BIPOLAR CUP DESIGN MAY LEAD TO OSTEOLYSIS AROUND THE UNCEMENTED FEMORAL COMPONENT

Thirty-eight patients with a total of 45 primary uncemented bipolar he miarthroplasties with smooth femoral components were observed for an a verage period of 5 years, Clinical results were excellent in 17, good in 15, fair in 10, and poor in 3 hips, Radiographs showed stem looseni ng and bipolar head migration in 12 and 2 hips, respectively, Endocort ical femoral osteolysis developed in 26 hips (58%). Eight hips require d revision surgery, All the revised hips showed circumferential wear o f the polyethylene insert rims without substantial wear of the bearing surface, Histologic examination of the fibrous membrane around the st em showed many polyethylene particles that had been phagocytized by ma crophages or surrounded by foreign body giant cells, No metal particle s were observed at this site, The intrinsic functional mechanism of th e bipolar endoprosthesis leads to persistent cyclic impingement of the stem neck on the rim of the bipolar head, producing polyethylene part icles and frequent stem loosening by periprosthetic osteolysis, Unless this structural problem can be corrected and the durability of the pr osthesis increased, the bipolar system should not be used for hemiarth roplastic surgery.