Within the Temporal Bone · Chapter 10 · Part II · Core dissections
Canal Wall Down Dissection and Cavity Design
One cavity, open to the meatus
Bench mode: one step per screen, large type, screen kept awake. Tap, use the arrow keys or say “next”.
- Specimen
- Adult temporal bone with a recorded canal-wall-up mastoidectomy; soft tissue of the concha if available
- Bench time
- One session for the bone work; longer if a meatoplasty is practised
- Burrs
- Large and medium diamond for the canal wall and ridge; cutting burr only far from the nerve
- View
- Low power for the cavity outline; higher power at the facial ridge
- Before this
- Chapters 6–8: cortical mastoidectomy, attic, facial recess
By the end you can
- Record the intact-wall stage before you change it
- Separate the ossicular chain without transmitting force to the stapes
- Take down the bridge and posterior canal wall in stages, following the nerve
- Shape one continuous cavity without ledges or dependent pockets
- Explain what a meatoplasty adds and what a bony cavity alone cannot show
One continuous, smooth cavity open to the meatus: facial ridge low with the nerve covered, no ledge under the tegmen, tip saucerised, every part visible through the meatus.
Choosing a patient’s operation. Canal-wall strategy in patients depends on disease, anatomy, reconstruction, surveillance and follow-up; the bench shows exposure and cavity geometry only.
The landmark chain
Each landmark leads to the next. Tap any one to see every station where it appears.
10.1 Understand the change
Canal wall down dissection removes the partition between the external canal and the mastoid, creating one common space; it changes both the exposure and the future form of the ear. The bench shows exposure and cavity geometry; it does not choose a patient’s operation.
In patients, the choice between canal-wall strategies depends on disease, anatomy, reconstruction, surveillance and follow-up; the aims include disease control and a manageable ear, with hearing reconstruction considered in the overall plan.
Document the canal-wall-up stage of Chapters 6 to 8 first: the original relationship cannot be recreated once the wall is gone. With the wall intact, the mastoid reaches the middle ear only through the aditus and any facial recess. With it down, canal, attic and mastoid become one cavity, divided only by the facial ridge over the mastoid facial nerve, and the meatus becomes the only access to all of it.
Know the facial nerve before you lower the ridge. Removing the wall does not resolve an unknown nerve course.
10.2 · The 6 stations
Record the intact wall
- Inspect the canal-wall-up cavity: tegmen, sinus plate, lateral canal, incus and posterior canal wall.
- Trace the expected course of the facial nerve from below the lateral canal dome to the digastric ridge.
- Photograph and record the stage, with any defects already present.
- Agree the limits of the wall to be removed and whether the chain will be kept or removed.
Intact wall recorded; nerve, tegmen and lateral canal placed.
You cannot place the facial nerve. Return to Chapter 8 first.
Separate the chain
- If the ossicles are to be removed, separate the incudostapedial joint first, under vision, through the facial recess or the canal.
- Only then mobilise and remove the incus; never lever it while it is still joined to the stapes.
- Divide the malleus neck or remove the head as the exercise requires, releasing its attachments under vision.
Chain separated, stapes untouched and stable.
The stapes moves when you work on the incus. Stop and separate the joint before going on.
The bridge and the wall
- Remove the bony bridge over the aditus to open the attic into the cavity.
- Lower the posterior canal wall in stages, working from the known mastoid and canal surfaces in turn.
- As the wall approaches the level of the nerve, keep the burr moving parallel to it.
- Inspect the transition between canal, epitympanum and mastoid after each stage.
Attic and canal open into the mastoid; the wall down to the level of the facial ridge.
You are lowering the wall faster than you can follow the nerve. Stop and trace it again.
▶ Canal Wall Down Mastoidectomy · Temporal Bone Dissection Part 2
Opens on YouTube (needs internet). Cadaveric dissection, GeNext MedEd Foundation workshop, Dr Raman Sharma.Lower the facial ridge
- Identify the facial ridge: the remnant of the posterior canal wall over the mastoid segment of the nerve.
- Thin it with strokes parallel to the nerve, keeping a covering of bone over it.
- Let the instructor show how a safe contour relates to this nerve and the rest of the cavity. No universal ridge height replaces that relationship.
- Check that the ridge no longer stands as a wall between the meatus and the mastoid tip.
A low, smooth ridge with the nerve still covered.
You see a pale line of nerve through a defect. Stop, record and obtain review.
Shape the cavity
- Smooth the tegmen and sinodural angle so that no ledge remains above.
- Remove overhangs of the anterior and inferior canal walls so that the canal floor runs into the open cavity floor.
- Saucerise the mastoid tip: a large superior opening can still leave a dependent recess below.
- Do not chase a flat surface across a neural or labyrinthine boundary: the lateral canal and the facial ridge stay covered.
One continuous, smooth cavity, every part visible from the meatus.
A recess cannot be seen from the meatus. Lower its rim or record why you cannot.
Open the meatus
- If the specimen has its concha, plan a meatoplasty proportionate to the cavity: the meatus must let the whole cavity be seen and cleaned.
- Incise the posterior meatal skin and excise a crescent of conchal cartilage behind the meatus, keeping the skin to line the opening.
- Check that the skin reaches the cavity edge without tension.
- In a bone without soft tissue, state the limitation rather than present bone work as a finished reconstruction.
The whole cavity, including the tip and the facial ridge, is seen through the meatus.
The meatus shows only part of the cavity. Enlarge the plan, or record the limitation.
10.3 Beyond bone
A bony cavity is not a finished clinical ear. Meatoplasty, lining, graft support, obliteration or canal reconstruction may form part of the operation, depending on its purpose. If a reconstruction exercise is planned, preserve or harvest the material before discarding tissue, and check that the lining or graft covers the intended surfaces without excessive tension. Graft placement can be demonstrated; healing cannot be assessed in the laboratory.
10.4 Inspect the final cavity
Examine the cavity from several angles for retained overhangs, hidden recesses and unintended defects; confirm the state of the facial nerve, labyrinth and stapes region; record the ossicles kept or removed and any deliberate exposure of dura or sinus.
Describe rather than label: “modified radical” and similar terms are used differently between centres. The exercise is complete when you can describe one connected space, say how removing the wall improved access, name the structures that now provide orientation and state what soft-tissue work a patient would need.
A good cavity is one you can see completely through the meatus.
10.5 Fault finding
| What you see | Probable cause | Correction |
|---|---|---|
| High facial ridge, cavity in two parts | Ridge left as a wall | Lower it along the nerve, keeping bone over it |
| Ledge under the tegmen | Superior overhang left | Smooth the tegmen and sinodural angle |
| Anterior canal bulge hides the drum | Anterior overhang left | Drill the bulge to an even canal |
| Cavity judged from its size alone | Access through the meatus not checked | Look at every part through the meatus |
10.6 Viva at the bench
Why must the intact-wall stage be recorded before the wall is removed?
The original relationship between canal, attic and mastoid cannot be recreated once the wall is gone; the record is the only reference for what was changed and why.
What limits how far the facial ridge can be lowered?
The mastoid segment of the facial nerve beneath it. The ridge is lowered along the nerve with a covering of bone kept, to a contour judged against this nerve, not to a universal height.
Why is a large superior opening not enough to describe a satisfactory cavity?
Because a dependent recess, typically at the mastoid tip, or a ledge under the tegmen can remain hidden from the meatus. A satisfactory cavity is one in which every part can be seen and cleaned through the meatus.
One best answer
10.7 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 |
|---|---|---|---|
| Intact-wall stage recorded first | |||
| Incudostapedial joint separated before the incus moved | |||
| Wall taken down in stages, nerve followed | |||
| Facial ridge low, nerve covered | |||
| No ledge, no dependent pocket | |||
| Whole cavity seen through the meatus |
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 a canal wall down mastoidectomy
Sources and credits
- Mathew R, Harris T, Patel P. Lateral temporal bone resection. 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/
- Companion video: Canal Wall Down Mastoidectomy, Temporal Bone Dissection Part 2 (GeNext MedEd Foundation workshop, Dr Raman Sharma). youtu.be/d3CDVlz2cJk
- Companion video: Facial Nerve Decompression, canal wall down mastoidectomy. Dr Bhushan Kathuria & Team. youtu.be/961KhvOiNMU
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 9: The Round Window and Cochlear Implant CorridorChapter 11: The Ossicular Chain and Reconstruction Principles ›
Appears in: