Cartilage and Joints

There are three types of cartilage. Hyaline cartilage, the most common, forms the articular surfaces of joints and also the costal cartilages, larynx and trachea. Fibrocartilage, tougher and rich in type I collagen, forms structures such as the menisci and the intervertebral discs. Elastic cartilage, with abundant elastic fibres, is found in the ear and epiglottis. Most joint surfaces are articular hyaline cartilage.

Articular cartilage is built from a framework of type II collagen, large water-attracting proteoglycans such as aggrecan, and a high water content, maintained by a sparse population of cells, the chondrocytes, which are the only cells present and sit in small spaces called lacunae. It has no blood supply, nerves or lymphatics, and the chondrocytes are nourished by diffusion from the synovial fluid, working at low oxygen levels. The proteoglycans carry a strong negative charge that draws in water, and it is this trapped, pressurised water, restrained by the collagen mesh, that gives cartilage much of its stiffness and its ability to spring back after a load. Because the fluid moves only slowly through the dense matrix, cartilage deforms gradually under a sustained load and recovers gradually once it is removed. Its structure is zoned: a superficial zone with collagen running parallel to the surface, which resists shear and gives a smooth gliding layer, a middle transitional zone, and a deep zone with collagen running perpendicular to the bone, which resists compression, then a calcified zone separated from the deep zone by the tidemark and anchored to the subchondral bone.

The collagen framework and the water held by the proteoglycans together let cartilage act as a load-bearing, shock-absorbing cushion, with the fluid pressurised within the matrix bearing much of the load. Because it is avascular and the chondrocytes cannot migrate, cartilage heals poorly: a partial-thickness defect within the cartilage has little capacity to repair, whereas one that breaches the subchondral bone may generate a fibrocartilaginous repair, which is mechanically inferior to normal hyaline cartilage.

Joints are classified by how the bones are joined. Fibrous joints, such as the skull sutures, allow little or no movement. Cartilaginous joints, such as the pubic symphysis and the joints between vertebral bodies, allow limited movement. Synovial joints, which include most of the limb joints, are freely movable and are the ones that matter most in sport. A synovial joint has bone ends capped with articular cartilage, enclosed by a capsule whose outer layer is fibrous and whose inner layer, the synovial membrane, produces synovial fluid into the joint cavity. That fluid, rich in hyaluronan and lubricin, both lubricates the surfaces and nourishes the cartilage, and the joint is reinforced by ligaments and, in some joints, by menisci or a labrum.

Synovial joints are further grouped by shape into hinge, pivot, saddle, condyloid, plane and ball-and-socket types, which gives them their different ranges and directions of motion. The shape of the joint surfaces, together with the capsule, ligaments and surrounding muscles, sets both how far a joint can move and how stable it is. Many joints also have surrounding bursae and fat pads that cushion and reduce friction between the moving structures.

Cartilage depends on movement. Because it has no blood supply, its nutrition relies on load: cyclical compression during activity pumps fluid, and with it nutrients and waste products, in and out of the matrix, so regular, moderate loading helps keep cartilage healthy, whereas prolonged immobilisation causes it to thin. Unlike bone or muscle, mature cartilage is metabolically responsive to loading but has very limited intrinsic capacity to repair, which is why protecting the joint surface and managing load matter so much. The muscles around a joint share this protective role, absorbing force and controlling movement, so strengthening them improves function, control and how load is distributed across the joint. Appropriate, graded loading is therefore better for a joint than rest, provided it stays within the joint's current tolerance.

Cartilage and joint problems are a large part of the sports medicine caseload. Osteoarthritis is a whole-joint disorder of the cartilage, subchondral bone, synovium and surrounding structures, not simply cartilage loss, and typically causes activity-related pain, stiffness and reduced movement. A focal chondral or osteochondral injury can follow a single loading event, and meniscal injuries give joint-line pain and mechanical symptoms such as catching or locking. Because cartilage heals so poorly, management centres on load and on strengthening the surrounding muscles rather than on regrowing the surface. One pattern must not be missed: a hot, swollen, very painful joint, especially with fever, may be septic arthritis, a hospital emergency.

• •The three cartilages are hyaline (articular), fibrocartilage (menisci, discs) and elastic (ear).
• •Articular cartilage is hyaline: type II collagen, proteoglycans and water, with only chondrocytes.
• •It is avascular and aneural, nourished by synovial fluid; defects reaching the subchondral bone may produce inferior fibrocartilage.
• •Cartilage zones run from a shear-resisting surface layer to a compression-resisting deep layer.
• •Joints are fibrous, cartilaginous or synovial; synovial joints are freely movable.
• •Synovial fluid, rich in hyaluronan, both lubricates and nourishes the joint.