Myositis Ossificans

     Myositis ossificans is an extra-osseous non neoplastic growth of new bone. A synonym for myositis ossificans is heterotopic ossification. There are several clinical subtypes of myositis ossificans. Myositis ossificans circumscripta refers to new extra-osseous bone that appears after trauma. Progressive myositis ossificans is a rare, inherited disorder characterized by fibrosing and ossification of muscle, tendon and ligaments of multiple sites often in the upper extremities and back that is disabling and ultimately fatal. I myositis ossificans is also a recognized complication of paralysis that occurs below the level of spinal cord injury.
     Myositis ossificans occurs most commonly in the second and third decade in the arms and thighs of patients who have experienced some sort of trauma. The quadriceps and brachialis are the most affected. Myositis ossificans
presents as a rapid enlargement and significant pain one to two weeks after injury. The patient has swelling and warmth at the site as well as an increased ESR and serum alkaline phosphatase. The clinical picture differs from that of osteosarcoma (which is often on the differential as the age groups are the same) as the pain from myositis ossificans decreases as time goes on where osteosarcoma pain increases.
     Myositis ossificans appears on plain-film at approximately 2-4 weeks after injury. The lesion begins to calcify at the periphery and works toward the center. At less than three weeks post-trauma, bone scan demonstrates increased uptake in the area. Osteosarcoma calcifies at the center and continues to the periphery. CT scan helps delineate a central radiolucency surrounded by a dense periphery.
     On gross examination, myositis ossificans has a shell of bone and a soft red-brown center. The mass may be attached to the bone by a stalk or might be in continuum with the periosteum. Alternatively, the mass may be attached only to the muscle.
     Microscopically, myositis ossificans appears different at the periphery and the center. Acutely, there is proliferation of undifferentiated mesenchymal cells that infiltrates the muscle. At approximately 2-3 weeks, osteoid
production begins at the periphery and fibrous tissue begins to form around the shell. The center of the lesion will be an irregular mass of immature fibroblasts. Moving towards the edge, there will be islands of disorganized osteoid. At the edge of the lesion, trabeculae of lamellar and woven bone are present. There may be a cartilage component as well. This process is in contrast to osteosarcoma where the ossification begins at the center of the lesion.
     There are several theories on the etiology of myositis ossificans. It has been suggested this is merely an organizing hematoma. Other authors think the periosteum has been pushed into the muscle or the osteoblasts escaped the periosteum. Metaplasia of connective tissue cells is also a possibility.
     Treatment for myositis ossificans is generally conservative. Early surgery is contraindicated. Excision is an option after decreased activity is evident on bone scan. If left alone, the mass will shrink in size.1

' Bullough, Peter, Orthopaedic Pathologv (third edition), Times Mirror International Publishers Limited, London, 1997.

Huvos, Andrew. Bone Tumors: Diagnosis. Treatment and Prognosis, W.B. Saunders, Co., 1991.

Cushner, FD and RM Morwessel, Myositis Ossificans Traumatica, Orthopaedic Review, p. 1319-1326, November, 1992.






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