Implantable Hearing Devices

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Chapter 32 Implantable Hearing Devices

William H. Slattery, III

It is estimated that 32 million Americans have a hearing loss severe enough to cause problems with communication. The severity of this loss ranges from mild, in which the individual may have difficulty only when significant background noise is present, to profound, in which the patient is unable to understand and communicate even in the quietest situation. Most hearing loss is sensorineural in nature. Less than 10% of hearing-impaired individuals have losses correctable by medical or surgical means. Although hearing loss may affect all frequencies, it is most common for individuals to have some component of high-frequency sensorineural hearing loss. Patients with a mild loss may receive no treatment other than instructions on modifying their acoustic environment to diminish background noise when selecting seating arrangements to allow improved listening conditions. Individuals with conductive losses usually have the opportunity to undergo surgical therapy to have the loss corrected. In some cases, owing to congenital abnormalities or infection, surgical correction is not an option. Individuals with profound sensorineural loss may receive cochlear implants, which provide electric stimulation directly to the cochlear nerve.

Most individuals with sensorineural hearing loss must rely on amplification to provide a better means of improving communication. This amplification is most commonly accomplished with conventional air conduction hearing aids. In recent years, hearing aids have decreased in size, and improved microchips have been developed, resulting in improved signal processing capabilities. Hearing aids have become easier to program with digital processors; they are able to provide much better individualization of amplification according to each patient’s needs. Despite the improvement in conventional hearing aid technology, only approximately 20% of hearing-impaired individuals who could receive benefit from amplification actually use these devices.

There are many reasons why patients do not wear a hearing aid. One of the most common reasons is that conventional air conduction hearing aids do not provide enough amplification of the sounds the individual actually wishes to hear. In addition, they produce troubling amplification of unwanted sounds, especially when background noise is present. Another major problem with conventional hearing aids is the limited high-frequency response. High-frequency output is limited by the high-frequency feedback that occurs when the microphone and receiver of conventional air conduction hearing aids are in close proximity. Although new circuitry (feedback cancellation) has diminished some of these complaints, some users of conventional air conduction hearing aids still complain of feedback problems. Poor fit can result when the ear canal enlarges because of long-term use of the device, causing feedback problems. As microchip technology has allowed hearing aids to become smaller to improve their cosmetic acceptance, it can result in greater problems with feedback as the microphone and speaker are placed closer together. Limited frequency output may produce problems with distortion, and, in particular, limited high-frequency amplification restricts sound localization abilities.

Cosmesis is still a major complaint of conventional air conduction hearing aids. Many patients believe that wearing such a device indicates a disability or carries a stigma of old age. Ear molds may cause an occlusive effect in blocking residual hearing, and produce discomfort, skin irritation, and the potential for increased infections of the ear canal (external auditory canal [EAC]). Discomfort is especially noted with smaller devices that fit more medially in the EAC. The occlusion effect, in addition to being uncomfortable, can result in loss of low-frequency information. There is a significant breakdown rate in conventional hearing aids because of wax in the receiver. Cerumen not only can occlude the receiver of the hearing aid, but also may cause wax impaction by the medial displacement of wax in the EAC.

Implantable hearing devices have become a viable alternative to conventional hearing aids. There are two main types of implantable devices. The more commonly used device is the bone-anchored cochlea stimulator (Baha). Used in Europe since 1977, the Baha received FDA clearance in 1996 as a treatment for conductive and mixed hearing losses. In 2002, the Baha was also approved for treatment of unilateral sensorineural hearing loss or single-sided deafness. The other class of implantable devices is the middle ear implant, in which stimulation of the ossicular chain or direct stimulation to the cochlea is performed. This chapter reviews both of these devices.

REQUIREMENTS OF IMPLANTABLE DEVICES

The goal of the middle ear implant is to improve the efficiency of the device by improving gain, sound quality, hearing, and noise, and eliminate acoustic feedback. The implant should improve the quality of life. The ideal implantable hearing device should be easy to implant. It should cause no trauma or damage to the normal auditory system. It is crucial that the auditory system remain intact in case of device failure. Experience with cochlear implants has shown the reliability of devices implanted in the postauricular and mastoid area, and revealed potential complications. Potential risks of implantation include further sensorineural hearing loss, damage to the dura, cerebrospinal fluid leak, and the possibility of cholesteatoma or skin implantation into the middle ear or mastoid area. Skin complications can occur with any type of implant, and infection of the device is always a risk. Additionally, difficulties arising from coupling of the device to the ossicular chain or inner ear may directly damage these structures. The facial nerve and chorda tympani may also be at risk with surgical approaches for implantation, resulting in facial paralysis or weakness, or taste disturbance. Increased stiffness of the ossicular chain resulting from device fixation to the ossicles may impede low-frequency response, whereas implants that increase the mass effect on the ossicular chain reduce the high-frequency response.

The ideal device must prove to be safe over the long-term. Batteries are required for these fully implantable devices; changes should be able to be easily performed in the office or on an outpatient basis. It is to be expected that these devices, worn for years, will require future upgrades. The devices should allow technology that is easily upgraded allowing better speech processing strategies to be programmed when made available.

Theoretically, a device that worked all the time—24 hours a day, 7 days a week—would offer a significant advantage over conventional air conduction hearing aids. The ability to use a device in all normal daily activities such as water exposure during bathing or swimming is also a significant benefit. Patients who wear conventional air conduction hearing aids are unable to use them while sleeping or during water exposure. The ideal implant offers significant cosmetic benefit by being as invisible as possible. The lack of maintenance or need to clean the EAC is a significant advantage to hearing impaired patients. A long battery life is essential to reduce the need for additional surgical procedures to change the battery.

CONVENTIONAL VERSUS IMPLANTABLE HEARING AIDS

Gain is the amount of acoustic energy a device is able to deliver above the incoming signal. An implantable hearing aid must provide better gain than conventional air conduction devices, or some other real benefit to justify its use in an individual patient. Amplification of high frequencies is expected, and gain must be significant enough to provide real benefit to the patient. Most patients with significant hearing loss require significant gain levels in the high frequencies to receive any benefit from a device. If the device does not provide enough amplification in the high frequencies, the amount of benefit the patient receives would be reduced. The maximum level of output in decibels SPL (Sound Pressure Level) required to accommodate various levels of hearing loss is approximately 50 dB above the hearing threshold. To provide benefit to patients with moderate hearing loss to moderate to severe hearing loss, a hearing aid must provide a maximum output level equivalent to 90 to 115 dB SPL, whereas a flat frequency response of 8 kHz is desirable for maximum speech comprehension.

Conventional air conduction hearing aids amplify sound before it reaches the middle ear. A microphone converts the incoming acoustic signal into an electric signal, and the amplifier and signal processor modify the electric signal to increase its strength. The receiver converts the amplified electric signal into an acoustic signal for presentation to the tympanic membrane for transmission via the middle ear to the inner ear in the normal physiologic manner.

In contrast, implantable devices provide acoustic energy to the middle ear or inner ear, bypassing the external ear canal, and in some cases the middle ear space. The microphone converts the incoming acoustic signal into an electric signal. The amplifier and signal processor modify the electric signal to increase its strength. The receiver converts the amplified electric signal into a vibratory signal for presentation to the ossicular chain or to the cochlea directly, bypassing the tympanic membrane. Middle ear implants take advantage of the direct vibration of the middle ear ossicles to drive the ossicular chain.

Middle ear implants may be totally or partially implantable. The partially implantable device consists of a microphone and a speech processor connected to a transmitter fitted with an external coil that transmits electric energy transcutaneously to the internal device. An internal receiving coil connected to a receiver provides electric energy to a transducer connected to the ossicular chain. The external device also has a battery to power the system.

A fully implantable device contains essentially all of the elements of a partially implantable device with the exception of a transducer coil and receiver. A microphone is placed under the skin, or is attached to the middle ear space and is connected to the internal speech processor. The entire system is powered by a rechargeable battery.

All middle ear implantable devices stimulate the ossicular chain or cochlear fluids; however, they differ primarily by the type of transducer used to connect to the ossicular chain or cochlea. These devices provide a broad frequency response with low linear and nonlinear distortion. They are able to amplify high frequencies without the problem of acoustic feedback seen in conventional hearing aids, allowing the potential for better hearing in background noise with a more natural sound quality. A fully implantable device can also eliminate the perceived social stigma of visible conventional hearing aids.

TYPES OF MIDDLE EAR IMPLANTS

An implantable hearing device is any surgically implanted device that converts acoustic energy to mechanical energy, which delivers vibratory stimulation to the inner ear. A transducer is a device that converts one form of energy to another. Essentially two types of transducers currently are used in middle ear implants: piezoelectric and electromagnetic. Each type has advantages and disadvantages related to power, efficiency, frequency response, and reliability.

Piezoelectric devices make use of ceramic crystals that change shape when voltage is applied. This change in shape can provide mechanical energy to stimulate the ossicular chain or inner ear. The change in the shape of the ceramic is temporary; the ceramic reverts to its original shape when the electric current is no longer applied. There are two types of piezoelectric ceramic crystals: monomorph and bimorph. The monomorph piezoelectric crystal consists of a single layer, which expands and contracts to create the vibrations directly. The bimorph consists of two bonded ceramic layers, which are arranged in opposing electric polarities. When the current is passed through the bonded layers, the entire structure bends, creating the vibration. Anatomic size restrictions limit use of piezoelectric ceramic materials because the amount of bending is proportional to the length of the crystal, reducing the amount of transductive power available.

The electromagnetic transduction of sound involves the creation of mechanical vibration by passing a current through a coil proximal to a magnet. As the electricity passes through the coil, an electromagnetic field is created, vibrating the magnet, which by direct or indirect contact causes movement of the middle ear structures or cochlear fluids or both. The magnet may be attached directly to the middle ear vibratory pathway—the tympanic membrane, incus, or stapes. A fluctuating magnetic field is generated when the coil is energized by electric signals that correspond to the acoustic input. The magnetic field causes the magnet to vibrate, inducing vibration of the ossicular chain or the cochlear fluids directly. The force generated by this system is directly proportional to the proximity of the magnet with the induction coil.

One method to produce electromagnetic stimulation is to separate the magnet from the induction coil. The coil is housed in a separate device, usually within the external ear canal while the magnet is attached to the ossicular chain. It can sometimes be challenging, however, to control the spatial relationship of the magnet and the coil when the magnet is attached to one part of the ear, and the coil is located within the ear canal, which may result in a wide variation in device performance. This can manifest as varying frequency responses, or fluctuation of output levels, if the distance between the alignment of the coil and magnet changes.

Another method of electromechanical stimulation is to house the coil and magnet together, often in a single assembly. If the magnet and coil are housed together, a probe extending from this assembly must be in contact with the ossicular chain. As current is passed through the assembly, vibrations from the probe are sent directly to the ossicular chain. This form of electromechanical stimulation optimizes the spatial and geometric relationships to avoid the problem of changing alignment that may occur between the coil and magnet. The major limitation of housing the magnet and coil together is the attachment of the stimulating device, or coupler, to the ossicle or inner ear. If the device shifts relative to the position of the ossicles or inner ear, there may be a reduction in the optimal transmission of auditory stimulus.

HISTORY OF MIDDLE EAR IMPLANTS

The use of a magnetic field to stimulate the ossicles is not new. This concept can be traced back to 1935, when Wilska placed iron particles directly on the tympanic membrane. A magnetic field was generated by an electromagnetic coil inside an earphone, which caused the iron fillings to vibrate in synchrony with the magnetic field, producing vibration of the tympanic membrane, simulating hearing. Later, Rutschmann glued 10 mg magnets onto the umbo, causing it to vibrate via the application of a modified magnetic field with an electromagnetic coil. The resulting vibration of the ossicles produced hearing sensation. Attachment of devices into the middle ear space did not occur until the 1970s with the RION device, which is discussed later in this chapter. Frederickson and colleagues developed the first mechanical device at Washington University in St. Louis, in 1973. This device, which used a multichannel digital signal processor that transmitted power to the implanted coil via a transcutaneous link, was implanted in 12 rhesus monkeys. After 2 years of implantation, there was found to be no damage to the cochlea or peripheral auditory system. Frederickson and colleagues found that the results from mechanical stimulation were similar to results produced by acoustic stimulation, and that high-intensity signals could be delivered to the middle ear effectively.

Mamiglia at Case Western Reserve University in Cleveland, Ohio, demonstrated the use of an electromagnetic device in the cat. This group used the malleus as the microphone for a totally implantable device, which was implanted in cats for approximately 9 months. As with Frederickson’s experiments, the results showed thresholds from mechanical stimulation that were comparable to acoustic stimulation with no adverse effects on the middle or inner ear. Dumond, in Bordeaux, France, worked on piezoelectric devices placed in contact with the round window membrane. Twelve guinea pigs were stimulated over a 7 month period. This group attached the piezoelectric devices to the round window without removal of any other component of the ossicular chain.

SPECIFIC DEVICES

A variety of implantable devices have undergone investigation. Devices that are currently approved and that have been implanted into humans are reviewed here (Table 32-1). Patient selection criteria depend on the type of transducer and the method by which the sound is stimulated.

TABLE 32-1 Specific Implantable Devices

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RION

The RION (Fig. 32-1), developed at Ehime University and Teikyo University in Japan in collaboration with the Rion Company by Yanagihara and colleagues, was first implanted in 1984. This is a partially implantable middle ear device that uses a piezoelectric transducer approach. The device consists of a microphone, speech processor, and battery that are contained in an external behind-the-ear unit. The internal component consists of an ossicular vibrator and internal coil, which are coupled. The essential component is the vibratory element consisting of a bimorph, or two piezoelectric ceramic elements pasted together with opposite polarity, which have been coated with layers of biocompatible material. The free end of the bimorph is attached to the stapes, and is attached to a housing unit screwed into the mastoid cortex providing fixation. The bimorph vibrates in response to applied electric current.

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FIGURE 32-1 RION system. A, Outer unit contains the microphone, amplifier, and battery. Internal component contains the ossicular vibrator. B, Canal wall down mastoidectomy (1). Canal wall up mastoidectomy (2). Cross section, canal wall up mastoidectomy (3). EAC, external auditory canal.

The indications for the RION include mixed hearing loss and significant mastoid disease. Its primary use has been in patients with conductive hearing loss from chronic otitis media. Patient selection criteria are listed in Table 32-2. Frequency responses are attenuated after approximately 5000 kHz, so patients with significant sensorineural hearing loss above this frequency may not receive as much benefit as patients with pure conductive hearing loss. The RION has been implanted in patients in Japan, but is not currently available in the United States.

TABLE 32-2 RION Device Patient Selection Criteria

Average bone conduction speech frequency hearing level (500, 1000, 2000 Hz); ≤50 dB
Moderate to severe deafness in contralateral ear
Intraoperative vibratory hearing test shows effectiveness of unit

The device may be placed in the postauricular area during a canal wall down mastoidectomy for treatment of chronic ear disease or through a transmastoid facial recess approach. The housing unit fits in the mastoid with a tip that extends to the stapes. The open mastoid cavity is the preferred approach because it allows adequate exposure to the ossicles, but this requires the ear canal to be closed off. The device is fixed to the mastoid cortex, and the bimorph container is placed over the stapes. A seat is surgically created for the internal coil and electric unit.

The RION device has been worn by some patients for more than 10 years. Implanted patients report natural sound quality without feedback or discomfort, which is very close to perceived normal hearing. Long-term sensorineural hearing loss has not occurred with the use of the RION. There have been no complications during implantation in more than 39 patients in Japan.

Totally Implantable Cochlear Amplifier

The Totally Implantable Cochlear Amplifier (TICA) device (Fig. 32-2) was developed at the University of Tubingen, Germany, in collaboration with Implex Corporation in Munich in the mid-1990s. The company went bankrupt, and the device is no longer available for use. The TICA is reviewed for historical purposes because it represents the first fully implantable device that was used in humans, and allowed the development of some important concepts that have been used elsewhere. The TICA was a fully implantable device that used a piezoelectric transducer to stimulate the ossicular chain. In addition, it used a microphone implanted in the ear canal that picked up sound. This sensor provided an electric input into the fully implanted unit or can, which was implanted subcutaneously in the mastoid area. The can was a hermetically sealed titanium container that included the speech processor, battery, and a receiving coil. The actuator attached to the body of the incus, causing vibration of the ossicular chain. The induction coil within the titanium can was used to receive electric impulses to permit recharging of the battery.

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FIGURE 32-2 TICA system. A, TICA device with the fully implantable microphone in the ear canal. The speech processor battery unit is housed in a subcutaneous pocket in the postauricular area. The stimulator attaches to the incus to stimulate vibration. B, Surgical view (1). Coronal view showing position of transducer (2). I, incus; M, malleus; S, stapes.

The patient would recharge the battery by wearing a small headband that could also be used for programming the device. A wireless remote control could be used by the patient to select four programs, adjust the volume, and switch the device on and off. The battery life was 50 hours, and recharging took approximately 2 hours. The speech processor consisted of a digitally programmable three channel audioprocessor. The induction coil could be used to receive input similar to cochlear implants for processing. Also similar to cochlear implants, fitting would start about 8 weeks postoperatively, and several programming sessions were usually required.

There were several advantages to the TICA, such as having no external components and being MRI-compatible. It had a wide frequency range from 100 Hz to 10,000 kHz. The battery life was estimated to be 3 to 5 years before a replacement was needed.

Approximately 20 patients were implanted with the TICA device. All patients had bilateral moderate to severe sensorineural hearing loss, and the patients had not benefited from hearing aids. One problem that arose after implantation of the TICA was feedback. The incus was stimulated by the actuator. The feedback occurred as sound was generated by the stimulation of the ossicular chain and picked up by the microphone implanted in the ear canal. The stimulation of the ossicular chair caused sound vibration to be transported to the cochlea, but this also resulted in the eardrum acting as a speaker, generating sound into the ear canal. This generation of sound necessitated disarticulation of the ossicular chain between the malleus and incus. To avoid feedback or sound coming out the ear canal from vibration of the malleus and eardrum, the neck of the malleus was removed.

Patients with the TICA implant described their hearing as being distortion-free and transparent. They reported excellent speech intelligibility, and an improved ability to listen to music, especially in the presence of background noise. Patients were able to use the device during sporting events, including swimming, and in the shower. The experience of the TICA implant showed that a fully implantable device was possible, and allowed the development of many of the components used in subsequent devices.

Vibrant Soundbridge

The Vibrant Soundbridge (Fig. 32-3) is the first FDA-approved implantable middle ear hearing device to treat sensorineural hearing loss, and has been implanted in thousands of patients worldwide. It is a partially implantable middle ear hearing device initially developed by Symphonix Devices, Inc. (San Jose, CA). Subsequently, Med-El Corporation of Innsbruck, Austria, took over the production and distribution of the device. The Vibrant Soundbridge is a semi-implantable hearing aid consisting of two parts: the speech processor worn externally, and the implantable vibrating ossicular prosthesis. The vibrating ossicular prosthesis is surgically placed subcutaneously in the postauricular area. The floating mass transducer (FMT) is connected to the internal receiver, and is attached to the stapes. The FMT is a unique electromagnetic transducer that contains a magnet of inertial mass within two electromagnetic coils. When activated, the magnet mass vibrates within the FMT between the two coils causing the entire unit to vibrate. Titanium strips are attached around the long process of the incus to hold the device in place. The FMT is oriented in the direction of the stapes so that the device vibrates directly into the inner ear, parallel to the plane of the stapes. The external auditory processor is held in place over the internal receiver by a magnet.

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FIGURE 32-3 Symphonix Vibrant Soundbridge system. A, Vibrant Soundbridge with external microphone, with coil for transcutaneous stimulation of internal device. Internal device connects to the floating mass transducer, which attaches to the incus. B, Location of internal receiver (1). Position of transducer (2). I, incus; M, malleus; S, stapes.

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FIGURE 32-4 Frequencies for Vibrant Soundbridge.

The auditory processor contains a microphone that picks up sound from the environment and converts it into an electric signal. The auditory processor is contained within the external unit, which also contains an induction coil to transmit the electric signal to the internal vibrating ossicular prosthesis. A receiving coil picks up the signal and transmits it to the FMT, causing it to vibrate, which stimulates the cochlea.

Placement of the internal device requires an outpatient mastoidectomy similar to cochlear implantation. The facial recess is widely opened to visualize the incudostapedial joint, and to allow the FMT to pass through easily. The FMT is crimped onto the incus after the vibrating ossicular prosthesis is embedded in the cortical bone in a seat behind the mastoid posterior to the sigmoid sinus. The external processor is attached 6 weeks after surgery, at which time the device is programmed. Table 32-3 lists current indications for the Vibrant Soundbridge.

TABLE 32-3 Vibrant Soundbridge Patient Selection Criteria

Adult (≥18 yr)
Word recognition score ≥50%
Normal middle ear function
Realistic expectations
Pure tone air conduction thresholds within frequencies shown in Figure 32-4

Clinical trials for the Vibrant Soundbridge began in 1996, and the device was approved by the FDA in 2000 The FDA trials showed greater than 94% of patients reported improvement in their signal quality satisfaction rating with the Vibrant Soundbridge compared with their previous conventional air conduction hearing aids. Of patients who had complained of feedback problems with their presurgery air conduction hearing aids, 97% reported no feedback with the Vibrant Soundbridge. Eighty-eight percent of patients reported improved sound quality satisfaction rating of their own voice. Satisfaction with the overall fit and comfort of the Vibrant Soundbridge was reported by 98% of patients.

The Vibrant Soundbridge has been implanted more recently over the round window membrane as a means to stimulate the cochlear fluid directly. Colletti implanted seven patients with atresia or chronic otitis media in which the stapes was unavailable to be stimulated. The classic transmastoid approach was used; however, the bony lip of the round window was drilled out to allow the FMT to fit over this area. Thresholds in the normal range were accomplished for all patients implanted with the FMT placed over the round window. Long-term effects of this type of stimulation are unknown; however, additional studies are currently investigating this mode of stimulation.

Soundtec

The Soundtec Direct Drive Hearing System (Fig. 32-5) system was developed by Hough of Oklahoma City, Oklahoma. Although new devices are no longer available, it is reviewed for historical purposes because many patients in the United States have this device. The Soundtec used an electromagnetic approach that separated the magnet and the induction coil. A tiny magnet, approximately the size of a grain of rice, was attached to the incudostapedial joint. The magnetic coil, which would drive the magnet, was located in the ear canal in a conventional in-the-ear mold. The sound processor, microphone, and battery were located in a behind-the-ear external unit and were linked to the coil in the external ear canal. The magnet was placed during an outpatient procedure via the transcanal approach, with the tympanic membrane elevated to attach the magnet to the incudostapedial joint A deep-seated ear canal fitting was required for the in-the-ear unit after complete healing occurred.

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FIGURE 32-5 A, SoundTec device. The magnet is attached to the incus between the incus and stapes joint. B, Direct Drive Hearing System. Electromagnetic driver in ear canal (1). Magnet at incudostapedial joint or on tympanic membrane (2). Magnetic middle ear problems (3). I, incus; M, malleus; Tm, tympanic membrane.

A total of 103 patients were implanted at 10 sites for the FDA trial before approval was given in 2001 for treatment of sensorineural hearing loss. The Soundtec direct system (see Fig. 32-4B) gave an average of 7.9 dB increase in functional gain over optimally fitted air conduction hearing aids. Patients’ AFAB and speech discrimination scores were much higher for the Soundtec direct system compared with the optimally fitted air conduction hearing aids. This device is no longer available for implantation.

Envoy Esteem

The Envoy Esteem (Fig. 32-6) has been developed by St. Croix Medical, Inc. (Minneapolis, MN). This is a fully implantable system that uses a piezoelectric transducer for reception and transduction. The transducer consists of two internal plates separated by a thin conduction material. The first transducer (sensor) detects movement of the malleus in response to sound stimulation. This sensor acts as a microphone sending signals to the processor. The second piezoelectric transducer (the driver) is placed on the stapes. The piezoelectric units use the bimorph design for the sensor and driver. One of the sensors is fixed to the malleus to detect vibration and stabilized by fixation to the cortical skull bone. A small amplifier in the base increases the gain. The fully implantable device also contains a speech processor powered by a lithium iodine battery.

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FIGURE 32-6 Envoy esteem fully implantable system. A, Eardrum is used as a microphone to pick up sound via a transducer attached to the malleus head. Internal component consists of a battery and speech processor. There is an internal coil for programming the device. The stimulation of the ossicles occurs via attachment to the stapes. B, Implant assembly (1). Coronal view showing position of transducer (2). Sensor attached to malleus, driver in contact with stapes (3). EAC, external auditory canal; M, malleus; Tm, tympanic membrane.

The Esteem device is implanted through a postauricular mastoidectomy, with the attic area opened widely to allow adequate room for placement of the sensor and driver. A bed is created in the cortical bone to attach the sound processor and battery unit. The facial recess is opened up to allow adequate visualization of the chorda tympani, facial nerve, and stapes, and to allow adequate space for the piezoelectric driver to be positioned in contact with the stapes. The bone of the posterior ear canal must also be thinned carefully to allow adequate visualization. The incudostapedial joint is separated, and a 2 mm section of the long process of the incus is resected. Disarticulation is required to separate the vibrating malleus from the stapes, which is to be stimulated. In some cases, full removal of the incus may be necessary, with the sensor attached to the malleus head.

The Envoy has gone through two generations of devices. The first device, the Esteem 1, had a battery life of 2.5 to 5 years, and no recharging of the battery was required. The Esteem 2, or second-generation device, has been used more recently in clinical trials. This device has a longer battery life—5 to 8 years without the need to recharge. There is also a broader fitting range resulting in improved gain of 10 to 20 dB over the Esteem 1.

The first devices were implanted in Europe in March 2000. Phase I trial (safety study) for the Esteem 1 began in March 2002. The Esteem 1 is currently approved with its CE mark for European implantation (the CE mark is equivalent to the U.S. FDA). The Esteem 2 was approved for CE mark in 2008. Phase II clinical trials for the Esteem 2 began in 2008. At this time, neither the Esteem 1 nor the Esteem 2 is approved by the FDA for use in the United States.

MET

The MET (Fig. 32-7) is a fully implantable ossicular stimulator produced by Otologics (Boulder, CO). The initial device tested was a partially implantable device that consisted of an external digital speech processor and an implanted unit. The external components consisted of a microphone, speech processor, battery, and transmitter that were housed in a disc that fit in the internal implanted device. The external unit is held in place with magnets that align it to the internal unit, similar to a cochlear implant. The implanted components consist of a subcutaneous electric package containing a transcutaneous receiver and a transducer motor in a hermetically sealed case. The electromagnetic motor drives a biocompatible probe tip, which is placed in a hole in the body of the incus. Activation of the device causes mechanical motion of the probe tip, which vibrates the ossicular chain.

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FIGURE 32-7 Otologic MET. A, Microphone is placed in subcutaneous pocket behind the ear. Internal device consists of a speech processor, battery, and coil. Electromechanical stimulator is attached to the body of the incus. Ossicular chain is kept intact. B, Surgical view (1). Coronal view showing position of transducer (2). Probe tip seated in body of incus (3). EAC, external auditory canal; I, incus; M, malleus; S, stapes.

The transducer was found to have a linear input/output curve to beyond 1000 dynes at 1 kHz with low distortion. The frequency response is flat, varying only about 10 dB up from 1 kHz to 10,000 kHz.

The device is placed during an outpatient surgical procedure through a postauricular transmastoid approach. A mounting device is placed into the mastoid cortex that secures the electromechanical motor, and a seat is created to place the electronic housing container. A laser hole is created in the incus body for the probe tip to rest. The device can be activated and programmed 6 to 8 weeks postoperatively.

The partially implantable device is currently available in Europe. FDA trials for the partially implantable device were not completed in the United States because the fully implantable device has subsequently become available. The fully implantable device is currently undergoing European and U.S. FDA trials at the time of this writing for the treatment of conductive and mixed hearing loss and congenital atresia.

The fully implantable device consists of the same transducer stimulator as the partially implantable device. In addition, the internal case contains the battery, speech processor, receiving coil for programming and battery recharging, and a separate microphone that is attached to the receiving coil. The microphone is placed in the postauricular area subcutaneously. It is anticipated that the battery will last approximately 12 years. The battery requires daily recharging that takes less than 1 hour, during which time the patient can still use the device. The induction coil in the internal receiver receives a transcutaneous charge from a battery charger, which is also the method by which the device is programmed. The MET has a remote control that is used for volume control and turning the device on and off. The receiver coil on the internal unit is used to access information.

The MET has several different attachments that may be used on the transducer for stimulation. The classic or original device was designed to attach to the incus. Subsequently, additional adapters have been created, allowing the device to attach to the stapes, oval window directly, or round window for direct stimulation. Clinical trials of these new attachments have not yet begun.

CONCLUSION

Much work has been done over the past several decades to develop implantable hearing devices. Although there have been many successes, this field must be considered still in its infancy, with virtually all of these devices undergoing modification. Although development is rapid, and the outcomes from current devices are encouraging, patients must be informed that the long-term results are presently unknown. At the time of this writing, the field seems to be moving toward the use of these devices for conductive or mixed hearing losses, as opposed to pure sensorineural hearing loss. Further clinical trials need to be conducted to learn more about this emerging field.

Experience has taught that clinical trials for implantable devices always take longer and cost more than originally estimated. Additionally, many manufacturers have had difficulty in the mass production of reliable products. In addition to manufacturing problems, cost has been a major issue because many government agencies and insurance carriers have not routinely paid for these devices. As outcome studies show measurable improvement by the use of implantable devices, however, the trend is beginning to turn for coverage of the cost of surgery and the device.

Subjects describe the results from these devices as being better than their conventional air conduction hearing aids. The patient’s subjective assessment of improvement far outweighs the objective measures that have been used in clinical trials. It seems that the benefit described by patients is not measured by functional gain alone. Although the exact rationale for this perceived improvement is unknown, better amplification of high frequencies than conventional air conduction hearing aids is theorized to be responsible for much of this benefit. Development of new measurement tools may be necessary to measure adequately the device’s benefit as this area of otology continues to grow.

REFERENCES

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4. Rutschmann J. Magnetic audition: Auditory stimulation by means of alternating magnetic fields acting on a permanent magnet fixed to the eardrum. IRE Transactions Med Electron. 1959;6:22-23.

5. Fredrickson J.M., Tomlinson D.R., Davis E.R., Odkuist L.M. Evaluation of an electromagnetic implantable hearing aid. Can J Otolaryngol. 1973;2:53-62.

6. Fredrickson J.M., Coticchia J.M., Khosla S. Ongoing investigations into an implantable electromagnetic hearing aid for moderate to severe sensorineural hearing loss. Otolaryngol Clin North Am. 1995;28:107-120.

7. Maniglia A.J., Ko W.H., Garverick S.L., et al. Semi-implantable middle ear electromagnetic hearing device for sensorineural hearing loss. Ear Nose Throat J. 1997;76:333-338. 340-341

8. Dumon T., Zennaro O., Aran J.M., Bebear J.P. Piezoelectric middle ear implant preserving the ossicular chain. Otolaryngol Clin North Am. 1995;28:173-187.

9. Yanagihara N., Aritomo H., Yamanaka E., Gyo K. Implantable hearing aid: Report of the first human applicatoins. Arch Otolaryngol Head Neck Surg. 1987;113:869-872.

10. Yanagihara N., Sato H., Hinohira Y., et al. Long-term results using a piezoelectric semi-implantable middle ear hearing device: The Rion device E-type. Otolaryngol Clin North Am. 2001;34:389-400.

11. Zenner H.P., Leysieffer H. Total implantation of the Implex TICA hearing amplifier implant for high frequency sensorineural hearing loss: The Tubingen University experience. Otolaryngol Clin North Am. 2001;34:417-446.

12. Luetje C.M., Brackman D., Balkany T.J., et al. Phase III clinical trial results with the Vibrant Soundbridge implantable middle ear hearing device: A prospective controlled multicenter study. Otolaryngol Head Neck Surg. 2002;126:97-107.

13. Colletti V., Soli S.D., Carner M., Colletti L. Treatment of mixed hearing losses via implantation of a vibratory transducer on the round window. Int J Audiol. 2006;45:600-608.

14. Hough J., Dyer R., Matthews P., Wood M. Early clinical results: SOUNDTEC implantable hearing device phase II study. Laryngoscope. 2001;111:1-8.

15. First chronic implant performed. Envoy-Voices. Minneapolis, MN: St. Croix Medical; March 2000.

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