Abstract

1. Energy transfer to the cochlea using a round window implantable hearing deviceJonathan H. Spindel1 and Geoffrey Ball2Department of Integrated Science and Technology and Department of Communication Sciences and Disorders, James Madison University, Harrisonburg, Virginia, USA1Institute for Applied Physics, University of Innsbruck, Innsbruck, Austria2 Abstract This study investigates placement of an implantable hearing device vibrational transducer on the cochlear round window membrane. Ten fresh frozen human temporal bones were implanted with floating mass transducers (FMT; Vibrant Med-El, Innsbruck, Austria). These bones were tested in three stages of implantation: (a) un-implanted, (b) implanted with a standard incus FMT placement (I-FMT), (c) implanted with an FMT placed on the round window membrane (RW-FMT). Derived measurement of induced displacement provided objective measurement of the vibratory input to cochlea and throughout the middle ear. These data indicate that that for a similar electric signal, the RW-FMT provides 10-15 dB greater linear displacement than the I-FMT. Normalization of the RW-FMT data to account for the difference in area between the round and oval windows, however, indicates that the two attachments will result in a similar volume displacement. Continuing studies in patients will provide greater insight into perceived loudness differences between these two methods of cochlear stimulation and help define surgical technique and clinical applicability of the round window approach. Methods and Materials Ten fresh frozen temporal bones were examined visually for abnormalities. Each bone was fitted with a probe microphone (ER-7C, Etymotic Research, Elk Grove, IL, USA) and acoustic transducer (ER-2, Etymotic Research, Elk Grove, IL, USA). Each bone was tested using acoustic stimuli, stimuli from an FMT attached to the incus (I-FMT) and stimuli from an FMT placed on the round window (RW-FMT). Laser Doppler Vibrometry (LDV, (Sensor OFV303, Controller 3001 S, Polytec, Waldbronn, Germany) data were obtained. Measurement of vibrational velocity was obtained from the stapes footplate and round window membrane for each test condition. (Data of round window during RW-FMT stimulus was measured from the FMT itself.) Figure 3: The displacement of the stapes footplate in response to I-FMT stimuli Figure 4: The displacement of the round window in response to RW-FMT stimuli • Background • Research and development over the past two decades has been directed at defining implantable hearing devices that can be used to circumvent issues associated with conventional acoustic amplification. As a result of these efforts a variety of implantable transducers have been developed, tested and applied clinically for the rehabilitation of hearing loss. Within the context of these efforts, debate continues to focus on the site of implantation that can maximize the transfer of vibrational energy to cochlea. • The objective of the current study was to investigate the response of this device when applied to the cochlear round window. • Potential advantages of a round window-based implant may include: • Increased ability to deliver energy to the cochlea • Use in treating conductive or mixed losses • Ability to treat patients with middle ear abnormalities • Potential for use as a electro-acoustic device A B Figure 1: Schematic of the experimental set-up for incus (A) and round window (B) FMT testing. (Probe microphone (ER-7C), external auditory canal (EAC), oval window (OW), round window (RW), Laser Doppler Vibrometer (LDV), incus FMT (I-FMT), round window FMT (RW-FMT)) A B The Vibrant Soundbridge (Vibrant Med-El, Innsbruck, Austria) is an approved clinical implantable hearing device that uses a floating mass transducer to impart vibrational energy to the cochlea through an attachment to the incus. This device has been shown to effective for use in the rehabilitation of sensorineural hearing loss Figure 5: Comparison of displacement responses for the I-FMT and RW-FMT. The solid blue line indicates the RW-FMT response corrected for round window area. Conclusions The corrected area response above implies that the volume displacement of the cochlear fluid is roughly equivalent in both incus and round window FMT driven stimuli. Nevertheless, these findings clearly demonstrate that a round window FMT stimulus is fully capable of driving the ear at or above levels achieved by an ossicular placement. Placement of an FMT, or other vibrational direct drive system, on the round window membrane bypasses the middle ear and delivers vibrational energy almost directly to the cochlear fluid. Avoiding reliance on ossicular placement permits a round window based device to be used to treat a range of middle ear pathologies outside of the inclusion criteria for current middle ear implants. Further human clinical investigation to define and refine surgical placement methodologies and clinical objective and subjective responses are necessary and will confirm the validity of this approach. Figure 2: Surgical view of the round window without (A) and with (B) the RW-FMT in place. • Results • Measurement of the movement of the stapes footplate in response to the I-FMT is shown in Figure 3 for three stimulation levels. The response of the RW-FMT to these same driving signals is show in Figure 4. Direct comparison of these responses shows a greater vibrational amplitude for the round window stimulus. If the linear displacement is corrected for the smaller area of the round window relative to that of the stapes footplate, however, this difference is significantly reduced.