Fluid composition influences wear testing of artificial knee implants

Multi-axial simulator testing is a standard tool in development and evaluation of artificial hip and knee joints. Close to in-vivo conditions are desirable in terms of implant set-up, input parameters, and environment. Thus, specimens are subjected to loads and displacements measured for patients with artificial joints, and a lubrication medium is provided to simulate human synovial fluid. Implant wear is then commonly assessed through periodic gravimetric measurements. However, ultra-high weight polyethylene (UHMWPE), the typical material for tibial plateaus, has been shown to gain weight through fluid absorption, which influences the results of such tests. A recent study performed on a 4-DOF knee joint simulator reveals that composition variations in the lubricating medium highly influences fluid uptake of UHMWPE, and alters the wear behavior of tested specimen. Two lubricants, both within boundaries of ISO and ASTM testing standards but with different compositions, have been compared. They showed significant deviations in fluid soak and wear results. It is suggested that wear measurements are only comparable if testing parameters are kept in very tight tolerances and identical lubrication media are used.