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Ultra-high molecular weight polyethylene knee prostheses of two different sizes (i.e. characterised by different contact areas) were run on a knee wear simulator in two tests differing for the applied load. Gravimetric and micro-Raman spectroscopic analyses were performed to investigate at macroscopic and molecular levels the effects of contact area and applied load on the morphology of the knee components. The larger prostheses showed a higher mass loss in both tests, while Raman spectroscopy showed a more significant polymer degradation in the smaller prostheses, suggesting that a lower contact area is responsible for a higher wear at the molecular wear, but for a lower gravimetric wear. Raman measurements showed that in the second test (using a higher applied load), the wear mechanism changed with respect to the first test: the less severe conditions used in the latter resulted in an increase in the orthorhombic content, while the more severe conditions used in the former determined an increase and rearrangement of the amorphous phase. The results here reported allow to gain more insights into the effects of contact area, applied load and contact stress on wear. Preliminary micro-Raman analyses on in vivo worn knee prostheses confirmed the results obtained in vitro: the retrievals of smaller size were found more degraded at molecular level than those of larger size. These findings showed that the conditions used in the in vitro tests well simulate the knee kinematics of the knee joint and thus the in vivo wear. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Small and large sized-mobile inserts (i.e. size #2 and size #6) were tested on a knee wear simulator for 2 million cycles, using bovine calf serum as lubricant. Gravimetric and micro-Raman spectroscopic analyses were performed on polyethylene inserts. Roughness analysis was performed on the metallic components. The larger size, i.e. #6, had almost twice weight loss with respect to the smaller one, i.e. #2 (37.8±2.8mg and 19.46±0.93mg, respectively). Significant statistical differences were observed in wear rates (p=0.001, Kolmogorov–Smirnov test for two samples). Raman spectroscopy showed a greater increase in crystallinity for the smaller inserts: upon wear, the orthorhombic content (α o) significantly increased at the expense of the third phase content (α b). [Copyright &y& Elsevier]