radio-²

The radius bone.

Latin radius, staff, spoke, ray.

The radius is the thicker and shorter of the two bones in the human forearm; radio‑ is found in a few adjectives that describe the radius in conjunction with some other part of the body: radio-carpal, of the radius plus the carpal bones of the wrist; radio-ulnar, of the radius and the ulna, the other long bone in the forearm.