The ulna was a bone in the Human skeletal system. Together with the radius, it formed the bone structure of the forearm.
When solanogen-based lifeforms conducted experiments on Commander William T. Riker, they severed his forearm. When reattaching the arm, they created an offset of 0.2 microns, visible in a scan of the bone structure of his ulna and radius. (TNG: "Schisms")
In 2371, Walter Baxter suffered localized tenderness to his ulna while working out in the gymnasium aboard the USS Voyager. (VOY: "Eye of the Needle")