There are limited treatment options available today to slow down progression of osteoarthritis in its early stages and most interventions, such as highly invasive partial and total joint replacement surgeries, are performed only at the late stages of the disease. Understanding the mechanism of early articular cartilage stress-mediated wear and failure can aid in the design of new treatment options that are introduced at earlier stages of the disease, presenting the potential to slow down osteoarthritis progression and thus significantly improve patient outcomes. This dissertation aims to provide a basic science understanding of wear propagation and repair of articular cartilage in the absence of traumatic events under the normal reciprocal sliding motion of the articular layers at physiologic load magnitudes. In this dissertation there are three main thrusts: (1) characterize cartilage delamination wear under normal sliding (2) define a chemical environment that promotes cartilage explant homeostasis to enable long-term wear-and-repair studies (3) investigate a practical treatment modality capable of stopping or slowing down structural degeneration of articular cartilage in OA.
We hypothesize that the mode of cartilage damage is delamination wear that progresses by fatigue failure of the extracellular matrix (ECM) under physiologic sliding, even when cartilage layers are subject to physiologic load magnitudes and contact stresses and even when the friction coefficient μ remains low (H1a). Based on prior literature findings regarding the role of synovial fluid (SF) boundary lubricants on the reduction of friction and wear, we also test the hypothesis (H1b) that SF delays the onset of cartilage delamination when compared to physiological buffered saline (PBS). We then test a third hypothesis (H1c) that loading cartilage against cartilage delays the onset of delamination wear compared to testing glass on cartilage, since contacting porous cartilage layers exhibit a much smaller solid-on-solid contact area fraction than impermeable glass contacting porous cartilage.
Next, we hypothesize that the homeostatic dysregulation previously observed in cultured immature cartilage explants results from the presence of non-physiologic levels of important metabolic mediators in the culture medium. To this end, we hypothesize that: (H2a) immature bovine cartilage explants cultured in native synovial fluid will maintain homeostasis as characterized by maintenance of their mechanical properties and ECM contents at initial (post- explantation) levels, and (H2b) explants cultured in a physiologic-based medium, consisting of physiologic levels of key metabolic mediators, will maintain a similar homeostasis over long- term culture.
Finally, a laser treatment strategy is explored that has the capability to reform collagen crosslinks, replacing those lost during OA progression. This novel therapy acts without injuring the cells and without any chemical additive or thermal ablation. The laser treatment protocol used in this application can specifically target the subsurface region, located 200 μm of the articular surface. By strengthening this region with enhanced crosslinking, we hypothesize (H3a) that cartilage will demonstrate greater resistance to fatigue failure than untreated controls. We then hypothesized (H3b) that this treatment protocol would also be effective on devitalized fibrillated human articular cartilage from OA joints with overall Outerbridge score OS1-3.
We find that for both cartilage-on-cartilage and glass-on-cartilage sliding configurations at physiologic applied loads, long-term sliding with a low friction coefficient causes wear in the form of delamination. We show that the use of synovial fluid as a lubricant delays the onset of wear; and, similarly, that sliding with a cartilage counterface also reduces the incidence of wear. In subsequent studies we fully characterize a homeostatic culture medium to emulate cartilage in vitro behavior in synovial fluid. We show that explants cultured in this medium can maintain their properties for at least one month and have no loss in cell viability. Laser treatment is then tested on both living and devitalized bovine and devitalized human cartilage and the treatment is shown to improve the wear resistance of the tissue without harming embedded cells.
Overall this work has led to novel insights that have clinical applicability. One strength of the in vitro investigations described in this body of work is the ability to separate out mechanically-mediated events from biochemically-mediated events, which would be impossible in vivo. Parsing out such specific mechanisms of cartilage wear can help guide better understanding of disease progression and drive therapeutic intervention. Intervening during the early stages of OA offers the promise of preventive care that currently does not exist and could provide significant benefits to a patient’s quality of life. This dissertation asserts that focusing on delaying or preventing wear by improving the resiliency of the extant intact cartilage in early OA is a viable strategy to improve patient outcomes and offers an innovative approach over existing regenerative techniques.
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