| Abstract: | The fact that a vibration of the skull causes a hearing sensation has been known since the 19th century. This mode of hearing was termed hearing by bone conduction (BC) or BC sound. Although more than a century of research on hearing by BC, its physiology is not completely understood. Recently, the field of BC physiology has gained interest due to the use of the Bone Anchored Hearing Aid (BAHA®) and the use of BC transmission for communication in noisy environments, where BC transmission offers potential advantages for picking up the sound as well as receiving the sound. The theory identifies several contributing factors to BC hearing of which five are presented here: 1. sound radiated into the external ear canal, 2. middle ear ossicle inertia, 3. inertia of the cochlear fluids, 4. compression of the cochlear walls, and 5. pressure transmission from the cerebrospinal fluid. Of these five, inertia of the cochlear fluid seems to be the most important.It is also suggested from tone cancellation and model experiments that the stimulation mode of the basilar membrane is identical for air conduction (AC) and BC, i.e. the hydrodynamics in the cochlea always sets up a traveling wave on the basilar membrane in the basal part wherever the stimulation is. However, these theories can sometimes not fully explain clinical BC results. The pressure distribution in the cochlear fluids is very different for BC and AC stimulation; a spatial pressure generation for BC is set up. Inertial effects of the osseous spiral lamina and organ of corti may give stimulation directly on the basilar membrane not considered by classical BC theories.The transmission of the vibrations in the skull influences the hearing by BC. The stimulation position, resonances and anti-resonances of the skull together with attenuation of vibrations in the skull, result in unequal stimulation of the cochleae and lateralization of the BC sound. Even if BC sound is believed to reflect the true cochlear function, external lesions such as middle ear diseases affect the BC sensitivity. However, a middle ear lesion affects BC sensitivity considerably less than it does AC sensitivity. The BC route can also be used for hearing aids; since the BC route is less efficient than the AC route, BC hearing aids are mainly used with hearing losses where AC hearing aids are contraindicated. Of the BC hearing aids, the Bone Anchored Hearing Aid shows the best result Lately it has been shown that, even if BC sound from one stimulation position is transmitted to both cochleae, a bilateral fitting of BAHAs is beneficial for the patient. Further, BAHA® is also being used to aid persons with severe unilateral hearing loss or single sided deafness. |
