Simulation of wear rates and mechanisms in total knee prostheses by ball-on-flat contact in a five-station, three-axis test rig

A five-station, three-axis knee wear simulator with ball-on-flat contact was designed and built. The motions included in the simulator were flexion–extension (FE), anterior–posterior translation (APT) and inward–outward rotation (IOR). The 10 mm thick, 40 mm diameter test disks were machined from an ultra-high molecular weight polyethylene (GUR 1050) bar. The balls were polished CoCr of 54 mm diameter. The lubricant was diluted calf serum, test length 3.3 million cycles and load 2 kN. Five similar tests were run first with non-irradiated disks, and then with newly gamma-sterilized (2.5–4 Mrad in air), non-aged disks. The principal wear mechanism was adhesive and manifested as burnishing of the wear zone. The mean wear rates and standard deviations for non-irradiated and gamma-irradiated polyethylene were 14.5±1.6 and 12.0±0.5 mg per one million cycles, respectively. The p-value of the difference in means was 0.005, strongly suggesting a significant difference. The results agreed with published studies so that initially, before possible deterioration due to ageing, gamma-irradiation improves the wear resistance of polyethylene. This has earlier been observed particularly in hip wear simulations, and has been shown to be due to crosslinking. In conclusion, the new simulator proved to be a valid wear test device for prosthetic knee materials.