Hearing aid implants are medical devices that amplify sounds for people who are hard of hearing that are implanted in the person's body instead of being worn externally. Implanted devices are generally more costly than externally worn hearing aids, but can be useful for people for whom conventional hearing aids are inconvenient or uncomfortable to wear or who suffer from forms of hearing loss that ordinary hearing aids cannot effectively correct. Commonly used examples of hearing aid implants include middle ear implants and bone-anchored hearing aids. The term is also used to refer to cochlear implants, though cochlear implants work on a very different principle from conventional hearing aids.
All hearing is based on the detection of vibrations in the cochlea, a structure in the inner ear. These vibrations stimulate sensory receptors in the cochlea called hair cells, which respond by producing electrical impulses that are sent to the nerve cells and onto the brain, where they are interpreted and perceived as sound. There are two types of hearing loss, sensorineural and conductive. Sensorineural hearing loss is the result of damage to the brain, the vestibulocochlear, or auditory, nerve, or the hair cells in the cochlea, with the latter being the most common cause. Conductive hearing loss is caused by a deformity, injury, or obstruction that interferes with the conduction of sound waves through the ear.
Bone-anchored hearing aids are hearing aid implants that conduct sound through bone. The internal part of the device, a small titanium implant, is surgically implanted behind the ear. It then begins osseointegration, a process by which the titanium bonds with the bones of the skull. After a few months, usually around three for adults and six for children, the bonding process is complete and an external audio processor is attached to the implant.
When the audio processor picks up sound, that sound is transmitted to the titanium implant. The implant vibrates, and this vibration is conducted through the skull to the inner ear. In people with normal hearing, these vibrations are channeled to the cochlea through the outer and middle ear, but vibrations through the skull can simulate it in the same way.
Bone-anchored hearing aids are often helpful for people with conductive hearing loss, because they bypass the outer and middle ear entirely. They are also helpful for people who cannot wear conventional hearing aids due to infection or inflammation in the ear canal. In some cases, bone-anchored hearing aids are also used by people whose hearing is impaired in only one ear, a condition called unilateral hearing loss, though they often have better results with a specialized external device called a CROS (contralateral routing of signals) hearing aid.
Middle ear implants are another type of implanted hearing aid. Sounds are picked up by an external microphone, and an audio processor worn is behind the ear. The sounds are interpreted and converted into an electrical signal that is transmitted through the skin to an internally implanted receiver. The receiver transmits this signal down a wire to a small transducer implanted in the middle ear. This vibrates in response to the signal, and these vibrations are conducted to the inner ear and perceived as sound.
Cochlear implants are also frequently included in the category of hearing aid implants, though they work very differently and are actually a substitute for hearing rather than an aid to it. Like middle ear implants, a cochlear implant has an external microphone that picks up sound. These sounds are analyzed by a speech processor, which can differentiate speech from other noises and filters out these other sounds. The processed sound is sent through a cable to the external transmitter, which transmits the signal to an implanted receiver that sends electrical signals to a set of electrodes implanted in the cochlea.
The electricity is transmitted to nerves in the cochlea and then onto the brain where it is perceived as sounds. This does not restore normal hearing, as many sounds are intentionally filtered out and the user's perception of sound is produced by a small number of electrodes rather than the thousands of hair cells in the cochlea, but the user usually regains the ability to comprehend human speech. Cochlear implants can be used by people who have severe hearing loss or are completely deaf, provided they still have a functioning auditory nerve, as they bypass most of the normal auditory system entirely and send information directly to the nervous system. These implants are usually ineffective for adults who have been deaf since they were children, however, because the brain's ability to interpret sound will not develop normally during childhood if it receives no sounds to interpret.