The cardinal function of articular cartilage is to evenly distribute the load between the joint surfaces, absorbs the shocks during physical activity, and it provides smooth gliding surfaces of low friction.
The duality in biomechanics is due to the two major proteins in the hyaline cartilage, i.e. collagen and proteoglycan. Collagen fibers provide the strength of the cartilage against any sheer force and in connection with the aggrecan molecules gives the resistant character to the articular cartilage.
Aggrecan molecules are found in aggregates wherein approximately 200 molecules are non-covalently anchored to long stretches of hyanluronan 6 stabilized by a third component, link protein. Due to the highly negatively charged GAG side chains these multi molecular complexes associate with hydrated cations and entrap substantial amount of water inside the tissue (65-80% of wet weight7). Its configuration with matrix collagen molecules prevents free flow of water within the matrix and thereby provides the visco-elastic properties of the cartilage. Depending on the load on the cartilage, part of the solvent can be released and later regained when the compression discontinues. Nevertheless, the more water is extruded, the harder it gets to compress the tissue as the repulsion among the negatively charged GAG chains becomes more manifest. This compression-dependent change in solvent content provides the tissue its resilience, and its ability to withstand compressive loads.
The healthy joint fluid and the escaping water from the compressed cartiage tissue both attribute to the low friction movement of the articular surfaces, as it glides on a very thin fluid film. Aging cartilage and the fraying of the surfaces increases friction within the joint (loss of even fluid film), thus it induces further fraying and degeneration of the cartilage tissue.
