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 × 10⁶ 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 R² = 0.80, while the average roughness R_a, used in most previous studies, presented an R²=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.