## Radiological Features of Chronic Osteomyelitis **Key Point:** Sequestrum (dead bone) and involucrum (new bone formation) are the pathognomonic hallmark findings of chronic osteomyelitis on plain radiographs. ### Definitions and Pathophysiology | Term | Definition | Pathophysiology | |------|-----------|------------------| | **Sequestrum** | Fragment of dead cortical bone | Result of vascular compromise; appears as dense radio-opaque fragment within lucent zone | | **Involucrum** | New bone formation around sequestrum | Periosteal response to infection; surrounds and attempts to wall off dead bone | | **Cloaca** | Opening in involucrum | Allows drainage of pus and necrotic material | | **Sinus tract** | Chronic draining pathway | May persist for years; risk factor for squamous cell carcinoma | ### Chronological Radiological Evolution 1. **Early phase (1–2 weeks):** Soft tissue swelling; bone changes minimal 2. **Acute phase (2–3 weeks):** Periosteal elevation, metaphyseal lucency 3. **Chronic phase (>3 weeks):** Sequestrum formation, involucrum, cortical thickening **High-Yield:** Sequestrum appears as a **dense (radio-opaque) fragment within a lucent (dark) zone**—this is the classic teaching point. The involucrum is the surrounding new bone shell. **Clinical Pearl:** In chronic osteomyelitis, plain radiographs may be sufficient for diagnosis if sequestrum and involucrum are visible. MRI is superior for early detection and soft tissue involvement, but the radiological hallmark remains sequestrum + involucrum. ### Why This Distinguishes Chronic from Acute Osteomyelitis - **Acute osteomyelitis:** Periosteal elevation, early metaphyseal lucency; no sequestrum yet - **Chronic osteomyelitis:** Sequestrum, involucrum, cortical thickening, sclerosis; may have sinus tract **Mnemonic:** **SIC** — Sequestrum, Involucrum, Cloaca (the triad of chronic osteomyelitis imaging) [cite:Robbins 10e Ch 26] 
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