The human elbow is made up of three bones that join together to form a complex hinge consisting of three separate joints. The upper arm bone or humerus connects to the forearm bones, the radius and the ulna, forming the radiohumeral and ulnohumeral joints. The radius and ulna, in turn, form a joint of their own called the proximal radioulnar joint. The entire elbow joint is surrounded by a strong, water tight sac called the joint capsule.
This capsule contains lubricating fluid called synovial fluid.