THE WEAR OF ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE SLIDING ON METALLIC AND CERAMIC COUNTERFACES REPRESENTATIVE OF CURRENT FEMORAL SURFACES IN JOINT REPLACEMENT

A number of studies have investigated the influence of surface roughne ss on the wear of ultra-high molecular weight polyethylene (UHMWPE) in total joint replacement. The results of these studies have shown that the wear factor is proportional to the counterface roughness raised t o a power greater than one. In this laboratory study, the effect of su rface finish of several biomaterials on the wear of UHMWPE was studied . The study was conducted using reciprocating pin-on-plate wear tests with bovine serum as a lubricant. The biomaterials investigated as the counterface material included stainless steel, cast cobalt chrome (Co Cr), CoCr (ASTM F799), alumina ceramic and zirconia ceramic. The count erface topographies of the wear plates were produced using techniques representative of current manufacturing methods. The surface roughness of the wear plates was varied in the range R-a = 0.005-0.04 mu m; thi s was representative of femoral heads and femoral knee components curr ently used clinically. Metals and ceramics with a similar surface roug hness produced a similar wear rate of UHMWPE. For the limited range of smooth counterfaces used in this study only a moderate correlation wa s found between the surface roughness and the wear factors. For a chan ge in counterface roughness R-a of 0.005 to 0.04 mu m, the wear factor increased from 7.4 +/- 1.6 to 16.5 +/- 2.4 x 10(-9) mm(3)/N m (mean /- standard error). This variation in counterface roughness had much l ess effect in wear than previously reported for rougher counterfaces. For an extended range of counterface roughness, a stronger correlation was found using an exponential function for the regression fit. The e xponential function shows the benefits of decreased wear with decrease d surface roughness. Although the wear rate decreased less rapidly wit h decreased counterface roughness for R-a values below 0.05 mu m, ther e were significant advantages to be gained from improved femoral head roughness to below 0.01 mu m R-a.