Temporal Bone Surgical Dissection Manual

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TemporalBoneSurgicalDissection'Manual by

RalphA. Nelson,M.D. HouseEanInstituteandotologicMedicalGroup,Inc.,Los Angeles

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Publishedby HouseEar lnstitute, Los Angeles

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House Ear Institute,256 South Lake Street, Los Angeles, California 9OO57O f 982 by House Ear Institute. All rights, including that of translation into othelanguages, reserved.No part of this manual maybe repr rced,str system, or transmitted, in any form or by any mp. photocopying, recording, oi' otherwise, without, publisher.

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Preface This voiume is intended as a basic introduction to surgical dissection of the temporal bone. The idea for this project originated with my initial attempts in my residency to dissect a temporal bone. I found no single source that would guide me in setting up a dissection bench and then drilling a bone in such a way that I could learn the anatomy from a surgical approach. I later found that not only was a surgical approach important, but also that an organized approach that would enable me to utilize each and every bone to its fullest extent was just as vital.

Drawings were selected over photographs in order to concentrate on specific details and to obtain the depth of field felt necessary to adequatelv depict the dissections. In certain areas, actual surgical procedures are used to better illustrate the anatomic detail and its applicability to the dissection. This manual is organized to introduce a student of the temporal bone to basic techniques and basic anatomy in a stepwise fashion, with each dissection building upon a previous one. All of the procedures in this manual can be accomplished in ten drilling sessions with a maximum of two temporal bones. Each chapter occupies a full two- or three-hour dissection period for the introductory student. Midway through the exercises, time may be taken for review. The volume has been designed for ready reference during drilling in the dissection laboratory. Again let me emphasize that this manual is an introduction to temporal bone dissection and in no way should be construed as a definitive treatise on the subject. I feel that it will aid those who have previously studied the temporal bone to review already acquired skills. The repeated use of these dissections will allow the temporal bone surgeon to become skilled and knowledgeable enough to complete actual surgeries with confidence. Rar.pg A. NELSoN, M.D.

Table of Gontents Intnoduction: Equipment, General Considerations . . 1 1. Basic Mastoidectomy 2 . Facial Recess,Epitympanum. 3. Postauricular Facial Nerve Decompression

......19 . . .26

4. Endolymphatic Sac and Extended F ac ialR e c e s s D i s s e c t i o n s.

......3 6

5. Tympanic Ring Remonal

......47

6. Postaurieularlabyrinthectomy 7. Int ernalA u d i t o r y C a n a l . 8 . M iddle Fo s s aA p p r o a c h . . . 9 . Middle Ear Dissection 10. Wall Dow n T e c h n i q u e s . Index

....49 ......5 7 ....6 7 . . .73 ......8 5 ... 9 1

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BenchlFig. 1l The bench is important to a temporal bone dissection laboratory. Although the dissection could be performed on any available table in any availabTeroom, a pleasant environment increases the likelihood of repeating the exercises. This is also increased by (1) the ability quickly to aid break down the equipment, (2) adequate gquipm€nt' and (3) ""?.tp use oi the same equipment and techniques that will be used in the operating room. Ideally, the temporal bone dissection bench should: (1) always be available and not be in use for Some other purpose, (2) be large and deep enough to allow prolonged sitting without undue discomfort, (3) have a readily accessible powei source for either an electrically driven drill o^ran air-driven driil, ( i have an easily cleaned and serviceably large trap for a suction apparatus so that prolonged dissections can be performed _ without interruptions for emptying the trap; (5) allow for the installation of an arm capable of supporting a microscope similar to the one used in the operating suite (unfortunately, the large floor stands used in the operaiing suJte are not flexible enough to be easily used around the a'oerageSench), and (6) contain enough storage so that the instruments to be-used are readily available for anyone to perform a dissection.

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Ghair The chair should be comfortable, similar to the chair in the operating suite, and adjustable for individual preferences for height-. Choose a chair with castors ior mobility so that the dissector can move closer or farther away from the bone for proper extension and support of the arms.

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Drill The drill ideally should resemble the equipm_ent in the operating suit-e' Any drilt with i variety of burr sizes will suffice, but reliability should be a cbnsideration. Burr shank support is necessary to prevent whippin$; the electrically driven Wullstein-Wpe (geared) drill with that support has been widety ana successfully used. Whatever system is used should allow enough too- for movement of the drill e-ngi-neon the bench so that both left- ind right-handed persons can use the bench. Both diamond and cutting bulrs should b-eavailable and cutting burrs should be sharp'

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The microscope should closely resemble the unit used in the operating room. Again, ihis i" to familiarize the dissection student with instrum-ents to be used in Surgery. We have found the Zeiss operating microscope to be durable and satisfactory.

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Equipment Suction-lrrigationlFig. 2l

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We advise the use of suction-irrigation in both the operating room and the bone dissection laboratory. Adherence to techniques that mimic operatin$ room procedure is important to instill confidence and familiarity in the dissector and thereby decreaseanxiety about actual operating room procedures. In the laboratory, the use of the suctionirrigator is not only to mimic operating room procedure, but also to insure that no bone dust or other debris obscures the fleld being dissected. A constant flow of cool moisture cools the drill and also keeps the burr free of bone dust, thereby allowing for better cutting edges.Any easily available water source can supply irrigation. It should flow simply by force of gravity, as it does in the operating room. We prefer suction tips of a simple configuration to avoid problems with handling. A straight House suction-irrigator has the least error in transfer from a suctionirrigator to a straight suction. The suction unit should be reliable and attached to a large, easily accessible,and easily cleaned trap.

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Although almost any device that stabilizes the bone for drilling purpos€s can be used, we have found the House-Urban s+ainleqs steel bone holder very satisfactory. It is easily cleaned, stable, and allows three-point flxation of the bone, which enables movement of the bone to an5r desired position for dissection. The bone cup should always be placed in a faqger pan that will contain spilled irrigation fluids and accumulated bone dust-

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Temporal bones cut from specimens should include all the mastoid air cells, a small portion of the squamosal part of the bone, the tympanic ring, and the petrous tip. Bones can usually be obtained during postmortem examinations without damage that would interfere with preparation of the body for viewing. Complaints from morticians can be avoided by fixing the facial bones to the cranial vault by wire sutures, as in repair of a facial fracture, and by filling in any defects in the base of the skull with plaster of paris to prevent leakage of embalming fluids. This manner of taking bones leaves no external deformity of the skull.

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Examinationand Orientation of Bones Before dissection, the temporal bone should be examined thoroughly in al effort to learn the external anatomy. Knowledge of this anatomy will ultimately be useful in identiffing external features during surgical exposure and also in gaining three-dimensional orientation. Once the external topography has been reviewed, soft tissue overlying the mastoid area should be removed. A11excess tissue that is not to be used in the dissection should be remor-ed from the bone before it ls secured in the stainless steel bone cup. The bone is then flxed in the stainless steel bone cup. It is oriented under the microscope for the standard surgical approach: u-ith the postauricular axis of the mastoid tip and the middle fossa floor in the horizontal plane and the temporomandibular joint away from the surgeon.

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GeneralGonsiderations

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The dissector should be comfortably positioned to prevent strain on extension of the arms or backache due to slouching. The chair should be adjusted to maintain a comfortable position at all-.times.A waterproof apron is recommended to protect clothing since dissection with copious a'mounts of irrigating fiuici will result in widesprbad moist bone dust. The microscope sho"utd bE adjusted to maintain constant focus regardless of the powei of magnification used. This is done by setting the proper distance between the eyes before starting and by focusing ",?"! "y.^ indiviaually at the highest power of the microscope-' You will then find that upon iurning to"lower powers you remain parfocal. Suction and irrigation shouldbe checked and regulated so that when suction is not useE, there is a constant stream of water, and when the thumb is placed upon the suction valve, the stream is directly shunted into the suction aiparatus. The drill should be tested to verify that the burr turns in the piop.t direction and that its cutting edges are not reversed. With all fqr-tip-.nt in proper working orderl the surgeon may now proceed with the dissection.

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Large-scale bone removal, such as on the cortex, is done u'ith the largest porEibt" cutting burr. The burr should be angled so that the flukes that 'are largest o.t ih. sides of the burr are directll- against the surface' Cuttin! with the smaller flukes at the tip of tl e burr results in more heat and sldwer cuttin$. Burring with the tip also obscures the area that is being dissected.

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A fundamental principle of temporal bone dissection is no blind dissection. Drilling should nevel proceed on the sense of touch alone. For better visualizatioil of underlying bone while it is being cut, bone dust and accumulated blood shouid be removed by adjusting the suctionirrigation. This is done by shifting the suction tip to various positions aroind the burr so that travel of the burr pushes irri$ation fluid and, bone dust into the suction tip. This keeps the drill burr cool, the field clear, and the flukes of the burr free of bone dust, which would otherwise interfere with good cutting.

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The handpiBce of the drill should always be grasped firmly' Torque from the drill, especially if it is air-driven, will commonly cause unwanted movement in the Leginning. Once the burr is at its full speed, larger cutting burrs tend t6 "r,-tn"because the burr chews ag-ainst the bone' This w-ill be accentuated by pressure or placement of the burr agailsta bone on which the buir flukes can catch. To prevent this difflculty' flp "f the burr so that the flukes cut perpendicular to the ridge' In "ttift particularly ti$ht areas, a diamond burr should be substituted and' 'preferably,-thJdri1 reversed so that it tends to run in the opposite direction.

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Temporal bone surgery is based upon laldmanks. Landmarks should cutting and should always be preserved until uf*"jo be identifi"d'b.fo.. at deeper levels. The most complete exp-osure 5 located othei landmarks are available landmarks' A general rule of i]' of assures the best utilization is This area' '*, u for a burr possible largest the $iven dissection is to use plunge could burr a where areas l* approaching especially true when cavity. a into plate or through dural .*Fj.,S*.r**

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\ Surface topography on exposure of the lateral surface of the mastoid bone before dissection is the external acoustic meatus (ear canal) anteriorl]-. the suprameatal spine (of Henle) at the superior posterior portion of the canal, and the suprameatal triangle (of Macewen) immediately behind the spine of Henle, where subperiosteal abscesses might form secondary to mastoid infection. The temporal line, forming a ridge continuous from the superior border of the zygomatic arch posteriorly onto the mastoid cortex, is the inferior limit of the temporalis muscle insertion. The lateral wall of the mastoid process (tip) is the point of insertion of the sternocleidomastoid muscle.

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External auditory canal Spine of Henle Macewen's triangle Temporal line Mastoid process

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The largest available burr and the largest available suction-irrigator should be used during initial cortical exposure. An absence of important structures in the cortex allows safe and rapid removal of this bone, conserving the surgeon's time and energr for more tedious and delicate dissection elsewhere. A high rate of irrigation is necessary to prevent overheating of the burrs and clogged burr flukes, which decrease cutting ability.

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The surface of the bone is wet with irrigation fluid from the suctionirrigator and the drill is applied to the mastoid cortex immediately posterior to the spine of Henle. A straight cut drawn along the temporal line posteriorly into the sinodural angle delineates the upper portion of rhe dissection. A second cut is made perpendicular to the first and rov-ard the mastoid tip. This cut is immediately posterior to the posterior cana-lrvall. The mastoid cortex is then removed in a s\-stematic fashion of saucerizatron. $-ith the deepest portion of penetration at the junction benreen the tu-o perpendicr:lar lines. This area behind the spine of Henle rrhich acruallr or-erlIes thr mlastoid antrum is ca-lled the suprameatal iangle of llacerl-en.

Landmarks: External auditory canal Spine of Henle Macewen's triangle Mastoid tip Temporal line

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\ To avoid a constricted dissection that prevents adequate angular visualization of deeper-lying structures, wide cortical removal should be completed before deeper penetration into the antrum. The mastoid cortex should be unroofed from the posterior canal wall back to and slightly beyond the sigmoid sinus and at an adequate distance into the mastoid tip. The posterior canal wall should be thinned so that the shadow of an instrument can be seen through the bone when the canal skin is elevated. This wide saucerization cannot be emphasized enough. Insufficiently wide saucerization is the single most common reason for- inadequate recognition of landmarks and awkward exposure during deeper dissection.

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External auditory canal Posterior canal wall Tegmen tympani Mastoid tip

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When adequate cortical removal has been accomplished, a kidney beanshaped cavity will result.'The inferior portion of the shape is formed by the mastoid tip below the sigmoid sinus and ear canal. The upper portion is above the sigmoid sinus, extending posteflorly into the sinodural angle and anteriorly into the zygomatic root. Anteriorly, the posterior bony canal wall will constrict the center of the cavity. This type of cortical removal is basic to all posterior approach procedures on the mastoid cavity. Again, be reminded that failure to perform this [pe of exposure while deepening the dissection toward the antrum can lead to a bothersome constriction of the cavity at more medial levels.

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Step 4 lFig. 6r: ldentificationof the MiddleFossaDural Plate Dtrlmg mastoidectomy, it is important to expose the middle fossa dural plate {tegmen tympani) for best possible access into the antrum and epin-rnpanic areas. This exposure is critical to a well dissected sinodural angle. u'hich provides visual access into the internal auditory canal area after labl-rinthectomy. Failure to locate the middle fossa plate usually leads to insufficient femoval of cells along the superior border and causes compromised exposure and decreased access to the epitympanum and hard angle.

Landmarks: Posterior canal wall Tegmen tympani (middle fossa dural plate)

\Ve have found that the best way to expose the middle fossa plate is to burr the cortex into the area of the linea temporalis until bone color changes indicate that the middle fossa dura has been reached. The dissection should round off the edge of the superior lip roughly paralleling the curve of the dura. This plate exposure can be followed into the antrum without lacerating the dura. At this stage of the dissection, do not use #24 burrs for extensive thinning of the plate, since larger cortical burrs tend to fracture the thin plate and create large dural dehiscences that mav become lacerated. Once the heavier bone is removed., thiniring cin be performed with a diamond, which results in less bleeding.

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When the middle fossa plate is well established, the surgeon locates the sigmoid sinus by drilling out the posterior portion of the mastoid cavity a sufficient distance from the bony canal wall. Well aerated mastoid bone is frequently found in neurotologic dissections, and it is possible to conlinue drilling in the posterior direction until the occiput is exposed. If a sufficient amount of bone has been removed posteriorly, the dissection may proceed deeper into the mastoid cavity in an attempt to find the sigmoid sinus. The sigmoid sinus generally appears in the posterior portion of the dissection as a blue discoloration of smooth dural bony plate. Dural plate of both the middle fossa and the posterior fossa tends to be somewhat uniform and is not cellular; therefore, one should inspect any area 6f smooth plate carefully for color changes that indicate an underlying soft structure, such as the sigmoid sinus. Because this plate can be quite thin, one proceeds with caution. Thinned dural plate usually can be identified by changes in the sound of the burr vibrating on it. Once the sigmoid sinus has been located under its plate, the area between the sigmoid and the middle fossa plate, the sinodural angle, can be fully evacuated of air cells. In dissection of the sinodural angle, penetration of the plate results in exposure of the superior petrosal sinus. This sinus lies immediately deep to the sinodural angle in its entire extent', and represents the posterior superior lip of the temporal bone where the middle fossa and posterior fossa meet. The si$moid sinus usually lies a few millimeters deep to the cortex in the mastoid ca\-iq-. This structure is the posterior limit of the standard mastoid dissection. It is not more widely exposed unless the posterior fossa is to be entered. Inferior to the sigmoid sinus lies the largely air-containing mastoid tip' Laterally, there is no danger of entering any vital structures, so the tip may be cleaned of air cells to gain better exposure in the jugular bulb area.

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Posterior canal wall Tegmen tympani Sinodural angle Mastoid tip

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The areas dissected and the structures identified thus become landmarks for deeper penetration into the temporal bone. The cortical landmarks of the external canal, spine of Henle, linea temporalis, and mastoid tip now become the mid-level landmarks of the middle fossa plate, sinodural angle plate, sigmoid sinus plate, mastoid tip air cells, and the thinned posterior external canal bony wall. Knowledgeable and safe penetration deeper in the temporal bone depends upon complete identification of these structures. The principle of temporal bone surgery is to move from one known landmark to the next, guided by the relationships between each. Only in this way can the temporal bone surgeon avoid the danger of being lost within a nebulous network of vital structures.

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The most important landmark at the next level of this dissection is the mastoid antrum. Although the size of this air-containing pocket varies considerably between pneumatized and nonpneumatized bones, one rarely encounters a totally absent antrum. The antrum lies immediately below the deepest point of penetration into the temporal bone posterior to the spine of Henle and the zygomatic root. Koerner's septum (Fig. 5) often lies deep to the mastoid cortex within the air cells of the well pneumatized temporal bone. It is a segment of the petrosquamous suture line representing the fusion of the squamous and petrous bones. It is usually a solid wall of nonpneumatized bone extending across the entire mastoid cavity separating the more superficial mastoid cortex (squamous) cells from the deeper (petrous) cells and antrum. It extends from the posterior canal wall at the tympanomastoid suture line and blends with the air cells in immediate approximation to the middle fossa plate, sinodural angle, and sigmoid sinus plate. This structure is often initially mistaken for the hard bone of the labyrinth and horizontal semicircular canal. These structures, of course, lie deep to Koerner's septum. After the septum is penetrated in the anterior superior quadrant of dissection, the true antrum will be seen as a very large air-containing cavity. By keeping the canal wall bone thin and avoiding the nearby middle fossa dura. frogressively deeper penetration will reveal the antrum. Normallv. the antrum can be identified as a larger air-containing space at whose bottom lies the basic landmark of the smoothly contoured. hard" labyrinthine bone of the horizontal lateral semicircular canal. Location of the horizontal semicircular canal (lateral canal) allows exposure of the fossa incudis, the epitympanum anteriorly and superiorly, and the external genu of the facial nerve medially and inferiorly. The hard labyrinthine bone extends posteriorly from the horizontal canal to the posterior canal, which lies anterior and medial to the sigmoid sinus. By removing cells between the horizontal canal and the sinodural angle, one encounters the hardest bone of the body, the socalled "hard angle," which is part of the otic capsule. Posterior to the labyrinth there may be some air cells in continuity with the petrous apex. Inferior to the posterior canal, the posterior fossa dural plate overlies the endolymphatic sac. More inferior to this area the bone of the mastoid tip may be thinned to expose periosteum of the digastric muscle. The digastric ridge runs in a posterior-to-anterior and lateral-to-medial direction, starting inferior to the sigmoid and ending at the stylomastoid foramen where the mastoid segment of the facial nerve exits the temporal bone. The periosteum generally is arranged in a semi-lunar shape and seems to turn superiorly to blend with the facial nerve sheath at the foremen. Finding the digastric is one method of locating the facial nerve in the mastoid cavity. Cells medially lead through the retrofacial area above the jugular bulb. After the mastoid portion of the facial nerve (discussed later) has been deflnitively identified, the continuity between the jugular bulb and sigmoid sinus can be followed along the smooth dural-type plate.

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Tegmen tympani Posterior canal wall Sinodural angle Sigmoid sinus Lateral semicircular canal

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The fossa incudis is most easily identified by removing bone in the zygomatic root overlying the antrum. The incus can normally be seen before actual exposure because of the light-bending refraction of the irrigating fluid (Fig. 9A). As the fluid is removed, the incus appears to disappear. As fluid fills the antrum, the incus once again appears. The facial nerve is normally located inferior and slightly medial to the horizontal semicircular canal by thinning the posterior canal wall bone and carefully removing bone in the facial recess area. The facial recess is delineated by the fossa incudis, the chorda tympani, and the facial nerve (Fig. 9E}). Generally, under increased magnification, a cutting burr is used during irrigation with copious amounts of fluid to remove bone dust and to provide optimal visualization of underlying bone. In this way, the blood vessels and sheath of the facial nerve will impart discoloration to the bone, normally pink in the living specimen and stark white in the cadaver. upon location of this discoloration, the sheath of the nerye can be followed inferiorly toward the mastoid tip and superiorly and anteriorly into the facial recess.

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Posterior canal wall Tegmen tympani Lateral semicircular canal Fossa incudis and incus Sinodural angle Digastric ridge Posterior semicircular canal

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We have now delineated a new set of landmarks that are used for further dissection of the temporal bone. The dissection thus far has created n-hat is commonly called a simple mastoidectomv.

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Topography The facial recess is a collection of air cells lr'ing immediatelr- lateral :c :ht facial nerve at the external genu. It occasionallv sen'es as a roule ior middle ear disease to extend into the mastoid area via cells other than the antrum. Cholesteatoma that frequently invades these cells u-ill be extremely difficult to remove via standard transcanal approaches. We feel that opening of the facial recess in any chronically diseased ear is of value additional avenue of mastoid aeration. This exposure also in providin{an allows better visualization of the middle ear cavity in chronic ear disease and exposure of the horizontal portion of the facial nerve during facial It is also the route to the round window for nerve deco-p.ession. insertion of ftre cochlear implant electrode. The landmarks used to expose the facial recess are the external genu of the facial nerve medially, th^e fossa incudis superiorly, the chorda tympani nerve laterally, and the tympanic membrane anteriorly and laterally.

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Landmarks: Posterior canal wall Horizontal canal Fossa incudis Facial recess cells fif p re s e n t ) External qenu

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{ { { Facial recess

Cell tract

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Zygoma tic root

Posterior canal i,

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llasroid tip

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Figure 11

Incus buttress

-Emo id s inus Horizontal

semicircular

canal

Step 3 lFig. 121=

t{

Gompletingthe Recess

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j

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With the new landmark of the facial sheath, the nerve is skeletonized distally along its descending portion in the mastoid and then medially as it foUows thJfloor of the faciat recess into the mtddle ear space. Smaller burrs will be necessary to accomplish most of the dissection in the facial recess since the recesi itself rarely exceeds two or three millimeters' Inferiorly the chorda tympani nerve is detected as it leaves the facial nerve. Dissection doei not sacrifice this structure. The chorda t1'rnpani nerve joins with the tympanic membrane anteriorly and laterally at the annulir edge; thus, following the chorda tympani generally prevents disruplion of the tympanic membrane.

Posterior canal wall Chorda tymPan Descending segment of facial nerve

Landmarks: Facial nerve Incus Fossa incudis Chorda tympani Stapes Horizontal canal

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= 5r

= = =

Long process of incus Buttress

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Fossa incudis

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Figure 12 Digastric muscle Sigmoid sinus

Tegmen Stapes Horizontal semicircular canal External genu Posterior semicircular canal

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suep4 tFig.131: The Middle Ear Throughthe FacialRecess T[ ith the facial recess fully opened, one can easily visualize the horizontal por-tion of the facial nerve, the lenticular process of the incus, the lncudostapedial joint, the capitulum of the stapes, the stapedial tendon, al-ld. n-ith proper angulation, the cochleariform process. The round u'indou' ma]'be easilr- identified inferior to the stapedial nandrrrarks. Superiorlr-. a burtreis of bone is presen-ed betg-een the short process of the incus a,nd rhe -aana-:ecess. This ls cornn-lon-lvterrned -the htl:t:ress - Dnllrng ihr:u{h :he b,ut--r-eii r-A1n r-

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7

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JugulanBulb/GarotidArtery Carotid artery (ghost)

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Jugulocarotid septum Jugular bulb (ghost)

Round window

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Figure 32 Sigmoid sinus

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Step 1lFig. 331:

G

Removingthe TympanicBone

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Removal of the tympanic ring is often indicated tt'hen there is a carcinoma of the conchal area of the ear or the external ear canal A procedure for cancer must be performed in €rn en bloc fashion u'ith the skin of the ear canal, the tympanic membrane, and the bony canal walls removed together. This can be done by a combination of some procedures described previously and a dissection through the area of the 4rgomatic root and the temporomandibular joint. A standard postauricular simple mastoidectomy is performed, followed by an extended facial recess dissection under the posterior canal wall but lateral to the facial nerve. More anteriorly, the lateral portion of the jugular bulb is the most medial part of the dissection. The lateral wall of the jugular bulb will eventually intersect with the posterior wall of the internal carotid artery in the anterior hypotympanum medial to the eustachian tube. Superiorly, the dissection started in the antrum continues through the epitympanum and zygomatic root below the middle fossa dura until the area of the temporomandibular joint is entered. The auricle or the concha is then removed en bloc with a sleeve of tissue representing the bony and cartilaginous external auditory canal, including the tympanic membrane and malleus. The incus will normally be attached to the malleus and it must be disarticulated from the stapes.

Lendrnan' ks:

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Zygomatic root Jugular bulb Temporomandibular joint

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tb

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Frgure 33

a

Epitympanum

Extended facial recess Jacobson's nerve

Promontory

Cochlearifonn process Eustachian tube

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step 1tFig. 341:

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d

Topography

i i

The postauricular labyrinthectomy is designed to eradicate labrrinthine Landmarks: vertigo by complete removal of the semicircular canals and all soft tissue Posterior canal wall of the vestibule. It is also the commonest surgical route to the internal Incus auditory canal. A complete simple mastoidectomy is performed with the Horizontal semicircular posterior canal wall left intact. The sinodural angle must be completely canal drilled out to provide adequate exposure of the area of the vestibule later. Posterior semicircular The middle fossa plate must be thinned completely to provide access to canal the superior semicircular canal. A large cutting burr is used to open the Tegmen tympani sinodural angle posterior to the labyrinth. In this way, the flukes on the side of the cutting burr may be used to better advantage in removal of the Facial nerye Sigmoid sinus extremely hard labyrinthine bone anterior to it. Sinodural angle

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Posterior canal wall Labyrinth

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Figure 34 Tegmen

Mastoid cortex

Sigmoid sinus

Sinodural angle

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srep 2 tFig.351: Openingthe SemicircularGanals Qr" crrrring into the labyrinth, one should be able to fenestrate the hrorizontal semicircular canal and, by enlarging this dissection, unroof dhe posterior canal and the anterior portion of the superior canal as well. The fenestrated canals are followed in their extent to provide continuing -a:1i,r:rarxis" The surgeon should eventuallv have a dissection that showJ :he ou'-eentarionof eaLchcanal to its neighbors. Anterior to the horizontal canal. tht esrernal genu and honizonral portions of the facial nene can be skeletonized. Preserradon ot the anterior n-ell of the horizontal canal *r.t-es as a protection for these pordons ot the facial nen-e until further :}::mrng or-er rhe facial is n'ecessary- ro epos€ more of the r-estibule.

Landmarks: Semicircular canals Facial nerve Incus Tegmen Sigmoid sinus

kcu':se r:l{aaon is necssa-r1: to remove t?st quantities of bone dust and ailso to prmqn[ trictirnat burning of nearbv bone. The largest burr that is emfortabrle sburld be used to enable qrrick bone removal, decrease fuictional heat- and prerent burr clogging.

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a a G

Horizontal semicircular canal Posterior canal

Incus

Facial nerve

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Figure 35 Posterior semicircular canal

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Superior semicircular canal

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Step 3 tFig.361:

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Followingthe SemicincularGanals

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The semicircular canals must be followed as they are highways into the Landmarks: vestibule. Transection without following will leave -5nake eyes- and create Facial nerne confusion about which direction to continue the dissection. Try to Tegmen develop a three-dimensional concept of the canal planes. Semicircular canals Sigmoid sinus Superiorly, the superior semicircular canal arches in a posterior-medial direction underneath the middle fossa plate. This canal is traced posteriorly and medially until it becomes the common crus in its junction with the posterior semicircular canal. The subarcuate artery usually penetrates the hard labyrinthine bone in the center of the circle inscribed by the superior canal. The posterior canal is then followed inferiorly and anteriorly under the descending portion of the facial nerve. Look for the endolymphatic duct as it courses from its opening in the medial wall of the vestibule next to the common crus. It travels posteriorly in the medial wall of the common crus and then curyes inferiorly and laterally as it passes under the posterior canal to enter the endolymphatic sac in the posterior fossa dura. There is frequently a bony operculum or small bony cap overlying this portion of the sac as the duct enters it. The sac often causes confusion by appearing to represent torn dura until the true nature of the soft tissue is recognized. The endoh-rrphadc duct appea,rs as a pearly white, thread-like discoloration ire the ha-rd labryrinthlne bone-

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Followingthe SemicircularGanals Facial nelve

Horizontal semicircular canal

Posterior semicircular canal

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Figure 36

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subarcuate a::ren' Superior semicircular

canal

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Step 4 tFigs. 37, 381:

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Openingand Gleaningthe Vestibule

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The surgeon follows the common crus forward into the vestibule and opens it widely. The bone over the facial nerve is then thinned well to provide good visual access to the vestibule. All the semicircular canals are completely connected, and any soft tissue is removed from the vestibule and semicircular canals to eliminate any remaining vestibular function. Before removing the soft tissue, note the maculae or ttre utricle and saccule. Note the spherical recess of the saccule in the anterior portion of the vestibule and the elliptical recess for the utricle posteriorly. Frobe the -common opening of the endolymphatic duct at the anterior end of the crus. Palpate the stapes footplate from its medial side and notice its proximity to the saccule and utricle. The postauricular labyrinthectomy is now complete.

Landrnarksi

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Facial nerve Tegmen Semicircular canals Elliptical recess Spherical recess Footplate of stapes Endolymphatic duct

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Membranous labyrinth remnant !-

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Figure 37

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gp,emimg and Gleaningthe Vestihule Postrrftrr

n-nal wall

Ampulla of posterior semicircular canal Ampulla of horizontal semicircular canal Ampulla of superior semicircular canal

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Figure 38 :iic durat ndrotq-cotr frn"n

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Step 1lFig. 391:

R

Topography

r.l

rl I

The simple mastoidectomy and postauricular labyrinthectomy are the first two stages of the approach to the internal auditory canal. This new and much deeper dissection requires that all the previous steps be accomplished properly to provide wide apcess for adequate visualization and working room deep within the petrous bone. During labyrinthectomy in this procedure, the ampulla of the superior semicircular canal is preserved as a landmark to find the end of the superior vestibular nerye, Bill's bar, and the facial canal. In approaching the internal auditory canal, one must remember that the medial wall of the vestibule represents the lateral wall of the internal auditory canal fundus, where the nerves enter the inner ear structures. Therefore, minimal bone removal on the medial wall of the vestibule will expose the internal auditory canal. Posteriorly, at the posterior fossa dura, the route to the porus acusticus is much deeper because the internal auditory canal is slanting away from the dissector. The plane of the canal, in an anteriorposterior direction, is roughly from the external genu to the sinodural angle. The superior border of the internal auditory canal extends from the facial canal to sinodural angle. The inferior border of tJ e internal auditory canal is at a point approxi-mating the junction of the e-.rternal genu and descending portion of the facial nen'e and parallels the super-ftor border posteriorll' to its junction with the pcterior f6sa-

Landmarks:

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Facial newe Vestibule Tegmen Posterior fossa plate Superior canal ampulla

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ra 3 3

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A sharp pick is often used to perfor.ate ttre very thin bone of the internal auditory canal fi.rndus in the area of the elliptical recess. Frequent\'a blue-line discoloration appears.

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Inferior border of internal auditory canal Vestibule

3

(

External genu of VII

t

t]

t / 2 ra

Tegmen

{/ ta

3 \a-

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Sigmoid sinus

ts Figure 39

Subarcuate artery 58

Sinodurd

angle

Superior border of internal auditory canal

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RemouingBoneover the Internal Auditory Ganal

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h 7t

I

7 t:t

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-ds seen from above, the bone to be removed for exposure of the internal auditory canal forms a wedge or triangle. One side is the roof of the internal auditory canal and the medial portion of the dissection; the second side is the floor of the labyrinthectomy dissection; and the third side is the posterior fossa dura as it extends between the other two sides. The thicker posterior portion of the bone is usually more difficult to remove because it is composed of hard labyrinthine bone. The internal auditory canal can be recognized by color changes in the bone. In the cadaver specimen or the living surgical specimen without a tumor, the color of the canal will be dark blue as in blue-lining of any hollow structure. In the specimen with a tumor, the color is usually ruddy from the vasculariq' of the tumor and overlying stmctures.

Landmarks: Tegmen Sinodural angle Vestibule Ampulla of superior canal Posterior fossa plate Sigmoid sinus Internal auditory canal

Posteriorh- in the sinodural anglg, the blueline of the superior petrosal sinus will show as it runs along the posterior superior border of the temporal bone from the apex to its entrance into the transverse sinus.

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't ?l

I -t t

Facial nerve

Superior semicircular canal ampulla Superior border of internal auditory canal

Vestibule

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Figure 40 ;osterior fossa dural plate "nal auditonr canal

Bone wedge

Subarcuate artery

-.-

Step 3 lFig. 411l.

d

Blue-liningthe Internal Auditory Ganal

rt

Skeletonization of the internal auditory canal for dissection purposes is accomplished through removal of bone for 180 degrees around the posterior portion of the canal extending from the area of the fundus to the porus acusticus. Such extensive surgical exposure is to prevent the bony overhang that would make work within the canal difficult. Blind dissection under bony ledges carries the possibility of injury to a nerve or blood vessel within the canal. For such extensive exposure, the surgeon must remove all bone superior to the canal extending from the area of the facial canal and petrous apex to the superior midpoint of the porus acusticus. This dissection is usually performed with small diamond burrs and a small suction-irrigator, which prevents the accumulation of bone dust from obscuring the underlying dissection area. The middle fossa dura is located and followed in an ever-deepeningtrench, with preservation of a thin layer of bone over the superior portion of the internal auditory canal. At the deepest portion of this dissection, which is at the superior lip of the porus acusticus, exposure is extremelv limited. With patience and persistence, however, the posterior superior portion of the internal auditory canal can be well exposed. With the posterior portion of the poms acusticus well defined at the juncfion of the pcterftrr fmsa dura and internal auditory canal" the inferior border of the intcrnal auditor-r canal can be better defined. A trench with the jqgular hrlb inferirt-v and the internal auditory canal superior\r is c.onstnrcted between the posterior fossa dura and hard labyrinthine bone anteriorly. This dissection is aarried anteriorly until a small white discoloration in the bone appears. This represents the cochlear aqueduct which, when entered, will often release cerebrospinal fluid. Extension of the dissection anterior to the cochlear aqueduct will involve the jugular bulb and the ninth, tenth, and eleventh cranial nerves. If proper removal of bony covering has occurred throughout the exposure of the internal auditory canal, there should now be an eggshell-thin coverin$ of bone from the fundus of the canal to the area of the jugular bulb inferiorly, to the superior petrosal sinus and middle fossa dura superiorly, and to the sigmoid sinus posteriorly. This bone may be removed for a wide exposure of the internal auditory canal and the posterior fossa dura. For procedures involving section of the vestibular nerye, such edensive exposure is not necessary. However, we encourage wide extrrosureto eliminate blind dissection through a small keyhole into the internal auditory canal fundus. Limited exposure also precludes the abililv to deal with the potential problem of a bleeder from the sectioned nerve.

e{

e LAndrnarks:

Superior petnosaf sinuLs Superior cpnal arnpulla Vestibule Facial nerye Cochlear aqueduct Tegmen Jugular bulb Sigmoid sinus and posterior fossa plate

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Blue-liningthe Internal Auditory Ganal Internal auditorv canal "blue-lined" Superior semicircular canal ampulla

Facial nerye Jugular

egmen

bulb

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Figure 4l

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Sigraoid sinus

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step 4 tFig. 42lz

d

Openingthe Internal Auditory Ganal

c

The bone removed, the exposed dura of the internal auditory canal is slit along the long axis of the canal at its inferior border. This precludes injury to the facial nerve superiorly in the occasional cases of variation (sometimes a tumor pushes the nerve posteriorly). Within the internal auditory canal, the vestibular nerves are both posterior, whereas the facial nerve is anterior superior and the auditory nerve anterior inferior. To locate these structures from a lateral dissection approach to the internal auditory canal, we use the facial nerve as the principal landmark. The facial nerye is located anterior to the superior vestibular nerve. Therefore, we ordinarily preserve the ampulla to the superior semicircular canal, which makes for easier identification of the superior vestibular nerve. A diamond burr is used to penetrate the medial wall of the superior ampulla. The thinned bone then exposes the superior vestibular nerve as it enters the labyrinth at that point. If the bone has been removed from the superior border of the internal auditory canal far enough anteriorly, the facial canal may be seen descending through its labyrinthine portion from the genicr-rlate into the internal auditory canal.

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Opemingthe Internal Auditory Ganal

a 4 r'a

Transverse crest

Superior vestibular nerve Superior semicircular canal ampulla

Facid nerve

Tegmen

Jugular bulb

-t

n lt?t

-

-t tt

-3

a rt rt rt rt rt

Figure 42 {

SingElar

Porrrs acustlsus

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Pm;terior fossa plate

rlretree

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Internal

auditory

Inferior vestibular neFne lt rt

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Step 5 lFigs. 43, 441=

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ldentificationof the Nerves When the superior ampulla is preserved, its medial wall represents the last remaining bone over the superior vestibular nerve at its termination in the ampulla. Wtren this bone is removed, a brush-like ending of the nerve is encountered. After the superior vestibular nerve is identified, a one-millimeter hook may be inseited deep to the vestibular nerve directly anterior and medial to the superior vestibular nerve to palpate the shelf of bone that separates the superior nerve from the facial canal. The superior vestibular nerve occupies a recess medially. The recess may be safely probed and the posterior edge of it palpated. Bill's bar, as this shelf of bone is called, represents the posterior wall of the fallopian canal. With identification of the bar and the fallopian canal, the superior vestibular nerve may then be avulsed from its attachments in the ampulla area. When gently lifting the superior vestibular nerve, one should carefully look for the facial nerye as it exits the fallopian canal into the internal auditory canal. Vestibulofacial anastomoses that occur here should be carefullv sectioned. Directly inferior to the superior vestibular and facial nen'es. a bonlprominence protrudes into the fundus of the internal auditory canalThis shelf of bone. called the transverse crest, falciform crest- or cristadivides the canal into superior and inferior portions. It is €sident during exposure of the fundus and allows easy identification of the inferior vestibular nerve laterally and the auditory nerve medially. The singular nerve to the posterior semicircular canal is an offshoot from the inferior vestibular nerve within the internal auditory canal. This method of vestibular internal auditory canal exposure is used for translabyrinthine approach for acoustic neuroma nerve sections and the translabyrinthine removal. .If bone is removed from the fallopian canal starting at Bill's bar, the facial nerve may be decompressed to the geniculate ganglion and internal genu a few millimeters anteriorlY.

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Landmarks: Falciform crest {tranverse } Superior vestibular nsrvre Bill's bar (vertical crest) Facial newe

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Transverse crest

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Recess of superior vestibular nerve

Inferior ves

Bill's bar

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Figure 43 Porus acusticus

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Facial nerve

Cochlear nerve $ingular

nen'e

Recess of superior vestibular nerve Fallopian

canal

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Bill's bar

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Figure 44

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Superior vestibular nerve Inferior vestibular newe

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step 1tFig. 45A, Bl:

d ,I

Middle FossaTopography In the middle fossa disseetion of the temporal bone, placement of the bone within the bone cup usually presents some difficulty. The dissector should visualize the bone as if looking through a craniotomy window that has been inscribed in the squamosal portion of the temporal bone (Fig. 45A). This is from the vertex of the head (the surgeon sits at ttre headof the table, not at the side as in the standard postauricular approach). The bone is positioned so that the surgeon looks direcily down upon the middle fossa floor. Then dura should be stripped back from the roof of the temporal bone, as in the surgical procedure for this approach. The topography should be studied (Fig. 458). As dura is reflected away from the squamosal portion of the bone, the floor of the middle fossa ii revealed. Laterally, this is the tegmen overlying the aerated mastoid portion of the bone and epitympanum. Anteriorly beyond the epitympanum, thin bone covers the eustachian tube. More anteriorly, the middle meningeal artery is the flrst major landmark to be encountered in elevating the dura. The foramen spinosum is located where the flat lateral portion of the squamosa intersects with the floor of the middle fossa. Posteriorly the floor of the middle fossa drops abruptlv into the posterior fossa where the tentorium intersects \Aith the temporal bone. The _ superior petrosal sinus is located in the re{lecred dr.ma-}{edral to dre aerated epitympanic bone, the arcuate eminence of the srrtrtritr cqnal is the second major landmark- Anterior to the arcuatc anirsroe rrxl approximately one centimeter medial to the mitrh rrnri4g'ral iltcry is the greater superficial petrosal nerve running anterior to pctcrir into the facial hiatus, where it urill join ttre facial nerve and gpnicrrlate ganglion. This key landmark allows the surgeon to locate the geniculate ganglion for unrooflng the facial nerve. It eventually will become the prime landmark in the middle fossa dissection.

Lanrlrn"arks:

Squanosa Arcuate erninence Middle meningeal artery Superior petrosal sinus

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tiliddle Fossa Topography Styloid process

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External auditory canal

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llastoid tip

Tegmen

Foramenspinosum process | "rgomatic

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Figure 45A Temporoparietal suture Arcuate eminence Superior semicircular cenai I ghostl Supeu-ior petrosal

sinus grosFe

Sru.rLperiorresfbullar

rrenre Ighost j

Petrous apex Greater superflcial petrosal nerve hl ea {ghost) Facia_lnen'e lghostJ Cochlea.r nerre Lghostl

Inferior resdbular nerve tghosc] Arcuate erninence

Forarnen spinosum

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Figure 45B 69

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Step 2 tFig, 46A, B, Gl

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SuperficialDissection

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The first portion of the dissection involves unrooflng the geniculate ganglion and tracing the facial nerve from the internal genu into the middle ear space. The greater superficial petrosal nerve should be followed to the geniculate ganglion (Fig. 46A). The geniculate ganglion will be superflcial; therefore, a minimum of bone should be removed. The largest diamond burr is used for this procedure with copious amounts of irrigating fluid to remove the bone dust. At the geniculate ganglion the nerve turns laterally, posteriorly, and inferiorly into the epitympanum. Although the nerve can be followed for quite a distance through this approach, the cochleariform process is the normal limit of dissection. Lateral to the facial nerye, the ossicular chain will be noted with the head of the malleus particularly prominent. Because the ampullated end of the superior canal is close to the medial portion of this dissection, one must be careful not to fenestrate the canal through the hard labyrinthine bone. The canal is usually bluelined for definitive identification. From the geniculate ganglion, the labyrinthine portion of the nerve may be followed posteriorly, medially, and inferiorly. past the ampullated end of the superior canal as it turns deep into the anterior superior ponion of the internal auditory canal (Fig. 468). With adequate thi.rrn,me of bone o -er the medial end of the facial nen-e. one can identiLS-the shelf of borae ttrat separates the superior vestibular nen:e as it enters the superior ampulla next to the facial nen€. This [s Bill's bar tFig. 46C]-

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Topography or a Middle ear procedures can be practiced throu$h a canal approach patient'^ ttre in as bone as easily fostauricul,a, approach on the temporal wifh actual operating t*T surgeon the familiarize will ih"". techniqu^es *o:tpio".a,tt. ..t& in"r."se dexterity in procedures--that.are among qt in bone temporal ilmanAing of the otologist's armamentarium. With the placed a speculum the surgicil position, tfre dissector o-peratesthrough the within the external auditory canal. The skin in the canal and and does leather like tympa.ri" membrane of prelerved bones tends to be are bones not simutate the feel of st21- 2[L3f , 51, 58, 88; Fig. u-r& r7-lA,21 2Il,39 xt. 23. 30'3r' 51; Fful. horiznntal gls|Ff 14,19 internal$nufl. m labyrinthine- 34- fr) mastoid s€Ecnt. 14. Uf. 3f , 34 3l- 34 middle ear s4mL sheath, 16,2l-t2" 2!1.3i135: FIE 2L,24 tympanic scgmcd- f!+ f5. L7, L9,23 facial recess, f6, 2[>25' 283f . 4f -43, 8u; Fig. 10-16, r8,za.gt (extended), se ffi frcial reccss facial ridge, 86, 88 crEst falciform crest, s€e lrarwrcrs Fallopian canal, 38,81,88: Q' 14: medial porous, 34 foramen spinosum, 68; Fig; 4!iltr{!B footplate, stapes, Fig. 55 fossa incudis, 14, 16, 20, 2a,3(), 41. 84: Fi11 8-10, 12, 20, 26-29, 3r, 57

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incudomallear joint, 82; Fig. 14 incudostapedialjoint, 23, 80; Fig. 13,52 incus, 16, 23-25, 30,48, 80, 82; Fig. 11, 13-14, L9-21, 23,28-31, 34-36, 464, 46C, 54, 58-59 body of, 26; Fig. 18 buttress, 3O, 41; Fig. 10-12,29-30 lenticular process, 23; Fig. 14, 19 long process,3O; Fig. L2,18,52 replacement prosthesis, Fig. 56 short process. 23 internal auditory canal, ll, 50, 58-64, 70,72; Fig. 39-44,468 "blue-lined", 60, 70, 72t Fig. 4l inferior border, 6O, 62; Fig. 39-40 posterior superior, 6O superiorborder, 62; Fig. 39-40 dura, Fig. 42 internal carotid artery, see carotid artery internal genu offacial nerve, 64 J

manubrium, Fig. 16, 52-53, 55 mastoid. 8, 70, 88 antrum, see antrum cavTty,12,21, 4l cortex 9-1O, 14; Fig. 5,34 process, 8; Fig. 3 t ip , 9 - 1 2 , 1 4 , . 1 6 , 3 9 , 4 4 , 8 8 ; F i g . 4 - 7 , L O - L I , 18, 24-25, 34, 454, 44, 57 meatoplasty, 86, 88 middle ear cavity,20,72 dissection. 74-83: 48-56 middle fossa. 12, 34 approach, 68-72: Fig. 45-47 dural plate (tegmen tympani), ll-12, 14,25, 48, 50, 52, 60, 88; Fig. 6 middle meningeal artery, 68, 7O, 72; Fig. 464 my'ringoplasty knife, Fig. 55

1{ nasopharynx, 72 ninth cranial nerve, 6O

o occiput. l2 operculum (bony), 52 ossicles. 25, 82: Fig. l7 ossicular chain, 3O, 70, 80 otic capsule, 14 oval window, 82; Fig. 56 overlay graft technique, 76 P

Jacobson's nerve, 80; Fig. 15, 33 j ugularbulb, 12 , 14 ,38 , 4l- 42, 44, 48, 60; 3r-33,4L-42 jugulocarotid septum, see crotch jugulocarotid spine, see crotch

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K Koerner's septum, lO, 14; Fig. 5 L labyrinth, 5O-51, 62; Fig. 34,37 labyrinthectomy, 50-55, 68,74,86; Fig. 34-38 transcanal, 82 labyrtinthine bone, 14, 39, 50, 52,59-60,70 lateral graft technique, see overlay graft technique lateral sinus, see sigmoid sinus le:iticular process of incus, see incus ligament of short process, Fig. f 4 linea temporalis, I 1-12 long process of incus, see incus long process of malleus, see malleus

T Macewen's triangle, 8, 9; Fig. 3-4 maculae of utricle and saccule, 54 nalleus, 48, 70,78, 80, 82; Fig. 46A, 46C, 51, 54, 58-59 handle (mucoperichondrium), see malleus, 'nanubiium her.l 24: Fig. f4-15 i-'-r:,Er ccess, 26 short Process, Ffu' 49

peritubular cells, Fig. 16 petrosal nerve (superficial, greater), 68, 70; Fig. 45A-458, 464' 46C, 47 petrosal sinus inferior, 44 superior, 12,59-60,68 groove, Fig. 45A petrosquamous suture line, 14 petrous apex, 14, 26, 60, 72; Fig. 45A, 47 petrous bone, 58 porus acusticus, 58, 6O, 64; Fig. 4L-43,47 posterior approach, 88 posterior canal wall, 9-12, 14, 21, 25,28, 38-39, 4t, 48,50, 88; Fig. 5-r3, 14, L7-2L, 24-28, 3r,34-35, 38, 57-6r posterior crus, Fig. 52 posterior fossa, 12, 68, 72 dural plate, 12, 14,38, 41, 52, 58-6O;Fig. 40.42 promontory, 23, 8O; Fig. 13, 15-16,33,53 pyramidal process, 23; Fig. 13, 15-16, 29, 32,6L

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Y v'asculirr strip. 76: Fig 49,5f vertical s€st- s€e Bill's bar v.estibular nerrte; 6O, 62. 8[ inferior, ltg. +%aa superior, 6.2, 64,72: Flg. 4244,4dC' vestibule, 50-54, 58, 82; Fig. 39-41"t6A

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T tegmen tympani, LO-LI,24,30, 48' 6O, 88; Fig. 5-6, 8-10, 12-16, 19, 23, 25-27, 30' 32-36. 39, 4L-42, 45A, 57-58' 60 temporal line, 8-9; Fig. 3-'1 temporalis muscle, 8 temporomandibular joint, 48' 78; Fig. 33 temporoparietal suture, Fig. 45A tensor ty'rnpani, canal, Fig. 16, 33 tenth cranial nerve, 6O tentorium, 68 transcochlear approach, 41 translaby'rinthine approach, 4l' 64 transverse crest, 64: Fig' 42-44 transverse sinus, 59 tympanic membrane, 20,22' 4l-42' 4a.74' 76' 80, 82; Fig' 17,30 tympanic plexus, 80 tympanic fir:g, 44,48; Fig' 33' 48 tympanic Sleeve,Fig. 33 ty"rnpanomastoid suture, L4' 76' 78; Fig;. 5O tympanomeatal flaP, Fig. 52-54 qrmpanosquamous suture, 76, 78: Fig. 50

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