Implications for Mechanisms of Bone Conduction from Thresholds of Post Mastoidectomy & Petrosectomy Patients

Bone conduction (BC) is elicited by applying a bone vibrator to the head. The resulting skull vibrations are conducted via bone, reaching the ear where they induce threshold by the following classical osseous BC mechanisms acting in parallel: the occlusion effect of the external ear, ossicular chain inertia of the middle ear and inner ear fluid inertia and distortion (compression-expansion) of the walls of the inner ear. Following description of these BC mechanisms, implications of mastoidectomy and petrosectomy surgery on these mechanisms will be presented. As a result of the surgery, the occlusion effect and ossicular inertia are lost, and inner ear fluid inertia and distortion would be severely reduced. Nevertheless, in 28% of radical mastoidectomy, and in 8% of petrosectomy patients, BC thresholds were normal. It is possible that the normal BC thresholds following the surgery may be due to the loss of one or more of the classical osseous BC mechanisms, leading to less mutual cancellation, so that BC threshold may remain normal. It is also possible, if not more likely, that BC threshold is induced by a different non-osseous mechanism, and the onset ("threshold") of the classical osseous BC mechanisms may be somewhat higher.