Bone-Anchored Hearing Aids 

Bone-anchored hearing devices allow patients with conductive hearing loss, mixed hearing loss, or unilateral profound sensorineural hearing loss (single-sided deafness) to achieve improved auditory acuity by transmitting the sound directly through the bone into the inner ears.

Currently, 3 devices are available that use bone conduction as a means of sound perception. Of these, 2 have been approved by the US Food and Drug Administration (FDA): the Baha, by Cochlear Corp, and the Ponto, by Oticon Medical. The Sophono Alpha 1(S), which is a bone conduction device used with a soft headband without an implant, is also approved by the FDA, but its implantable counterpart, the Alpha 1(M), is still awaiting FDA approval.

The Sonitus Soundbite is another device that uses teeth to conduct the sound to the inner ear. It has been approved by the FDA for single-sided deafness as well. The Soundbite will not be discussed in this article, however, because it is not a surgically bone-anchored hearing device.

Category

Hearing aid, bone-anchored

Device Details

Currently available bone-anchored hearing aids include the following:

• Cochlear Corporation - Baha (including Baha 3 AKA BP100, Intenso, Baha 3 Power, and Cordelle II; see the image below)Components of Cochlear Baha system. Used with permission from Cochlear Corp.
• Oticon Medical AB - Ponto Pro
• Sophono, Inc - Alpha 1(S) and 1(M) (see the images below)Sophono external processor, which couples magnetically with internal component. Used with permission from Sophono, Inc. Sophono internal implant. Used with permission from Sophono, Inc.

The Cochlear Corporation's Baha system uses a titanium post that is implanted through the skin into the skull. After the implant has become integrated into the bone, a hearing processor is attached to this post, transmitting the sound directly through the bone into the inner ear and bypassing the middle ear (see the image below). Cochlear has introduced a newer implant, the BI300, which has a wider diameter than the original one (4.5 mm vs 3.75 mm) and is designed to improve stability and enhance the integration of the implant into the skull.

There are 3 Cochlear sound processors on the market, all of which use the same implant. The Baha 3 (BP100) uses 12 channels with a frequency range of 250-7000 Hz. Its gain at 1600 Hz is 34 dB, and its peak output is 126 dB. The Intenso has a frequency range of 250-7000 Hz. Its gain at 1600 Hz is 45 dB, and its peak output is 124 dB.^{[1, 2]}

Cochlear has received approval from the US Food and Drug Administration (FDA) for the Baha 3 Power sound processor, which builds on the design of the BP100. This device is meant to replace the Intenso processor, which is used for patients with higher degree of hearing loss. The Baha Cordelle system consists of an ear-level vibration transducer that is connected to a body-worn microphone and amplifier.^[3]

Like Cochlear's Baha, Oticon Medical’s Ponto Pro uses a surgically inserted implant with an attached abutment that connects the sound processor to the skull. It has 10 channels with a frequency range of 125-8000 Hz and a maximum gain of 33 dB. The implant and abutment systems used by the Ponto Pro and Baha systems are interchangeable; for example, a Cochlear sound processor can be fitted on an Oticon brand titanium implant and abutment, and other mixtures are also possible.^[4]

The Sophono Alpha 1 operates 8 channels at 16 frequency bands with a frequency range of 280-5400 Hz. Its gain at 1600 Hz is 29 dB, its maximum gain is 38 dB, and its peak output is 115 dB. With a single 1.3 V battery, the Alpha 1 can be used for more than 200 hours.^[5]

The Alpha 1(M) differs from other hearing systems in that it uses a fully implantable internal magnet with no implant protrusion from the skin and no use of an abutment. An external magnet connects with the internal magnet through the skin and allows transmission of data from the sound processor to the implant.

The Alpha 1(S) functions without an implant and uses a soft headband to attach the processor to the scalp. This model is typically used in adults who have not committed to implant surgery or in children younger than 5 years. Soft headbands are available for the Baha and Ponto systems as well.

Bone-anchored hearing devices are indicated in patients who have been diagnosed with conductive or mixed hearing loss or in patients with single-sided deafness. The appropriate processor is selected on the basis of the patient's hearing level.

For patients with conductive or mixed hearing loss, the Cochlear Baha 3 (BP100), Oticon Ponto Pro, and Sophono Alpha 1 processors are indicated when the average bone conduction hearing impairment is greater than or equal to 45 dB. The Cochlear Intenso processor, which will be replaced by the Baha 3 Power, is indicated for patients with impairment greater than or equal to 55 dB. The Cochlear Cordelle II processor is indicated in patients with impairment of 60 dB or more.

In patients with single-sided deafness, the Cochlear Baha , Oticon Ponto Pro, and Sophono Alpha 1 sound processors are indicated when the hearing ear has an average bone conduction threshold of 20 dB or greater.

The majority of research on bone-anchored hearing devices in the literature involves the Cochlear Baha system, probably because this system has been on the market longer. The newer devices produced by Oticon and Sophono have not yet been evaluated extensively in the literature.

An early study with the Cochlear Baha system in patients with conductive or mixed hearing loss demonstrated that aided sound thresholds in the high-frequency region were better than thresholds obtained with traditional air-conduction hearing aids.^[6] Improvement in hearing thresholds at the high-frequency levels has been shown to be important for better speech recognition, especially in loud situations. In addition, it was found that patients had a better speech-in-noise (S/N) ratio with the Baha than they did with their air-conduction hearing aids.

When compared with patients who had a Baha device implanted unilaterally, patients with bilaterally implanted Baha devices fared significantly better with respect to sound localization at various frequencies, speech recognition in quiet and noisy settings, and binaural masking level difference measurements.^[7]

The Baha system has been approved for the treatment of single-sided deafness in patients with normal hearing in the contralateral ear for nearly a decade. Although it has not been shown to improve sound localization in such patients, it significantly improves both speech perception, as measured by the speech reception threshold (SRT), and speech recognition in noise, as measured by the hearing-in-noise test (HINT)^{[8, 9]} ; the latter is improved even in patients with mild to moderate hearing loss in the nondeaf ear.^[10]

Christensen et al conducted a study of teenagers who were fitted with the Divino Baha system for treatment of single-sided deafness. Results demonstrated that there was a significant improvement in acoustic acuity in both quiet and noisy situations, as measured by the HINT. Additionally, both the patients and their parents related satisfaction with the device’s performance.^[11]

Contralateral routing of signals (CROS) is one of the original treatment options for patients with single-sided deafness. Although it has the obvious benefit of not requiring a surgical procedure, it does require the patient to wear either a behind-the-ear or an in-the-ear hearing aid on the good ear in addition to the microphone/transmitter on the deaf ear.

Compared with CROS, the Baha performed significantly better on HINT testing, as well as scored higher in patient satisfaction. Additionally, patients described a greater degree of perceived benefit with the Baha than with CROS, as measured by the Abbreviated Profile Hearing Aid Benefit (APHAB).^{[12, 13]}

With regard to the pediatric population that uses the Baha Softband system, Verhagen et al reported that in children with congenital bilateral aural atresia who are too young for the implantable system, the Baha Softband achieved nearly the same level of hearing thresholds as the implantable system.^[14] Additionally, the speech development of these children is on par with that of well-hearing children.^[14]

Adaptive directionality is one of the unique features of the Oticon Ponto Pro device. Compared with a traditional bone-anchored hearing system in which the microphone position is fixed, the Ponto Pro device works by determining the direction from sound is most dominant and automatically adjusting the angle at which the microphone picks up sound.

At present, there is no published clinical research on the Oticon Ponto system. However, in data presented at the annual meeting of the Danish Technical Audiological Society, speech recognition thresholds in patients fitted with the Ponto Pro sound processor with adaptive directionality were higher than in patients fitted with bone-anchored hearing devices with fixed directionality.^[15]

Research from Germany on the use of the Sophono Alpha 1 demonstrated that the average patient who received the Alpha 1 system experienced a 38 ± 8 dB increase in sound field thresholds. Word recognition scores improved nearly 75% in the same group of patients. The holding strength of the magnets used to hold the sound processor to the scalp is equivalent to that of the traditional implant with attached abutment. In addition, patient comfort is significantly greater with the Alpha 1 than with conventional bone-anchored hearing aids (see the image below).^[16]

Surgical implantation of the traditional bone-anchored hearing aid (Cochlear and Oticon brands) is described in depth in Surgical Placement of Bone-Anchored Hearing Systems. Implantation of the Sophono Alpha 1(M) is slightly different in that the implant is placed completely under the skin, with no protruding fixture.

From a surgical standpoint, adequate skin care and proper hygiene of the abutment are critical in patients who receive the traditional implant plus abutment. Daily cleansing of the surgical site and surveillance for skin changes around the abutment are required. For patients in whom the Sophono Alpha 1(M) has been implanted, no additional skin care is required after the surgical incision has healed, though the strength of the magnet may have to be modified if the skin shows signs of inflammation or breakdown.

The processor is adjusted to patient’s level of hearing, much as in traditional hearing aid fitting.

Surgical complications are well described in Surgical Placement of Bone-Anchored Hearing Systems. Briefly, the majority of the surgical complications are related to skin issues surrounding the implant. These issues include formation of granulation tissue around the abutment, as well as possible overgrowth of the abutment by new skin. Another complication is extrusion of the implant, which is a result of failure of osteointegration of the fixture into the temporal bone.