Myositis ossificans

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Overview

  • 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  

Age

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

Presentation

  • 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

Physical /laboratory findings

  • 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

Plain films

  • 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

Bone scan

  • 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

CT scan

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

Differential diagnosis

  • Osteosarcoma
  • Myositis ossificans progressiva

Natural history

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

Pathology

  • 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 

Diagnosis and treatment

  • 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

Recommended reading

Bullough P. Orthopaedic Pathology (fourth edition). 2004 Elsevier Limited  2004.

Beiner JM, Jokl P. Muscle Contusion Injuries: Current Treatment Options. JAAOS 2001  9:227-237

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