Otosclerosis involving the oval window of the middle ear cleft may cause varying degrees of stapedial ankylosis. Impairment in the mobility of this essential link in the ossicular chain is reflected in the acuity of hearing. Thus a mechanical defect is introduced into the hearing system, which is expressed audiometrically as tympanic deafness. The bone conduction thresholds according to pure tone audiometry are normal or variably increased and the air conduction thresholds are elevated to a level proportional to the degree of stapedial fixation but not exceeding 50 to 60 decibels without some loss of bone conduction acuity. (Fig. 1) When the maximum loss in air conduction acuity is reached, the stapes is generally considered to be firmly fixed, at least in so far as sound pressure excitation is concerned. However, when the air-bone conduction thresholds do not differ by more than 30 decibels, the stapes may be partially mobile. (Fig. 2) This difference between the air and bone conduction thresholds is referred to as the cochlear reserve and indicates the potential of cochlear function, were the mechanical defect of stapes fixation to
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