22 Implantable Middle Ear Hearing Devices: Esteem and Maxum
Hearing loss is present in between 9 and 13% of the population of the United States, the United Kingdom, Germany, and France, and yet only between 25 and 39% of the people who need hearing aids actually own them (Anovum EuroTrak Study 2009). Furthermore, a 2009 MarkeTrak survey revealed that more than 20% of the hearing aid wearers in the United States were less than “somewhat satisfied” with their hearing aids and only 56% are “satisfied” or “very satisfied.” As many as 7.5% of the hearing aid owners did not use their hearing aids. Areas in which hearing aid wearers were most likely to be dissatisfied were use in noisy situations, chewing/swallowing, discomfort with loud sounds, sound fidelity, inability to hear soft sounds, and feedback. Other shortcomings include acoustic feedback, occlusion effect, potential device loss, battery costs, difficulties with cerumen, tissue reaction to the earmold, inability to wear overnight, and difficulties with dexterity in putting in and taking out the hearing aid.1
Implantable middle ear hearing devices (IMEHDs) were developed to address many of the disadvantages of conventional hearing aid technology. Through direct stimulation of the ossicles, the sound fidelity can be improved. More comfort can be achieved by allowing the ear canal to stay open, which would also eliminate the occlusion effect. Further decoupling the microphone and receiver could eliminate feedback. Ear canal hygiene issues can be obviated by the use of partially or totally implanted hearing systems. Finally, totally implanted IMEHDs obviate cosmetic issues and allow for continuous amplification while bathing or swimming.
While conventional hearing aids are adequate for the majority of people with mild hearing loss, the people with more moderate to severe hearing loss are more likely to be plagued by these issues. IMEHDs have advantages over conventional hearing aid technology in the moderate to severe hearing loss ranges because the lack of feedback allows the application of more gain. Indeed, IMEHDs are a good fit for those patients who fall into the “gray” area between hearing aids and cochlear implants.
Types of Implantable Middle Ear Hearing Devices
There are generally two categories of IMEHDs based on their underlying mechanism of action: electromagnetic and piezoelectric. Electromagnetic devices are composed of a magnet, which vibrates in response to the electromagnetic field that results from passing an electric signal through a wire coil. The key issue in the design of implants using this technology is to get the coil as close to the magnet as possible because the power of the magnetic field is inversely proportional to the cube of the distance between the magnet and the coil. The magnet can be attached to the tympanic membrane or ossicles. The coil can be placed in the external ear canal as in the extra coil electromagnetic transducer or can be fixed close to the magnet as in the intracoil transducer design.
Piezoelectric devices contain a material such as ceramic or crystals that move in response to an electric signal and, conversely, create an electric signal in response to movement. When attached to a diaphragm, which collects sounds over a wide area and applies the movement to a smaller area, it acts like a microphone. In another configuration where one end of the material is fixed and the other is free floating as in a springboard, an electric signal can be applied to the fixed end, which causes movement in the free end. The free end can be attached to an ossicle to transmit sound through movement. Limitations to this technology include the size of the material, which determines the power output, and the fixation of one end of the material, which can affect residual hearing.2
History of Implantable Middle Ear Hearing Devices
The first IMEHD is generally attributed to Wilska in 1935, who placed iron particles on the tympanic membrane and subjected them to an electromagnetic coil in the external auditory canal. Pure tones were perceived via a variable-frequency oscillator.3 In 1957, Djourno et al implanted a silver wire-wrapped iron core into the middle ear of a patient who was ultimately able to hear whistling sounds and a few spoken words.4 A magnet was glued onto the umbo and an electromagnetic stimulus applied by Glorig et al in 1972, who were able to reproduce speech sounds.5
Goode et al experimented with various coil designs and magnets placed in the canal throughout the 1970s and 1980s.6 At the same time, others were developing electromagnetic devices to be placed into the middle ear. These eventually led to the IMEHDs that became commercially available in the late 1990s.
Concurrently in the 1970s and 1980s, piezoelectric research in Japan yielded partially and totally implantable devices. The partially implanted design yielded the Rion device E-type, which was indicated for use in patients with conductive and mixed hearing loss due to chronic otitis media. It is composed of an external, ear-level microphone, amplifier, and primary inductions coil, which transmits an electric signal via electromagnetic induction. The internal secondary induction relays the signal to a ceramic bimorph (piezoelectric) vibrator, which is attached to the stapes with a hydroxyapatite tube that is interposed between the tip of the vibrator and the head of the stapes.7,8
In a 2010 study on the long-term follow-up of 30 of 39 patients who were initially implanted, only 11 (36.7 %) were still wearing the device. Five of these individuals experienced a decline in benefit, but all were satisfied with its use. The remaining 19 patients experienced a malfunction of the device or had to have it removed for other reasons.9
Another piezoelectric device, the totally integrated cochlear amplifier (TICA; originally marketed by Implex), has a microphone, which 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. The TICA received the CE mark in Europe in the late 1990s, but has not undergone studies in the United States. In 2004, Cochlear acquired Implex and it is no longer commercially available.
Commercially Available Implantable Middle Ear Hearing Devices
There are several commercially available IMEHDs on the market worldwide. This chapter explores two of these: the Maxum, and the Esteem. Med-El Corporation’s Vibrant Soundbridge will be covered in a separate chapter.
Maxum by Ototronix
Description
The Maxum by Ototronix is a partially implanted electromagnetic system based on technology previously marketed as the Direct System by Soundtec, which Ototronix acquired in 2009. The Maxum consists of a magnet, which is implanted transcanal onto the incudostapedial joint and an extra coil electromagnetic transducer located in the external auditory canal.
The magnet is neodymium-iron-boron encased in a titanium canister, which is attached to the ossicular chain by positioning a collar around the neck of the stapes. The external sound processor (SP) gathers and processes speech and sounds and then sends the electric signals to an electromagnetic coil positioned far medial in the external auditory canal adjacent to the tympanic membrane. The electromagnetic energy created by the coil vibrates the magnet and moves the ossicles.
The SP has been redesigned with digital circuitry. As a result, it is easier to program and incorporates directional microphones, noise cancellation circuitry, and wide dynamic range compression. The new processor is compatible with the original Soundtec magnet and is available in either an in-the-canal (IPC) (Fig. 22.1) or a behind-the-ear (BTE) (Fig. 22.2) configuration. The BTE 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. The IPC configuration consists of an integrated processor and electromagnetic coil that fits in the canal similar to an ITC or CIC hearing aid.
Because of the direct stimulation of the ossicles by the magnetic implant, no acoustic speaker or receiver is used. This eliminates the use of sound energy in the canal, which offers several possible advantages over hearing aids. Because it works by electromagnetic energy through the ear canal, the system 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. Also, because acoustic energy is not used, there is no sound in the canal to cause feedback as in hearing aids. As a result, functional gain can be improved without necessarily precipitating feedback.
Figure 22.1 Maxum implant—in-the-canal configuration.
Printed with permission from: Ototronix LLC, Houston, Texas, United States.
Figure 22.2 Maxum implant—behind-the-ear configuration.
Printed with permission from: Ototronix LLC, Houston, Texas, United States.
Indications
Maxum is indicated in people older than 18 years with moderate to severe sensorineural hearing loss. It is recommended that the patients have prior experience with conventional hearing aids. Contraindications include active middle ear infections, conductive hearing loss, neural hearing loss, disabling tinnitus, or tympanic membrane perforation.
Procedure
Surgery for the Maxum is done in an outpatient setting through a transcanal procedure with local anesthetic. After a standard ear canal block with local anesthetic and vasoconstriction, a tympanomeatal flap is elevated and, if necessary, the scutum is curetted until the stapes and incudostapedial joint are adequately visualized.
The Maxum magnet is then placed on the stapes. This magnet is encased in a titanium case with a wire attached that will ultimately secure the magnet to the distal incus and stapes. It attaches to the stapes loosely so that it can be positioned to line up well with the external transducer. To verify that the flat, lateral portion of the magnet is perpendicular to the axis of the ear canal, light should reflect from the entire surface in a “full moon” pattern. Once this alignment has been accomplished, a small amount of glass ionomer cement may be applied to the wire attachment to fix it to the stapes and long process of the incus. These cements flow easily and encase the magnet’s connection and ossicles like a cast. In case the magnet has to be removed for any reason, the cement can be readily chipped away for removal. This might be necessary if the patient needed a magnetic resonance imaging (MRI) examination. A photograph of the magnet is taken at the end of the procedure, and this is used by the company along with the deep external auditory canal impression to produce the transducer and make sure that it lines up correctly with the magnet for maximum performance.
Results
A clinical study was recently completed on the digital processor. The 10 individuals in the digital processor study were all patients with original Soundtec device. They were fit with the new digital processor, and their results were compared with their original processors.
The results of the study showed that the new digital processor can be expected to provide similar performance to the original clinical results,10,11 which included a statistically significant average increase of 7.0 to 7.9 dB in functional gain in the pure tone average (PTA) and an increase of 9.2 to 10.8 dB in the high frequencies (2000, 3000, and 4000 Hz); 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 primarily owing 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.10,11
In the original Soundtec device, patients complained of hearing the magnet move or “rattle” when the processor was not being worn. It was thought to occur from the movement of the magnet around its single point of fixation with the ossicular chain. This effect was improved, but not completely eliminated, by further stabilizing the implant by placing adipose tissue between the implant collar and the neck of the stapes. 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.
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