The key evolutionary function of a joint is to allow macrostructural (i.e. skeletal) movement whilst causing minimal tissue damage. Each structure involved in the joint has a role to play in dissipating the forces involved in joint motion whilst allowing the joint to return to a normal or resting position. Depending on the configuration of a joint and surrounding supportive structures, joints can move in a variety of ways.
The forces on a joint in motion can be broken down mechanically into compressive, tensile, rotational and shear forces. The joint capsule and ligaments resist tensile, shear and rotational forces on the joint. The articular cartilage and subchondral bone resist compressive forces on the joint. The synovial fluid aids in both resisting compressive and shear forces as well as distributing forces more evenly onto joint structures.
In the normal joint at NO POINT do the articular surfaces come into contact with each other. As the joint is compressed the synovial fluid distributes the compressive forces more or less evenly over the articular surfaces. Pressure on the articular cartilage compresses the proteoglycan molecules and the water in them is "squeezed" out of the cartilage matrix. This water forms a thin film over the surface of the articular cartilage along with HA and Lubricin, further aiding in lubrication of the joint surface. This movement of water molecules absorbs a very small amount of energy, however most of the energy is transmitted by the articular cartilage onto the underlying subchondral bone. When the compression is released the water is reabsorbed and the cartilage returns to its normal shape. The ability of the cartilage to compress with no structural damage is crucial for normal joint function. The compression causes the expulsion of water molecules that further aid in lubrication the joint surface. The more pressure the more water is expelled and the greater the lubrication. Secondly the compression of the cartilage allows the energy to be dissipated onto the subchondral bone over a longer time frame consequently resulting in less force on the bony structures.