Defect initiation at subsurface grain boundary as a precursor of delamination in ultrahigh molecular weight polyethylene

In order to examine the initiation mechanism of delamination in ultrahigh molecular weight polyethylene (UHMWPE) knee components, a bi‐directional sliding fatigue test was performed for three types of UHMWPE specimens: nonirradiated, gamma‐irradiated (25 kGy) and gamma‐irradiated (25 kGy) with 0.1% vitamin E added. Sliding surfaces of post‐tested UHMWPE specimens were observed using an optical microscope and a scanning electron microscope. Also, surface roughness was measured at the sliding surfaces of UHMWPE specimens. Delamination was observed only in gamma‐irradiated specimens. A networked structure of surface asperity that resembled grain boundary was observed prior to delamination in gamma‐irradiated specimens. Surface roughness in the gamma‐irradiated specimens, higher than in any other specimen, showed a rapid increase prior to delamination. Detailed observation using an optical microscope and a scanning electron microscope showed microscopic crack initiation along subsurface grain boundaries in gamma‐irradiated specimens. These results suggest that subsurface crack initiation is a precursor of delamination and is accelerated by oxidative degradation due to gamma irradiation. Of the three types of specimens, UHMWPE with vitamin E added showed the lowest surface roughness values at all measuring points. The addition of vitamin E is effective in improving wear resistance and fatigue performance of UHMWPE. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res 67A: 276–284, 2003