Vibrant Soundbridge Direct Drive Middle Ear Implant: Issues and Review

Overview and Historic Review:
Direct drive, implantable middle ear hearing devices represent a new category of hearing devices. Rather than delivering acoustic energy into the external auditory canal (as with traditional hearing aid systems), direct drive middle ear implant systems use mechanical vibrations delivered directly to the ossicular chain, while leaving the ear canal completely open.

One major advantage of direct drive devices is the ability to provide improved sound quality to hearing impaired subjects (Luetje, Brackman, Balkany et al, 2002).

Patient reports of improved sound quality for direct drive devices of various types have been published previously by many authors (Goode 1978, Hough & Vernon 1987, Maniglia 1982, Yanigihara 1988). In a recently published report, patients using the Vibrant® Soundbridge™ middle ear implant systems using the Floating Mass Transducer™ (FMT) have also reported improvements in overall sound quality, clearness of sound and tone, and improved sound quality with respect to their own voice (Luetje, Brackman, Balkany et al, 2002).

Candidacy Profile:

Appropriate candidates for direct drive middle ear hearing devices include adults aged 18 years and older with moderate-to-severe sensorineural hearing loss. Candidates should have experience with traditional hearing aid fittings and should desire an alternate hearing system. Word recognition as determined under headphones should be at least 50 percent correct in the designated ear. Normal middle ear function should be apparent based on clinical history, tympanometry and observation. The patient should be counseled regarding realistic expectations.

Patient Profile:

Often, patients who are interested in seeking direct drive middle ear hearing devices have experienced dissatisfaction regarding the sound quality of their own voice. Despite multiple office visits to their hearing healthcare professional, these individuals are unable to produce speech sounds that sound "normal" or comfortable to them and they are unable to overcome this obstacle. Additionally, some of the patients who have been successful with the direct drive middle ear hearing device have experienced physical discomfort while wearing traditional hearing aids.

Other commonalities across these patients includes; frustration with amplification secondary to multiple hearing aid repairs and multiple office visits, cerumen issues relating to occluded hearing aid receivers and potential cerumen impaction of the external auditory canal, inability to wear traditional hearing aids due to sensitive ear canal skin, exostosis, miscellaneous physical complaints related to the presence of a foreign object in the ear canal, and the inability to overcome acoustic feedback issues with traditional amplification.

Sound Quality:

Sound quality measures are difficult to grasp and quantify. Almost by definition, the word "quality" implies a subjective measure. There are several theoretical reasons why direct drive middle ear implant hearing devices might indeed, produce better sound quality than conventional acoustic hearing aids.

The theoretical issues which might impact qualitative judgments include; increased high frequency gain (more high frequency energy can be delivered via the Vibrant Soundbridge than would typically be anticipated using traditional hearing aid technology), improved signal coupling (bypassing the tympanic membrane, yielding a potentially more efficient high frequency sound transfer system (see Goode and Ball, 1996), reduction in acoustic feedback (because the signal is not acoustically delivered into the external auditory canal, less acoustic feedback is likely) no insertion loss, no occlusion effect and reduced distortion from external auditory canal resonances, because the external auditory canal is not occluded while using direct drive middle ear implant systems.

Objective measurements of complex signals delivered to the stapes footplate by middle ear transducers have not yet been reported. Such evidence may be of value in supporting patient reports of improvement in sound quality when using direct drive systems. The ability to obtain and analyze such measurements may prove to be a useful tool for researchers and developers of middle ear systems.

Recent Reports:

The first Food and Drug Administration (FDA) approval for a direct drive middle ear implant system occurred on August 31, 2000. The Vibrant Soundbridge was shown to be safe and effective in clinical studies. A review of 81 patients, studied as part of the FDA approval process, determined that the participants could hear as well with the device, as with more traditional hearing aids (FDA, 2000).

Fisch et.al. (2001) reported results from 47 patients in their multi-center European clinical trials. They determined the Vibrant Soundbridge can be used safely to treat moderate to severe sensorineural hearing loss. Additionally, they reported changes in pre-versus-post operative hearing thresholds (under headphones) were clinically insignificant (within 5 dB).

Fraysse et.al (2001) reported results from 25 patients using the Vibrant Soundbridge direct drive middle ear implant system. Objective and subjective tools were used to determine results. Their results indicated that no significant changes were determined regarding the status of pre-versus-post operative hearing thresholds. Fraysse et. al. also reported significant improvements in communication across various listening situations while using the Vibrant Soundbridge, as compared to traditional (acoustic) hearing aid fittings, for the majority of their patients.

Laser Doppler Vibrotomy (LDV):

We have measured ossicular vibrations of the middle ear in response to pure tones using Laser Doppler Vibrotomy (Goode & Ball, 1996). In essence, 3-dimensional motion and dynamic acoustic parameters of the ossicles and related structures can be recorded and studied using LDV. Laser Doppler Vibrometers are commercially available (Ometron) and as sophistication improves, the applicability of this advanced technology to hearing sciences is likely to yield greater and more precise observations and measurements.

Floating Mass Transducer (FMT):

The FMT is a totally enclosed transducer that uses inertial drive to impart mechanical vibrations directly to the vibrating structure of the middle ear, i.e., the ossicles. Though small in size, the mechanical energy that the FMT imparts to the vibratory structure can be comparable to very high sound pressure levels. The FMT has been specifically designed to mimic the vibratory responses of the middle ear. It is capable of delivering mechanical stimulation to the middle ear throughout the entire speech frequency range of human ears.

The FMT has two electromagnetic coils. The coils are wound around a hermetically sealed titanium housing. Residing within the housing is a permanent magnet supported by a pair of springs. An electrical signal is supplied to the coils, which in turn causes the magnet and the entire transducer to vibrate. The driving force of the transducer is imparted to both the ossicular chain and the driving mass through their mutual reaction. This type of inertial drive transducer is referred to as a "Floating Mass Transducer" or FMT. The transducer is the key component of the Vibrant Soundbridge.

Applying LDV to the FMT:

For the 1999 poster presentation (AAA, 1999) we used a human temporal bone model and recorded the output of the LDV (Polytec SLV-200-1) measuring the stapes footplate vibration. Rather than convert the laser output into displacement, we recorded the vibrometer voltage output with an audio mini-disk system (Sony Corporation). We then used an audio analysis system (Spectra Plus, version 4.0) to analyze and display the spectral content of the recordings. We delivered the same complex signals consisting of either speech or music to transducers for analysis by the system.

Using the temporal bone protocol demonstrated in the 1999 AAA poster, we recorded the audio signal produced by stapes vibration. We obtained several recordings for complex signals including speech and music when driving the ossicular chain with a Floating Mass Transducer (FMT) and an industry standard hearing aid receiver (Knowles, Ed-Series). We compared the signal delivered to the stapes footplate by the FMT directly to that delivered by the hearing aid receiver.

The primary technical challenge associated with this method was that the noise floor of the LDV was high. By testing several temporal bones and using various sound isolation techniques, we were able to reduce the noise floor. However, in order to achieve the adequate dynamic range necessary for complex signal testing, all measurements were recorded at 110 - 115 dB SPL. In the future, obtaining recordings at lower levels may become possible as LDV technology improves.

Results:

The resultant plots compared the recorded audio signal produced by the FMT to the hearing aid receiver. The output signal from the hearing aid receiver was 110 - 115 dB sound pressure level, as measured approximately 5 mm from the tympanic membrane. The signal supplied to the FMT was calibrated to impart the equivalent amount of stapes footplate vibration that was obtained when the ear was driven with sound. The following observations were made.

1- With the FMT placed in a temporal bone and driven at a level equivalent to 110 - 115dB SPL, accurate reproduction of speech and music at the stapes footplate in the temporal bone was noted.

2- Via the FMT, high frequency spectral information was present at frequencies above 7 kHz.

3- Via the hearing aid receiver, high frequency information was largely absent.

Discussion:

Although key frequencies for speech perception are typically considered to be in the range between 0.5 kHz and 3 kHz, words such as "Northwest" often contain significant amounts of high frequency content at 4 kHz and higher. This high frequency information is important for understanding speech with a significant amount of high frequency content and for hearing high frequency environmental sounds.

Regarding the absence of high frequency information recorded for the hearing aid receiver, there are at least two possible reasons for this observation. The first is the middle ear rolls off amplitude as a function of frequency. Specifically, it may be more difficult for higher frequency, lower amplitude acoustic signals to adequately vibrate the stapes footplate. A second reason is that hearing aid receivers roll off higher frequencies. Importantly, the FMT has been specifically designed to work within the middle ear and maximize signal coupling to the ossicular chain, with specific attention to middle and high frequencies.

Output levels of 110 to 115 dB are required to address the hearing requirements of patients with moderately severe to severe hearing losses. This places significant demand on conventional hearing aids to adequately drive the tympanic membrane and the middle ear with a complete frequency spectrum, while not introducing unwanted distortion and feedback. At such high output levels, acoustic feedback and occlusion become significant issues for conventional hearing aids. It is possible for middle ear implants to operate at 110-115 dB output levels without feedback or occluding the ear canal.

Using LDV in the temporal bone model to study middle ear implants continues to prove itself as an unparalleled research tool. This method, when coupled with sophisticated audio analysis techniques, demonstrates the ability and the potential for direct drive middle ear implants to stimulate the stapes footplate with a complete frequency spectrum and with enhanced speech information. These findings suggest that implanted patients may be able to perceive subtle high frequency speech components and may also be able to better distinguish high frequency environmental sounds. The ability to deliver a complete and accurate audio spectrum to the hearing impaired patient, with unusually good high frequency information, may be the reason the "qualitative" reports demonstrate a positive reaction form patients.

Conclusion:

We believe the results reported from the original pilot study (AAA, 1999) demonstrated the ability of middle ear transducers to deliver significant energy to the stapes footplate. Playback of audio recordings using this method clearly demonstrated the high fidelity nature of the FMT.

We believe the FDA approval of the Vibrant SoundBridge (FDA, 2000) along with recent reports from Europe (Fisch et al., 2001, Fraysse et.al., 2001) help demonstrate the utility and efficacy of the direct drive system.

We believe the clinical results (subjective and objective) demonstrated in the recent Luetje, Brackman, Balkany et al (2002) paper support and demonstrate improvements in patient satisfaction, hearing instrument performance and patients preference for the direct drive system.
Importantly, of the 53 patients they reported, none were lost to follow-up, and at 5 months post-op, each preferred their Soundbridge to their hearing aid.

References:

AAA, 1999. A preliminary version of this article was presented as a poster session, at the 1999 American Academy of Audiology annual meeting. Miami, Florida.

FDA Talk Paper (August 31, 2000). FDA Approves New Implanted Hearing Device.
https://www.fda.gov/bbs/topics/answers/ans01031.html

Fisch, U., Cremers, C.W.R., Lenarz, T., Weber, B., Babighian, G., Uziel, A.S., Proops, D.W.,O'Connor, A.F., Charachon, R., Helms, J., and Fraysee, B. IN: Otology and Neurotology 2001; 22:962-972. "Clinical Experience with the Vibrant SoundBridge Implant Device."

Fraysse, B., Lavieille, J.P., Schmerber, S., Enée, V., Truy, E., Vincent.C., Vaneecloo, F.M., Sterkers, O., In: Otology & Neurotology 001;22:952-961. "A Multicenter Study of the Vibrant Soundbridge Middle Ear Implant: Early Clinical Results and Experience."

Goode, R.L. Implantable Hearing Aids. Otolaryngologic Clinics of North America, Vol. 11, No. 1, (1978).

Goode, R.L., Ball G.R. et al, Laser Doppler Vibrometer (LDV) - A New Clinical Tool for the Otologist, American Journal of Otology, 17:813-822, (1996).

Hough, J. Vernon, J., et al. A Middle Ear Implantable Hearing Device for Controlled Amplification of Sound in the Human: Preliminary Report. Laryngoscope, 97:141-51 (1987).

Luetje, C.M., Brackman, D., Balkany, T.J., Maw, J., Baker, R.S., Kelsall, D., Backous, D., Miyamoto, R., Parisier, S., and Arts, A.: Phase III clinical trial results with the Vibrant Soundbridge implantable middle ear hearing device: A prospective controlled multicenter study. Otolaryngology - Head and Neck Surgery. Feb 2002, Vol 126, No 2.

Maniglia, A.J. Design Development and Analysis of a Newer Electro-Magnetic Semi-implantable Middle Ear Hearing Device. Transplants and Implants in Otology II, (W.B. Saunders and Co.) pp 365-369 (1982).

Ometron. OMETRON VIBRATION MEASUREMENT SYSTEMS. Website location
https://www.imageautomation.com/ometron/home.html.

Yanigihara, N. Suzuki J.I. et al. Middle Ear Implant: Implantable Hearing Aids. (Karger, 1988).

Additional Recommended Readings:

Implantable hearing device technologies. Spindel, J., Hearing Journal, David Kirkwood, Ed., Volume 54, Number 8, Pages 43-44, August 2001.

An overview: Implantable hearing devices. Spindel, J., Audiology Today, Jerry Northern Ed., Volume 14, Number 1, Pages 11-13, January/February 2002.

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