## Anatomical Site of Otosclerosis **Key Point:** Otosclerosis is a primary disease of bone remodeling that affects the **stapes footplate at the oval window niche**, specifically at the anterior oval window margin (fissula ante fenestram). ### Pathological Focus The disease process involves abnormal bone remodeling (osteoclastic resorption followed by disorganized osteoblastic new bone formation) in the otic capsule. The **stapes footplate** becomes progressively fixed to the oval window, preventing normal ossicular chain vibration. ### Why the Stapes? The oval window is the only mobile membrane in the ossicular chain. When the stapes footplate becomes fixed due to otosclerotic bone, conductive hearing loss results because the stapes cannot transmit vibrations to the perilymph of the inner ear. **High-Yield:** The anterior oval window margin (fissula ante fenestram) is the **most common site** of otosclerotic focus — this is the embryological remnant of the otic capsule and is the weakest point for bone remodeling. ### Natural History - Begins at oval window niche → progressive fixation of stapes footplate - May later involve cochlear capsule → sensorineural component (cochlear otosclerosis) - Malleus and incus are NOT primarily affected in otosclerosis **Clinical Pearl:** Carhart's notch (bone conduction loss at 2 kHz) on audiometry is a signature finding when cochlear otosclerosis develops. 
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