Lubrication of and by Articular Cartilage

Publisher Summary This chapter discusses lubrication of articular cartilage by articular cartilage. The study of joint lubrication started with procrustean attempts to force joints into one or other category of bearings then familiar to engineers, but their behavior did not match the expectations for either hydrodynamic or boundary lubrication. They were members of a new category—weeping bearings—that exploited physical principles not knowingly used by engineers. Weeping lubrication was supplemented by an odd sort of boundary lubrication provided by the synovial fluid. Since then, other explanations have been proposed, some peculiar to joints, but one—elasto-hydrodynamic lubrication—reflected advances in the understanding of man-made bearings. Experiments show that even gross changes in the cartilage surface—such as fibrillation caused by arthritis or removal of the outer layer with a knife—leave it quite slippery. Synovial fluid has side groups identical to those found in one component of saliva and in the antifreeze glycoprotein that keeps the blood of some species of fish from freezing at a temperature lower than that predicted by a straightforward application of Raoult's law. Compressive creep data predicts how fast a joint cartilage will get thinner under load.