The radial collateral ligament of the elbow, which strengthens the side of the wall of the joint capsule of the elbow, is composed of a fibrous band extending between the lateral epicondyle of the humerus (upper arm bone) and the annular ligament of the radius. The annular ligament, in turn, is attached to the margin of the trochlear notch of the ulna, and it encircles the head of the radius, functioningmycontentbreak to keep the head in contact with the radial notch of the ulna. The elbow joint capsule also encloses the resulting radioulnar joint, so that its function is closely associated with the elbow.