POLYETHYLENE WEAR AFTER TOTAL HIP-ARTHROPLASTY - THE EFFECT OF A MODULAR FEMORAL-HEAD WITH AN EXTENDED FLANGE-REINFORCED NECK

The use of modular components in total hip arthroplasty has been thoug ht to contribute to accelerated polyethylene wear. Specifically, a mod ular femoral head with a flange extension and a longer neck may cause increased wear. The purpose of the current study was to evaluate the e ffect of a flange extension on polyethylene wear. Ninety-one patients who had had a total of 100 primary total hip arthroplasties were evalu ated after an intermediate duration of follow-up. All of the acetabula r components consisted of a hemispherical titanium-alloy fiber-mesh po rous-coated shell with a nonelevated modular polyethylene liner; they were inserted without cement and with use of supplemental screws throu gh the dome after so-called line-to-line reaming, All of the femoral c omponents consisted of a modular head with a diameter of twenty-eight millimeters and either a long neck (with a flange extension) or a shor t or medium neck (without a flange extension). The study group compris ed sixty-two patients (sixty-six hips) who had had radiographic evalua tion that was adequate to allow the valid measurement of polyethylene wear, Thirty-two hips were in men, and thirty-four were in women. The mean age of the patients was fifty-six years, the mean weight was seve nty-three kilograms, and the mean duration of follow-up was 6.1 years (range, four to eight years). The rate of polyethylene wear in the ele ven hips in which the femoral component had a flange extension was sig nificantly greater than that in the fifty-five in which the femoral co mponent did not have a flange extension (mean, 0.17 compared with 0.11 millimeter per year; p = 0.009), Multivariate analysis showed that th e presence of a flange extension was associated with increased polyeth ylene wear to a greater degree (F = 2.86) than were all other variable s that were measured, including a younger age (F = 1.72), a more verti cal angle of the acetabular component (F = 0.49), a heavier weight (F = 0.14), male gender (F = 0.11), and a smaller initial thickness of th e polyethylene (F = 0.02). These data support an association between t he presence of a modular femoral head with a flange extension and an a ccelerated rate of polyethylene wear, The presumed mechanism is an inc rease in peripheral, or so-called rim, impingement of the flange-reinf orced neck on the acetabulum due to a decrease in the ratio between th e diameters of the femoral head and neck.