The ulna is a long bone, prismatic in form. In anatomical position the ulna is placed at the medial side of the forearm closest to the body, parallel with the radius on both arms. Articulations The ulna articulates with: trochlea of the humerus, at the right side elbow as a hinge joint with semilunar trochlear notch of the ulna. 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 radi… (More on Ulna)