The Bone-Anchored Cochlea Stimulator—BACS

Historical Aspects

In addition to the disadvantages of traditional bone conduction devices, new developments in research contributed to the development of the Baha. From the mid-1970s, the use of osseointegrated implants in the treatment of edentulousness was in clinical practice. In conjunction, Professor Per-Ingvar Brånemark sought an acoustical means to evaluate the degree of osseointegration. In one experiment, a patient with implants in the upper jaw had a special adaptor secured to a dental implant and an Oticon bone transducer attached. While it was not possible to measure the stability of the implant at that time, it was clear that the patient experienced sound very clearly, even when the stimulation was of a low level. This had important implications regarding the ability to transmit sound via bone. In his PhD thesis, Per Kylén at Linköping University in Sweden measured the sound reaching the cochlea during ordinary mastoid drilling. His findings also contributed to the Baha concept. He found that the high noise levels reached during drilling produced a temporary threshold shift.

In 1977, in close cooperation with Bo Håkansson at Chalmers University of Technology in Göteborg, the first prototype of the Baha was manufactured and tested on three patients with chronic ear disease. The initial results were most encouraging and during the following 5 years another 14 patients underwent surgery. The patients were very satisfied with the hearing results and the program continued to expand. In June 2014, the total number of Baha patients worldwide was estimated to be over 120,000.

By following the original guidelines set by Professor Brånemark for establishing osseointegration, the success rate for stable implants was as high for Baha as was that for the oral cavity. Skin irritation was noted around the coupling in some patients. To reduce the relative movement between coupling and skin, reduction of the soft tissues was introduced. This is still a technique used by many surgeons. However, due to the new design and surface characteristics of the abutment a technique without soft tissue reduction is gaining popularity. Both techniques will be described.

Hearing Through Bone Conduction

von Békésy, Barany, Tonndorf, and others have presented theories related to bone conduction. These investigators did not have access to direct bone conduction in humans with percutaneous implants. During the last decades, Håkansson, Goode, and Stefelt are three of the leading scientists in this area. ^, Hearing through bone conduction is highly complex and several factors contribute to the hearing process:

• • Sound radiated in the ear canal

  • Middle ear ossicle inertia

  • Inertia of the cochlea fluids

  • Compression of the cochlear walls

  • Pressure transmission from the CSF

Middle ear ossicle inertia is most important between 1.5 and 3.1 kHz and cochlear fluid inertia the most dominant contributor below 4 kHz.

These factors are often interrelated and it is difficult to isolate one from another. Interestingly, almost regardless of the location of bone stimulation, the waves of the basilar membrane travel from the base of the cochlea, where the membrane is stiffer, toward the helicotrema, as is the case with air-conducted sound. This means that the cochlea has difficulties in differentiating between bone-conducted and air-conducted sound. Cancellation experiments have verified this. Stenfelt has also shown that bone-conducted sound from 0.1 to 10 kHz for levels up to 77 dB HL is linear. This is important, as the distortion level of the bone-conducted sound will be low.

Osseointegration

Osseointegration, OI, is a term coined by Per-Ingvar Brånemark that refers to a direct contact between the bone and an implant that can withstand a functional load. ^, OI is a fundamental prerequisite for direct bone conduction. Without OI, the Baha will not function.

Among the factors identified by Brånemark as key to establishing OI was the choice of implant material. Commercially pure titanium, cpTi, has been the material of choice since the start. The cpTi has a purity of 99.75%. When a cpTi implant is machined, the surface will be covered with an oxide layer within milliseconds. The implant can thus be regarded as a ceramic. It is the oxide layer which has the unique biocompatible properties. It is also dense and adheres to the bulk metal. Proteins from the host will not denature when in contact with the oxide. Titanium oxide has a high electrochemical value, which means that the surface will attract foreign material that could contaminate the implant. This is why the implant should be handled with the utmost care, avoiding contact even with sterile particles from gloves and draping during surgery.

The surgical procedure, which will be described in detail below, is focused on reducing surgical trauma as much as possible. Excessive heat will interfere with healing. An osteocyte will only tolerate 42°C for one minute before apoptosis. If too many osteocytes are damaged, there is a risk of fibrous tissue healing and impaired OI. The screw-shaped design provides initial stability. Micromovements during the healing phase could, if too large, result in soft tissue encapsulation and prevent OI.

In the treatment of edentulousness with implants, early loading often caused loss of OI, and therefore a two-stage procedure was initially recommended for Baha. However, the load produced by a Baha is very low compared to the forces produced during chewing. Based on a study comparing a one-stage with a two-stage procedure in adults, the one-stage procedure is now recommended as no difference in OI could be found. In irradiated bone, a two-stage procedure with a 6-month interval is suggested, often in combination with hyperbaric oxygen treatment.

Implants in children will be discussed separately.

Patient Selection

Two groups of patients are potential candidates for a BACS:

• • Patients with conductive or mixed hearing loss

  • Patients with single-sided deafness (SSD)

Conductive or Mixed Hearing Loss

Patients who have mixed hearing loss with a conductive loss greater than 30 dB may not always be helped by conventional air conduction aids and could benefit from a BACS.

Chronic ear disease

The Baha was originally designed for patients with bilateral or unilateral conductive or mixed hearing loss that needed amplification but who could neither be helped through reconstructive surgery nor use an air conduction hearing aid. Suitable candidates included patients with ears that drain in spite of trials to make the ear dry and those whose ears start to drain when the external ear canal is occluded with a hearing aid mold. Patients with dry canal wall-down mastoid cavities who experience acoustic feedback with an air conduction aid may also benefit from a BACS.

Congenital malformations

Patients with bilateral aural atresia often have a normal or near-normal cochlea function and are ideal patients for a BACS. As reconstructive atresia surgery is both complex and not without risks, a lasting hearing improvement is not always possible to achieve. In patients with a Jahrsdoerfer rating of 7 or poorer, we often suggest a bone-anchored hearing implant instead of reconstructive trials.

While atresiaplasty surgery has the potential to restore natural sound conduction to patients who are appropriate surgical candidates, even the best results often have some residual air-bone gap. The hearing outcomes are reliably better with a BACS compared to atresiaplasty. In a systematic review comparing hearing outcomes in atresiaplasty compared to the BACS, Nadaraja et al. showed that of the aggregate data for atresiaplasty ears, 73.8% (95% CI, 62.2% to 85.4%) had an SRT less than 30 dB (338 ears), 60.3% (95% CI, 45.8% to 74.8%) had a PTA less than 30 dB (390 ears), and 68.9% (95% CI, 59.4% to 78.3%) had an ABG less than 30 dB (852 ears). The average hearing gain was 24.1 dB (95% CI, 21.62 to 26.51) for 516 ears. The hearing outcomes deteriorated with time. Of the BACS patients, 95.9% (95% CI, 91.5% to 100.0%) had a PTA less than 30 dB (77 ears), and 98.2% (95% CI, 94.5% to 100.0%) had an ABG less than 30 dB (47 ears); the average hearing gain was 38.0 dB (95% CI, 33.14 to 45.22) in 100 ears.

Another advantage of a BACS for these patients is that the treatment does not interfere with atresia or microtia surgery should this become necessary later on. While regulations regarding the minimum age for implantation vary from country to country, a BACS has been successfully used in children as young as 2 years of age. In young children, a BACS on a SoftBand has been very successful and implant surgery can be delayed until the child is older ( Fig. 31.1 ). Children can thus benefit from hearing with the BACS until they are old enough for a final evaluation of the anatomy and decision regarding the best means of hearing rehabilitation.

Unilateral external ear canal atresia was historically thought to have little detrimental effect on child development. However, it has now been well established that unilateral hearing loss has significant adverse effects on childhood development of speech, language, and cognition and that children benefit from early hearing rehabilitation in cases of unilateral hearing loss as well as bilateral loss.

Conductive loss in only hearing ear

A conductive loss in a patient with contralateral deafness presents a particular problem. If the conductive component is large, an air conduction hearing aid may have difficulty overcoming this gap. Any middle ear surgery intended to improve the hearing in the only hearing ear will have some risk of causing a sensorineural loss. The BACS does not put the inner ear at risk in the only hearing ear. For example, a patient with a deaf ear on one side following stapes surgery and a maximum conduction loss on the other side due to bilateral otosclerosis is an excellent candidate for a BACS.

Chronic external otitis or chronic otitis media preventing the use of an air conduction hearing aid

Some patients in need of amplification will experience problems with air conduction ear canal molds. Several different materials have often been tried but, for some, the mold will cause irritation, infection, and inflammation. As a result, these patients can only use their hearing aid for very limited periods or not at all. For them, a BACS is a good alternative. This group represents approximately 10% of the Baha patients at our Implant Unit in Göteborg, Sweden.

Patients with chronic otitis media due to tympanic membrane perforation refractory to surgical repair, a mastoid bowl with mucosalization, or deep retraction pockets which collect squamous debris may suffer from chronic otorrhea when traditional hearing aids are worn. Traditional hearing aids can trap moisture in the ear canal and exacerbate certain types of chronic otitis media. In these patients, a BACS allows the ear canal to remain open to air and overcomes the patient’s conductive hearing loss.

Patients with Down syndrome

In Down syndrome conductive hearing impairment is common. Narrow external ear canals and middle ear malformations are prevalent. Serous otitis media is also frequent among these children and can sometimes be hard to treat. It is clinically proven that a BACS improves the quality of life for these patients.

Single-Sided Deafness

Unilateral sensorineural deafness is not unusual. The etiology includes acoustic tumor surgery; however, the majority of cases are idiopathic. Patients with SSD often experience verbal communication problems even when the opposite ear functions normally. Problems with understanding speech are most commonly experienced in noisy surroundings. Difficulty with sound localization is another frequent problem, partly due to the head shadow effect, but other factors are also involved. Interestingly, SSD patient satisfaction with a BACS is often greater than that which would be indicated by hearing test results alone.

Patient Counseling

Patients considering a BACS should be thoroughly counseled in all aspects. The patient must have realistic expectations and be aware that this sound processor has limitations. The surgical procedure should be described and the possible intra- and postoperative complications discussed. A model implant may be helpful for demonstrating to the patient the small size of the implant when discussing the implant procedure. Many patients experience some postoperative numbness around the implant site. This is more common in patients where a large amount of soft tissue has been removed. The patient should also be able to come for regular follow-up visits. One of the main causes of adverse skin reactions is inadequate hygiene. Therefore, the need for a high level of personal hygiene must be stressed preoperatively. The sound processor requires proper maintenance. Detailed instruction of its function and physical care should be given.

Age is not a contraindication per se. The oldest patient who has undergone surgery at the Sahlgrenska unit was over 90 years of age and the youngest only 18 months. The authors suggest that, as surgery becomes both easier and less risky with age, the Baha® SoftBand is used until the child is 4 to 5 years old ( Fig. 33.1 ). See the section on Baha in Children.

The need for future MRI should be considered. The level of ferromagnetic contamination is in the range of 0.02% to 0.05 % in the bulk metal but no traces are found in the oxide layer; therefore, the implant will not jeopardize magnetic resonance imaging. However, it is advisable for clinicians to follow the individual manufacturer recommendations. ^, Due to the difference in density between bone and titanium, artifacts will appear around the implants but will typically be less than 1 mm.

When clinical and audiometric criteria have been evaluated and the final decision to proceed with implantation has been made, the patient should be informed that the BACS procedure is completely reversible. If he/she for whatever reason is not satisfied with the hearing result, it is easy to remove the abutment and, if so desired, also the implant, returning the patient to his/her original state. Very few procedures in otology offer this possibility.

Bone-Anchored Cochlea Stimulator Models And Audiometric Criteria

As a rule of thumb, the better the cochlear function, the better the result. The air-bone gap is of no importance since the BACS will overcome even a maximal air-bone gap, provided that the bone line is within a range that can be stimulated.

In 2020, there are two major manufacturers of percutaneous bone-anchored cochlea stimulators. One is the Baha from Cochlear Bone-Anchored Solutions, Göteborg, Sweden, a division of Cochlear Ltd. The other is the Ponto from Oticon Medical, also in Göteborg, Sweden. The basic construction of the vibrator in both devices is very much the same and goes back to the first prototype of the Baha made by Professor Bo Håkansson at Chalmers University of Technology in Göteborg, Sweden. Each of these companies have several models to fit different types of hearing loss. As technology advances rapidly, the reader is referred to the manufacturers to obtain the most up-to-date information on what devices are currently available. At the time of this writing, both companies suggest that their strongest processors can be used for patients with bone conduction demonstrating up to 65 dB hearing loss.

It is important to understand that the levels of hearing impairment discussed above are only guidelines. Some patients have not been satisfied even when they are well within the audiometric criteria, while others are satisfied even though their hearing capacity is lower than technically required.

In the preoperative evaluation, a testband of the SoftBand with a BACS or similar type of simulator can be very helpful. The BACS is attached to a coupling on a soft band around the head. With this test device, the patient still has skin between the transducer and skull bone. If the patient is satisfied with this, the chances that implant surgery will be successful are very high. A test rod can also be used as an alternative means to simulate sound through bone conduction and give the patient an idea regarding the hearing outcome. The BACS is attached to the coupling of a plastic rod and pressed onto the skin over the mastoid. To reduce the damping effect of the skin, the patient can also hold the rod firmly between the teeth, which will simulate direct bone conduction.

Contraindications

There are in fact no absolute contraindications to provide a patient with a BACS. However, psychiatric or neurodegenerative disease rendering the patient unable to follow instructions and participate in follow-up should be discussed. A referral for psychiatric or neurological evaluation may be helpful in these cases. The same could be said for patients with extremely poor general hygiene. Inadequate skull thickness is a relative contraindication, although it would be unusual to not find some location on the skull to implant the device. Patients with diabetes, psoriasis, scleroderma, or other skin problems could experience a slightly increased frequency of adverse skin reaction, which should be mentioned during patient counseling. Bone diseases such as osteogenesis imperfecta and Paget disease might constitute a higher risk of implant losses but is not an absolute contraindication.