Myositis Ossificans - Morphology
Microscopic Features: Myositis ossificans (MO) passes through three parallel stages of clinical, radiologic and histopathologic changes. It begins as a highly cellular fibroblastic process within 1-2 weeks of trauma and matures over the course of several weeks to a lesion with peripheral lamellar bone. The different levels of maturation can be seen simultaneously in the form of 3 distinct zones in properly oriented specimens (as in this image). The zonal pattern is an important diagnostic clue and is well-established in lesions that are at least 3-4 weeks old. The initial stage (seen here on the right) is highly cellular and consists of fibroblastic/myofibroblastic proliferation which can mimic nodular fasciitis or a sarcoma. The intermediate stage shows transition from fibroblastic stroma to chondro-osseous matrix with formation of immature woven bone (seen near the center of the image). The mature stage consists of abundant thick trabeculae of lamellar bone (seen in the left one-third of the image).