10Canal Wall Up Versus Canal Wall Down and Hearing

FUp or down: what the choice means

When cholesteatoma or chronic disease reaches the mastoid, the surgeon faces a decision that echoes through the rest of the operation: keep the posterior bony wall of the external ear canal, or take it down. In a canal-wall-up (CWU)— or intact-canal-wall — mastoidectomy, that wall is preserved, so the external canal keeps its normal tube shape and the middle ear stays a separate, enclosed space. In a canal-wall-down (CWD) mastoidectomy the wall is drilled away and the facial ridge lowered, merging the mastoid and middle ear into a single open cavity that communicates freely with the meatus [1996].

The primary purpose of either operation is the same: eradicate disease. CWD exteriorises the disease and is the safer choice for extensive or recurrent cholesteatoma, poorly accessible attic and sinus tympani disease, the only-hearing ear, or where reliable follow-up is doubtful. CWU spares the patient an open cavity and its lifelong care but accepts a higher risk of residual or recurrent disease hidden behind the intact wall. This module is not about which controls disease better — it is about the downstream acoustic consequences of the choice, because the same decision that shapes safety also shapes the environment every prosthesis must live in.

FWhy air space matters for hearing

A reconstructed ossicular chain is an inert strut suspended in an air-filled space. For it to deliver sound, three things must hold: the strut must move freely, the air around it must let it move, and the cochlea must have somewhere to push fluid. The middle ear is fundamentally an air-filled impedance-matching cavity, and its volume is part of the machine. Fill it with fluid, fibrous tissue or a collapsed drum and the system stiffens; the reconstruction can no longer swing, and a conductive gap persists despite a structurally perfect repair [1996].

A useful rule of thumb is that an aerated residual middle-ear space — on the order of half a millilitre or more — is needed to avoid stiffening the reconstruction and to give the prosthesis room to excurse. The reason CWU tends to protect hearing is simple: by keeping the posterior wall it preserves the native contour of the canal and a larger, enclosed middle-ear pocket. CWD, by removing the wall and lowering the ridge, trades some of that protected air volume for surgical exposure. The hearing question, then, begins not with the prosthesis but with how much aerated space is left for it to work in [2006].

TThree acoustic costs of taking the wall down

It is tempting to imagine that removing a piece of bone behind the drum is acoustically neutral. It is not. Taking the canal wall down imposes three specific penalties on the reconstruction, each of which the surgeon can partly mitigate.

• Reduced middle-ear volume.Lowering the facial ridge and removing the posterior wall shrinks the residual air-filled space. Less air means a stiffer system, poorer impedance matching and less room for prosthesis excursion — a direct mechanical drag on the result [1996].
• An exposed round window. In an intact ear the round window sits in a recessed, naturally shielded niche, so it is driven out of phase with the oval window and the cochlea sees a useful pressure difference. In an open cavity the round window can be left exposed to the same sound field as the reconstruction; both windows then move nearly in phase, the inter-window pressure difference collapses, and cochlear fluid displacement falls. This is the round-window baffle problem, the dominant acoustic argument for shielding it [1998].
• Mucosal loss and adhesions.The aggressive drilling that an open cavity demands denudes mucosa over a wide area. Healing by fibrosis tethers and stiffens the reconstruction, drags prostheses medially and raises the risk of displacement and extrusion — the same hostile-environment failures that dominate every ossiculoplasty prognostic index [2001].

Notice that none of these penalties is “the missing canal wall” in itself. They are its consequences: less air, an unshielded round window, a scarred bed. That distinction matters, because each consequence can be addressed at operation even when the wall cannot be put back.

TWhat the comparative evidence shows

Given those mechanisms, one might expect CWD to give clearly worse hearing. The literature is more nuanced. The classic teaching, from large surgical series, is that hearing results do not differ significantly between CWU and CWD provided disease is fully eradicated and the chain is appropriately reconstructed [1996]. A staged-ossiculoplasty cohort bore this out directly: mean air-bone gap closure was 10.9 dB in canal-wall-up ears and 13.5 dB in canal-wall-down ears, a difference that did not reach significance [2010].

Other series do detect an advantage for keeping the wall up. In single-stage cholesteatoma surgery the intact-canal-wall technique gave better hearing than the canal-wall-down approach, with the gap most apparent around 1 kHz — precisely the speech-relevant region where the transformer mechanism matters most [2017]. The honest synthesis is that mastoidectomy type is a weak independent predictor. What really moves the outcome is the condition of the ear: an intact, mobile stapes superstructure and a healthy, well-aerated middle ear predict success regardless of the wall, while disease extent and a hostile environment predict failure regardless of it [2006, 2005]. A well-aerated open cavity can hear as well as an intact one; a scarred, airless CWU ear can hear badly.

CReconstructing in the open cavity

Because the penalties of an open cavity are consequences rather than the wall itself, much of the hearing result is recoverable through technique. The reconstructive aims in a CWD ear are to re-create a protected, aerated pocket around the reconstruction and to restore the inter-window pressure difference.

• Rebuild an air space. A cartilage or fascia tympanoplasty set at an appropriate depth, sometimes combined with mastoid obliteration to reduce dead space and a small canal-wall reconstruction where feasible, restores an enclosed, aerated pocket in which a prosthesis can move [1996].
• Shield the round window.Interposing cartilage or fascia over an exposed round-window niche — without immobilising it — re-establishes the acoustic baffle so the windows are not driven in phase. The round window must stay mobile; the goal is to screen it, not to pack it solid [1998].
• Respect the stapes and choose the prosthesis to suit. With an intact superstructure a PORP onto the stapes head usually performs best; with an absent superstructure a TORP onto the footplate is needed, and its stability in a shallow cavity depends on the rebuilt pocket above [2005].

Equally, the open, freshly drilled cavity is often a poor bed to reconstruct into at the same sitting. Many surgeons therefore stagethe ossiculoplasty in CWD ears, deferring the prosthesis until the cavity has healed, re-mucosalised and proven dry — a strategy that also doubles as a check for residual disease [2001].

CChoosing wall and timing together

The wisest framing keeps two decisions separate but linked. The wall is chosen for disease control: take it down when safety demands it, and do not compromise eradication to protect a few decibels. The hearing is then protected by managing the environmentthe chosen wall leaves behind — air space, round-window shielding, mucosal health and stapes status — rather than by the wall decision itself [2006, 1996].

From that follows a short, defensible policy. In a favourable CWU ear with an intact stapes, single-stage reconstruction closes the gap reliably. In a CWD ear, plan for the cavity’s acoustics: rebuild an aerated pocket, baffle the round window, choose the prosthesis to match the stapes, and stage the reconstruction when the bed is hostile or disease clearance is uncertain. Counsel patients accordingly — that an open cavity is not a sentence to poor hearing, and that an intact wall is no guarantee of good hearing if the ear inside it is airless or diseased. Match the operation to the disease, then make the environment fit to receive the prosthesis, and the canal wall ceases to be the thing that decides whether the patient hears [2001, 2005].