|Year : 2013 | Volume
| Issue : 2 | Page : 183-187
Middle ear implants
KS Gangadhara Somayaji1, Rajeshwary Aroor2
1 Department of ENT, Yenepoya Medical College, Karnataka, India
2 KS Hegde Medical Academy, Mangalore, Karnataka, India
|Date of Web Publication||13-Dec-2013|
K S Gangadhara Somayaji
Department of ENT, Yenepoya Medical College, Mangalore - 575 018, Karnataka
Source of Support: None, Conflict of Interest: None
Hearing loss is becoming more common in the society living in cities with lot of background noise around, and frequent use of gadgets like mobile phones, MP3s, and IPods are adding to the problem. The loss may involve the conductive or perceptive pathway. Majority of the patients with conductive hearing loss will revert back to normal hearing levels with medical and/or surgical treatment. However, in sensorineural hearing loss, many factors are involved in the management. Though traditionally hearing aids in various forms are the most commonly used modality in managing these patients, there are some drawbacks associated with them. Implantable middle ear amplifiers represent the most recent breakthrough in the management of hearing loss. Middle ear implants are surgically implanted electronic devices that aim to correct hearing loss by stimulating the ossicular chain or middle ear. Of late, they are also being used in the management of congenital conductive hearing loss and certain cases of chronic otitis media with residual hearing loss. The article aims to provide general information about the technology, indications and contraindications, selection of candidates, available systems, and advantages of middle ear implants. (MEI)
Keywords: Envoy system, hearing loss, middle ear implants, vibrant sound bridge
|How to cite this article:|
Gangadhara Somayaji K S, Aroor R. Middle ear implants. Arch Med Health Sci 2013;1:183-7
| Introduction|| |
Implantable middle ear amplifiers represent the most recent breakthrough in the medical and rehabilitative management of patients with hearing loss. Although traditional hearing aids are good enough in managing these patients, their inherent shortcomings and consumer dissatisfaction resulted in the evolution of this technology.  Implantable hearing devices, as compared to conventional hearing aids, are a spectrum of prosthetic devices that are wholly or partially implanted through surgery to ameliorate hearing loss. They are designed for individuals who have serviceable hearing. They serve to overcome the disadvantages of conventional hearing aids such as signal distortion and biofeedback.  By driving the ossicular chain directly, the output speaker component of the hearing aids is eliminated, providing a more natural, less distorted signal to be introduced into the cochlea. This reduces acoustic feedback; the ear remains open eliminating the occlusion effect while enhancing sound quality via the natural resonance of an open ear canal. 
| History|| |
Historically, the first clinically available MEIs (by Drs. Suzuki and Yanagihara in Japan) were designed for those with irreversible middle ear conductive/mixed losses.  Wilska is credited with being the first to use electromagnetic induction to stimulate the middle ear. Iron particles were placed on the tympanic membrane of a human subject and stimulated by an electromagnetic coil placed in the ear canal.  Since then, and particularly over the past 30 years, much work has been done looking at different sites and the methods of attachment of microphone, amplifier, and transducer. The vibrant sound bridge was invented by Geoff Ball. A major problem with these implants has been to produce a device that is small enough to fit within the confines of the middle ear and yet powerful enough to produce the required gain. Technological advances in recent years have fulfilled these requirements.
| Transducer Technology|| |
Based on the different transducers used in the design, current technology offers three types of middle ear implants: Piezoelectric, electromagnetic, and electromechanical.  While Dr. Wilska's work in the 1930s used an electromagnetic approach, the first clinically wearable middle ear implants were of the piezo-electric type. They are further categorized as partially or totally implanted. Piezoelectric implants use ceramic structures that are capable of temporary bending if electric current is applied.  When attached to the ossicles, the implants create vibrations, which can transmit high frequency energy. Due to relative small space of the middle ear, piezoelectric transducers are likely to be limited in low frequency output. They have an advantage that they are compatible with MRI.  Electromagnetic implants convert electromagnetic signals into ossicular vibrations via a nearby coil without physical contact. A magnet is implanted onto the ossicle and vibrates according to the electromagnetic signals that are transmitted to the coil from the sound processor. A major limitation of this device is the distance between the coil, usually in the external auditory canal, and the magnet. As the distance increases, the associated output significantly decreases. Electromechanical transducers have the coil and the magnet in the same assembly and hence the output does not vary. Piezo-electric hearing aids will only be useful for someone with a moderate or moderately-severe hearing loss in the mid and high frequencies. (Maximum output of 110 dB SPL) The electromagnetic approach can be useful for those with severe hearing loss. In both the technologies, there is greater gain and output in the mid-and-high frequency region, than for lower frequency sounds. Because of regulatory concerns, during FDA trials, the electromagnetic MEI manufacturers have limited the output to about 110 dB SPL (similar to the piezo-electric MEIs), but theoretically, they should be able to transduce up to about 140 dB SPL.
| Clinical Implementation|| |
Few major biomechanical issues must be addressed in the development of implantable middle ear devices. The first is that the device should not affect the normal functioning of the middle ear. Optimally, it 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 impair 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, which is enough to render the device ineffective. The long-term stability of the fixation must also be considered. The mechanical forces acting at the interface could affect the life expectancy of the device. 
| Candidacy and Selection Criteria|| |
- Functioning middle ear without infection
- Adults >18 years of age
- Bilateral, stable, moderate (hearing loss between 40 and 70 dB) to severe (hearing loss between 71 and 90 dB) sensorineural hearing loss as defined by Pure Tone Average with >60% speech discrimination
- Realistic expectations of device performance
- Previous experience with hearing aids is recommended.
| Indications|| |
Medical indications: Microtia, aural atresia, exostosis, recurrent chronic otitis externa, fire victims, excessive wax production, irregular pinna that will not support a hearing aid, allergy to ear mould materialsand previous radical mastoidectomy. All these patients could be equally benefitted from the bone-anchored hearing aids as well.
Audiological indications: Noise-induced hearing loss with high frequency loss and good speech discrimination. Recently, soundbridge device has been proposed for use in conductive and mixed hearing loss. However, the hearing loss needs to be stable.
Social indications: Patients requiring a free ear canal for their professions like musicians, swimmers, and physicians.
Bone conduction and bone-anchored hearing aids may be used in conductive and mixed hearing loss. However, their use is complicated by the need for accurate placement and sufficient pressure to ensure good coupling between the bone vibrator and the skull.  Device failures, risk of poor osseo-integration, and skin reactions have been reported with BAHA.  The superiority of middle ear implants as compared to BAHA in the treatment of conductive hearing loss is still not proven.
| Contraindications|| |
Chronic otitis media, previous middle ear surgery, otosclerosis, ossicular erosion, retrocochlear hearing loss, tinnitus, and skin diseases in the canal. 
| Outcomes of the Implant|| |
A middle ear implant does not restore normal hearing and requires time to acclimatize to the sound quality. The degree of benefit varies with each individual. Variables influencing the benefits each person will derive include factors such as age, degree and duration of hearing loss and integrity of the auditory system. The advantages of having a middle ear implant are more natural sound quality, lack of feedback, absence of occlusion and distortion, and favourable cosmetic appearance. Middle ear implants may improve many of the problems associated with hearing aid use, and fully implantable middle ear implants also allow the patients to swim and wash while wearing the device. However, there are some safety issues. Implantation involves a surgical procedure, which might injure facial nerve, chorda tympani, and ossicular chain. Some implants may require restriction to the use of magnetic resonance, electroconvulsive therapy and radiotherapy to the head. 
| Patient Management Protocol|| |
The expected length of assessment from referral to the final consent appointment is typically 18 weeks.
The initial appointment is meeting with audiologist who will:
- Explain the assessment process
- Take a detailed case history
- Discuss the middle ear implant and issues surrounding implantation, expectationsand potential outcomes
- Conduct basic audiological testing.
Suitable patients will subsequently undergo detailed evaluation
- Further clinic appointments will be issued to assess the hearing, amplification needs and functional aided hearing abilities.
- Hearing aid fitting- to optimize hearing aid setting
- Auditory brainstem response test-an objective, non-invasive hearing test
- Speech perception tests
- Pre-operative questionnaires
The meeting with the ENT surgeon will involve discussion regarding:
- Medical history
- Decisions made regarding further assessments required
- Magnetic Resonance Imaging (MRI) and possibly Computer Tomography (CT) may be discussed and booked to check the integrity of the cochleae and hearing nerves.
- Medical issues surrounding the middle ear implant surgery
- Surgical risks and side-effects (temporary taste disturbance, tinnitus, dizziness, balance impairment, infection, device failure, etc.)
- Surgical consent
The device will then be activated about 8 weeks after surgery.
| General Precautions|| |
Device working on electromagnetic technology are not MRI-compatible.
Monoploar surgical instruments are not to be used in the vicinity of these implants
Diathermy must never be applied over the implant. 
| Available Systems|| |
Piezoelectric devices 
- Rion device E-type
- Cochlear totally integrated cochlear amplifier
- Envoy esteem
Electromagnetic hearing devices 
- Med-El Vibrant sound bridge
- Otologics middle ear transducer and carina
- Soundtec directdrive hearing system
Envoy system and the vibrant sound bridge system are discussed below.
| Envoy System|| |
The Envoy is a totally implantable hearing system consisting of a sensor, a driver and a sound processor. The tip of the sensor makes contact with the malleus. As the tympanic membrane moves, the vibrations of the ossicular chain are detected by the sensor, which in turn generates a voltage proportional to malleus vibration.  The voltage is routed to the sound processor via the sensor lead, where the signal is processed and amplified.  The sound processor sends the information to the driver, whose tip makes contact with the head of the stapes resulting in its vibration.The Esteem envoy system 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 is identical to that received by a person with normal hearing. 
This device is indicated for patients with bilateral non-progressive sensorineural hearing loss with atleast 40% speech reception threshold, who have not been benefitted from hearing aids.
The procedure involves cortical mastoidectomy and facial recess approach.The current battery has a predicted lifetime of 5-7 years, depending on use; it can be replaced under local anaesthesia.Removal of portion of the incus is involved with reversibility of the procedure requiring repair of the incus. Glue is required to anchor portions of the driver and sensor. The long-term efficacy of this device is yet to be proven  [Figure 1].
|Figure 1: The Envoy system: a: Implant with sensor and actuator. b: Positioning of the implant.|
Click here to view
| The Vibrant Soundbridge System|| |
The vibrant soundbridge device consists of an external audio processor and an electromagnetic system referred to as the vibrating ossicular prosthesis(VORP). The VORP contains a receiver, conductor link and the floating mass transducer (FMT). The FMT consists of two energizing coils wrapped around hermetically sealed titanium housing with a magnet inside. Sound picked up by the audio processor is processed, amplifiedand sent via radiofrequency signals to the VORP. The VORP receives the signal and transmits the information to the FMT that, via an electromagnetic field, directly vibrates the ossicular chain. The surgery performed is called vibroplasty, which means vibratory stimulation of the ossicles in the middle ear. It could be a round window vibroplasty or TORP/PORP vibroplasty.
The patient needs to have normal middle ear, stable hearing loss with no improvement from conventional hearing aids. The procedure involves a cortical mastoidectomy and a posterior tympanotomy and is similar to the cochlear implantation.  It does not involve disarticulation of the ossicular chain.  FMT should be positioned as superiorly as possible on the incus and crimpled firmly. The long-term effects of an attached mass and the pressure created by the crimpled attachment clip are yet to be assessed.The Vibrant Soundbridge has been implanted in many patients worldwide.  The device has now been implanted in some cases for over ten years, and the long-term results appear favorable. There are no major complications reported. Minor taste disturbances and tinnitus were reported in some studies.  Alternate positions for fixing the device are being explored [Figure 2].
|Figure 2: The vibrant soundbridge system. a: Connection to long process of incus; b: Connection to round window. c: Laterally on a bell prosthesis; d: Linearly as TORP vibroplasty; e: Next to a stapes piston|
Click here to view
| Adverse Effects|| |
Commonly reported adverse events following implantation include middle ear effusion, tympanic membrane perforation, ossicular discontinuity, hematoma of the ear canal and tympanic membrane, impairment of taste and ear pain. In the studies reporting adverse events, device malfunction occurred in 5.7% of the patients. 
| Conclusion|| |
The utility of middle ear implants is only beginning to be appreciated. As additional information accumulates, the medical and rehabilitative fields will continue to gain experience with the effectiveness of current device technologies and witness the continued advancement of this technology towards the treatment of hearing loss. Preliminary reports indicate that the technology is safe and effective for those adults seeking as alternate means of amplification. Binaural amplification is advantageous. Studies are being done on the advantages of fully implantable systems, which could greatly enhance the quality of life, bringing all the benefits of performance, comfortand convenience to patients. 
| Future Research|| |
The possibility of hydroacoustic transmission via a water-filled tube either to the ossicles or directly to the round window membrane has been under investigation.  Clinical trials of round window stimulation using the vibrant soundbridge are currently underway. Research is being done in developing MRI compatible totally implatable devices.
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[Figure 1], [Figure 2]
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