A synovial joint is a joint that is covered by synovial membrane and is divided into three parts: the joint cavity, the joint surface, and the joint capsule. The articular surfaces are the contact surfaces of the associated bones involved in making up the joint. They are generally convex and concave to accommodate each other; the convex one is called the articular head and the concave one is called the articular fossa. The cartilage on the surfaces of the several bones that make up the joints, the surfaces of the articular bones are covered with cartilage, allowing the joints to have less friction during movement, and once the cartilage wears out, the relative friction of the joint activity produces pain. The joint capsule is wrapped around the joint, both ends attached to the bone surface adjacent to the circumference of the joint surface, can be divided into the outer fibrous layer and the inner surface of the synovial layer. It is the soft tissue structure, such as ligaments, that surrounds the articular surface, making it a confined space. The joint cavity is a closed, potential cavity surrounded by articular cartilage and synovial layer of the joint capsule, containing a small amount of synovial fluid, which can lubricate the joint and reduce friction, and the cavity is negative pressure, which is conducive to the stability of the joint.