The mechanical point impedance of the human head, with and without skin penetration

The fact that a titanium screw can be implanted into the mastoid portion of the human skull, at the same time establishing a permanent, reaction-free skin penetration, has made it possible to attach a new bone conduction hearing aid directly to the skull. To understand and improve this new method of bone stimulation, the mechanical point impedance of the titanium screw-skull system was measured. The conventional point impedance of the skin-covered mastoid portion of the temporal bone was also measured and the difference in magnitude between the two impedances was calculated. An impedance head (Brüel & Kjaer 8001) and an FFT analyzer (Hewlett-Packard 5423) were used for mechanical point impedance measurements. Seven patients have been investigated. The magnitude of the impedance for the screw-skull system was found to be generally between 10 and 30 dB higher than that for the conventional skin-covered mastoid bone. One conclusion is that the conventional point impedance of the skin-covered mastoid portion of the human skull is essentially due to the properties of the skin and subcutaneous soft tissue. Another conclusion is that a much lower stimulation velocity is needed, with skin penetration, to produce a given hearing sensation.