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Abstract: A study of the temporal bone collection of the University of Minnesota was carried out to verify the patterns of growth and expansion of the otosclerotic lesion, and evaluate its relationship to hearing loss. From 144 human temporal bones with otosclerosis, 117 (81.25%) were found to have an otosclerosic lesion anterior to the oval window. These were evaluated according to their evolution, activity, size, degree of cochlear endosteum involvement, fixation of the stapedial footplate; and with sensorineural hearing loss. Results showed that these lesions had a variable pattern of expansion from the region anterior to the oval window, sometimes towards the margin of the window, sometimes towards the apex the cochlea, on occasions superiorly, or towards round window region. However, in these patterns expansion the lesions showed an almost circular shape found by the measurement of their vertical and horizontal sizes. The authors found that 74.35% of the lesions were symmetric. Lesions invading the cochlear endostem without the concomitant presence of lesions anterior to the oval window was found in only 1.4% of the cases. The activity of the lesions increased directly in relationship to their size; smaller lesions being predominantly inactive and medium and larger ones being predominantly active. The correlations among size, activity, and sensorineural hearing loss showed that: 1) The lesions do not necessarily follow the sequence of an initial active stage to a final inactive one. 2) Lesions that could theoretically cause sensorineural hearing loss without fixation of the stapedial footplate were extremely uncommon, and even in these cases we found that the degree of cochlear endosteum involvement would not cause hearing loss of a worse degree than that found for bones without endosteal involvement. Based on our findings there is no histopathological basis to consider "clinical cochlear otosclerosis" as a common disease entity, this occurring theoretically in less than. 15% of the caucasian population.
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