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Myositis Ossificans - Morphology

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Microscopic Features: The zonal pattern is an important diagnostic clue for myositis ossificans (MO) and is well-established by 3-4 weeks after onset. In properly-oriented specimens, 3 distinct zones of maturation can be seen simultaneously (as captured in this image).

The innermost zone (on the right) can resemble nodular fasciitis, granulation tissue or a sarcoma and consists of an irregular proliferation of immature fibroblastic/myofibroblastic cells in a highly vascular, collagenous background.

In the intermediate zone (in the middle of the image), there is transition from fibroblastic to osteoblastic stroma with formation of irregular trabeculae of immature woven bone. The osteoid trabeculae are rimmed by plump osteoblasts.

In the outermost layer (periphery of the lesion; seen here on the left), the osteoid undergoes mineralization and eventual maturation to form parallel arrays of lamellar bone with large round osteocytes (left one-third of the image). Such foci may be mistaken for an osteoma.

See higher magnification in the next image.

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