ulnar notch of the radius ulnar notch of the radius the articular surface for the ulna is called the ulnar notch (sigmoid cavity) of the radius; it is in the distal radius, and is narrow, concave, smooth, and articulates with the head of the ulna forming the distal radioulnar joint. "sigmoid cavity" redirects here. for greater sigmoid cavity, see trochlear notch. ulnar notch of the radiusbones of left forearm. anterior aspect. (ulnar notch visible but not labeled.)detailsidentifierslatinincisura ulnaris radiita98a02.4.05.019ta21228fma75082 23530, 75082anatomical terminology