Ossicular chain

Malformation of the ossicular chain rarely occurs as a solitary lesion; it is usually associated with other temporal bone anomalies, most commonly atresia of the external auditory canal (see external ear). The most common isolated congenital anomaly is disconnection between the incus and stapes: the long process of the incus is absent or abnormal and the superstructure of the stapes may also be malformed (Fig.1).

Erosion of the ossicular chain is seen commonly in cholesteatoma, but also occurs in non-cholesteatomatous otitis media. The long process of the incus is the most vulnerable part of the ossicular chain. Ossicular fixation may occur as an end-stage of chronic otitis media: this is called fibroadhesive chronic otitis media; on CT, soft tissue thickening may be seen around the ossicular chain. Also, see tympanosclerosis.

The conductive hearing loss, resulting from ossicular erosion, may be correctable by surgical insertion of an ossicular implant. The most common procedure is interposition of an incus (originating from a cadaver, or from the patient), sculptured to fit between the stapes and the manubrium of the malleus or tympanic membrane (Fig.2).

Ossicular chain interruption is seen with and without associated temporal bone fracture (see longitudinal fracture temporal bone, transverse fracture temporal bone). Because of poor ligamentous support, the incus is the ossicle most commonly affected by trauma. The most common lesion is luxation of the incudostapedial joint (Fig.3), followed by complete separation of the incus, at both the incudostapedial and malleoincudal joint. Subluxation of the incus is easily recognized on axial CT by disturbance of the normal ice cream cone image (Fig.4); in subtle cases, the dislocation is only apparent on coronal CT images as the Y sign.

RH