The Influence of Loading Time and Lubricant on the Friction of Articular Cartilage

Abstract

Friction of cartilage on metal, metal on cartilage and cartilage on cartilage contact configurations, within a mixed lubrication regime, was measured using synovial fluid, Ringer's solution or with no lubricant present. The main test variable was the period of stationary loading which ranged from 5 s to 45 min, prior to sliding and consequently measuring friction. The coefficient of friction rose gradually with increasing stationary loading time, up to a value of approximately 0.3 at 45 min for all the contact configurations. Following the re-application of load, after short periods of load removal, friction was also found to drop sharply. The flow of liquid in the biphasic cartilage and load carriage by the fluid phase was highlighted as being an important factor in reducing friction within the mixed or boundary lubrication regime. Movement of the contact zone over the cartilage counterface ensured very low friction as the slider moved over fully hydrated cartilage. For the cartilage--cartilage contacts synovial fluid significantly reduced friction compared to Ringer's solution. This was attributed to an effective boundary lubrication action, which was not as effective for the cartilage--metal contacts.

Alteration of articular cartilage frictional properties by transforming growth factor β, interleukin‐1β, and oncostatin M

“…All explants treated with cytokines started at a minimum and monotonically increased to an equilibrium (Figure 2), similar to controls and consistent with a biphasic lubrication phenomenon observed in other experiments using cartilage on glass (24,25,54).…”

“…As such, differences observed between the time constants of the monotonic (t) curve in this study ( Figure 2) and others (24,25,54) are the result of testing cartilage samples of various diameters. Boundary mode lubrication was achieved upon equilibrium, as evidenced by invariance in eq over a range of applied normal strains (20-40%) (44,46).…”

“…Boundary lubrication was clearly regulated by treatments known to promote accumulation of lubricants on the tissue surface. Boundary mode lubrication is most likely to occur at slow speeds, often at the beginning of the gait cycle after the joint has experienced high loads for sustained periods of time (24,51). In boundary mode, eq is highest, and as such, tissue damage is most likely to occur.…”

“…The transition from 0 to eq occurs with a time constant that is governed by fluid flow within the cartilage and is dependent on the magnitude of applied strain (24,44), the strain rate (44), and the square of the sample radius, which is the path length the fluid must travel to exude from the tissue (47). As such, differences observed between the time constants of the monotonic (t) curve in this study ( Figure 2) and others (24,25,54) are the result of testing cartilage samples of various diameters.…”

“…Lubricin has been observed to lubricate a range of bearings, from latex-glass to cartilage-cartilage, in the boundary mode regime (18,(21)(22)(23), although the mechanism of lubrication is not fully understood. The investigation of the role of lubricin in cartilage lubrication is further complicated by a biphasic or pressure-mediated lubrication mecha-nism, which occurs when cartilage tissue is pressurized (24,25).…”