Otosclerosis is a disease of bone that is limited to the third hearing bone, the stapes, and the bone surrounding the inner ear (cochlea). In the early stages of the disease, otospongiosis, areas of "softened" bone appear, particularly around the front portion of the stapes. The soft patches of bone contain chemicals (enzymes) that are capable of dissolving bone. With time, the bone destruction stops, and the otospongiotic bone hardens (calcifies). In non-obliterative otosclerosis, the stapes footplate has a "blue" appearance in contrast to the normal yellow-white color of the normal footplate.

Otosclerosis is most often caused when one of the bones in the middle ear, the stapes, becomes stuck in place. When this bone is unable to vibrate, sound is unable to travel through the ear and hearing becomes impaired. Why this happens is still unclear, but scientists think it could be related to a previous measles infection, stress fractures to the bony tissue surrounding the inner ear, or immune disorders. Otosclerosis also tends to run in families. It may also have to do with the interaction among three different immune-system cells known as cytokines. Researchers believe that the proper balance of these three substances is necessary for healthy bone remodeling and that an imbalance in their levels could cause the kind of abnormal remodeling that occurs in otosclerosis.