Surgical Placement of Bone-Anchored Hearing Systems Treatment & Management

Updated: Jun 11, 2018
  • Author: Stephen P Cass, MD, MPH; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Treatment

Medical Therapy

Other than treatment of infection with antibiotics and treatment of some rare forms of sensory hearing loss with steroids, no specific medical therapy is available for most common forms of hearing loss. Before the Baha, patients unable to wear air-conduction hearing aids were destined to use conventional bone conductors in order to amplify hearing. A conventional bone conductor consists of an amplifier and transducer attached to a headband or spectacle frames. The bone transducer is applied with a certain force to the skin covering the mastoid process and transmits sound vibrations transcutaneously to the skull base and the cochlea.

The conventional bone conductor has numerous drawbacks, such as variations in speech recognition owing to variation on pressure between the transducer and the mastoid, discomfort for the user, and poor cosmetic appearance. The high static pressure needed to maintain sufficient contact between the transducer is frequently reported to produce pain, skins irritations, and/or headaches. Furthermore, the listening environment becomes unnatural when the microphone and vibrational transducer of the conventional bone conductor are on opposite sides of the head. [27]

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Surgical Therapy

The introduction of Baha offers an alternative hearing amplification system to patients not satisfied with the conventional bone conductor. The main advantages with the new system are removal of the problematic transcutaneous transducer and elimination of the sound-attenuating tissue layers between the transducer and the skull.

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Preoperative Details

Surgery in adults

In adults, surgery is usually performed as a one-stage procedure. [33, 34] Adult patients are usually handled in day care units, where surgery is performed under local anesthesia in the operation room. In some case, general anesthesia is preferred.

Surgery in children

In children, special surgical considerations are to be taken into account because of pediatric bone being thinner and softer with lower mineral content. In children, a 2-stage procedure is recommended, with an osseointegration period of 3-6 months in between stages. [34] At the first stage, the skin over the implant site is incised, continuing through the subcutaneous tissue and periosteum. The fixture without abutment is placed and the soft tissue is closed. In young children, the cranial bone thickness is often less than 4 mm. In this situation, the implant can be screwed in as far as dura, then left “proud” and covered with periosteum. New bone forms to fully secure the implant. In the second stage, the abutment is placed after a soft tissue reduction and skin penetration with skin punch. After a healing period of 2-3 weeks, a Baha is fitted according to clinical standard.

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Intraoperative Details

The operative technique includes 2 components: (1) subcutaneous tissue reduction to create thin, immobile skin around the abutment and (2) bone work for fixture placement performed in such a way as to maximize the opportunity for osseointegration. Several techniques are available to perform the necessary subcutaneous tissue reduction and include a U-flap, linear incision, and use of a dermatome. Another alternative technique uses a longer abutment without skin thinning. [35] However, although promising, long-term outcomes using this technique are not yet available. The linear incision technique has proven to be safe and reliable, and a comparison of techniques suggests the linear technique is also associated with fewer complications. [36, 37]

Linear incision technique  [38]

For positioning the fixture, the site of fixture is located and marked with the surgical pencil using a sound processor template. The optimal location is approximately 50-55 mm from the ear canal, posterior and superior to the auricle along the temporal line. The sound processor should not touch the auricle, overlie a prior mastoid cavity or craniotomy site, and, in cases of auricular microtia, be placed far from any tissue that may be used for auricular reconstruction—this is important (see the image below). The area is next injected with 2% lidocaine with 1 to 100,000 epinephrine mixed in equal part with 0.5% bupivacaine (10-20 mL is used).

Bone-anchored hearing systems. Template for determ Bone-anchored hearing systems. Template for determining site of implanted fixture-abutment.

For the linear incision technique, a 40- to 60-mm skin incision is planned (see image below). The length of the skin incision can be varied according to the patient’s scalp thickness. (One good rule of thumb is that the incision should be 6 times the scalp’s thickness.)

Bone-anchored hearing systems. Linear incision wit Bone-anchored hearing systems. Linear incision with separate site for fixture-abutment.

Subcutaneous skin flaps are created on both sides of the incision. The length of each skin flap should be half the length of the skin incision. The flaps are developed in the immediate subdermal plane (see the image below).

Bone-anchored hearing systems. Skin flaps created. Bone-anchored hearing systems. Skin flaps created.

Subcutaneous tissue is removed down to the cranial periosteum, and the cranial periosteum is preserved. Electrocautery is used to assist in hemostasis (see the image below).

Bone-anchored hearing systems. Subcutaneous soft t Bone-anchored hearing systems. Subcutaneous soft tissue removal.

Six mm of periosteum is removed at the site of the fixture placement (see the image below).

Bone-anchored hearing systems. Periosteum removed Bone-anchored hearing systems. Periosteum removed at implant site.

A guide hole is created using a 1.8-mm drill bit with a 3-mm stop. The guide hole is probed to check if additional bone is present, and, if so, the guide hole is drilled further, to a depth of a 4 mm. All drilling is performed under continuous irrigation at low speed to avoid thermal injury to the bone that could impair osseointegration (see the image below). Prudent drilling, in order not to penetrate the dura or the transverse dural sinus, is necessary.

Bone-anchored hearing systems. Guide drills: 3 mm Bone-anchored hearing systems. Guide drills: 3 mm then 4 mm depth.

The guide hole is then drilled to 3.8 mm diameter, and a slight countersink is created using the countersink bit (see the image below). Drilling at a right angle to the surface of the skull is important, as well as limiting the countersink so that strong cortical bone remains, which facilitates the initial stability of the implant necessary for osseointegration.

Bone-anchored hearing systems. Hole widened with d Bone-anchored hearing systems. Hole widened with drill countersink.

The fixture/abutment assembly is basically a self-tapping screw that is implanted using a torque-limiting drill set at 30-40 Ncm torque (see the image below).

Bone-anchored hearing systems. Fixture implanted u Bone-anchored hearing systems. Fixture implanted using torque-limiting drill.

The skin incision is closed and the abutment exteriorized using a 4-mm skin punch (see the first image below). A conforming, bolster-type dressing is applied and secured using a healing cap. This dressing is used to keep the skin flaps down to the periosteum, eliminating all subcutaneous dead space (see the second image below). Finally, a mastoid-type dressing is placed.

Bone-anchored hearing systems. Closure and exterio Bone-anchored hearing systems. Closure and exteriorization of abutment through separate skin punch.
Bone-anchored hearing systems. Dressing with heali Bone-anchored hearing systems. Dressing with healing cap.

Postoperatively, the mastoid dressing is removed after 24 hours, and the conforming dressing is removed after 7 days. Sutures are removed at 7 days.

Osseointegration is sufficient by 3-6 months, at which time the sound processor is provided and programmed. [39, 40] (see the images below).

Bone-anchored hearing systems. Surgical site posto Bone-anchored hearing systems. Surgical site postoperative day 7.
Bone-anchored hearing systems. Position of process Bone-anchored hearing systems. Position of processor following surgery.
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Postoperative Details

Aftercare is critical for long-term stability of the implant. The site should be cleaned daily with soap and water and a soft brush facilitates hygiene. If inflammation develops around the interface of the abutment and skin, then additional care is required. This may include use of topical antibiotics or steroid-containing ointments. Occasionally, the healing cap should be used at night with the topical antibiotic administrated by wrapping ointment-soaked ribbon gauze around the abutment . Skin care and abutment hygiene are critical to maintain normal usage of the Baha. Parents and caretakers usually have to perform this role in children with developmental disabilities.

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Complications

Operative complications

The following complications are rare (< 1%) and most often seen see in children:

Extrusion of the fixture

The rate of extrusion ranges from 3% [41] to 10% [42] and is generally higher in children than adults. All fixture extrusions represent a failure of osseointegration and are influenced by the age of the patient, the surgical technique, and the state of the bone (ie, previous irradiation). Extrusion of the fixture within the first 3 months suggests a technical surgical issue. Failures over time are usually related to either trauma or chronic infection. [27]

Skin complications

Soft tissue reactions can be graded using the Holgers scale or a modified version. [43, 44] Grade 2 and higher skin reactions (red and moist) develop at some time in about 25% of patients. These skin reactions typically required re-education on hygiene or topical therapies. More severe skin reactions, including formation of granulation tissue or skin thickening leading to growth of skin over the abutment, occur less frequently, in the range of 3-10% of patients. These skin reactions can be treated with cautery, steroid injections, or revision surgery.

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Outcome and Prognosis

After more than 25 years of clinical experience, the bone conduction implants (BCIs) are a well-established treatment for patients with conductive or mixed hearing loss. Owing to their success and excellent performance for hearing impaired individuals, their use has spread, and the indications for application have gradually become broader.

Baha in conductive or mixed hearing loss

The use of the BCIs in patients with unilateral conductive or mixed hearing loss has proven to be successful in achieving binaural hearing with only few complications and no interference with the function of the normal ear.

The statistics for closure of air-bone gap are as follows [9] :

  • 10 dB in 80% of patients

  • 5 dB in 60% of patients

In audiological terms, the Baha results are superior to those obtained with a conventional bone conduction device (external headband–mounted sound processor).

Air-conduction hearing devices should not be used in patients with therapy-resistant otorrhea, making a bone conduction device a better option. Additionally, in patients with mixed hearing loss in whom the air-bone gap exceeds 30 dB, audiological performance is likely better with a bone conduction device than with an air-conduction device.

In studies that include patients with aural atresia, chronic otitis media, chronic otitis externa, and otosclerosis, hearing improvement with the Baha was good; Lustig et al report a mean pure tone average of 28 dB and a gain in hearing of 33 dB. [9] Liepert et al report a similar average gain of 30 dB in speech recognition threshold. [45] Wazen et al report an improvement in speech recognition threshold from 52 dB to 27 dB.

In cases of aural atresia, bone conduction devices provide predictable and long-term stable hearing results that do not depend on the degree of external and middle ear malformations; it is placed during a simple surgical procedure with a low morbidity rate and a very high rate of patient satisfaction.

The satisfaction level of patients with conductive hearing loss is well reported in the literature. Concerning general satisfaction, the average scores are very good. The Baha is better than the other types of equipment, with high indices of satisfaction (index = 9 among 24 patients, [46] index = 8.11 among 52 patients, [31] and index = 8.3 among 165 patients). [47] Nearly 89% of the patients preferred the Baha to the conventional equipment tested beforehand. [14]

BCIs in single-sided deafness

Bone conduction amplification on the side of a deaf ear has been shown to provide greater benefit in subjects with monaural hearing than did contralateral routing of signals (CROS) amplification. [48] Advantages may be related to averting the interference of speech signals delivered to the better ear, as occurs with conventional CROS amplification, while alleviating the negative head-shadow effects of unilateral deafness. Newer CROS devices have advances in their signal processing and features that have not been fully explored and compared with bone conduction devices.

The advantages of head-shadow reduction in enhancing speech recognition with noise in the hearing ear outweigh disadvantages inherent in head-shadow reduction that can occur by introducing noise from the deaf side. The level of hearing impairment correlates with incremental benefit provided by the Baha. Patients with moderate sensorineural hearing loss in the functioning ear perceived greater increments in benefit, especially in background noise, and demonstrated greater improvements in speech understanding with Baha amplification. [15]

Results for single-sided deafness are as follows [49] :

  • 70% improvement in quality of life

  • 88% better performance at a dinner table, when a person sitting on their deaf side

  • 88% better performance while talking to one person among a group of people.

  • Average satisfaction score is 8 out of 10

BCIs in children

In children with binaural congenital conductive hearing loss, intervention should take place as soon as possible after birth. This is possible using the bone conduction sound processor attached to a Velcro headband (ie, Baha soft band). Placement of the osseointegrated fixture or use of a magnet-based system is FDA approved for age 6 and older.

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Future and Controversies

Bone conduction implants (BCIs) and single-side deafness

Because normal sound localization requires 2 hearing ears, bone conduction devices in single-sided deafness would not be expected to provide normal sound localization, and most studies observe no improvement of objective localization ability, either with the contralateral routing of signals (CROS) or with the Baha system. [15, 49] However, several investigators report some improvement in sound localization with the Baha, raising the possibility that some sense of directionality may be possible when using the Baha. [13]

Note that intelligibility in a noisy environment is improved with conventional CROS systems as well. Therefore, an acquired pseudo-binaural audition does not seem to be specific to the Baha. The current hypothesis to explain the benefits provided by the Baha predicts its ability to allow patients to optimize their use of the head-shadow effect. However, this hypothesis has not yet been formally verified and might not be sufficient to fully explain all observed audiological benefits.

The exact mechanisms of Baha-based improvements in hearing remain unclear. The current reasoning is that sounds captured on the side of the deaf ear are transmitted via bone conduction through the skull to the contralateral hearing cochlea. At this site, 2 auditory signals are encoded, one originating from air-conducted ipsilateral sound waves and the second coming from the contralateral side via bone conduction. Some studies could identify, using the Baha vibrator system, the characteristics of transcranial transmission of sound. [5] Results showed that skull transmission acts as a low-pass filter with almost no attenuation of low-frequency sounds (below 700 Hz), while higher-frequency sounds have their intensity decreased by 12 dB per decade above 1 kHz.

Regarding temporal dynamic aspects, the current thought includes the possibility of completing the travelling of sound via bone conduction from the ipsilateral to the contralateral ear with frequency-dependent delays ranging from 0.2-0.6 seconds, thereby causing some dispersion of conducted sounds. [50] Therefore, the pseudo-binaural signal provides redundant but delayed low-frequency information that may mimic the effects of interaural time differences naturally existing between 2 ears and used for low-frequency sound localization.

However, the pseudo-binaural acoustical signal is highly complex and contains particular specific characteristics owing to the summation of 2 monaural signals transmitted through 2 different modalities (air/bone). Results of the same study suggest that the acoustical difference induced by the Baha system, which is presumed to support pseudo-binaural audition, is presumably lost at a superior level and therefore not taken into consideration by the central nervous system. [16]

Furthermore, such an electrophysiological study does not completely explore the nervous processes induced by Baha. From this point of view, functional cerebral imaging using the same experimental procedure is needed to explore the possible central auditory system modifications induced by such a hearing aid.

Since bone conduction devices acting to transfer sound transcranially from the deaf side to a working cochlea inherently cannot provide true sound localization, their binaural benefit is limited. To address this limitation, there recently has been interest in using cochlear implantation in single-sided deafness; early reports indicate favorable patient satisfaction with measureable improvements in hearing in noise ability and sound localization. More developments in this area of study are certain in the future.

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