Intracochlear Sound Pressure Measurements in Normal Human Temporal Bones During Bone Conduction Stimulation

Bone conduction (BC) is heavily relied upon in the diagnosis and treatment of hearing loss, but is poorly understood. For example, the relative importance and frequency dependence of various identified BC sound transmission mechanisms that contribute to activate the cochlear partition remain unknown. Recently, we have developed techniques in fresh human cadaveric specimens to directly measure scalae pressures with micro-fiberoptic sensors, enabling us to monitor the input pressure drive across the cochlear partition that triggers the cochlear traveling wave during air conduction (AC) and round-window stimulation. However, BC stimulation poses challenges that can result in inaccurate intracochlear pressure measurements. Therefore, we have developed a new technique described here that allows for precise measurements during BC. Using this new technique, we found that BC stimulation resulted in pressure in scala vestibuli that was significantly higher in magnitude than in scala tympani for most frequencies, such that the differential pressure across the partition-the input pressure drive-was similar to scala vestibuli pressure. BC (stimulated by a Bone Anchored Hearing Aid [Baha]) showed that the mechanisms of sound transmission in BC differ from AC, and also showed the limitations of the Baha bandwidth. Certain kinematic measurements were generally proportional to the cochlear pressure input drive: for AC, velocity of the stapes, and for BC, low-frequency acceleration and high-frequency velocity of the cochlear promontory. Therefore, our data show that to estimate cochlear input drive in normal ears during AC, stapes velocity is a good measure. During BC, cochlear input drive can be estimated for low frequencies by promontory acceleration (though variable across ears), and for high frequencies by promontory velocity.