The human body has about 327 joints, which vary greatly in size and function. Diarthrodial joints have a joint capsule that encloses the bone ends of a joint space. The bone ends are covered by articular cartilage, and the joint space is partially lined by a synovial membrane. The fluid is secreted by the synovial membrane and is mainly a transudate of water and solutes from bone marrow vessels. Hyaluronic acid and glycoproteins are added by the lining cells of the synovial membrane. The cartilage of articular surfaces is known as hyaline cartilage and consists of chondrocytes scattered throughout an abundant extracellular matrix. Collagen type II is the most abundant organic component of hyaline cartilage. The deepest zone of cartilage directly adjacent to subcortical bone is calcified. Supporting tissues of the joint allow frictionless motion of one bone end against the other. Joint motion and loading are necessary to maintain healthy articular cartilage. Immobilisation and casting lead to atrophy and degeneration of the cartilage.