3Incus Interposition Step by Step

FWhat the operation rebuilds

When chronic ear disease eats away the long process of the incus, the ossicular chain falls silent at its weakest link. The drum still vibrates and the stapes can still rock the inner-ear fluids, but the broken bridge between them lets sound energy leak away as a conductive hearing loss. Incus interposition— sometimes called incus transposition — is the elegant, century-old answer: take the very ossicle that has failed, lift it out, reshape it, and reseat it so that it bridges the gap once more. The bulky body of the incus, the part disease usually spares, becomes a small sculpted strut that re-couples the handle of the malleus above to the head of the stapes below [1994].

Because the graft is the patient’s own bone, there is no rejection, no risk of disease transmission, and no cost beyond operating time. It integrates with minimal foreign-body reaction, and by reconnecting the native malleus and stapes it preserves much of the chain’s natural lever— the mechanical advantage the middle ear uses to match sound from air into fluid. This module is about the doing: how the incus is delivered, carved, and seated, and the geometry that separates a durable repair from a disappointing one. The five operative steps below frame the rest of the chapter.

FBefore you carve: confirming the anatomy

Interposition only works when the two anchors it bridges are sound. The reshaped incus must seat against something above and something below: the malleus handle (or, if the handle is gone, the drum) at the top, and a mobile stapes superstructureat the bottom. That pairing — malleus present, stapes arch present and mobile — is exactly the configuration the Austin classification calls type A, and it is the home territory of incus interposition [1971]. Lose the stapes superstructure and you can no longer interpose to the capitulum; you must reach the footplate with a total prosthesis instead.

Two further checks decide whether to proceed. First, the incus itself must be reusable: its body has to be preserved enough to carve a strut and, critically, free of cholesteatoma, because squamous matrix can invade an ossicle microscopically in a way the naked eye cannot exclude. Second, the middle-ear environmentmust be favourable — a well-aerated cleft, healthy mucosa, a working Eustachian tube and quiescent disease — since the autograft is living tissue that an inflamed, fibrotic ear will punish. Palpate the stapes for mobility before committing: a fixed footplate makes any superstructure repair futile.

TDisarticulation, delivery and off-field carving

The operation begins by freeing the incus without injuring its neighbours. Separate the incudostapedial joint first: working from the stapes side means the fragile capitulum is never levered against while the incus is still tethered above. Then divide the incudomalleolar joint, taking care of the chorda tympani, which drapes across the incus neck and is easily avulsed by a careless sweep. With both joints open the incus lifts cleanly out of the epitympanum. Inspect the delivered ossicle: a healthy, disease-free body is your raw material; matrix on or in it means you must sterilise the remnant or abandon the autograft.

Carving is then done off the field— on a separate platform or held on a sucker, away from the open vestibule — so that drilling debris and any slip of the burr cannot be driven into the inner ear. Under the microscope, a fine diamond burr drills the long and short processes back to the level of the body, leaving a compact block to shape. The discipline of carving off the field is not fussiness: a momentary skid of a spinning burr against a mobile stapes or an open oval window can cause irreversible sensorineural loss. This is the single most important safety habit of the technique [1994].

TDrilling the acetabulum and seating the strut

Two features are then cut into the block. On one surface a shallow cup — the acetabulum, roughly 1 mm across — is drilled to cradle the head of the stapes; on another a groove or facet is fashioned to receive the malleus handle [1994]. The acetabulum matters more than its size suggests: a true cup self-centres the strut on the capitulum so it cannot slide off as the ear heals, whereas a flat contact tends to migrate. The goal is a strut whose length and angle let it sit snugly between the two anchors without tension and without tipping — close to perpendicular to the drum so the lever is preserved [1994].

With the graft shaped, seat it gently and watch the chain. The strut should rest with light, stable contact at both ends, and palpation should set the whole reconstruction moving freely. If the malleus bows laterally or the chain feels splinted, the graft is too long and must come out for trimming — you trim the bone, never the irreplaceable stapes. Many surgeons protect the contact points with a wisp of fascia or a sliver of cartilage and ensure the reconstruction is buttressed by a well-supported tympanic membrane or graft, then stabilise it with small pledgets of absorbable gelatin sponge so the strut is not displaced as healing proceeds. The reward for this fiddly carpentry is a bridge made entirely of the patient’s own, perfectly biocompatible bone.

CGetting the length right

Geometry is everything, and length is the variable the surgeon controls at the burr. Too short, and the strut loses contact with one anchor — the air-bone gap simply persists despite an apparently completed reconstruction. Too long, and it tensions and stiffens the chain, bowing the malleus laterally and threatening to sublux the very stapes the operation set out to use; a splinted chain conducts poorly and may fail late. The target is the narrow middle ground where the strut bridges both anchors with gentle, stable contact and the chain still moves freely. The explorer below lets you slide the sculpted length through these zones and see the mechanical consequence.

Where the malleus is awkwardly medialised or foreshortened, even a perfectly carved strut may not seat well; the surgeon may then carve the incus to bridge drum to stapesinstead, or abandon interposition for a prosthesis whose fixed geometry handles the anatomy better. The practical rule is conservative: shape long, seat, assess, and trim in small increments rather than risk an over-long strut that loads the stapes. A strut that is a fraction short of ideal but mobile usually outperforms one that is technically “in contact” but tensioning the chain.

CWhat the technique buys you

Done well, the sculpted autograft delivers hearing that rivals a manufactured prosthesis with a durability few alloplasts can match. In a representative series of 137 interpositions the mean air-bone gap fell from about 27 dB to roughly 19 dB, with two-thirds of ears closed to within 20 dB and — importantly — no extrusions; retesting more than a year later showed the gap had barely moved [2005]. Longer follow-up confirms the durability: at a mean of more than nine years, 70% of ears still held an air-bone gap of 20 dB or less, the longest such series reported [2004]. The bony bridge does not slowly loosen the way a poorly coupled prosthesis can.

Two comparisons place the technique. Against a modern titanium incus prosthesis, a large series reported air-bone gap closure within 20 dB in 81% of stable ears — excellent — but at the price of a 7% extrusion rate and a frequent need for cartilage interposition beneath the drum, exactly the failure modes the autograft avoids [2017]. Against bone cement, which rebridges a short residual long process directly to the stapes, cement edges ahead for very short defects — in one head-to-head series 78% of cement repairs closed to within 20 dB versus 63% for interposition — but cement needs a substantial stump, whereas the sculpted incus can span a longer gap [2013]. The honest summary is that incus interposition remains the most physiological, most biocompatible, and least-extruding reconstruction available, and in a clean, well-aerated Austin type A ear it is an excellent default — provided the surgeon is willing to invest the carving time and respect the geometry that makes or breaks it [1994].