Bruce Black MD
Jane Black PhD
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In recent years, substantial progress has been made with respect to implantable hearing technology. Cochlear implants are the most famous example, but are accompanied by the development of other technology that functions on different principles. There are four patterns of devices
These devices are used to repair or replace diseased middle ear ossicles. Total/Partial ossicular replacement prostheses are commonly used in the surgery of chronic ear diseases. Columellar variants are placed between the eardrum membrane and the entrance to the middle ear. Assembly types bridge the gap left by disease-induced dissolution of parts of the ossicular chain. Stapedectomy piston prostheses are applied in the surgery for otosclerosis.
The Med El Vibrant Soundbridge (VSB) is an active middle ear implant that stimulates the cochlea using a tiny electromagnetic “floating mass transducer” containing an electromagnet. Implantation requires a small (3 cm) incision on the rear of the ear. The body of the implant is similar to a cochlear implant, but with a fine electrode line leading to the microtransducer. The body of the implant rests within a pocket created to the rear of the external ear. The transducer is passed into the middle ear and is then clipped to the chain of ossicles, vibrating the chain to produce sound. A small external “Amade” processor, sited unobtrusively on the head behind the ear, powers the implant. The processor induces current in a complementary coil within the implant , which is then processed into vibration through the “barrel” of the implant, vibrating the skull and producing audible sound by bone conduction.
These implants (MED EL Bonebridge, and Cochlear BAHA) act by vibration of the skull itself. This produces hearing by vibration of the hair cells of cochlea of the ear implanted. Conventional hearing aids stimulate the ear by vibration of the normal middle ear mechanisms, via the external ear canal. If this is not possible (e.g. chronic infection, canal obstruction etc.), a bone conduction aid is used to bypass the obstruction, acting directly on the cochlea. This may be done in cases of conductive, sensorineural, or mixed deafness. Application in severe conductive deafness is beneficial. As the implant does not need to overcome the mechanical deafness aspect, and therefore does not need to amplify sound excessively (which also amplifies unwanted extraneous sound). Alternatively, if the implanted ear has suffered a severe nerve deafness, as in Meniere’s Disease, the implant may be used to stimulate the better hearing opposite ear, eliminating the “head shadow” dead zone out from the deaf ear. The better ear must have excellent hearing in such cases.
The Med El Bonebridge implantimplant is an active bone conduction device. It is similar to the Soundbridge in design and is implanted via similar minimal-trauma surgery, but is fixed to the head behind the ear. As with the Soundbridge, the device is powered by the magnetically attached external “Amade” processor. This implant is used for same-side conductive, mixed or sensorineural deafness, or for opposite-ear stimulation when the implanted hear has a profound nerve deafness. Hearing is produced by vibration of the electromagnet contained within the “head “ of the device.
More recently, the Attract model employs magnetic attraction to stabilise the external processor on the skin to the rear of the ear, without a visible screw mount (transcutaneous technique)..
All three devices are particularly useful when the external canal has problems that impede the efficient use of traditional (air conduction) hearing aids that function via the external canal:
| Disease States | Other Factors |
|---|---|
| Atresia | Excessive wax |
| Narrow | Perspiration |
| Exostoses | Canal Occlusion effect |
| Otitis Externa | Mastication Noises |
| Manual disability e.g. rheumatoid | Feedback Howl |
The VSB is usually employed to stimulate the ossicular chain of the ear implanted, although modifications may permit its use in other circumstances, when the chain is damaged In expert hands the implants are very effective over a range of mild to moderately severe hearing losses, whether conductive, mixed or sensorineural.
Bone conduction devices serve a similar range, but are particularly useful to bypass conductive deafness, as they can stimulate the inner ear directly. This is valuable in mixed deafness cases when the severity of loss taxes the amplification ability of air conduction aids.
Bone conduction devices are also useful in cases of one-sided severe nerve deafness. If the better ear retains excellent function, an implant in the severely affected side can recover “head shadow” (the “blind side” deafness) by stimulating the better ear. This does not provide direction-finding, stereo or overall awareness, but is very beneficial at the table or in groups, to improve deaf-side communication.
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Queensland Otology is a comprehensive medical and surgical practice providing excellence in diagnostic, medical and surgical treatment of diseases of the ear, including cochlear implantation and other implantable hearing devices.