Heterotopic ossification may occur following head or spinal cord trauma, burns, or major surgery on the musculoskeletal system. A heterotrophic bone may cause severe physical impairment by surrounding a joint such as the hip or the elbow and fixing the joint in a nonfunctional position (i.e., contracture). Heterotopic ossification is also very common following open reduction and internal fixation of acetabular fractures.

Heterotopic ossification is the process of bone formation in extraskeletal soft tissue. Roberts in 1968 was first to note the association between head trauma and periarticular ectopic bone formation, and others have since demonstrated a positive correlation between the extent of head trauma and functional severity of heterotopic ossification. The pathogenesis of heterotopic ossification following head injury is uncertain. However, recent theories suggest that head trauma causes an increase in inflammatory mediators and bone morphogenic proteins, which induce soft tissue osteoprogenitor stem cells to differentiate into osteoblasts. Increased serum levels of prostaglandin E₂ after head trauma are also believed to play a role in the proliferation of differentiated osteoblasts. The osteoblasts secrete osteoid peripherally about the area of trauma within 7 to 10 days. Primitive cartilage appears after 14 days, and trabecular bone forms within 2 to 5 weeks. Mature lamellar bone in the periphery surrounding immature and undifferentiated central tissues is present by the sixth week. Plain radiographs and computerized tomography scans show a very characteristic zoning pattern of mature bone at the periphery and a lucency in the center. The lucent area represents immature musculoskeletal tissues.

The hip is the most commonly involved joint, followed by the elbow, shoulder, and knee. The incidence of heterotopic ossification of the
hip following head trauma varies from 11% to 76%, with ankylosis of the joint occurring in 11% to 20% of those patients. Clinical signs that should cause one to suspect heterotopic ossification include pain and decreased range of motion at the joint, as well as inflammatory markers such as fever, erythema, swelling, and warmth at the joint. Making the diagnosis is also dependent on identifying patients who are more likely than others to develop heterotopic ossification. Risk factors include male gender, underlying systemic disorders such as ankylosing spondylitis and diffuse idiopathic skeletal hyperostosis, previous heterotopic bone formation, and limb spasticity.