Prosthetic teeth turn out to be effective carriers of vibrations, making them suitable as potential hearing aids.
In a research paper titled "The sensitivity of bone conduction for dental implants," published earlier this month in The Journal of the Acoustical Society of America, eggheads at Tongji University in Shanghai describe tests to determine how well implanted teeth conduct sound.
The authors, Fengxuan Ren, Yutong Li, Lidan Chen, Jiaqi Huang, and Jianxiang Tao, explain how it's well-known that bone conduction through natural teeth can produce an auditory effect. But sound transmission through artificial teeth hasn't received as much study.
So the researchers assembled 38 subjects who met the desired criteria and had them listen for a signal transmitted by a vibration device placed on either a tooth implant, a natural tooth, or the mastoid bone (located just behind the ear). The stimulus took the form of tones with frequencies between 250 Hz and 4000 Hz at 30 dB, and at lower decibel levels if the participants signaled that they heard the sound.
The results suggest "dental implants were more sensitive to [bone conduction] than natural teeth and mastoids at some frequencies," the authors observe.
The study results also indicate that implants in front (anterior) teeth have a lower threshold to detect vibrations than back (posterior) teeth. "[A]nterior dental implants showed better [bone conduction] sensitivity than posterior ones," the authors state, speculating that this is due to differences in bone density in the front and back of the jaw.
The first bone anchored hearing aid (BAHA) was implanted by Anders Tjellström, of the University of Gothenburg, in 1977. Presently, these devices take the form of products including the Cochlear Baha System, Oticon Ponto, and Medtronics Alpha 2 MPO ePlus. Some are non-surgical – worn on a headband – and others, depending on the specific condition being treated, require the surgical implantation of a metal stud or magnet that supports an external sound processing unit.
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BAHAs and cochlear implants are both options for treating profound hearing loss. BAHAs can be either surgical or non-surgical and rely on vibrations to transmit sound waves to the inner ear and auditory nerve. Cochlear implants are always surgically installed and carry sound directly to the auditory nerve via electrical impulses.
The development of conductive hearing technology has had some obstacles – for instance, US federal rules for Medicare insurance reimbursement. About a decade ago, a company called Sonitus Medical raised $80 million to develop its non-surgical SoundBite Hearing System, "a prosthetic device that makes novel use of the established principle of bone conduction by transmitting sound via the teeth and thereby replacing the function of the impaired ear."
Three years later, the company declared bankruptcy after a Centers for Medicare & Medicaid Services (CMS) decision not to reimburse patients for SoundBite hardware – a tooth-fitted mouthpiece and microphone receiver. The federal agency overseeing Medicare classified the device as a hearing aid rather than a prosthetic because no surgery was required.
So it may be that vibrating prosthetic teeth can survive the bureaucracy of the American health system because they're implanted rather than worn.
"Since dental implants exhibited excellent [bone conduction] properties, dental implants could be used as potential [bone conduction] hearing assistive devices in the mouth," the researchers conclude. "Suppose these devices could be contained in the superstructures of dental implants. In that case, dental implant hearing aids will provide unique advantages in practical applications, such as excellent concealment, good comfort, and improved quality of sound." ®