Middle Ear Implants: 2002

Introduction & Historical Review:Historically, the first clinically available MEIs (by Drs. Suzuki and Yanagihara in Japan) were designed for those with unresolveable middle ear conductive/mixed losses. However, modern MEIs require a well functioning ossicular chain. Middle Ear Implants (MEIs) are useful for patients with purely sensorineural hearing loss.

Middle ear implants (also called "soundbridges") have been around in one form or another since 1935 when Dr. Wilska sprinkled some iron filings onto a person's eardrum. A magnetic field was generated by a coil of wire inside an earphone and was applied to the iron filings. The subject reported 'hearing', despite the fact there was no acoustic sound energy coming from the earphone. The magnetic field from the earphone caused the iron filings to vibrate in synchrony with the magnetic field. This electro-magnetically induced vibration caused the eardrum to vibrate, which allowed sound to be transduced from the middle ear to the inner ear in the normal fashion.

Dr. Wilska's experimental device had some obvious limitations- its bulky size, the amount of energy required to transduce sound, (28,000 mA to produce 85 dB SPL) and of course, the person had to be lying down in order to keep the iron filings correctly positioned on the tympanic membrane!

Since the 1930s, a number of research teams around the world have tried to create a wearable MEI. Current MEIs can generate 85 dB using less than 3 mA!

What is a middle ear implant?

A middle ear implant can be thought of as a hearing aid, where either the receiver, or the entire unit, is surgically inserted into the middle ear. The potential advantages of MEIs are three-fold;

First, if the ossicles can be driven directly (also referred to as "direct drive") there may indeed be improved sound quality, with essentially no acoustic feedback.

Second, a completely implantable MEI may one day be available, with no external components at all. Indeed two manufacturers have designed completely implantable middle ear devices.
Thirdly, depending on the MEI, if there is no device in the ear canal, there is no insertion loss with a net boost in high-frequency sound transmission. Resulting in a higher fidelity, more useful sound product.


Are all middle ear implants the same?

No, MEIs come in two general categories; electromagnetic and piezo-electric. While Dr. Wilska's work in the 1930s used an electromagnetic approach, the first clinically wearable middle ear implants were of the piezo-electric type.

Piezo-Electric:

A piezo-electric crystal has some interesting properties. When such a crystal is bent, it generates an electric charge, and when an electric charge is applied to the crystal, it bends. Indeed, many of the miniature transistor radios of the early 1960s were piezoelectric and were simply called "crystal sets." The crystal acted as a small ''microphone'' for the radio waves that bent according to the incoming waves. This bending of the crystal generated a small electric current-- enough to drive the radio set. Such a crystal can also be used in the middle ear. When attached to the middle ear bones, a small electric current from the microphone would cause it to bend and flex. This in turn causes the middle ear bones to vibrate and thereby transduce sound to the inner ear. Since 1984, this type of middle ear implant has been used successfully in Japan on over 60 patients and was pioneered by Drs. Suzuki and Yanagihara.

Several other implant programs around the world used the piezo-electric approach with variations which either improved the surgical technique or improved the sound quality. One piezo-electric implant program that is currently in the initial stages of FDA regulatory approval in the United States is from St. Croix Medical. Another, the Implex device from Germany, has already received European CE approval. Essentially, with these devices, either the person's eardrum acts as a microphone (St. Croix Medical) or a small microphone is placed in the posterior portion of the ear canal wall (Implex). Sound is transduced from the environment to the piezo-electric crystal attached to one of the middle ear bones. This causes the bones to vibrate -- sending the sound energy directly to the inner ear.

Some advantages of the piezo-electric approach are that the components of the implant are physically small and the design is quite simple. In contrast, the electromagnetic approach uses bulkier components, and depending on its implementation, can be quite complex. Interestingly, the magnet should weigh less than 50 mg. If the magnet's mass is above this, there will be "loading" of the ossicular chain with the introduction of an associated high-frequency conductive component.

Electromagnetic:

In contrast to the piezo-electric method of transducing sound, there are several programs around the world (mostly in the United States) using the electromagnetic approach. With this method, the microphone sends sound energy to a coil of wire that creates a magnetic field. The magnetic field communicates (through induction) with an implanted magnet -- much like a cochlear implant. Instead of a crystal being implanted, a magnet is connected to the ossicular chain which vibrates in synchrony with the magnetic field. The mechanical vibration of the ossicular chain is transferred to hydraulic, and then bio-electric signals via the inner ear.

There are technical issues regarding where to place the magnet, as well as the nature and orientation of the magnetic coil. Nonetheless, many of the research protocols and findings are similar. Essentially, the magnet has to be medial enough on the ossicular chain to derive benefit from the high-frequency rotational characteristics of the ossicular chain, yet lateral enough to be near the transducing magnet to optimize gain and output. A compromise location is near the incudo-stapedial joint.

One major advantage of the electromagnetic approach is that this technolgy can generate significantly more gain and output than can piezo-electric technology. Piezo-electric hearing aids will only be useful for someone with up to a moderate or moderately-severe hearing loss in the mid and high frequencies (maximum output of 110 dB SPL), the electromagnetic approach can be useful for those with severe hearing loss. In both technologies, there is greater gain and output in the mid-and-high frequency region, than for lower frequency sounds.

Because of regulatory concerns, during FDA trials the electromagnetic MEI manufacturers have limited the output to about 110 dB SPL (similar to the piezo-electric MEIs), but theoretically, they should be able to transduce up to about 140 dB SPL.

Four (or five) MEIs:

Based on the most recent review (January, 2002) there are four (or five) middle ear implants that have either received regulatory approval or are in regulatory trials. Of the five MEIs, three are electromagnetic and two are piezoelectric.

Electromagnetic:
(1) Symphonix Vibrant Soundbridge
(2)Otologics MET (Middle Ear Transducer)
(3) Soundtec Direct System

Piezoelectric:
(4)St. Croix Medical Envoy
(5)IMPLEX TICA (Totally Implantable Cochlear Amplifier)

The first three MEIs are only partially implantable. Specifically, only the receiver (i.e., the magnet and associated transducing coil) is implanted. The microphone and the amplifier are worn externally. While this approach may not be as cosmetically appealing as the smaller piezo-electric approach of St. Croix Medical or Implex, it may be useful for a wider range of hearing losses.

Unique Features:

The Vibrant Soundbridge from Symphonix uses a well designed ''floating mass transducer'' magnet that is crimped onto the ossicular chain without any necessary ossicular disarticulation during surgery. The Otologics LLC implant uses a probe that is connected to a hole drilled with a laser to the medial side of the incus. The Soundtec system initially used a doughnut shaped magnet that was situated at the incudo-stapedial boundary, but now uses a magnet that is held alongside the ossicular chain by a coil of wire that has been connected to the ossicles. The current implementation of the Soundtec device is to house the electromagnetic coil in a shell that sits in the ear canal coupled to an amplifier in a behind-the-ear housing, however an application has been submitted to FDA for approval of a fully in-the-canal external component. Other MEI manufacturers use disk-shaped cases or a behind-the-ear hearing aid case situated behind the ear. Greater high-frequency gain (boost) will be achieved with MEIs that do not have anything obstructing the ear canal.

The differences among the various electromagnetic MEIs may appear subtle. Indeed, the decision to use one, rather than another, may be based on many factors-- type and extent of surgery, availability in one's geographic region and preferences of the otolaryngologist. All approaches use surgical techniques which appear to be safe, reliable, and reversible. Of the various MEI surgeries, it appears that the Soundtec device is the shortest and least invasive and can be implanted under local anesthesia.



The differences between the two piezo-electric MEIs are also subtle. It is not clear whether there is any advantage to use the eardrum as a microphone or to situate a microphone in the ear canal wall. The Implex device uses a Walkman type head set that recharges the battery every 3 or 4 days. The recharging takes 90 minutes. In contrast, the St. Croix Medical device uses a battery that lasts about 5 years. Surgery for the piezo-electric MEI tends to be more involved than that for the electromagnetic MEIs, and may not be as easily reversible.

MEI Candidacy Issues:

For all modern MEIs, patients must have completely normal middle ear function in the ear of interest. Patients must be 18 years of age or older. Typical MEI candidates are those who have not been successful with conventional amplification due to physical discomfort, unresolved occlusion, feedback, or limited high frequency amplification. Appropriate hearing losses (for electromagnetic systems) range from mild to moderate to severe hearing losses.

Binaural fittings are reasonable. Binaural MEIs makes good intuitive and audiologic sense, as has been demonstrated with hearing aids, binaural Bone Anchored Hearing Aids (BAHA) and binaural cochlear implants.

High Frequency Emphasis Issues:

MEIs that do not have a coil housed in a hearing aid shell residing in the outer ear canal, will be able to generate more high frequencies. MEIs transduce much more efficiently in the mid-and-high frequency regions, and as such, would be better for flat or sloping sensorineural hearing losses. Indeed, MEIs may be less than optimal for reverse slope hearing losses. As was mentioned earlier, greater high-frequency gain (boost) will be achieved with MEIs that do not involve obstructing the ear canal.

One can argue that completely-in the-canal (CIC) hearing aids already provide a cosmetic alternative for those who may have considered the totally implantable piezo-electric MEI. In this sense, the number of people opting for a piezo-electric implant may be significantly lower than what would have been the case ten years ago (before the introduction of CIC hearing aids). However, the improved fidelity obtained from driving the middle ear bones directly without occlusion and without insertion loss from ear canal obstruction, may yield a form of amplification which performs significantly better than conventional air conduction hearing aids.

The range of hearing losses successfully fit with an electromagnetic middle ear implant is vast. Whether or not electromagnetic devices become totally implantable, many people with severe hearing loss will be able to obtain significant benefit without acoustic feedback and without occlusion.

Regulatory Issues & Approval:

To date, only two MEIs have received FDA regulatory approval for use in the United States, those are the Symphonix (June 2000) and Soundtec (September 2001) devices. In addition, Symphonix carries the CE mark which approves it for use in Europe and certain parts of South America. The Soundtec device received approval for use in Canada in the fall of 2001. IMPLEX has received the CE mark. Unfortunately, IMPLEX has not responded to attempts to contact them, and there have not been any published reports of the device. The other manufacturers are well into FDA trials- either phase I or phase II, and contacting their websites or companies directly can keep you up to date on the regulatory status.

References (and comments):

1- Maniglia, A., & Proops, D. (ed.). (2001). "Implantable Electronic Otologic Devices: State of the Art", The Otolaryngologic Clincs of North America, Vol. 34, No. 2., Philadelphia: W.B. Saunders Company.
(This is an excellent technical overview of the field of middle ear implants and bone anchored hearing aids written by those surgeons, engineers, researchers, and audiologists who are intimately involved with MEIs.)

2- Chasin, M. (guest ed.) (2002). "Trends in Implantable Hearing Aids", Vol. 6 (in press- spring 2002), New York: Westminster Publications, Inc.
(This issue of Trends in Amplification, edited by Todd Ricketts with Marshall Chasin as guest editor, brought together representatives from middle ear implant and bone anchored hearing aids to describe clinical results and issues relating to these devices)

3-Middle Ear Implant: An Audiologist's First Hand Experience, Lisa Evans-Smith AuD. Published on the internet website Audiology Online (https://www.audiologyonline.com) /audiology/newroot/articles/arc_disp.asp?id=273&catid=1

4- Middle Ear Implant: My Personal Experience, 1/14/2002, Lisa Evans-Smith , Audiologist, Denver, Colorado. Published on the internet website Healthy Hearing (www.healthyhearing.com). https://www.healthyhearing.com/healthyhearing/newroot/patient/displayperspective.asp?id=57

Contact Information:
Symphonix Devices, Inc.,
2331 Zanker Rd.,
San Jose, CA, 95131-1109
www.symphonix.com
Otologics LLC,
Suite106-5445 Airport Blvd.
Boulder, CO, 80301, www.otologics.com
Soundtec,
Suite 100-5400 Grand Boulevard,
Oklahoma City, OK, 73112, www.soundtecinc.com
St. Croix Medical,
#113-5301 East River Rd.,
Minneapolis, MN, 55421, www.stcroixmedical.com

NOTE: An earlier version of this article was published on Audiology Online in August of 2000. The previous version is available on the internet at: /audiology/newroot/articles/arc_disp.asp?id=231&catid=1

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