Joints are classified as per their degree of movement.
(1) Synovial joints are freely movable and flexible which allow for a variety of movement.
(2) The bones of synovial joints are joined and stabilized by ligaments that form a capsule and the capsule is lined with synovial membrane.
(3) Bones are separated by a thin fluid-filled cavity.
(4) Different types of synovial joints allow different types of movements. A hinge joint, such as the knee and elbow, allows movement in one direction only like the hinges on a door. Ball-and-socket joints form the hip and shoulder and allow movement in all planes including rotational movement.