Articular cartilage is a smooth viscoelastic tissue that covers the joints and is responsible for the mechanical distribution of loads across the joints and aids in the motion of the joints. Once damaged, it does not have the capacity to heal itself.
The majority of the cartilage structure and function is controlled by chondrocytes (cartilage cells) that regulate the turnover of the articular cartilage components and maintain tissue equilibrium.
Imbalance in chondrocytes function leads to degenerative joint diseases such as osteoarthritis.
Articular cartilage is a highly specialized tissue with unique mechanical behaviour and poor regenerative capacity.
The structure of articular cartilage is divided into 4 layers:
There are two types of cartilage in the knee:
Cartilage is composed of:
Within the cartilage matrix, there are numerous proteoglycans attached by a linked protein to hyaluronic acid all woven into collagen to form an elastic and compressible structure.
The proteoglycans in the cartilage maintain a structural matrix giving cartilage tissue its form and structure.
Chondrocytes are responsible for the production of the building materials for the biomechanical properties of the cartilage.
Chondroitin sulphate and hyaluronic acid provide elasticity and high water binding properties enabling cartilage to act as a shock absorber.
Cartilage does not have blood vessels. Therefore, chondrocytes rely on diffusion of nutrients and metabolites from the cartilage surface.
The cartilage is dependent on constant lubrication with hyaluronic acid to prevent dryness and receive nutrients.
Synovial fluid acts both as a source of nutrients and to drain metabolic waste from the cartilage.
Chondrocytes produce cartilage components including collagen, proteoglycans and hyaluronic acid, which occur in the superficial and mid layers of the cartilage.
Collagens are the most abundant large molecule in cartilage tissue (60% of the dry weight of the cartilage) providing tensile and shear strength to the tissue.
The composition of the cartilage tissue and the organization of the chondrocytes and their response to external factors is dependent of the age of the tissue.
During the course of aging, chondrocytes move away from the superficial region to the deeper layers which leads to decreased hydration and increased stiffness.
The age-related decrease in proteoglycans within cartilage tissue may be due to decreased levels of hyaluronic acid without the replacement by larger chain hyaluronic acid molecules.
Increased mechanical forces exerted on the cartilage tissue leads to further tissue calcification.
The breakdown of proteoglycans leads to a reduction in the compressive strength of the cartilage which accelerates the loss of cartilage tissue (collagen).
Destructive enzyme production, which is increased by repeated joint movement, subsequently digests cartilage tissue which disrupts cartilage equilibrium.
Chondroitin sulphate and hyaluronic acid are chondro-protective agents that can provide for the protection of cartilage components and delay the structural change and damage.