Tribological behaviour of a synthetic synovial fluid and polyurethane in biomedical implants

The purpose of this research is to evaluate the lubrication properties of a synthetic synovial fluid in combination with biocompatible polyurethanes, versus materials commonly used in biomedical implants. This combination is found in endurance testing of meniscal implants made from polyurethane. Two different polyurethanes were used for friction measurements, applying a synthetic lubricant, containing a Ringer's solution, hyaluronic acid and bovine serum albumin. The results were compared with friction measurements, using a polyurethane sphere against bovine cartilage, lubricated with bovine synovial fluid. The influence of the lubricants was tested by comparing water, synthetic- and bovine synovial fluids with the various material combinations, found in existing knee implants. From the measurements it was shown that the friction pairs including metal surfaces did not show the common Stribeck behaviour, with respect to transitions from the boundary regime to full film lubrication, and friction remained relatively constant over the whole velocity range. Friction pairs including the polymer counter surfaces and the water lubricated contacts, showed the expected transitions from boundary to mixed lubrication. From this it was concluded that protein adsorption mainly defined the frictional behaviour when using metal surfaces, leading to a coefficient of friction (COF)≈0.2 using synthetic synovial fluid, and COF≈0.15 when using bovine synovial fluid. PEEK samples showed higher values in the boundary lubrication region, which decreased to values of COF≈0.1 at higher velocities. Polyethylene samples showed higher friction results, which was attributed to the surface roughness. From the observed friction results and wear tracks it was concluded that a synthetic synovial lubricant performs very well with all material combinations, when more attention is paid to the polyethylene surface finish.