The predictive power of surface profile parameters on the amount of wear measured in vitro on metal-on-polyethylene artificial hip joints

Various studies report a weak correlation between the average surface roughness R-a of metallic ballheads and the observed wear rate of the polyethylene cups coupled to them. The aim of this study is to verify, with controlled in vitro experiments, whether other parameters such as the total roughness R-t, and the skewness R-sk are better predictors of the polyethylene wear when the metallic heads present a surface conditioning that varies substantially from specimen to specimen, as is usually the case with retrieved specimens. Twelve CoCrMo (cobalt-chromium-molybdenum) ballheads were intentionally damaged in order to reproduce a wide spectrum of surface conditioning and were then subjected to the standard wear test against polyethylene cups, using a hip joint wear simulator. After 2 x 10(6) cycles the weight lost by the cups was assessed with a gravimetric procedure, and the surface roughness of the metallic ballheads was qualified in terms of R-a, R-t, and R-sk. The various parameters were correlated to the weight loss using a linear regression analysis. The skewness R-sk showed a coefficient linear regression R-2 = 0.80, while the average roughness R-a, used in most previous studies, presented an R-2 = 0.56. It was concluded that when specimens with substantially different surface conditioning are compared, as in retrievals analysis, it is also important to report the skewness R-sk so that qualify the surface roughness of the specimens can be qualified.