Broken arm in plaster cast. Coloured X-ray of the arm of a young woman set in a plaster cast from wrist to upper arm. The fractured bone itself has healed and so it is not clearly visible here. The larger radius and smaller ulna bones of the forearm are seen, together with bones of the wrist (carpals), palm of the hand (metacarpals) and fingers (phalanges). A plaster cast is used to immobilise an area, preventing movement and allowing the fractured bone to heal.