Oxidation of ultra-high molecular weight polyethylene and its influence on contact fatigue and pitting of knee bearings

Tibial bearings of total knee replacements are generally made from ultra-high molecular weight polyethylene (UHMWPE). Failure of those bearings has been found to result in many cases from subsurface crack initiation and propagation. Two types of subsurface-originated failure modes are investigated in this study: macroscopic contact fatigue and microscopic pitting. The cracks responsible for both modes of failure initiate in polyethylene material that lias been embrittled by oxidation; this oxidation is an undesirable outcome of the gamma irradiation (in air) used to sterilize the tibial bearings. Accelerated aging was used in this work to achieve oxidation levels similar to those found in retrieved bearings. Tribotesting of the aged bearing materials was carried out under simulated service conditions using a rolling/sliding tester. Macroscopic fatigue cracks and microscopic surface pitting developed in UHMWPE specimens tested on the rolling/sliding tester, and the damage was similar to that found in retrieved tibial bearings. The fatigue cracks invariably initiated in the embrittled oxidized layer, and the number of cycles before initiation of the fatigue cracks was dependent on the oxidation level and on the contact stress.