This experimental study tests two hypotheses which address outstanding questions in cartilage lubrication: can the friction coefficient remain low under sustained physiological loading conditions? How effective is synovial fluid (SF) in the lubrication of articular cartilage? Based on theory, it is hypothesized that migrating contact areas can maintain elevated cartilage interstitial fluid pressurization, thus a low friction coefficient, indefinitely. It is also hypothesized that the beneficial effects of SF stem from boundary lubrication rather than fluid-film lubrication.
Five experiments were conducted on immature bovine femoro-tibial joints, to compare the frictional response under migrating vs stationary contact areas; the frictional response in SF vs saline; the role of sliding velocity and the role of congruence on the friction coefficient.
Migrating contact area could maintain a low friction coefficient under sustained physiological conditions of loading for at least 1 h. SF reduced the friction coefficient by a factor of approximately 1.5 relative to saline. However, interstitial fluid pressurization was far more effective, reducing the friction coefficient by a factor of approximately 60 relative to equilibrium (zero-pressure) conditions. It was confirmed that SF acts as a boundary lubricant.
These results emphasize the importance of interstitial fluid pressurization on the frictional response of cartilage. They imply that the mechanical integrity of cartilage must be maintained to produce low friction in articular joints. The more limited effectiveness of SF implies that intra-articular injections of lubricants in degenerated joints may have only limited effectiveness on their tribological properties.