THE INFLUENCE OF ABDUCTION ANGLE ON THE WEAR CHARACTERISTICS OF A SECOND GENERATION HIGHLY CROSSLINKED POLYETHYLENE

Abstract

The introduction of highly crosslinked PE with improved wear performance has allowed for the marketing of thin liners. Previous studies have shown that steep angles reduce femoral head coverage thereby decreasing contact area and can subject the acetabular rim to excessive stresses. This can be especially concerning for thinner PE constructs. Previous work with thicker (9.9mm) non-crosslinked PE show a correlation of decreased wear with increased abduction angle. Therefore, the objective of this study was to isolate and examine the effects of varying cup abduction angles on the wear of a thin second generation highly crosslinked polyethylene. Five sets of sequentially crosslinked Trident^® design inserts with a wall thickness of 3.9mm were evaluated. Sequentially crosslinked liners were machined from compression molded GUR1020 UHMWPE that had been γ-irradiated followed by annealing 3 times (X3). Testing was conducted using a hip joint simulator for 3 million cycles. All cups were fixed, positioned superiorly at a neutral version angle, and divided into five groups of varying inclination angles: 0°, 20°, 30°, 50° and 70°. A physiological load was applied to each couple at a rate of 1Hz using Alpha Calf Fraction serum. Weight was converted to volume and plotted as a function of cycle count. In addition, all PE inserts were microscopically analyzed for any gross damage and areas of deformation. Wear rates plotted against inclination angle exhibited poor correlation between wear rate and angle (R2=0.253). Student’s t-tests revealed significant differences (p< 0.05) between 0° and 70°, and between 50° and 70° angles. There was no statistical differences for any of the other tested angles. Visual inspection of the tested liners revealed wear scars of increased areas of polishing on inserts positioned at lower abduction angles. No deformation, cracking or pitting of the liners was observed. Visual inspection of the liners revealed an increase in overall area of polishing with a reduction in abduction angle. This indicates that load is concentrated over a smaller area for higher angles resulting in increased contact stress for steeper cups; however, this did not translate into a correlation of high abduction angle and high wear. These results do not correlate with our previous work, however that study was conducted on smaller diameter thicker non-highly crosslinked material. We believe the difference in results is due to fundamental material response. Although visual burnishing indicates a trend in contact area, there may be a role of deformation in the results. Future work will involve finite element analysis to study these differences. The results in this study suggests that the sequentially crosslinked polyethylene is able to maintain its low wear characteristics at various abduction angles even with a thin (3.9 mm) liner.