8TORP Design and Biomechanics

FWhen the superstructure is gone

A total ossicular replacement prosthesis (TORP) is the device the surgeon reaches for when the stapes superstructure — the capitulum and the two crural arches — has been destroyed, leaving only the mobile footplate sitting in the oval window. Where a partial prosthesis (PORP) can rest its foot on a present, cup-shaped stapes head, a TORP has no such ledge to perch on. It must instead bridge the entire gap, from the smooth, flat footplate at the floor of the oval window all the way out to the reconstructed drum or the malleus handle. In that single strut the TORP recreates, in synthetic form, the ancient idea of a columella: one continuous element connecting the sound-collecting membrane to the cochlear window, a concept that runs through avian and reptilian middle ears and that Wullstein formalised when he brought synthetic struts into reconstructive tympanoplasty [1956].

The distinction between a PORP and a TORP is therefore not about material— both are made in titanium, hydroxyapatite, and composite forms — but about where the foot lands. A PORP lands on the stapes head; a TORP lands on the footplate. That single anatomical fact changes everything about how the prosthesis behaves, because the footplate is a far less forgiving landing site than the stapes head, and the longer strut is far harder to keep upright. The whole of TORP design is an answer to one question: how do you keep a long column standing on a slippery, socketless plate while it carries sound faithfully for years?

FAnatomy of a TORP construct

A TORP has three working parts, and each presents the surgeon with a decision:

• The head (or shield)— a broad plate that meets the drum. It must be wide enough to couple sound from the membrane but is a foreign body pressing on thin epithelium, so it is almost always protected from the drum by a thin cartilage interposition. A bare alloplastic head tends to erode and extrude; capping the head with cartilage drove extrusion rates down to low single figures in the hydroxyapatite era and remains standard practice [1992].
• The shaft — the column that sets the length and therefore the tension on the chain. A few tenths of a millimetre decide whether the construct is over-tight (stiffening the annular ligament) or loose (losing contact). Some shaft flexibility is desirable so the strut conforms to the conical drum and forgives small sizing errors.
• The foot (or footplate shoe)— the base that meets the footplate. This is the TORP-specific problem child: the footplate is flat and offers no socket, so the foot must be widened, grooved, or stabilised with an interposed graft to stop it tilting and sliding.

The ideal TORP, then, is a light, biocompatible column with a cartilage-protected head, a slightly flexible shaft cut to the loosest stable length, and a foot designed to stay centred on the footplate. No single material achieves all of this perfectly, which is why the literature on TORP design is really a literature on coupling and stabilisation rather than on which alloy is best.

TThe two-interface instability problem

The PORP has one delicate interface to manage well; the TORP has two, and they pull in different directions. At the lateral end, the head presses on the drum and tends to extrude— the failure mode shared with the PORP, managed by cartilage interposition and a central, light contact. At the medial end, the foot must stay upright on the footplate and tends to tilt and migrate— the failure mode unique to the TORP. Because the strut is long, any small rotation at the foot is magnified into a large displacement at the head, and vice versa, so the two interfaces are mechanically coupled: instability at one end levers the other.

Why is the footplate such a hostile landing site? The stapes head a PORP uses is a concave capitulumthat gently captures and centres a prosthesis foot. The footplate offers nothing of the sort: it is a smooth, slightly convex oval plate suspended on the annular ligament, with no rim, socket, or lip. A foot set on it is balanced rather than seated. Add the ordinary perturbations of healing — scar contracture, a haematoma, a retraction pocket, or even the swings of ambient pressure across the drum — and a TORP that was perfectly placed at surgery can tilt, slip off the plate, or lever the head out of the drum over the following weeks [2008]. Tilting is doubly damaging: it not only loosens the construct but also angles the foot so that force is delivered obliquely rather than axially into the cochlear fluids, wasting the very transmission the surgery was meant to restore.

TStabilising the foot on the footplate

Because tilting and migration of the foot are the dominant TORP failure modes, much of TORP technique is devoted to building the socket that the footplate lacks. Several strategies are used, often in combination:

The most studied of these is the cartilage shoe, in which an oval cartilage plate roughly 2.5 × 3.5 mm with a central perforation is placed in the oval window niche; the hole captures and centres the prosthesis foot on the footplate, and at second-look the graft is found to have integrated stably into the niche[2008]. Purpose-built titanium footplate prosthesesachieve the same end by engineering a positive seat. In a clinical series, adding a titanium footplate-stabilising device to a TORP improved closure of the air–bone gap to within 20 dB from 44% to roughly 70% of ears and reduced prosthesis displacement [2015]. The recurring principle is the same one that governs the head: create a stable, central, low-mass contact— here by manufacturing the socket the footplate never had.

Crucially, stabilising the foot must never mean fixing it. The annular ligament supplies most of the chain’s compliance; a shoe that splints the footplate, or bone pate packed rigidly around the niche, would convert a mobility problem into a fixation one. The aim is a centring cushion that holds position while letting the footplate piston freely, sized for the loosest configuration that remains positionally stable.

CWhy TORPs trail PORPs — mechanics and selection

Pooled outcome data consistently show that, on average, TORPs close the air–bone gap less reliably than PORPs. A meta-analysis of 40 studies and more than 4,000 ears found PORP reconstruction significantly more effective at restoring the chain, with a combined risk ratio around 1.28 in its favour and better long-term stability [2013]. It is tempting to read this as a simple verdict that the TORP is the inferior device, but two quite different mechanisms are tangled together, and separating them is what matters at the clinic.

The first mechanism is mechanical, and it is everything described above: the longer strut, the socketless footplate, the two coupled interfaces, and the loss of the natural lever and stable coupling that a present stapes arch would have provided. A drum- or malleus-to-footplate column is intrinsically harder to keep aligned and transmits force less faithfully than a PORP seated on a mobile capitulum, and malleus coupling helps but does not fully restore the advantage [2004].

The second mechanism is selection, and it is easy to overlook. The very indication for a TORP — an absentstapes superstructure — is itself a marker of more aggressive disease. Ears that have lost their entire stapes arch have usually endured more severe or more chronic cholesteatoma or otitis media, with worse mucosa, more fibrosis, and poorer aeration. These host-bed factors independently degrade outcome, so the TORP cohort is sicker before a single prosthesis is chosen. The same staging work that established the malleus as a favourable factor identified an absent superstructure as an independent adverse one [2001]. In other words, part of the TORP’s apparent inferiority is the company it keeps.

This distinction drives two clinical rules. First, preserve and use any mobile stapes arch: a usable superstructure should be reconstructed with a PORP, never sacrificed to standardise on a TORP. Second, when the superstructure is genuinely gone, the TORP is the correct and effective device — the response is to optimise its mechanics and its host bed, not to abandon it. Occasionally a surgeon will choose a TORP despitea present arch — for a medialised, rotated, or otherwise unusable stapes — a deliberate, situation-specific decision rather than a default [2015].

COperating habits for a stable TORP

The biomechanics converge into a short list of habits that separate a TORP that holds and hears from one that tilts and fails:

• Build a socket for the foot.The footplate offers none, so make one — a grooved foot, a cartilage shoe in the niche, or a titanium footplate-stabilising device — to centre and capture the column[2008, 2015].
• Recruit the malleus when you can. A malleus-coupled head gives a firm, central lateral anchor that steadies the whole strut and recovers part of the lever; release the tensor tympani to lateralise a medialised handle if needed [2004, 2001].
• Cap the head with thin cartilage. Protect the drum from the bare alloplastic plate to prevent extrusion, using the smallest disc that will do the job so as not to load the high frequencies[1992].
• Size for the loosest stable, slightly flexible fit. Avoid over-lengthening, which stiffens the annular ligament and can subluxate the footplate; err loose, because healing tightens the construct.
• Fix the environment, not just the chain. No TORP overcomes a non-aerated, fibrotic, or actively diseased ear; mucosal status and ventilation set the ceiling, and a staged approach may be wiser than forcing a reconstruction into a hostile bed [2001].

The unifying idea is that the TORP is a columella reinvented in metal and ceramic, asked to do the work of an entire ossicular chain across a longer span with fewer natural landmarks. Its design problems — extrusion at the drum, tilting at the footplate, over-tensioning of the annular ligament — are all problems of keeping a long, light column stable and well coupled at both ends. Solve those, choose the device for the defect rather than by habit, and respect the diseased bed that made the superstructure disappear in the first place, and the TORP earns its place as the indispensable tool for the most damaged ears [2013].