The bone-anchored bearing aid, a study of the mechanical point impedance of the buman head with and without skin penetration

Abstract

The possibility of implanting a skin penetrating titanium screw into the mastoid portion of human skull and of establishing a permanent reaction-free skin penetration has made it possible to develop and attach a new bone conduction hearing aid directly to the skull. To understand and improve this new hearing system, the mechanical point impedance of the titanium screw-skull system ZMT has been measured. The conventional point impedance of the skin-covered mastoid portion of the temporal bone ZMS has also been measured and the difference in magnitude between the two impedances, 2010 log |ZMT/ZMS|, was calculated. Seven patients have been investigated. The magnitudes of ZMT were found to be between 10 and 35 dB higher than those of ZMS. One conclusion is that the conventional point impedance of the skin-covered mastoid portion of the skull is essentially due to the properties of the skin and subcutaneous soft tissues. Another conclusion is that much lower velocity levels are needed, with skin penetration, for a given hearing sensation, which makes it possible to develop an “all in one housing” bone-conduction hearing aid.