Implantable Hearing Aids

10.1

Bone Conduction Mechanisms

One of the most important, and likely the most important, finding in bone conduction physiology research was when in 1932 reported the cancellation of the perception of a bone conducted (BC) tone by an air conducted (AC) tone. … The perception of a 400 Hz tone, 57 dB above threshold, delivered to both ears by a BC transducer, could be cancelled by careful adjustment of the amplitude and phase of the AC stimuli produced by binaural earphones. … The accomplishment of canceling a BC tone by an AC tone of the same frequency lead von Békésy to conclude that, although the transmission to the inner ear is different, the final processes for AC and BC stimulation are the same.

aiming to further test the findings of von Békésy, carried out loudness matching at each frequency and at 30–80 dB hearing level (HL). They fixed the sound pressure from the earphones and the subject adjusted the output level of the bone transducer for equal loudness. They found a non-unity relation between the different loudness functions for air-conducted (AC) and bone-conducted (BC) sound with slopes between 0.51 and 0.92. This resulted in a 6–10 dB difference in the AC and BC loudness functions for the normal hearing group at 250–750 Hz. At 1–4 kHz the difference was only 4–5 dB over the same dynamic range ( Fig. 10.1 ). Similar results were obtained for the sensorineural hearing-impaired group.

further investigated this discrepancy between air and bone conduction at low frequencies by using another loudness estimation method (adaptive categorical loudness scaling) in 20 normal hearing subjects. When the stimulation was by bone conduction, the loudness functions were steeper and the ratios between the slopes of the AC and BC loudness functions were 0.88 for the low-frequency sound and 0.92 for the high-frequency sound. These results were almost identical to those of using the equal loudness estimation procedure. Thus, the findings could not be attributed to the loudness estimation procedure. described the AC and BC paths as schematically illustrated in Fig. 10.2 . The transmission of sound for the AC path is by way of the outer ear canal to the middle ear via the ossicles, resulting in a motion of the stapes that gives a motion of the cochlear fluid. The AC sound pressure at the different parts of the ear induces some BC sound via pathways indicated in Fig. 10.2 . Due to the large impedance difference between air and bone this sound transfer is negligible for a normal hearing ear. The BC path has, as indicated in Fig. 10.2 , a rather complex form. The transducer is applied either directly to the bone or to the skin covering the bone. The vibrations at the temporal bone radiate sounds into the outer ear canal by way of relative jaw movements and sound radiation from the cartilage and soft tissues and may induce sounds in the middle ear cavity by compression of the cavity. More importantly, they also cause the relative motion of the middle ear ossicles and the compression of the cochlea together with the fluid inertia ( ).

For understanding the action of bone conduction devices, the dominant contributions to perception are the inertial component of the cochlear fluid and the compression of these fluids (highlighted in Fig. 10.2 ). Important here is the mobility of the cochlear windows. Compression of the cochlear shell occurs for wavelengths smaller than the size of the audiovestibular system. Compression of cochlear fluids requires one mobile cochlear window, whereas the inertial response requires two mobile windows. Because cochlear fluids are incompressible, the inertial component in bone conduction is dominant up to approximately 4 kHz ( ). In addition, based on a model study, concluded that: “Inner ear compression and middle ear inertia were within 10 dB for almost the entire frequency range of 0.1 to 10 kHz. Ear canal sound pressure gave some contribution at the low and high frequencies, but was around 15 dB below the total contribution at the mid frequencies. Intracranial sound pressure gave responses similar to the others at low frequencies, but decreased with frequency to a level of 55 dB below the total contribution at 10 kHz.”

As we have seen, the middle ear ossicles contribute to BC hearing primarily by the inertial forces acting on them when the skull is vibrating. This middle ear inertia is effective in the mid frequencies (1–3 kHz), i.e., the resonance frequencies of the middle ear. described that for a fixed stapes footplate, as in otosclerosis, the ossicular inertia is removed and a loss in BC sensitivity of approximately 20 dB around 2 kHz is normally seen, referred to as the Carhart notch. This depends on the mobility of the oval window, which is typically absent for stapes fixation ( ). No major alteration of the BC thresholds at the high and low frequencies is expected from otosclerosis with fixation of the stapes. Other lesions of the middle ear usually affect the BC thresholds only 10 dB or less. A conductive impairment in the middle ear affects the BC thresholds similarly whether the stimulation position is on the mastoid or on the forehead. noted that when a bone conductor is applied to the head: “the skull develops vibrations in all three planes and rotational motion. This complex motion of the skull is reflected in the motion of the cochlea; it also moves in all three dimensions in space without any dominating direction. The transcranial attenuation of BC vibration energy is on the average –5 to 10 dB. This indicates that during BC testing, masking of the non-test ear is always required if the hearing status of a single ear is tested. The skull has several resonances and anti resonances in the frequency range of hearing; the first appears around 0.8 to 0.9 kHz. The resonances are highly damped and do not affect hearing by BC, whereas the anti resonances can cause up to 20 dB attenuation for narrow bandwidths. The sensitivity of BC sound depends on the position of the transducer; the mastoid site is approximately 10 dB more sensitive than the forehead.”

10.2

Bone-Anchored Hearing Aids

A BAHA is an auditory prosthetic based on bone conduction, which can be surgically implanted. It is an option for patients without external ear canals and when conventional hearing aids (CHAs) with a mold in the ear cannot be used. BAHAs are especially useful in case of chronic otitis media ( www.snikimplants.nl ). The BAHA uses the skull as a pathway for sound to travel to the inner ear. For people with conductive hearing loss, the BAHA bypasses the external auditory canal and middle ear, stimulating the functioning cochlea. For people with unilateral complete deafness, often referred to as single-sided deafness (SSD), the BAHA is placed on the deaf side and uses the skull to conduct the sound from the deaf side to the side with the functioning cochlea. Individuals under the age of 2 (five in the United States) typically wear the BAHA device on a Softband. This can be worn from the age of 1 month as babies tend to tolerate this arrangement very well. When the child’s skull bone is sufficiently thick, a titanium percutaneous abutment can be surgically embedded into the skull with a small abutment exposed outside the skin. The BAHA sound processor sits on this abutment and transmits sound vibrations to the external abutment of the titanium implant ( Fig. 10.3 , left). The implant vibrates the skull and inner ear, which stimulate the nerve fibers of the inner ear, allowing hearing. Two companies manufacture BAHAs today—Cochlear and Oticon ( https://en.wikipedia.org/wiki/Hearing_aid ; accessed November 1, 2015).

Bone conduction implants (BCIs) come in two forms: percutaneous (BAHA) and transcutaneous. described these as “while the advantage of the transcutaneous approach such as that used with the Audiant™ is one of cosmetics (i.e., no abutment protruding through the skin), the improved transduction (by up to 20 dB) of a ‘hard wired’ percutaneous implant such as the BAHA™ appears to be a much more significant factor in the successful fitting of these patients.” Other early comparisons of the two types (BAHA vs a transcutaneous temporal bone stimulator) resulted in the BAHA as the better choice ( ).

10.2.1

General Performance

10.2.1.1

Single-Sided Deafness

BCIs are often used as a “Contralateral Routing of Signals” device, which improve directional hearing in single-sided deafness (SSD). examined sound localization in azimuth in patients with acquired severe unilateral conductive hearing loss. All patients were fitted with a BAHA to restore bilateral hearing. The patients were tested in the unaided (monaural) and aided (binaural) hearing condition. They found that the BAHA significantly improved sound localization in 8/12 of the unilateral hearing loss patients. collected data of six patients with SSD, seven with a mild to severe hearing loss at the BAHA side and (near-) normal hearing at the other side. They found that SSD patients listened mainly with their normal ear, although the BAHA lifted the head shadow effect. also observed that these patients even regained limited binaural sensitivity with the device. The six patients with severe bilateral hearing loss in this study listened predominantly with their BAHA and only regained limited directional hearing. also evaluated the sound localization capabilities of patients with unilateral, profound sensorineural hearing loss who had been implanted with either a bone-anchored hearing device (BAHA BP100) or a TransEar 380-HF bone conduction hearing device. They found that: “Neither the BP100 nor the TransEar device improved sound localization accuracy in patients with unilateral, profound sensorineural hearing loss compared with performance in the unaided condition.”

found that restoration of aural sensitivity in the deaf hemifield with an integrated bone conduction hearing aid enhances speech intelligibility under complex listening conditions after 3 months of unstructured use. evaluated 196 patients with SSD, 93% of these patients suffered from an acquired hearing loss, who were enrolled for a trial period of 2 weeks. The most important reason mentioned by 66% of all the patients who declined the BAHA was lack of improvement of speech understanding in noise. evaluated the efficacy of BAHA for SSD in unilateral profound hearing loss with normal or mild high-frequency hearing loss in the hearing ear (pure-tone average (PTA) ≤25 dB HL measured at 0.5, 1, 2, and 3 kHz). After a 6-month trial, these adult patients benefitted most in speech understanding in challenging listening situations.

implanted nine adults with SSD for more than 1 year and normal hearing on the contralateral side with a Bonebridge (cf. Fig. 10.3 , right). They found that: “Speech discrimination scores showed a mean signal-to-noise ratio improvement of 1.7 dB SPL for the aided condition compared with the unaided condition in the setting where the sound signal is presented on the side of the implanted ear and the noise source was in the front.” It is interesting that the benefit was only 1.7 dB when one would expect an improvement of about 5 dB (the head shadow for broadband signals, Snik, personal communication). The potential underlying variability is described in the work of , who noted that patients with a moderate SNHL in the functioning ear perceived greater increments in benefit, especially in background noise, and demonstrated greater improvements in speech understanding with BAHA amplification.

Summarizing, in unilateral hearing loss sound localization accuracy and speech understanding in noise can be improved by a BAHA, in SSD only when the speaker is localized on the deaf side.

10.2.1.2

Bilateral Hearing Loss

systematically reviewed the literature and found some evidence that bilateral BAHAs provided additional benefit compared to unilateral BAHA. investigated the subjective benefit from a BCI sound processor in 14 patients, who were fitted with a Cochlear BAHA Compact, 23 with a BAHA Divino, or 7 with a BAHA Intenso. The used a survey with a median follow-up time of 50 months. At that time, 86% of the patients still used their sound processor and reported that: “Speech understanding in noisy situations is rated rather low, and 58% of all patients reported that their BCI benefit was less than expected.”

tested 24 successive patients equipped with the Bonebridge. They measured the overall average functional hearing gain of all patients ( N =23) was 28.8±16.1 dB (mean ± SD). Monosyllabic word scores at 65 dB SPL in quiet increased statistically significantly from 4.6±7.4% to 53.7±23.0%. Evaluation of preoperative bone conduction thresholds revealed three patients with thresholds higher than 45 dB HL in the high frequencies starting at 2 kHz. These three patients had a very limited benefit of their BCIs. concluded that: “The Bonebridge bone-conduction implant provides satisfactory results concerning functional gain and speech perception if preoperative bone conduction lies within 45 dB HL.” found that “the maximum output of the Bonebridge ranges from 55 to 71 dB HL, depending on frequency. Accepting a minimum dynamic range of 35 dB with the Bonebridge, fitting of the Bonebridge in a linear program is advocated in patients with a sensorineural hearing loss component of up to 30 dB HL.”

compared the new transcutaneous bone conduction hearing aid, the Sophono Alpha 1, with the percutaneous BAHA system ( Fig. 10.4 ). They found: “The BAHA-based outcome was slightly better compared with Sophono-based results in sound field thresholds, speech recognition threshold, and speech comprehension at 65 dB.” They also remarked that “the Sophono offers appealing clinical benefits of transcutaneous bone conduction hearing; however, the audiologic challenges of transcutaneous application remain, as the Sophono does not exceed percutaneous application regarding audiologic output.” See also www.snikimplants.nl .