General Considerations of Implantable Hearing Devices
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 30 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.
The search for alternatives to conventional hearing aids motivated the development of implantable hearing devices that deliver sound energy more directly to middle and inner 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. Although the chapter is mainly devoted to implantable middle ear hearing devices (IMEHDs), a bone-anchored hearing aid (BAHA) is a common implantable alternative to the conventional hearing aid and is briefly discussed.
Risks associated with middle ear device implant surgery include sensorineural hearing loss, ossicular chain disruption, facial nerve injury, external canal laceration, and cerebrospinal fluid 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 IMEHDs are very exciting for both patients and care providers.
Perioperative risks associated with BAHA surgery include cerebrospinal fluid leak and wound problems surrounding the osseointegrated implant. The most common long-term complication is skin overgrowth over the abutment, as common as one in five patients, and has been shown to correlate with incomplete skin graft survival in the early postoperative period. However, an important distinguishing feature of the BAHA from all middle ear implants is that it is safe in MRI scanners with forces up to 9.4 T. Because of its minimal risks, the BAHA has become a popular alternative for those with conductive and sensorineural hearing losses and cannot tolerate or use hearing aids.
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.
The BAHA consists of a titanium post that is osseointegrated in the postauricular region. Attached to the post is a sound processor that contains a microphone. The microphone picks up sound and the processor sends vibrations to the post to stimulate the cochlea via bone conduction.
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 a 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 life span 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.
