Articular cartilage allows active articulation of diarthrodial joints. This is made possible by a unique set of properties afforded by a very specialized macromolecular organization of an extensive extracellular matrix that is secreted by chondrocytes. Cells within the superficial layer of the cartilage secrete a special lubricant that, together with hyaluronic acid in the synovial fluid, creates an almost frictionless articulation. Articular cartilage is integrated with subchondral bone, the interface of which is partly calcified. Peripherally, cartilage gives way to the synovium, which not only lubricates the joint but also maintains an aseptic environment (cartilage is extremely sensitive to proteolysis induced by contaminant bacteria).