Acoustic and Implant Driven Vibrations of the Round WindowJonathan H. Spindel1, Richard L. Goode2, Alex Huber3, Geoffrey Ball4

Department of Integrated Science and Technology and Department of Communication Sciences and Disorders, James Madison University, Harrisonburg, Virginia, USA 1

Division Of otolaryngology – HNS, Stanford University, Palo Alto California2

Department of Otorhinolaryngology, University Hospital, Zurich, Switzerland3

Institute for Applied Physics, University of Innsbruck, Innsbruck, Austria4

Abstract

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 is to investigate the response of a round window implantable hearing device. Potential advantages of a round window-based implant could include increased efficiency for delivering energy to the cochlea, use in treating conductive or mixed losses and an ability to treat patients with middle ear abnormality.

A series of bench top studies using human temporal bone were employed to examine round window vibrations in non-implanted bones and bones implanted with floating mass transducers (FMT; Vibrant Med-El, Innsbruck, Austria). Bones were tested using scanning and single point laser Doppler vibrometry (LDV). Scanning measures were obtained for round window vibrations in non-implanted bones while single point LDV measures were taken under 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 measures 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 area differences between oval and round windows, however, indicates that the two attachments will result in similar volume displacements. 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.

Results

Measurement of displacement of the stapes footplate in response to the I-FMT is shown in Figure 3 for three stimulation levels. Figure 4 shows the response of the RW-FMT to these same driving signals. Figure 5 shows a direct comparison of average cochlear window displacements measured for an external ear canal (EAC) acoustic stimulation of 100 dB SPL compared to I-FMT and RW-FMT stimuli. RW-FMT responses at the round window are between 8 dB and 17 dB greater than that obtained from the I-FMT at the stapes footplate for the equivalent drive current to the transducer.

Single Point Measures

Ten fresh frozen temporal bones were examined visually for abnormalities. Each bone was fitted with a probe microphone and acoustic transducer. Each bone was tested using acoustic stimuli, stimuli from an FMT attached to the incus (I-FMT) and stimuli from an FMT in contact with the round window membrane (RW-FMT). A 0.5 mm long, 0.5 mm diameter nylon rod glued to one end of the RW-FMT facilitated placement and contact with the round window membrane. Laser Doppler Vibrometry data provided measurement of vibrational velocity. Velocity data was obtained from the stapes footplate and round window membrane for each test condition and mathematically converted to displacement data.

Conclusions

The corrected area response shown in Figure 5 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 including chronic conductive and mixed losses. The potential for higher levels of stimulation to the ear for equivalent drive current may permit use of this approach for high gain applications.

Further human clinical investigation to define and refine surgical placement methodologies and clinical objective and subjective responses are necessary to support and confirm the validity of this approach.

• 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 2: Surgical view of the round window without (A) and with (B) the RW-FMT in place. Contact with the RW membrane was accomplish using a 0.5 mm long 0.5 mm diameter nylon rod attached to the FMT.

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 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

The objective of the current study is to investigate the response of this device when applied to the cochlear round window.

Advantages of a round window-based implant may include:

Scanning LDV Measures

Figure 1: Scanning Laser Doppler Vibrometery (SLDV) captures time dependent “images” of round window vibrations under conditions of acoustically induced vibration of the tympanic membrane at 1 kHz (adapted from Huber, Goode and Ball data).

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.