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 t ranslation (APT) and inward-outward rotation (IOR). Preliminary tests were done with non-irradiated, and with gamma-irradiated, artificially aged ultr a-high molecular weight polyethylene disks. The counterface was a 54 mm dia meter, polished CoCr ball. The load was static 2 kN, and lubricant diluted calf serum. The wear of non-irradiated polyethylene proved to be insensitiv e to the disk thickness. Gamma-irradiation and aging resulted in higher wea r 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 w ear zone was burnished in all disks, the dominating wear mechanism being ad hesive. Severe delamination occurred only in a disk made from a gamma-irrad iated tibial component which had been on the shelf for 10 years. In accorda nce with clinical findings, the majority of the polyethylene wear particles had a diameter between 0.1 and 1 mum, with an average of 0.7 mum. As the t ibial components made of polyethylene are often damaged by oxidation, the e ffects of aging conditions on polyethylene wear are important subjects of f urther studies. (C) 2001 Elsevier Science B.V. All rights reserved.