The effect of adding a second axis of motion was investigated for pin-on-disk wear testing of ultra-high-molecular-weight polyethylene (UHMWPE) for orthopaedic implant applications. In addition to linear reciprocation of the UHMWPE or metal disk, axial rotation of the metal or UHMWPE pin was conducted. The added rotation reproduces the cross-shear on the UHMWPE surface that is generated in clinically relevant wear simulator tests and in vivo. The wear rates that result from the multi-axis pin-on-disk tests are significantly higher (one to two orders of magnitude) than those seen in the linear-only tests. This supports the findings of other researchers (Bragdon et al., 1996; McKellop, 1995; Walker et al., 1996; Wang et al. 1997) in that the application of nonlinear motion increases the wear of UHMWPE substantially. This is further validated by the comparison of a hip simulator wear test conducted with three axes of motion — rotation, flexion, and abduction — to a test conducted with two axes of motion — rotation and flexion. The absence of the abduction eliminated a significant degree of nonlinear motion (cross-shear) and, consequently, the wear rate was significantly lower than that seen in the test with abduction.

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