Abstract
Introduction
Surface wear of polyethylene is still considered a long-term risk factor for clinical success, particularly as life expectancy and activity levels increase. Computational models have been used extensively for preclinical wear prediction and optimization of total knee replacements (TKR). In most cases, the input wear parameters (wear factors and coefficients) to the computational models have been experimentally measured under average contact stresses to simulate standard activities. These wear studies are not therefore applicable for more adverse conditions that could lead to edge loading and high stress conditions, including higher levels of activities and severe loading conditions. The current study investigated the multidirectional pin-on-plate wear performance of moderately cross-linked ultra-high molecular weight polyethylene (UHMWPE) under high applied nominal contact stress, to be used as inputs to a computational model investigating adverse high stress conditions.
Materials/Methods
Moderately cross-linked UHMWPE (GUR_1020,5Mrad gamma irradiation) pins were tested against cobalt–chrome alloy (CoCr) plates in a multidirectional pin-on-plate wear simulator. The CoCr metallic plates were polished to an average surface roughness of 0.01μm. The pin rotation and the plate reciprocation of ±30º and 28mm were in phase, having a common frequency of 1Hz, and resulted in a multidirectional motion at the pin-plate contact surface in a flat-on-flat configuration. Six different pin diameter and applied load combinations were tested, resulting in applied nominal contact stresses from 4 to 80[MPa](Fig.1). Each set was run for 1million cycles in 25% bovine serum as a lubricant. The volumetric wear was calculated from the weight loss measurements using a density 0.93mg/mm3 for the UHMWPE material. The wear factor and wear coefficient were calculated as (volumetric wear/(load x sliding distance)) and (volumetric wear/(contact area x sliding distance)) respectively[1]. Statistical analysis of the data was performed in ANOVA and significance was taken at p<0.05.
Results
Changing the load from 80 to 216[N] (5mm_pins) and from 212 to 283[N] (3mm_pins), increased the volumetric wear from 1.38±0.12 to 1.66±0.07 and from 0.8±0.12 to 1.03±0.05[mm3] respectively (mean±95% confidence interval (CI), n=6). However, under the same load of 216N, changing the pin diameter from 5 to 3 [mm] decreased the volumetric wear from 1.66±0.07 to 0.86±0.02[mm3] (mean±95% CI, n=6) (Fig.1). For stress levels from 4 to 30[MPa] the wear factor significantly decreased from 3.06±0.27 to 0.67±0.11×10−7mm3/N.m] (mean±95%CI, n=6, p<0.001). Any further increase in the stress level (up to 80MPa) did not affect the measured wear factor (ANOVA, p=0.44) (Fig.2-a). In contrast, the measured wear coefficient increased from 1.25±0.11 to 4.88±0.14×10−9] (mean±95% CI, n=6, p<0.001) while increasing the stress from 4 to 80[MPa](Fig.2-b).
Discussion
For the same level of motion at the contact surfaces, the two main parameters that significantly contributed to the volumetric wear were the applied load and contact area. The measured wear parameters were significantly dependent on the applied nominal contact stress. Future work will consider different motions at the contact surfaces with different degrees of cross-shear.
Conclusion
The stress level significantly affected the wear performance of moderately cross-linked UHMWPE in this pin-on-plate configuration. Computational simulations of TKR should therefore account for these effects when considering adverse high stress conditions.
