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Abstract
More than two million artificial knee and hip joints are implanted every year, mainly due to osteoarthritis. The clinical survivorship of artificial joints (AJs) decreases significantly after the first 10 years of being implanted, in particular for patients younger than 55 years, with a reported clinical survivorship of 76% after 10 years. Combined with the extension of life expectancy, there is an urgent need to improve the performance and extend the lifespan of AJs.
This project aims to improve wear performance of ultra-high molecular weight polyethylene (UHMWPE), the most widely used material for bearing surfaces in AJs. The polymer was enhanced using two different approaches: (a) reinforcing the polymer matrix with multi-walled carbon nanotubes (MWCNTs) in different concentrations and through an improved mixing process using sonication, and (b) by implementing a novel design of a three-layered composite based on a hard-soft-hard structure. For the latter approach, the top layer was made of the MWCNT-reinforced UHMWPE, textured with large-diameter dimples to provide the stringent mechanical properties of the three-layered composite and improve wear resistance and promote self-lubrication. The middle layer was made of hydrogel, a soft material with a low Young’s modulus and high-water content (~70%), providing energy absorption and improving lubrication conditions. These two layers were supported by a third layer made of neat UHMWPE.
Wear test and mechanical properties were tested and compared. Although the MWCNT/UHMWPE composites did not evidence a significant change in the hardness with increasing the concentrations of MWCNTs, a reduction in mechanical wear rate of 32% respect to the neat UHMWPE was found when the composite was reinforced with 0.5 wt% of MWCNTs. The three-layered composite demonstrated a significant further wear reduction to around 60% using neat UHMWPE as the top layer and around 66% wear reduction when 0.5 wt% MWCNT/UHMWPE composite was used as the top layer of the three-layered structure.
This study not only provides better understanding of the effects of MWCNT concentrations on the mechanical properties and wear performance of UHMWPE, but also proposed a new design to further improve the wear resistance through better load distribution and self-lubrication.