Effect of crosslinking and Pt-Zr quasicrystal fillers on the mechanical properties and wear resistance of UHMWPE for use in artificial joints

The use of artificial joints is common for restoration of comfort and functionality in joints that have been afflicted with cartilage loss due to disease or injury. These implants incorporate an articulating interface of ultra-high molecular weight polyethylene (UHMWPE) sliding against a polished metallic counterface such as 316L stainless steel. While the design of these joints has been refined over several decades, there are still significant limitations to their pain-free lifetime due to osteolysis induced by UHMWPE paniculate. Crosslinking has been used recently to increase the wear resistance of the polymer, but there are significant tradeoffs involving reduced elastic modulus and impact toughness. The authors have proposed using Pt-Zr quasicrystals (QC) as fillers in UHMWPE as a method of increasing the wear resistance of the polymer while avoiding significant losses in mechanical properties. UHMWPE samples that had been irradiation crosslinked, filled with 20 wt.% Pt-Zr quasicrystals, or both, were tested in a dynamic mechanical analyzer to determine their viscoelastic properties. Furthermore, Charpy impact tests were performed on these materials, as well as multi-directional sliding wear tests in the dual axis wear simulator (DAWS), a machine designed to simulate in vivo joint wear conditions. It was found that while crosslinking reduced elastic modulus of UHMWPE over 30%, the use of QC fillers led to a slight increase. Additionally, the reduction in impact toughness when using QC fillers was not as great as with crosslinking. Finally, it was found that both QC fillers and crosslinking provided the same significant reduction in wear amounts over untreated UHMWPE. The reduction in wear is explained in terms of the wear mechanisms. This involves inhibition of polymer chain orientation in the case of crosslinking, and shear load shielding effects of quasicrystals in the case of QC-filled polymer. These results suggest that Pt-Zr quasicrystal filler may be a desirable alternative to crosslinking when attempting to increase the wear resistance of UHMWPE for biomedical applications.