Surgical Placement of Bone-Anchored Hearing Systems Treatment & Management

Updated: Oct 29, 2019
  • Author: Stephen P Cass, MD, MPH; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Treatment

Medical Therapy

Other than management 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 BCI, 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. [32]

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Surgical Therapy

BCIs offer an alternative hearing amplification system to patients not satisfied with the conventional bone conductor.  The main advantages with the percutaneous BCI system are removal of the problematic transcutaneous transducer and elimination of the sound-attenuating tissue layers between the transducer and the skull. However, this introduces its own set of limitations, to address which, alternative BCI systems have been developed. Four surgical methods pertinent to different BCI systems will be discussed:

  • Percutaneous implant placement (Cochlear's Baha Connect)
  • Percutaneous implant placement, minimally invasive point surgery (MIPS) (Oticon Medical's Ponto)
  • Transcutaneous magnet implant placement (Cochlear's Baha Attract, Medtronic's Sophono)
  • Active BCI system placement (MED-EL's Bonebridge)
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Preoperative Details

Surgery in adults

In adults, surgery is usually performed as a one-stage procedure. [38, 39] Adult patients are usually handled in day surgery units, where surgery is performed under local anesthesia in the operation room. In some case, general anesthesia is preferred.

Surgery in children

In young 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. [39] 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 skin penetration with a biopsy punch. After a healing period of 2-3 weeks, a BCI is fitted according to clinical standard.

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Intraoperative Details

The main goal of surgery is to perform fixture placement in a way that maximizes the opportunity for osseointegration and minimizes complications. Soft tissue reduction is no longer a part of standard surgical technique for BCI placement; indeed, instead of modifying the patient's anatomy to fit the implant, a newer assortment of abutment lengths allows the surgeon to modify the device to the patient's anatomy.  

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 or overlie a prior mastoid cavity or craniotomy site, and, in cases of auricular microtia, it must be placed far from any tissue that may be used for auricular reconstruction (see the image below). Before injection of local anesthetic, the scalp thickness over the planned implant area is measured by inserting a hypodermic needle down to bone, gripping it at the epidermal level with a hemostat, removing the needle, and measuring the distance from the skin surface to the bone with a ruler.  

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

Percutaneous implant technique

Once the implant site has been chosen, for the curvilinear incision technique, the incision should be marked out in a radius at least 1.5 cm away from the implant site. This may be posterior or anterior to the planned implant, depending on the clinical circumstances. Various surgeons will use different incision lengths (radii), but it should be long enough to ensure that the drill may be placed directly perpendicular to the implant placement site once the flap is lifted, and a 180˚ incision offers this. A smaller incision may be attempted first and lengthened as necessary once the drill is brought into the field. The area is next injected with a 1:1 mixture of lidocaine with epinephrine, and bupivacaine with epinephrine.

A subcutaneous, supraperiosteal, posteriorly based skin flap is created, working toward the implant site. At this time, the skin flap is replaced in its original position, and a hypodermic needle is placed through the desired skin area into the bone; the flap is then lifted and the surgeon views the location at which the needle enters. This area is marked with a marking pen and used to make a cruciate periosteal incision before drilling commences. 

When the drill is used, a drill indicator is placed on the drill so that its perpendicular nature may be more readily assessed to ensure that an absolute perpendicular angle to the bone is taken. 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 (2,000 RPM) to avoid thermal injury to the bone osteocytes 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 widening/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 is the term given to the screw that osseointegrates into the cortical bone. The abutment denotes a post that attaches to the fixture and protrudes through the skin. The fixture is essentially a self-tapping screw that is implanted using a torque-limiting drill set at 30-40 newton-centimeters (N-cm) torque (see the image below); alternatively, a hand drill with a torque wrench may be used to limit torque to the appropriate 30-40 N-cm. The titanium oxide coating on the fixture that enables osseointegration should not be touched by anything after being taken out of its holder and before it is screwed into the bone. Again, ensuring that the drill is truly perpendicular to the cortical bone is essential.  

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

The implant may be placed in a one- or two-stage procedure. In a one-stage procedure, most common in adults with good bone stock, the abutment inserter is used to pick up the abutment, which is attached to the fixture. This entire complex is carefully drilled into the existing hole using the torque-limited drill at 30-40 N-cm. The skin incision is then closed in layers, and the abutment is exteriorized using a 4 mm skin punch. Closing before making the punch ensures that the punch site will be directly over the abutment. A conforming, bolster-type dressing is applied and secured using a healing cap, eliminating all subcutaneous dead space.

In immunocompromised patients, those with previously irradiated bone, and children, a two-stage procedure is recommended. In this case, the fixture alone is inserted via the implant inserter, using the torque-limited drill at 30-40 N-cm (without an the abutment attached). A cover screw is inserted to the top of the fixture to ensure no bone or tissue growth into the implant threads, and the incision is closed in layers. In the second stage, a 4 mm punch biopsy is used to visualize the implant, the cover screw is removed, and the abutment is screwed into the fixture. The bolster and healing cap are applied at this point.

Bone-anchored hearing systems. Dressing with heali Bone-anchored hearing systems. Dressing with healing cap.

 

Bone-anchored hearing systems. Position of process Bone-anchored hearing systems. Position of processor following surgery.

Percutaneous implant placement, MIPS

Oticon Medical's Ponto device is designed to be placed through a single 5 mm biopsy skin punch, being inserted through the soft tissue and periosteum down to bone. A cannula fits into this soft tissue hole and helps guide the drills. With regard to all drilling, saline, used for cooling, should be employed to fill the cannula before drilling starts. During the first drill, a spacer is kept in the cannula to prevent penetrating more than 3 mm deep. If dura is still present at the drill hole's depth, the spacer may be removed and the drill inserted deeper to 4 mm. Then, similar to implantation of Cochlear's Baha device, a widening drill is used to widen and countersink the existing hole. The cannula is then flushed. The abutment inserter is used to pick up the abutment and the implant, and this is placed into the bone with a torque-limited drill (40-50 N-cm). A healing cap and dressing is then applied. 

Transcutaneous magnet implant placement

Cochlear's Baha Attract system and the Medtronic's Sophono system both use subcutaneous magnets to transduce externally processed sound to convey information to the cochlea. The Baha Attract system incorporates osseointegration, similar to Baha Connect, whereas the Sophono system is fixed to the skull via screws that do not osseointegrate. Though placement is similar to the abovementioned percutaneous implants, there are a few differences, with the curvilinear incision incorporating a larger radius to fully cover the magnet with the skin flap. Similar to the percutaneous devices, the scalp thickness is measured with a hypodermic needle and a hemostat. A subcutaneous, supra-periosteal plane is used to elevate the flap.

Insertion of the fixture proceeds similar to the percutaneous devices, except that there is no abutment to be placed. Once the fixture is inserted (30-40 N-cm), the bone bed indicator is twirled around to ensure that the fairly large magnet will be able to be fully implanted into the drill hole without premature contact; if this contact is detected, removal of periosteum or bone is necessary until the bone bed indicator is able to move freely in 360˚. After this, the magnet is screwed in at 25 N-cm, ensuring that the "up" arrow is pointing superiorly. The soft tissue gauge is used to ensure that the scalp is of sufficient thinness to allow magnetic contact between the internal and external magnets. After this, the incision is closed in layers. (See the images below.)

A large curvilinear incision is planned and marked A large curvilinear incision is planned and marked.

 

Scalp thickness is measured prior to injection of Scalp thickness is measured prior to injection of local anesthetic.

 

The skin flap is raised superficial to the periost The skin flap is raised superficial to the periosteum.

 

A bone bed indicator is used to assess the need to A bone bed indicator is used to assess the need to remove any other tissue in order for the magnet to lie flat.

 

The magnet is shown here, screwed in place to 30-4 The magnet is shown here, screwed in place to 30-40 N-cm.

 

The skin flap thickness is tested using the skin f The skin flap thickness is tested using the skin flap gauge.

 

The operation concludes with a skin incision close The operation concludes with a skin incision closed in layers.

Active BCI system placement

MED-EL's Bonebridge system is the only currently available active BCI. To implant this device, local anesthetic is injected, and a standard postauricular incision is made, exposing the mastoid cortex. Similar to a cochlear implant procedure, a subperiosteal pocket is created, with this being superior and posterior to the incision. The T-sizer template is used to check the size of the required hole, and this is marked.

The mastoid is drilled in a circular manner to a depth of 8.7 mm, enough to encompass the Bonebridge's floating mass transducer (FMT). If an 8.7 mm depth cannot be achieved, the BCI lifts are required to elevate the transducer out of the drilled hole; the number of lifts required depends on the depth of the drill hole. This may be assessed using the depth gauge. Also, the skin flap gauge is used to ensure that the skin thickness is 7 mm or less.

Once this has been resolved, the receiver and demodulator are placed in the subperiosteal pocket, and screws are placed to anchor the transducer to the mastoid (10-32 N-cm torque), with our without the BCI lifts as needed. Depending on the ideal placement of the transducer and receiver, there is some flexibility in the implant: +/- 90˚ in plane and -30˚ out of plane. Finally, the incision is closed in layers. (See the images below.)

Postauricular incision and T-sizer template. Court Postauricular incision and T-sizer template. Courtesy of MED-EL (https://www.medel.com/bonebridge-surgical-support/#prettyPhoto).
Marking the template's outline on the bone. Courte Marking the template's outline on the bone. Courtesy of MED-EL (https://www.medel.com/bonebridge-surgical-support/#prettyPhoto).

 

The mastoid bone is drilled in a standard manner w The mastoid bone is drilled in a standard manner with normal otologic bits. Courtesy of MED-EL (https://www.medel.com/bonebridge-surgical-support/#prettyPhoto).

 

Skin flap gauge to check skin thickness, and T-siz Skin flap gauge to check skin thickness, and T-sizer template to check the drilled hole. Courtesy of MED-EL (https://www.medel.com/bonebridge-surgical-support/#prettyPhoto).

 

Bonebridge floating mass transducer (FMT) screwed Bonebridge floating mass transducer (FMT) screwed into the bone cortex surrounding the drilled mastoid hole. Courtesy of MED-EL (https://www.medel.com/bonebridge-surgical-support/#prettyPhoto).

 

The implant may be bent to accommodate the patient The implant may be bent to accommodate the patient's skull curvature. Courtesy of MED-EL (https://www.medel.com/bonebridge-surgical-support/#prettyPhoto).

 

 

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Postoperative Details

Postoperatively, the conforming dressing is removed after 7 days. 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 the use of topical antibiotics or steroid-containing ointments. Occasionally, the healing cap should be used at night, with the topical antibiotic administered by wrapping ointment-soaked ribbon gauze around the abutment. Skin care and abutment hygiene are critical to maintain normal usage of the BCI. Parents and caretakers usually have to perform this role in children with developmental disabilities.

Osseointegration was formerly deemed sufficient by 3-6 months; however, earlier loading of the processor has been shown to be effective with no significant increase in failure rates. [40, 41]  McLarnon et al showed good results with loading at 4 weeks postoperatively, [42] Faber et al and Wazen et al at 3 weeks, [43, 44] and Høgsbro et al at 1 week. [45]  

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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% [46] to 10% [47] 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. [32]

Skin complications

Soft tissue reactions can be graded using the Holgers scale or a modified version. [48, 49] 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

Over decades of clinical experience with bone conduction implants (BCIs), these devices have become 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.

BCIs 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 audiologic terms, BCI 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 a BCI 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. [50] 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 BCI is better than the other types of equipment, with high indices of satisfaction (index = 9 among 24 patients, [51] index = 8.11 among 52 patients, [36] and index = 8.3 among 165 patients). [52] Nearly 89% of the patients preferred a BCI 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. [53] 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 BCI. 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 BCI amplification. [15]

Results for single-sided deafness are as follows [54] :

  • 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, softband). 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, 54] However, several investigators report some improvement in sound localization with the BCI, raising the possibility that some sense of directionality may be possible when using the BCI. [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 BCIs. The current hypothesis to explain the benefits provided by the BCI 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 BCI-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 BCI 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. [55] 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 acoustic 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 acoustic difference induced by the BCI 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 electrophysiologic study does not completely explore the nervous processes induced by BCIs. 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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