Chapter 67. Implantable Middle Ear Hearing Devices

Societal Factors

Hearing loss is a common disability among adults. In the aging population, 25% of individuals between age 65 and 74 and 50% of individuals age 75 and older have hearing problems. Overall, approximately 30 million adults in the United States have moderate-to-severe sensorineural hearing loss. For this group, acoustic amplification (a conventional hearing aid) is an important rehabilitative strategy that often restores hearing to a serviceable level.

Despite the potential benefits of acoustic amplification, many hearing-impaired patients do not accept hearing aids. Some common complaints about hearing aids include feedback annoyance, ear canal discomfort, stigma of wearing an external appliance, and psychological rejection. It is estimated that only 20% of individuals within the United States who may benefit from a hearing aid own one. Only half of those who own a hearing aid use their device on a long-term basis.

Advantages of Implantable Hearing Devices

The search for alternatives to conventional hearing aids motivated the development of implantable hearing devices that deliver sound energy more directly to middle and inner ear structures. This design eliminates many of the disadvantages of conventional hearing aids. Implantable hearing devices endeavor to deliver more natural sound quality, increase gains across the frequency spectrum, reduce feedback, improve comfort and cosmesis, and eliminate ear canal occlusion. Although the chapter is mainly devoted to implantable middle ear hearing devices (IMEHDs), a bone-anchored hearing aid (BAHA) is a common implantable alternative to the conventional hearing aid and is briefly discussed.

Implantation-Related Risks

Risks associated with middle ear device implant surgery include sensorineural hearing loss, ossicular chain disruption, facial nerve injury, external canal laceration, and cerebrospinal fluid leak. Beyond surgical risks, other considerations associated with implantable hearing devices are higher costs compared with conventional hearing aids, incompatibility with magnetic resonance imaging (MRI), and uncertain need for future explantation (ie, device removal) and reimplantation. Nevertheless, emerging technologies in IMEHDs are very exciting for both patients and care providers.

Perioperative risks associated with BAHA surgery include cerebrospinal fluid leak and wound problems surrounding the osseointegrated implant. The most common long-term complication is skin overgrowth over the abutment, as common as one in five patients, and has been shown to correlate with incomplete skin graft survival in the early postoperative period. However, an important distinguishing feature of the BAHA from all middle ear implants is that it is safe in MRI scanners with forces up to 9.4 T. Because of its minimal risks, the BAHA has become a popular alternative for those with conductive and sensorineural hearing losses and cannot tolerate or use hearing aids.

Hearing Device Components

An IMEHD is a device that converts acoustic energy to mechanical energy and delivers it to a vibratory structure in the middle ear. The basic components of an IMEHD consist of an acoustic signal detector (receptor), a transmission link, and an actuator that vibrates the ossicular chain (effector). The two basic transducer types used to drive the ossicular chain are electromagnetic and piezoelectric systems. Electromagnetic fields generated by induction coils can put magnets into oscillatory motion. Piezoelectric transducers are generally ceramic materials that vibrate in response to applied electrical energy. The general design of an IMEHD consists of separate receptor and effector limbs. For semi-implantable devices, the receptor limb is an external, removable component that houses the microphone, the speech processor, and the power supply. It is held in a stable position relative to the fixed internal component across the scalp interface by using a centering magnet. Acoustic information is transferred from the external receptor component to the internal effector system through radiofrequency coupling. For totally implantable devices, the receptor and effector limbs are completely internalized; there is no external component. Transcutaneous technologies are used to power and replenish energy to the internal batteries. The effector limb of implantable hearing devices differs in the location of ossicular chain stimulation. The sites of contact are the incus head, body, and lenticular process, and the stapes superstructure.

The BAHA consists of a titanium post that is osseointegrated in the postauricular region. Attached to the post is a sound processor that contains a microphone. The microphone picks up sound and the processor sends vibrations to the post to stimulate the cochlea via bone conduction.

Acoustic, Imaging, and Other Device Considerations

Acoustic considerations for IMEHDs relate to increased stiffness and mass loading of the ossicular chain, which may result in a deepening of the existing hearing loss. Because middle ear mechanics may be impacted by all IMEHDs, normal middle ear function is a strict criterion in selecting patients for implantation. Ossicular chain stiffness is increased when there is a rigid coupling between the device and the ossicles. Mass loading is increased when an effector component is attached to the incus or the stapes.

Currently, none of the IMEHDs is compatible with MRI. Unforeseen clinical problems in the future might warrant device explantation for diagnostic and therapeutic interventions. Once an IMEHD is implanted, electrocautery cannot be used in surgical procedures because electrical discharges might damage the device. Electromagnetic interference from other environmental sources might possibly interact with IMEHDs in unknown ways. Device-related uncertainties include life span of IMEHDs, output protection safeguards to prevent noise-induced hearing loss, hermitic seal dependability to reduce device failure rates, ease of upgrade from a semi- to a fully implantable model, and performance capacity to accommodate progressive hearing loss.

The semi-implantable Vibrant Soundbridge® (Med-El, Innsbruck, Austria), the totally implantable Envoy Esteem™ (Envoy Medical, Minneapolis, MN), and the totally implantable Carina™ (Otologics LLC, Boulder, CO) devices featured in this section are examples of innovative IMEHD technologies. The Vibrant Soundbridge® is currently the only FDA-approved device within the United States and is also available in Europe. The Envoy Esteem, a totally implantable device, has recently been recommended for approval by an advisory panel to the FDA in the United States. It is currently available in many European countries as well as Brazil, Iran, and India. The Carina™ is currently undergoing Phase II FDA trials in the United States.

Vibrant Soundbridge

The Vibrant Soundbridge is a semi-implantable device that uses an electromagnetic effector to drive the ossicular chain. The external component is the audio processor, and the internal component is the surgically implanted vibrating ossicular prosthesis. The audio processor houses the microphone, the speech processor, and the battery. The vibrating ossicular prosthesis (Figure 67–1) contains the radiofrequency link, the demodulator, and the ossicular stimulator–the floating mass transducer–which is attached to the incus lenticular process with a titanium clip. The floating mass transducer is an electromagnetic effector with a magnet housed within an induction coil.

Implantation & Candidate Criteria

The surgical procedure consists of a mastoidectomy with a facial recess approach to place the floating mass transducer onto the lenticular process. After a 2-month period of healing, the device is activated. The speech processor delivers electronically controlled currents to drive the floating mass transducer into vibratory motion. Candidates for implantation are adults (⩾18 years) with a moderate-to-severe sensorineural hearing loss and speech discrimination scores >50%. Recent studies have also shown that placement of the floating mass transducer either directly on the round window membrane or on the stapes in the absence of the incus have promising results, suggesting that this is an alternative form of hearing amplification for patients with otosclerosis and aural atresia.

Testing

For the Vibrant Soundbridge Phase III FDA study, statistically significant improvements in average functional gain on the order of 10–15 dB across the frequency spectrum were reported. Mass loading of the incus did not adversely affect hearing in a clinically significant manner. Subjects reported improved satisfaction and performance; they preferred the Vibrant Soundbridge to a heterogeneous group of conventional hearing aids. Occlusion and feedback were virtually eliminated. Of note, aided speech recognition was comparable between the Vibrant Soundbridge and conventional hearing aids.

Safety

The trial also demonstrated acceptable safety. Most patients did not have a significant change in residual hearing (ie, change in pure-tone average <10 dB). However, a small percentage of patients (4% or 2 of 53 patients) experienced a 12–18 dB decrease in residual hearing. Other adverse effects have been reported during the U.S. trial. There were six device failures; these devices were successfully reimplanted after the manufacturer revised the product. One subject had a disconnection of the floating mass transducer; the device was successfully reimplanted.

Envoy Esteem™

The Envoy Esteem™ is a totally implantable hearing device that uses piezoelectric transducers. A major design challenge of totally implantable hearing devices is the management of mechanical and acoustic feedback. By necessity, receptor (sensor) and effector (driver) limbs of the system are in close proximity. At high output levels by the effector limb, feedback may occur because the sensor detects the output signal. This results in feedback oscillation. The Envoy system addresses this difficult problem by segregating the receptor and effector limbs through controlled ossicular discontinuity. The device has now received FDA approval and is available in the United States.

Implantation & Candidate Criteria

The surgical procedure is a mastoidectomy with a facial recess approach to vaporize the distal 2–3 mm of the incus lenticular process with a laser (Figure 67–2). Bone cement is used to stabilize the sensor and driver to the mastoid and to affix the device tips to the incus body and stapes capitulum. When incoming sounds vibrate the native drum, the incus head is set in motion. The sensor tip, which is firmly attached to the incus head, deflects the piezoelectric transducer. Electrical signals are generated and transmitted to the speech processor. Outflow electrical signals from the processor guide the movements of the driver tip, which is transmitted to the stapes. Candidates for the Envoy device implantation are adults (⩾18 years) with a mild-to-severe sensorineural hearing loss and speech discrimination scores ⩾60%.

Phase I Testing and Safety Data

For the Envoy Esteem™ Phase I FDA study in seven patients, five of seven perceived benefit over their best-fit hearing aid at the 2-mo activation period. Two of the five patients who ultimately experienced benefit required revision surgery after initial implantation because immediate benefit was insufficient. In the original cohort of seven patients, three were explanted owing to infection or patient request. Functional gain with the Esteem™ was similar to hearing aids. Cochlear reserve in study patients appeared to be preserved following Esteem™ implantation, whereas air conduction thresholds for frequencies greater than 1 kHz were increased by 10–20 dB at 12 months after implantation. Device modifications were implemented prior to Phase III trial.

Otologics Carina™

Initially developed as a semi-implanatable device, the Otologics Carina™ device is now a fully implantable device that incorporates a microphone, speech processor, battery, and transducer into a prosthesis. The microphone, located under the postauricular skin, amplifies sound and converts acoustic signals to electrical signals that are transmitted to the transducer. Thereafter, the ossicular stimulator vibrates the ossicles. The subcutaneous battery is charged daily with a radiofrequency coil that is placed over the implant site. The implant is programmable and the volume can be adjusted with a remote control that sits over the implant.

Implantation & Candidate Criteria

The surgical procedure is a modified atticotomy that exposes the body of the incus and the head of the malleus. A mounting plate with a retaining ring is affixed to the mastoid cortex with self-tapping screws that allows the electronics capsule to be secured. A laser is used to create a hole in the midbody of the incus. The transducer is mounted to the retaining ring with its probe tip is advanced into the hole of the incus. Electrical stimulation of the transducer translates into mechanical stimulation of the ossicular chain. After a 6- to 8-week healing period, the device can be activated. Candidates for the Otologics device are similar to that for the Envoy Esteem.

Phase I Testing and Safety Data

A study of 20 implanted patients demonstrated that although their conventional hearing aids had slightly better pure-tone averages and monaural word recognition scores, patients preferred hearing with the Carina™, noting improved quality of sound, improved ability to hear soft sounds, comfort, and ability to use the device in noisy conditions. Complications included implant extrusion in three of the patients, requiring explantation in two, increased charging times in seven patients, resulting in two no longer using their implants, and loss of external communication with the implant, making it unable to be charged. It was also noted that at 6 months after implantation, there was decreased usage of the implant secondary to decreased speech perception. It is believed that this is a result of the migration of the microphone and processors. Since this trial, modifications to the device to address these issues have been made as it enters Phase II testing.

Bone-Anchored Hearing Aids

While it is not an implantable middle ear device, the BAHA (Cochlear Corporation, Sydney, Australia) is an FDA-approved implant for patients with conductive hearing loss, mixed hearing loss, or unilateral sensorineural hearing loss who cannot tolerate or have limited benefits from conventional air conduction hearing aids. Traditional bone conduction hearing aids require the use of a head band to secure the transducer to the head. With the BAHA, not only is the band no longer required but also the coupling between the transducer and the microphone is significantly better, as much as by 10–15 dB. Disadvantages of the BAHA include a higher cost and requirement of a surgical procedure. However, the improvement in hearing and communication has made the BAHA an appealing option for certain groups of patients with hearing loss.

Implantation & Candidate Criteria

The surgical procedure consists of creating thin skin flap, about the thickness of a full-thickness skin graft. A titanium abutment is installed into the temporal bone and the thin skin flap with a center hole is draped over the implant. The wound is then allowed to heal for 3–4 months for osseointegration to take place.

The BAHA was originally indicated for patients with a conductive or mixed hearing loss with a conductive component >30 dB. Patients with chronically draining ears despite vigorous treatment, bilateral congenital aural atresia, and conductive loss in the only hearing ear have been the primary beneficiaries. More recently, patients with single-sided deafness have been shown to benefit from the BAHA.

We would like to acknowledge Kenneth C.Y. Yu, MD for his contribution to this chapter in the previous editions of CDT.