Implantable Hearing Devices 

Background

Hearing loss affects up to 10% of the population in the United States. The prevalence increases with age, and more than one third of people older than 65 years have a significant hearing loss. Only approximately 20% of people with hearing loss seek assistance from hearing aids. Of these, as many as 20% do not wear their hearing aids, and another 17% are dissatisfied with them.

Although technical improvements and modifications have improved the fidelity of conventional aids, hearing aids still have many limitations. The aids may be difficult to maintain, require frequent cleaning, dehumidification, and battery changes. Some patients may perceive them as being uncomfortable because they simply cannot tolerate an object in the ear canal. Patients often report the occlusion effect of an object occluding or blocking the entire ear canal.^[1]

Chronic otitis externa, canal exostoses, or frequent cerumen impactions make it difficult for some to wear hearing aids. Poorly fitting ear molds, faulty circuitry, or canal issues can lead to annoying feedback. Feedback also limits the amount of functional gain that may be delivered to the patient. Poor sound quality and problems hearing background noise are frequent complaints of those who wear hearing aids. Finally, some patients may perceive a social stigma associated with hearing-aid use.

Several types of devices can be considered implantable hearing devices.^{[2, 3, 4, 5]} These include cochlear implants, auditory brainstem implants (ABIs), bone-anchored hearing devices, and implantable middle-ear devices. Implantable middle-ear devices are generally available in 2 types—piezoelectric and electromagnetic—and may be further categorized as either partially or totally implanted.

Implantable middle-ear devices improve fidelity by directly stimulating the ossicles and improve comfort by allowing the ear canal to remain open and not occluded. In addition, most implantable middle-ear devices almost completely eliminate feedback. Improved cosmesis by means of miniaturization and concealment of the components is another benefit. Finally, totally implanted devices allow the patient to continue receiving amplification while swimming or bathing.

Many challenges remain in developing the ideal implantable hearing device. Limitations in the capacity and recharging cycles of available batteries necessitate transducer energy efficiency. Restrictions due to the size of the middle ear challenge the devices to deliver enough gain to aid more severe hearing loss.

Costs associated with the development of these devices, as well as the costs of surgical implantation, make these devices more expensive than conventional hearing aids. This increased expense, coupled with the technical difficulty of implanting some of these devices, may limit their widespread acceptance and availability.

For patient education resources, see the Ear, Nose, and Throat Center, as well as Hearing Loss.

Indications

In general, candidates for implantable middle-ear devices should have tried conventional aids with limited success. Most current devices are designed for patients with mild-to-severe sensorineural hearing loss. Mild-to-moderate loss can often be adequately improved with conventional aids. Patients with severe sensorineural hearing loss often have difficulty with feedback because of the amount of gain required.

Furthermore, the tight-fitting earmold required for severe hearing loss leads to discomfort and hygiene issues. Implantable middle-ear devices, which do not require occlusion of the ear canal and uncouple the receiver from the microphone, are particularly well suited for these more severe hearing losses. The most recent generation of high-output middle ear drivers may allow effective rehabilitation of patients in the so-called gray area between hearing aids and cochlear implants.

Totally implantable middle-ear devices may be particularly attractive to athletes and swimmers in whom water or sweat limit their ability to hear when active. Actors, performers, or any individuals who desire a completely concealed hearing aid will find totally implantable middle-ear devices an attractive option.

Recent applications to the round window make rehabilitation of conductive hearing losses within the realm of possibilities for implantable middle-ear devices.^[6] Further modifications of existing implantable middle-ear devices may further adapt them for conductive hearing loss use.

Currently, implantable middle-ear devices are indicated for patients aged 18 years or older. Cochlear implants and osseointegrated implants are indicated in children as well.

Contraindications

Patients should be free of significant middle-ear disease or infection. Hearing loss should be stable, and word recognition scores should be sufficient to allow adequate discrimination of sounds. Additionally, the patient’s expectations of benefit should be reasonable.

Overview

Several types of devices can be considered implantable hearing devices. These include cochlear implants, auditory brainstem implants (ABIs), bone-anchored hearing devices, and implantable middle-ear devices. Implantable middle-ear devices are generally available in 2 types: piezoelectric and electromagnetic. They are now additionally categorized as either partially or totally implanted.

The implantable middle-ear devices discussed in this article were developed to treat conductive and sensorineural hearing loss. Two implantable middle-ear devices have previously been approved by the US Food and Drug Administration (FDA); only one of these is currently available in the United States. Two totally implanted implantable middle-ear devices are currently undergoing FDA investigation in the United States.

Implantable middle-ear devices improve fidelity by directly stimulating the ossicles, and they improve comfort by allowing the ear canal to remain open and not occluded. In addition, most implantable middle-ear devices almost completely eliminate feedback, one of the most annoying adverse effects of conventional aids. Improved cosmesis by means of miniaturization and concealment of the components is another benefit. Finally, totally implanted devices allow the patient to continue receiving amplification while swimming or bathing.

Cochlear Implants

A cochlear implant provides sound perception by means of an electrode surgically implanted into the cochlea (see the image below). Candidates must have bilateral profound hearing loss and meet strict audiologic criteria. These devices are covered more fully in other articles (see Cochlear Implant Indications and Cochlear Implant Surgery).

Auditory Brainstem Implants

ABIs were designed to be used in cases of neurofibromatosis type 2 (NF-2) in which tumors involving complexes of both cranial nerve VII and cranial nerve VIII render the patient anacusic. These devices, which bypass the cochlea and cochlear nerve, are implanted into the lateral recess of the fourth ventricle adjacent to the cochlear nucleus to provide the patient with auditory perception.^{[7, 8]} They are usually implanted after the tumor is resected, during the same operation.

House and Hitselberger implanted the first ABIs in 1979. The device was a single-channel percutaneous implant with a ball electrode, which they implanted in approximately 13 patients. The second-generation device incorporated the cochlear implant speech processor (3M/House) and was implanted in 25 patients with NF-2, starting in 1986.

In 1992-1993, the first multichannel device (which had an 8-channel array) was developed. In 1994, a multicenter investigation was conducted to evaluate a multichannel ABI powered by a stimulator (Cochlear Corporation Nucleus 22) with transcutaneous signal transmission.

The latest ABI incorporates a digital speech processor (Nucleus 24). It offers 4 user-selectable programs, as well as programmable volume and sensitivity controls. The processor uses the SPEAK speech coding strategy and has the flexibility to add future strategies as they are developed.

The ABI is indicated in patients with NF-2 aged 12 years or older. Implantation may occur during tumor removal on the first or second side or as a separate procedure. The patient should be medically and psychologically suitable, and no audiologic criteria are applied. Because of possible injury to the cochlear nucleus from radiotherapy, prospective ABI recipients who have undergone gamma-knife irradiation should be considered with extreme caution.

A study on 61 patients receiving the multichannel ABI showed in most patients with NF-2, the provides useful auditory sensations both effectively and safely.^[9] The majority of patients can derive enough auditory information from the ABI to improve their lip-reading abilities, and a few are able to achieve open-set (no lip-reading cues) speech understanding. Performance may improve for up to 8 years after implantation.

ABIs have been used with varying success in patients born with cochlear nerve aplasia; those with traumatic cochlear nerve avulsion, cochlear fractures, or cochlear ossification; and those in whom cochlear implantation is unsuccessful (for salvage treatment).

Bone-Anchored Hearing Devices

The Baha device (Cochlear Corporation) is a percutaneous implantable device primarily used for conductive hearing loss, or more recently, for single-sided sensorineural hearing loss (see the image below).^{[10, 11, 12, 13, 14, 15]}

Developed in Gothenburg, Sweden and used in Europe since 1977, the original device was designed to treat conductive or mixed losses and has become popular for hearing rehabilitation in patients with congenital ear malformations or refractory chronic ear disease. The Baha device can close the air-bone gap to within 10 dB of the preoperative bone-conduction thresholds in as many as 80% of patients and to within 5 dB in 60%.

An osseointegrated titanium fixture with a percutaneous abutment is implanted in the postauricular area (see the first image below). An external sound processor is attached to the abutment at will (see the second image below). A microphone in the processor, which vibrates the bone in the skull by means of the fixture, picks up sound. The sound is transmitted directly to the inner ear on the side with conductive hearing loss or, in the case of single-sided deafness, the ear with the better sensorineural hearing.

The external processor is available in 3 models and is chosen on the basis of the bone thresholds of the ear to be aided. A body-worn external processor, the Cordelle II, allows for the implantation for pure-tone-average bone thresholds as low as 70 dB HL. The 2 smaller processors, the Intenso and the BP100, are for pure-tone-average bone thresholds better than 45 dB HL. The newer BP100 has a programmable signal processor and built-in directional microphone.

The Baha device can be placed bilaterally in patients with bilateral disorders to allow for sound localization and to improve speech recognition in noise.

The surgery is typically performed in a single stage in adults. About 3-4 months must be allowed for osseointegration before the external processor can be attached and its benefits realized. A 2-stage procedure is recommended in children in whom the fixture is placed into the bone at the first stage. After 3-6 months to allow for osseointegration, a second-stage operation is done to connect the abutment through the skin to the fixture. Complications are few and mostly limited to local infection and inflammation at the implant site and a failure of osseointegration.

The Baha device is FDA-approved for use in children aged 5 years or older, but it has been used successfully in Europe in children as young as 1.5 years. Indeed, 1 group with considerable experience suggests that the most suitable age is 2-4 years. A bone-augmentation technique may aid implantation in children of this age.

Before the FDA approved the Baha device for single-sided deafness, the contralateral routing of signal (CROS) hearing aid was the only option available for rehabilitation. Poor performance and aesthetic considerations limited the use of CROS aids.

The Baha device can be implanted on the side of the deaf ear, and it transmits the sound by means of bone conduction to the contralateral cochlea. This process eliminates the head-shadow effect and allows hearing from both sides of the head. As compared with the CROS aid, the Baha devices yields substantially improved speech recognition in quiet and in composite noise.

In 2009, Oticon Medical released a bone-anchored hearing system in the United States. Designated the Ponto (see the image below), the devices has a digital sound processor that features noise reduction technology. Like the Baha, the Ponto has an osseointegrated screw and a transcutaneous abutment placed postauricularly. It is also indicated for conductive and mixed hearing loss as well as single-sided deafness.

Piezoelectric Implantable Middle-Ear Devices

Piezoelectric devices operate by passing an electric current into a piezoceramic crystal, which changes its volume and thereby produces a vibratory signal. The major disadvantage is that power output is directly related to the size of the crystal. Studies of early designs indicated that such an approach benefits only people with up to moderate (about 60 dB) hearing loss; however, newer designs have challenged this view. Piezoelectric transducers have the advantage of being inert in a magnetic field and therefore compatible with magnetic resonance imaging (MRI).

Rion device E-type

One of the earliest piezoelectric implantable middle-ear devices, the Rion device E-type (RDE), has been used for both conductive and sensorineural losses. The RDE is a partially implanted device composed of an external ear-level microphone and amplifier and an internal electromagnetic coil and vibrator element. The piezoelectric vibrator element is anchored to the squamous portion of the temporal bone with a titanium screw. It is attached to the stapes with a hydroxyapatite tube, which is interposed between the tip of the vibrator and the head of the stapes.

In 39 patients in Japan, an initial hearing improvement of 36 dB at 3 months after surgery eventually decreased to 21 dB with long-term follow-up.^[16] The reason for diminished performance was thought to be to a decrease in the sensitivity of the ossicular vibrator caused by aging and tissue reaction around the vibrator element. The authors reported that sensorineural hearing was not affected and that all patients preferred the device to a conventional aid.

Totally Integrated Cochlear Amplifier device

The Totally Integrated Cochlear Amplifier (TICA; Implex American Hearing Systems, now owned by Cochlear Corporation) is totally implantable (see the image below). The microphone is implanted subcutaneously in the external ear adjacent to the tympanic membrane. A digitally programmable processor located subcutaneously on the mastoid bone processes the signal. A piezoelectric transducer is coupled to the body of the incus and drives the ossicular chain by vibratory actions. Many European and some North American research programs have used this approach.^[17]

The TICA received the CE mark in Europe in the late 1990s but has not undergone studies in the United States. Since 2004, when Cochlear acquired Implex, no further studies have been published.

Envoy Esteem device

Another totally implantable piezoelectric device is the Esteem by Envoy Medical (originally St. Croix Medical; see the image below). This device uses the eardrum as the microphone, taking advantage of the natural acoustics of the ear canal without obstruction, interference, or any external devices. Therefore, the input signals are identical to those received by a person with normal hearing.

The mechanical signal is detected by a piezoelectric transducer at the head of the incus (the sensor) and converted to an electrical signal. The electrical signal is amplified, filtered, and converted back to a vibratory signal. The processed vibratory signal is then delivered by means of a piezoelectric transducer (the driver) attached to the stapes capitulum (see the image below). The incus lenticular process is removed to prevent feedback to the sensor. The second-generation piezoelectric transducer can produce a sound pressure level (SPL) approaching 110 dB.

An audiologist programs the implant using a device called the commander. After the device is programmed, patients are given a personal programmer that allows them to turn the device on or off, to adjust the volume, and to remotely modify background noise filters.

The advantages of totally implantable technology are notable. Without an appliance in the external auditory canal, the occlusion effect is eliminated. Uncoupling of the sensor and the driver eliminates most feedback. Even more important, for some patients, is the fact that the device is completely concealed in the body.

The Esteem device faces some hurdles as it is further developed. Whereas the first-generation battery was expected to last, the current battery has a predicted lifetime of 5-7 years depending on use. The current battery can be replaced with the aid of a local anesthetic.

In addition, removal of a portion of the incus permanently alters the ossicular mechanism and makes it difficult to attain full recovery of hearing to preimplantation baseline levels if the device fails or is in the off position. A new prosthesis, the Kraus K-Helix System (marketed by Grace Medical), was developed to reconstruct the defect in the incus created by placing the Esteem.

Finally, functional gain decreased by more than 3000 Hz in the phase I study of the Esteem device; however, functional gain was substantially improved in the phase II studies.

The phase I clinical trial of the Esteem device included 7 patients, 5 of whom had working implants in the 2-month postactivation period.^[18] The remaining 2 had no benefit from the device as a result of a breach of the hermetic sealing of the device, which allowed moisture to pass into the circuitry. This breach was corrected in the current version of the device.

Overall, the patients with functioning devices perceived a benefit with the Esteem device, as compared with their hearing aids, in terms of ease of communication in favorable conditions, background-noise reverberation, and aversiveness of sound. Speech discrimination showed marked (17%) improvement over hearing-aid conditions. Functional gain and speech reception thresholds were similar for the Envoy device and for the hearing aids.

Phase II of the FDA clinical trial was composed of two parts. In the first part, consisting of over 70 patients, pure tone averages and word recognition scores (WRS) improved significantly. However, a significant number of revision surgeries were required, which prompted a second part of the FDA clinical trial. This “Pivotal Clinical Study” involved 54 patients implanted in 3 sites in the United States.

Results presented at the FDA panel meeting in December 2009 indicated an improvement of 11.4 +/- 1.8 dB over the pre–implant-aided condition at 10-month follow-up. WRS at 50 dB yielded an improvement in 56% of the subjects in comparison with the pre–implant-aided condition. The WRS was unchanged in 37% and decreased in 7% at 4 months in comparison with the pre–implant-aided condition. There was no postoperative decline in bone thresholds in comparison with the pre–implant-aided condition.

Formal FDA approval was granted in March 2010. The Esteem received the CE mark in 2006 and is currently being implanted in several countries.

Electromagnetic Implantable Middle-Ear Devices

Electromagnetic hearing devices function by passing an electric current into a coil, which creates a magnetic flux that drives an adjacent magnet. The coil may be separate from the magnet or integrated with the magnet. The small magnet, or a magnetic piston, is attached to one of the vibratory structures of the middle ear (eg, tympanic membrane, ossicles, round window). In some cases, the external coil can be housed in a completely-in-the-canal (CIC) type of hearing-aid shell.

The major disadvantage is that power is decreased by the square of the distance between the coil and the magnet; therefore, the coil and magnet must be close. A slight shift of coil position in the external ear results in unpredictable or insufficient power output. Furthermore, the anatomy of the middle ear space restricts the size of the magnet or coil.

Vibrant Soundbridge device

One example of an electromagnetic device is the Vibrant Soundbridge device (see the image below). Originally developed by Symphonix Devices, Inc., the Soundbridge was the first FDA-approved implantable middle-ear device to treat sensorineural hearing loss. It was marketed and implanted in the United States for a few years until the technology was purchased by Med-El of Austria. It is now marketed by Vibrant Med-El and implanted worldwide.^{[19, 20]}

The Soundbridge device is a semi-implantable device composed of an external sound processor and amplifier, an audio processor, and an internal vibrating ossicular prosthesis (VORP). Sound passes into a microphone on the postauricular audio processor and is transmitted through the skin to an implanted receiver on the VORP.

The VORP, which is implanted postauricularly (similar to a cochlear implant) conducts the sound to a magnet surrounded by a coil called the floating mass transducer (FMT). The transducer is attached to the long process of the incus and the magnet hugs the long axis of the stapes, which causes it to vibrate.

The phase III FDA trial was completed in 2000, and results from the 53 patients submitted to the FDA were published in 2002.^[21] The device was safe, with no notable change in preoperative and postoperative bone thresholds. Furthermore, functional gain and WRS were improved with the Vibrant device compared with the patients’ conventional hearing aids. Self-assessment inventories indicated that 94% of the patients believed that the overall sound quality of the Vibrant Soundbridge device was better than that of their conventional hearing aid.

As of 2007, the use of the Soundbridge was expanded. It was successfully implanted on the round window membrane in patients with aural atresia and mixed hearing losses.^{[22, 23, 24]} The Soundbridge was also applied to the incus in a more traditional sense in patients with otosclerosis.^{[25, 26]}

The audio processor has evolved from the original analog Vibrant P unit to a digital 3-channel Vibrant D unit to the current digital 8-channel Vibrant Signia unit. The Signia device modestly increases functional gain and speech-in-noise understanding results compared with the Vibrant D device.

Otologics Middle Ear Transducer and Carina devices

The Middle Ear Transducer (MET; Otologics LLC, Boulder, Colo) was first introduced as a semi-implantable device (see the image below). The original device was composed of an implanted transducer mounted in a laser-drilled hole in the body of the incus. The transducer translates the electrical signals into a mechanical motion that directly stimulates the ossicles and enables the wearer to perceive sound. The transducer is coupled with an externally worn audio processor (Button processor), which contains the microphone, battery, and signal processor.

The device has a CE mark for pan-European marketing approval. To date, more than 300 patients have received the implant in Europe and in the United States.

A totally implantable version subsequently was developed and called the Carina (see the image below). The Carina consists of 4 primary components: the implant, the programming system, the charger, and the remote control.^[27]

The implant consists of 2 main parts: the electronics capsule and the MET. The capsule contains the microphone, battery, magnet, digital signal processor, and connector. A microphone located under the skin picks up sounds, which are amplified and converted into an electrical signal. The signal is sent down the lead and into the transducer, which is the same as that used in the semi-implantable version. The semi-implantable version can be upgraded to the fully implantable version in a single surgical procedure with the patient under local anesthesia.

The programming system coil is placed over the implant site and held in place magnetically. The coil couples with the implant by means of a radiofrequency signal that is used to program the device in the same manner as a traditional digital hearing aid. The programming system also allows for extensive testing and diagnostics of the stimulator.

The charger system consists of the base station, charging coil, and charger body. To charge the implant, the wearer removes the charger body from the base station and places the coil on the skin, over the implant site. The charger body contains a clip that allows the charger to be attached to the belt of the wearer during charging. Typically, charging time will be about 1 hour if charging is performed daily. While recharging the implant, the wearer can perform normal daily activities, turn the implant on and off, and adjust the volume.

A remote is used to control the stimulator when the device is not being charged. The remote allows the wearer to turn the implant on and off and to adjust the volume. To use the remote control, the wearer holds the remote against the skin over the implant.

The US phase I trial results yielded a 15-20 dB functional gain across audiometric frequencies in 20 patients. The pure-tone averages and monaural WRS were better with the hearing aid in the same ear preoperatively, whereas the patients generally perceived more benefit in the postoperatively implant-aided conditions.^[28]

The Otologics Carina received European approval in October 2006. Phase II of the FDA clinical trial is currently ongoing in the United States.

Soundtec Direct System device

The Soundtec Direct System (see the image below) was introduced to the US market in 2001 and voluntarily withdrawn in 2004. This semi-implantable device converts sound energy to electromagnetic energy to directly stimulate the ossicles.^[29]

A surgically implanted neodymium-iron-boron (NdFeB) magnet is attached to the ossicular chain by positioning a collar around the neck of the stapes. An earmold coil assembly consisting of an acrylic skeleton mold with an embedded electromagnetic coil stimulates the magnet. The earmold coil assembly is inserted deeply into the ear canal, ideally about 2 mm away from the tympanic membrane. It is attached to a sound processor that is fitted either in the canal or behind the ear, similar to a hearing aid.

Such a design offers several possible advantages. Because it works by electromagnetic energy through the ear canal, the Soundtec device does not require an acoustic seal, which may lead to the occlusion effect or alter the resonance qualities of the ear canal (which causes distortion). In addition, functional gain can be improved without necessarily precipitating feedback, a common problem with traditional aids that occurs when sound pressure escapes the ear canal and cycles back through the microphone.

A relative disadvantage is that the procedure requires separation and then reconstitution of the incudostapedial (I-S) joint. In the clinical study, residual hearing was not affected in most subjects. Average bone conduction thresholds decreased by 1.1 dB from 250 to 4000 Hz. This effect may be a result of movement of the mobile stapes into the vestibule during disarticulation of the I-S joint, which may cause sensorineural hearing loss.

There was also an average decrease of 4.2 dB in air-conduction thresholds from 250 to 8000 Hz, primarily as a consequence of the loading effect of the magnet implant on the ossicles.

The phase II FDA clinical trial included 103 patients with moderate to moderately severe sensorineural hearing loss who had previously worn hearing aids for at least 45 days.^[30] The Direct System demonstrated a statistically significant improvement in average soundfield threshold and functional gain from 500 to 4000 Hz. The additional average functional gain (over hearing aids) was an increase of 7.0-7.9-dB in PTA and a 9.2 dB increase in the high-frequency average (2000, 3000, and 4000 Hz).

In addition, there was a statistically significant increase of 5.3% in speech discrimination. Furthermore, subjective measures of feedback, occlusive effect, perceived aided benefit, patient satisfaction, and device preference over the patient’s optimally fitted hearing aid showed that 89% preferred the Direct System in overall satisfaction, 99% preferred the Direct System as having the least amount of feedback, and 89% preferred the Direct System for sound quality.

A long-term follow-up retrospective review of 64 patients receiving the Soundtec device revealed a significant average functional gain of 26 dB.^[31] About 55% of patients complained of hearing the magnet move or “rattle” when the processor was not being worn. This effect was improved, but not completely eliminated, when the implant was further stabilized by placing adipose tissue between the implant collar and the neck of the stapes.

A patient questionnaire to assess sound quality, speech in noise, and satisfaction with the Soundtec compared with conventional hearing aids failed to show a significant difference.

The authors concluded that the ideal patient is one younger than 70 years with a moderate sensorineural hearing loss, speech discrimination scores equal or better than 60%, appropriately sized ear canals, and sufficient manual dexterity to insert the processor.

To date, around 600 Soundtec devices have been implanted, with the most users in the United States. The device was voluntarily withdrawn from the market in 2004 when the company identified ways to improve it and to eliminate the magnet “rattling” some patients experienced. The “rattling” sound, which occurred primarily when the external processor was not used, occurred in as many as 7% of patients. It was thought to occur from movement of the magnet around its single point of fixation with the ossicular chain.

Additional studies of the Soundtec in a magnetic field indicate that it should be mechanically stable and nondestructive during 0.3-T open MRI with a modified MRI protocol.^{[32, 33]}

Ototronix Maxum System

The Ototronix Maxum System is based on the Direct System technology, which Ototronix acquired from Soundtec in 2009. The general concept of stimulation is the same. The Maxum is a partially implantable device that converts sound energy to electromagnetic energy so as to stimulate the ossicles directly.

There are 2 components: the implant and the external processor. The implant consists of a surgically implanted neodymium-iron-boron (NdFeB) magnet encased in a titanium canister, which is attached to the ossicular chain by positioning a collar around the neck of the stapes.

The external processor consists of a digital sound processor and electromagnetic coil. The sound processor gathers and processes speech and sounds. The processor sends signals to an electromagnetic coil located in the ear canal near the tympanic membrane to directly stimulate the magnetic implant and, thereby, the ossicles.

Because of the direct stimulation of the ossicles by the magnetic implant, no acoustic speaker or receiver is used (unlike hearing aids). This eliminates the use of sound energy in the canal, which offers several possible advantages over hearing aids.

Because the Maxum works by electromagnetic energy through the ear canal, it does not require an acoustic seal, which may lead to the occlusion effect. Distortion may be reduced because there is no increase in sound energy in the canal, which can alter the resonance qualities of the ear canal and cause distortion. In addition, functional gain can be improved without necessarily precipitating feedback, which occurs when sound pressure escapes the ear canal and cycles back through the microphone.

The processor is available in 2 configurations: in the canal and behind the ear. The in-the-canal configuration consists of an integrated processor and electromagnetic coil that fits in the canal in much the same fashion as an in-the-canal or CIC hearing aid (see the image below).

The behind-the-ear configuration has an earmold coil assembly consisting of an acrylic skeleton mold with an embedded electromagnetic coil inserted deeply into the ear canal, ideally approximately 2 mm away from the tympanic membrane (see the image below).

Ototronix has redesigned the processor with digital circuitry. This can offer several possible advantages. The digital design is easier to program and incorporates directional microphones, noise cancellation, and wide dynamic range compression. It is also compatible with the original Soundtec device and patients can be upgraded by simply changing to the Maxum digital processor.

A clinical study was recently completed on the digital processor. The patients in the digital processor study were all originally Soundtec patients. They were fitted with the new digital processor, and the results were compared to the results from their original processors.

The findings of the study indicated that the new digital processor can be expected to provide similar performance to the original clinical results, which included a statistically significant average 7.0-7.9 dB increase in functional gain in the pure-tone audiometry and a 9.2-10.8 dB increase in the high frequencies (2, 3, and 4 kHz); a statistically significant increase of 5.3% in speech discrimination; and improved subjective measures of feedback, occlusive effect, perceived aided benefit, patient satisfaction, and device preference over the patient’s optimally fit hearing aid.

As in the original study, the Maxum implant will have an average decrease of 4.2 dB in air-conduction thresholds across the frequency range of 250 to 8000 Hz due primarily to the loading effect of the magnet implant on the ossicles. Average bone conduction thresholds decrease by 1.1 dB over the frequency range of 250 to 4000 Hz.

With the original Soundtec device, patients complained of “rattling” when the processor was not worn, an effect thought to result from movement of the magnet around its single point of fixation with the ossicular chain. This effect was improved, but not completely eliminated, by placing adipose tissue between the implant collar and the neck of the stapes to stabilize the implant. It has recently been found that the use of a very small amount of glass ionomeric cement (as typically used with a prosthesis) can fixate the implant and not allow it to move.

A relative disadvantage of the original Soundtec device was that the procedure required separation and then reconstitution of the I-S joint. However, the Maxum will be available with a connection design, which will not require separation of the I-S joint. Like the original Soundtec device, the Maxum should be mechanically stable and nondestructive during 0.3-T open MRI with a modified MRI protocol.^{[32, 33]}

Issues and Challenges in Developing Implantable Middle-Ear Devices

Many challenges remain in developing the ideal implantable hearing device. Limitations in the capacity and recharging cycles of available batteries necessitate transducer energy efficiency. Restrictions due to the size of the middle ear challenge the devices to deliver enough gain to aid more severe hearing loss.

Costs associated with the development of these devices, as well as those associated with surgical implantation, make these devices more expensive than conventional hearing aids. This increased expense, coupled with the technical difficulty of implanting some of these devices, may limit their widespread acceptance and availability.

Another hurdle is in measuring the devices’ perceived benefit. All implantable middle ear devices have shown benefit over hearing aids in the patient’s perception of the sound fidelity, a very important quality. These implants rely on the Abbreviated Profile of Hearing Aid Benefit (APHAB), a subjective questionnaire of sound qualities that patients complete preoperatively with their hearing aid and postoperatively with the implant. However, quantitatively measuring the psychoacoustic qualities of sound that are so important to the patient is difficult.

A few major biomechanical issues must be addressed in developing implantable middle-ear devices. First, the device should not affect normal function of the middle ear. In the optimal situation, the device should not alter air-conduction thresholds. If the device is unsuccessful, the added mass of the unit attached to the vibratory structure of the middle ear should not affect that structure’s ability to vibrate.

Another important issue is the anchoring of the device to the ossicular chain. Even a little laxity at the interface between the prosthesis and bone could diminish the transmitted power enough to render the device ineffective. Long-term stability of the fixation must be considered as well. The mechanical forces acting at the interface could affect the life expectancy of the device.

The direction of the transducer’s vibration must be coincident with the axis of normal sound transmission through the ossicular chain. In effect, the device must be attached to the tympanic membrane, the long process of the incus, or the head of the stapes. Otherwise, the transmitted force is reduced. Recent cases involving implantation in the round window membrane challenge this more traditional approach.

Finally, with the totally implantable devices, battery life must be sufficient to avoid frequent replacements requiring reoperation. A rechargeable battery offers a potential solution to this problem as long as the battery is able to maintain a charge over a long period.

Current Status of Implantable Middle-Ear Devices

Currently, the semi-implantable Vibrant Soundbridge and the totally implantable Envoy Esteem are approved by the US Food and Drug Administration (FDA) for implantation in the United States. The Vibrant, Esteem, and the Otologics Carina are currently available outside the United States. The Carina is currently undergoing FDA investigation in the United States.

Many believe that these devices may represent a new era in hearing rehabilitation, much as the largely successful, but more narrowly indicated, cochlear implants have done. Although hurdles still remain, the significant advantages of these devices and the increasing number of people who may benefit from them support their use and development.