Otosclerosis is caused by the accumulation of abnormal bone growth onto the smallest of the three hearing bones—the tiny stirrup-shaped bone called the stapes. The stapes is the third hearing bone, and its motion on the footplate of the inner ear (located behind the ear drum) moves the fluid within the inner ear and allows us to hear. The over-growth of bone associated with otosclerosis may cause the stapes bone to become fixed, thus restricting its motion and resulting in hearing loss.