A 28-year-old woman with a 5-year history of chronic suppurative otitis media and conductive hearing loss undergoes high-resolution CT temporal bone. The scan shows a mass in the middle ear eroding the long process of the malleus and stapes superstructure. What is the most common ossicle eroded in cholesteatoma?

Explanation

## Most Common Ossicle Eroded in Cholesteatoma **Key Point:** The incus is the most frequently eroded ossicle in cholesteatoma, occurring in 50–80% of cases, followed by the malleus and stapes. ### Why the Incus Is Most Vulnerable #### Anatomical Factors 1. **Precarious blood supply** — The incus has a limited vascular pedicle, making it susceptible to ischemic necrosis 2. **Exposed position** — The long process of the incus is directly exposed to the attic and sinus tympani 3. **Lack of periosteal protection** — Unlike the malleus, the incus has minimal periosteal coverage in the attic 4. **Pressure erosion** — Expanding cholesteatoma matrix directly compresses the long process #### Mechanism of Erosion - Enzymatic degradation (collagenase, proteases from cholesteatoma epithelium) - Pressure necrosis from expanding mass - Osteoclastic resorption induced by inflammatory mediators - Ischemic necrosis from vascular compromise ### Frequency of Ossicular Erosion | Ossicle | Erosion Frequency | Mechanism | Clinical Impact | |---------|-------------------|-----------|------------------| | Incus | 50–80% | Pressure + ischemia | Severe CHL | | Malleus | 30–40% | Pressure erosion | Moderate CHL | | Stapes | 20–30% | Rare; late finding | Severe CHL if superstructure eroded | | Tensor tympani | <5% | Very rare | Minimal functional impact | **High-Yield:** "Incus first, malleus second, stapes last" — this is the classic order of ossicular erosion in cholesteatoma. ### Clinical Correlation - **Incus erosion alone** → Air-bone gap 30–40 dB - **Incus + malleus erosion** → Air-bone gap 40–50 dB - **All three ossicles eroded** → Air-bone gap >50 dB (ossicular chain reconstruction often needed postoperatively) **Clinical Pearl:** On CT, look for the "absent incus sign" or erosion of the long process of the incus as a hallmark of cholesteatoma. Ossicular erosion pattern helps predict postoperative hearing outcome and need for ossiculoplasty. **Mnemonic:** **IMS** = **I**ncus (most common) → **M**alleus (second) → **S**tapes (least common) [cite:Dhingra 8e Ch 9]