Joint Loading and Biomechanics
Loading of a joint can have variable effects on the articular cartilage. Sudden impact loading can cause specific damage and in addition repetitive loading may also promote degenerative change whereas appropriate load within the required range will help with nutrition and function of the joint.
The articular cartilage acts as a sponge in addition to acting as a shock absorber. On load bearing, synovial fluid is pushed out from the superficial layer of the articular cartilage and becomes trapped against the opposing layer, effectively maintaining a high pressure level. This prevents formation of high friction between the more solid elements of the articulating surface. Deeper layers within the articular cartilage have less fluid and act more as shock-transducers.
Following surgery this knowledge needs to be taken into account when identifying specific exercises. Reducing load on a joint is important to relieve symptoms but the absence of load can in itself be deleterious. Repetitive low loading will boost nutrition of the articular cartilage by encouraging flow of the synovial fluid.
Normal movement of the knee joint consists of relatively unconstrained rolling and gliding of the surfaces. In the presence of joint stiffness then these normal physiologic actions can become restricted resulting in increased abnormal load distribution over the surface. This is particularly important in the patello femoral joint when accurate tracking is required.
Different activities result in different effects on the articular cartilage and knee bending, for example results in a small reduction in cartilage volume on the patella of about 5% (after 15 knee bends). Running causes the largest amount of deformation to the articular cartilage whereas walking does not produce any reduction in cartilage volume.