The BAHA and beyond: Bone conduction is market-ready

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Published on 03 April 2014

The Bone Anchored Hearing Aid (BAHA) approaches its thirtieth birthday since market introduction. A lot is happening on the BAHA-front, says its inventor, professor Bo Håkansson. A bone anchored hearing aid that doesn’t need a skin penetration and at the same time compensates the loss of 10 to 15 dB due to a wireless solution is technically ready. The first patients have already been implanted with the Bone Conduction Implant (BCI) in a clinical study. Market introduction is scheduled for this year, but it is still uncertain under which brandname the BCI will see the light.

In 1984, professor Bo Håkansson got his Ph.D. for his thesis ‘The Bone Anchored Hearing Aid – Engineering aspects’ at Chalmers University of Technology, Göteborg, Sweden. This thesis arguably marks the birth of the Baha®, a brandname registered by Cochlear Corp. This name has also become the general name of the device. BAHA’s are by now implanted in more than 100,000 people all over the world. They offer a hearing solution to patients with mechanical hearing impairment, following for example chronic otitis or bone diseases. Also, they can be used in the case of congenital malformations of the outer ear, the hearing canal or the middle ear.

Håkansson looks back at the development: “One of my promotors was Professor Brånemark, the father of dental implants. One other important person was Professor Anders Tjellström who was the Ear-Nose-Throat surgeon who developed the surgical technique and promoted this system worldwide. The thought at the outset of research in 1977 was, that if titanium implants in the jaw where possible, why not then also behind the ear to improve bone conduction hearing?” The principle of bone conduction was already well known. Conductive hearing loss, as opposed to nerve-related hearing loss, can be totally reversed by restoring conduction. “A person can get back to the same natural hearing that he had before. Bone conducting hearing aids already existed. These had, however, major drawbacks because they were applied against the skin. In order to benefit from the bone conduction, the transducer had to press relatively hardly on soft tissue, causing discomfort and skin irritation. Moreover, a headband was needed to apply the pressure. Still, the vibration first had to pass the skin and soft tissue before entering the skull bone and being transmitted to the inner ear. We believed that an implant would be able to guide and more effectively transmit the vibration directly to the inner ear and would therefore provide an attractive alternative.”

The new configuration consisted of an implant anchored in the bone, abutment through the skin and a coupling arrangement to an external soundprocessor, which had the microphone and the transducer in the same housing. This configuration demanded a completely different hearing device, with adjusted signal processing. Also, feedback problems had to be tackled. “I worked with three patients for three years until I had something that worked”, Håkansson remembers. “But in my thesis I was already able to evaluate the first fifty patients with an implant connected to an optimised single housing design.”

Delicate structure

Håkansson was involved in the business developments that followed the invention, but eventually he resumed his research at Chalmers University to further develop the product. “For instance to address the problems around skin penetration. Twenty to thirty per cent of the patients develop some skin problem within ten years. In ten to fifteen per cent, the implant is lost at some time. Furthermore, whereas in Northern Europe the abutment is often accepted, in Southern European countries patients are far less willing to have an abutment for aesthetical reasons.”

So the question for further research was: could permanent skin penetration be avoided by placing the transducer in the bone under the skin and making the transmission from the sound processor via a wireless link through two coils – one implanted and one in the external unit? That is exactly what is used in Cochlear implants and in some middle ear implants. Håkansson: “But 10 to 15 dB of the signal gets lost through the wireless induction transmission. Not all patients can afford that loss. I wondered whether it also could be realised without this loss.”

According to Håkansson, the answer depends on the attachment in the bone. He went looking for an alternative more sensitive transducer location. What if the vibrator could be moved from the thinner skull 55 millimeter behind the ear (where the receiving coil will remain), to 20 millimeters behind the earcanal, in the mastoid portion of the temporal bone? “The difficulty is, that it is a spongeous kind of internal bone, a delicate structure where it is difficult to attach a retension screw.

Another complicating factor is that the facial nerve is embedded in this bone, at a location which is not exactly known on beforehand. You cannot just drill in it. So it is not easy, but it can be done by placing the vibrator not deeper than 5 to 6 millimeters in the mastoid bone. Moreover, it can be anchored without a screw.”

With a laser Doppler beam measurement via the ear canal different bone conduction systems, depending where they are attached, can be compared. As it turns out, the gain lost to the wireless connection is won back by the higher sensitivity to vibrations at the alternative location in the mastoid bone.

Novel transducer

The use of such an alternative implant location could not be attained without altering the transducer. “As it happened, MED-EL owned the Vibrant Soundbridge®,a middle ear implant, invented by Geoffrey Ball in the 1990s”, says Håkansson. “We decided to do a joint study in cadavers to investigate the possibilities of a bone anchored hearing aid without skin penetration. This led to the conclusion that a transcutaneous system without skin penetration would be viable. The introduction of MED-EL’s BoneBridge® paved the road also for us. It included CE-approval for the BoneBridge, something we also need for our Bone Conducting Implant (BCI), as we call it. The BoneBridge is different from our device. Because of the location we chose in the mastoid bone, and because of a unique Balanced Electro Integrated Separation (BEST) transducer which is less than half the size of MED-EL’s present transducer.”

The BEST transducer is at the same time small, efficient and reliable, and has also been developed in one other application for audiometric hearing threshold diagnostics in cooperation with the Danish company Ortofon Microtech, renowned for its turntable cartridges. They are now also specialised in these bone conductors for audiometry and this product is marketed by Interacoustics under the trade name Radioear B81.

Interacoustics is a worldwide operating company in audiometry under the William Dermant Holding (WDH) group in Denmark. “In our development, together we focused on minimising distortion in low frequences. The resulting new transducer principle BEST has improved properties, combining low distortion with high output. It is about to enter the market in its own right.”

Ortofon Microsystems manufactures the BEST transducers under license from Osseofon - the IP holding company emerged from Chalmers University of Technology. Apart from transducers for hearing aids and for hearing diagnostics there is also an application for communication systems. Håkansson: “It is also applicable for use in communication systems for safety and military applications. For these applications it is attractive to have communication coming in, but at the same time to leave both ear canals open to still enable hearing natural environmental sounds. The rights for this particular application were developed in a company called Oiido emeraged from Chalmers Entrepreneur school and licensed to 3M subsidiary Peltor. This application never took off commercially as was expected. The license was therefore later terminated but is still a pending application that even has been considered by Google in their “Google glasses.”

CE-marking forthcoming

For the BCI the BEST transducer was an important step, but not the final one. A significant grant from Vinnova (Swedish Governmental Agency for Innovation Systems) in 2008 enabled further development of the system. “In October 2012, we got approval from the Swedish Medical Device agency (Läkemedelsverket) for doing BCI surgery in a clinical study at Sahlgrenska University Hospital in Göteborg, Sweden, as basis for a future CE-marking. In December 2012 the first patient received the BCI, thanks to surgery by doctor Måns Eeg-Olofsson. In February 2013 the second patient received a BCI. As results were successful, the project moved on with two other patients in May and two in October.

“Six patients are most likely sufficient, but in order to get CE-marking we have to successfully conclude a six month follow-up after the last implantations”, Håkansson states. “Results in safety and effectiveness are very good, we have not witnessed any significant complication and hearing rehabilitation results are in general very good indeed.”

Now, a few months before CE-marking can be attained, things are getting really exciting. The Intellectual Property (IP) of the new system is kept by Håkansson’s company Osseofon. “Negotiations with various parties are ongoing, about what company will manufacture the BCI system. As yet, it is still unknown what brandname will feature on the device.”