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.