A critical review. Heterotopic ossification in total hip replacement.

Heterotopic ossification (HO) status post total hip arthroplasty is a relatively common phenomenon with clinical significance in approximately 5% of all cases. Risk factors appear to include males with osteoarthritis, particularly with marked osteophyte formation, and those with ankylosing spondylitis or diffuse idiopathic spinal hyperostosis. Previous hip surgery, or previous ectopic bone in the same or contralateral hip are definite predisposing factors. Although meticulous surgical technique is critical in any operation, the suggestions that carelessness in dissection or tissue handling, or inadequate hemostasis or debridement of devitalized tissues or of bony debris can cause HO are unproved. Similarly, there is no solid evidence that the surgical approach, prosthesis type, use of trochanteric osteotomy, or the presence of cement influence the incidence of HO. Whether postoperative complications such as infection, dislocation, or hematoma are causally related is speculative; and the role of alkaline phosphatase in predicting those at risk remains controversial. Despite the number of studies designed to elucidate risk factors, critical analysis suggests that this question remains largely unanswered and that there is a need for well-designed, prospective, controlled studies to determine which hip arthroplasty patients are at risk. Treatment of established HO depends upon recognizing the "maturity" of the ectopic bone, which can best be determined by serial scans but is approximately one year postop. Excision followed by prompt initiation of radiotherapy or of one of several reported nonsteroidal anti-inflammatory drug protocols will produce successful results in a majority of cases. Prophylaxis depends upon recognizing those at significant risk and initiating the appropriate protocol within the first few postoperative days.