Improved performance of ultra-high molecular weight polyethylene for orthopedic applications



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A considerable number of total-joint replacement devices used in orthopedic medicine involve articulation between a metallic alloy and ultra-high molecular weight polyethylene (UHMWPE). Though this polymer has excellent wear resistance, the wear particulate produced leads to the limited lifetime of the devices ? osteolytic bone loss. Crosslinking has been shown to reduce the wear rate of UHMWPE, but can cause a reduction in various mechanical properties such as impact toughness. This study presents two alternate approaches to improving the wear performance of UHMWPE in orthopedic applications Previous work has shown that UHMWPE-based composites have wear resistance comparable to the irradiation-crosslinked polymer. Zirconium has been shown to have excellent corrosion resistance and biocompatibility, and the authors have used the material as reinforcing filler in UHMWPE with promising results. Compression-molded UHMWPE composites with up to 20 weight percent (wt%) of micro-sized zirconium particles were investigated with regards to wear behavior and impact toughness. These composites showed a significant reduction in wear compared to unfilled polymer while still maintaining impact toughness. These results reinforce the paradigm of using polymer composites for orthopedic applications and may provide a viable alternative to the property tradeoffs encountered with irradiation crosslinking. Apart from UHMWPE, novel materials including hydrogels and bio-derived polymers show great potential in orthopedics, but such materials require the development of innovative fixation techniques [1-3]. The development of controlled porous UHMWPE morphologies offers the opportunity to utilize and expand these developing technologies. Interconnected porous structures were prepared by dry mechanical mixing of NaCl particles and UHMWPE powders followed by compression molding. Samples were soaked in water to remove the embedded salt, leaving a porous UHMWPE structure. Computational simulations of porogen distribution and leaching predicted leaching to be 95% effective when initial salt concentrations were 60wt% and higher, which was found to match very well with the experimental data. It was found that varying the concentration and particle size of the porogen can tailor the final pore morphology to a specific application, while DMA results showed that storage and loss moduli depend greatly on porosity, but not on pore size. Finally, porous UHMWPE scaffolds were successfully impregnated with gelatin, confirming the compatibility of UHMWPE with hydrogel-based fillers.