Surgical Placement of Bone-Anchored Hearing Systems 

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

Overview

History Of The Procedure

Bone-anchored hearing system

A bone conduction implant (BCI) uniquely combines the concepts of osseointegration and bone conduction hearing. 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.

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

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]

Problem

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

Epidemiology

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.

Etiology

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

Pathophysiology

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]

Presentation

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]

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.[28]

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.[29]

Indications

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, 30, 31, 12, 32]

  • Congenital ear malformation or atresia[33]

  • Chronic external otitis preventing use of conventional equipment[30, 31]

  • 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.

Contraindications

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.[34] In another study, patients received significant benefit—both documented audiologically and subjectively in activities of daily living—and the complication rate was low.[35]

 

Workup

Audiometric Testing

Basic audiometry including pure-tone audiogram and speech audiometry should be performed. A trial of the Baha using an external headband is useful. Several questionnaires are also available in order to obtain the participating patient’s subjective view of different aspects of Baha fitting, and these include the study-specific questionnaire,[36] the International Outcome Inventory for Hearing Aids, the Meaningful Auditory Integration Scale, the Meaningful Use of Speech Scale, and the H70.[37]

 

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]

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)

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.

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).

 

 

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]  

Complications

Operative complications

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

  • Bleeding from a dural sinus

  • Cerebrospinal fluid (CSF) leak

  • Subcutaneous hematoma

  • Abscess formation

  • Subdural hematoma

  • Meningitis

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.

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.

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.