Acceleration levels at hearing threshold with direct bone conduction versus conventional bone conduction.

Some patients with hearing disorders cannot use a conventional air conduction hearing aid, but have to use a bone conduction hearing aid. The technique of introducing a permanent, skin-penetrating titanium implant has made it possible to develop a bone-anchored hearing aid with all parts in a single housing. Ten patients have been using such an aid for almost 2 years. This investigation deals with absolute acceleration threshold measurements in 7 of these patients. The measuring apparatus consisted mainly of a Békésy audiometer and an accelerometer, Brüel & Kjaer 4344. In the frequency range from 250 to 6 000 Hz, the acceleration levels at hearing threshold decreased by between 16 and 28 dB when measured directly on the titanium screw, as compared with when measured on the intact skin surface. This lowering in acceleration thresholds can be explained by the fact that skin and subcutaneous soft tissues act as a shunt for acceleration.