ELECTRONIC
IMPLANTABLE HEARING DEVICES
Middle Ear
Implants
Introduction & Historical
Review:
Historically, the first clinically available MEIs (by Drs. Suzuki and
Yanagihara in Japan) were designed for those with irresolvable 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 technology 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.
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.
The Symphonix device
