ELBOW

in human anatomy, hinge joint formed by the meeting of the humerus (bone of the upper arm) and the radius and ulna (bones of the forearm). The elbow allows the bending and extension of the forearm, and it also allows the rotational movements of the radius and ulna that enable the palm of the hand to be turned upward or downward. The elbow forms from the expansion of the lower end of the humerus into two thick knobs, or condyles: the humerus' dome-shaped lateral condyle articulates with a shallow depression on the end of the radius, and the humerus' spool-shaped trochlea fits into a notch in the ulna. In addition, the edge of the radius' head fits into a shallow groove on the side of the ulna. The bending and extension of the elbow joint are achieved, respectively, by contractions of the biceps and triceps muscles. These movements chiefly involve only the humerus and ulna; rotation of the forearm involves the smaller radius bone as well. The elbow is especially susceptible to stress injuries, although its surrounding capsule contains cushioning synovial membranes and is reinforced by ligaments. Thick lateral ligaments support the hinge action of the humerusulna junction, and a strong annular ligament around the upper part of the radius helps to hold that bone in place. These ligaments prevent the forward displacement of the forearm bones, but acute stresses can produce rearward dislocations of them. Attempts to force elbow movement past full extension of the arm (180 degrees) tear the joint's protective capsule, producing elbow sprains, while chronic and repeated stressful motions, such as the rotation of the forearm in some sports, can cause pain from overuse of the joint (tennis elbow).