Total hip replacement, coloured X-ray. The replacement hip consists of a ball and shaft, attached to the top of the femur (thigh bone), and a socket, which is fitted into the pelvis (upper left). The ball fits into the socket. Surrounding the upper part of the shaft is a metal cage held in place by wires. This cage helps the shaft to graft to the bone. However, in this patient the shaft has not properly grafted, a state known as non-union.