9Prosthesis Extrusion: Mechanisms and Prevention

FWhat extrusion is, and why it is slow

Of all the ways an ossiculoplasty can fail, extrusion is the most literal: the prosthesis you placed against the stapes slowly works its way back outthrough the eardrum and ends up sitting in the ear canal, or is found lying on the drum at a follow-up visit. It is not a sudden event. It is a slow march — a migration measured in months — in which the lateral end of the prosthesis, the part nearest the drum, gradually erodes through the epithelium covering it until there is nothing left to hold it in. The hearing that was excellent at the first postoperative audiogram drifts back down, the ear may feel full, and on microscopy the glistening head of the prosthesis becomes visible through a thinning, tented drum before it finally breaks the surface.

It is worth fixing the direction in your mind, because two complications are easily confused. Extrusion is movement laterally, out through the drum. Displacement— the prosthesis slipping off the stapes head, tilting, or plunging medially— is the opposite vector and a different problem, one of coupling and stability rather than of the drum interface. Both reopen the air–bone gap, but their mechanisms and their fixes differ. This module is about the lateral march: the forces that drive a prosthesis out, the interface biology that permits it, and the single most effective defense against it, the interposed cartilage cap. The diagram below introduces the mechanism you can toggle through as we go.

The crucial early lesson, learned in the first large clinical series of synthetic prostheses, is that extrusion is not mainly a property of the material. When Brackmann and colleagues reviewed more than a thousand porous-polyethylene reconstructions, extrusion was common with a bare head but fell sharply once cartilage was routinely placed between the prosthesis and the drum [1984]. That observation — that an interface problem has an interface solution — is the spine of everything that follows.

FPressure plus poor environment: the two forces

Two forces drive the slow march, and they act together. The first is pressure. A rigid prosthesis head meets the underside of the drum over a tiny area. If the drum then retracts — draping down onto and being tented over that small hard point — the force of retraction is concentrated into high local pressure on a patch of epithelium only a millimetre or two across. Pressure is force divided by area, and a point contact gives the smallest possible area and therefore the highest pressure. Sustained pressure on living epithelium does what sustained pressure always does to tissue: it squeezes out the blood supply, the epithelium becomes ischaemic, thins, and is resorbed. Each retraction cycle advances the head a little further, and the drum heals lateral to it. That is the march.

The second force is the middle-ear environment. The retraction that loads the head is itself the product of poor ventilation — eustachian tube dysfunctiondrawing the drum inward, often with frank atelectasis. Add active disease — recurrent infection, granulation tissue, myringitis — and the epithelium is not just compressed but inflamed and fragile, far less able to tolerate a foreign body against it. This is why extrusion is overwhelmingly a late complication of the unhealthy ear. In a careful analysis of hydroxyapatite prostheses, late extrusion was strongly and significantly associated with postoperative atelectasis, recurrent otitis media and myringitis, while ears that stayed dry and aerated tended to retain their prostheses [2002]. Pressure provides the mechanism; a poor environment provides both the retraction that applies it and the fragile tissue that yields to it.

These same environmental variables are the ones that prognostic staging systems use to predict any ossiculoplasty result. Drainage, mucosal disease and the overall middle-ear status drive the achievable hearing outcome and the durability of the reconstruction alike [2001]. Extrusion is, in a sense, the most dramatic expression of a bad environment: not merely a poorer audiogram, but the loss of the prosthesis altogether.

TThe interface biology behind the march

Why does the epithelium give way to the prosthesis rather than simply healing over it and holding it down? The answer lies in the biology of the implant’s surface. Most prosthesis materials are bioinert: the tissue tolerates them but does not bond to them, walling them off in a thin fibrous capsule rather than embracing them. An implant held only by a fibrous capsule and its mechanical seating is free to micromotion, and under the repeated loading of a retracting drum that micromotion becomes migration. The implant is, in effect, never anchored; it is only resting in place, and a steady lateral force will eventually move it.

The contrast is a bioactivesurface, one whose chemistry the host recognises and onto which living tissue is laid down directly, with no fibrous gap. Early ossicular work with bioactive glass-ceramics showed that such a surface could integrate and resist extrusion in a way an inert encapsulated surface did not — the conceptual origin of friendly-interface and coated-prosthesis strategies [1988]. But this bonding is conditional: it requires a dry, healthy, well-aerated bed. The very ear that drives extrusion — wet, retracted, inflamed — is precisely the ear in which a bioactive surface fails to bond. So surface chemistry helps at the margin, but it cannot rescue a hostile environment, and it does nothing to relieve the underlying problem of concentrated pressure. For that, you need to change the geometry of the contact itself.

TCartilage capping as defense

The defense that addresses pressure directly is the cartilage cap: a thin disc of cartilage — typically harvested from the tragus or concha — interposed between the head of the prosthesis and the drum. Toggle the mechanism widget above to the capped state and the logic is visible. The cap takes the same retraction force and spreads it over a broad surface instead of a point, so the local pressure on the drum epithelium falls below the threshold that causes ischaemia. Just as importantly, cartilage is a tough, largely avascular tissue that tolerates direct contact with the drum and resists resorption, so it interposes a durable buffer between a hard foreign body and a fragile membrane. It converts a point load on living epithelium into a distributed load on inert cartilage.

This is not a theoretical argument. In a direct clinical comparison of hydroxyapatite prostheses placed with versus without an interposed cartilage disc, the extrusion rate fell from 13.2% to 1.9%— an order of magnitude — with no penalty to hearing [2002]. The same lesson runs through the larger synthetic-prosthesis series: extrusion that was troublesome with a bare head became uncommon once cartilage interposition was routine [2001]. A cartilage cap is cheap, autologous, immunologically silent, and the most reliable single manoeuvre the surgeon controls. The widget below lets you build a particular ear from its risk factors and watch how decisively the cap and the environment dominate the composite risk.

A reasonable question is whether the cartilage muffles the sound it protects against extrusion. It does not, to any clinically meaningful degree: a thin cap adds negligible mass and stiffness, and the controlled comparisons that show the extrusion benefit show equivalent hearing with and without it [2002]. The cap is very nearly a free lunch — large protection, trivial acoustic cost — which is why it has become the default at any interface where a rigid head would otherwise sit against the drum.

CReading the numbers: what actually moves extrusion

Put the figures side by side and the hierarchy of causes becomes clear. It is tempting to attribute extrusion to the material on the head, and material does contribute something — the abandoned porous polymers extruded freely, and even modern inert titanium is not extrusion-proof. A PRISMA review of eighty titanium PORP and TORP series found a pooled extrusion or dislocation rate of about 5.2% (range 0–35%), rising to nearly 14% in children, whose more active eustachian and adenoidal disease worsens the environment [2023]. So inertness lowers the floor but does not abolish the risk. The chart sets the key figures against one another.

Read across the bars and the message is unambiguous. The gap between a bare hydroxyapatite head (13.2%) and the same prosthesis under a cartilage cap (1.9%) dwarfs the difference between any two modern materials; the late rate in atelectatic ears (14%) shows the environment doing the same damage from the other direction [2002] [2002]. The two large levers on extrusion are the interface (capped or bare) and the environment (aerated or retracted). The choice between, say, titanium and hydroxyapatite is a refinement at the margins of a decision whose centre lies in the drum and the eustachian tube, not in the catalogue [2023].

CA practical prevention strategy at the table

The mechanism dictates the prevention, and it falls into two halves — relieve the pressure, fix the environment.

• Cap the interface. Interpose a thin cartilage disc between any rigid head and the drum, especially in the atelectatic, revision or otherwise marginal ear. This is the single most reliable manoeuvre and costs almost nothing acoustically [2002].
• Respect aeration. Extrusion is a disease of the unventilated ear. Assess and, where you can, improve eustachian tube function; do not place a definitive prosthesis into a frankly atelectatic, retracting drum and expect it to stay [2002].
• Stage the wet or active ear.Granulation, persistent infection and irreversible mucosal disease all raise extrusion risk. Settle the ear first — staging the reconstruction if necessary — rather than reconstructing into inflammation [2001].
• Avoid undue length and tension. A prosthesis cut too long sits under axial load and presses harder on the drum; size it to seat snugly without tenting, so the cap is protecting against retraction rather than against your own over-pressure.
• Use an inert, friendly material — but keep it in proportion. Titanium and hydroxyapatite both extrude far less than the abandoned polymers, and a bioactive surface helps at the margin [1988]. But material is the smallest of the levers; the cap and the environment matter far more [2023].

The unifying idea is simple enough to carry into every case. A prosthesis extrudes when a concentrated pressure acts on a fragile, retracting drum for long enough. Take away the concentration with a cartilage cap, take away the retraction by protecting aeration, and the slow march has nothing to drive it. Extrusion then becomes the rare event it should be, rather than the predictable late failure of a bare head under a collapsing drum.