The effect of counterface roughness on the wear of UHMWPE for rectangular wear paths

Ultra-high molecular weight polyethylene (UHMWPE) is used worldwide as a bearing material in total joint replacement prostheses. Multi-directional motion has been identified as a major factor affecting the wear rate of UHMWPE in total hip replacement prostheses. The trajectory of relative motion between a femoral head and an acetabular cup takes a general quasi-elliptical or rectangular shape during the patient's gait cycle. Differences in motion pattern can affect the in vivo wear rates of UHMWPE cups in patients when all other factors are equal. In a previous study that utilized smooth Co-Cr counterfaces, we compared wear factors, k, for UHMWPE articulated in a series of rectangular wear paths (width = A, length = B) with systematically increasing aspect ratios (B/A) and linear tracking (A = 0), all with identical path lengths per cycle. The results showed that the wear factor significantly decreased in the rectangular wear path with the highest aspect ratio and in linear tracking. The goal of our current study was to quantify the effect of a roughened counterface on the cross-path wear of UHMWPE. UHMWPE pins were articulated against both smooth and rough Co-Cr disks in diluted calf serum using a multi-directional wear tester under physiological loading conditions. Five different rectangular wear path geometries and linear tracking, all with identical path lengths per cycle, were employed for each wear test. Gravimetric weight loss was converted into volumetric wear rates and wear factors, k. The results showed that roughened counterfaces produced a larger increase in the wear factor in rectangular wear paths with higher aspect ratios. The ratio of krough/ksmooth decreased monotonically as a function of increasing width of rectangles, normalized by total path length, or A/(A + B).