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Synovial joints are typical movable joints present in elbow, shoulder, hips and knee etc. These joints contain two articular surfaces which are covered by a glassy clear hyaline cartilage. The cartilage is thickest in the area of maximum friction. The cartilage appears bluish in colour and do not have blood supply. Joint capsule is a simple tube like structure made up to fibrous cartilage surrounding the articular surfaces. Outer layer of joint capsule is fibrous ligament and provides rigidity and strength to joint. Inner vascular layer of joint capsule is called synovial membrane and it secretes synovial fluid which is meant for lubrication of joint. Synovial fluid is thick gelatinous fluid resembling egg white. The cavity in which synovial fluid is filled called joint cavity or synovial cavity which is bounded by synovial membrane and articular cartilage.
The greatest amount of movement between bones occurs at synovial joints. Here the bones are separated by a fluid-filled cavity, the synovial cavity. A cross-section of a simple synovial joint shows two layers articular cartilage separated by synovial fluid. This is normally straw-coloured and slightly viscous. Synovial fluid serves as a joint lubricant; it also helps in the nutrition of the articular cartilage. It is produced by the synovial membrane which lines the synovial cavity. The joint capsule is composed of the synovial membrane plus overlying fibrous tissue; at the edges of the joint the fibrous tissue is continuous with the periosteum of the bones. In some joints, part of the fibrous joint capsule is thickened to form ligaments which provide stability to the joint and restrict movement in certain directions.
Synovial joints may contain intra-articular structures which provide further stability to the joint. At the knee joint, a complex synovial joint, there are two cruciate ligaments which run from the femur to the tibia, and provide stability to the joint. Rupture of one of these ligaments (usually the cranial or anterior cruciate ligament) is quite common and leads to the joint being unstable, with lameness of the affected leg. Intra-articular discs of hyaline cartilage, called menisci (singular: meniscus), are also present in the knee joint, lying between the femur and tibia. They act as intra-articular shock absorbers, preventing concussive damage to the articular cartilage. These are the notorious cartilages which trouble sportsmen.
The term “arthritis” means inflammation of a joint and usually involves a synovial joint. There are many causes, ranging from direct trauma to infection. Arthritis leads to a painful swelling of the joint which reduced the range of movement which may be undertaken. Long term, or chronic, arthritis leads to an alteration in the nature of the synovial fluid which loses its normal lubricant properties. In addition, there may be destruction of articular cartilage, with loss of the normal smooth surfaces, and these changes lead to a further reduction in joint function resulting from pain, friction and reduced range of movement.
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