Otosclerosis is an abnormal, microscopic growth of bone in the walls of the inner ear which causes the stapes bone commonly called the "stirrup" to become frozen in place or "fixed". Normally the stapes, the smallest bone in the body, vibrates freely to allow the transmission of sound into the inner ear. When it becomes fixed to the surrounding bone, it prevents sound waves from reaching the inner ear fluids and hearing is impaired.

In a normal ear, sound (which consists of vibrations of air molecules) is funneled by the auricle, or "outer ear", through the ear canal to the eardrum. Movements of the eardrum are transferred to the ossicles, the small bones of the middle ear. The eardrum and ossicles act as an amplifier and transducer, changing sound into a mechanical energy. 

At the end of this chain, the stapes vibrates in and out of the oval window, a thin membrane between the air-filled middle ear and fluid-filled inner ear. The vibration of the stapes sets up a wave of movement of the inner ear fluids. This, in turn, stimulates the fine sense organs (the hair cells) of the inner ear, which then stimulate the auditory (hearing) nerve, which carries the sound energy to the brain. It is this sequence of events that results in normal hearing.