Characterization of ossicular chain vibration at the umbo: implications for a middle ear microelectromechanical system design.

We propose the use of a microelectromechanical system (MEMS) accelerometer as a middle ear microphone for future totally implantable cochlear prostheses. The MEMS accelerometer would be attached to the umbo to detect and convert the natural bone vibration that occurs in response to external sounds into an electrical signal that represents the acoustic information. The signal could be further processed to stimulate cochlear implant electrodes. To determine the feasibility of our proposal, we conducted a study to investigate whether the characteristics of umbo vibration along two orthogonal axes-one axis being perpendicular to the tympanic membrane and the other axis being parallel to the tympanic membrane plane but orthogonal to the long process of the malleus-differ significantly enough to compromise the sensing performance of the proposed accelerometer should a position misalignment occur during the implant procedure. We used laser Doppler vibrometry to measure the displacement of the umbo along the two orthogonal axes in 4 cadaveric temporal bones at multiple frequencies within the audible spectrum. We found that the peak-to-peak displacement frequency response along both axes was nearly flat from 250 Hz to 1 kHz, and it gradually rolled off with a slope of approximately -20 dB and -40 dB per decade above 1 kHz and 4 kHz, respectively. At each frequency, the displacement exhibited a linear function of the input sound level with a slope of 20 dB per decade. A comparison of measurements along the two axes indicated a similar frequency response, with an average amplitude difference of 20%. The characterization data suggest that the performance of a miniature ossicular vibration-sensing device attached on the umbo would not be degraded in the event of a position misalignment. The data also indicate that a MEMS accelerometer needs to achieve a resolution of 35 i g/sqrt[Hz] to detect normal conversation.