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 22 mm to 44 mm, and thicknesses ranging from 3.9 mm to 19.6 mm. 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 8.2 mm 3 /mc, with a mean of 1.8±1.6 mm 3 /mc and median of 1.6 mm 3 /mc. Individual bearing sizes and thicknesses reveal non-normal distributions for the 22 mm, 32 mm, and 40mm groups and for the 10 mm thickness group. The sample with the highest calculated linear penetration rate, 22.2 μm/mc, was for a 22 mm bearing size with 19.6 mm thickness. Before and after outlier removal, regression results of bearing size, thickness, and shelf age produce linear coefficients of correlation, R2 Published clinical data performance at 5 years documented with 32 mm bearings show clinical values ranging from 0.008 mm/yr to 0.015 mm/yr. The 32 mm bearing size represents 127 samples in this data set, and the mean calculated linear penetration is 0.008 mm/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.