Microabrasion--a simple method to assess surface degradation of UHMWPE following sterilisation and ageing.

The ageing behaviour of ultra-high molecular weight polyethylene (UHMWPE) has been studied following gamma irradiation (25 or 40 kGy) in air. Accelerated ageing procedures used elevated temperature (70 degrees C) and/or pressurised oxygen (5bar). Shelf-aged UHMWPE was also studied. The variation in surface density and mechanical properties were determined following the various sterilisation and ageing treatments. Microabrasive wear testing was also performed. Wear rates were found to correlate well with stress at break for sterilised and aged UHMWPE but not with elongation to failure. It is proposed that the wear mechanism is fracture dominated and occurs following some disentanglement of the polymer chains. Wear also depends upon embrittlement of the surface layer due to its processing and ageing. Elongation to failure in a tensile test is not a good measure of this embrittlement whereas the microabrasion test provides more surface sensitive information concerning this property.