Fracture behavior of UHMWPE in non-implanted, shelf-aged knee prostheses after gamma irradiation in air.

We studied non-implanted ultra-high molecular weight polyethylene (UHMWPE) knee components, that were gamma-sterilized in air and shelf-aged during 6 and 7 years, to explore fracture mechanisms in the degraded material. Prior to the fractography study, we assessed degradation through FTIR and optical density measurements in microtome sections. Then, we morphologically compared the surface fractography of degraded, oxidized UHMWPE with that of non-degraded material in specimens from the same implants, fractured after uniaxial tensile tests in standardized samples. Fractography surface analysis clearly differentiated the behavior of non-degraded and degraded UHMWPE. Failure in non-degraded samples occurred through craze formation, followed by fast crack propagation. Quantitative fracture analysis suggested a ductile behavior. This was supported by the identified local plasticity in the region close to the initial flaw. In the fractography surface analysis of degraded material, a fibrillar behavior was observed. A heterogeneous domain structure appeared, showing different coarseness with multiple void sizes. Our results, based on mechanical measurements and fracture morphology in the subsurface region, confirmed the embrittlement of oxidized UHMWPE.