Myositis Ossificans - Morphology

Comments:
Myositis Ossificans (MO) - Intermediate Zone: The intermediate zone is located at some distance from the center of the lesion and shows transition from fibroblastic stroma to chondro-osseous matrix. There is appearance of irregular trabeculae of immature woven bone lined by plump osteoblasts. The trabeculae will subsequently mineralize to form lamellar bone seen in the more mature peripheral zone.

The findings can be concerning for a sarcoma due to hypercellularity, haphazard arrangement, and increased mitotic activity. However, the osteoid trabeculae of myositis ossificans are thicker (unlike the irregular, lace-like osteoid of osteosarcoma) and there is no pleomorphism or atypical mitoses.