Scan of Riker's arm showing ulna and radius
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")
After having sex with Grilka in 2373, Quark had a compound fracture of the right radius, two fractured ribs, torn ligaments, strained tendons and several bruises, contusions and scratches. (DS9: "Looking for par'Mach in All the Wrong Places")
The ChoH'a' Species reassignment protocol involved shortening the radius. (DIS: "Despite Yourself")