It is activity itself that combats the natural deflation process associated with interstitial lubrication."įuture work for Burris and his colleagues includes exploring the implications for osteoarthritis (OA), which is associated with the degradation of cartilage.Rheumatoid arthritis is a debilitating form of arthritis that is caused by the body’s own defenses. "We know that cartilage thickness is maintained over decades in the joint and this is the first direct insight into why. "We observed a dynamic competition between input and output ," Burris said. Burris believes that hydrodynamic pressures, which force fluid flow into the cartilage, must have counteracted the fluid lost to exudation. Since their experiment involved stationary contacts - in which contact between glass and cartilage occurs at a single site rather than moving across the entire surface of the cartilage plug - their results couldn't be explained by migrating contact theory, like the Ateshian group's. They found that at slow sliding speeds (less than would occur in a joint at typical walking speeds) cartilage thinning and an increase in friction occurred over time, but as the sliding speed increased toward typical walking speeds, the effect was reversed. student, placed larger-than-average cartilage samples against a glass flat to ensure the presence of the necessary wedge. To investigate whether hydrodynamic pressurization could refill deflated cartilage, Burris and A.C. If the tire were porous, however, the exterior fluid pressure could force fluid back into the tire. For example, when a normal tire travels over water at a high speed, pressure builds until a film forms to lubricate the interface this is called hydroplaning, and results in a complete loss of frictional control. "However, it was unclear to us how our joints could prevent deflation given the long periods of time we spend sitting and standing each day without some active input mechanism." That is, there must be some way for the cartilage to reabsorb the fluid that leaks out when we're not moving.īurris had a hunch that the reabsorption process was driven by hydrodynamic pressurization, which occurs whenever the relative motion of two surfaces causes fluid between them to accelerate in the shape of a triangular wedge. "This study was the first direct evidence that interstitial pressure is a viable mechanism of long term load support and lubrication," Burris said. In 2008, Ateshian's group demonstrated this phenomenon for the first time using a small sphere articulated against a cartilage plug, showing that interstitial pressure was maintained indefinitely if the contact area moved faster than the diffusive speed of the synovial fluid. In 1995, a group at Columbia led by Gerard Ateshian used theory to show that continuous knee movement could prevent the deflation process if it occurred faster than the fluid could respond. 18-23 in San Jose, Calif.īurris and his colleagues are not the first to study cartilage deflation. Burris will speak about his research during the AVS 62nd International Symposium and Exhibition, held Oct. Burris and his colleagues have proposed a mechanism that explains how motion can cause cartilage to reabsorb liquid that leaks out. "The important question is why cartilage doesn't deflate over the course of days, months or years in our joints," said David Burris, an assistant professor in the Mechanical Engineering Department at the University of Delaware. Yet the symptoms associated with osteoarthritis usually take decades to develop. Since cartilage is porous, fluid is readily squeezed out of the holes over time. Loss of this fluid, called synovial fluid, results in a gradual decrease in cartilage thickness and increase in friction, which is related to the degradation and joint pain of osteoarthritis. Cartilage is filled with fluid - about 80% of the volume of the cartilage tissue - that plays the essential roles of supporting weight and lubricating joint surfaces.
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