Hearing Prosthetics: Surgical Techniques

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Chapter 49 Hearing Prosthetics

Surgical Techniques

Wade W. Chien, Howard Francis, John K. Niparko

Tumors in the cerebellopontine angle and internal auditory canal can result in hearing loss. In addition, surgical resection of these tumors often leads to hearing impairment due to manipulation and/or sacrifice of the cochlear nerve. This chapter examines the current technologies that enable a restored perception of sound and the clinical application of implantable hearing technologies. We describe the application of an osseointegrated implant, cochlear implant, and auditory brain stem implant to the solution of hearing loss in the neurosurgical patient.

Hearing Loss

Hearing loss can be divided into two broad categories: conductive and sensorineural. In conductive hearing loss, the sound conduction mechanism in the outer and middle ear is impaired. This results in a decrease in the perception of air-conducted sound delivered through the external auditory canal. If the transduction of sound to neural signals continues normally in the inner ear, the perception of sound transmitted to the cochlea through the skull (bone conduction) is not disturbed. 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. Both conductive and sensorineural hearing loss are present in patients with mixed hearing loss.

Hearing loss is usually evaluated through audiologic testing. A typical audiogram has two parts: pure tone audiometry and speech audiometry. In pure tone audiometry, air- and bone-conduction thresholds are tested at different frequencies (Fig. 49-1). 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 from a standardized list presented at suprathreshold levels that are recognized and repeated by the patient.

FIGURE 49-1 Pure tone audiometry in a patient with bilateral sensorineural hearing loss. Frequency (in hertz) is plotted on the x-axis, and loudness of sound (in the decibel hearing level) is plotted on the y-axis. The air- and bone-conduction thresholds are elevated in both ears (right ear in red, left ear in blue), particularly at higher frequencies.

Some patients may have mild hearing loss on pure tone audiometry but significant impairment in speech audiometry. Therefore, sound awareness does not guarantee the presence of useful hearing. It is always important to consider both types of audiometry when evaluating patients with hearing loss. Furthermore, the presence of normal hearing in one ear underestimates the negative impact of acquired unilateral hearing loss on the ability to comprehend speech and access information in the presence of background noise.

The rehabilitation challenges posed by hearing loss are modest when threshold elevation is the primary deficit but become increasingly complex as speech perception is impaired. Referral to and close collaboration with specialists in otolaryngology–head and neck surgery and audiology provides comprehensive consideration and delivery of options that meet the specific needs of the patient. Threshold deficits in the setting of normal or near-normal speech discrimination can be overcome by a conventional hearing aid through a combination of amplification and sound filtering. In patients with lower SDS, more complex filtering, other speech processing strategies, and a directional microphone are required to overcome speech perception challenges, particularly in background noise. If the hearing loss is primarily conductive and the ear canal has been surgically altered so that it cannot accommodate a conventional hearing aid, a bone-conduction hearing aid can be used to deliver sound transcutaneously or via an osseointegrated implant. When the loss of acuity and speech discrimination is too advanced for conventional hearing aids to provide benefit, a normal contralateral ear may serve as the conduit for sound input from the deaf side using wireless transmission from an ear-level microphone to a receiver by the contralateral routing of sound (CROS) hearing aid or using bone transmission across the skull using osseointegrated technology. The presence of significant sensorineural hearing loss and speech perception deficits in both ears poses the greatest challenge. In this scenario, the delivery of relevant neural codes to the central nervous system can be accomplished by electrical stimulation of the cochlear nerve or the auditory brain stem using implantable prostheses.

Bone-Anchored Osseointegrated Implants

Osseointegrated implants were first introduced into clinical practice in Scandinavia in the late 1960s, ostensibly for intraoral rehabilitation.¹ Since then, osseointegrated implants have gained widespread acceptance in the fields of dental, oral/maxillofacial, craniofacial, and orthopedic surgery. 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 now considered 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, averts many of these limitations of conventional bone conducting devices.²

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. The cutaneous-implant interface as originally conceived by Brånemark ³ is based on biologic principles observed throughout nature. Teeth and nails (as well as talons, tusks, or claws as found in other species) all interface with a thin, firmly attached cutaneous or mucosal border with little to no hair. This tissue architecture limits tissue mobility and preserves stability of tissue planes while inhibiting the penetration of microbes and subsequent inflammation or infection.^4,⁵ By mimicking these attributes in the skull, the surgeon creates a permanent cutaneous-implant border that the patient may easily maintain.

Advances in metallic biomaterials facilitate the creation of permanent, well-tolerated implant fixtures upon which the transducer is placed. Titanium is the most notable among several materials that have found clinical application in anchoring dental prostheses. This is because of its ability to create a corrosion-resistant oxide layer on the surface of the implant that confers osseointegration potential.^6,⁷ Because the implant may be worn for several decades or longer, the toxicity and carcinogenicity of the oxide coating takes on particular importance.⁸ Reports have shown that titanium is superior to stainless steel, lacking steel’s high potential for corrosion or the toxicity of its components.^9,¹⁰ To date, pure titanium appears free of the adverse sequelae seen with other metals and thus continues to represent an ideal implant material.⁷

Currently, the most widely used osseointegrated hearing device is the Baha implant (Cochlear Corporation, Englewood, CO), shown in Fig. 49-2. 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.

FIGURE 49-2 Baha osseointegrated implant. It consists of three parts: the sound processor, the connecting abutment, and the titanium implant.

(Courtesy of Cochlear Corporation, Englewood, CO.)

Indications

Baha implants were first developed for patients with conductive hearing loss and chronically draining ears, those with discomfort from the sound levels required from a traditional hearing aid, and patients unable to tolerate a hearing aid because of a large mastoid bowl or meatoplasty following chronic ear surgery. Patients with aural atresia are also candidates for Baha implants. Patients who have undergone external auditory canal closure following extensive skull base surgery with preserved inner ear function are also not amenable to traditional hearing aids. This group of patients may benefit from a device that offers bone-anchored hearing.¹¹ Baha implants are also approved for patients with single-sided 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. 49-3).

FIGURE 49-3 The osseointegrated implant can be used for single-sided deafness, because it relies on bone conduction, which stimulates both cochleae simultaneously. As sound comes in from the deaf side, it is picked up by the sound processor, which activates the contralateral cochlea by bone conduction, achieving sound transmission.

(Courtesy of Cochlear Corporation, Englewood, CO.)

Surgical Technique

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. A dermatome is used to raise an anteriorly based skin flap. The underlying and surrounding soft tissues are removed, leaving the periosteum intact. A small circle of the periosteum (about 6 mm²) 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. 49-4). The skin flap is reflected back to its original position, with a central opening to accommodate the abutment. The edges of the skin flap are then sutured to the surrounding periosteum and skin. 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.

FIGURE 49-4 Surgical placement of the Baha implant. The connecting abutment and the titanium implant are placed securely into a previously drilled hole in the skull.

(Courtesy of Cochlear Corporation, Englewood, CO.)

Complications

In a study looking at postoperative complications of Baha implant in 149 patients, House and Kutz ¹² found that the most common complication is skin growth over the abutment (7.4%). This is managed by either steroid application or skin flap revision with removal of the underlying scar tissues. In some cases, a split-thickness skin graft may be needed. Other complications include implant extrusion (3.4%), wound infection (1.3%), and flap necrosis (0.7%). Implant extrusion is more likely to occur in radiated bone and in the pediatric population. Pretreatment with hyperbaric oxygen is suggested in radiated patients. The delay to use is extended in this population and in children by 2 to 3 months to allow for optimal osseointegration.

Outcomes

In one of the largest series to date studying the Baha implant, Håkansson et al.¹³ reported results from 147 patients over 10 years. Patients were divided into three groups based on their pure tone average (PTA) bone-conduction thresholds: 0 to 45 dB, 46 to 60 dB, and more than 60 dB hearing level. The authors noted a strong relationship between PTA and successful rehabilitation. In the group with the best cochlear reserve (PTA of up to 45 dB), 89% of patients said their hearing was subjectively improved by the implant, while 8% felt their hearing was worse. Conversely, in the groups with progressively less cochlear function (the 46 to 60 dB and the more than 60 dB groups), 61% and 22% of patients reported subjective hearing improvement, respectively. Furthermore, SDSs improved on average from 14% unaided and 67% with a traditional hearing aid to 81% with the Baha. This number increased to 85% if people with a sensorineural loss greater than the 60 dB hearing level were excluded and to 89% if subjects with a PTA worse than 45 dB were excluded. Based on these results, the authors recommended that to be in consideration for a “high success rate” with the Baha, patients should have a PTA by bone conduction that is less than the 45 dB hearing level, though improvements in hearing should still be expected for a PTA of up to 60 dB.

Lustig et al.¹⁴ performed a review of experience with the Baha in the United States. The most common indications for implantation included chronic otitis media and external auditory canal stenosis and/or aural atresia. Patients who had undergone skull base surgery and had complete closure of the external auditory canal were also included. Overall, each patient had an average improvement of 32 ± 19 dB with the use of the Baha. Complications were limited to local infection and inflammation at the implant site in 3 of 40 patients and failure to osseointegrate in 1 patient. Patient response to the implant was uniformly satisfactory.

In addition to implantation for purely conductive or mixed hearing losses, emerging data indicate the value of Baha amplification for patients with unilateral profound sensorineural hearing loss. The Baha on the deafened ear effectively expanded the sound field for the patient and improved the patient’s speech understanding in noise, much like a CROS hearing aid or transcranial CROS system.^15,¹⁶ However, in contrast to CROS, the Baha does not require the placement of an ear mold in the better-hearing ear. As the better-hearing ear functions normally, the acoustic “head shadow” can be used to isolate sounds incident to the deafened side but heard in the better ear through transcranial bone conduction from the Baha. This avoids the potential discomfort and perceptual costs of wearing an ear mold on the better-hearing ear. Preliminary results show subjective improvement in both sound quality and speech understanding in noise.¹⁷