Effect of incus removal on middle ear acoustic sensor for a fully implantable cochlear prosthesis.

System miniaturization and steady progress towards a totally implantable prosthetic system is the current trend in cochlear implant technology. To achieve this objective, the external microphone of present implants needs to be implantable. This goal can be accomplished by placing a miniature accelerometer on the ossicular chain in the middle ear to detect and convert bone vibrations into an electrical signal for further processing and stimulating cochlear electrodes. This paper describes the characterization of the umbo of a human temporal bone before and after the removal of the incus to determine the impact of the resulting change in umbo mechanics and attached accelerometer performance. With the removal of the incus, the umbo vibration acceleration frequency response in the direction perpendicular to the tympanic membrane increases by 5 dB below 2 kHz. Above 2 kHz the response diverges due to the change of ossicular chain resonant frequency caused by the removal of the incus. However, at each frequency the umbo vibration acceleration exhibits a linear function of the input sound pressure level (SPL) with a slope of 20 dB per decade before and after removal of the incus. A commercial accelerometer attached to the umbo shows similar characteristics. From the measurement results of umbo characterization, a miniaturized implantable accelerometer with a packaged mass below 20 milligrams, a sensing resolution of 35 microg rms/square root Hz, and a bandwidth of 10 kHz would be required to detect normal conversation.