Understanding the lubrication of diarthrodial joints has been an important objective in the field of biomechanics since the early part of this century. The process by which this efficient lubrication is maintained is not yet fully understood although several theories on this topic have been advanced through the years. While research in the biotribology of diarthrodial joints has been relatively limited recently, the applicant believes that there currently exists a unique opportunity to achieve major strides in this area. The objectives of this study are to provide a new quantitative theory for predicting the frictional response of articular cartilage, and to perform experiments that directly test the predictive ability of this proposed friction theory. The dependence of cartilage frictional properties on the presence of a boundary lubricant and on the integrity of the collagen ultrastructure will also be investigated. These objectives will be achieved using the latest developments in the theoretical modeling of cartilage, as well as the latest literature findings from frictional experiments. A correct theoretical model of cartilage friction should be expected to predict all experimentally observed phenomena, or most of them if mixed lubrications modes prevail. These include the ability to predict that the cartilage friction coefficient is time-dependent, velocity-dependent, and load-dependent, while being nearly independent of the viscosity of synovial fluid. The development of such a predictive model is significant at several levels. First, a fundamental understanding of the biotribology of diarthrodial joints has been an elusive basic science goal for several decades. A comprehensive and accurate theory would shed light into a lubrication mechanism heretofore poorly understood, and would provide insights on how that mechanism is suited for the intermittent motions and high loads of diarthrodial joints. Second, a theoretical knowledge of the lubrication mechanism will provide a better ability to predict how that mechanism might deteriorate or be defeated as a result of material property changes (e.g., from cartilage or synovial fluid degeneration), or geometry changes (e.g., cartilage thinning, surface remodeling, surgical interventions). Third, precise knowledge of the behavior of the frictional coefficient of cartilage may provide a greater understanding of the role of frictional surface tractions on the state of stress within cartilage, and whether a deterioration of the frictional properties may promote tissue degeneration through mechanical pathways. One of the long-term goals of this proposal is to demonstrate that the wide variation of frictional properties reported in the literature are not necessarily inconsistent with each other, and can be related to fundamental mechanisms that can be described theoretically, and verified experimentally.
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