A three-axis knee wear simulator with ball-on-flat contact

A three-axis knee wear simulator with ball-on-flat contact was designed and built for basic wear and friction tests of prosthetic knee materials. The three-axis motion consisted of flexion–extension (FE), anterior–posterior translation (APT) and inward–outward rotation (IOR). Preliminary tests were done with non-irradiated, and with gamma-irradiated, artificially aged ultra-high molecular weight polyethylene disks. The counterface was a 54 mm diameter, polished CoCr ball. The load was static 2 kN, and lubricant diluted calf serum. The wear of non-irradiated polyethylene proved to be insensitive to the disk thickness. Gamma-irradiation and aging resulted in higher wear rates, which further increased with decreasing disk thickness. The steady-state wear rates of the disks varied from 10.7 to 47.1 mg per one million cycles, and the average coefficients of friction from 0.043 to 0.063. The wear zone was burnished in all disks, the dominating wear mechanism being adhesive. Severe delamination occurred only in a disk made from a gamma-irradiated tibial component which had been on the shelf for 10 years. In accordance with clinical findings, the majority of the polyethylene wear particles had a diameter between 0.1 and 1 μm, with an average of 0.7 μm. As the tibial components made of polyethylene are often damaged by oxidation, the effects of aging conditions on polyethylene wear are important subjects of further studies.