Abstract
During the past five years, much research has focused on the effects of sterilisation on UHMWPE wear and mechanical properties. Gamma irradiation sterilisation of UHMWPE causes polymer chain scission and oxidation, which adversely affects both wear and mechanical properties. However, gamma irradiation can also produce crosslinking of the polymer chains, which improves wear resistance. Enhanced polyethylenes or highly crosslinked polyethylenes have been developed to further improve the wear resistance of the material. Highly crosslinked polyethylenes demonstrate markedly improved wear behaviour in hip simulator studies, but they also have a decrease in mechanical properties (yield strength, ultimate tensile strength, and fatigue strength). In a highly conforming joint such as the hip where contact stresses are relatively low due to the large surface area of contact, surface wear mechanisms (abrasion and adhesion) predominate while in a less conforming joint such as a fixed bearing knee replacement, where contact stresses are high, fatigue wear mechanisms occur more typically (delamination and pitting). Modifications to improve the wear resistance of UHMWPE such as the highly crosslinked materials may therefore be more appropriate for hip replacements than for fixed bearing knee replacements.
Gravimetric wear measurements of UHMWPE from hip simulators have been used to predict in vivo behaviour of new materials. However, UHMWPE implants absorb fluid so that measurements of weight loss from wear simulators must be corrected with use of fluid soaked control specimens. There are still some inaccuracies in this technique leading to negative reported wear rates for some highly crosslinked polyethylenes.
The biologic response to particulate debris may be more dependent on the size and number of particles rather than the volume of debris generated. The average particle size appears to decrease with greater amounts of crosslinking suggesting that for some highly crosslinked polyethylenes the number of particles may be greater than for non-crosslinked polyethylene, even though the volume of wear debris is decreased.
Previous efforts to improve the wear behaviour of polyethylene such as the addition of carbon fibres (carbon reinforced polyethylene), hot isostatic pressing (Hylamer), and heat pressing have not demonstrated improvements in vivo. While current joint simulator studies may predict in vivo wear behaviour, clinical studies will ultimately be necessary to determine if highly crosslinked polyethylenes enhance the longevity of total joint arthroplasty.
The abstracts were prepared by Mrs Dorothy L. Granchi, Course Coordinator. Correspondence should be addressed to her at PMB 295, 8000 Plaza Boulevard, Mentor, Ohio 44060, USA.
