The radial head lies at the proximal end and articulates with the capitulum of the humerus and the ulna medially. The radial head has a cup-shaped depression on the top that mates with the capitulum, a round articular prominence of the distal humerus. It functions as limited ball-and-socket joint enabling sagittal plane flexion and extension at the elbow and rotation about the long axis of the forearm (pronation and supination). The surface of both the radial head and capitulum are covered by articular cartilage, which provides a tough, smooth surface for joint movement. Ligaments keep the bones from separating during activity.
The lateral surface of the radial head articulates with the radial notch of the proximal ulna to form the proximal radio-ulnar joint. The radio-ulnar joint is contained by the annular ligament that wraps around the radial head. The proximal radio-ulnar joint allows the forearm to spin around its long axis (pronation and supination).
