Joints are the bone connectors, on which the body’s flexibility and range of movement depends. Joint function and freedom of movement are also linked to a good bone, ligament and muscle condition. Each end of the bone that makes up a joint is covered by cartilage, which is essential for its smooth and consistent movement. The cartilage helps to reduce friction so that bones don’t rub against each other, as well as to distribute the load, and to reduce the loads generated during movement.
Structure of the joint cartilage
The joint cartilage is made of hard, flexible tissue and has a smooth surface. Cartilage is made up of a fibrous material, and has no blood vessels or lymph vessels.
The chondrocyte cells make up between 2 and 10% of the volume of the articular cartilage, with the rest being the intracellular material synthesised by these cells, which is made up of water and the large-molecule protein collagen, proteoglycans (proteins bound to carbohydrates – mucopolysaccharides) and non-collagenous proteins.
The fibrils of the protein collagen bind together to form a fibrillar network that gives rigidity and strength to the cartilage in the joint. Cartilage proteoglycans (also known as “aggrecan”) have a high molecular weight and a characteristic “bottle-brush” structure, in which the glycosaminoglycan chains of chondroitin sulphate and keratan sulphate glycosaminoglycans bind to the protein core.
The aggrecan holds a lot of water, interfering with the collagen fibril network and giving the cartilage its flexibility, elasticity and resistance to pressure.
In a healthy joint, collagen makes up about 60% of the cartilage’s dry mass, proteoglycans make up 25-35% and non-collagen proteins and glycoproteins make up the remaining 15-20%. The optimal percentage of joint cartilage components and the unchanged structural order of the joint cartilage ensures the good mechanical properties of this tissue.
