Abstract
Total Joint Replacement (TJR) devices undergo standardized wear testing in mechanical simulators while submerged in a proteinaceous testing solution to mimic the environmental conditions of artificial joints in the human body. Typically, bovine calf serum is used to provide the required protein content. However, due to lot-to-lot variability, an undesirable variance in testing outcome is observed. Based on an earlier finding that yellowish-orange serum color saturation is associated with wear rate, we examined potential sources of this variability, by running a comparative wear test with bilirubin; hemin; and a fatty acid, oleic acid, in the lubricant. All these compounds readily bind to albumin, the most abundant protein in bovine serum. Ultrahigh molecular weight polyethylene (UHMWPE) pins were articulated against CoCrMo discs in a pin-on-disc tribometer, and the UHMWPE wear rates were compared between lubricants. We found that the addition of bilirubin increased wear by 121%, while hemin had a much weaker, insignificant effect. When added at the same molar ratio as bilirubin, the fatty acid tended to reduce wear. Additionally, there was a significant interaction with respect to bilirubin and hemin in that UHMWPE wear rate decreased with increasing fatty acid concentration. We believe the conformational change in albumin by binding bilirubin makes it more likely to form molecular bridges between UHMWPE and the metal counterface, thus increasing adhesive wear. However, fatty acids compete for binding sites on albumin, and can prevent this conformational change. Hence, the protein is stabilized, and the chance for albumin to form bridges is lowered. Ultimately, UHMWPE wear rate is driven by the competitive binding of bilirubin and fatty acid to albumin.
Highlights
Total Joint Replacement (TJR) is a successful clinical procedure to alleviate pain and restore the function of damaged articular joints in the human body
In our previous work [18], we extended the concept of albumin adhesion by Widmer et al [19] and proposed that in vivo PE wear is generated by an adhesive binding process, driven by protein interactions with both joint counterfaces
For pins run in Low fatty acids (FA)/Bovine Serum Albumin (BSA), moderately sized protrusions the case of BSA.protrusions
Summary
Total Joint Replacement (TJR) is a successful clinical procedure to alleviate pain and restore the function of damaged articular joints in the human body. Most of the replacement bearings use a hard-on-soft combination, with ultrahigh molecular weight polyethylene (UHMWPE) as the soft partner of the bearing couple. Prosthetics are subjected to standardized [1,2,3,4] simulated wear tests, while samples are submerged in a proteinaceous solution. This solution is intended to replicate the synovial fluid, and contains water, albumin (the most abundant protein in synovial fluid [5]), buffers, and often antibiotics and antifungal agents. Previous studies dealing with metal-on-UHMWPE bearings [6,7]
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