HO in Orthopedics: Heterotopic Ossification
In orthopedics, HO stands for Heterotopic Ossification, which is the pathologic formation of bone in soft tissues and muscles where bone normally does not exist. 1, 2
Definition and Clinical Significance
- HO is defined as the presence of mature lamellar bone in extraskeletal soft tissues, representing a tissue repair process that has gone awry 1, 3
- This abnormal bone deposition most commonly occurs in the hip or elbow regions, though it can affect any soft tissue area 4
- The condition can significantly impair patients through pain, stiffness, and contractures that prevent activities of daily living 4
Common Clinical Contexts
Acquired Forms
- The acquired form of HO most frequently occurs after musculoskeletal trauma, spinal cord injury, or central nervous system injury 1
- Patients who have recently undergone total hip arthroplasty are at particular risk for developing HO 1
- Blast-related amputations in military casualties show increased incidence, with blast injury, amputation through the zone of injury, increased injury severity, and associated brain injuries being significant risk factors 5
Neurogenic HO (NHO)
- Traumatic brain injury or spinal cord injury may prompt formation of neurogenic heterotopic ossification, involving complex interplay between demineralized bone scaffold, CNS signaling, and local tissue mediators 4
Diagnostic Challenges
- Early HO presents with fever, swelling, erythema, and occasional joint tenderness that can be difficult to distinguish from cellulitis, osteomyelitis, or thrombophlebitis 1
- Radiographs should be obtained first to identify the presence of heterotopic ossification 6
- CT is superior to all other modalities for visualizing and quantifying established heterotopic bone formation 6
- Bone scanning is frequently used to confirm diagnosis and distinguish between diagnostic possibilities 1
Critical Management Consideration
In the rare genetic condition fibrodysplasia ossificans progressiva (FOP), surgical excision is absolutely contraindicated as it triggers more extensive ossification 7, 6. This represents a critical pitfall to avoid, as FOP is characterized by widespread progressive heterotopic ossification of skeletal muscle and soft connective tissues 7.