Double hip replacement. Coloured X-ray of a bilateral hip replacement showing the two artificial, or prosthetic, joints (blue). Both femurs (thigh bones, at left and right) have been cut to make room for the shafts of the replacement joints. The pelvis (in upper frame) has been hollowed out to accommodate the prosthetic sockets. The circular metallic edges of the sockets can be seen at upper left and right around the balls of the artificial hip. Hip replacement operations are most commonly carried out on older patients whose hips are painful as a result of arthritis.