Myositis ossificans

Solitary, non-progressive, benign ossifying condition of soft tissues
Synonymous with heterotopic ossification
Not to be confused with fibrodysplasia (myositis) ossificans progressiva, a rare, inherited fibrosing and ossifying disorder of multiple soft tissue sites (often upper extremity and back); progressively disabling and fatal

Athletic adolescent or young adult (second through third decades of life)

Enlarging mass in arm or thigh
Patient generally has experienced a recent, often low-grade limb trauma
Commonly involved muscles include quadriceps, brachialis, and deltoid
Patient may present with significant pain 1 to 2 weeks after a trauma with rapid enlargement of lump
Pain often decreases as time goes on, differentiating myositis ossificans from osteosarcoma, in which pain is often unrelenting and progressive

Swelling, warmth at involved site
Lesions typically distant from joints within substance of involved muscles (quadriceps, brachialis, deltoid)
Increased ESR and serum alkaline phosphatase levels

Radiograph taken soon after onset of symptoms may not reveal calcification
Within 1-2 weeks, a poorly defined area of peripheral calcification may appear
At 2-4 weeks after injury, X-ray shows round mass with distinct margin of mature peripheral calcification; well delineated from surrounding tissue
- Center of mass more radiolucent, consisting of immature osteoid and primitive mesenchymal tissues; mature calcification will progress toward center of mass
  - In contrast, osteosarcoma calcifies early at its center and continues to periphery, a key distinguishing feature between the two entities
Lesions usually 2-5 cm in diameter

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

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

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

Gross: Shell of bony tissue with soft reddish brown central area; mass may be attached to bone by a stalk, may be in continuum with periosteum or may be solely adherent to 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 periphery of the lesion
- Bone usually immature and woven with large, round, and crowded osteocytes; longstanding lesions, however, may mature to a lamellar pattern along the periphery
- Center of lesion contains irregular mass of active, immature fibroblastic cells with foci of interstitial microhemorrhages that are rarely extensive

Early biopsy of lesion may make it difficult to differentiate a focus of myositis from sarcoma based on histology alone
Clinical and radiographic correlates essential, with critical distinction being whether lesion is most mature at its periphery (ie, myositis) or in its central core (ie, osteosarcoma).
Treatment generally conservative
Early surgery contraindicated
Excision an option after decreased activity is evident on bone scan