The elbow joint is composed of three bones. The humerus (arm bone) articulates against both forearm bones, the radius and the ulna. The two forearm bones, in turn, rotate one around the other to allow rotation of the forearm. As a result, the elbow is responsible for flexion and extension, as well as rotation of the forearm. Rotating the forearm so that the palm is facing down is called pronation and rotating so that the palm is facing up is called supination. Stability of the joint is dependent upon the bony architecture as well as a system of ligaments and muscles. These also allow for painless motion.
Injuries or degenerative processes can lead to tearing and degeneration of these support structures of the elbow. This can result in pain, stiffness, weakness, instability, or loss of function.