The radius was a bone in the Humanoid skeletal system. Together with the ulna, it formed the bone structure of the forearm. When solanogen-based lifeforms conducted experiments on Commander William T. Riker in 2369, 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" )