Sometimes the whole hip joint must be replaced. The whole hip joint is the top (head) of the thighbone (femur) and the surface of the socket into which the head of the thighbone fits. This procedure is called total hip replacement or total hip arthroplasty. The head of the thighbone is replaced with a ball-shaped part (prosthesis), made of metal. The prosthesis has a strong stem that fits within the center of the thighbone. The socket is replaced with a metal shell lined with durable plastic.