Heterotopic Ossification

Heterotopic ossification is the pathological formation of mature, lamellar bone in extraskeletal soft tissues such as muscles, tendons, and ligaments, without direct continuity to the periosteum. Heterotopic ossification is a frequent and clinically significant complication encountered in various rehabilitation and surgical settings, particularly following orthopaedic procedures such as total hip arthroplasty (THA), trauma, burns, spinal cord injury (SCI), traumatic brain injury (TBI), stroke, and joint replacement surgeries. First described as “paraosteoarthropathy” in paraplegic soldiers during World War I, heterotopic ossification continues to present a substantial burden in civilian and military populations. The pathogenesis of heterotopic ossification involves an initial inciting event, typically soft-tissue trauma, followed by an inflammatory cascade that recruits mesenchymal stem cells and induces their differentiation into chondrocytes and osteoblasts, ultimately leading to ectopic bone formation via endochondral ossification. While acquired heterotopic ossification remains the primary concern in orthopaedic and neurorehabilitation practice, rare hereditary forms include fibrodysplasia ossificans progressiva, progressive osseous heteroplasia, and Albright’s hereditary osteodystrophy. These genetic disorders are governed by distinct mechanisms and are not the focus of this discussion.