Prevention of heterotopic bone formation: clinical experience with diphosphonates.

Heterotopic bone formation after total hip arthroplasty is a common occurrence in patients with osteoarthritis, and severe amounts of ectopic bone may limit motion or cause pain. Diphosphonates have been suggested as a method of preventing ectopic bone formation, but no long-term clinical evaluation of their effectiveness has been published. Because patients with osteoarthritis appeared to respond well to diphosphonate therapy in an earlier study, we thought that they would be an appropriate group of patients to study. We evaluated the results of 177 patients with 200 total hip arthroplasties performed for primary osteoarthritis. Considerable postoperative heterotopic bone formation (classes III and IV according to the classification system of Brooker and associates) was found in 36 hips (18%). The incidence of heterotopic bone formation was found to be as high as in the patients who had received either a placebo or no drug therapy. The postoperative range of motion of the hips, as well as ratings for pain, walking, and function, did not differ significantly between the treated and untreated groups. Diphosphonates (EHDP) have been demonstrated to inhibit the growth of hydroxyapatite crystals in vitro by chemisorption onto the crystal surface and thus have been thought to have the potential of preventing pathological calcification in vivo. However, diphosphonates have no inhibitory effect on the formation of osteoid matrix, and the delay in mineralization of matrix is reversed when therapy is discontinued. Although this delay in mineralization was known at the onset of these clinical trials, we hoped that the ultimate amount of heterotopic bone would be less in the treated patients and that the range of motion would be improved as a result of delaying the process of mineralization. Unfortunately, the final range of motion in the diphosphonate-treated patients did not differ significantly from that in the untreated group, and the final amount of heterotopic ossification was not reduced. Therefore diphosphonate therapy must be considered ineffective.