Fig. 2A–B.

(A) The plane perpendicular to the calculated axis of the ulna passing through the midpoint of the PRUJ is called “plane-U.” (B) The RNA is the angle between the line created by joining the ends of the radial notch (normal ulna: Line Xn, affected ulna: Line Xa) and the line passing through the ulnar posterior border and the tip of the coronoid process (Line Y).