Normal and pathological adaptations of articular cartilage to joint loading.

Joints are functional units that transmit mechanical loads between contacting bones during normal daily or specialized activities, e.g., sports. All components of the joint, i.e. articular cartilage, bone, muscles, ligaments/tendons and nerves, participate in load transmission. Failure in any of these components can cause joint malfunction, which, in turn, may lead to accumulation of damage in other joint components. Mechanical forces have great influence on the synthesis and rate of turnover of articular cartilage molecules, such as proteoglycans (PGs). Regular cyclic loading of the joint enhances PG synthesis and makes cartilage stiff. On the other hand, loading appears to have less evident effects on the articular cartilage collagen fibril network. Continuous compression of the cartilage diminishes PG synthesis and causes damage of the tissue through necrosis. The prevailing view is that osteoarthrosis (OA) starts from the cartilage surface through PG depletion and fibrillation of the superficial collagen network. It has also been suggested that the initial structural changes take place in the subchondral bone, especially when the joint is exposed to an impact type of loading. This in turn would create an altered stress pattern on joint surfaces, which leads to structural damage and mechanical failure of articular cartilage. The importance of the neuromuscular system to the initiation and progression of OA is still poorly understood. Many surgical extra- and intra-articular procedures have been used for the treatment of OA. Although some of the new methods, such as autologous chondrocyte transplantation and mosaicplasty, have given good clinical results, it is reasonable to emphasize that the methods still are experimental and more controlled studies are needed.