LVII Animal Experiments in Bone Conduction: Clinical Conclusions

Abstract

A comprehensive study on bone conduction (BC), yielded results some of which have direct bearings on clinical BC problems. A combination of a large otosclerotic focus on the footplate with minimal fixation may produce appreciable BC losses which may stimulate a concomitant sensorineural impairment. Experiments in cochlear models have indicated that the compressional mode of BC stimulation can function even in the absence of any cochlear pressure outlets. Animal experiments showed that, although a compliant round window improves the response to compressional BC, occlusion of both cochlear window does not have a very pronounced detrimental effect. The cochlear aqueduct was found to be an auxiliary pressure outlet of scala tympani, but only for BC stimulation. Carhart''s notch was shown to be due to the elimination of the middle ear. In particular, the magnitude of the loss is given by the relative contribution of the middle ear to the total BC response, and the place along the frequency scale by the resonant point of the ossicular chain in response to BC stimulation. The occlusion effect of the external ear canal (Bing test) is caused by a combination of a) elimination of the high-pass filter effect of the open ear canal and b) alteration of the resonant properties of the external ear canal. The first factor is responsible for the low-frequency emphasis, the latter for the sharply defined changes in the middle to high frequencies. Both affect the so-called osseotympanic component of BC. Explanations are also given for the clinical tests of Runge, Weber, and Gelle. It is to be noted that, although the phenomena incurred are quite similar in all four cases (including the Bing test), the explanations differ slightly, but significantly, from one another.