Lubrication mechanisms of lipids and proteins in model synovial fluids

Abstract

Artificial joints restore function which has been impaired due to disease, trauma or genetic conditions. Although many prostheses function satisfactorily for the lifetime of the patient, in some cases failure occurs due to wear of the bearing surfaces. To assess the wear life of artificial joint systems extensive testing in simulators is performed, yet there are discrepancies between the predicted performance and wear experienced in vivo. The lubricant used in simulators has a different chemical composition to synovial fluid. As the composition affects the performance of a lubricant, specifying a model fluid closer to synovial fluid will improve simulator testing, and consequently joint design. The difficulty in specifying a model fluid is that it is not fully understood which elements of synovial fluid determine the lubrication performance. This thesis aims to develop an understanding of how model synovial fluids can provide boundary lubrication in artificial joints. Static and dynamic measurements are used to reconcile the adsorption and lubrication behaviour of synovial fluid proteins and lipids. The boundary lubrication of both current prosthesis materials and cutting-edge artificial cartilage materials is considered. Static tests show changes to adsorbed films dependent on surface interactions, pH and interactions between proteins and lipids. This affects the hydration and macromolecular content of adsorbed layers, which impacts on the layer properties. Under loading and shear, pH and macromolecule interactions govern the formation of lubricating films, through both adsorption and the formation of thicker, aggregated films. Adsorbed film properties were strongly correlated with the lubricating film thickness for protein solutions, but affected by structural changes for solutions containing lipid vesicles. Comparing the lubrication of current and cutting-edge materials shows that the permeability of the surface influences the active lubrication mechanisms. Based on the findings of this work, recommendations for the composition of model synovial fluids are made.