Abstract
Introduction Sterilisation of UHMWPE prosthetic components by high-energy radiation in air induces an oxidative degradation of the polymer, which may compromise the mechanical performances of the whole implant. Many manufacturers shifted to gas sterilization with EtO and gas plasma or to radiation sterilization in inert atmosphere and with barrier packaging. Aim of the present study was to investigate the relationship between sterilisation method, packaging, oxidation and mechanical properties of current orthopaedic UHMWPE.
Materials 100 sterilised UHMWPE hip, knee, and shoulder components by 19 orthopedic manufacturers were studied. The components were cut in half and sectioned using a microtome into slices of controlled thickness (0,1–0,3mm) which were analysed by FTIR. The UHMWPE packaging was also evaluated by FTIR, in order to correlate the extent of oxidation to the storage conditions. Mechanical properties were evaluated using the small punch test, as described in ASTM F2183-02.
Results The UHMWPE packaging was classified, when possible, into one of the following types; A: PET blister(s) with Tyvek® gas-permeable cover; B: packaging involving a polymeric multilayer bag; C: packaging involving at least one Aluminium foil. Using Type A, air permeable packaging for radiation-sterilized UHMWPE is the equivalent to irradiation in air. Many radiation sterilized implants packaged using Type A materials were severely oxidized. In the case of packaging type B, we detected moderately low oxygen index (OI) in the majority of samples, but an average high hydroperoxide level, even though type B packaging has well-documented oxygen barrier properties. UHMWPE components contained in packaging type C exhibit low OI and hydroperoxide level, due to the impermeable Al foil. The small punch test measurements showed that the oxidised sample exhibit generally diminished mechanical properties. Reductions in the yield load (up to 15%), in the ultimate load (up to 33%) and in the ultimate displacement (up to 21%), compared to the original or EtO sterilised material, have been measured on the majority of the oxidised samples.
Discussion The present results confirm that oxidation has a negative effect on the mechanical properties of UHMWPE. It remains difficult to generalise about the overall effectiveness of barrier packaging at protecting UHMWPE from oxidation, but it is our opinion that a complete absence of sterilisation-induced oxidation can only be guaranteed by gas sterilisation.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
