Abstract
We propose and have evaluated a method to measure hip friction during wear testing on a popular multi-station hip simulator. A 6-degree-of-freedom load cell underneath the specimen sensed forces and torques during implant wear testing of simulated walking. This included internal–external and adduction–abduction rotations which are often neglected during friction testing on pendulum-type machines. Robust mathematical analysis and data processing provided friction estimates in three simultaneous orthogonal rotations, over extended multi-million cycle wear tests. We tested various bearing couples including metal-on-plastic, ceramic-on-plastic, and metal-on-metal material couples. In one test series, new and intentionally scratched CoCrMo 40-mm-diameter femoral heads were tested against conventional ultrahigh-molecular-weight polyethylene, highly cross-linked, and highly cross-linked with vitamin E versions. The scratching significantly increased friction and doubled the wear of all groups. Before scratching, friction levels for the aforementioned plastic groups were 0.056 ± 0.0060, 0.062 ± 0.0080, and 0.070 ± 0.0045, respectively, but after scratching increased to 0.088 ± 0.018, 0.076 ± 0.0066, and 0.082 ± 0.0049, respectively, all statistically significant increases (p = 0.00059, 0.00005, 0.0115, respectively). In another test series of 44-mm femoral head diameter hips, metal-on-plastic hips with conventional ultrahigh-molecular-weight polyethylene showed the lowest friction at 0.045 ± 0.0085, followed by highly cross-linked with 0.046 ± 0.0035 (not significantly different). In a ceramic-on-plastic design with conventional ultrahigh-molecular-weight polyethylene, higher friction 0.079 ± 0.0070 was measured likely due to that ceramic surface being rougher than usual. Metal-on-metal hips were compared without and with a TiN coating, resulting in 0.049 ± 0.014 and 0.097 ± 0.020 friction factors, respectively (statistically significant, p < 0.001), and the coating wore away on all coated hips eventually. Higher friction mostly correlated with higher wear or damage to femoral heads or implant coatings, except for the highly cross-linked wear resistant ultrahigh-molecular-weight polyethylene which had slightly higher friction, confirming the same finding in other independent studies. This type of friction measurements can help screen for clamping and elevated wear of metal-on-metal and resurfacing total hip replacements, surgical malpositioning, and abraded and otherwise damaged surfaces.
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