Surgical Placement of Bone-Anchored Hearing Systems

Bone-anchored hearing system

Several bone conduction systems are now available. The first BCI system developed was Baha. Baha uses a percutaneous osseointegrated fixture to create a method for direct transmission of vibration to the skull using a bone conduction hearing device. Baha was introduced by Tjellstrom et al, who established the first 3 patients in 1977. ^[1] The US Food and Drug Administration (FDA) approved use of the Baha for conductive and mixed hearing loss in 1996 and for single-sided deafness in 2002. More recently, BCI systems that use implanted osseointegrated magnets and the first BCI system using an implanted bone conduction transducer have been developed.

An image depicting bone-anchored hearing systems can be seen below.

Osseointegration

The concept of osseointegration was discovered and developed in Gothenburg, Sweden, by Professor P.I. Branemark, who recognized the potential for growth of bone tissue in contact with the surface of a titanium implant. Branemark defined osseointegration as a direct structural and functional connection between ordered living bone and the surface of a load-carrying implant.

Most materials fail to osseointegrate, and, instead, a foreign body reaction leads to formation of a fibrous capsule around the material. ^[2] Titanium has proven to be the material of choice for osseointegration; the use of titanium implants in the field of dental implantation, first introduced in 1965, has exploded worldwide. ^[3] Osseointegration is reliably achieved in BCI systems using commercially pure titanium (99.75 %), which is machined, then covered with an extremely biocompatible thin oxide layer.

Osseointegration is dynamic process that develops gradually over 6-12 weeks following fixture implantation. Many factors influence successful osseointegration, including the material, the macrostructure and microstructure of the implant, the quality of bone at the site of implantation, and surgical factors. ^[4] The implant must remain completely immobile during the initial period of osseointegration. This is critical; otherwise, osseointegration fails, with the formation of a fibrous capsule around the implant instead of new bone formation. The initial stability of the implant is mechanically achieved via the use of a screw-shape implant that is secured to bone with precise torque parameters.

Bone conduction hearing

Bone conduction hearing is unique in that it can produce clear sound perception regardless of outer and middle ear function, as long as inner ear function (cochlea) is intact. ^[5] Several factors contribute to bone conduction hearing, including the sound pressure within the external ear canal, the middle ear and middle ear ossicle motion, and cochlear fluid movement.

The ear canal and walls of the middle ear contribute to bone conduction hearing via skull vibration, which produces radiated sound in the ear canal and middle ear. However, this effect is small because sound pressure found in the open external auditory canal is 10 dB less than bone conduction threshold levels. During skull vibration, inertia of the middle ear ossicles produces motion of ossicles relative to the skull vibration. This effect contributes primarily to the low-frequency and mid-frequency bone conduction hearing. The inertial effects of cochlear fluid relative to vibrating bone is the most important contributor to bone conduction hearing, and this effect produces basilar membrane motion that is the same as air-conduction hearing.

Transcranial attenuation of bone conduction hearing refers to the decrement in sound energy that occurs when one side of the skull is stimulated and hearing thresholds in the cochlea on the opposite side are measured. This is chiefly relevant when using the BCI to route sound from the deaf side to the hearing ear in people with unilateral sensorineural hearing loss. Transcranial attenuation is frequency dependent; it is lowest for low-frequency vibrations and highest for high-frequency vibrations. Overall, subjective attenuation measured in humans is about 10 dB in the range of 0.25-4 kHz. ^[6]

Conductive and mixed conductive/sensory hearing loss

A bone conduction implant (BCI) is used to treat 2 basic problems: (1) conductive and/or mixed hearing loss and (2) deafness in one ear (single-sided deafness). These devices are considered when use of a conventional (air-conduction) hearing aid is not possible. For the case of conductive or mixed hearing loss, they are used most commonly in patients with chronic ear infections, cholesteatoma, and chronic otorrhea in which the diseased eardrum and/or middle ear ossicles are not able to conduct sound to the cochlea and use of a conventional hearing device often is not possible. The other common situation is congenital aural atresia in which absence of the ear canal and eardrum causes conductive hearing loss and a conventional hearing aid cannot be used.

Before BCIs, the only device available to treat these situations was a conventional bone conduction hearing aid. This device consists of a bone conduction hearing aid (vibrator) attached to a headband (see the image below). These devices, while very helpful over the years, have several inherent disadvantages that limit their benefit and their acceptance, including the following:

• Discomfort caused by the constant pressure of the vibrator against the scalp

• Poor sound quality and volume caused by the indirect and variable coupling of the vibrator to the skull due to intervening hair and scalp tissues

• Variable and unstable positioning affecting the quality of transduction

• Bilateral use is not possible

• Poor aesthetics ^[7]

  Bone-anchored hearing systems. Conventional bone-conducting hearing aid.

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Frequency

BCIs have been used in both adults and children for more than 30 years. ^{[8, 9, 10]} According to manufacturer information, more than 45,000 patients worldwide are fitted with one today.

Incidence of conductive hearing loss

According to the National Institute on Deafness and Other Communication Disorders (NIDCD), hearing loss affects approximately 28 million Americans and approximately 17 in 1000 children and adolescents younger than 18 years. About 6 or 7 of every 1000 children in the United States are born with mild-to-moderate hearing loss. Some of these patients (true incidence unknown) have unilateral severe-to-profound sensorineural hearing loss or conductive hearing loss that is not correctable with surgery or aided with traditional hearing aids; these patients could be candidates for a BCI. For example, the incidence of aural atresia is estimated to be 1 in 10,000 births. In about one quarter of the cases, atresia is bilateral. ^[11]

Incidence of single-side deafness

The incidence of single-side deafness in adults is not known for certain. However, an estimate of 23 cases per 100,000 population can be established by looking at the incidence of the 3 most common causes of single-sided deafness in adults, as follows:

• Vestibular schwannoma accounts for 1 in 100,000 cases.

• Ménière disease causing profound hearing loss accounts for 10 in 100,000 cases.

• Sudden sensorineural hearing loss resulting in profound hearing loss accounts for 12 in 100,000 cases.

Conductive hearing loss

Ear canal problems are as follows:

• Chronic eczema

• Recurrent infection of the ear canal

• Congenital aural atresia (syndromic and nonsyndromic)

• Acquired stenosis or surgical closure of the external auditory canal (ie, trauma or temporal bone tumors; ie, glomus jugulare)

Tympanic membrane problems are as follows:

• Chronic tympanic membrane perforation due to otitis or cholesteatoma

• Severe tympanic membrane atelectasis

Middle ear ossicle problems are as follows:

• Syndromic congenital ear malformation (ie, hemifacial microsomia, Goldenhar syndrome, Treacher Collins syndrome, Pierre Robin syndrome, Branchio-oto-renal syndrome, Down syndrome)

• Cholesteatoma causing incus/stapes necrosis

• Otosclerosis (primary, revisions, only hearing ear)

• Trauma causing ossicular dislocation

Mixed hearing loss

This type has the same etiologies described for conductive hearing loss, but this condition generally involves older patients in whom some degree of sensorineural hearing is present as well.

Single-sided deafness

Causes of this type are as follows:

• Eighth nerve tumors (ie, acoustic neuroma)

• Idiopathic sudden unilateral sensorineural hearing loss

• Sensorineural hearing loss secondary to trauma or surgery of the middle ear

• Ménière disease

• Congenital unilateral sensorineural hearing loss

Conductive hearing loss

Conductive hearing loss results from abnormalities of the external ear canal, tympanic membrane, middle ear, or ossicle that reduce the effective intensity of the air-conducted signal reaching the cochlea. Examples of abnormalities include occlusion of the external auditory canal by cerumen, infection, a mass, or atresia; middle ear infection and/or fluid; perforation of the tympanic membrane; or ossicular abnormalities. By definition, the threshold for conductive hearing loss is pure-tone air-conduction thresholds poorer than bone conduction thresholds by more than 10 dB. The maximum threshold in conductive hearing loss is 60 dB.

Mixed hearing loss

Bone conduction is far less effective than air conduction for amplifying sound in patients with purely sensorineural hearing loss. However, in patients with sensorineural hearing loss and conductive hearing loss (mixed hearing loss), air-conduction hearing aids lose effectiveness because in contrast with a BCI, an air-conduction hearing aid has to compensate for the conductive hearing loss. This requires larger amounts of sound energy, pushing the limits of amplification and output levels of an air-conduction hearing aid (owing to increased susceptibility to feedback and saturation of the amplifier).

Accordingly, as the width of the air-bone gap increases, patients’ performance with the air-conduction hearing aid gradually approaches that with the Baha. At a certain point, a break-even point occurs at an air-bone gap of 25-30 dB. ^[12] Thus, in a patient with mixed hearing loss in whom the air-bone gap exceeds 30 dB, a BCI system has the potential for better performance than an air-conduction device.

Single-sided deafness

Current therapeutic strategies for treating single-sided deafness are based on the use of specialized hearing instruments, as such frequency-modulated (FM) systems, or semi-implantable devices. The common purpose of all these devices is to reconstruct some amount of binaural hearing with one ear, ie, by providing the healthy ear with acoustical information transferred from the deaf side. The first systems based on this general idea were labeled contralateral routing of offside signal hearing aids or contralateral routing of signals (CROS).

In these systems, a microphone placed on the deaf side transfers acoustical information by wired electrical transduction to a receiver placed on the contralateral ear. Current CROS systems are still based on the same theoretical approach but use wireless technologies, such as FM transduction, that significantly improve wearing comfort and aesthetics by eliminating visible wires connecting the 2 required hearing devices.

An alternative approach to transferring the signal electrically from one ear to the other is use of transcranial bone conduction, a principle sometimes also referred to as transcranial-CROS. This strategy is based on fitting the deaf ear with a highly powerful hearing instrument. The output of the hearing aid increases to such a level (100-120 dB sound pressure level) that the resulting intense vibrations of the receiver may be conveyed via bone conduction, along the cranial bone and ultimately encoded by the contralateral cochlea.

Therefore, in contrast to the standard CROS system approach based on electrical transmission of sound and final air-conduction hearing, transcranial-CROS systems use bone conduction for the ear-to-ear transmission. Although both have proven efficient in some cases, patients often reject these systems for cosmetic (presence of wires or large and visible behind-the-ear housings), acoustic (noise caused by transcranial-CROS, absence of powerful-enough devices or efficient enough digital feedback cancellers), or efficiency reasons; the benefits provided by either system often remaining limited.

Bone conduction devices effectively provide transcranial transmission of sound via bone conduction, thereby overcoming the limitations of conventional CROS aids and transcranial CROS. The chief benefit in single-sided deafness is providing the ability to hear speech spoken into the deaf ear by elimination of the head-shadow effect (speech sounds arising from the deaf side are blocked from reaching the hearing ear by the head).

In addition, transcranial stimulation improves speech intelligibility in noise. This effect is most pronounced when noise is presenting to the hearing ear and the speech signal of interest is directed toward the deaf ear. Because normal sound localization requires two hearing ears, the transcranial stimulation in single-sided deafness is not expected to provide normal sound localization.

Despite the fundamental limitation of hearing only through one ear, several investigators have noted improvement in BCI users in binaural hearing performance, especially in sonorous localization and intelligibility in a noisy environment (see more discussion in the Future and Controversies section). ^{[13, 14, 15, 16]}

Description and characteristics of the bone conduction implants (BCIs)

Transcutaneous semi-implantable osseointegrated hearing systems

Transcutaneous semi-implantable osseointegrated hearing systems include the Baha, manufactured by Cochlear Ltd, and the Ponto, manufactured by Oticon Medical. The transmission of sound to bone is accomplished via an osseointegrated titanium fixture surgically implanted in the temporal bone. These systems offer the following advantages:

• Elimination of discomfort related to the pressure of the vibrator used in a conventional bone conduction hearing aid

• Improved sound quality and audibility of sounds because of the direct bond between the sound processor and the temporal bone

• Precise and stable positioning, supporting the quality of transduction

• More aesthetic device with digital signal processing and wireless connectivity

Surgical kit

The titanium implant, also called the fixture, is available in 3- and 4-mm sizes. Most surgeons use a 4-mm fixture whenever the thickness of the skull bone permits. ^[17]

The percutaneous abutment (passing through the scalp) is attached to the fixture either from the start (1 stage) or secondarily (2 stage). The abutment length is variable from 6-12 mm. Choice of abutment length depends on the type of surgical technique used and clinical characteristics of the patient. There is a strong trend toward using surgical techniques that preserve the subcutaneous tissue around the abutment. Instead of thinning the scalp tissue to accommodate a fixed length abutment, the abutment can be sized to the thickness of the scalp. ^[18]

Several models of sound processors are available, and their use depends on the patient’s average bone conduction thresholds (ie, the patient's current sensory hearing ability).

The Baha BP400 (Cochlear Ltd) sound processor uses digital processing of the acoustic signal and includes a directional microphone, adaptive signal processing, and integrated wireless technology. It is effective up to 45-dB average bone conduction threshold. The Baha BP310 (Cochlear Ltd) adds 10 dB more power output than the Baha BP400 and is effective up to 55-dB average bone conduction threshold. The Baha Cordelle II (Cochlear Ltd) is a body-worn unit with 13 dB more output than the BP400 and is effective up to 60 dB average bone conduction threshold.

The Ponto Pro Plus (Oticon Medical) sound processor has a new, more efficient transducer, is digital and programmable, and includes automatic adaptive directionality and new feedback and noise management strategies. It also has new wireless capabilities and is available in regular and power versions. The Ponto Pro Power (Oticon Medical) can handle conductive and mixed hearing loss up to 55-dB average bone conduction thresholds. It is also effective for patients with single-sided deafness with sensory hearing in the hearing ear better than 20-dB average bone conduction thresholds.

Osseointegrated magnet-based BCIs

Osseointegrated magnet-based BCIs are now available that eliminate the need for a percutaneous abutment. Instead, a magnet is implanted under the skin and the sound processor is attached to a second external magnet. The two magnets are attracted to each other, permitting transmission of sound via vibration to the cochlea. These systems offer the following advantages and disadvantages:

• Elimination of the abutment greater reduces the incidence of soft tissue reactions and complications

• Elimination of the percutaneous abutment makes the device more acceptable

• Performance of the magnet devices on the average is 5-10 dB poorer than the direct-connect systems

• Skin complications can still occur if the strength of the magnets used is overly strong

• Retention of the processor in a magnet system is not as reliable as in a direct-connect percutaneous system

The two magnet-based systems that are currently FDA approved and available are the Sophono Alpha 2 and the Cochlear Baha Attract (Cochlear Ltd). Soft tissue thickness is a key consideration in the performance of a transcutaneous magnet device, as there is frequency-dependent attenuation with increasing soft tissue thickness in the range of 10-20 dB. ^[19]  Data describing patient outcomes suggest good patient satisfaction and good hearing performance when osseointegrated magnet-based systems are used in audiologically appropriate patients. ^[20] .  

Active BCI systems

Active BCI systems involve an implanted transducer combined with a transcutaneous radiofrequency audio processor. Currently, the only device in this category is Bonebridge (Med El Corp). ^{[21, 22]} This device has CE Mark for use in Europe and was approved for use in the United States in July 2018 for patients aged 5 years and older. Bonebridge works similarly to a cochlear implant, with a magnet serving to secure the processor but not as a medium for sound transduction to the skull (as occurs with the magnet-based BCIs). The processed signals are transmitted via radiofrequency to the implanted transducer, which decodes the signals and causes the device implanted within the mastoid bone to vibrate, conducting sound to the cochlea. This type of system offers the following advantages:

• Elimination of the abutment greatly reduces the incidence of soft tissue reactions and complications

• Having intact skin is more acceptable to patients than having a percutaneous abutment

• Performance of implanted transducers can be equivalent to the direct-connect percutaneous system

• Radiofrequency-based audio processors are a proven technology used for many years in the cochlear implant industry

An initial study by Sprinzl et al of 12 patients fitted with the Bonebridge device reported significant audiologic improvements, from 14% word recognition scores (WRS) and 62 dB speech reception threshold (SRT), preoperatively, to 83% WRS and 42 dB SRT at 1 month postoperatively, with continued improvement to 93% WRS and 37 dB SRT at 3 months postoperatively. ^[23] A study by Riss et al of 24 patients fitted with the Bonebridge device reported satisfactory functional gain and speech perception outcomes associated with the implant if preoperative bone-conduction hearing-loss thresholds were no higher than 45 dB. ^[24]

A retrospective study by Rader et al found that patients with conductive or mixed hearing loss who received an active transcutaneous bone conduction implant (BCI) demonstrated long-term improvement in speech intelligibility. The mean word recognition score with Freiburg monosyllables was 79% at 6 months postoperatively (short-term) and 75% between 6 and 37 months postoperatively (long-term), compared with 25% for unaided hearing. In addition, short-term evaluation revealed an improvement in the speech reception threshold in noise of 3.6 db signal-to-noise ratio in the implanted ear. ^[25]  Long-term (>1 year) efficacy was also shown by Schmerber et al to be maintained in 16 implanted patients; in addition, no adverse effects, skin or otherwise, were found in this study. ^[26]

A systematic review by Sprinzl and Wolf-Magele supported the efficacy of the Bonebridge device in conductive or mixed hearing loss and single-sided deafness. Improved hearing thresholds and speech recognition were reported in adults and children, with, based on studies evaluating the safety of the device, a 5% rate of minor adverse events found (6 out of 117 patients); only one patient required surgery (superficial revision for recurrent infection). ^[27]  As with other BCIs, Bonebridge was found to be less effective overall in rehabilitation for single-sided deafness than for mixed and conductive loss. ^[26]

A study by Irmer et al indicated that patient quality of life is improved by implantation of the Bonebridge device. Patient responses to the Assessment of Quality of Life 8 dimensions (AqoL-8D) questionnaire, given an average of 40 months after implantation, pointed to an increase in the perceived quality of life (overall utility score mean = 0.76 out of 1). The device’s impact on social life was also rated as positive, according to the Audio Processor Satisfaction Questionnaire (APSQ), the average social life subsection score being 4.17 out of 5. ^[28]

Overall, the Bonebridge device, utilizing direct-drive bone vibration, seems to be associated with audiologic gains similar to those occurring with the percutaneous BCI systems, while incorporating the advantages associated with the transcutaneous systems regarding lack of skin issues and infections. 

Adhesive BCI systems

The ADHEAR device was approved for use in the United States in April 2018. The system is not surgically implanted, representing an intermediate technology between traditional bone conduction hearing aids and the surgically implanted devices. An adhesive adapter is stuck to the non–hair-bearing skin of the postauricular area. As in other BCIs, the audio processor, which may be directly connected to this adapter, converts sound received by the microphone into vibrations. These vibrations, transmitted by the adhesive adapter to the skull, are perceived as sound by the wearer. The adhesive adapter is single use and water-resistant and may be changed every 3-7 days. ADHEAR offers the following advantages and disadvantages:

• Avoidance of undergoing a surgical procedure
• Elimination of skin complications associated with percutaneous devices, as well as any pressure-related complications associated with transcutaneous magnet devices
• Transducing vibrations through skin may not amplify high-frequency sounds as much as traditional BCIs do
• May be used in patients who are not candidates for a traditional BCI (very young children, patients who refuse surgery)

Dahm et al tested 12 patients suffering from conductive hearing loss with the ADHEAR device. They found a mean improvement in aided threshold from 45.1 dB hearing loss to 30.8 dB hearing loss, compared with 47.5 dB to 28.2 dB with a conventional softband bone-conduction hearing aid. They also noted improvement in SRT from 56.8 dB sound pressure level (SPL) to 44.5 dB SPL, and improvement in the word recognition score (WRS) from 29% to 59% (at 65 dB SPL) and 40% to 68% (at 70 dB SPL) for the ADHEAR device. These improvements were most noticeable up to 2 kHz, with lower average gain at 4 kHz and 6 kHz. ^[29]

Mertens et al tested the ADHEAR device in 17 patients suffering from single-sided deafness, with a CROS hearing aid used as a control. They found a 5˚ improvement in sound localization but no improvement in speech perception in noise. The study questionnaires showed that 71% of participants left the adhesive on for 7 days or longer before changing it; only 12% noted the adhesive falling off during normal use, although 24% of patients experienced at least some skin irritation. Overall, 47% of patients found the device partially useful, while another 24% found it useful or very useful. ^[30]

Conductive and mixed hearing loss

Audiologic criteria

A BCI is indicated for patients presenting an average hearing impairment in bone conduction better than or equal to 45 dB hearing loss (on the 0.5, 1, 2, 3 kHz) for BP300 or Pronto Pro sound processors, 55 dB hearing loss for the BP310 and the Ponto Pro Power, and 60 dB hearing loss for the Cordelle sound processor.

Medical criteria

A BCI is indicated in the event of conductive or mixed hearing loss when use of a conventional air-conduction hearing aid is impossible, contraindicated, or ineffective. The most common situations include the following:

• Chronic otitis media with chronic otorrhea ^{[12, 31, 32, 12, 33]}

• Congenital ear malformation or atresia ^[34]

• Chronic external otitis preventing use of conventional equipment ^{[31, 32]}

• Discomfort using conventional equipment

• Air-conduction hearing aid ineffective owing to large conductive hearing loss (inadequate gain, uncomfortable occlusion, and feedback effects)

BCIs for single-side deafness

Audiologic criteria

The hearing ear should have 20 dB air conduction.

Medical criteria

The BCI is for patients who cannot or will not use conventional hearing aids or CROS hearing aids.

The audiometric contraindications are relative in most cases. To guarantee good functional results, for conductive and mixed hearing loss, the average loss in bone conduction must be lower or equal to 45 dB for the Baha BP400 or Pronto Pro Plus, to 55 dB for the BP310 and Ponto Pro Power, and to 60 dB for the body-worn Baha (Cordelle).

The minimum age of establishment must be taken into account. In very young children, the thinness of the temporal bone can be the cause of higher failure rates and 2-stage surgery should be used. When cranial bone is found to be less than 4 mm, an implant that is longer (ie, a 4-mm implant) than the bone thickness (ie, 2-3 mm) can be used with the implant screwed in as far as the dura. This leaves a portion of the implant protruding (ie, "proud") from the cranial bone surface that should then be covered with periosteum. During the staging interval, new bone growth osseointegrates the entire fixture.

Any major defect of hygiene is a contraindication because of increased risks of local infection. Patients should be able to follow the instructions given and to take part in regular follow-up. However, this contraindication is relative; a recent study on a group of patients presenting with important cognitive deficits (trisomy 21) showed a high rate of skin complications in which treatment was easy and fast; and, despite these problems, patients and their families expressed high satisfaction. ^[35] In another study, patients received significant benefit—both documented audiologically and subjectively in activities of daily living—and the complication rate was low. ^[36]