Heterotopic Ossification is an uncommon development where extraossues bone is laid down around joints. The most commonly involved joint is the hip joint, typically after total hip replacement. Non-steroidal therapy and radiotherapy are both used to prevent it from occurring in medium and high risk groups.
Pathology and Presentation
Pathogenesis is not completely understood. The current theory is that pluripotential mesenchymal cells are in some way stimulated to develop into osteoblasts by the event (eg. trauma or surgery). These then proliferate rapidly in the peri-operative period and lay done extraosseus bone. There are several genetic disorders where patients develop progressive ossification around many joints within the body.
Most patients complain of stiffness in the affected joint. Pain is less commonly a problem.
The risk of developing heterotopic ossification is increased by:
- A past history of heterotopic ossification
- Ankylosing spondylitis
- Male gender
- Presence of excessive osteophyte formation in response to osteoarthritis.
Heterotopic Ossificaiton is graded by the Brooker Classification:
Grade I - Islands of bone, asymptomatic
Grade II - Bony spurs arising from the femur or acetabulum, > 1 cm between surfaces
Grade III - Bony spurs arising from the femur or acetabulum, < 1 cm between surfaces
Grade IV - Ankylosis
Management
Surgery is required for established heterotopic ossification.
In patients at high risk (eg. ankylosing spondylitis, previous heterotopic ossification) prophylaxis is recommended.
- NSAIDs are a simple way of reducing heterotopic ossification. Indomethacin is most commonly used, with a dose of 100 mg/day for 14 days post operatively together with a proton pump inhibitor
- Radiotherapy is slightly more effective at controlling the disease but by < 2% when compared to indomethacin. It is also associated with potential long term second malignancies although there is only a few case reports in the literature. It is essential to time the radiotherapy either 4 hours prior to surgery or within 3 days post-operatively. In general, pre-operative irradiation is easier for the patient but more difficult logistically.
- A single randomised study which compared indomethacin alone with indomethacin + radiotherapy has been done (Pakos et al 2009), demonstrating reduced development of extraosseus ossification with combined therapy.
This might be considered evidence that radiotherapy + indomethacin should be the standard of care.
Radiotherapy Technique
The patient is simulated supine with a vacuum bag to immobilise the leg. A CT scan is performed through the region of interest.
Opposed anterior and posterior fields are used. The field aims to cover the joint space with enough margin for patient movement and to account for penumbra. Total dose is 7 Gy in 1 fraction (most established regiment and higher doses do not seem to be superior). The superior extent of the field is 3 cm above the acetabulum; the inferior extent is 2/3rds of the femoral stem component of the joint surgery.
There is some concern that for non-cemented prostheses (that require bone growth into the prosthesis for fixation), radiotherapy might impair the eventual fixation of the joint. There is no evidence for this in the published studies and most authors agree that this is unnecessary; shielding these parts may in fact impair the ability of radiotherapy to stop heterotopic ossification from occurring.
