In-vitro wear simulation of sequentially crosslinked and annealed polyethylene acetabular liners: 14 years of results

Abstract

Sequentially crosslinked and annealed polyethylene (SXL) is a second generation highly crosslinked polyethylene that has been available for joint replacement with clinical success. The purpose of this study is to characterize SXL cups through an analysis of almost 14 years of historical in vitro wear simulation data. Three characteristics were evaluated in this study (1) Wear rate, (2) linear penetration and (3) shelf-life effect on wear.Specimens were SXL acetabular cups with internal diameters ranging from 22mm to 44mm, and thicknesses ranging from 3.9mm to 19.6mm. All cups were tested for 1–5 million cycles. Wear rates were tested for normality and suspected outliers were tested using the generalized extreme studentized deviate (ESD).A total of 290 samples were evaluated, which included suspected outliers. After ESD statistical outlier removal, distribution was non-normal and the range of wear rates was −4.3 to 8.2mm3/mc, with a mean of 1.8±1.6mm3/mc and median of 1.6mm3/mc. Individual bearing sizes and thicknesses reveal non-normal distributions for the 22mm, 32mm, and 40mm groups and for the 10mm thickness group. The sample with the highest calculated linear penetration rate, 22.2μm/mc, was for a 22mm bearing size with 19.6mm thickness. Before and after outlier removal, regression results of bearing size, thickness, and shelf age produce linear coefficients of correlation, R2<0.1, for all comparisons.Published clinical data performance at 5 years documented with 32mm bearings show clinical values ranging from 0.008mm/yr to 0.015mm/yr. The 32mm bearing size represents 127 samples in this data set, and the mean calculated linear penetration is 0.008mm/mc. This value compares favorably to the reported penetration values, assuming that 1 million hip wear simulation cycles approximates closely to one year of in-vivo cycles. In addition, wear analysis showed no effect of shelf-life on wear and maintained its low wear characteristics regardless of polyethylene thickness or head size.