Vibrational spectroscopy study of the oxidation of differently sterilised UHMWPE explanted acetabular cups.

Ultra high molecular weight polyethylene (UHMWPE) is used from more than 40 years as acetabular cup in total hip joint prosthesis. In order to improve the longevity of the implants, in 1991 Hylamer™ was patented and marketed by the DePuy-DuPont Orthopaedics joint venture. Hylamer™ is a hot isostatically pressed material with high crystallinity (65-71%) and density. Despite the good pre-clinical results, since 1996 some Authors have shown early loosening due to extensive oxidation and phase transformation. In order to analyze the reasons of the high tendency to oxidation of this type of cups, we have examined differently y- sterilised (air and N2) explanted acetabular cups at different distances from the surface using FT-IR spectroscopy. The oxidation is reported as oxidation index (Ol), calculated from the ratio between the area of the carbonyl peak (1670-1850 cm-1) and the area of the reference peak (1327-1394 cm-1). The results obtained indicate that the air-sterilised acetabular cups are more oxidated than the N2 ones. The oxidation of the N2-sterilised cups is only due to the in vivo environment (presence of the typical esther band at 1738 cm-1) while the oxidation of the air-sterilised cups is due both to the environment and the y rays (presence of the cheton band at 1717 cm-1). In the more oxidated sections of the air-sterilised cups we have observed a PE delamination. Moreover, a correlation between Ol and crystallinity of the cups (calculated by differential scanning calorimetrie measurements) can be observed. Our results show how sterilization method can change the PE properties. In particular, the cups sterilized in air demonstrated a severe oxidation caused by both sterilization and in vivo permanence. Oxidation has been associated to a great crystallinity increase together with a deterioration of the material properties.