Micromechanics of shelf-aged and retrieved UHMWPE tibial inserts: Indentation testing, oxidative profiling, and thickness effects

Understanding the surface micromechanical properties of ultra-high-molecular-weight polyethylene (UHMWPE) may allow for improvement in its wear characteristics. Microtomed sections of two UHMWPE tibial bearings, one that had been irradiation sterilized and shelf-aged, and the other irradiation sterilized and used in a patient, were subjected to depth sensing indentation testing. The microtomed sections exhibited a white band in the subsurface region that is characteristic of oxidation, and the indentation testing, followed by FTIR analysis at the same testing locations, was performed across this region and into the bulk of the material. Indentation testing yielded data leading to hardness, modulus, and energy dissipation factor (EDF). FTIR profiling generated information about oxidation; oxidation indices were calculated by taking a ratio of peak heights at 1716 cm^-1 (ketone) to 2022 cm^-1 (methylene). The mechanical properties showed a strong linear correlation with oxidation index above a minimum thickness. Modulus, hardness, and EDF all increased with increasing oxidation. The appropriate thickness-to-indentation- depth ratio was determined by two methods and was found to be approximately 20:1. The mechanical properties through the oxidized region were seen to vary with depth into the sample in a profile similar to the oxidation profile. The differing aging environments of the tibial bearings are hypothesized to have had an effect on both the mechanical and oxidation profiles. The retrieved bearing exhibited a narrower oxidation profile with peaks closer to the surface, and oxidation indices of lower magnitude. The mechanical properties proved similar, with less intense and narrower readings for the retrieved sample. This research is consistent with much of the literature.