MODELING FLEXIBILITY IN THE HUMAN OSSICULAR CHAIN: COMPARISON TO OSSICULAR FIXATION DATA

Evidence exists for flexibility in the ossicular chain. This evidence includes the observation that patients with otosclerotic lesions sufficient to cause a substantial reduction in stapes mobility and conductive hearing loss show only small reductions in low-frequency umbo velocity. In models of the middle ear, ossicular flexibility has often been assigned to the incudostapedial (I-S) joint the incudomalleal (I-M) joint is assumed rigid and this presumed flexibility is generally not well described. We introduce flexibility in the I-M joint in a simple middle-ear model to explain the discrepancy in reductions in umbo and stapes velocity at low frequencies observed in human temporal bones with simulated stapes fixation by Nakajima et al. (2003qb). We determine the magnitude of I-M joint flexibility from measurements of low-frequency relative rotational velocity (Willi et al., 2002) and assume the I-S joint is rigid. The relationship between umbo and stapes velocities predicted by the model for normal ears and the reduction in umbo velocity predicted for various degrees of stapes fixation (modeled by reductions in annular ligament compliance) are a good match to experimental data in temporal bones and to clinical data in patients with stapes fixation. Reductions in umbo velocity due to stapes fixation are successfully predicted by varying a single model parameter, the annular ligament compliance, at the site of fixation.