Ectopic bone formation after total hip arthroplasty.

Two hundred and forty consecutive Japanese patients with 280 primary total hip arthroplasties (THA) were analyzed to clarify the incidence of ectopic bone formation and its predisposing factors and to examine its effect on the clinical results. Ectopic bone formation after THA was found in 61 joints (22%). The predisposing factors were male patients and a hypertrophic type of osteoarthritis. It was revealed that extensive ectopic bone formation, class 3 according to Brooker's classification, restricted flexion and abduction motion of the hip joint.