Towards a New Model for Synovial Fluid Lubrication and Wear of Artificial Joints

Surface wear of metal-on-metal (MoM) hip joints is an ongoing problem that can result in the formation of nano-wear particles and premature failure of the prosthesis. Wear is essentially controlled by the properties of the synovial lubricating film and the nature of the articulating surface. The interfacial film thickness formed within the loaded contact zone during the gait cycle is the most important parameter, as this determines surface separation and damage. Film thickness depends on the operating conditions (contact pressure, entrainment speed) and synovial fluid properties (rheology, chemistry). It is usually assumed that a ‘fluid film’ separates the articulating surfaces; however lubricant film thickness has not been measured directly in artificial joints. At present the only insight we have is from mathematical models that calculate film thickness for a simple fluid. Synovial fluid (SF) has extremely complex rheological and chemical properties, which are not captured in the calculation models, and thus these have not been validated for in vivo joint operation. Our objective in this study is to critically examine SF lubrication mechanisms and to develop a physically valid model to predict film formation. To do this film thickness was measured for a model synovial fluid (bovine serum) in a test device that simulates hip joint articulation. Initial studies have focused on the role of SF proteins as these are known to influence wear levels. Such knowledge will provide us with a scientific framework for the development of new prosthesis designs and strategies to reduce wear.