The structure, physiology, and biomechanics of articular cartilage: injury and repair

Hyaline cartilage which surrounds the articular surfaces and allows for painless movement of synovial joints by reducing friction is composed of chondrocytes embedded within an extracellular matrix made up of a macromolecular framework and water. Due to its avascular, aneural, and hypocellular structure, its response to trauma is very restricted. The main components of the extracellular matrix are water, proteoglycans, collagen, and other proteins and glycoproteins. Homeostasis of the hyaline cartilage is maintained mainly by chondrocytes. Collagen provides the tensile and shearing properties of the cartilage and immobilizes the proteoglycans within the matrix. The mechanical behavior of the cartilage depends on the interactions between collagen, proteoglycans, and interstitial fluid components. The mechanical strength of the hyaline cartilage is determined by the permeability of the solid matrix, flow of the interstitial fluid in the tissue, and the balance between the solid and fluid phases. Injuries that only disrupt the hyaline cartilage do not have a healing potential; but those involving the subchondral bone as well as articular cartilage cause fibrin clot formation, release of growth factors, and increase in cell activities like migration, proliferation, differentiation, and matrix synthesis.