Hearing in Response to Soft Tissue Stimulation – Bone Conduction or an Alternative Pathway to the Inner Ear? Evidence from Bone Anchored Implant Users
2Department of Otolaryngology and Head and Neck Surgery, Shaare Zedek Medical Center, Jerusalem, Israel
3Speech & Hearing Center, Hebrew University School of Medicine-Hadassah Medical Center, Jerusalem, Israel
4Department of Communication Disorders, Hadassah Academic College, Jerusalem, Israel
5Department of Medical Neurobiology (Physiology), Institute for Medical Research - Israel-Canada, Hebrew University-Hadassah Medical School, Jerusalem, Israel
Background: Although hearing via bone-conduction (BC) is often implemented in aural rehabilitation, its mechanisms are not fully understood. For example, it is not clear whether stimulation to sites other than bone utilizes BC hearing. To better understand whether BC is involved when soft-tissue is stimulated, the present study compared an objective measure of skull-vibrations to subjective hearing-thresholds in response to bone and soft-tissue stimulation.
Objective: To compare hearing-thresholds to skull-vibration magnitudes in patients with percutaneous bone-anchored implant (BAI) in a within-subject design. The assumptions were that (1) because the BAI integrates with the skull, abutment vibrations will reflect `true` skull-vibration, and (2) if hearing thresholds and skull-vibration magnitudes will respond similarly to stimulation of soft-tissue, that would imply that bone and soft-tissue stimulation share a similar mechanism.
Methods: Five hearing-impaired adults with BAI participated in the study. We assessed skull-vibrations using laser-Doppler vibrometer focused on the implant, and hearing thresholds in response to stimulation of the neck, the mastoid and the forehead with and without a layer of gel. We then compared relative hearing thresholds and skull vibrations (neck vs. mastoid and gel vs. forehead).
Results: Bone vibrations magnitudes were correlated linearly with stimulus intensity at all sites of stimulation, which allowed extrapolating the vibration to lower levels. Stimulation of soft-tissues led to hearing threshold elevation and an equivalent reduction of skull vibrations.
Conclusions: Theoretically, the findings support the notion that bone and soft-tissue stimulation share a common mechanism. Clinically, skull-vibration magnitude may predict hearing-thresholds and therefore could be used to verify BC device fittings.
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