= HETEROTOPIC BONE FORMATION = ECTOPIC OSSIFICATION = MYOSITIS OSSIFICANS

Common complication following surgical manipulation, total hip replacement (62%) and chronic immobilization (spinal cord injury / neuromuscular disorders)

Mechanism: pluripotent mesenchymal cell lays down matrix for formation of heterotopic bone similar to endosteal bone

Causes: para- / quadriplegia (40–50%), myelomeningocele, poliomyelitis, severe head injury, cerebrovascular disease, CNS infections (tetanus, rabies), surgery (commonly following total hip replacement)

Evolution: calcifications seen 4–10 weeks following insult; progression for 6–14 months; trabeculations by 2–3 months; stable lamellar bone ankylosis in 5% by 12–18 months

• largest quantity of calcifications around joints, especially hip, along fascial planes
• disuse osteoporosis of lower extremities
• renal calculi (elevation of serum calcium levels)

Radiographic grading system (Brooker):

• 0 no soft-tissue ossification
• I separate small foci of ossification
• II >1 cm gap between opposing bone surfaces of heterotopic ossifications
• III <1 cm gap between opposing bone surfaces
• IV bridging ossification

Bone scan:

• tracer accumulation in ectopic bone
• assessment of maturity for optimal time of surgical resection (indicated by same amount of uptake as normal bone)

Cx: ankylosis in 5%

Rx: 1000–2000 rad within 4 days following surgical removal