Prosthetic hip. View of a prosthetic (artificial) hip joint. The metal shaft (black) ends in a ball (silver) embedded in a plastic socket. Prosthetic hip joints are used to replace diseased hip joints that have become stiff and painful as a result of arthritis or injury. In a hip replacement, the ball" at the end of the thigh bone is removed and the "socket" in the pelvis is enlarged. The artificial components are then cemented in their place, the shaft fitting into a specially made hollow inside the thigh bone. Hip replacement is a successful operation that has transformed many lives, but the long-term outlook is uncertain as the artificial joint may loosen with time."