Wear properties of ultra high molecular weight polyethylene material

The Ultra High Molecular Weight Polyethylene (UHMWPE) standard reference material (HSS Poly Hi Solidur) was used to study fatigue properties in torsion. Cylindrical disks of 75 mm diameter by 10 mm thick were carbon coated (Lawrence Livermore Lab.) and gamma sterilized at 2.5-4.5 mrad. They were tested while immersed in a 1% bovine serum albumin solution. The uncoated controls with and without gamma sterilization were also tested for comparison. Each specimen, preloaded to a 600 lbf. Through a 32 mm cobalt chrome modular head (Biomet, Inc.), was cycled ±200 of rotation in an Instron hydraulic biaxial machine (Model 1321). The von Misses stress at the contact surface, estimated by the ANSYS 5.2 finite element analysis was found to be 13.8 MPa for 2 mm depth of penetration by the modular head into the UHMWPE disk. The carbon coating came off the coated specimen after 230,000 cycles (average) of testing. The uncoated samples with and without gamma sterilization, were tested to one million cycles. The particles collected from each of these tests, when characterized by Oil-Red-O (ORO) stain, showed that the non irradiated and uncoated control specimens generated the string like particles. The gamma sterilized disks with and without coating produced predominantly irregular smaller fragments. This change in morphology of the wear particles may be related to the increased brittleness caused by the gamma sterilization. The thin slices cut from interior sections and stained by ORO failed to show the presence of a subsurface white band. This may be due to the relatively shorter period of storage used in this study. It was concluded from this study that this type of carbon coating is not useful to reduce the wear particle generation in the UHMWPE