## Pathological Site in Otosclerosis **Key Point:** Otosclerosis is characterized by abnormal bone remodeling (osteoclasia followed by osteosclerosis) that occurs specifically at the **oval window**, most commonly at the anterior oval window margin. ### Mechanism of Stapes Fixation The disease process involves: 1. **Osteoclastic resorption** of normal otic capsule bone 2. **Replacement with abnormal, hypervascular bone** (otosclerotic focus) 3. **Gradual fixation of the stapes footplate** within the oval window niche 4. **Loss of stapes mobility** → conductive hearing loss ### Why the Oval Window? The oval window is the **only mobile ossicle articulation** in the middle ear. The stapes footplate normally vibrates freely within the oval window membrane. When otosclerotic bone encroaches on this space, the footplate becomes progressively fixed, preventing sound transmission to the inner ear. **Clinical Pearl:** The anterior oval window margin is the most common site (>90% of cases), followed by the posterior margin. Rarely, otosclerosis can extend to involve the round window or cochlear capsule (cochlear otosclerosis). **High-Yield:** Stapes fixation at the oval window causes a **progressive conductive hearing loss** with a characteristic **Carhart bone** (bone conduction better than air conduction at 2 kHz). 
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