1Prostheses Types, Biomechanics and Selection: Chapter Overview

FWhy an alloplastic prosthesis?

When disease has destroyed the ossicular chain beyond what the patient’s own tissue can rebuild, the surgeon reaches for a manufactured strut — an alloplastic prosthesis. The modern era began when Wullstein systematised reconstructive tympanoplasty and brought the first synthetic devices into the middle ear, reframing the surgeon’s task as the deliberate rebuilding of a sound-conducting apparatus [1956]. Alloplasts answered real shortcomings of biological tissue: they are sterile, of consistent geometry, immediately available off the shelf, spare the labour of sculpting an autograft, and carry no donor-site morbidity or disease-transmission risk.

This chapter is the map of that territory. It is not about a single material but about the logic of prosthetic reconstruction: what we want from an implant, the two basic shapes it comes in (the PORP and the TORP), the mechanics that decide whether sound actually reaches the cochlea, and the judgement that picks the right strut for the ear on the table. The detailed material chapters — titanium, hydroxyapatite, bone cement, hybrids — sit downstream of these principles; this overview is the scaffold on which they hang.

One caveat frames everything that follows. Across the comparative literature, well-chosen modern prostheses give broadly similar hearing, and the dominant determinants of success are the middle-ear environment and the surgeon’s execution, not the brand of strut. Keep that humility in mind as we catalogue the options: the prosthesis is a tool, and the ear is the patient.

FThe ideal-prosthesis checklist

Generations of otologists have converged on a short list of properties an ideal ossicular prosthesis should possess. It must be biocompatible, provoking minimal foreign-body reaction; it must resist extrusion through the drum and resist resorption that would re-open the gap; it must be mechanically and positionally stable through healing; it must be easy to handle and shape under the microscope; and it must couple sound efficiently, with appropriate mass and stiffness. No material satisfies every criterion, so each is a bundle of trade-offs.

• Hydroxyapatite— a calcium-phosphate ceramic mirroring bone mineral — is exceptionally biocompatible and tolerates direct drum contact, with hybrid HA-head designs reaching extrusion rates around 4%; its weakness is brittleness and difficult intraoperative trimming [1992].
• Titanium is the current benchmark: light, rigid, infinitely configurable, with extrusion of only 1–2%— but a bare titanium head against the drum tends to extrude, so it is usually capped with cartilage [2004].
• The patient’s own autograft incus, where salvageable, is biologically unbeatable: in a randomised comparison for eroded-incus defects it closed the air–bone gap to within 20 dB in 65% of ears versus 35% for a titanium prosthesis, with fewer complications [2017].

Reading the checklist as a set of competing demands — rather than a wish-list any one implant fulfils — is the conceptual skill this section builds. Explore how each criterion pulls against the others below.

TPORP and TORP: the two configurations

Whatever its material, an alloplastic strut comes in one of two basic lengths, defined entirely by where its medial end couples to the chain:

• A partial ossicular replacement prosthesis (PORP) rests on the head of an intact, mobile stapes superstructure and bridges up to the malleus handle or the drum.
• A total ossicular replacement prosthesis (TORP) is used when the superstructure is absent or unusable; it spans the whole gap from the stapes footplate to the drum or malleus.

This single anatomical fact — superstructure present or absent — is the most important branch point in prosthetic reconstruction, because the two configurations do not perform equally. A PORP couples to the stapes head, a stable platform that keeps the strut aligned and well seated; a TORP balances on the footplate, is more prone to displacement and subluxation, and demands precise tension and angulation. The clinical consequence is consistent: PORPs out-perform TORPs. In a meta-analysis of 40 studies and over 4,300 ears, the PORP was significantly more effective at restoring the chain (combined risk ratio 1.28) and more stable in long-term follow-up, except within staged and cholesteatoma subgroups [2013]. Series-level data echo this, with PORPs typically closing the air–bone gap to within 20 dB in roughly two-thirds of ears against a markedly lower fraction for TORPs.

The practical lesson is to preserve a mobile stapes superstructure whenever it exists: it is the single best prognostic feature an ear can offer, and removing a usable arch to simplify placement discards the very thing that predicts a good result. A TORP is sometimes preferred even with an arch present — for a medialised or rotated stapes, or a shallow middle-ear cleft — but that is an exception driven by access, not a default [2015].

TBiomechanics: mass, stiffness, length and angle

A prosthesis is, acoustically, a passive mechanical element inserted into a finely tuned transformer. Four physical properties decide whether it helps or hinders sound transmission.

Two principles deserve emphasis. First, the native middle ear is dominated by the annular ligament, which contributes the great majority of the system’s stiffness; a prosthesis that over-tensions the footplate or is seated too rigidly fights that compliance and chokes off stapes motion. The classical ideal is a loosest stable construct — firmly seated yet freely mobile, aligned yet flexible. Second, the malleus matters. A reconstruction that incorporates the malleus preserves more of the native ossicular lever and aligns the strut with the drum’s vibratory axis; constructs that retain the malleus and a mobile stapes give smaller residual gaps than drum-to-footplate struts that bypass them. In one large series the residual air–bone gap was 11.6 dB with the malleus handle present versus 16.9 dB when absent [2001]. Where the malleus is grossly medialised, gentle lateralisation or a shaft bent toward the promontory can restore an efficient angle and a stable interface.

CSelecting a prosthesis at the microscope

Selection is a decision made with the ear open, reading the ossicular remnant and the host bed. A few principles convert the catalogue above into operative judgement:

• Let the remnant choose PORP versus TORP. A mobile superstructure mandates a PORP; only its absence justifies a footplate-mounted TORP, which then needs a cartilage shoe and meticulous tension [2013, 2015].
• Match the smallest solution to the smallest defect. Isolated incudostapedial erosion with an intact, mobile chain is better rebridged in situ with bone cement than spanned with a whole strut, closing the gap to within 20 dB in roughly 80–94% of such cases [2014].
• Retain the malleus and protect the head. Couple to the malleus where you can, and cap an alloplastic head with cartilage to keep it from eroding through the drum [2001, 2004].
• Remember the autograft in the favourable ear.When the incus is salvageable and the ear is dry and aerated, the patient’s own bone gives excellent, low-extrusion results at no cost [2017].

Work the remnant through the decision aid below: the state of the stapes superstructure and the long process of the incus together point to a construct, while the malleus and a cartilage cap improve stability throughout.

CThe environment beats the implant

The chapter closes where it opened, with a discipline-defining truth: the middle-ear environment usually matters more than the prosthesis. Mucosal health, aeration, eustachian-tube function, the absence of active infection, and the integrity of the drum and stapes dominate the outcome, and no strut rescues a fibrotic, non-aerated, or actively diseased ear. This is why hearing results converge across well-chosen modern materials and why the strongest prognostic factors in large series are ossicular-chain status, mucosal status, otorrhoea, and revision surgery rather than the implant chosen [2001].

For the surgeon, the implications are practical. Prepare the ground before you place the strut:clear disease, restore aeration, keep the windows free, and stage the reconstruction in unfavourable ears rather than forcing a prosthesis into a hostile cleft. Choose the construct from the remnant, retain native lever where you can, and accept that a modest, durable gain in a healthy ear beats an ambitious reconstruction in a diseased one. The unifying idea of this overview — carried into every material chapter that follows — is that prostheses are tools, not totems: the skilled surgeon keeps the whole shelf in mind and matches the strut to the ear and the defect, knowing that the biology will have the final word.