Hearing Prosthetics: Surgical Techniques

Hearing Loss

Hearing loss can be divided into two broad categories: conductive and sensorineural. In conductive hearing loss, the sound conducting mechanism in the outer and middle ear (ear canal, tympanic membrane, and ossicles) is impaired. In sensorineural hearing loss, the inner ear, cochlear nerve, or central auditory pathway are affected, disturbing the perception of sound delivered through both air and bone conduction. Mixed hearing loss is a combination of conductive and sensorineural hearing losses.

Audiometric testing has two primary parts: pure tone audiometry and speech audiometry. In pure tone audiometry, air- and bone-conduction thresholds are tested at different frequencies ( Fig. 37.1 ). In conductive hearing loss, the disparity between air- and bone-conduction thresholds results in an air–bone gap. In sensorineural hearing loss, both air- and bone-conduction thresholds are elevated. In speech audiometry, the ability to perceive and discriminate words and sentences is tested. The speech reception threshold is a measure in decibels (dB) of the speech perception threshold, whereas the speech discrimination score (SDS) is a measure as a percentage of words that are correctly repeated by the patient.

Some patients may have mild hearing loss on pure tone audiometry but significant impairment in speech audiometry, therefore, the common complaint of, “I can hear, but I cannot understand.” It is always important to consider both types of audiometry when evaluating patients with hearing loss.

Hearing loss is often helped by conventional hearing aids. For conductive hearing loss, ear canal atresia or deformity, a transcutaneous or an osseointegrated implant is an option. With a unilateral loss, CROS hearing aid (contralateral routing of signal) or osseointegrated implant can send sound to the good ear. With severe hearing loss beyond the ability of hearing aids, a cochlear implant or ABI is the remaining option.

Bone-Anchored Implants

Osseointegrated implants were first introduced into clinical practice in Scandinavia in the late 1960s for intraoral rehabilitation. Application of this technology to bone-anchored hearing devices represents a refinement of conventional bone-conduction hearing aids. In bone-conduction hearing aids, sound is transmitted through vibration of the skull to the cochlea. However, the utility of conventional bone-conduction devices is limited due to patient discomfort and limited sound fidelity secondary to soft tissue attenuation. Coupling the bone vibrator to an osseointegrated implant, as originally performed by Tjellström and his team at the Institute of Applied Biotechnology in Sweden in 1977, overcomes many of these limitations of conventional bone conducting devices.

The success of this technology relies on two basic principles: the creation of a permanent percutaneous connection and the placement of an osseointegrated titanium abutment upon which a transducer is coupled. Advances in metallic biomaterials facilitate the creation of permanent, well-tolerated implant fixtures upon which the transducer is placed. Titanium has the ability to create a corrosion-resistant oxide layer on the surface of the implant that confers osseointegration potential. ^, Because the implant may be worn for several decades or longer, the toxicity and carcinogenicity of the oxide coating takes on particular importance. Reports have shown that titanium is superior to stainless steel, lacking steel’s high potential for corrosion or the toxicity of its components. ^, To date, pure titanium appears free of the adverse sequelae seen with other metals and thus continues to represent an ideal implant material.

The most widely used osseointegrated hearing device is the Baha implant (Cochlear Corporation, Englewood, CO), shown in Fig. 37.2 . The Baha consists of a pure titanium implant and a sound processor. The processor couples directly to the titanium implant via a skin-penetrating abutment, utilizing a force-fit, plastic coupling.

Modifications of osseointegrated hearing devices now include transcutaneous systems, (Sophono, Inc., Boulder, CO) in which the driver is recessed in the skull but not osseointegrated ( Fig. 37.3 ). Transcutaneous bone conduction systems use magnetic retention to connect the sound processor to the implant. A major advantage of the transcutaneous system is the absence of the skin-penetrating abutment used in percutaneous systems. Soft tissue attenuation potentially limits the gain of these devices compared to osseointegrated devices, but improved driver technology has lessened this difference. These transcutaneous bone anchored systems are well tolerated and have a high patient satisfaction rating. ^,

Indications

Bone conducting implants were first developed for patients with conductive hearing loss who could not tolerate traditional hearing aids (chronic drainage, large mastoid bowl or meatoplasty following ear surgery, aural atresia, external auditory canal closure, etc.). This group of patients may benefit from a device that offers bone-anchored hearing. Percutaneous and transcutaneous bone anchored implants are also approved for patients with single-sided sensorineural deafness, for example, due to the effects of a tumor in the cerebellopontine angle or its treatment. ^, In this instance, sound is delivered to the skull on the side lacking sensorineural function and transmitted by bone conduction to the normal contralateral ear, where it is perceived ( Fig. 37.4 ).

Surgical Technique

Percutaneous system—the postauricular area is cleaned and shaved, and the site of the abutment placement is marked. The abutment is typically placed 50 to 55 mm posterior to the ear canal along the temporal line. The underlying and surrounding soft tissues are removed, leaving the periosteum intact. A small circle of the periosteum (about 6 mm ² ) is removed, exposing the underlying bony cortex. A 4-mm hole is drilled at the center of the exposed bony cortex. A countersink is then used to enlarge the hole. The abutment is slowly threaded into the previously prepared hole using the abutment inserter ( Fig. 37.5 ). The skin flap is reflected back to its original position, with a central opening to accommodate the abutment. The abutment is left undisturbed for 3 to 4 months, which allows osseointegration to occur. Once the abutment is osseointegrated, it is ready for coupling to the sound processor.

Transcutaneous system—the incision and scalp elevation is very similar to the percutaneous system, except the skin flap is seldom thinned and there is no skin perforation for the abutment. A bony well is created to allow flush seating of the bone conducting device.

Cochlear Implants

Cochlear implants are neural prostheses that convey sound information to the auditory cortex via electrical stimulation of the auditory nerve, bypassing the cochlea in individuals with bilateral severe to profound sensorineural deafness. A typical cochlear implant consists of an external component, which includes the microphone and the speech processor, and an internal component, which includes the receiver-stimulator and the stimulating electrodes. The speech processor is battery powered and is housed in a behind-the-ear unit similar to a hearing aid. A microphone captures acoustic input and delivers it to the speech processor that, in turn, converts it into electrical signals. An external antenna transmits the encoded signals across the scalp via radiofrequency to the antenna of the internal device. The signals are then sorted tonotopically and are delivered to different auditory nerve fibers along the cochlear spiral. An example of a cochlear implant is shown in Fig. 37.6 .