The structure of a synovial joint
Hyaline (articular) cartilage: the articular surfaces of the bones are covered with hyaline (articular) cartilage, which forms a smooth, white, shiny mass on the surface. This protects the bone tissue, and helps to reduce friction between the bones.
Joint (articular) capsule: this is a strong, fibrous tissue completely surrounding the joint, attached to the bones near the edge of the articular surfaces. The capsule blends with the periosteum of the bones, and is perforated only by blood vessels and nerves (both periosteum and the joint capsule are composed of collagen fibres). The capsule adds stability to a joint, and stops unwanted material from entering the joint. Capsules are reinforced with ligaments. The joint capsule is made up of two layers. The external layer is a tough, flexible fibrous capsule that is continuous with the periosteum of the articulating bones. It is made up of dense, irregularly arranged masses of interwoven (and some elastic) fibres in a viscous matrix. It is impact resistant (bearing stress in all directions), contains few cells, and is minimally vascularised.
Synovial membrane: this is a fine membrane which lines the inside of the joint capsule, but does not cover the hyaline cartilage. Its role is to produce synovial fluid.
Synovial fluid: this is a yellowish oily fluid that lubricates the articulating surfaces, forms a fluid cushion between surfaces, provides nutrients for the hyaline cartilage, and absorbs debris produced by friction between joint surfaces. Synovial fluid becomes more viscous (thicker) in cold weather. A warm up should include mobilising exercises taking joints through a full range of movement to ensure good joint lubrication. If the synovial membrane of the knee becomes inflamed (synovitis) because of injury or illness, the feeding capillaries become more permeable and fluid rushes into the joint – commonly known as water on the knee.
Ligaments: these are strong fibrous bands, such as the cruciate and collateral ligaments of the knee that join articular surfaces. They help control movement and aid the stability of joints. Ligaments are composed of densely aligned collagen fibres. There is very little elasticity in ligaments and they can only stretch to about 6% of their length before they snap. If stretched, they may not return to their original length. They do not have a blood supply and, therefore, take a very long time to repair once damaged, resulting in joint instability and greater risk to the joint affected. Dense, parallel-arranged, masses of collagen (regular collagen fibres) and elastic fibres running in the same direction and parallel to the direction of pull, form ligaments. This results in tissue with great resistance to pulling forces where the tension is exerted in a single direction. The collagen fibres are slightly wavy allowing the ligament to stretch a little, but once the fibres have been straightened, there is no more ‘give’.
Articular discs (menisci): these lie between the articular surfaces and are attached to the capsule at the outer edge of the joint. Their function is to absorb shock, maintain joint stability and protect the bone surfaces.
Bursae: these are closed sacs filled with synovial fluid, and are located wherever friction may develop, such as between the tendons and the bones. Bursitis is the inflammation of a bursa as a result of overuse. It can cause pain and restricted movement. Common sites of bursitis are the knees, elbows and the Achilles tendon.
Pads of fat: these fill the crevices in and around the joints, and form protective cushions for vital joint structures.