Ossicular-Chain Dislocation

Excerpt

Hearing loss following head injury is a common clinical finding, although typically delayed in its presentation due to the immediate investigation for more significant injuries. Hearing loss can be conductive, sensorineural, or mixed. When conductive hearing loss persists more than 6 weeks after injury, ossicular chain dislocation must be considered.

The ossicular chain consists of three bones; the malleus, incus, and stapes joined by two synovial joints, the incudomallear and incudostapedial joints. Together, they comprise the primary sound-conduction apparatus to transmit vibratory stimulus from the tympanic membrane to the oval window. In addition to simply conducting sound, amplification occurs due to the compound lever effect of the ossicles and their natural resonance. Complete loss of this conductive pathway would result in a conductive hearing loss of around 50-60dB.

The entire ossicular chain is encased within the petrous portion of the temporal bone within the middle ear cavity. While the temporal bone offers significant bony protection, the structure remains vulnerable to damage from varying mechanisms. The relatively fixed position of the malleus and stapes compared to the incus may contribute to the finding that dislocation of the ossicles is far more common than fracture.