Bone conduction activation through soft tissues following complete immobilization of the ossicular chain, stapes footplate and round window

Abstract

Classically it has been thought that bone conduction activation at the mastoid leads to relative motion between the stapes footplate and the oval window due to inertial and to compression (distortion) mechanisms. However, several recent clinical findings and experimental manipulations may point to additional mechanisms. These manipulations were extended in the present study. In ten fat sand rats, following obliteration of one ear, auditory nerve brainstem evoked response (ABR) thresholds were recorded in response to broad band click stimuli, either air conducted (AC) through insert earphones or bone conducted (BC) delivered directly to the exposed skull bone. Following this, the entire ossicular chain, stapes footplate and round window were completely immobilized with super glue, leading to a mean AC threshold elevation of 44 dB, but to a mean BC threshold change (elevation) of only 3.5 dB. In this state of complete immobilization, the bone vibrator was applied to a pool of saline in the surgical area and ABR was elicited with a mean threshold which was not significantly different from that of the BC threshold. When the bone vibrator was then applied to the eye without touching the bone at the orbit, the resulting ABR threshold was about 20 dB greater than the BC threshold. In conclusion, BC stimulation can activate the cochlea without two mobile windows. Furthermore, the cochlea can be activated by a fluid pathway and by application of a bone vibrator to non-osseous sites (soft tissue conduction).