Myositis ossificans - Orthopaedia

Overview

This review focuses on myositis ossificans circumscripta. This is a solitary, non-progressive , benign ossifying condition of the soft tissues, synonymous with heterotopic ossification. This is not to be confused with fibrodysplasia (myositis) ossificans progressiva which is a rare, inherited fibrosing and ossifying disorder of multiple soft tissue sites (often upper extremity and back) that is progressively disabling and fatal.

Age

Athletic adolescent or young adult (2nd-3rd decade)

Presentation

Usually presents as an enlarging mass in the arm or thigh of a patient who has experienced some sort of recent, often low grade limb trauma. Commonly involved muscles include quadriceps, brachialis and deltoid. Patient may present with significant pain 1-2 weeks after a trauma with rapid enlargement of the lump. The pain often decreases as time goes on, differentiating this entity from osteosarcoma where the pain is often unrelenting and progressive.

Physical /Laboratory findings

Swelling and warmth at the site of involvement

Typical lesions are distant from joints within substance of involved muscles (quadriceps, brachialis, deltoid)

Increased ESR and serum alkaline phosphatase

Plain films

Radiograph taken soon after the onset of symptoms may not reveal any calcification. Within 1-2 weeks, a poorly defined area of peripheral calcification may appear. At 2-4 weeks after injury, x-ray shows a round mass with distinct margin of mature peripheral calcification that is well delineated from surrounding tissue. The center of the mass is more radiolucent consisting of immature osteoid and primitive mesenchymal tissues. Mature calcification will progress towards the center of the mass. In contrast, osteosarcoma calcifies early at its center and continues to the periphery, a key distinguishing feature between the two entities.

Size

2-5 cm in diameter

Bone scan

At less than 3 weeks post-trauma, a bone scan will demonstrate increased uptake in the area. Bone scan also helpful in determining timing for surgical excision. Lesions should only be excised after bone scan demonstrates decreased activity.

CT Scan

Often helpful in delineating a central radiolucency surrounded by a dense periphery

Differential Diagnosis

Osteosarcoma

Myositis ossificans progressiva

Natural history

Over time, the volume of heterotopic bone formation will decrease and the soft tissue mass will shrink in size.

Pathology

Gross specimen: Shell of bony tissue with a soft reddish brown central area. Mass may be attached to the bone by a stalk, may be in continuum with the periosteum or may be solely adherent to the surrounding muscle

Microscopic: Acutely, undifferentiated mesenchymal cells proliferate and infiltrate the muscle. At 2-3 weeks, osteoid production begins with progressively more distinct trabeculae at the periphery of the lesion. Bone is usually immature and woven with large, round, and crowded osteocytes. However, longstanding lesions may mature to a lamellar pattern along the periphery. The center of the lesion contains an irregular mass of active, immature fibroblastic cells with foci of interstitial microhemorrhages that are rarely extensive.

Treatment

Treatment is generally conservative. Early surgery is contraindicated. Excision is an option after decreased activity is evident on bone scan.

Diagnostic Problems

Early biopsy of the lesion may make it difficult to differentiate a focus of myositis from sarcoma based on histology alone. Clinical and radiographic correlates are essential, with the critical distinction being whether the lesion is most mature at its periphery (i.e. myositis) or in its central core (i.e. osteosarcoma).