Tonndorf (1968) found that there are three different forces that contribute to the forces needed to stimulate the cochlea: Distortional, Inertial (Ossicular), and External canal (Osseotympanic)[1] As vibrations compress the bones of the skull, pressure is put on the otic capsule and the membranous labyrinth.
Some of this energy hits the tympanic membrane and combines with inertial bone-conduction, stimulating the inner ear.
The goal of bone ABR is to estimate cochlear function and to help identify the type of hearing loss present.
Techniques and results for bone-conduction auditory brainstem responses are presented in a review chapter by Stapells,[3] as well as in a detailed assessment protocol by the British Columbia Early Hearing Program (BCEHP).
Atresia, microtia, otitis media and other outer/middle ear abnormalities, as well as infants with sensorineural hearing loss, will require the use of bone-conduction ABR testing.
Infants who have a considerable amount of amniotic fluid in their middle ear space may need to be tested with BCABR.
With Bone ABR, the waves are typically more rounded that with traditional auditory brainstem response.
Mauldin & Jerger (1979) found that for adults, the Wave V latencies derived from bone-conduction ABR are approximately 0.5 ms longer than the same intensity level of air conduction.
Stapells and Ruben, in 1989, demonstrated bone-conduction tone burst ABRs in infants with conductive hearing loss.
This asymmetry and higher travelling wave velocity at the base explains why the ABR is biased towards the high frequencies.
Stapells recommends using alternating polarity to reduce stimulus artifact, especially with tone burst stimuli.
(Contrary to some suggestions, there is no evidence that thresholds for single-polarity tone bursts (e.g., rarefaction) are better than those to alternating polarity.