Methods to predict the clinical output levels of acoustic implants

Abstract

Over the last years, Active Middle Ear Implants (AMEIs) and Direct Acoustic Cochlear Implants (DACIs) have become well established as a therapy for hearing impaired patients who are not suited for a provision of a hearing aid or of a cochlear implant. To improve the outcome of such devices and to develop new therapies a method is needed that allows to determine the efficiency and the clinical output level of the implant before clinical data are available. ASTM standard F2504–05 describes a procedure to quantify the output levels of AMEIs as equivalent sound pressure levels from stapes vibration measurements in human cadaveric temporal bones (TBs). However, this standard is intended only for stimulations at the ossicular chain and requires a mobile and visible stapes. Common applications such as the round window (RW) stimulation and the mechanical inner ear stimulation with a DACI are thus outside the scope of ASTM standard F2504–05 and earlier studies indicate that stapes vibration is no reliable reference for the outcome of those stimulations. Therefore, in the present thesis it was first tested whether ASTM standard F2504–05 can be modified to quantify the output level of RW stimulations and mechanical inner ear stimulations. For this purpose these and further stimulation modes were performed with a DACI actuator in human cadaveric TBs and the output levels were quantified based on vibration measurements at the stapes and at the RW. The results show that such an adaption of the ASTM standard is possible but the determined output levels showed a pronounced variation and should be used only for rough estimates, especially at frequencies > 1 kHz. Therefore in the present thesis an alternative method was developed to quantify output levels of AMEIs and DACIs in cadaver studies. This method uses the intracochlear sound pressure difference (ICPD) between the inner ear compartments scala vestibuli and scala tympani as a reference, because this measure has been shown to correlate with auditory evoked potentials in animals and it is considered as the input to the inner ear independent from the mode of stimulation. In order to create the basis for a commonly accessible method it was first tested whether ICPDs are measurable in human TBs with off-the-shelf pressure sensors. The results demonstrate that two pressure measurement systems are usable to measure ICPDs during acoustic stimulation with sufficient SNR and sensitivity. Next, it was investigated whether ICPD can be used to quantify output levels of an AMEI in human cadaveric TBs. For this purpose an AMEI Incus stimulation was performed in TBs and the output levels were quantified as equivalent sound pressure levels from both stapes vibration measurement and ICPD measurement. To validate the results the experimentally obtained levels were directly compared to clinical data. Although ASTM standard F2504–05 has been published already in 2005 and is commonly used today, such a comparison demonstrating that the output levels predicted from cadaver studies actually match the real outputs in patients, had never been performed before. It was found that output levels estimated from stapes vibrations according to ASTM standard F2504–05 and clinical data match within 9 dB but ICPD as reference provided even more accurate results being almost identical to clinical data. So the results of this thesis demonstrate for the first time that both ICPD and stapes motion can be used as a valid measure to predict the clinical output level of AMEIs in cadaver studies. Based on the here presented findings vibration measurement as the faster and easier to conduct method is preferable to assess mechanical stimulations at the ossicular chain, whereas ICPD measurement is preferable if even more accurate results are needed and to assess stimulations outside the scope of ASTM standard F2504–05.