Ectopic bone formation following low friction arthroplasty of the hip.

Ectopic bone formation after low friction arthroplasty (LFA) occurs nearly three times as often in males as in females and, in significant amounts, in 10 per cent of the total number of patients. The more limited the range of motion preoperatively, the more severe the degree of ectopic bone formation. The diagnosis does not closely relate to ectopic bone formation though the incidence is highest in osteoarthritis. The removal of osteophytes does not increase the incidence of ectopic bone formation. Trochanteric osteotomy in the surgical exposure is not an important cause of ectopic bone formation. A patient with ectopic bone after LFA on one side has a 92 per cent chance of developing it in a subsequent LFA on the opposite side. Systemic factors appear to play a dominant role in the production of ectopic bone.