The effect of model and human synovial fluid chemistry on wear of CoCrMo in bench tests

Abstract

Prostheses, for example hips and knees, are used to relieve pain and restore function in degenerated synovial joints. They are tribological devices and are susceptible to wear of the bearing surface which can lead to early failure. Human synovial fluid (HSF) is produced to lubricate the joint, however in tribological testing of implants 25%w/w Bovine Calf Serum (25BCS) is used as a screening fluid, although the composition is different. The key questions addressed in this thesis are: the lubrication and wear mechanisms similar? Is 25BCS an appropriate replacement for HSF in wear tests? A ball-on disc CoCrMo wear test was developed to use small lubricant volumes suitable for HSF, which was supplied by the Musculoskeletal Group at Imperial College. Post-test the wear was measured, and surface deposits analysed to provide insights into lubrication mechanisms. Friction and wear of HSF samples were compared to BCS and model fluids comprising of proteins (albumin and globulin), lipids and hyaluronic acid. The wear depth for most HSF samples was significantly higher (up to 77%) compared to 25BCS. Deposited films were observed in and around the wear scar and the pattern was similar for all protein containing fluids including HSF. The deposits appeared to be viscous, insoluble and adherent to the surface. Micro-IRRAS analysis showed they were formed of denatured proteins although in some instance lipids were present. Analysis of the amide I band showed the protein secondary structure was transformed to an increased native β-sheet content with some fibular β-sheet structures. Thioflavin T (ThT) was used to confirm the presence of β-sheet amyloid fibrils. The research showed that 25BCS isn’t an appropriate substitute for HSF. The lubrication and wear mechanisms are similar, however 25BCS has a greater tendency to form high β-sheet content deposits due to their chemical composition that protect surfaces from wearing.