Concept and Mechanism of Action
The bone-anchored hearing aid (BAHA) system operates based on the principle of bone conduction. It comprises a microphone, sound processor, and transducer that are attached to the skull, allowing amplified sound signals to be directly transmitted to the skull, where they generate vibrations in the cochlea that are perceived as hearing.
BAHA is a partially implantable hearing device consisting of three main components:
- A titanium screw, which is implanted into the skull at the mastoid bone.
- An abutment, resembling a bolt, that passes through the skin and connects to the screw implanted in the bone.
- An external sound processor, which is an integrated unit that houses the microphone, speech processor, oscillator, and battery. This processor connects to the abutment via a rotating axis, transmitting sound vibrations directly to the skull and cochlea, where they induce vibrations in the inner ear lymphatic fluid.
Indications
BAHA is primarily used in patients with conductive hearing loss, mixed hearing loss, or sensorineural hearing loss with bone-conduction thresholds in the mild to moderate range, particularly for those who cannot use air-conduction hearing aids.
- Patients with narrowing or closure of the external auditory canal, chronic inflammation in the middle ear or auditory canal, or unmanageable otorrhea with hearing loss.
- Patients with moderate to severe hearing loss who cannot tolerate air-conduction hearing aids due to discomfort from occlusion or intolerable feedback whistling.
- Patients with complete unilateral deafness who desire binaural hearing effects.
For hearing compensation, the average pure-tone bone-conduction threshold (PTAbc) should be ≤45 dB HL, and the maximum speech recognition score (SRSmax) should be ≥80%. Cases with PTAbc ≥70 dB HL and SRSmax ≤60% are generally not considered suitable.
Outcomes and Complications
BAHA directly transmits sound vibrations to the inner ear through the skull, with minimal sound attenuation and distortion. This results in significant improvements in post-operative hearing sensitivity, speech resolution, and sound localization ability. Statistics indicate that 78% to 89% of patients are satisfied with the outcomes and continue to use the device long-term.
As a partially implantable hearing device, BAHA involves an abutment that crosses the skin and attaches to an external sound processor. Some patients may experience discomfort or infection at the implantation site, which has motivated ongoing development of fully implantable bone-conduction hearing devices and vibrating sound bridges.