## Chronic Osteomyelitis: Pathological Features **Key Point:** Sequestrum and involucrum formation is the pathognomonic histological hallmark of chronic osteomyelitis, distinguishing it from the acute phase. ### Definitions and Pathophysiology **High-Yield:** Understanding the bone remodeling response in chronic osteomyelitis is essential: 1. **Sequestrum** — a fragment of dead (necrotic) bone that has lost its blood supply due to suppuration and pressure necrosis. It is surrounded by pus and granulation tissue. 2. **Involucrum** — new bone laid down by the periosteum in response to the infection. It attempts to wall off the infection but is often incomplete, allowing chronic drainage and fistula formation. ### Histological Progression | Phase | Timeline | Histology | Key Features | |-------|----------|-----------|---------------| | **Acute** | 0–2 weeks | Neutrophilic infiltrate, vascular congestion, edema | Suppuration, minimal bone necrosis | | **Subacute** | 2–4 weeks | Granulation tissue, early fibrosis, osteoclastic resorption | Transition to chronic phase | | **Chronic** | >4 weeks | Sequestrum + involucrum, fibrosis, chronic inflammation | Pathognomonic finding; may persist for years | **Clinical Pearl:** The sequestrum appears as a radiopaque fragment within a lucent cavity on radiographs — the **"bone within bone"** sign. The involucrum is the surrounding shell of new bone. **Mnemonic:** **SIS** = *Sequestrum* (dead bone) + *Involucrum* (new bone shell) = chronic osteomyelitis Signature. ### Why This Matters Clinically Sequestrum must be surgically removed (sequestrectomy) because: - It is avascular and cannot be reached by antibiotics - It acts as a nidus for persistent infection - Its removal is essential for cure of chronic osteomyelitis [cite:Robbins & Cotran Pathologic Basis of Disease 10e Ch 26] 
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