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Boundary lubrication of articular cartilage : contribution of hyaluronan in health and injury

Abstract

Healthy articular cartilage provides low friction properties to the synovial joint through both fluid film and boundary lubrication. Boundary lubrication, mediated by synovial fluid (SF), is important in reducing articulation-induced shear and wear. SF is composed of molecules including hyaluronan (HA), proteoglycan-4 (PRG4), and surface active phospholipids (SAPL) that have been shown to contribute to boundary lubrication. Joint injury predisposes articular cartilage to deterioration, and the mechanism of this may involve impairment of boundary lubrication function, possibly due to altered concentrations of HA, PRG4, and/or SAPL. However, clear relationships between the composition--boundary lubrication function of SF in health and injury, particularly at physiological test surfaces, remains to be established. Therefore, the goal of this dissertation was to further the understanding of the contributions of SF macromolecules, with a focus on HA, to the boundary lubrication of articular cartilage under normal and pathophysiologic conditions, and to what extent chemical augmentation of injured SF by HA can restore lubrication function of SF and articular cartilage. SF lubrication function and composition were altered coordinately after acute joint injury. Steady-state friction coefficients were higher, while HA concentration and molecular weight (MW) were shifted to lower levels in injury SF. Supplementation in vitro of the abnormal SF with HA of physiologic MW and concentration led to a marked recovery of boundary lubrication function. Articular cartilage lubrication by HA was markedly dependent on concentration and MW, approaching the low friction coefficients of native SF particularly at the MW and concentration of HA found in normal SF. The depletion of HA from SF led to a marked increase in friction coefficients for both normal and injury SF. Additionally, lubrication function of the HA-depleted SF was recoverable to normal levels upon repletion with high MW HA. Collectively, these results elucidate the role of HA as a boundary lubricant for articular cartilage, contributing to the understanding of the altered low-friction properties of cartilage following joint injury, and may also aid in the development of targeted clinical treatments and repair strategies designed to restore failed joint lubrication and prevent further cartilage damage

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