Groundbreaking research in bone conduction technology

“It sounds different because your own voice is also heard, not only out from the mouth via air conduction, but also via so-called bone conduction. In bone conduction, vibrations are conducted from the speech organ through the skull bone to the inner ear, but the only sound that reaches the microphone during a voice recording is what comes from the mouth and does therefore not have the same characteristics as the sound we experience when we hear ourselves speaking.”

What is bone conduction used for?

An important application of bone conduction is within the field of hearing aids, where the benefit is that the vibrations are conducted through the skull bone to the inner ear and thus by-passing a hearing loss originating in the outer or middle ear. For example, some patients can lack a normal functioning outer or middle ear so the sound cannot reach the inner ear via the ear canal. In that case, the person has a so-called conductive hearing loss. Bone conduction can also be used for bone conduction headsets, which produce audible vibrations in the skull bone – these are used instead of regular headphones and allow you to keep the ear canals free to hear what is happening nearby, for example in traffic. Diagnosis of balance disorders can also be facilitated by use of bone conduction, and yet another area of application is audiometry, to find out the type and degree of a hearing impairment.

“When you measure the bone conduction thresholds, you use test signals from a vibrating transducer (speaker that produces vibrations) behind the ear. The difference between the air conduction and the bone conduction hearing thresholds shows if a conduction hearing loss exist and thus if a bone conduction hearing aid may help," says Sabine Reinfeldt, Associate Professor at Biomedical Signals and Systems.”

BAHA and BCI

BAHA, Bone Anchored Hearing Aid, is anchored through the skin and into the bone with the help of a screw, stimulating the skull bone without skin in between. The hearing aid itself can be taken off and put on.

“One of the advantages of BAHA, which Bo Håkansson, Professor at Biomedical Signals and Systems, has developed through collaboration with Sahlgrenska University Hospital and the Brånemark Osseointegration Center, is that the microphone and the transducer can be worn on the same side of the head. Previously, you had to have microphone and transducer on different sides of the head to avoid feedback problems. Another advantage is that no static pressure over the skin is needed as for previous bone conduction hearing aids and that there is no attenuation of the vibrations over the skin since the vibrations reach the skull bone directly via the titanium screw," says Sabine Reinfeldt.

BAHA has been given to approximately 400,000 patients since it all started, and this hearing system is manufactured by two competing companies in the Gothenburg area.

BCI, Bone Conduction Implant is an implanted bone conduction hearing aid that is a further development of the BAHA system by completely implanting the transducer so that the skin can be kept intact. The sound is transmitted wirelessly through the intact skin via magnetic induction from an external sound processor. A major advantage of BCI is to conduct sound into the skull bone without a permanent skin penetration. The clinical study for the BCI started in 2012, and today there are 16 patients with this implant and the first surgeries were performed by MD Assoc Prof. Måns Eeg-Olofsson at the Sahlgrenska University Hospital in Gothenburg. The BCI patients have been followed for up to five years and the follow-ups have shown that the BCI is safe and effective for the patients.

Since the transducer must be small enough to fit in the bone behind the ear, Bo Håkansson developed a new transducer technology for the BCI system, called BEST, Balanced Electromagnetic Separation Transducer. It is a smaller transducer compared to the transducer in the BAHA and it is placed in the bone during surgery. It is also more reliable and efficient and provides less distortion of the sound. Other applications for the BEST transducer is the B250, which is used for balance and dizziness diagnostics, as well as in the B81, which is an audiometric transducer used for bone conduction hearing threshold measurements.

Verification of bone conduction hearing aids

To know if the patient has received the best possible fitting of their hearing aid, certain tests need to be done. For conventional air conduction hearing aids, there are objective, standardised methods, but for bone conduction hearing aids, you mostly rely on what the patient says about different types of sounds after the basic fitting is made. Therefore, the research group is focusing on developing a new method to optimize fitting that works for all types of bone conduction hearing aids. The proposed method is to use a skin microphone to measure the sound on the patient's forehead which is radiated from the bone conducted vibrations in the skull bone. Speech played through the implant at conversational level is then measured by the skin microphone, and it is controlled that the measured sound is within the patient's hearing range – otherwise the fitting for the hearing aid will need to be adjusted. The research group has an ongoing clinical study on 29 patients with several different types of bone conduction hearing aids to evaluate the benefit of the method. This research is an important part of the doctoral thesis of Audiologist and PhD student Ann-Charlotte Persson at Gothenburg University who will defend the work during late spring 2024.

The skin microphone has several areas of use, and the research group is now also looking at the benefits of using it as a kind of bone conduction stethoscope. With a stethoscope, you can listen to the vibrations in the skull bone radiating from the head, and thus you can listen to the vibrations from the hearing aid that is approximately as the patient hears the sound.

“It would be very good if an Audiologist could hear the sound generated by the device in order to get an idea of what or how well the patient hears certain sounds," says Karl-Johan Fredén Jansson.

Improved diagnostics of dizziness and balance disorders

B250 and VEMP

With the help of the B250 transducer technology, low-frequency vibrations can be used to improve diagnosis of dizziness and balance disorders. With vibration stimuli towards the head at low frequencies, a reflex is triggered in the vestibular organ, causing eye- and neck muscles to react. Using electrodes on the skin, one can measure muscle responses from the eye- and neck muscles related to the vibration stimulation. This measurement is called VEMP and is short for “vestibular evoked myogenic potential”, where the sound stimulation is done with either air or bone conduction sound. B250 has been shown to improve the quality of bone conduction VEMP responses which is beneficial for the diagnosis of so-called “Superior Canal Dehiscence Syndrome” (SCDS). SCDS means that the patient has a small opening in the superior semicircular canal of the vestibular system, which causes dizziness and makes them hypersensitive to low-frequency vibrations. For example, these patients can hear their own eye movements or heart beats, a phenomenon called autophony or hypersensitivity to internal body sounds. If the patient has SCDS, the VEMP measurement often results in increased myogenic amplitudes, especially at low-frequency stimulation.

Another diagnostic method for SCDS patients is ankle audiometry (AA) with B250, which is similar to a regular hearing test, but instead of placing a transducer on the head, the B250 is placed on the ankle. For a certain and relatively low sound level at the frequency 250 Hz, only patients with SCDS can hear the sound as they are more sensitive to sound that propagates in the body via bone conduction. With the B250, the stimulation is more precise than other methods, and the method is a simple way to distinguish between patients with and without SCDS. The AA method has been developed together with MD PhD Luca Verrecchia at Balance clinic, Karolinska University Hospital who is conducting his Postdoc in our research group in a where he also performs VEMP studies on infants as a part of the program for screening of the balance function. This examination becomes safer using bone conduction as it can be done with lower stimulation levels. In another project, the possibility of performing simultaneous measurements of VEMP for eye- and neck muscles is being tested. Clinically, VEMP measurement of eye and neck responses are performed separately, but with bone conduction, it is possible to do these simultaneously to save time. Several projects where bone conduction is used in diagnosis of balance disorders are also in progress.

Auditory Brainstem Response – a test where the patient does not need to be active

Another ongoing project linked to the use of B250 is “auditory brainstem response” (ABR), which is an objective hearing test where the patient does not need to actively participate in the hearing test by pressing a button when they hear sounds. Instead, the propagation of nerve signals from the inner ear to the brain's inner hearing organs is measured using electrodes on the skin behind the ear when the ear is exposed to sound. A challenge for these patients is that the bone conduction transducer causes electromagnetic noise also picked up by the electrodes as an artifact that complicates the interpretation of the measurement result. This is especially challenging when measuring on children or patients with smaller heads where the electrodes need to be placed close to the transducer. However, it has been shown that B250 radiates much lower electromagnetic noise than conventional audiometric transducers and a project is ongoing to further reduce this noise with different types of shielding methods.

Collaborations

• Sahlgrenska University Hospital
• Habilitation and Health, Hearing Organization, Region Västra Götaland
• Karolinska University Hospital, Stockholm
• Universitätsklinikum Halle, (Saale), Germany
• University of Alberta, Edmonton, Canada
• Audioscan, Ontario, Canada
• The University of Sydney, Australia
• Ortofon A/S, Nakskov, Denmark
• Oticon Medical, Askim
• Universitätsklinikum Bonn, Bonn, Germany