## Distinguishing Acute from Chronic Osteomyelitis ### Key Radiological Difference **Key Point:** Sequestrum (dead bone fragment) and involucrum (new bone formation around dead bone) are pathognomonic features of chronic osteomyelitis and are absent in acute disease. ### Pathological Timeline Acute osteomyelitis progresses to chronic osteomyelitis over weeks to months. The hallmark transition involves: 1. **Acute phase (0–2 weeks):** Suppuration, vascular compromise, bone necrosis begins 2. **Chronic phase (>3–4 weeks):** Dead bone (sequestrum) is walled off by new bone (involucrum), forming a sinus tract ### Comparative Features Table | Feature | Acute Osteomyelitis | Chronic Osteomyelitis | | --- | --- | --- | | **Sequestrum & Involucrum** | Absent | Present (diagnostic) | | **Fever** | High, acute onset | Absent or low-grade | | **ESR/CRP** | Markedly elevated | Mildly elevated or normal | | **Blood culture** | Often positive | Usually negative | | **X-ray findings** | Soft tissue swelling, early periosteal reaction | Sequestrum, involucrum, sinus tract | | **Duration of symptoms** | Days to 2 weeks | Weeks to months | | **Discharge/sinus** | Absent | Present | ### Clinical Pearl **Clinical Pearl:** Sequestrum appears on X-ray as a dense bone fragment surrounded by lucency (dead bone is radio-opaque because it is not remodelled). Involucrum is the new cortical bone laid down by the periosteum around the sequestrum. Together, they are virtually diagnostic of chronic osteomyelitis and do NOT occur in acute disease. ### High-Yield Point **High-Yield:** While fever, elevated inflammatory markers, and positive blood cultures are common in acute osteomyelitis, they are NOT specific discriminators—they can be absent in chronic disease or present in other infections. Sequestrum and involucrum are the ONLY pathognomonic radiological signs that distinguish chronic from acute osteomyelitis. [cite:Robbins 10e Ch 26] 
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