A 26-year-old, previously healthy man sustains blunt force trauma to the left upper arm. On physical examination, there is focal swelling and redness. Three weeks later, the superficial contusion has resolved, but now a slightly tender mass is palpated in the outer aspect of the upper left arm. A radiograph of the left arm shows a 5-cm mass in the soft tissue. There is a radiolucent center and surrounding irregular bone formation. One month later, the mass is now 3 cm and painless. CT scan of the arm shows a well-circumscribed mass within muscle with areas of bright calcification throughout. What is the most likely diagnosis?

Myositis ossificans is an uncommon, exuberant repair reaction following soft-tissue trauma to muscle in which there is metaplastic bone formation. The keys to diagnosis are the location within soft tissue, calcification beginning at the periphery, and decrease in size over time. Gouty tophi can form in soft tissues, but there is typically a history of gouty arthritis first, and the lesions do not calcify. A hem- arthrosis forms with joint trauma and hemorrhage in and around the joint capsule, but does not involve calcification. An osteochondroma is a bony exostosis projecting from bone into soft tissue. An osteosarcoma that rarely arises in soft tissue must be distinguished from myositis ossificans; the latter is characterized by the mature shell of bone, lack of enlargement, and lack of bone or soft-tissue destruction. Polymyositis involves inflammation with degeneration and regeneration of muscle fibers, but there is no mass effect and no calcification.