon, Claes H Dohlman, C Stephen Foster, Roger F Steinert, Deberoah P Langston, Mark B Abelson, Michael D Wagoner, Jeffrey P Gilbard, Arthur S Boruchoff, and Ann M Bajart for all their dedication and effort in teaching surgical and medical skills relating to cornea and external diseases while I was a 2-year Clinical Cornea Fellow at Harvard. I wish to thank Dr Kenyon, under whose expert guidance I did my research work both at the Schepens Eye Research Institute and at the Massachusetts Institute of Technology (MIT), in Boston. I am fortunate to have worked with my colleagues, cornea fellows and residents at MEEI during my fellowship years, to name a few, Drs Mitchell C Gilbert, Eduardo C Alfonso, Kazuo Tsubota, Scheffer CG Tseng, Dimitri T Azar, John R Wittpenn, and Oliver D Schein. Special thanks to Drs James V Aquavella and Gullapalli N Rao for what they taught me in corneal surgery including epikeratoplasty, refractive surgical procedures, and keratoprosthesis. I am thankful to all my teachers in my formative years during my ophthalmology residency at the University of Pennsylvania. Although, not an all inclusive list, special thanks to Drs Ralph C Eagle, Jr, Myron Yanoff, John H Rockey, Irving M Raber, Alexander J Brucker, David M Kozart, William C Frayer, Harold Scheie, and Madeleine Q Ewing. I am especially thankful to Ralph C Eagle, Jr, MD, for all his support and professional inspiration, and for teaching me the various pathological basis of disease processes as it relates to the eye. I thank Myron Yanoff for accepting me into the ophthalmology residency program at the University of Pennsylvania. Teachers are one of the greatest assets of any society. I thank all my teachers from kindergarten to completion of my formal education both in the medical and pre-medical years. Without these teachers, I will be lacking in knowledge and I am indebted to each and every one of my teachers. I wish to acknowledge my wife, Annita, and the kids, Michelle, Andrea and Olivia for putting up with my late night academic work and for all their understanding and loving support. Thanks to Laura Phelps for the excellent medical illustrations in this book. To my office staff, for their patience and understanding. To all my patients, from whom I continue to learn everyday. Learning is a continuous and dynamic process that stimulates the mind and makes ophthalmology an even more interesting and challenging field in our life’s journey.

Introduction..................................... Sutures have been a necessary evil in most forms of corneal surgery. Sutures have historically been necessary to obtain a secure incision during the healing phase. The introduction of sutures in cataract surgery, utilizing large re-usable needles that the surgeon had to thread, in a manner like a tailor, represented a major breakthrough in rehabilitation after cataract surgery. As the needles and suture material became smaller and the number of possible sutures in a single incision increased, the patient rehabilitation time improved dramatically while the complication rate fell. No longer were cataract patients restricted to bed rest with their heads stabilized by sand bags, awaiting for healing of a limbal incision secured only by an overlying conjunctival flap. The problem with sutures in cataract surgery, of course, was the impossibility of precise control of the suture tension and placement. After prolonged healing, typically lasting several months, the patient might have high amounts of astigmatism representing either excessively tight or excessively loose sutures. These issues are now largely a historical footnote, as cataract surgery has come full circle. The drive for smaller incisions in cataract surgery allowed the creation of a self-sealing “valve” incision that, because of its inherent water tightness and structural stability, permits surgeons to use no sutures in many cases. In coming full circle and returning to sutureless cataract surgery, the evolution of the “valve” style incision represented a re-learning of the incision shape that helped cataract surgeons with sutureless incisions in the early 20th century, because the Graefe knife incisions of that era also created the same valve effect, unfortunately limited by the extreme width of the incision necessary to perform the whole lens surgery of that era. The lessons of sutures in cataract surgery apply even more to corneal surgery. In corneal transplantation, until recently, sutures have been mandatory to align and stabilize the junction of the donor and the recipient cornea. Because the cornea is slow to heal, those sutures must be retained much longer than in cataract surgery. In almost all cases, the sutures are also considerably closer to the optical center and the limbus. This proximity dramatically increases the negative impact of suture tension. Despite decades of improvement in suture materials, needles, and ingenious variations in suturing technique and suture patterns, the problem of distortion and slow healing of corneal incisions has remained as a powerful impediment to high quality vision after corneal transplantation. Indeed, another of the ironies in this story of sutures in ophthalmology came with a shift from silk sutures to fine nylon sutures for corneal transplantation. Silk sutures caused intense inflammation, vascularization, and higher rejection rates as well as patient discomfort. However, if the transplant survived, the patient benefited by the stimulation of more rapid incision healing and full suture removal much earlier than is possible with nylon sutures. The use of non-inflammatory material, therefore, caused a further shift in the direction of prolonged dependency on sutures and vulnerability to the negative impact of those sutures in corneal surgery. This outstanding text, conceived and edited by Thomas John, MD thoroughly explores the dramatic shift under way toward corneal transplantation without corneal sutures. The text thoroughly develops the background technologies that are the foundation of lamellar endothelial transplantation. Current endothelial transplant surgery typically still involves a few limbal sutures, but the future is clearly in the direction of transplantation of endothelial cells alone. That will complete the transformation to a completely sutureless corneal transplantation. Roger F Steinert MD Chair of Ophthalmology Director of the Gavin Herbert Eye Institute Professor of Ophthalmology and Biomedical Engineering Department of Ophthalmology University of California, Irvine, CA, USA

Perspective ....................................... Seems like just a year ago that we witnessed publication of Thomas John’s definitive magnum opus of lamellar keratoplasty, “Surgical Techniques in Anterior and Posterior Lamellar Corneal Surgery” (Jaypee Brothers, New Delhi, 2006). Can it really be time for yet another multifaceted, multi-authored work on posterior lamellar keratoplasty ?? Answer: Absolutely !! The Back to the Future of century old anterior lamellar keratoplasty techniques have now been extended and adapted for posterior corneal application plus technologically enhanced by current imaging and surgical instrumentation. This Paradigm Shift to Targeted TissueSpecific Keratoplasty is clearly the greatest advance in corneal surgery to occur within the last half century (Top 10 Ophthalmic Innovations of the Past 25 Years, Ocular Surgery News, Dec. 2007). Such rapid developments of the past decade, thanks to Dr. John and several of the nearly 50 international authors contributing to the current treatise, have propelled Endothelial Keratoplasty to have become the nearly standard approach for the surgical management of corneal endothelial disease. Little more than a year ago has passed since my own pilgrimage to Chicago (?or was it Lourdes or perhaps Mecca…?) to witness and learn from the Master K-Plaster, Tom John, himself. Thusly converted, I can now personally attest to the Miracle of Posterior Lamellar Keratoplasty. True, this surgery and its variations requires mastery of new surgical skills as well as problem solving and complication management, as does any novel operative technique. Yet having witnessed the surgery and its results (including the increasingly long-term published series, as are also included herein), the Bottom Line is all too obvious: Once you go DSEK, you never go back ! And so it is that Dr. John and friends rapidly push forward and relentlessly expand the frontiers through their current approaches to the state of the sutureless keratoplasty art. Including sections devoted to corneal basic science, tissue imaging, surgical instrumentation, basic themes plus multiple variations of endokeratoplasty technique, and future directions, this opus is a definitive work of equally magnum magnitude which should convince even more conservative corneal surgeons that the Back to the Future of lamellar keratoplasty is unquestionably now. Finally, I must also express my personal and professional appreciation to Thomas for both the honor of including me among his illustrious cast of coauthors but especially for the Epiphany of helping me to Perceive the Light. Kenneth R Kenyon MD Founder, Cornea Consultants International Boston, MA, USA & Munich, Germany Associate Clinical Professor of Ophthalmology Harvard Medical School, Boston, MA, USA Senior Scientist, Schepens Eye Research Institute Boston, MA, USA

Contents ........................................... Section 1: Cornea 1. Corneal Hysteresis and Biomechanics of the Normal Cornea ...................................................................................... 3 Mujtaba A Qazi, Jay S Pepose (USA) 2. Corneal Physiology ............................................................................................................................................................. 13 Wisam A Shihadeh (Jordan), Almamoun Abdelkader (Egypt), Herbert E Kaufman (USA) 3. Corneal Endothelium in Health and Disease ................................................................................................................ 23 Pedram Hamrah, Eric C Amesbury, Richard A Eiferman (USA)

Section 2: Corneal Imaging 4. Optical Coherence Tomography (OCT) of the Anterior Segment ............................................................................... 39 George Baikoff (France) 5. Optical Coherence Tomography in Corneal Implant Surgery ..................................................................................... 47 Roger F Steinert (USA) 6. Use of Optical Coherence Tomography (OCT) in Descemet’s Stripping with Endothelial Keratoplasty (DSEK) and Descemet’s Stripping Automated Endothelial Keratoplasty (DSAEK) ....................... 53 Leejee H Suh, William W Culbertson (USA) 7. Imaging of the Cornea and Anterior Segment with High-Frequency Ultrasound ................................................... 61 Ronald H Silverman, Monica Patel, Omer Gal, Harriet O Lloyd (USA) D Dan Z Reinstein (UK), D Jackson Coleman (USA) 8. Confocal Microscopy of the Cornea ......................................................................................................................................................71 Jasmeet S Dhaliwal (USA), Auguste G-Y Chiou (Switzerland), Stephen C Kaufman (USA)

Section 3: Next Generation Operating Microscope 9. Next Generation Operating Microscope: 3D Digital Microscope and Microsurgical Workstation ....................... 85 Ramagopal Rao, David Miller (USA) 10. Role of Surgical Slit-lamp in Endothelial Transplantation and Anterior Segment Surgery ................................... 95 Thomas John (USA)

Section 4: Surgical Instruments 11. New/Useful Surgical Instruments in DSAEK .............................................................................................................. 107 Thomas John (USA)

Section 5: Artificial Anterior Chambers 12. Artificial Anterior Chambers .......................................................................................................................................... 123 Thomas John (USA)

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Section 6: Classification of Lamellar Corneal Surgery 13. Definition, Terminology and Classification of Lamellar Corneal Surgery .............................................................. 133 Thomas John (USA), Enrique S Malbran (Argentina)

Section 7: History 14. History of Lamellar and Penetrating Keratoplasty ...................................................................................................... 143 Thomas John (USA), Luiz F Regis-Pacheco (Brazil), José G Pecego (Brazil), Mark A Terry (USA)

Section 8: Deep Lamellar Endothelial Keratoplasty (DLEK) 15. Deep Lamellar Endothelial Keratoplasty (DLEK): Large Incision Technique ....................................................... 157 Mark A Terry, Paula J Ousley (USA) 16. Deep Lamellar Endothelial Keratoplasty (DLEK): A Procedure for Special Cases of Endothelial Dysfunction .................................................................................................................................. 171 Mark A Terry (USA) 17. Deep Lamellar Endothelial Keratoplasty: Small Incision Technique Combined with Phacoemulsification and Posterior Chamber IOL .......................................................................... 183 Mark A Terry (USA) 18. Deep Lamellar Endothelial Keratoplasty (DLEK) Combined with Scleral-fixated Posterior Chamber Intraocular Lens Implantation ...................................................................................................................... 201 Ashraf Amayem, Magdi Helal (Saudi Arabia)

Section 9: Descemet’s Stripping Automated Endothelial Keratoplasty (DSAEK) 19. Eye Banking and Donor Corneal Tissue Preparation in DSAEK ............................................................................. 217 Anastasios John Kanellopoulos (Greece) 20. Endothelial Keratoplasty: A Step by Step Guide to DSEK and DSAEK Surgery ................................................... 225 Mark A Terry (USA) 21. Improved DSAEK Surgery for Enhanced Endothelial Survival ................................................................................ 237 Massimo Busin, Vincenzo Scorcia (Italy) 22. Endothelial Keratoplasty: Visual and Refractive Outcomes ...................................................................................... 245 Marianne O Price, Francis W Price (USA) 23. DSAEK Simplified Surgical Technique ....................................................................................................................... 253 Thomas John (USA) 24. Surgical Technique for Descemet Stripping Automated Endothelial Keratoplasty (DSAEK) ............................ 281 Mark S Gorovoy (USA) 25. Descemet’s Stripping Endothelial Keratoplasty (DSEK), Through a 3 mm Incision using the Tri-fold Technique ........................................................................................................................................................... 289 Keith A Walter, Marshall E Tyler (USA) 26. Femtosecond Laser (Intralase®) – Descemet’s Stripping Endothelial Keratoplasty (Femto-DSEK): Initial Studies of Surgical Technique in Human Eyes ............................................................................................... 293 Ciro Tamburrelli, Agostino Salvatore Vaiano, Emilio Balestrazzi (Italy)

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27. Techniques to Facilitate Disk Adherence to Recipient Cornea in DSAEK ............................................................. 303 Thomas John (USA) 28. Complication Management in DSAEK ......................................................................................................................... 311 Thomas John (USA) 29. Unanswered Questions in DSAEK ................................................................................................................................ 327 Thomas John (USA) 30. Use of Eye Bank Pre-cut Donor Tissue in DSAEK ...................................................................................................... 331 Anthony Kuo, Terry Kim (USA)

Seciton 10: DLEK Versus DSAEK 31. Comparison of Wound Architecture in DLEK Versus DSAEK ................................................................................. 343 Thomas John (USA) 32. Use of Dyes in DSAEK and DLEK ................................................................................................................................. 349 Thomas John (USA) 33. Comparative Visual Recovery in DSAEK, DLEK and PKP ........................................................................................ 361 Mark S Gorovoy (USA)

Section 11: DMEK and Future Directions in Posterior Lamellar Keratoplasty 34. Posterior Lamellar Keratoplasty Using Tissue Adhesive ........................................................................................... 367 Juan M Castro-Combs, Naima B Jacobs-El, Ashley Behrens (USA) 35. Novel Approach for Corneal Endothelial Cell Transplantation using Descemet Membrane as a Carrier ........ 377 Naima B Jacobs-El, Juan M Castro-Combs, Ashley Behrens (USA) 36. True Endothelial Cell (TEnCell) Transplantation ........................................................................................................ 389 Panagiotis Georgoudis, Michael J Tappin (UK) 37. Descemet Membrane Endothelial Keratoplasty (DMEK) ......................................................................................... 399 Massimo Busin (Italy) 38. Corneal Endothelial Reconstruction with a Bioengineered Cell Sheet .................................................................... 405 Jui-Yang Lai, Ging-Ho Hsiue (China) 39. Future of Posterior Lamellar Keratoplasty .................................................................................................................... 421 Thomas John, Kenneth R Kenyon (USA)

Index ..................................................................................................................................................................................... 425

Mujtaba A Qazi, Jay S Pepose

Corneal Hysteresis and Biomechanics of the Normal Cornea

1

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Corneal Endothelial Transplant

Introduction The structural characteristics of the cornea facilitate its essential functions, specifically to serve as both a transparent barrier and the predominant refractive element of the eye. Given the integral relationship between form and visual function, the biological and mechanical responses of the cornea to surgical interventions impact its optical performance. While major advances have occurred in the refinement and standardization of corneal surgical techniques, our ability to predict individual biological responses to surgery remains limited and can influence the predictability and stability of visual outcomes after corneal surgery. Some of these biomechanical responses are seen immediately, e.g., following lamellar keratectomy and/or laser ablation.1 Others may manifest in shape instability over time or with further surgery, or result in serious complications such as wound dehiscence, scarring, haze formation, and induction of irregular astigmatism. Corneal biomechanical characteristics often change with wound healing, and also may be better understood in the context of whole eye rigidity.2 In this chapter, we highlight the biochemical and ultrastructural features that most strongly contribute to the biomechanical properties of the normal cornea. Further understanding of these factors provide the basis for improving outcomes and reducing complications of corneal surgery, by identifying individual response outliers and developing strategies for regulating or compensating for these biomechanical features.3 While more familiar to engineers than to most ophthalmologists, we also review the definition of corneal hysteresis and other metrics that have been applied to studies of corneal biomechanics — a subject that encompasses the effects of corneal hydration, regional pachymetry, viscoelasticity and other inherent corneal characteristics that may not yet be fully defined.

Collagen Structure of the Cornea The tensile integrity and refractive curvature of the cornea is determined in large part by the stroma, which represent the bulk of the corneal thickness. On a weight basis, the stroma is approximately 78% water, 15% collagen and 7% non-collagenous proteins and proteoglycans.4 Approximately 300 collagen lamellae, spanning from limbus to limbus, comprise the center of the cornea.5 This number increases to about 500 as the cornea thickens toward the periphery.4,5 Presumably, this occurs from branching of the lamellae, with some lamellae branches merging with others.6 Branching is seen more extensively in the corneal periphery, where there is primarily a circumferential

orientation of the fibrils.7,8 Branching and interlacing of lamellae has been implicated to play an important role in corneal tensile strength.9 The orientation and spacing of the collagen fibrils appears to be controlled by stromal proteoglycans. Swelling studies have shown that the interlamellar adhesive strength of the central cornea depends upon proteoglycan bonding, whereas branching and interlacing of lamellae provides additional adhesive strength peripherally.7 Changes in the proteoglycan matrix may explain the increased pliability of the central cornea in keratoconus and may potentially impact the corneal response to keratorefractive surgery, contact lens wear and tonometric testing.10 The anterior-most stromal lamellae have oblique branching and interweaving fibers that insert into Bowman’s layer.11 Because of these features, the anterior stroma swells less and is about 25% stiffer than its posterior counterpart.12 Similarly, as there is greater interlacing of peripheral fibers, swelling of the peripheral cornea is usually less than in the central stroma.13 These findings suggest that peripheral and/or posterior incisional surgery may have less of a profound impact on corneal biomechanics than anterior, central surgery (Table 1-1). It appears that corneal shape is not determined on a random basis, but results from a steady state balance between the biomechanical properties of the cornea and intraocular pressure (IOP).14 The cornea assumes the shape for which its potential energy content is minimal and for which its stromal fibrils are in a relatively relaxed state, as a function of variables such as tissue elasticity, thickness, fibril length, rate of change of IOP, among others. External physiologic corneal stresses, such as from normal blinking or diurnal variation in IOP, and non-physiologic corneal stresses, caused by increases in IOP from forceful lid closure or rubbing, may potentially impact the corneal shape. However, normal corneas have been found to show low extensibility, measured by changes in anterior surface sagittal height, for a wide range of physiologic conditions and even with marked elevations in IOP in order to maintain refractive stability. 15 Conversely, when the corneal biomechanical properties are altered via incisional surgery TABLE 1-1: Local variation in corneal lamellar ultrastructure Collagen lamellae in the peripheral cornea: 1. Greater number 2. Greater branching and interlacing 3. Circumferential orientation 4. Greater resistance to swelling Collagen lamellae in the anterior cornea: 1. Anterior strands insert into Bowman’s layer 2. Greater proteoglycan bonding centrally 3. Greater stiffness than posterior cornea

Corneal Hysteresis and Biomechanics of the Normal Cornea

5

such as radial keratotomy, diurnal variation in IOP can lead to fluctuation in corneal refractive power by greater than one diopter (D).15

Metrics of Corneal Biomechanical Properties In the terminology of material science, the cornea is a complex composite (of collagen, other proteins, proteoglycans, water, and salts) with non-linear elastic and viscoelastic properties characterized by important local variation in organization in central versus peripheral and anterior versus posterior dimensions. Mathematical modeling of such a complex system is therefore quite difficult, but begins with identification of intrinsic properties of corneal tissue, as described below (Table 1-2). TABLE 1-2: Descriptors of corneal biomechanical properties 1. 2. 3. 4. 5. 6. 7. 8. 9.

Elasticity Viscoelasticity Hysteresis ORA corneal resistance factor Creep Stress relaxation Sheer strength Ocular rigidity PASCAL ocular pulse amplitude

Elasticity (Young’s modulus, E) is an indicator of material stiffness, with a higher modulus corresponding to a stiffer material. For example, a metallic rod would have a higher modulus than a wood rod (Figure 1-1). A perfectly elastic material returns to its original form, when an external stress is withdrawn, in a completely reversible and symmetric manner, i.e., along the same stress-strain pathway.16 Elasticity is traditionally measured ex vivo with an extensiometer that records the force generation required during steady axial elongation of a tissue sample. The slope of stress (force per unit area) over strain (the current length divided by the starting length) is calculated for a representative portion of the curve. A linear approximation can be obtained from the instantaneous slope of the stressstrain curve (tangent modulus) or as a chord between two points on the curve (secant modulus). A limitation of extensiometer measurement of elasticity is that the range of in vivo corneal elasticity modulus in healthy or diseased tissue is unknown,17 and reports on animal and human tissues can span orders of magnitude.15,18 Additionally, while most biological soft tissues approximate linear elastic behavior when a small range of stresses are introduced, their overall elastic behavior is highly non-linear. Nevertheless, understanding elastic

Figure1-1: The influence of structural and material properties upon the ability to deform the cornea. Bending a single chopstick is usually easy. However, bending three of the same type of chopsticks at once is much more difficult (top row). Hence, a larger deformation will be generated for thinner corneas given the same applied force. This partially explains the underestimation of IOP in eyes with thinner corneas. In contrast, it requires greater pressure to applanate or indent a thicker cornea, which contributes to overestimation of IOP in eyes with thicker corneas. Similarly, much more force is required to bend a steel rod than a wood rod of the same dimensions (middle row). The difference in this case is the elastic properties of the material, specifically Young’s modulus. Steel has a much higher Young’s modulus (w200 000 MPa) than wood (w10 000 MPa); therefore, if all other parameters are the same, it is much harder to deform a steel structure than a wood structure. Corneal curvature is another variable that can affect the accuracy of IOP measurement, possibly because of the difference in the volume of the displaced fluid after a given area is flattened (bottom row).17

properties is important for evaluating the instantaneous response of the cornea to surgery and can affect its subsequent viscous behavior. Some studies have demonstrated, for example, a decrease in stiffness and increase in extensibility of keratoconic tissue relative to normal tissue.19 The elastic modulus was identified by Guirao 20 in mathematical modeling as the most influential risk factor for posterior corneal steepening after keratorefractive surgery. Viscoelasticity extends the biomechanical response of biological tissues into complex mathematical descriptions of viscous fluids, where elastic responses are time and rate dependant.21 Viscoelastic materials return to their pre-stress shape via different stress-strain pathways that depend upon loading rates. Viscoelastic properties can be described through metrics of hysteresis, stress relaxation and creep. Viscoelastic creep is a time-dependent elongation of tissue (or increasing strain) that occurs under a sustained or constant stress (such as IOP).21 The effect of creep is a reduction in effective tissue stiffness, which can lead to a decrease in resistance to stretch. Creep may be a precursor to ectasia, where stressed collagen fibrils undergo a

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Figure 1-2: Experiments illustrating elastic and viscoelastic properties in a 7 mm, full-thickness horizontal corneal strip from a 63-year-old donor. Elliptical polarization allows visualization of non-homogeneous internal stresses. Progressive stretching of the sample (1, 2 and 3) and measurement of the induced load (stress) allows calculation of the elastic (Young’s) modulus from the slope of the stress-strain relationship. The relationship is non-linear. A second experiment in which a constant displacement is imposed in the same sample demonstrates time-dependent stress relaxation, a viscoelastic property of biological soft tissues (4 and 5). (Courtesy of W.J. Dupps, Jr., MD, PhD and T. Doehring, PhD.16)

pathologic weakening without an initial change in length. Once the collagen fibrils are weakened, a gradual stretching then occurs under constant stress or IOP. Viscoelastic stress relaxation refers to a situation where strain is increased then held constant (no more tissue elongation) while a slow but quantifiable time-dependent relaxation of the load is observed (Figure 1-2). Hysteresis, in general, is a property of physical and biological systems that do not instantly follow the forces applied to them but react slowly or do not return completely to their original state.21 Hysteresis describes a lag between making a change, such as increasing or decreasing power, and the response or effect of that change. Whereas a rubber band can be described as elastic because is springs back to its original shape at the same rate as when it is stretched, a putty exhibiting viscoelastic behavior quickly assumes a new shape when pushed upon but will not immediately return to its original shape when the mechanical pressure is released. Another example of hysteresis is a thermostat set at 80 degrees, which actually regulates the room temperature between 78 and 82 degrees. In broad terms, corneal hysteresis can be thought of as a metric of the ability of the cornea to absorb energy. Shear strength describes stromal resistance to lamellar sliding and bending. The shear resistance provided by collagen interweaving and other matrix forces has been

estimated from metrics such as the interlamellar cohesive strength.7 Corneal shear strength is low relative to its tensile strength, but provides a mechanism for tensile load transfer between lamellae.9 Volumetric distension experiments provide a measure of whole globe stiffness, or ocular rigidity. The slope of a pressure-volume curve can be recorded during such experiments. Ocular rigidity is non-linearly dependent upon IOP and has been shown to increase with age.2 The utility of metrics describing ocular rigidity may be limited with respect to their impact on the understanding of corneal surgery, given the contributory role of the scleral and uveal tissue to ocular rigidity.

New Techniques for in vivo Measurement of Corneal Biomechanical Properties While ex vivo diagnostic techniques, such as extensiometry, have provided valuable information on the biomechanical nature of normal and pathological corneas,22 a new era is dawning in biomechanical research with the development of techniques to measure structural and biomechanical properties in vivo. Imaging of the cornea can now be performed by confocal microscopy,23 very high frequency,24

Corneal Hysteresis and Biomechanics of the Normal Cornea and optical coherence technology (See Section 2: Corneal Imaging). Holographic interferometry uses optical comparison to evaluate corneal elasticity.25,26 Another method, dynamic corneal imaging, uses stepwise central indentation of the cornea and computer analysis of videokeratography images during indentation to assess corneal elastic properties in vivo. 27 A commercially available device that uses a dynamic, bidirectional, airpuff applanation to measure in vivo IOP and corneal viscoelastic properties is the Ocular Response Analyzer ([ORA], Reichert Ophthalmic Instruments, Depew, New York, USA).28

Biomechanics and Intraocular Pressure Increasing attention has focused on the impact of corneal parameters, particularly central corneal thickness (CCT), on the measurement of IOP.29 IOP measurements have been demonstrated to vary with CCT using the Goldmann applanation, 30 pneumotonometry,31 and non-contact tonometry. 32 The deformation of the cornea during applanation is determined by an interaction of the external applied force with the intrinsic properties of the cornea. With the same applied force, a larger deformation will be produced for less rigid corneas. This partially explains the underestimation of IOP in eyes with thinner corneas. In contrast, it requires greater force to applanate a more rigid cornea, partially explaining the overestimation of IOP in eyes with thicker corneas. Additionally, alteration of corneal biomechanics by LASIK flap creation and excimer laser ablation affects the postoperative measurement of intraocular pressure (IOP) using Goldmann applanation tonometry (GAT).33-35 The impact of pachymetry on intraocular pressure readings has been recently highlighted by the Ocular Hypertension Treatment Study,36 which demonstrated an inverse relationship between CCT and the risk of developing glaucoma. Goldmann tonometry is a static measurement, calculating IOP from the force applied during a steady state applanation of the cornea.37 Its design is based upon a number of assumptions, including that all corneas were of uniform thickness (i.e., 500 microns), that the eye’s volume was spherical and that the cornea behaved biomechanically as an infinitely thin and perfectly flexible membrane. Corneal biomechanics embody far more than central pachymetry alone, and include viscosity, elasticity, hydration, regional pachymetry and other factors.17 As an example, whereas corneas from patients with keratoconus are generally thinner than average and biomechanically

7

“floppy”, corneas from patients with Fuchs’ dystrophy are thicker than normal while they are also biomechanically similar to keratoconus.28 Other illustrative examples are the decrease in corneal rigidity following radial keratometry29 and hyperopic LASIK33 with subsequent drop in static applanation tonometry readings, yet little or no change in central corneal pachymetry. These examples highlight the complexity and potential flaws of simplistic attempts to linearly offset IOP based upon pachymetry alone. Alternative techniques that dynamically derive IOP from the corneal movement in response to a rapid air pulse simultaneously assess and compensate (to varying degrees) for the effect of the cornea’s viscous and elastic qualities on IOP measurement. Reichert’s ORA28 utilizes a metered collimated air pulse to applanate the cornea and an infrared electro-optical system to record inward and outward applanation events. The air-pulse deforms the cornea through an initial applanation event (peak 1), then beyond into concavity and then gradually subsides, allowing the cornea to rebound through a second applanation (peak 2) (Figure 1-3). Corneal hysteresis (CH) is defined as the difference between the applanation pressure at peak 1 (P1) and peak 2 (P2), so that CH = P1–P2. This dynamic assessment of corneal biomechanical properties yields metrics of both the cornea’s viscous and elastic qualities. Whereas corneal hysteresis may reflect mostly corneal viscosity, corneal resistance factor (CRF, defined by a linear function of P1 and P2) may predominantly quantify corneal rigidity. The equation used to determine CRF is: CRF = P1– 0.7 × P2. Pascal Dynamic Contour Tonometry (PDCT, Swiss Microtechnology AG; Port, Switzerland) employs a concave

Figure 1-3: The waveform generated from the ocular response analyzer identifies the pressure difference, or hysteresis, during inward (peak 1) and outward (peak 2) applanation events during non-contact tonometry.43

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Figure 1-4: Diurnal measurement of corneal hysteresis (left) and intraocular pressure (right) with the ocular response analyzer. While there was a statistically significant reduction in ORA IOP during the course of the day, diurnal measurements of hysteresis did not statistically differ.42

tip to “contour match,” rather than applanate, a convex segment of the central cornea and, thus, may be relatively independent of the effects of CCT or surgical intervention on IOP assessment.38-40 The instrument dynamically records over 100 IOP measurements per second, measuring IOP fluctuations throughout the cardiac cycle and digitally displaying the average diastolic IOP. Ocular Pulse Amplitude (OPA), the difference in IOP between systole and diastole (IOP systolic – IOP diastolic), is also reported and may be a marker for overall ocular rigidity,2 although it is also affected by ocular blood flow.

Clinical Applications of Hysteresis and Corneal Biomechanics Corneal hysteresis in normal eyes has been reported to range from 5.0 to 18.7, with a mean hysteresis of 9.6 to 12.7 mm Hg.28,41,42 Hysteresis values did not show a statistical difference in a cohort of 21 normal patients between right and left eyes, with a mean difference of 0.4 mm Hg (p>0.08). 42 Hysteresis values appear to be relatively insensitive to diurnal effects (Figure 1-4), although intrasubject variations have been observed. The correlation of CCT with CH and CRF was 0.59 and 0.62, respectively.41 Lower hysteresis was associated with visual field progression in a glaucomatous population.43 Corneas with keratoconus (Figure 1-5), Fuchs’ dystrophy and post-refractive surgery demonstrate a general decrease in corneal hysteresis compared to corneas in normal eyes.28 The low corneal hysteresis in the Fuchs’ eyes is seen despite unusually thick, but edematous, corneas. However, the large 99% confidence interval of corneal hysteresis seen in normal controls has considerable overlap with diseased

Figure 1-5: Distribution of hysteresis values in 339 normal (blue) and 60 keratoconic (red) eyes. Mean hysteresis for normal and keratoconic eyes was 9.6 mm Hg and 8.1 mm Hg, respectively.28

and post-surgical corneas, limiting its diagnostic value as a single metric in individual cases. What may turn out to allow better diagnostic differentiation are the significant changes seen in applanation waveform in diseased or postsurgical corneas. Keratoconic and post-LASIK corneas appear to have similar applanation signal morphology, indicating reduced or low corneal viscoelastic properties in both cases (Figure 1-6). Investigations to quantify morphologic characteristics of the ORA waveform are now underway that attempt to extract additional corneal biomechanical information. Corneal hysteresis may be useful as a qualification factor for LASIK in corneas that have similar CCT but display significantly different waveform properties. Thus, the ORA waveform, along with the derived biomechanical metrics of corneal hysteresis and resistance factor, may provide a more complete characterization of corneal biomechanical properties than corneal thickness alone and is perhaps a better tool for assessing refractive surgery qualification and outcomes.

Corneal Hysteresis and Biomechanics of the Normal Cornea

9

Figure 1-6: ORA applanation signal in keratoconic and post-LASIK eyes shows depressed applanation peak amplitudes and altered applanation peak widths. A: Case 1-4 are from keratoconic eyes. B: pre- and post-LASIK waveforms show a decrease in hysteresis postoperatively, along with reduced applanation peak amplitudes. C: waveform post penetrating keratoplasty shows marked alternation in the applanation signal, with increased noise during the applanation events.28

Modeling Based Upon Biomechanical Metrics: Implication for Surgical Planning Models of the cornea have taken many forms, including complex computational models that integrate structural, biomechanical and optical corneal properties.44 Dupps and

Wilson16 have proposed a strategy (Figure 1-7) for such modeling. By comparing these models to clinical experiments, useful models can be created, which can significantly improve our understanding of corneal biomechanics and allow us to better predict refractive effect after corneal transplant procedures.45 Results of finite element simulations indicate that significant changes in corneal refractive power could be introduced if refractive

Figure 1-7: An approach to biomechanical modeling of surgery and disease in the cornea. Disease is simulated by alteration of the substructural components or their material properties. Surgery is simulated by imposing an ablation profile or incisions. The model is optimized retrospectively by comparing model simulations to analogous experiments in tissue or clinical models. A model optimized with clinical data can then be used prospectively to design and evaluate patient-specific treatment algorithms.16

10

Corneal Endothelial Transplant wound healing and viscoelastic features of the normal and post-surgical cornea. As our understanding of these processes improves, so will our ability to offer rational interventions and strategies for further improving the predictability of keratorefractive surgery and minimizing its complications.

References

Figure 1-8: Major biomechanical loading forces in the cornea and a model of biomechanical central flattening associated with disruption of central lamellar segments. A reduction in lamellar tension in the peripheral stroma reduces resistance to swelling and an acute expansion of peripheral stromal volume results. Interlamellar cohesive forces and collagen interweaving, whose distribution is greater in the anterior and peripheral stroma and is indicated by grey shading, provide a means of transmitting centripetal forces to underlying lamellae. Because the central portions of these lamellae constitute the immediate postoperative surface, flattening of the optical surface occurs, resulting in hyperopic shift. The degree of flattening is associated with the amount of peripheral thickening. This phenomenon is exemplified clinically by PTK-induced hyperopic shift but is important in any central keratectomy, including PRK and LASIK. Simultaneous elastic weakening of the residual stromal bed may occur, and the threshold for inducing irreversible (plastic) or progressive (viscoelastic) steepening (or ectasia) is a matter of great clinical concern.16

procedures (Figure 1-8) are combined with corneal transplants. This requires high precision resections of corneal grafts, which may improve with the application of femtosecond laser technology.46 Treatment of the cornea with riboflavin and UVA to increase collagen crosslinking47 may also allow us to modulate the stiffness of the cornea before or after corneal transplant procedures.

Conclusion A review of the histology of corneal fibrils indicates evidence for inextensibility, under a wide range of physiologic conditions, which appears to be the basis for stability of refraction and corneal curvature. Given the major biomechanical loading forces of the normal cornea, surgical disruption of the central corneal lamellae may lead to central corneal flattening and peripheral thickening.1 Studying the dynamics of corneal shape changes that occur in response to collimated air pulses (via the Ocular Response Analyzer) may provide a basis for understanding the biomechanical effects of incisional and lamellar corneal surgery. The in vitro interactions of corneal fibroblasts and a fibrillar collagen substrate48 can be combined with advanced structural and functional in vivo diagnostic imaging techniques to develop mathematical models of the

1. Roberts C. The cornea is not a piece of plastic. J Refract Surg 2000;16:407-13. 2. Pallikaris IG, Kymionis GD, Ginis HS, Kounis GA, Tsilimbaris MK. Ocular rigidity in living human eyes. Invest Ophthalmol Vis Sci 2005;46:409-14. 3. Reinstein DZ, Roberts C. Biomechanics of corneal refractive surgery. J Refract Surg 2006;22:285. 4. Maurice DM. The cornea and sclera. In: Davson, H. (Ed.), The Eye. Academic Press, Orlando, FL, 1984;1-158. 5. Meek KM, Boote C. The organization of collagen in the corneal stroma. Exp Eye Res 2004;78:503-12. 6. Radner W, Zehetmayer M, Aufreiter R, Mallinger R. Interlacing and crossangle distribution of collagen lamellae in the human cornea. Cornea 1998;17:537-43. 7. Smolek MK, McCarey BE. Interlamellar adhesive strength in human eyebank corneas. Invest Ophthalmol Vis Sci 1990;31: 1087-95. 8. Meek KM, Newton RH. Organization of collagen fibrils in the corneal stroma in relation to mechanical properties and surgical practice. J Refract Surg 1999;15:695-9. 9. Dupps WJ, Roberts C. Effect of acute biomechanical changes on corneal curvature after photokeratectomy. J Refract Surg 2001;17:658-69. 10. McMonnies CW, Schief WK. Biomechanically coupled curvature transfer in normal and keratoconus corneal collagen. Eye Contact Lens 2006;32:51-62. 11. Komai Y, Ushiki T. The three-dimensional organization of collagen fibrils in the human cornea and sclera. Invest Ophthalmol Vis Sci 1991;32:2244-58. 12. Muller LJ, Pels E, Vrensen JM. The specific architecture of the anterior stroma accounts for maintenance of corneal curvature. Ophthalmology 2001;85:437-43. 13. Doughty MJ, Bergmanson JPG. Collagen fibril characteristics at the corneo-scleral boundary and rabbit corneal swelling. Clin Exp Optom 2004;87:81-92. 14. Sjontoft E, Edmund C. In vivo determination of Young’s modulus for the human cornea. Bull Math Biol 1987;49:217-32. 15. Jue B, Maurice DM. The mechanical properties of the rabbit and human cornea. J Biomech 1986;19:847-53. 16. Dupps WJ Jr, Wilson SE. Biomechanics and wound healing in the cornea. Exp Eye Res. 2006;83:709-20. 17. Liu J, Roberts CJ. Influence of corneal biomechanical properties on intraocular pressure measurement Quantitative analysis. J Cataract Refract Surg 2005;31:146–55. 18. Hoeltzel DA, Altman P, Buzard K, Choe K. Strip extensiometry for comparison of the mechanical response of bovine, rabbit, and human corneas. J Biomech Eng 2002;114:202–15. 19. Edmund C. Corneal topography and elasticity in normal and keratoconic eyes. A methodological study concerning the pathogenesis of keratoconus. Acta Ophthalmol Suppl 1989; 193:1-36. 20. Guirao A. Theoretical elastic response of the cornea to refractive surgery: risk factors for keratectasia. J Refract Surg 2005; 21:176-85. 21. Dupps WJ. Biomechanical modeling of corneal ectasia. J Refract Surg 2005;21:186-90.

Corneal Hysteresis and Biomechanics of the Normal Cornea 22. Andreassen TT, Simonsen AH, Oxlund H. Biomechanical properties of keratoconus and normal corneas. Exp Eye Res 1980;31:435-41. 23. Sherwin T, Brookes NH. Morphological changes in keratoconus: pathology or pathogenesis. Clin Exp Ophthalmol 2004;32: 211-7 24. Reinstein DZ, Silverman RH, Raevsky T, Simoni GJ, Lloyd HO, Najafi DJ, Rondeau MJ, Coleman DJ. Arc-scanning very high frequency digital ultrasound for 3D pachymetric mapping of the corneal epithelium and stroma in laser in situ keratomileusis. J Refract Surg. 2000;16:414-30 25. Smolek MK. Holographic interferometry of intact and radially incised human eye-bank corneas. J Cataract Refract Surg 1994; 20:277–86. 26. Jaycock PD, Lobo L, Ibrahim J, Tyrer J, Marshall J. Interferometric technique to measure biomechanical changes in the cornea induced by refractive surgery. J Cataract Refract Surg 2005;31:175-84. 27. Grabner G, Eilmsteiner R, Steindl C, Ruckhofer J, Mattioli R, Husinsky W. Dynamic corneal imaging. J Cataract Refract Surg 2005;31:163-74. 28. Luce DA. Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. J Cataract Refract Surg 2005:31:156-62. 29. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures: a review and metaanalysis approach. Surv Ophthalmol 2000;44:367-408. 30. Lleo A, Marcos A, Calatayud M, Alonso L, Rahhal SM, SanchisGimeno JA. The relationship between central corneal thickness and Goldmann applanation tonometry. Clin Exp Optom 2003;86:104-8. 31. Morgan AJ, Harper J, Hosking SL, Gilmartin B. The effect of corneal thickness and corneal curvature on pneumotonometer measurements. Curr Eye Res 2002;25:107-12. 32. Stabuc SM, Hawlina M. Influence of corneal thickness on comparative intra-ocular pressure measurements with Goldmann and non-contact tonometers in keratoconus. Klin Montasbl Augenheilkd 2003;220:843-7. 33. Jarade EF, Abi Nader FC, Tabbara KF. Intraocular pressure measurement after hyperopic and myopic LASIK. J Refract Surg 2005; 21:408-10. 34. Svedberg H, Chen E, Hamberg-Nystrom H. Changes in corneal thickness and curvature after different excimer laser photorefractive procedures and their impact on intraocular pressure measurements. Graefes Arch Clinic Exp Ophthalmol 2005; 243:1218-20.

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35. Chang DH, Stulting RD. Change in intraocular pressure measurement after LASIK: the effect of the refractive correction and the lamellar flap. Ophthalmology 2005:112:1009-16. 36. Brandt JD, Beiser JA, Kass MA, Gordon MO. Central corneal thickness in the Ocular Hypertension Treatment Study (OHTS). Ophthalmology 2001;108:1779-88. 37. Goldmann H, Schmidt T. On applanation tonography. Ophthalmologica 1965;150:65-75. 38. Siganos DS, Papastergiou GI, Moedas C. Assessment of the Pascal dynamic contour tonometer in monitoring intraocular pressure in unoperated eyes and operated eyes after LASIK. J Cataract Refract Surg 2005;31:458-9. 39. Duba I, Wirthlin AC. Dynamic contour tonometry for postLASIK intraocular pressure measurements. Klin Monatsbl Augenheilkd 2004:22:347-50. 40. Kaufman C, Bachmann LM, Thiel MA. Intraocular pressure measurements using dynamic contour tonometry after laser in situ keratomileusis. Invest Ophthalmol Vis Sci 2003;44: 3790-4. 41. Pepose JS, Feigenbaum SK, Qazi MA, Sanderson JP, Roberts CA. Changes in corneal biomechanics and in intraocular pressure pre- and post-LASIK using static, dynamic and non-contact tonometry. Am J Ophthalmol 2006; accepted. 42. Laiquzzaman M, Bhojwani R, Cunliffe I, Shah S. Diurnal variation of ocular hysteresis in normal subjects: relevance in clinical context. Clin Experiment Ophthalmol 2006;34:114-8. 43. Congdon NG, Broman AT, Bandeen-Roche K, Grover D, Quigley HA. Central corneal thickness and corneal hysteresis associated with glaucoma damage. Am J Ophthalmol 2006;141:868-75. 44. Buzard KA. Introduction to biomechanics of the cornea. Refract Corneal Surg 1992;8:127-38. 45. Cabrera Fernandez D, Niazy AM, Kurtz RM, Djotyan GP, Juhasz T. Biomechanical model of corneal transplantation. J Refract Surg 2006;22:293-302. 46. Sikder S, Snyder RW. Femtosecond laser preparation of donor tissue from the endothelial side. Cornea 2006;25:416-22. 47. Kohlhaas M, Spoerl E, Schilde T, Unger G, Wittig C, Pillunat LE. Biomechanical evidence of the distribution of cross-links in corneas treated with riboflavin and ultraviolet A light. J Cataract Refract Surg 2006;32:279-83. 48. Petroll WM, Cavanagh HD, Jester JV. Dynamic threedimensional visualization of collagen matrix remodeling and cytoskeletal organization in living corneal fibroblasts. Scanning 2004;26:1-10.

13

Corneal Physiology

Wisam A Shihadeh Almamoun Abdelkader Herbert E Kaufman

Corneal Physiology

2

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Corneal Endothelial Transplant

Introduction The cornea forms the transparent anterior part of the eye. It protects the contents of the eye and serves as the major refractive element in it. The principal layers of the cornea are the epithelium, Bowman’s layer, stroma, Descemet’s membrane, and the endothelium (Figure 2-1). The basis of understanding corneal physiology is to address the physiological importance of the corneal epithelial, endothelial barrier and metabolic pump functions. If either limiting layer is compromised, the cornea will increase in thickness, become edematous and show decrease in transparency. Loss of the corneal endothelial barrier will result in a much greater corneal edema than with loss of the epithelial barrier. This chapter addresses the fundamentals of corneal physiology to provide a foundation for understanding the normal as well as the pathological corneal conditions.

pachometers reported an average corneal thickness of 520 micrometers.4-7 Nowadays, because of practicality issues, automatic acoustic pachometers became the gold standard for measuring the corneal thickness despite calibration uncertainties. The corneal hysteresis phenomenon is a result of viscoelastic dampening in the cornea, i.e., the tissue’s ability to absorb and dissipate energy (See also Chapter 1, Corneal Hysteresis and Biomechanics of the Normal Cornea). Studies have shown that subjects whose corneas exhibit low corneal hysteresis, which can be thought of as having a “soft” cornea, are probable candidates for a variety of ocular diseases and complications. This parameter can be measured by the Ocular Response Analyzer from Reichert (Depew, NY),8 which utilizes a patented applanation process. Hysteresis, as an indicator of the viscoelastic properties of the cornea, can be used to enable a more accurate tonometry measurement. 9-12 Clinical research has shown that this measurement may be a valuable tool for identifying and classifying conditions such as corneal ectasia, Fuchs’ dystrophy as well as glaucoma.13

Physiology of the Corneal Epithelium The corneal epithelium serves as a barrier between the environment and the corneal stroma. Through its interaction with tear film, it forms a smooth refractive surface on the cornea.

Barrier Function

Figure 2-1: The cornea is a layered structure consisting of the epithelium (E), Bowman’s layer (Bw), stroma (St), Descemet’s membrane (De), and the endothelium (En).

Corneal Thickness and Hysteresis Corneal thickness is one of the important parameters of corneal health. Pachymetry is a useful tool in both cornea and glaucoma practices. Thin cornea can indicate keratoconus or other ectasic diseases, whereas a thickened cornea usually correlates with endothelial dysfunction and secondary corneal edema. Applanation tonometry underestimates the intraocular pressure in eyes with thin corneas and overestimates it in thick corneas.1-3 The early measures of corneal thickness using manual optical

The barrier is formed as a balance of cell shedding; basal cell division and the renewal of basal cells maintain epithelial cells by centripetal migration of new basal cells originating from limbal stem cells. The epithelial cells move from the basal layer to the surface of the cornea, progressively differentiating until the superficial cells form two layers of flattened cells. These are encircled by tight junction known as zonula occludens which serve as a semipermeable, highly resistant membrane. This barrier prevents the movement of fluid from tears into the stroma and also protects the cornea and intraocular structures from infection by pathogens.14

Refractive Function The microvilli on the surface of the most superficial epithelial cells are covered with a glycocalyx that interacts with the mucin layer of the tear film. This leads to smoothening of the surface of the cornea forming the smooth optical surface required for clear vision.15

Corneal Physiology Active Na+ Transport Na+

Transport of between tears and stroma has been demonstrated in the rabbit and frog.16-22 In humans,23 when factors such as corneal resting potential and other ion transport systems are taken into account, the net flow of Na + across the epithelium is from stroma to tears. Nevertheless, the corneal epithelium contains an active Na+ transport system directed from tear to stroma. The transepithelial transport of Na+ is most likely secondary to the Na+/K+ exchange activity of the deeper epithelial membranes, which accounts for the directionality of transepithelial Na+ transport.

Active Cl- Transport

15

coupling of the chloride secretion to sodium transport. In addition to the transport mechanisms above, the corneal epithelial cells also contain a sodium-hydrogen exchanger and a lactate-hydrogen co-transporter. These transport mechanisms serve to regulate intracellular pH by extrusion of lactate and hydrogen (H+) ions. Corneal epithelial cells have beta-adrenergic receptors that respond to stimulation by activation of adenylate cyclase and increase cyclic adenosine monophosphate (cAMP) levels in cells (Figure 2-2). cAMP increases the conductance of apical chloride channels, stimulating chloride transport. In vitro, the ion transport can osmotically move water from stroma to tears. However, in vivo epithelial ion transport probably has a minor role in corneal deturgescence as compared to the endothelium.29

Cl- transport across the epithelium is from the stroma to tears.19, 24, 25 Net Na+ absorptive transport and net Clsecretory transport can occur simultaneously only under special experimental conditions. In the living eye, the epithelium generates an electrical potential of about 30 mV, tear side negative, and in this situation the net movement of NaCl across the epithelium appears to be in the stromato-tear direction.21 The presence of active Cl- secretory transport in the epithelium raises the possibility that the epithelium might participate in the regulation of stromal hydration in addition to its role as a diffusion barrier. This has been demonstrated in frogs in which Cl- secretion by the epithelium produces significant stromal dehydration.26, 27

Electrophysiology of Corneal Epithelium and Ion Transport The mammalian cornea generates a transepithelial potential of 25-35 millivolts. This high voltage is consistent with the low ionic conductance of the optical epithelial cell membranes and high resistance of the tight junctions of the paracellular pathway. About 50% of the short circuit current across the corneal epithelium is carried by chloride ions moving through the apical membrane channels into the tears. This current is due to ionic gradients set up by epithelial transport of sodium and chloride ions. Ouabain sensitive Na+ - K+ ATPase present in the basolateral membrane of these cells, pumps sodium ions from the cells towards the stroma.28 A sodium-chloride co-transporter, also located in the basolateral membrane, facilitates the influx of sodium down its electrochemical gradient carrying with it chloride ions, which then diffuse through channels in the apical membrane. This chloride secretion is blocked when the Na+ - K+ ATPase is inhibited by Ouabain, demonstrating the

Figure 2-2: Scheme for the neuroregulation of Cl¯ transport in the corneal epithelium. It is proposed that serotonin and dopamine can evoke the release of norepinephrine from the sympathetic nerve fibers in the cornea. In turn, norepinephrine may activate adenocyclase via the β-adrenoreceptor to increase cell levels of cyclic AMP and, finally, to increase the chloride conductance of the apical epithelial membrane.

Physiology of the Corneal Stroma The corneal stroma is basically an extracellular compartment with keratocytes and nerves. It measures around 470 micrometers thick centrally in a human adult cornea. Collagen fibers approximately 22 to 32 nm30,31 in diameter appear to run uninterrupted from limbus to limbus in flat sheaths or lamellae. Although the concentration of Na+ and K+ may collectively be 35 mEq/L higher in the stroma than in the aqueous humor,32 the combined activity of these ions, and hence their effective osmolarity, is probably less in the stroma than in the aqueous humor. This fact is important for control of corneal hydration. When

16

Corneal Endothelial Transplant

the stroma swells, the diameter of collagen fibrils remains constant; swelling takes place in the ground substance, which is rich in glycosaminoglycans, and leads to an increased spatial separation of collagen fibrils.33 The stroma is maintained in a relatively dehydrated state, in comparison to its ability to swell. The stroma consists of 78% water, which is equivalent to a ratio of 3.45 parts water (by weight) to 1 part solid material. The corneal stroma scatters less than 10% of normal incident light. This is an unexpected property of the cornea given the disparity in refractive index between the collagen fibrils and the proteoglycan matrix. Maurice34 proposed that corneal transparency is a consequence of a crystalline lattice arrangement of collagen fibrils within the stroma. He also proposed that the light scattered by individual fibrils and uniform diameter is canceled by destructive interference with scattered light from adjacent fibers; therefore light is scattered only in the forward direction. Such an arrangement requires that all collagen fibrils be of equal diameter (275-350A) and that all fibrils be equidistant from each other. However, to maintain this transparency, it is required that the distance between the collagen fibrils be less than one half the wavelength of visible light. On the other hand, Goldman and Benedek35 and others36, 37 recognized that refractive elements in tissues whose dimensions are small (5.0 D) can actually prevent functional success of the clear graft.17 Most patients can be visually rehabilitated with spectacles or contact lenses, however, if these modalities fail, several surgical and suture adjustment/removal options exist that may improve the patients vision. A number of factors are thought to contribute to the vexing problem of PK induced astigmatism, one of which is suture tension. The tension necessary to achieve a watertight seal of the full-thickness vertical incision of PK produces a gathering of the donor-recipient junction. This suture-induced tissue stretching yields a topographically “unsmooth” corneal surface that has higher and/or more irregular astigmatism than either the donor or recipient corneas had preoperatively. Irregularities of the trephination margins, donor/recipient thickness disparity and irregular suture technique are other important factors for high astigmatism after penetrating keratoplasty.17,18 Interestingly, the full-thickness vertical incision of PK apparently never heals to preoperative levels of strength. This is supported by the fact that many years after a corneal transplant, a seemingly stable cornea can rupture with trivial trauma leading to possible loss of the eye.19 This is likely because blood vessels are necessary to meet the nutritional needs of the wound healing process, but the cornea is avascular and does not adequately supply these needs. Furthermore, slow recovery of vision,13,20 risk of suture related problems,21 and other interface complications,22 are all untoward outcomes that plague corneal surgeons. The leading causes of corneal transplant failure are allograft rejection and endothelial decompensation.23-27 Since the cornea is usually avascular, the overall cumulative probability of corneal graft rejection at 10 years

Novel Approach for Corneal Endothelial Cell Transplantation is 21%.10 Most of these rejections occur within the first few years after keratoplasty. 28 Once a graft rejection is suspected, it can sometimes be blocked by the use of topical corticosteroids, but up to 49% are irreversible.29

Posterior Lamellar Keratoplasty While PK remains the gold standard for surgical corneal diseases over the past decade, posterior lamellar keratoplasty (PLK) has gained increasing clinical acceptance in the treatment of corneal diseases that involve endothelial dysfunction. This is groundbreaking for corneal surgeons because corneal endothelial dysfunction is the most common indication for PK in developed countries. 30-32 PLK selectively targets the corneal endothelium by replacing some contiguous circular portion of recipient endothelium, Descemet’s membrane and posterior stromal tissue with that of donor tissue. In this manner, the functional donor endothelial cells are able to appropriately transport fluid and solutes away from the stroma leading to an optically transparent cornea. PLK offers several advantages over PK. The concept of PLK was first introduced by Barraquer wherein the posterior corneal tissue was accessed by creating a hinged anterior corneal flap. From this conception, two separate PLK techniques evolved: one which improved upon this hinged-flap approach and another which changed the approach such that the recipient’s anterior chamber is entered from a smaller curvilinear incision. The hinged-flap approach was named microkeratome-assisted PLK (MAPK also called endokeratoplasty and endothelial lamellar keratoplasty) while the small incision approach was designated deep lamellar endothelial keratoplasty (DLEK). The MAPK approach is technically straightforward, wherein a microkeratome is used to create a hinged corneal flap exposing the underlying posterior stroma which is then trephined and transplanted. Though promising results have been obtained, case series have shown that microkeratome-assisted PLK carries the possibility of flap problems in addition to several of the same problems that plague PK and DLEK. The DLEK approach was developed by Melles et al33 who proposed a limbal incision approach to manually dissect a pocket in the midstroma and then trephine the posterior stroma. In 2000, Terry et al31,34 slightly modified Melles’ technique re-designing the instrumentation and performed the first DLEK in the United States. Due to the high difficulty level and other concerns, DLEK has been the subject of several investigations and modifications over the past six years.

379

A dilemma common to both MAPK and DLEK is postoperative mismatch between donor and recipient stromal thickness. Even if there is a perfect preoperative match between the thickness of the donor tissue and that of the recipient’s, a discrepancy may be measured with the passage of time. This discrepancy occurs because the thickness of donor tissue in corneal preservation media is greater than the thickness of normal physiological corneal tissue due to edema. Thus, if tissue deturgescence is not done before the donor disk is prepared, the donor stroma may become thinner than anticipated during the postoperative period when the donor’s endothelial cells begin to function. This thinning of the donor disk may cause same degree of corneal flattening which can potentially lead to poor postoperative refraction.35 Consequently, donor tissuedeturgescence is usually calculated preoperatively to lessen or possibly prevent unanticipated postoperative donor thinning. Another problem shared by PLK and MAPK is the issue of haze in the horizontal interface between the donor and recipient stroma.36 This distortion is thought to lead to a limited maximum average Snellen visual acuity that can be attained with these procedures. While PK can attain visual acuities of 20/20, there has been a relative dearth of these cases in MAPK and DLEK; the average highest acuity attained in these procedures has been 20/40 to 20/50.37-42 Terry et al. had only 1 patient who could be fully correctable with spectacles to 20/20 vision in this DLEK series of 98 cases.37 What is interesting is that there is not yet a means for measuring this optical distortion of the transplant interface, and thus several investigations are underway. Clearly, PLK has not yet been fully standardized and will continue to be modified and improved upon. Any procedure that replaces the endothelium ideally should accomplish the following goals: 1. Smooth surface topography without significant change in astigmatism from preoperative to postoperative. 2. A highly predictable and stable corneal power. 3. A healthy donor endothelium that resolves all edema. 4. A tectonically stable globe, safe from injury and infection. 5. An optically pure cornea. An additional sixth goal is technical ease with a reasonable learning curve such that a wide array of corneal surgeons can successfully carry out the procedure.34 The further the instrumentation and technique of any given procedure strays from those classically used by corneal surgeons, the greater the technical difficulty. With technical difficulty comes increased inaccuracies, which can yield intersurgeon variability in success rates and possibly unhappy patients. These 6 goals will be referred to throughout the remainder of the text.

380

Corneal Endothelial Transplant

Microkeratome-assisted PLK (MAPK) In microkeratome-assisted PLK, the opening incision is a hinged anterior stromal flap made with a microkeratome, similar to the flap made during laser-assisted in situ keratomileusis (LASIK) procedures. This flap approximates 130-480 μm in thickness and 8.5-9.5 mm in diameter. The flap is retracted with a flat spatula and it is used to expose the remaining cornea which is then trephined with a 7.08.0 mm diameter trephine to create a lamellar disk (posterior stroma, Descemet’s membrane and endothelium). This trephine diameter is dependent on the flap diameter and hinge width. This recipient lamellar disk carries with it the dysfunctional endothelial cells and is accordingly removed from the eye. Viscoelastic is then placed in the anterior chamber to stabilize it. The donor disk is then prepared (as will be described later) with a similar diameter trephine and positioned into place in the recipient’s stromal bed. The donor-recipient interface is self-adhering, but 8-16 interrupted absorbable or non-absorbable nylon sutures are used to secure the donor disk into place.43 In clinical studies, 50% of patients had a BCVA greater than or equal to 20/60 (range: 20/30-20/200) at a maximum of 1 year of follow up.43,44 One study reported a 1 month average spherical equivalent of -1.25 D.43 At twelve months, Ehlers et al44 found that the endothelial cell density was in the range of 1200-2300 cells/mm2. None of these studies had episodes of graft rejection or wound dehiscence. An important principle of lamellar refractive and LASIK surgeries is that in order to preserve the tectonic integrity of the cornea, a minimum of half or more corneal tissue must remain after the creation of a flap to prevent refractive instability. As this principle is abided by during the MAPK procedure, Behrens et al.32 showed that average intraocular pressures of up to 88 mmHg could be withstood in laboratory eyes when a 200 μm flap is used for the MAPK procedure. This stability renders the eye resistant to any subsequent injuries that may take place during patient’s daily activities. As microkeratomes have been enhanced, optical properties of corneal flaps have greatly improved. Interface scarring is almost absent after microkeratome dissection in LASIK. When compared with manual dissection, microkeratomes create a smoother donor-recipient interface and more uniform depth than manual dissection. Interestingly, although interface scarring is essentially absent, there is a limit to the maximum best average Snellen visual acuity seen in the small clinical series of MAPK. LASIK has proven that corneal dissection performed by

means of microkeratome produces an optical quality which is compatible with 20/20 vision, thus further investigations are needed to determine why the average maximum visual acuity is limited with MAPK procedure. In comparing PK to MAPK, it is widely accepted that both procedures accomplish goals 5 and 3 above. Both procedures also accomplishes goal 6 of technical ease with a very reasonable learning curve. However, as was indicated previously, PK falls short in terms of goals 1, 2, and 4. Although MAPK preserves the original central corneal surface, when flap and intrastromal sutures are put into place, some astigmatism is induced. The MAPK astigmatism is not nearly as high or irregular as that of PK because less sutures are used; thus, goal 1 is relatively (compared to PK, but not to DLEK as we will see later) achieved by the MAPK procedure. Similarly, literature data shows that the MAPK procedure achieves goal 2. Short term studies have indicated that MAPK also achieves goal 4, but large scale studies are needed to determine if goal 4 still holds true over a longer period of time. The MAPK method has less wound complications than PK because the combined anterior flap/posterior trephine wound is inherently stronger than the full thickness/ complete circumferential wound used in PK. Furthermore, the MAPK flap can be lifted for suture removal (with an argon laser to cut intrastromal sutures or a sharp blade after flap elevation), and although speculative, LASIK over the posterior button may correct residual refractive errors.45 In addition, the decreased amount of sutures used during this procedure improves postoperative visual recovery as well as imparts fewer suture-related problems as compared to PK. Clearly, MAPK specifically has potential benefits, mostly related to its conventionality. It allows for easy automated access to the stromal bed as well as classic trephination and transplantation, making the procedure relatively reproducible.46 This procedure also allows easy access for concurrent intraocular surgeries. Furthermore, with the ease and automation of surgery, MAPK is afforded a shortened surgical time. Some possible pitfalls are also incurred specifically with MAPK. This procedure produces a shift in postoperative average corneal power and astigmatism, similar to the transfer of stromal flattening to the corneal surface in LASIK. In MAPK there is a donut effect of donor-recipient interface due to sutures, which is transmitted to the surface flap. Surface sutures may also contribute to this astigmatism and high absolute power. As with LASIK, flap complications may also occur; epithelial ingrowth in the flap-graft interface can decrease the BCVA and has been reported in clinical cases of MAPK.47 As with LASIK, it is suspected

Novel Approach for Corneal Endothelial Cell Transplantation that corneal melting and micro/macrostriae are also possible adverse outcomes of MAPK.

Deep Lamellar Endothelial Keratoplasty (DLEK) In the DLEK method, a temporal or 12 o’clock curvilinear incision (either 9.0 mm or 5.0 mm) is made into the sclera, 1 mm away from and parallel to the limbus. A specialized spatula is then used to dissect from this incision into the stroma to a corneal depth of about 75-80% so as to create a lamellar pocket. Once the desired depth is reached, a special dissector is used to extend this lamellar plane over the whole cornea. A specialized 7.0-8.0 mm interlamellar trephine is then inserted into the created plane to begin cutting out a lamellar disk (posterior stroma, Descemet’s membrane and endothelium) from the posterior cornea. Once the trephine slightly enters the anterior chamber it is withdrawn from the lamellar plane and specialized interlamellar scissors are inserted to complete the trephination to a full 360 degrees. This completes the creation of the recipient’s posterior corneal lamellar disk which is removed from the eye through the temporal incision. The resultant recipient bed diameter is then measured with external calipers (7.08.0 mm). Air is then insufflated into the anterior chamber through a 0.5 mm limbal stab wound at the 2 o’clock position. This air functions to create a fluid free working space for careful insertion and manipulation of the donor tissue and to enhance self-adhere of the donor tissue to that of the recipient. The donor lamellar disk is then prepared (as will be described later) to match the diameter of the recipient bed and viscoelastic is layered on it’s endothelium. The donor disk is then either folded in a taco-like fashion (endothelial side inside) or placed endothelial side down on a specialized transfer spatula (decision depends on the size of the scleral incision), and inserted into the host anterior chamber through the scleral incision. The donor disk is positioned into the recipient’s posterior corneal defect using the transfer spatula. Minor adjustments to further fit the donor into the recipient bed can then be made with a Sinskey hook through the 2 o’clock limbal stab wound. In this method, no sutures are necessary to secure the donor tissue because it is self-adhering. The scleral incision is also selfsealing and sutures may or may not be used to close it. Several corneal surgeons have performed large volumes of DLEK procedures and sufficient data is available. On average, BCVA is 20/48 at maximum 2 years follow up.48-51 Melles et al reported that all patients operated on

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with the initial PLK procedure who did not have concomitant ocular disease have a BCVA of 20/30 or better; several have 20/20 (Netherlands Institute for Innovative Ocular Surgery, unpublished data). Similarly, these same studies also showed that DLEK yields minimal postoperative astigmatism; on average, astigmatism is 1.46 D. One study showed an average postoperative spherical equivalent of -0.369 D at 1 year.50 Postoperative endothelial cell density averages 1790.5 cells/mm2 at 36 months at a maximum of 3 years.48,51 As with MAPK, there appears to be a limit to the maximum best average Snellen visual acuity with DLEK; this is likely associated with postoperative interface opacities caused by stromal scarring from manual dissection.52 The lack of significant postoperative changes in astigmatism and corneal power is due to lack of disruption of the surface of the cornea in this procedure. This eliminates postoperative need for special corrective contact lenses which is seen with PK. Tectonic stability of the eye is in part provided by the continuity of the cornea and limbus. The DLEK technique does not disrupt this continuity and consequently appears to leave the cornea stable, though there are no long-term studies to confirm this. In Terry et al34 laboratory study stability of completed transplant was crudely tested by manually shaking and “pounding” on the globe which did not dislocate the donor disk from the recipient bed. This apparent increased stability renders the eye more resistant to any subsequent injuries that may take place during patient’s daily activities. The lack of sutures to secure the donor into place imparts some risk of graft dislocation with the DLEK procedure. Terry experienced a 6% donor detachment rate in his first 90 DLEK cases (Ophthalmology Times, May 15, 2005). Sano reported 1 donor detachment in his first 3 DLEK cases.49 Price had a 5% detachment rate in his first 101 PLK surgeries.53 In comparing PK to DLEK, it is widely accepted that both procedures accomplish goals 5 and 3 above. PK also accomplishes goal 6 of technical ease with a very reasonable learning curve, but it falls short in terms of goals 1, 2, and 4 (the reasons for this were discussed earlier). In contrast, DLEK leaves the corneal surface untouched and consequently achieves goals 1 and 2. Short-term studies have indicated that this procedure achieves goal 4, but large scale studies are needed to determine if goal 4 still holds true for DLEK over a longer period of time. Clearly, DLEK specifically has potential benefits, mostly due to its lack of corneal incisions and suture use. There are less suture-related complications as compared with PK

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because the sutures are placed on the sclera rather than the cornea. Most outstanding, there is negligible postoperative astigmatism or changes in corneal power because the corneal topography is essentially unchanged. In addition, because DLEK leaves the corneal surface untouched, the recovery time is considerably hastened such that the astigmatic and corneal power advantages are seen immediately after surgery. Moreover, unlike PK and MAPK, no postoperative suture removal visits are necessary which keeps visual fluctuations at a minimal during the healing process. However, some possible pitfalls are also incurred specifically with DLEK, which largely lie in it’s difficulty level. During the DLEK procedure, manual lamellar dissection, interlamellar trephination and scissor excision are difficult and tedious skills which could potentially traumatize the anterior chamber structures or the lens. Similarly, graft folding, and transplantation of donor corneal disk supported only by an air bubble are difficult surgical skills. The accuracy (and thus success) of these techniques is strongly dependent on the surgeon’s skills, making the procedure difficult to reproduce because the learning curve is quite steep. On the other hand, Seitz et al have demonstrated in a laboratory model the feasibility of using the femtosecond laser for DLEK to create both the opening incision as well as the stromal dissection, which may allow an easier clinical procedure.55 Other authors have also corroborated this with further experimental studies.56-58

Donor Tissue Preparation For both the MAPK and DLEK methods, donor tissue is prepared a few different ways using an artificial anterior chamber (AAC). In one method, the donor corneoscleral button is mounted on the AAC endothelial side down and a lamellar pocket is dissected similar to the DLEK recipient procedure. Then the button is turned epithelial side down and trephined at a diameter that is dependent on the measured diameter of the recipient posterior lamellar disk. The donor posterior disk is then separated from its anterior corneal layers and used as indicated in the respective procedures. In another method, the donor corneoscleral button is mounted onto the artificial anterior chamber endothelial side down and trephined to approximately 75% depth with a 9.0 mm diameter suction recipient trephine. A special blade and dissector are then used to carefully excise the 9.0 mm anterior corneal disk and to extend the trephinate depth peripherally throughout the remainder of the corneal diameter. The donor tissue is then removed from the anterior chamber and placed endothelial side up on a donor

punch block where it is trephined at a diameter that is dependent on the measured diameter of the recipient posterior lamellar disk. The donor posterior disk is then carefully separated from its anterior corneal layers and used as indicated in the respective procedures. Yet another method involves the use of a microkeratome wherein a flap is made (as in the MAPK method) and a posterior corneal disk is trephined and used as indicated in the respective procedures. Femtosecond laser preparation of donor tissue from the endothelial side has been done in the laboratory and shows promising results.57 The concerns with donor preparation techniques lie in ensuring endothelial cell survival and technical ease. All procedures have shown comparable endothelial cell survival and no particular method is favored over the other in terms of standardization. Note that viscoelastic is used liberally on the endothelium in all donor preparation techniques.

Descemet’s Stripping with Endothelial Keratoplasty (DSEK) A very revolutionary refinement of the DLEK procedure was realized when Melles et al proposed a means of stripping Descemet’s membrane from the recipient’s stromal bed so as to create a smooth recipient surface. This smooth recipient surface would theoretically decrease the interface haze that was incurred with the DLEK technique so that there would no longer be a limited average Snellen visual acuity.59-61 Price slightly modified Melles’ technique and was the first surgeon in the United States to carry out such a procedure.62 In this procedure, there are two methods available, manual dissection and microkeratome-assisted which has been coined Descemet’s stripping automated endothelial keratoplasty (DSAEK). During manual dissection, the donor is mounted epithelial side up on an AAC and dissected 8090% corneal depth over the whole area of cornea with blades.61 In the DSAEK donor preparation method, a microkeratome is used to dissect to a 300-350 μm stromal plane. In both techniques, the donor tissue is then placed endothelial side up on a donor punch block and trephined at 8.0-9.0 mm diameter. The donor disk is then placed in storage media for later use. After marking the recipient cornea with a slightly larger trephine than used for donor tissue, the recipient surgery is then performed. Through a 5.0 mm scleral tunnel, modified Price-Sinskey hook is used to score Descemet’s membrane in a circular pattern beneath the area of the epithelial reference mark. Then, a 45 or 90-degree Descemet’s

Novel Approach for Corneal Endothelial Cell Transplantation stripping instrument is used to strip Descemet’s membrane and endothelium from the recipient’s stroma within the scored area. Once fully stripped, Descemet’s membrane and the endothelium were removed from the anterior chamber with a forceps. The donor disk is then folded and transplanted into the recipient stromal bed as was done in the graft folding method of DLEK. DSEK offers all of the benefits of DLEK in addition to technical ease and less trauma to anterior chamber structures.62 This procedure achieves goals 1 through 6 of the ideal corneal transplant procedure and recovery of useful vision occurs within weeks. Furthermore, in DSEK, there is no problem with donor/host thickness mismatch because Descemet’s membranes with its endothelial cells are not subject to edematous changes and consequently are all approximately the same thickness. Six months after DSEK, mean manifest cylinder was 1.5 ± 0.94 diopters, basically unchanged from the preoperative value of 1.5 ± 1.0 D. Mean manifest spherical equivalent refraction was 0.15 ± 1.5 D, also statistically comparable to the preoperative value. Preoperative mean BSCVA was 20/100. Statistically significant improvement in BSCVA was noted at the 3-month and 6-month examinations; six months after DSEK, 62% of the eyes refracted to = 20/40 and 76% saw = 20/50.63 A further study in DSEK compared microkeratomedissected and manually dissected donor tissue.64 Mean refractive astigmatism was 1.5 D preoperatively and 6 months postoperatively in both groups. Spherical equivalent refraction did not change in the microkeratome group but increased by 0.66 D in the hand dissection group. There were 7 primary graft failures. The major drawbacks of DSEK lie in graft dislocation. It was theorized in the DLEK procedure that good adhesion of donor to recipient tissue is dependent on inherent adhesive quality of bare stromal surfaces when pressed together when assisted by intraocular pressure and stromal bed dissection,65 however, in DSEK there is no stromal bed (perpendicular cuts) thus dislocation may occur. The dislocation rate of DSEK has been quoted as high as 50%,63 making it a significant area of investigation for corneal surgeons. Several successful efforts have been made to improve this high dislocation rate.64 Using techniques to remove fluid from the donor–recipient graft interface ultimately reduced the detachment rate to