The skeletal system consists of bones joined together at joints. Most of the joints in the human body articulate; that is, they are capable of movement. Also known as diarthroses, the synovial joints consist of various types of connective tissue that hold and cushion the ends of the bones, conferring flexibility and both allowing and limiting movement of the bones.
Cartilage covers the ends of the bones that meet at a synovial joint. Called articular, or hyaline, cartilage, this material has a low friction coefficient and an ability to bear large compressive loads. The articular cartilage is composed mostly of water, with an approximate 20% collagen and glycoprotein aggregate composition that gives the cartilage a hydrated property that resists compression, protecting the ends of the bone from damage during regular movement and stress. The exact arrangement and function of the cartilage components is still under study.
Synovial joints are surrounded by a joint capsule, which comprises various fibrous connective tissues. Also called the articular capsule, this structure binds the bone ends together and is responsible for directing the movement allowed at that joint. Specialized attachment zones join the connective tissue to the bone, and the tissue thickens to form the attachment capsule of ligaments. The composition of the capsule depends on the stress the joint needs to bear. More compressed regions of the capsules adapt with more fibrous cartilage, which increases with age (Journal of Anatomy, 1994).
The joint capsule is lined by the synovial membrane. This membrane secretes synovial fluid into the joint cavity as a lubricant. The viscosity of the fluid is determined by the presence of hyaluronic acid, but other components include lubricin, as well as proteinases and collagenases, enzymes involved in connective tissue restructuring. Joints with tendons crossing the capsule have additional synovial fluid between the joint and tendon. This second synovial capsule is called a bursa (plural: bursae), and it acts to reduce friction between the tissues during movement.
Movement occurs at the synovial joints because the bones are bound by the somewhat flexible connective tissues rather than fusion. The joint structure functions to both reduce friction and protect the ends of the bones during movement. The ligaments cross the joint to hold the bones in place, whereas tendons cross the joint to transfer the kinetic energy from the muscles, pulling on the bone with one end based on the contraction of the muscle at the other end of the connective tissue actually causing the movement of the bone by pressuring the joint to bend. The exact form of the joint capsule determines which direction and how far the joint can bend.