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A bone-anchored hearing aid is a type of hearing aid based on bone conduction. It is primarily suited for people who have conductive hearing losses, unilateral hearing loss, single sided deafness and people with mixed hearing losses who cannot otherwise wear 'in the ear' or 'behind the ear' hearing aids.

Background

Hearing impairment is the most common physical disability in the industrialized world. Two of the causes of impaired hearing are, lack of function in the inner ear, the cochlea and when the sound has problems in reaching the nerve cells of the inner ear. Example of the first include age-related hearing loss and hearing loss due to noise exposure. A patient born without external ear canals is an example of the latter for which a conventional hearing aid with a mould in the ear canal opening would not be effective. Some with this condition have normal inner ear function as the external ear canal and the inner ear are developed at different stages during pregnancy. With normal inner anatomy sound conducted via the skull bone improve hearing.

A vibrator with a steel spring over the head or in heavy frames of eyeglasses pressed towards the bone behind the ear has been used to bring sound to the inner ear. This has however several disadvantages like discomfort and pain due to the pressure needed. The sound quality is also impaired as much of the sound energy is lost in the soft tissue over the skull bone, particularly for the higher sound frequencies important for speech understanding in noise.

Patients with chronic ear infection where the drum and/or the small bones in the middle ear are damaged often have hearing loss but difficulties to use a hearing aid fitted in the ear canal. Direct bone conduction through a vibrator attached to a skin penetrating implant addresses these disadvantages.

In 1977 the first three patients were implanted with a bone conduction hearing solution by Anders Tjellström at the Ear, Nose, and Throat Department at Sahlgrenska University Hospital in Gothenburg, Sweden. A 4 mm long titanium screw with a diameter of 3.75 mm was inserted in the bone behind the ear and a bone conduction hearing aid was attached.

The term osseointegration was coined by professor Brånemark. During animal studies he found that bone tissue attached to the titanium implant without any soft tissue in between. He also showed that an such animplant could take a heavy load. His definition of osseointegration was "direct contact between living bone and an implant that can take a load".

The first clinical application was in oral surgery where implants were used for retention of dentures. Brånemark sought an acoustic way to evaluate osseointegration. A patient with implants in the jaws was fitted with a bone vibrator on one of his implants. When tested the patient experienced very loud sound even at low stimulation levels indicating that sound could propagate very well in the bone. It has later been shown by Bo Håkansson that the sound transmission in bone is linear indicating low distortion of the sound.

The implant in the bone is made of commercially pure titanium and will osseointegrate according to the research performed by professor Per-Ingvar Brånemark. The hearing instrument is impedance matched and developed by professor Bo Håkansson at Chalmers University of Technology in Gothenburg, Sweden under the name of Baha. The design has been updated first by Entific and later by Cochlear Bone Anchored Solutions both in Gothenburg. The Danish hearing aid company Oticon has also developed a device based on the same principle. Osseointegration has been defined as the direct contact between living bone and an implant that can take a load. No soft tissue at the interface.

When Entific seld the trademark to Cochlear, Cochlear trademarked the acronym BAHA, to avoid confusion between the "BAHA", a sound processor and technology, and the Baha hearing aid.

Use and operation

Bone-anchored hearing aids use a surgically implanted abutment to transmit sound by direct conduction through bone to the inner ear, bypassing the external auditory canal and middle ear. A titanium prosthesis is surgically embedded into the skull with a small abutment exposed outside the skin. A sound processor sits on this abutment and transmits sound vibrations to the titanium implant. The implant vibrates the skull and inner ear, which stimulate the nerve fibers of the inner ear, allowing hearing.

Bone conduction hearing solutions use the screw shaped implant placed in the bone behind the ear through a minor surgical procedure. A skin penetrating coupling, called an abutment, is secured to the implant in the bone. A sound processor attached to the abutment will transmit the sound by direct conduction through bone to the inner ear, bypassing the external auditory canal and middle ear. The implant vibrates the skull and inner ear, which stimulate the hair cells of the inner ear, allowing hearing.

The surgery is often performed under local anaesthesia and as a day-care procedure. An important piece of information for the patient is that if they for whatever reason is not satisfied with the Baha solution, it is very easy to remove and the patient is back in the preoperative situation. No other ear surgical procedure is reversible like this.

Both Cochlear and Oticon Medical's Baha bone conduction hearing solutions utilize a bone-anchored sound processor that convert incoming sound into vibrations. Cochlear Baha utilises a snap-lock coupling which allows them to clip onto the implanted abutment, The Oticon Ponto uses a spring based coupling. The sound processor units run on small circular batteries which last approx 6 to 14 days. The coupling is designed to detach upon impact as a safety feature to prevent damage to the bone or surrounding tissues.

Surgical procedure

In adults, surgery is often performed under local anesthesia and as an outpatient procedure. The bone behind the ear is exposed through a U-shaped or straight incision or with the help of a special designed Baha dermatome. A hole, 3 or 4 mm deep depending on the thickness of the bone, is drilled. The hole is widened and the implant with the pre-mounted coupling is inserted under generous cooling to minimize surgical trauma to the bone.

Some surgeons perform a reduction of the subcutaneous soft tissue. The rationale for this is to reduce the mobility between implant and skin to avoid inflammation at the penetration site. This reduction of the soft tissue has been questioned and some surgeons do not perform any or a minimum of soft tissue reduction. The rationale for this is that any surgery will result in some scar tissue that could be the focus of infection. The infections seen early during the development of the surgical procedure could perhaps be explained by the lack of seal between implant and abutment allowing bacteria to enter the space. A new helium tight seal may be advantageous and prevent biofilm formation. This will also allow the surgeon to use longer abutments should there be a need. Three to six weeks later or even earlier the audiologist will fit and adjust the hearing processor according to the patient's hearing level. The fitting will be made using a special program in the computer.

The original surgical procedure has been described in detail by Tjellström et al. 2001

Everyday handling of skin at implant site

An area where skin penetrated requires care and cleaning. There is a risk of inflammation around the abutment. Daily cleaning is required.

Daily cleaning is not needed with Cochlear's DermaLock procedure which leaves hair and skin intact.

Indications for Baha

Chronic ear disease

This is a fairly common condition often associated with continuous or intermittent drainage from the ear canal. These patients may also have a hearing loss and need amplification. A conventional air conduction aid with a mold placed in the ear canal opening is may not be appropriate due to the drainage, and may even provoke drainage. If the hearing loss is significant an air conduction aid may have difficulty overcoming the dysfunction of the eardrum and middle ear bones. Bone conduction hearing device bypassing the middle ear may be a more appropriate treatment for these patients. Good transmission of sound in the bone, with reduced attenuation and distortion may be possible.

The Baha System can be tested the device surgery by attaching it to a plastic rod pressed against the bone behind the ear or for longer periods of time on a testband or softband.

Single sided deafness

A person who is deaf on one side may have functional difficulty hearing even when the other ear is normal,particularly in demanding situations such as noisy environments and when several people are speaking the same time. A complication in single sided deafness is hearing impairment in the hearing ear. Conventional ear surgery involves a risk of hearing loss due to the surgical procedure. Most ear surgeons are thus reluctant to perform surgery on an only hearing ear. The Baha surgery avoids this risk may be an appropriate treatment. A small study (n=50) found an extended trial a BAHA system with a headband prior to surgery led to more realistic expectations. In the study 50% wished to proceed to surgery.

External ear canal problems

Irritation in the external ear canal can be due to inflammation or eczema may be a condition for which a conventional air conduction aid is not an appropriate treatment. Direct bone conduction may be an option.

Malformations

Patients with malformations are not always suitable for reconstructive surgery. Treacher Collins syndrome patients may have significant malformations with ossicular defects and an abnormal route of the facial nerve. These structures as well as the inner ear could be at danger at surgery.

Patients with Down Syndrome may have a narrow ear canal and middle ear malformation leading to impaired hearing. It has been suggested{by whom}} that some part of the cognitive delay seen in these children is partly due to their poor hearing.

The surgery can only take place once the skull is at least 2.5 mm thick. Children who suffer certain syndromes may have a slighter build, thinner bone or unusual anatomy. Other children may have a thicker skull at a younger age and therefore it is difficult to give a specific age for surgery.

In Canada, stage one surgery for Baha has been implanted into children as young as 13 months at the Hospital for Sick Children in Toronto. Stage two surgery has been done as early as 22 months. In the U.S., the Food and Drug Administration (FDA) only approves Baha implantation of children aged 5 years or older.

For infants and young children prior to surgery the sound processor can be worn on a head band or soft band which the infant wears to hold a Baha against the skull.

Benefits

The benefits of Bone Anchored Hearing Aid such as Baha are well documented. By bypassing the outer or middle ear, Baha can increase hearing in noisy situations and help localise sounds. The benefit is not only improved speech understanding, hearing with Baha results in a natural sound with less distortion and feedback when compared with conventional hearing aids. The ear canal is left open for comfort, and helps to reduce any problems caused by chronic ear infections or allergies. In patients with Single-sided sensorineural deafness (SSD), Baha sends the sound via the skull bone from the deaf side to the inner ear of the hearing side. This transfer of sound gives a 360-degree sound awareness.

Drawbacks

The main drawbacks of BAHA systems are: risk of skin infection, accidental or spontaneous loss of the bone implant, and patient refusal for treatment due to stigma.

The surgical procedure is simple both for the surgeon and the patient. Very few and minor risk are at hand for the experienced ear surgeon. Minimal discomfort and pain is reported. Some numbness of the area around the implant is not uncommon as small superficial nerves in the skin are sectioned during the procedure. However, these problems often disappear after some time. Most importantly, there is no risk of further hearing loss due to the surgery.

The Baha has been "drop-tested" to stand "normal" trauma but some sound processor units are quite fragile if dropped. If the sound processor does not work as before the audiologist should be contacted.

Both manufacturers have warranty and repair policies. An audiologist or the manufacturers may provide "loaner" unit.

Special concerns in small children

Hearing is of utmost importance for a normal speech, intellectual and social development. See review paper by Lieu 2004. The skull bone in children is often very thin and also softer than in the adult. Surgery is thus often delayed until the age of 4 – 5 years. In the meantime the child with bilateral atresia will be fitted with a sweatband round the head called the Baha Softband with a coupling for a Baha. This is often made already at the age of one month. These small babies often tolerate this arrangement very well. For further reading se Papsin et al. 1997.

New developments

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DermaLock technology

Cochlear's DermaLock procedure, in use since 2012, allows the skin around the abutment area to be preserved by using a hydroxyapatite coating on the implant. The DermaLock surface promotes soft tissue integration by allowing more cell binding proteins to bind to the hydroxyapatite surface and orientates the proteins in a way that makes it easier for the cells to attach to the abutment. The hydroxyapatite coating on the DermaLock Abutment is applied with a plasma-spray technique. In most patients, the area around the implant is healed just a few days after surgery. This minimally invasive technique reduces surgery time and risk of complications such as numbness. It is also considered more aesthetically appealing for the patient.

A more powerful sound processor

Oticon Medical has introduced a sound processor that supports hearing loss down to 55 dB. There is also a special focus on increasing the output in the mid and high frequency range to reproduce louder sounds. This 6k to 9k bandwidth is especially important for kids understanding of sounds such as the syllables “s”. Stelmachowicz et al.(2001)

Oticon Medical's Bone Anchored Hearing devices are part of the Ponto family.

Feedback (howling sounds) shield

Oticon Medical has created a novel frequency shift designed to give users less whistling and fewer artificial sounds as they move through changing sound environments. They are reusing the same system, called the "Inium Feedback shield" that is being used for Oticon hearing aids.

The Baha Attract System

Cochlears Baha 4 Attract System is the world's first fully magnetic Baha system. It uses magnets to connect the sound processor to the implant. The sound processor attaches to the external magnet which transmits the sound to the internal magnet, which is completely hidden under the skin. This means the need for a skin penetrating abutment bis eliminated. This is a more visual appealing solution but with less power (lower volume) since the sound has to move through skin compared to via the titanium abutment.

The Cochlear Baha Attract System was cleared by the FDA in November 2013 and received the CE-mark in September 2013. It will be fully launched in February 2014.

The original Baha prototype has over the years been refined and improved. This goes for the external design as well as the many new functions.

Costs

In America the cost of the Baha device is approximately $ 4000. In The Netherlands the cost of the device is approx € 3000 (in 2008). The cost of the titanium implant, surgery and aftercare from surgeon and audiologist must also be considered. In America surgery cost can cost as much as $ 30,000 including the device and can vary depending upon the type and hospital, though most costs are covered by insurance. In the UK the procedure is usually offered on the NHS, although not all areas will do this for patients who retain hearing in one ear.

Manufacturers

At present there are four device brands available: Vibrant (MED-EL GmbH), Cochlear BAS, Oticon Medical, and Sophono, Inc. The sound processors are basically the same, with differences in design, features, and atachment type.

Vibrant markets the BoneBridge, Cochlear markets the Baha 3 System, Oticon Medical markets the Ponto Pro Plus, and Ponto Plus Power, Ponto Pro, Ponto Pro Power and Ponto brand receivers, and Sophono offers the Alpha System. Cochlear and Oticon Medical units attach onto the titanium abutment and act as receivers sending sound through the implant to the skull and to the inner ear. The two brands are distinguished from each other easily by appearance as the Baha 3 BP100 is rectangular and the Ponto is teardrop shaped. Cochlear has registered the name Baha as a trademark .

The Alpha System from Sophono uses implanted magnets to secure the processor behind the ear, while the BoneBridge from Vibrant MED-EL uses a transducer imbedded in the same way. The devices lie completely under the skin and allow for the implant site to close and heal completely, so no abutment is necessary.

Models

There are different types of systems. The Baha Classic and Compact are not manufactured anymore. In 2009 the Baha 3 (BP100) and Ponto pro were released. In May 2011 Oticon Medical launched a more powerful version of the Ponto Pro the Ponto Pro Power. Cochlear released the Baha 3 BP110 Power Sound Processor which can be fitted to the same patients as the Intenso. In 2013, Cochlear launched the new Baha 4 Sound Processor with 2.4 GHz wireless technology and a new Feedback management system. In 2013 Oticon Medical launched the Ponto Plus with more Power, new feedback management system and the Ponto Streamer for wireless connectivity.

COCHLEAR:

• Cordelle II. A bodyworn Baha for people with a severe hearing loss who need more amplification than the other Bahas available. The Cordelle II consists of a transducer which snaps onto the abutment and a bodyworn unit. This is the only Baha to have an induction telecoil receiver built in.
• Baha Classic 300. This is the older Baha device which has been mostly superseded by the Compact and Divino but is still worn by those with more severe and mixed hearing losses. This was discontinued in February 2007.
• Baha Compact. Rated as identical to the Classic but found to be slightly less powerful by a few users. The Compact is 33% smaller than the Classic and has added AGCo and improved shielding from mobile telephone signals.
• Baha Divino. Released in July 2005 this was the first digital Baha which had a built-in directional microphone.
• Baha Intenso. More power and clearer sound quality in all types of listening environments plus far less irritation from feedback.
• Cochlear Baha 3 BP100 Sound Processor) - released in 2009 the BP100 is a fully programmable, multi channel digital sound processor
• Cochlear Baha 3 BP110 Power Sound Processor - released in 2011 the Baha 3 Power is a higher powered version of the fully programmable digital sound processor
• Cochlear™ Baha® 4 Sound Processor was launched in 2013 and features 2.4 GHz wireless technology and the powerful Ardium platform.

OTICON MEDICAL:

• Ponto Pro introduced in 2009 is a more streamlined teardrop shaped baha receiver from a well known manufacturer of hearing aids, Oticon.
• Ponto Power was added by Oticon in 2011 as a higher-powered version of Ponto Pro, offering more volume with no discernible feedback.
• Ponto Plus and Ponto Plus Power was added in 2013; with more Power, new feedback management system and the Ponto Streamer for wireless connectivity.

SOPHONO

• Alpha 1: Released in 2006 in Europe and 2011 in the US. Curved shape, omnidirectional microphone.
• Alpha 2: Introduced late 2012. Symmetrical, rectangular shape for use on either ear, dual microphones for directionality plus omnidirectional for ambient sounds, comes with a coupon for free Skinit decal sticker. Both come in four different colors: silver, black, champagne, or brown.

References

1. Raicevich, George; Burwood, Eric; Dillon, Harvey (2008). "Taking the pressure off bone conduction hearing aid users". Australian and New Zealand Journal of Audiology. 30 (2): 113–8. doi:10.1375/audi.30.2.113.
2. Mylanus, E.A.; Snik, A.F.; Cremers, C.W. (1994). "Influence of the thickness of the skin and subcutaneous tissue covering the mastoid on bone-conduction thresholds obtained transcutaneously versus percutaneously". Scandinavian Audiology. 23 (3): 201–3. doi:10.3109/01050399409047509. PMID 7997838. {{cite journal}}: Cite has empty unknown parameter: |month= (help)
3. US Patent 4,498,461. Coupling to a bone-anchored hearing aid
4. Tjellström, A.; Håkansson, B.; Granström, G. (April 2001). "Bone-anchored hearing aids: Current status in adults and children". Otolaryngologic Clinics of North America. 34 (2): 337–64. doi:10.1016/S0030-6665%2805%2970335-2. PMID 11382574.
5. Pennings, R.J.E.; Gulliver, M.; Morris, D.P. (2011). "The importance of an extended preoperative trial of BAHA in unilateral sensorineural hearing loss: a prospective cohort study". Clinical Otolaryngology. 36 (5): 442–9. doi:10.1111/j.1749-4486.2011.02388.x. PMID 21883961.
6. Tjellström A, Håkansson B, Granström G. Bone-anchored hearing aids: current status in adults and children. Otolaryngol Clin North Am. 2001 Apr;34(2):337-64.
7. Papsin BC, Sirimanna TK, Albert DM, Bailey CM. Surgical experience with bone-anchored hearing aids in children. Laryngoscope. 1997 Jun;107(6):801-6.
8. Flynn MC, Sadeghi A, Halvarsson G. Baha solutions for patients with severe mixed hearing loss. Cochlear Implants Int 2009;10 Suppl 1:43-7.
9. Lin LM, Bowditch S, Anderson MJ, May B, Cox KM, Niparko K. Amplification in the rehabilitation of unilateral deafness: speech in noise and directional hearing effects with bone-anchored hearing and contralateral routing of signal amplification. Otology and Neurology 2006;27(2):172-82
10. Hol MK, Snik AF, Mylanus EA, Cremers CW. Long-term results of bone-anchored hearing aid recipients who had previously used air-conduction hearing aids. Arch Otolaryngol Head Neck Surg 2005 Apr;131(4):321-5.
11. Watson GJ, Silva S, Lawless T, Harling JL, Sheehan PZ. Bone anchored hearing aids: a preliminary assessment of the impact on outpatients and cost when rehabilitating hearing in chronic suppurative otitis media. Clin Otolaryngol 2008;33:338–342.
12. Snik AF, Mylanus EA, Proops DW, Wolfaardt J, Hodgetts WA, Somers T, Niparko JK, Wazen JJ, Sterkers O, Cremers CW, Tjellström A. Consensus statements on the Baha system: Where do we stand at present? The Annals of Otology, Rhinology & Laryngology 2005 Dec;114(12) Suppl 195:1-12.
13. Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1159/000323715, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1159/000323715 instead.
14. 4. Lieu JE. Speech-Language and Educational Consequences of Unilateral Hearing Loss in Children. Arch Otolaryngol Head Neck Surg. 2004;130:524-530.
15. 5. Papsin BC, Sirimanna TK, Albert DM, Bailey CM. Surgical experience with bone-anchored hearing aids in children. The Laryngoscope, 1997;107(6):801-6.
16. Stelmachowicz PG, Pittman AL, Hoover BM, Lewis DE. Effect of stimulus bandwidth on the perception of /s/ in normal‐ and hearing‐impaired children and adults. J Acoust Soc Am. Oct 2001;110:2183‐2190
17. Trademark 783134 registered in Sweden on 11 June 2002

External links

• Baha implant activation video Folk musician Patrick Costello filmed the activation of his Baha implant.
• The Ear Foundation's pages. Detailed information about bone anchored hearing aid technologies from the UK-based charity.
• French BAHA website. Site francophone dédié à l'aide auditive à ancrage osseux: Baha, Ponto, Alpha 1
• Microta and Atresia Facebook Group

• Use dmy dates from March 2011
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