Implantable middle ear hearing aids directly drive the ossicular chain or its attachment by means of the implanted components. They are intended for use in patients with residual cochlear function who could benefit from amplification. Based on the different transducers used in the design, current technology offers two types of middle ear implants: piezoelectric and electromagnetic. Piezoelectric implants use ceramic structures that are capable of temporary bending if electric current is applied. The implants may be single layered or bilayered, that is, mono-morph or biomorph, respectively. When attached to the ossicles, the implants create vibrations with alternating current. The amount of the structural bending is proportional to the overall length of the implants that are attached to the ossicles, a limiting factor for amplification power in piezoelectric implants. Electromagnetic implants convert electromagnetic signals into ossicular vibrations via a nearby coil without physical contact. A magnet is implanted onto the ossicle and vibrates according to the electromagnetic signals that are transmitted to the coil from the sound processor. A major limitation of the device is the distance between the coil, usually in the external auditory canal, and the magnet. Its power output is inversely proportional to the cube of the distance.8
There are a number of middle ear implantable devices currently available: Direct Drive Hearing System (DDHS, SOUNDTEC, Oklahoma City, OK), Envoy (St. Croix Medical, Minneapolis, MN), Middle Ear Transducer (MET, Otologics, Boulder, CO), and Rion (Rion Company, Tokyo, Japan). The Totally Integrated Cochlear Amplifier (TICA, Implex, Munich, Germany) and Vibrant Soundbridge (Sympho-
nix, San Jose, CA) have been discontinued in the United States. Table 29-1 provides a summary of the devices.
The SOUNDTEC DDHS design consists of an external and an internal unit. The external part includes a processor that is worn behind the ear and an electromagnetic coil in the ear canal in a conventional ear mold. To minimize the distance between the coil and magnet, the magnet is placed at the incudo-stapedial joint (Fig. 29-3). The magnet is coated with Parylene C and sealed in a laser-welded titanium cylinder. A titanium alloy ring is used to secure the implant around the incudo-stapedial joint.9
The audiologic indications are bilateral mildmoderate to moderately severe sensorineural hearing loss (SNHL), bone thresholds within 10 dB of air thresholds, high-frequency pure tone average of 1000, 2000, and 4000 Hz between 35 and 70 dB hearing level, and discrimination scores greater than 60%. A recently completed phase II clinical trial demonstrates that the SOUNDTEC system provides statistically significant reduction in feedback and occlusive effect as well as a statistically significant improvement in the following categories: functional gain, articulation index scores, speech discrimination in a quiet environment, perceived aided benefit, patient satisfaction, and device preference over the patient's optimally fit hearing aid.10
The surgical technique is a transcanal stapedect-omy approach. A standard stapedectomy-type tym-panomeatal flap is elevated to expose the tympanic cavity. The incudo-stapedial joint is then separated in a careful manner so as not to disrupt the stapes superstructure. The long process of the incus is then separated enough to allow for placement of an
Table 29-1 Implantable Hearing Devices
Direct Drive Hearing System SOUNDTEC
Middle ear transducer (MET) Rion
*Totally Integrated Cochlear
Amplifier (TICA) *Vibrant Soundbridge attachment ring. A 4-0 silk suture is placed and advanced beneath the long process of the incus. This sling then allows the surgeon to use his or her nondominant hand to elevate the incus during implant insertion. The implant is then inserted using the suction-operated SOUNDTEC insertion instrument or the SOUNDTEC open-mouthed cylinder-holding forceps. Only special nonmagnetic instruments and specula are used during the implant process to allow for exact positioning of the implant. The lenticular process is then allowed to rejoin with the capitulum of the stapes through the ring. The magnet cylinder is axially aligned with the ear canal. A cast made from absorbable Gelfoam is used to stabilize the implant. Care is taken to ensure that the promontory, the posterior canal wall, and the under-surface of the tympanic membrane do not come into contact with each other to prevent synechia formation. The tympanomeatal flap can then be returned to its original position. If the magnet comes into close apposition with the undersurface of the tympanic membrane, then a cartilage cap obtained from the tragus should be placed in the undersurface of the tympanic membrane to minimize the risk of implant extrusion. Following implantation, a 10-week period is allowed for healing to take place. Then the SOUNDTEC search coil is used to locate the optimal area for placement of the electromagnetic coil within the external auditory canal (ECA). Once the ECA is fitted, the device is activated.10
The Envoy is a totally implantable hearing system developed by St. Croix Medical Products of Minnesota. The Envoy hearing aid consists of a sensor, a driver, and a sound processor. The sensor is placed in the mastoid cavity in such a way that the transducer tip can be adhesively attached to the
Transduction FDA status Implantability
EM Phase II completed Partial
St. Croix Medical PE
Rion Co PE
Phase I Phase II NA NA
Total Partial Partial Total
Phase III completed
EM, electromagnetic; NA, not approved; PE, piezoelectric.
stapes. The body of the sensor is secured to the mastoid cavity using bone cement. The driver is also positioned within the mastoid cavity (Fig. 29-4). The sound processor is typically implanted in a bony cavity created behind the ear, in a similar fashion as cochlear implant. The sensor acts as a microphone. It converts sound vibrations into electrical signals. The electrical signals are then amplified by the sound processor. The driver converts the electrical signals into mechanical vibration to drive the stapes.8,1 The Envoy system is currently undergoing phase I clinical trial in the United States.
The MET is produced by Otologics, LLC, of Boulder, Colorado. It is approved in both the United States and Europe for implantation. It has an implantable part and an external part in a behind-the-ear case. A probe is placed in the body of the incus and generates vibration in the ossicles (Fig. 29-5). The audiologic indication is moderately severe to severe SNHL.12 A postauricular approach is typically used for implantation. Currently, a phase II clinical trial is being conducted in the United States.
The Rion is manufactured by Rion Company in Japan. It is approved for implantation in Japan and is not available in the United States. Indications for the device include patients with conductive deafness and mild to moderate mixed hearing losses due to chronic otitis media, tympanosclerosis, or total loss of the sound conductive mechanism. The device's usage has been limited to patients with bilateral deafness. It is implanted in the ear with greater hearing loss (Fig. 29-6). The audiologic criteria for selecting candidates are (1) an average bone-conduction hearing threshold for speech frequencies not exceeding 50 dB; (2) speech discrimination score better than 70%; and (3) intraoperative hearing testing that demonstrates the effectiveness of the implant. The device is implanted via a mastoidectomy, canal wall up or down.13
The TICA has been developed by Implex Corporation, Munich, Germany, and approved in Europe for moderate to severe high-frequency SNHL. The device is implanted through a mastoidectomy and has a small remote control to adjust the device. The battery is charged transcutaneously with a portable charger within approximately 90 minutes. The running time is about 50 hours, and the battery may require replacement every 3 to 5 years. The indications for the new model TICA LZ0331 include lack of benefits from conventional hearing aids, normal pneumatization of mastoid, and SNHL from 50 to 90 dB above 3 kHz.8
The Vibrant Soundbridge system was developed by Symphonix Devices, Inc., of San Jose, California. In November 2002 the company was dissolved by its board and the system withdrawn from the market. Prior to its withdrawal, the device was approved by the Food and Drug Administration (FDA) and the European Union for implantation. A phase III study demonstrated effectiveness for moderate to severe SNHL in adults. The device consists of two parts: the external speech processor and the implanted vibrating ossicular prosthesis. The surgery includes a mastoidectomy, similar to cochlear implantation.
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