Middle Ear Implants

Introduction:

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

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 subjects 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 vibration in turn caused the eardrum to vibrate, which allowed sound to be transduced 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 a sound, (28,000 mA to produce 85 dB SPL) and the person had to be lying down on a bed 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 with less than 3 mA!

What is a middle ear implant?

A middle ear implant is a hearing aid where either the receiver, or the entire hearing aid is surgically inserted into the middle ear. The advantages of such an implant are two-fold.

First, if the ossicles can be driven directly, there may be improved sound quality, with no feedback. Second, a middle ear implant may be completely implantable with no external components at all. Indeed two manufacturers have now designed completely implantable middle ear devices. In addition, 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.

Are all middle ear implants the same?

No, MEIs come in two different types- 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.

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 &gravei;crystal setsî.

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 also 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.

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. This field then communicates (through magnetism) with an implanted magnet -- much like a cochlear implant. Instead of a crystal being implanted, a magnet is connected to the ossicular chain that vibrates in synchrony with the magnetic field. This vibration is then sent to the inner ear. There are technical issues regarding exactly where to place the magnet, as well as the nature and orientation of the magnetic coil, but the essence of many of the research programs 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 chain, and lateral enough to be near the transducing magnet in order to optimize gain and output. A compromise location is near the incudo-stapedial joint.

Some advantages of the piezo-electric approach include 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. In all cases, the magnet should weigh less than 50 mg. Above this mass, there will be loading of the ossicular chain with the introduction of an associated high-frequency conductive component. Nevertheless, the one major advantage of the electromagnetic approach is these hearing aids can generate significant more gain and output than can piezo-electric hearing aids. Whereas 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 the low-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.

Based on the most recent review, there are four electromagnetically based MEIs that have received some status in the FDA regulatory approval system. None have been approved completely. However, it is expected that at least one may receive final approval by the end of 2000. The four devices are:

(1) the Vibrant Soundbridge from Symphonix
(2) the Otologics LLC implant led by Dr. John Fredrikson of Washington University,
(3) the Soundtec system based on the work of Dr. Jack Hough in Oklahoma and,
(4) the device of Dr. Anthony Maniglia of Cleveland.

These electromagnetic devices are only partially implantable, in that only the receiver (i.e., the magnet and associated transducing coil) is implanted. the microphone, and 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.

Some 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 device of Anthony Maniglia uses an innovative combination of coils that helps to transduce the sound efficiently. The current implementation of the Soundtec device is to house the electromagnetic coil in a shell that sits in the ear canal. Other MEI manufacturers use disk-shaped cases or a behind-the-ear hearing aid case situated behind the ear. More net high-frequency boost will be achieved with those MEIs that do not have anything obstructing the ear canal.

The differences among the various electromagnetic MEIs may be subtle, and the decision to use one over another may be based on many factors-- type and extent of surgery, availability in one's geographic region, and comfort of the otolaryngologist. All approaches use surgical techniques which appear to be safe, reliable, and reversible.

Table I: A summary of six MEIs that either are in current use around the world or have obtained some level of FDA regulatory approval. Note that the two piezo-electric MEIs are completely implantable.



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.

Candidacy:

For all modern MEIs, the patient must have completely normal middle ear function in the ear of interest. It is not unreasonable to consider a binaural fitting, just as benefits have been shown for binaural Bone Anchored Hearing Aids (BAHA) and binaural cochlear implants. Typical MEI candidates are those who have not been successful with conventional amplification because of unresolveable occlusion, feedback, or limited high frequency amplification. Those MEIs that do not have a coil housed in a hearing aid shell that occupies the outer ear canal, will be able to generate more high frequency sound transmission than those that do have such an insertion loss. 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. MEIs may be less than optimal for reverse slope hearing losses.

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 with no occlusion and insertion loss from an outer ear obstruction, may yield a from of amplification which performs significantly better than conventional air conduction hearing aids.

The range of hearing losses that can be successfully fit with an electromagnetic middle ear implant is certainly great. Whether or not this form ever becomes totally implantable, many people with severe hearing loss will be able to obtain significant benefit without feedback and without the occlusion effect.

Other MEIs and the Future:

There are other MEIs discussed in the research literature which have not yet advanced to a point where FDA approval has been sought. The primary reason is undoubtedly funding, rather than engineering or scientific excellence. One such implant is based on the work of Jonathan Spindel at the University of Virginia and uses a round window location for the magnet. In this approach, the ossicular chain is left intact with the transduction entering by way of the round window (and of course, the unobstructed ossicular chain). Whenever two transduction routes coincide, interesting possibilities open up for noise suppression (for example, using phase). Other programs, have sought to situate the piezo-electric crystal or magnet against the semi-circular canal-- an arrangement that can also be used to suppress low-frequency noise. Surgical approaches will undoubtedly continue to be refined, as will the use of alternative materials for the MEI.

Table I: A summary of six MEIs that either are in current use around the world or have obtained some level of FDA regulatory approval. Note that the two piezo-electric MEIs are completely implantable.

Contact Addresses:

Anthony Maniglia, MD,
2074 Abington Rd.,
Cleveland, OH, 44106

Implex AG Hearing Technology
Munchner Strasse 101,
85737 Ismaning/Munich,
Germany,
email: kaul@implex.de or helberg@implex.de

Otologics LLC,
no.106-5445 Airport Blvd.
Boulder, CO, 80301

Soundtec,
no.100-5400 Grand Boulevard,
Oklahoma City, OK, 73112

St. Croix Medical,
no.113-5301 East River Rd.,
Minneapolis, MN, 55421
(612)574-0570
www.stcroixmedical.com

Symphonix Devices, Inc.,
2331 Zanker Rd.,
San Jose, CA, 95131-1109
www.symphonix.com