Abstract
Improving the wear resistance of polyethylene is considered paramount to improving knee implant longevity. Consequently, a range of polymer fabrication techniques have evolved in the quest for a highly wear resistant material. The objective of this study was to explore the wear performance of polyethylene as fabricated in a variety of ways.
The following materials were prepared, sterilised, artificially aged, and machined into wear specimens (n = 4 for each material): Compression molded GUR1050 with three levels of cross-linking (120 kGy, 65 kGy, and 0 kGy irradiation – control); ram extruded GUR4150 high modulus material; compression molded GUR4150 low modulus material; and HSS/PolySolidur/Hoechst reference polyethylene. Using a custom designed joint articular wear simulator, samples were loaded for 2 million cycles at a frequency of 0.5 Hz under loads of 2.1 kN. Tests were stopped every 250 000 cycles; and wear surfaces were examined microscopically for surface damage (pitting, cracking, delamination).
After 2 million loading cycles the following specimens were pitted and delaminated: 2 GUR1050 control samples, 3 GUR4150 high modulus specimens, and all 4 reference polyethylene specimens. Burnishing, but no pitting, was seen in all GUR1050 elevated cross-linked polyethylene specimens, and in all GUR4150 low modulus specimens.
The materials tested in this study represent a broad range of fabrication techniques. Differences in starting resin cannot fully account for the differences in wear behaviour seen between the groups; as damage was not limited to one resin group. The cross-linked specimens were melt-annealed, prior to cross-linking. It is possible that this processing step, and not the actual cross-linking, contributed to the improved wear performance of this group. However, of most interest is the comparable wear performance of GUR1050 cross-linked polyethylene and GUR4150 low modulus polyethylene suggesting that cross-linking polyethylene is not the only route towards obtaining a polyethylene with superior wear characteristics.
The abstracts were prepared by Nico Verdoschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, Universitair Medisch Centrum, Orthopaedie / CSS1, Huispost 800, Postbus 9101, 6500 HB Nijmegen, Th. Craanenlaan 7, 6525 GH Nijmegen, The Netherlands.
