Otosclerosis is an ear disease that can cause hearing loss over time. The bones in the middle ear develop spongy growths of new bone (sclerosis); the growths are usually on the stapes, a middle-ear bone that leads to the inner ear. Because the new bone growth makes it difficult for the stapes to move properly, it is difficult for sound to be conducted to the inner ear. This type of otosclerosis is sometimes called stapedial otosclerosis, after the stapes.
Another, less common type of otosclerosis is cochlear otosclerosis. In this kind of otosclerosis, the filaments in the cochlea (part of the inner ear) become less flexible and eventually die.