Within the Temporal Bone · Chapter 14 · Part IV · Inner ear and skull base
The Labyrinth and Endolymphatic Sac Region
Preserve the relationships first; open the labyrinth only by plan
Bench mode: one step per screen, large type, screen kept awake. Tap, use the arrow keys or say “next”.
- Specimen
- Adult wet bone after a complete mastoidectomy with sinus and tegmen defined; side recorded
- Bench time
- One session to skeletonise; the destructive stage is a separate, supervised session
- Burrs
- Medium and small diamond; small diamond to follow a lumen; no cutting burr on the capsule
- View
- Higher power; rotate the specimen to see each arc in full
- Before this
- Chapters 6–8 and 13: mastoidectomy, lateral canal, facial nerve
By the end you can
- Relate the otic capsule to the mastoid, facial nerve and middle-ear windows before drilling it
- Predict the posterior and superior canals and their common crus from the lateral canal and the CT
- Skeletonise the three canals to a blue line with the capsule intact
- Find the endolymphatic sac on the posterior fossa dura and corroborate it
- Open the canals and vestibule in a planned sequence, and record the last preserved and first destructive stage
Three canals and the common crus blue-lined with the capsule intact and photographed; the sac found by position and duct; and, only if agreed, the canals and vestibule opened in a stated sequence with the facial nerve covered.
Stations 4 and 5 open the labyrinth. They are a separate, destructive exercise, begun only after the intact stage has been photographed and recorded and the instructor has agreed the sequence.
The landmark chain
Each landmark leads to the next. Tap any one to see every station where it appears.
14.1 Before you drill the capsule
The bony labyrinth holds the cochlea, the vestibule and the three semicircular canals. In the mastoid only the lateral canal shows itself; the posterior and superior canals, their common crus and the vestibule must be found by relationship. This chapter holds two exercises that must be kept apart: skeletonisation, which keeps the otic capsule intact, and labyrinthectomy, which opens it and is destructive to inner-ear function.
Start from a wide mastoid with the tegmen and sinus plates thinned, the antrum open and the facial nerve traced in bone (Chapters 6, 7 and 13). Relate the exposed otic-capsule surface to the mastoid, the facial nerve and the middle-ear windows. The lateral canal prominence is the reference, but the full arrangement of the canals needs deliberate three-dimensional study: rotate the specimen and follow whole arcs, rather than labelling one small prominence in isolation.
Stations 1–3 preserve the labyrinth; Stations 4 and 5 open it. Do not begin Station 4 until the intact stage has been photographed and recorded.
The blue line is the end of skeletonisation, not its goal. Never let the burr dwell to make a shell look more dramatic.
| Skeletonisation (preserving) | Labyrinthectomy (destructive) |
|---|---|
| Surrounding bone removed; capsule intact | A separate exercise, planned with the instructor |
| Each canal outlined to a blue line | Lumina opened in a stated sequence |
| Facial nerve covered by bone | Vestibule entered from the ampullae |
| Photographed and recorded as an intact stage | Recorded from the first deliberate opening |
14.2 · The 5 stations
Map the canals from outside
- Find the dome of the lateral canal in the antrum floor, smooth dense bone with the incus short process in front of it.
- Trace the facial nerve in bone: the second genu lies just inferior to the lateral canal, and the tympanic segment runs forwards below its ampullated end.
- Project the posterior canal behind the posterior end of the lateral canal, in a plane at right angles to it, and the superior canal above, rising towards the tegmen.
- Check both projections on the CT. Place the common crus where the posterior limb of the superior canal meets the upper limb of the posterior canal.
Lateral canal, incus and facial nerve named; posterior and superior canals and common crus marked on the bone and the CT.
You cannot place the facial nerve. Return to Chapter 13 before drilling the capsule.
Skeletonise to a blue line
- Remove the cells and soft bone behind and above the lateral canal, the surface kept in view under continuous irrigation.
- Follow the posterior canal down its arc to its ampulla, medial to the facial nerve just below the second genu. Keep bone over the nerve.
- Follow the superior canal up from the common crus and forwards to its ampulla, beside the ampullated end of the lateral canal.
- Stop at a blue line, the lumen glimpsed through the last shell of capsule. Move the burr along the canal; never hold it on one point.
All three canals and the common crus outlined, each capsule intact; photographed before going on.
The lumen opens unintentionally. Stop, show the instructor and record it as an unintended opening.
The endolymphatic sac
- Thin and remove the presigmoid bone (Trautmann's triangle) between the sinus, the posterior canal and the superior petrosal sinus; lift the last flakes off the dura.
- Extend the plane of the lateral canal backwards across the posterior canal (Donaldson's line). The sac usually lies below it, but the line only starts the search; it does not fix the sac in every bone.
- Look on the posterior fossa dura between the posterior canal and the sinus for a pale, thickened dural duplication running down towards the jugular bulb.
- Corroborate it: its duct passes into bone deep to the posterior canal, in the vestibular aqueduct. Preserve that relationship and the dura where the specimen allows.
Sac identified by position and by its duct, dura intact.
You are about to name a thickened patch of dura as the sac without its duct or its relationship to the posterior canal.
Open the lateral and posterior canals
- Begin only after the intact stage has been photographed and the instructor has agreed the sequence.
- Open the lateral canal along its blue line and follow its lumen forwards to its ampulla, above the tympanic segment of the facial nerve.
- Open the posterior canal and follow its lumen down to its ampulla, the point nearest the mastoid segment of the facial nerve: keep a shell of bone on the nerve.
- Follow the lumina; do not create a broad, undirected defect.
Both canals opened along their whole arcs, facial nerve covered.
You have lost the lumen. Return to a recognisable surface and find it again before drilling on.
Superior canal, common crus and vestibule
- Follow the posterior canal up to the common crus and open the superior canal from there, over its arc, to its ampulla.
- Relate the three ampullae: superior and lateral side by side anteriorly, posterior below, medial to the facial nerve.
- Open the vestibule from the ampullae and the common crus, keeping the tympanic segment and second genu of the facial nerve, lateral to it, covered.
- Distinguish the vestibule from the internal auditory canal: the vestibule is lateral and opens from the ampullae; the canal lies deeper and medial (Chapter 15).
Three canals, common crus and vestibule opened and named; facial nerve intact.
You see dura deep in the vestibule. You have reached the fundus of the internal auditory canal: stop.
14.3 What the preserved bone cannot show
The membranes and fluids of the living labyrinth do not behave in preserved material as they do in a patient. Do not infer clinical tolerance from the ease with which a cadaveric canal can be opened. Clinical labyrinthectomy has a functional purpose and consequences that need specialist selection and counselling; the laboratory demonstration is anatomical.
Compare the specimen before skeletonisation, after it and after the planned opening. Identify where the exercise changed from preservation to destruction, show how the canal arcs, the vestibule and the facial nerve relate to one another, and record any unintended violation before the planned opening. Completion means a correct account of the labyrinth’s relationships and the extent of the work, not the maximum removal of surrounding bone.
◉ See it on real specimens
Real dissection and operative photographs published openly by other surgeons and institutions. They open on the original site with full credit to the authors; they are not copied into this companion. Compare them with the drawings above.
14.4 Fault finding
| What you see | Probable cause | Correction |
|---|---|---|
| A short arc taken for the whole canal | Outline incomplete | Rotate the specimen; follow the arc to both ends |
| Translucent shell, burr marks in one spot | Chasing a blue line | Move the burr along the canal; stop at the line |
| Posterior field narrower than expected | Forward sinus or low dura | Accept the limit; define it, do not force it |
| Superior canal not found above the lateral canal | Searching too far laterally | It lies deeper; follow it up from the common crus |
| Facial nerve bared at the posterior ampulla | Lumen followed without regard to the nerve | Keep a shell of bone on the mastoid segment |
14.5 Viva at the bench
What does a blue line tell you, and why should you stop there?
That only the last shell of capsule covers the lumen. It is the end of skeletonisation: further drilling opens the canal and turns a preserving exercise into a destructive one.
How do you confirm that a dural thickening is the endolymphatic sac?
By position, on the posterior fossa dura between the posterior canal and the sinus, usually below Donaldson's line, and by its duct passing into bone deep to the posterior canal in the vestibular aqueduct. Position alone is not enough.
Can an easily opened preserved labyrinth demonstrate the clinical tolerance of that manoeuvre?
No. Preserved membranes and fluids do not behave as in a patient, and clinical labyrinthectomy has functional consequences the specimen cannot show.
One best answer
14.6 My dissection log
Saved on this device only. Print it or send the summary to your fellowship coordinator for review.
Score the dissection · review order
| Item | 0 · not achieved | 1 · partly | 2 · achieved |
|---|---|---|---|
| Canals and common crus predicted from the lateral canal and CT | |||
| Three canals blue-lined, capsule intact, photographed | |||
| Sac found and corroborated by its duct | |||
| Labyrinth opened only in an agreed sequence | |||
| Facial nerve covered throughout | |||
| Last preserved and first destructive stage recorded |
The review order is a teaching method, not a validated scoring instrument. Structured end-product scales such as the Welling scale have been studied for rater reliability; they assess the dissection, not operative outcome, but they make feedback specific.
Before you assist at labyrinthine surgery
Sources and credits
- Harris T, Linder T. Mastoidectomy and epitympanectomy. In: Fagan J, ed. The Open Access Atlas of Otolaryngology, Head & Neck Operative Surgery. University of Cape Town. med.libretexts.org/
- Hua X. Temporal bone anatomy: cadaveric dissection. Iowa Head and Neck Protocols. University of Iowa; 2018. iowaprotocols.medicine.uiowa.edu
Real dissection photographs with numbered drawings: Bassett Collection, Stanford University (CC BY-SA 4.0). Classic anatomical and operative plates: Gray’s Anatomy of the Human Body (1918, drawings by H. V. Carter) and Burghard’s A System of Operative Surgery (1909), both public domain, credited beside each plate. Remaining diagrams and charts: Dr Bhushan Kathuria & Team, not to scale. Operating-theatre frames and clips: our theatre, patient consent taken, identity not shown. Atlas photographs: Open Access Atlas of Otolaryngology (CC BY-NC 3.0), with their authors credited beside each image. This edition uses no AI-generated images. For professional education alongside supervised instruction and local protocols.
Dr Bhushan Kathuria & Team · Reg. No. HN-017323, Haryana Medical Council · A GeNext MedEd Foundation surgical education initiative.
‹ Chapter 13: The Intratemporal Facial NerveChapter 15: The Internal Auditory Canal and Translabyrinthine Exposure ›
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