77. Microvascular Reconstruction

Print   Close Window

Note: Large images and tables on this page may necessitate printing in landscape mode.

Copyright 2007 The McGraw-Hill Companies.  All rights reserved.

Current Otolaryngology  > XVI. Implants > Chapter 69. Implantable Middle Ear Hearing Devices >
 

General Considerations of Implantable Middle Ear Hearing Devices

Societal Factors

Hearing loss is a common disability among adults. In the aging population, 25% of individuals between age 65 and 74, and 50% of individuals age 75 and older have hearing problems. Overall, approximately 14.4 million adults in the United States have moderate to severe sensorineural hearing loss. For this group , acoustic amplification (a conventional hearing aid) is an important rehabilitative strategy that often restores hearing to a serviceable level.

Despite the potential benefits of acoustic amplification, many hearing-impaired patients do not accept hearing aids. Some common complaints about hearing aids include feedback annoyance, ear canal discomfort, stigma of wearing an external appliance, and psychological rejection . It is estimated that only 20% of individuals within the United States who may benefit from a hearing aid own one. Only half of those who own a hearing aid use their device on a long- term basis.

Advantages of Implantable Middle Ear Hearing Devices

The search for alternatives to conventional hearing aids motivated the development of implantable hearing devices that deliver sound energy more directly to middle ear structures. This design eliminates many of the disadvantages of conventional hearing aids. Implantable hearing devices endeavor to deliver more natural sound quality, increase gains across the frequency spectrum, reduce feedback, improve comfort and cosmesis, and eliminate ear canal occlusion .

Implantation- Related Risks

Risks associated with middle ear device implant surgery include sensorineural hearing loss, ossicular chain disruption, facial nerve injury , external canal laceration, and cerebral spinal fluid (CSF) leak. Beyond surgical risks, other considerations associated with implantable hearing devices are higher costs compared with conventional hearing aids, incompatibility with magnetic resonance imaging (MRI), and uncertain need for future explantation (ie, device removal) and reimplantation. Nevertheless, emerging technologies in implantable middle ear hearing devices (IMEHD) are very exciting for both patients and care providers.

Hearing Device Components

An IMEHD is a device that converts acoustic energy to mechanical energy and delivers it to a vibratory structure in the middle ear. The basic components of an IMEHD consist of an acoustic signal detector (receptor), a transmission link, and an actuator that vibrates the ossicular chain (effector). The two basic transducer types used to drive the ossicular chain are electromagnetic and piezoelectric systems. Electromagnetic fields generated by induction coils can put magnets into oscillatory motion. Piezoelectric transducers are generally ceramic materials that vibrate in response to applied electrical energy. The general design of an IMEHD consists of separate receptor and effector limbs . For semi-implantable devices, the receptor limb is an external, removable component that houses the microphone, the speech processor, and the power supply. It is held in a stable position relative to the fixed internal component across the scalp interface by using a centering magnet . Acoustic information is transferred from the external receptor component to the internal effector system through radiofrequency coupling. For totally implantable devices, the receptor and effector limbs are completely internalized; there is no external component. Transcutaneous technologies are used to power and replenish energy to the internal batteries. The effector limb of implantable hearing devices differs in the location of ossicular chain stimulation. The sites of contact are the incus head, body, and lenticular process, and the stapes superstructure.

Acoustic, Imaging, and Other Device Considerations

Acoustic considerations for IMEHDs relate to increased stiffness and mass loading of the ossicular chain, which may result in a deepening of the existing hearing loss. Because middle ear mechanics may be impacted by all IMEHDs, normal middle ear function is a strict criterion in selecting patients for implantation. Ossicular chain stiffness is increased when there is rigid coupling between the device and the ossicles. Mass loading is increased when an effector component is attached to the incus or the stapes.

Currently, none of the IMEHDs is compatible with MRI. Unforeseen clinical problems in the future might warrant device explantation for diagnostic and therapeutic interventions. Once an IMEHD is implanted, electrocautery cannot be used in surgical procedures because electrical discharges might damage the device. Electromagnetic interference from other environmental sources might possibly interact with IMEHDs in unknown ways. Device-related uncertainties include lifespan of IMEHDs, output protection safeguards to prevent noise-induced hearing loss, hermitic seal dependability to reduce device failure rates, ease of upgrade from a semi- to a fully implantable model, and performance capacity to accommodate progressive hearing loss.

Specific Implantable Middle Ear Hearing Devices

The semi-implantable Vibrant Soundbridge (Med-El, Innsbruck, Austria) and totally implantable Envoy (Envoy Medical, Minneapolis, MN) devices are featured in this section to provide examples of innovative IMEHD technologies. The Vibrant Soundbridge was FDA approved for distribution in the U.S. market under Symphonix (San Jose, CA) corporation, but its availability under Med-El, which acquired Vibrant Soundbridge from Symphonix in 2003, has been on hold in the United States because FDA approval for the Med-El manufacturing facility is under review. The Vibrant Soundbridge is available in Europe. The Envoy is an investigational device undergoing clinical trials in the United States and Germany.

Vibrant Soundbridge

The Vibrant Soundbridge is a semi-implantable device that uses an electromagnetic effector to drive the ossicular chain. The external component is the Audio Processor, and the internal component is the surgically implanted Vibrating Ossicular Prosthesis. The Audio Processor houses the microphone, the speech processor, and the battery. The Vibrating Ossicular Prosthesis (Figure 691) contains the radiofrequency link, the demodulator, and the ossicular stimulatorthe floating mass transducerwhich is attached to the incus lenticular process with a titanium clip. The floating mass transducer is an electromagnetic effector with a magnet housed within an induction coil.

Figure 691.

The Symphonix Vibrant Soundbridge Semi-Implantable Device. The external Audio Processor receives and processes sounds. Signals are transmitted to the internal Vibrating Ossicular Prosthesis (inset, right) via radiofrequency coupling. The floating mass transducer, which is attached to the incus lenticular process with a titanium clip, vibrates the stapes.

(Reproduced, with permission, of the Symphonix Corporation, San Jose, CA.)

Implantation & Candidate Criteria

The surgical procedure consists of a mastoidectomy with a facial recess approach to place the floating mass transducer onto the lenticular process. After a 2-month period of healing, the device is activated. The speech processor delivers electronically controlled currents to drive the floating mass transducer into vibratory motion. Candidates for implantation are adults ( 18 years ) with a moderate to severe sensorineural hearing loss and speech discrimination scores > 50%.

Testing

For the Vibrant Soundbridge Phase III FDA study, statistically significant improvements in average functional gain on the order of 1015 dB across the frequency spectrum was reported. Mass loading of the incus did not adversely affect hearing in a clinically significant manner. Subjects reported improved satisfaction and performance; they preferred the Vibrant Soundbridge to a heterogeneous group of conventional hearing aids. Occlusion and feedback were virtually eliminated. Of note, aided speech recognition was comparable between the Vibrant Soundbridge and conventional hearing aids.

Safety

The trial also demonstrated acceptable safety. Most patients did not have a significant change in residual hearing (ie, change in pure-tone average < 10 dB). However, a small percentage of patients (4% or 2 of 53 patients) experienced a 1218 dB decrease in residual hearing. Other adverse effects have been reported during the U.S. trial. There were six device failures; these devices were successfully reimplanted after the manufacturer revised the product. One subject had a disconnection of his floating mass transducer; the device was successfully reimplanted.

Envoy

The Envoy is a totally implantable hearing device that uses piezoelectric transducers. The Envoy device is undergoing a Phase II FDA trial. A major design challenge of totally implantable hearing devices is the management of mechanical and acoustic feedback. By necessity, receptor (sensor) and effector (driver) limbs of the system are in close proximity. At high output levels by the effector limb, feedback may occur because the sensor detects the output signal. This results in feedback oscillation. The Envoy system addresses this difficult problem by segregating the receptor and effector limbs through controlled ossicular discontinuity.

Implantation & Candidate Criteria

The surgical procedure is a mastoidectomy with a facial recess approach to vaporize the distal 23 mm of the incus lenticular process with a laser (Figure 692). Bone cement is used to stabilize the sensor and driver to the mastoid and to affix the device tips to the incus body and stapes capitulum. When incoming sounds vibrate the native drum, the incus head is set in motion. The sensor tip, which is firmly attached to the incus head, deflects the piezoelectric transducer. Electrical signals are generated and transmitted to the speech processor. Outflow electrical signals from the processor guide the movements of the driver tip, which is transmitted to the stapes. Candidates for Envoy device implantation are adults ( 18 years) with a mild to severe sensorineural hearing loss and speech discrimination scores > 60%.

Figure 692.

The Envoy Totally Implantable Device. Sensor and driver limbs connect to the processor with detachable pins. The sensor is affixed to the incus body with bone cement. Note that the distal incus lenticular process is shortened to segregate sensor and driver vibrations (inset, upper right). The driver is also attached to the stapes capitulum with bone cement (inset, lower left).

(Reproduced with permission of St. Croix Medical Corporation, Minneapolis, MN.)

Phase I Testing and Safety Data

For the Envoy Phase I FDA study in 7 patients, 5 of 7 perceived benefit over their best-fit hearing aid at the 2-month activation period. Two of the 5 patients who ultimately experienced benefit required revision surgery after initial implantation because immediate benefit was insufficient. In the original cohort of 7 patients, 3 were explanted owing to infection or patient request. Functional gain with the Envoy was similar to hearing aids. Cochlear reserve in study patients appeared to be preserved following Envoy implantation, whereas air conduction thresholds for frequencies greater than 1 kHz were increased by 1020 dB at 12 months after implantation. Device modifications were implemented prior to Phase II clinical trial, which is currently under way.

Chen DA, Backous DD, Arriaga MA et al. Phase 1 clinical trial results of the envoy system: a totally implantable middle ear device for sensorineural hearing loss. Otolaryngol Head Neck Surg. 2004;131(6):904. (Phase I clinical study results in 7 patients.) [PMID: 15577788]

Fisch U, Cremers CW, Lenarz T et al. Clinical experience with the Vibrant Soundbridge implant device. Otol Neurotol. 2001;22(6):962. (Multicenter clinical study demonstrating successful implantation and fitting of the Vibrant Soundbridge.) [PMID: 11698826]

Fraysse B, Laveille JP, Schmerber S et al. A multicenter study of the Vibrant Soundbridge middle ear implant: early clinical results and experience. Otol Neurotol. 2001;22(6):952. (Study showing the Vibrant Soundbridge procedure is generally a safe procedure.) [PMID: 11698825]

Ko WH, Zhu WL, Kane M, Maniglia AJ. Engineering principles applied to implantable otologic devices. Otolaryngol Clin North Am. 2001;34(2):299. (Summary of engineering principles of possible actuators and sensors for totally implantable middle ear and cochlear hearing devices.) [PMID: 11382572]

Luetje CM, Brackman D, Balkany TJ 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(2):97. (Phase III study demonstrating safety and efficacy of the Vibrant Soundbridge system.) [PMID: 11870337]

Web Sites

http://www.envoymedical.com/Envoy_device.htm (This website provides information on their product.)

Copyright 2007 The McGraw-Hill Companies.  All rights reserved.
Privacy Notice. Any use is subject to the Terms of Use and Notice.  Additional Credits and Copyright Information.