Myositis Ossificans

Background

• Benign process characterised by heterotrophic ossification within large muscle groups
• It is important because it often mimics more aggressive pathological conditions
• Most common type is MO circumscripta – where new bone appears usually after trauma (common in young adults and paraplegics)
  • Other types/related conditions include MO progressiva (rare inherited disorder- progressive and ultimately fatal); panniculitis ossificans (similar to MOC but in subcut tissues)

Pathology

• MO is extraosseous bone formation without inflammation which occurs in muscle.  It has zonal organisation
  • Peripheral well organised mature/lamellar bone
  • Intermediate osteoid region
  • Central, immature non-ossified cellular focus
• The histopathology can appear similar to osteosarcoma- so a clear history is important to avoid inappropriate management

Investigation

• Xray- lesion of dense peripheral calcification (other imaging may also be useful e.g. CT/MRI)

Management

• Observation if otherwise asymptomatic, surgery (resection) if painful

Presentation

• Painful, tender, enlarging mass, most commonly in the large muscles of the extremities, often following trauma