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Introduction: Myositis ossificans (MO) is a benign, solitary, self-limited, ossifying mass that is usually found within the musculature of the extremities. The term is a misnomer on several counts. It can occur in non-muscle tissues, there is little or no inflammation and the initial stages show no bone formation. It is related to fibro-osseous pseudotumor of digits and soft tissue aneurysmal bone cyst.

The lesion passes through three parallel stages of clinical, radiologic and histopathologic changes. The initial stage (first 4 weeks) is highly cellular and can mimic a sarcoma (right half of the image). The intermediate stage (4-8 weeks) is characterized by the appearance of osteoblastic matrix and immature woven bone (center of the image). The mature stage (> 8 weeks) shows abundant mature lamellar bone and can mimic osteoma (left one-third of the image). The lesion becomes smaller as it matures and may eventually regress.
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