The relative influence of five variables on the in vitro wear rate of uncrosslinked UHMWPE acetabular cup liners

Abstract

The wear factors, k exp ( in 10 −6 mm 3 N −1 m −1) , of two sets of hemispherical uncrosslinked ultra-high-molecular-weight polyethylene UHMWPE acetabular cup liners, when articulated against 28-mm-diameter Co–Cr alloy femoral heads, were determined in a hip joint wear simulator. All test conditions used were the same for both sets, with the exception that in one set (comprising six liners), the bovine serum concentration in the test lubricant ( P) was 25 vol% , while it was 90 vol% for the other set (comprising seven liners). For each liner in each set, the values of the following independent or explanatory variables were determined: the tensile toughness ( U, in MJ m −3 ) of the UHMWPE; the degree of crystallinity (% C) of the UHMWPE; the mean surface roughness ( R a, in μm) of the liner; and the radial clearance between the femoral head and the liner (Δ L, in mm). All the results in both sets were combined and correlational analyses were performed between k exp, on the one hand, and various combinations of U, % C, R a,Δ L, and P, on the other hand. It was found that the strongest correlations were obtained when P was included, but that % C and U are also important variables. Within the range of values of the independent variables used, the recommended relationship is k exp =460U 0.72%C −5.29P −0.66 ( adjusted r 2=0.858). This relationship has many potential uses, such as for screening uncrosslinked UHMWPEs that are being considered as candidates for fabricating acetabular cup liners, and for conducting studies of the sensitivity of k exp to a change in a value of a specified independent variable (with the values of all the other independent variables held constant). These uses, as well as the study limitations, are discussed in detail.