Influence of skin flap thickness on the transmission characteristics of middle ear implant audio processors.

Objective: To measure signal transmission characteristics for audio processors of an active middle ear implant as a function of skin flap thickness, i.e., distance between audio processor and the implant's receiver coil.

Methods: Output sound pressure levels for 90 dB input sound pressure level (OSPL90), reference test gains as function of frequency for an input sound pressure level of 60 dB (RTG60), and reference test gains (RTG - high frequency averages) were recorded in a hearing aid test box for Samba 2 Hi, Samba 2 Lo, and AP404 audio processors (MED-EL, Innsbruck, Austria) positioned on an implant-in-the-box and distances of 0-10 mm between audio processors and the receiver coil.

Results: For all audio processors, the OSPL90 and RTG decreased linearly with increasing distance. The effect was dependent on audio processor type and the strongest reduction was observed for Samba 2 Lo. Between distances of 0 mm and 10 mm, the relative change of RTG was - 9,9 dB for Samba 2 Hi, -10,3 dB for AP404, and -27,7 dB for Samba 2 Lo.

Conclusions: Skin thickness is a clinically significant factor which has to be considered in VSB treatment. In combination with insufficient transducer coupling or in patients with hearing thresholds close to the indication criteria limit, a thick skin flap could lead to loss of transmitted energy resulting in insufficient audiological outcome with the active middle ear implant.