In-vivo characterisation of an implanted microphone and totally implantable active middle ear implant.

Abstract

OBJECTIVE\nTo objectively evaluate acoustic sensitivity of the implanted microphone, and maximum stable gain of a totally implantable active middle ear implant.\n\n\nDESIGN\nProspective, single centre evaluation.\n\n\nSTUDY SAMPLE\nFourteen adult patients.\n\n\nRESULTS\nMicrophone sensitivity is approx. 10\u2009dB lower than an externally worn conventional hearing aid, at frequencies up to 4000 Hz, and substantially lower at higher frequencies. The masking level due to microphone noise, which determines the softest test tones that can be detected, is estimated at <20 to <30\u2009dB HL up to 1000\u2009Hz, and <40\u2009dB HL at higher frequencies. Maximum stable effective gain is the maximum amplification achievable without causing feedback whistling. In sensorineural hearing loss (SNHL) cases, it is 30-40\u2009dB at frequencies up to 2000\u2009Hz, allowing to compensate for even the maximum recommended hearing loss (60-70\u2009dB HL). In both SNHL and mixed hearing loss (MHL) cases, maximum stable effective gain is lower (+20 to -30\u2009dB) around 3000-6000\u2009Hz.\n\n\nCONCLUSIONS\nMicrophone sensitivity is high enough to achieve aided thresholds of 20-40\u2009dB HL. A strong correlation between actuator coupling efficiency and maximum stable effective gain implies that any effort to improve actuator efficiency should also increase the available gain.