Within the Temporal Bone · Chapter 06 · Part II · Core dissections
Cortical Mastoidectomy
From cortex to antrum in six stations
Bench mode: one step per screen, large type, screen kept awake. Tap, use the arrow keys or say “next”.
- Specimen
- Adult temporal bone, wet, dry or printed; side and prior defects recorded
- Bench time
- One session; your instructor sets the target
- Burrs
- Large cutting for cortex and cells; diamond near the plates
- View
- Naked eye to map, low power for the cortex, higher power at the plates
- Before this
- Chapters 1–5: set-up, drill control, orientation, imaging
By the end you can
- Read the CT of your own specimen and agree the endpoint before the first cut
- Outline the cavity from surface landmarks and saucerise as you go
- Find the antrum by continuity and corroborating landmarks
- Define the tegmen and sinus plates as continuous surfaces, intact
- Review the cavity in a fixed order and record it honestly
Posterior canal wall intact; antrum recognised; superior and posterior limits identified and covered by bone.
Opening the facial recess, skeletonising the facial nerve, opening the labyrinth or removing the ossicles.
The landmark chain
Each landmark leads to the next. Tap any one to see every station where it appears.
06.1 Read your bone
Answer four questions from the axial and coronal CT of this specimen before the first cut. The CT does not replace the landmarks; it tells you which landmark will be difficult.
A bare specimen does not simulate incision, soft-tissue elevation, haemostasis or closure. Record the side, the condition of the bone and any earlier dissection, so a defect found later can be told apart from one you made. Mount the bone with the canal and mastoid surface in one view, and set the microscope and chair before taking up the drill: a comfortable posture at the surface becomes an awkward reach as the cavity deepens. Repositioning is part of the technique. Photograph the starting view.
06.3 · The 6 stations
Map the cortex
- Relate the canal to the mastoid surface. Find the temporal line, an approximate surface guide to the middle fossa floor, and the mastoid tip.
- Find the spine of Henle at the posterosuperior canal margin and the suprameatal (Macewen) triangle behind it: the antrum lies deep to the triangle.
- Draw the outline along the temporal line, parallel to the posterior canal wall and down towards the tip, with the back edge set from the sinus position on your CT.
Landmarks named and the outline drawn. Surface projections are approximations, not depths.
You cannot find the canal wall or the temporal line. Re-orient the specimen first.
Open the cortex
- Start in a fully visible cortical field with a burr suited to controlled bulk removal.
- Develop the opening progressively over the whole outline (cut 1 along the temporal line, cut 2 parallel to the canal wall, cut 3 joining behind and down to the tip) rather than drilling a narrow tract towards an expected destination.
- Preserve the posterior canal wall; do not shorten its superior or posterior margins.
- Before each pass, identify the surface you are removing and where the pass will finish; after it, clear the field and inspect what changed.
The cortex is lifted over the whole outline and the first cells are open.
The contact point disappears from view. Stop the drill before you correct the view.
Saucerise as you go
- As cells appear, remove the intervening bone so that the field stays broad and readable. Do not follow an attractive cell tract without relating it to the established boundaries.
- Reduce overhanging edges as the exposure advances: a deep cavity under a small opening hides the working surface and invites oblique instruments. If access is poor, widen the opening before reaching for a smaller burr.
- Keep the bone readable with irrigation and suction, and tell washable debris from a genuine plate. Where bone thins or its identity is uncertain, slow down and establish the relationship first.
A wide cavity with bevelled edges and the canal wall intact.
The cavity is deeper than it is wide: access has lagged behind depth. Widen it before going further.
Find the roof and the back wall
- Follow the tegmen as one continuous surface, not as isolated patches: it is the superior limit towards the middle cranial fossa.
- Follow the sinus plate in the same way as the posterior limit, and open the angle between them, the sinodural angle.
- Keep a protective covering of bone over both; their contours vary. Choose a burr whose shaft and opposite edge do not lie against a protected structure.
- If the roof descends or the sinus approaches the working field, reconsider access before working further medially.
Roof and posterior limit traced as continuous surfaces, both covered by bone.
You are relying on colour alone to name a plate. Trace its continuity first.
Find the antrum
- Advance along the roof, anteromedially towards the antrum: the superior route keeps a continuous reference.
- Do not search low: a low search brings the dissection towards the facial nerve.
- Confirm the antrum by its neighbours: the prominence of the lateral semicircular canal (dense, yellow-ivory bone) and the body and short process of the incus. Tell newly visible structures from residual cell partitions.
- Near the ossicles, direct the burr away from the chain and never let it touch it.
The antrum is open and the lateral canal and incus are seen.
You meet a dense plate with no lateral canal in view. Reassess the roof, canal wall and CT before crossing it.
Review and record
- Inspect in the same order every time: canal wall, roof, posterior boundary, antrum, inferior extent. Point with a stationary instrument, never a moving burr.
- Relate the deeper course of the facial nerve: its second genu lies just inferior to the lateral canal, and the digastric ridge leads to the stylomastoid foramen. Leave the nerve, the labyrinth and the ossicles covered and untouched.
- Save a final overview in the starting frame. Record whether the endpoint was achieved, any deviation from the plan with its anatomical reason, and, separately, any unintended damage.
Every boundary preserved, every structure named, and a record the next chapter can start from.
You cannot place the facial nerve. Do not move on to the facial recess.
06.4 When the specimen differs from the example
An air-cell pattern is not a template for the next bone. Keep three things apart in your record: what you have identified, what you expect, and what remains uncertain. Write the uncertainty down before more drilling obscures the evidence. If a cavity looks smaller than the example, reassess the boundaries before enlarging it: a reference specimen shows relationships, not compulsory dimensions.
If a plate defect or ossicular injury is suspected, stop, document the field and get your instructor’s review; do not keep going to make an imperfect result look finished. A cadaver does not reproduce bleeding, facial function or hearing, so a dry, clean field cannot show that the same manoeuvre would have been harmless in a patient.
Obscured needs a better viewUnidentified needs anatomical reassessmentInjured needs recognition and review
More drilling is the answer to none of them.
06.5 Fault finding
| What you see | Probable cause | Correction |
|---|---|---|
| Large tip cells, no antrum | Working too low and too far back | Return to the roof; work anteromedially |
| Posterior canal wall thinning | Burr axis too far forwards | Move the axis backwards; keep the wall thick |
| Unexplained smooth bone | A problem of identification | Stop and reassess roof, canal wall and CT |
| Displaced incus | Mechanical injury transmitted through bone | Stop; document; instructor review |
| Gouges on the cortex | Burr skating under pressure | Lighter pressure; use the side of the burr |
06.6 Viva at the bench
Why is the first large air cell not automatically the antrum?
Cell size is not identity. The antrum is confirmed by continuity along the roof and by its neighbours, the lateral canal and the incus; Körner’s septum can make superficial cells look like it.
Why must the cavity be widened before it is deepened?
A wide saucer shows the plates and keeps the working surface in view; a deep, narrow hole hides them and invites oblique instruments.
Why is a low search for the antrum dangerous?
It brings the dissection towards the facial nerve, whose second genu lies just inferior to the lateral canal.
One best answer
06.7 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 |
|---|---|---|---|
| Posterior canal wall | |||
| Roof (tegmen) | |||
| Posterior boundary (sinus plate) | |||
| Antrum, lateral canal and incus | |||
| Inferior extent (tip, digastric ridge) | |||
| Saucerisation (wide, bevelled edges) |
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 your first mastoid in the theatre
Sources and credits
- Butler NN, Wiet GJ. Reliability of the Welling scale (WS1) for rating temporal bone dissection performance. Laryngoscope 2007;117:1803–8. doi.org/10.1097/MLG.0b013e31811edd7a
- Fernandez SA, Wiet GJ, Butler NN, Welling B, Jarjoura D. Reliability of surgical skills scores in otolaryngology residents: analysis using generalizability theory. Eval Health Prof 2008;31:419–36. doi.org/10.1177/0163278708324444
- Harris T, Linder T. Mastoidectomy and epitympanectomy. The Open Access Atlas of Otolaryngology, Head & Neck Operative Surgery (Fagan J, ed.). University of Cape Town. CC BY-NC 3.0. med.libretexts.org/
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 5: Transcanal Exposure and the Middle EarChapter 7: The Antrum, Aditus and Epitympanum ›
Appears in: