Because ultrahigh-molecular-weight polyethylene (UHMWPE) is susceptible to frictional wear when used in sliding members of artificial joints, it is common practice to use cross-linked UHMWPE instead. However, cross-linked UHMWPE has low impact resistance; implant breakage has been reported in some cases. Hence, sliding members of artificial joints pose a major trade-off between wear resistance and impact resistance, which has not been resolved by any UHMWPE. On the other hand, multiwall carbon nanotubes (MWCNTs) are used in industrial products for reinforcement of polymeric materials but not used as biomaterials because of their unclear safety. In the present study, we attempted to solve this trade-off issue by complexing UHMWPE with MWCNTs. In addition, we assessed the safety of these composites for use in sliding members of artificial joints. The results showed the equivalence of MWCNT/UHMWPE composites to cross-linked UHMWPE in terms of wear resistance and to non-cross-linked UHMWPE in terms of impact resistance. In addition, all MWCNT/UHMWPE composites examined complied with the requirements of biosafety testing in accordance with the ISO10993-series specifications for implantable medical devices. Furthermore, because MWCNTs can occur alone in wear dust, MWCNTs in an amount of about 1.5 times that contained in the dust produced from 50 years of wear (in the worst case) were injected into rat knees, which were monitored for 26 weeks. Although mild inflammatory reactions occurred in the joints, the reactions soon became quiescent. In addition, the MWCNTs did not migrate to other organs. Furthermore, MWCNTs did not exhibit carcinogenicity when injected into the knees of mice genetically modified to spontaneously develop cancer. The MWCNT/UHMWPE composite is a new biomaterial expected to be safe for clinical applications in both total hip arthroplasty and total knee arthroplasty as the first sliding member of artificial joints to have both high wear resistance and high impact resistance.
Keywords: arthroplasty; artificial joints; multiwall carbon nanotubes; ultrahigh-molecular-weight polyethylene.