The ulna 's a long bone, prismatic in form, placed at the medial side of the forearm, parallel with the radius. Articulations The ulna articulates with: the humerus, at the right side elbow as a hinge joint. the radius, near the elbow as a pivot joint, this allows the radius to cross over the ulna in pronation. the distal radius, where it fits into the ulna notch. the radius along its length via the interosseous membrane that forms a syndesmoses joint. Proximal and distal aspects The ulna's… (More on Ulna)