Simultaneous measurement of friction and wear in hip simulators

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.

Effect of wear, acetabular cup inclination angle, load and serum degradation on the friction of a large diameter metal-on-metal hip prosthesis

“…The resultant values of the present study measured with a more simple, inexpensive method were still in agreement with those reported in (Haider et al, 2016;Kaddick et al, 2015;Sonntag et al, 2017). Haider et al (2016) obtained a friction factor of 0.08 ± 0.01 for the 44 mm diameter AoP. The corresponding values in the present study can be calculated by dividing the T values, 2.6 Nm ± 1.0 Nm (Table 1), by the peak load, 3 kN, and by the radius, 14 mm, which results 0.06 ± 0.02.…”

“…In these sophisticated, complex designs the three orthogonal components of T can be obtained separately, as the femoral head support is instrumented. The resultant values of the present study measured with a more simple, inexpensive method were still in agreement with those reported in (Haider et al, 2016;Kaddick et al, 2015;Sonntag et al, 2017). Haider et al (2016) obtained a friction factor of 0.08 ± 0.01 for the 44 mm diameter AoP.…”

“…Friction measurement has been added to three-axis hip joint simulators by (Haider et al, 2016;Kaddick et al, 2015;Sonntag et al, 2017). In these sophisticated, complex designs the three orthogonal components of T can be obtained separately, as the femoral head support is instrumented.…”

Three-dimensional friction measurement during hip simulation

“…However, in both cases, torque values around the taper axes during continuous ISO testing are small compared to those that are necessary to initiate mechanical loosening [ 42 , 43 ]. Haider et al [ 26 ] performed friction measurement on 40 mm metal-on-polyethylene bearings in a standard hip simulator intentionally used for wear testing. They reported a lower maximum frictional torque compared to the 40 mm ceramic-on-XPE bearing in the present study.…”

“…Online friction measurements have been performed in wear simulators (orbital bearing type according to ISO 14242–3 and standard wear-testing machines according to ISO 14242–1) using a multi-axis machine transducer [ 26 ] or custom-made measurement devices [ 27 – 31 ]. In these cases, three-dimensional friction results are mainly validated through a comparison with the existing literature.…”