A synovial joint is the type of joint found between bones that move against each other, such as the joints of the limbs (e.g. shoulder, hip, elbow and knee). Characteristically it has a joint cavity filled with fluid. Other types of joint allow little or no movement, including fibrous joints (e.g between the bones of the skull) and cartilaginous joints (e.g. between the ribs and the breastbone).
A synovial joint is made up of:
- cartilage – a smooth gristly material that covers the surface of the bones. This acts as a shock absorber and reduces friction as the bones move over each other.
- joint capsule – a fibrous material that encloses the joint. Together with the ligaments, tendons and muscles, the capsule keeps the bones of the joint in place.
- synovial fluid – a clear sticky substance that fills the synovial cavity and nourishes and lubricates the cartilage surfaces as they move against each other, like oil lubricating a piston.
- synovial membrane (or synovium) – a special layer of cells that lines the joint capsule and produces the synovial fluid.
The moving parts in synovial joints make them particularly vulnerable to injury, most commonly sprains, where ligaments become stretched or torn, and dislocations.
Synovial joints may also become inflamed, called arthritis. There are more than 100 different types of arthritis, arising from problems in different parts of the joint. For example in osteoarthritis, the cartilage becomes worn, and in rheumatoid arthritis the body’s immune system attacks the synovial membrane. However, most types of arthritis have similar early symptoms– joint pain, stiffness and swelling.