Heterotopic ossification (HO) is abnormal bone formation that occurs in soft tissues. The cause of HO is unknown; formation usually follows a traumatic event such as injury or surgery. HO after total hip replacement is thought to occur as a result of bony debris that spreads to the soft tissues when the proximal femur is prepped. The incidence of HO after total hip replacement varies from 1% to 90%. HO is twice as likely to occur in men and is more common in patients with hypertrophic osteoarthritis. HO is usually asymptomatic but may cause hip pain and stiffness in up to 7% of patients following total hip replacement. HO can be seen on hip radiographs as early as 4 to 6 weeks postoperatively.1,2
Routine prophylaxis for HO is not indicated for all patients following total hip replacement. In patients at higher risk for HO, such as those with prior HO or a history of Paget disease, hypertrophic osteoarthritis, ankylosing spondylitis, or diffuse idiopathic skeletal hyperostosis, prophylactic treatment after surgery is recommended. Indomethacin given for 6 weeks postoperatively was found to significantly decrease the rate of HO after THR. External beam radiation therapy given in multiple postoperative doses has been shown to prevent HO as well. Once HO can be seen on radiographs, only surgical excision will remove the bone from soft tissue. Surgical excision is delayed until the HO has matured, which usually occurs at 12 to 18 months. A decrease in bone scan activity is an indicator of bone maturation and is helpful for surgical timing. Surgical excision is only performed in rare cases when severe, painful HO significantly limits mobility. Surgical excision is approached cautiously in HO patients after total hip replacement as outcomes may vary. Excision is likely to improve hip function, but it is less reliable in relieving pain.1,2
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).
- Iorio R, Healy WL. Heterotopic ossification after hip and knee arthroplasty: risk factors, prevention, and treatment. J Am Acad Orthop Surg. 2002;10(6):409-416.
- Nagi ON, Dhillon MS, Batth HS. Heterotopic ossification after total hip arthroplasty: a review of etiopathogenesis, risk factors and treatment modalities. Indian J Orthop. 2002;36(4):225-233.