Abstract
UHMWPE implants are made from small powders which are formed by one of three methods. The powders are either compression molded into sheets and then implants machined from the compression molded material, ram extruded into rods and then machined into implants, or molded directly into the final shape. With each method the powders are exposed to variable temperatures and pressures to consolidate the material. It may not be possible to directly mold some implants such as those with complex geometries or modular locking mechanisms.
Clinical and implant retrieval studies of UHMWPE sterilized by gamma irradiation in air have demonstrated that wear behavior may be influenced by resin type and manufacturing method or both. Directly molded Hi-fax 1900 total knee tibial components were found to have more surface wear (scratching and embedded metallic debris) and less fatigue wear (delamination) than similar components which were machined from ram extruded GUR 415 resin (1). The molded Hi-fax 1900 components also demonstrated less oxidation than the machined GUR 415 components. Both groups of implants were sterilized by gamma irradiation in air suggesting that the resin type and manufacturing method or both may influence resistance to oxidative degradation and associated wear behavior. However, most currently available UHMWPE implants have not been sterilized by gamma irradiation in air and it is not clear if wear behavior of these implants will be affected by resin type or manufacturing method.
During the past five years, much research has focused on the effects of sterilization on UHMWPE wear and mechanical properties. Gamma irradiation sterilization of UHMWPE causes polymer chain scission and oxidation which adversely affects both wear and mechanical properties. However, gamma irradiation can also produce cross linking of the polymer chains which improves wear resistance. Enhanced polyethylenes or highly cross linked polyethylenes have been developed to further improve the wear resistance of the material. Highly cross linked polyethylenes demonstrate markedly improved wear behavior 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 cross linked materials may therefore be more appropriate for hip replacements than for fixed bearing knee replacements.
Previous efforts to improve the wear behavior of polyethylene such as the addition of carbon fibers (carbon reinforced polyethylene), hot isostatic pressing (Hylamer), and heat pressing have not demonstrated improvements in-vivo. While current joint simulator studies may accurately predict in-vivo wear behavior, clinical studies will ultimately be necessary to determine if highly cross linked polyethylenes enhance the longevity of total joint arthroplasty.
The abstracts were prepared by Professor Jegan Krishnan. Correspondence should be addressed to him at the Flinders Medical Centre, Bedford Park 5047, Australia.
