Highly crosslinked polyethylene wear rates and acetabular component orientation

© 2018 The British Editorial Society of Bone & Joint Surgery. Aims The aim of this study was to determine whether there is a difference in the rate of wear between acetabular components positioned within and outside the \u27safe zones\u27 of anteversion and inclination angle. Patients and Methods We reviewed 100 hips in 94 patients who had undergone primary total hip arthroplasty (THA) at least ten years previously. Patients all had the same type of acetabular component with a bearing couple which consisted of a 28 mm cobalt-chromium head on a highly crosslinked polyethylene (HXLPE) liner. A supine radiostereometric analysis (RSA) examination was carried out which acquired anteroposterior (AP) and lateral paired images. Acetabular component anteversion and inclination angles were measured as well as total femoral head penetration, which was divided by the length of implantation to determine the rate of polyethylene wear. Results The mean anteversion angle was 19.4° (-15.2° to 48°, SD 11.4°), the mean inclination angle 43.4° (27.3° to 60.5°, SD 6.6°), and the mean wear rate 0.055 mm/year (SD 0.060). Exactly half of the hips were positioned inside the \u27safe zone\u27. There was no difference (median difference, 0.012 mm/year; p = 0.091) in the rate of wear between acetabular components located within or outside the \u27safe zone\u27. When compared to acetabular components located inside the \u27safe zone\u27, the wear rate was no different for acetabular components that only achieved the target anteversion angle (median difference, 0.012 mm/year; p = 0.138), target inclination angle (median difference, 0.013 mm/year; p = 0.354), or neither target (median difference, 0.012 mm/year; p = 0.322). Conclusion Placing the acetabular component within or outside the \u27safe zone\u27 did not alter the wear rate of HXLPE at long-term follow-up to a level that risked osteolysis. HXLPE appears to be a forgiving bearing material in terms of articular surface wear, but care must still be taken to position the acetabular component correctly so that the implant is stable