Synovial joints are the most common type of joint in the body. A key structural characteristic for a synovial joint is the presence of a joint cavity. This fluid-filled space is where the articulating surfaces of the bones contact each other. Also, unlike fibrous or cartilaginous joints, the articulating bone surfaces at a synovial joint are not directly connected to each other with fibrous connective tissue or cartilage. This gives the bones of a synovial joint the ability to move smoothly against each other, allowing for increased joint mobility.
The walls of the joint cavity are formed by the articular capsule, a fibrous connective tissue structure attached to each bone just outside the area of the bone's articulating surface. Friction between the bones at a synovial joint is prevented by the presence of the articular cartilage that covers the entire articulating surface of each bone. Lining the inner surface of the articular capsule is a thin synovial membrane. The cells of this membrane secrete synovial fluid, a thick, slimy fluid that provides lubrication to further reduce friction between the joint's bones. This fluid also provides nourishment to the articular cartilage, which does not contain blood vessels. The ability of the bones to move smoothly against each other within the joint cavity, and the freedom of joint movement this provides, means that each synovial joint is functionally classified as a diarthrosis.
Outside their articulating surfaces, the bones are connected by ligaments, which are strong bands of fibrous connective tissue. These strengthen and support the joint by anchoring the bones together and preventing their separation. Ligaments allow for normal movements at a joint but limit the range of these motions, thus preventing excessive or abnormal joint movements. At many synovial joints, additional support is provided by the muscles and their tendons that act across the joint. A tendon is a dense connective tissue structure that attaches a muscle to bone. As forces acting on a joint increase, the body will automatically increase the overall strength of contraction of the muscles crossing that joint, thus allowing the muscle and its tendon to serve as a "dynamic ligament" to resist forces and support the joint. This indirect support by muscles is essential at the shoulder joint, for example, where the ligaments are relatively weak.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 9.4: Synovial Joints
