The so-called joint, also known as the indirect joint, is the most differentiated form of bone connection. The joints are very mobile because the opposing bony surfaces are separated from each other, with a cavity filled with synovial fluid in the middle and connected by connective tissue around it. The basic structure of the joint is divided into the joint surface, the joint capsule, and the joint cavity. The articular surface is the contact surface of the bones involved in forming the joint, and each joint includes at least two articular surfaces, called the joint head and the joint socket. The joint capsule is a capsule of fibrous connective tissue that attaches around the joint, surrounds the joint, and encloses the joint cavity, and is generally divided into an outer fibrous membrane and an inner capsule membrane. The joint cavity is a closed cavity composed of the synovial layer and the joint surface, which is useful for maintaining the stability of the joint.