Ultra-high-molecular weight polyethylene wear - An in vitro comparison of acetabular metal types and polished surfaces

The generation of debris from the wear of ultra-high-molecular weight polye thylene (UHMWPE) is a well-recognized factor in the development of osteolys is and the long-term failure of total joint arthroplasties. Wear between th e articulation of the femoral head and the polyethylene has been recognized for many years, but more recently, both retrieval and in vitro studies hav e demonstrated that convex surface wear or backside wear also occurs and ma y be of significance. Currently, modular acetabular components are being de signed with polished surfaces, fewer screw holes, various polyethylene lock ing mechanisms, and stiffer metal alloys in an attempt to reduce backside w ear. The purpose of this study was to determine if differences existed in U HMWPE wear based on the metal alloy used and the surface finish in modular acetabular components. Sixteen components in 4 groups were subjected to 10 million gait cycles using an in vitro, joint simulator. All components used 28-mm cobalt chrome femoral heads on cobalt chrome tapered stems. The 4 gr oups differed only in the type of metal backing and type of interior finish ed surface: polished cobalt chrome, unpolished cobalt chrome, polished tita nium, and unpolished titanium. UHMWPE changes were examined in terms of art icular (concave) surface wear, backside (convex) surface wear, and friction al torque. The overall linear and volumetric wear rates were 1.05 mm/10 mil lion cycles and 325 mm(3)/10 million cycles. No significant differences in linear and volumetric wear rates were detected between the cobalt chrome an d titanium acetabular components. Surface finish did not influence wear rat es, in terms of backside wear, all specimens in the 4 groups demonstrated t otal loss of all sputtered gold with the exception of those areas extruded through the screw holes. Extrusion through the screw holes was on the order of 0.0004 inch for all groups, and no significant difference was seen amon g the groups for this parameter. The measurements of articular frictional t orque demonstrated a significant difference among the polished and unpolish ed cobalt chrome components (17.3 N . m vs 11.5 N.m; P=.0039, 2-way analysi s of variance, Student's Newman Keuls method). Some designs in modular acet abular components have favored stiffer alloys, such as cobalt chrome, with polished concave surfaces to decrease wear on both the concave and the conv ex surfaces. In this study there was no significant difference in wear rate s noted between cobalt chrome and titanium acetabular components, and polis hing of the components had no appreciable affect in reducing backside wear.