The ins and outs of middle ear implants

Middle ear implants have been around since 1935 when a Dr. Wilska sprinkled iron filings onto the eardrum of a patient while the patient was lying prone on a medical couch.1 He applied a strong magnetic field encased in an earphone over the test ear and the patient reported hearing sound, despite the absence of an acoustic signal. The flux of the magnetic field caused the iron filings to vibrate in synchrony, which in turn vibrated the eardrum. From that point on, the transduction was identical to conventional acoustic stimulation. The problems with the Wilska device were obvious: First, the patient had to remain lying down. Second, the amount of current necessary to generate a sufficient magnetic field was prohibitively great. Because of these issues, very little progress was made in magnetic stimulation of the ear until the 1970s and 1980s. Now, however, as a result of improved technology, the problem of current has been all but resolved. Fifty years ago, generating 80 dB SPL required approximately 28,000 milliamps; with today's middle ear implants (MEIs), it takes only about 3 milliamps. The first clinically wearable device was made in Japan by Drs. J.-I. Suzuki and N. Yanagihara.2 The instrument was designed for persons with chronic middle ear dysfunction. Interestingly, all of the modern MEIs are intended for hearing-impaired patients whose middle ear function is normal. Thus, the ideal candidate is someone with a purely sensorineural hearing loss who has tried hearing aids with limited success. This limited success may be related to chronic acoustic feedback or to insufficient high-frequency amplification. (Candidacy is discussed elsewhere in this issue by Lisa Evans-Smith). THE ELECTRO-MAGNETIC MEI The original Wilska device used an electrically based magnetic field “transmitter” and a magnet “receiver.” This basic approach, although refined, continues to be used by some of today's middle ear implants. Specifically, a modern electromagnetic MEI uses an external coil connected to a microphone and amplifier and an implanted magnet, usually, but not always, situated on the ossicles. This is significantly more efficient than Wilska's approach in that the magnet receiver is implanted in the middle ear cavity instead of on the eardrum. Otherwise, though, there has not been any major change from the 1930s. The modern version of the electromagnetic MEI was first described by Fredrickson over 25 years ago,3 but only recently has achieved commercial status in North America. Three companies—Otologics LLC of Columbus, OH; SoundTec of Oklahoma City, OK; and Symphonix Devices, Inc. of San Jose, CA—have developed electromagnetic MEIs. To date, however, the Symphonix Soundbridge is the only MEI to have received approval from the FDA, while the SoundTec Direct Drive System and the Otologics Middle Ear Transducer™ Ossicular Stimulator are under FDA review. Three essential features characterize all electromagnetic MEIs. First, this type of transduction is very efficient (with an impedance on the order of 102 or 103), which means that levels in excess of 130 dB SPL can be transduced. This capability makes the electromagnetic MEI potentially useful for severe to profound sensorineural hearing losses. Regulatory concerns (related to safety and effectiveness) have limited output to about 110 dB SPL, but the potential is there for more gain and output. Secondly, no current electromagnetic MEI is small enough to be completely implanted in the middle ear. Consequently, all the electromagnetic MEIs that have been approved or are under review are only partially implanted devices, since they include an external coil situated either behind the ear or in the ear canal. The third common feature of existing electromagnetic MEIs pertains to where the magnet is implanted on the ossicular chain. The more medial (nearer the cochlea) the position of the magnet, the more high-frequency sound transmission is possible, due to the rotational characteristics of the ossicular chain at higher frequencies. The more lateral (toward the eardrum) the magnet's placement, the closer it is to the external magnetic coil and, therefore, the stronger the transduced sound is. The desired balance between transduced strength and high-frequency transmission determines the location of the magnet. In most electromagnetic MEI systems, the magnet is located at or near the incudo-stapedial joint. The magnet can be crimped to the ossicular chain (as with Symphonix), inserted on a holder that slips over the incudo-stapedial joint after a temporary disarticulation (SoundTec), or connected via a probe to the medial portion of the incus (Otologics). In each case, implantation involves safe and reliable surgical procedures. PIEZO-ELECTRIC MEIs During the 1970s, a second type of MEI, the piezo-electric device, was developed.2 This system uses a small piezo-electric crystal that is either connected to the ossicular chain or, in some implementations, replaces the more lateral portions of the chain. A piezo-electric crystal generates current when physically bent, and when a current is applied to it, it bends. Thus a piezo-electric crystal can function as both a microphone and a receiver—albeit with differing levels of efficiency. The structure is remarkably simple. An external microphone transduces sound to the crystal which is implanted in the middle ear. This received current causes the crystal to bend, which, in turn, causes the ossicular chain to vibrate in synchrony. Two piezo-electric MEIs have been developed. The system made by St. Croix Medical of Fridley, MN, is undergoing FDA review, while Implex AG Hearing Technology of Munich has received the CE European approval for its product. Two essential features characterize the piezo-electric type of MEI. First, piezo-electric MEIs are quite simple and very small—small enough that both the existing models are totally implantable. With the St. Croix device, the wearer's eardrum serves as the microphone, while the Implex device uses a small microphone situated under a flap of skin in the posterior portion of the ear canal wall. Since these MEIs are totally implantable, frequent battery changes are impractical. Thus, the Implex product has a rechargeable battery, while the battery used by St. Croix is reported to last 4 to 5 years. The second feature relates to the fact that piezo-electric transduction is significantly less efficient than its electromagnetic counterpart. Its impedance is on the order of about 107 to 109, which means that the maximum possible output is about 110 dB SPL. The gain, therefore, is no more than about 30 dB to 40 dB, which would limit its use to moderate or, at most, moderately severe sensorineural hearing losses. The most obvious advantage of the piezo-electric MEI in its current form is that it is completely implantable. With a maximum gain of 30 dB to 40 dB, one needs to question whether a completely-in-the canal (CIC) hearing aid would be a viable alternative if cosmetics were the only concern. However, a clear advantage of an MEI over a CIC is that it does not occlude the ear. Table 1 summarizes the salient characteristics of each of the MEI systems that are either currently being reviewed by the FDA or have received FDA or other (e.g., CE mark) approval from government regulatory bodies. The electromagnetic and piezo-electric MEIs are two implementations of a new concept in hearing aids, i.e., implantability. Both types can use the latest hearing aid technology and most models can be easily updated.Table 1: A summary of five middle ear implant systems. CE = European approval; piezo = piezo-electric; EM = electromagnetic.The differences within each type of MEI are minimal and are related to surgical technique, magnet, or microphone location. Patients who have tried and been dissatisfied with conventional amplification because of acoustic feedback or insufficient high-frequency amplification may find MEIs to be a viable alternative.

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