Oxidation of differently sterilized hylamer acetabular cups after in vivo wear.

The aim of this work was to investigate the oxidation profile, the oxidation products and their distribution in differently sterilized Hylamer acetabular cups explanted after in vivo wear. A microtome was used to obtain 200 Dm sections from spherical hearing portion of explanted Hylamer acetabular cups gamma ray sterilized in air. The thin sections were soaked in boiling exane and the extracted PE sections were scanned through their thickness in 0.2 mm deep increments using FTIR analysis. An explanted Hylamer cup sterilized in N2 was used as control. DSC measurements were also performed in order to correlate the oxidation with the crystallinity of PE. The cups sterilized in air show severe oxidation in the back and in the bulk with the highest oxidation index (OI = 6.3) at 1 mm from the back surface, while no sign of oxidation was found at the hearing surface. An high concentration of esters probably due to biological environmental oxidation was found on the back of the cups. In the bulk the oxidation was prevalently due to chetons, that are associated with an oxidation due to the sterilization with gamma ray in the presence of oxygen. In fact in the control cup the OI was lower and the presence of chetons as negligible. The oxidation profile was similar to that obtained by other author: maximum OI few mm from the hearing surface. The absence of oxidation at the hearing surface of the air sterilized cups could be due to the wear. In fact if wear progresses faster than oxidation, PE will be removed before its oxidation. The sections of PE sterilized in air with higher OI were delaminated and highly crystalline. Moreover, we have found a good correlation between oxidation profile and crystallinity profile. A possible cause of the poor wear resistance of Hylamer cups could be the high tendency to oxidation specially if they were sterylized in air.