GENERAL CONSIDERATIONS OF IMPLANTABLE MIDDLE EAR HEARING DEVICES
It is estimated that 32 million Americans have a hearing loss severe enough to cause problems with communication. The majority of hearing loss is sensorineural in nature and sound amplification (a conventional hearing aid) is an important rehabilitative method that often restores hearing to a functional level. The remaining individuals have conductive or mixed hearing losses that are potentially amenable to surgical treatment.
Despite the potential benefits of hearing aids, many patients with sensorineural hearing loss do not use them. It is estimated that only 1 in 5 of individuals within the United States who may benefit from a hearing aid actually own one. Only 50% of those who own a hearing aid actually use it on a long-term basis. Some common complaints about hearing aids include feedback distortion, ear canal discomfort, stigma associated with wearing a hearing aid, and psychological rejection. Conventional hearing aids are optimized to amplify sounds in the 500-Hz to 2000-Hz range, which corresponds to speech; the lack of upper and lower register amplification, also known as nonlinear frequency amplification, can also give sounds an artificial, hollow, or “tin can” sound.
Over the past 30 years, implantable hearing devices have become a viable alternative to conventional hearing aids because they correct many of the deficiencies found with conventional air-conduction hearing aids. Implantable hearing devices attempt to deliver a more natural sound quality, provide linear amplification across the frequency spectrum, reduce feedback, improve comfort and cosmesis, and eliminate ear canal occlusion. The more commonly used implantable hearing device is the bone-conduction system as a treatment for conductive and mixed hearing losses. These devices rely on direct vibration of the temporal bone to indirectly stimulate the cochlea. In 2002, bone-conduction devices were also approved for treatment of unilateral sensorineural hearing loss or single-sided deafness. The other class of implantable hearing devices is the middle ear implant, in which vibratory stimulation of the ossicular chain or cochlear fluids is performed. This chapter is focused on the technology and current status of middle ear implantable devices.
DEFINITION OF IMPLANTABLE MIDDLE EAR HEARING DEVICES
An implantable middle ear hearing device (IMEHD) is a surgically implanted device that converts acoustic energy to mechanical energy by delivering vibratory stimulation to the inner ear. The basic components of an IMEHD consist of a receptor limb and an effector limb. The receptor limb converts and amplifies the incoming acoustic signal to an electrical signal. The effector limb contains a transducer that converts the amplified electrical signal into a vibratory signal for presentation to the ossicular chain or the cochlea. The 2 basic transducer types used to drive the vibratory signal are electromagnetic and piezoelectric systems. In electromagnetic systems, electromagnetic fields generated by induction coils can put magnets into oscillatory motion. On the other hand, piezoelectric transducers are generally ceramic materials that ...