Hip replacement. X-ray images taken before and after a hip replacement operation. At left is an X-ray of a patient's hip with rheumatoid arthritis. This is an autoimmune disorder where the body's immune system attacks its own tissue. The head of the thigh bone (femur) forms a ball which fits into a socket in the pelvic bone. At the joint the tissue and bone are worn by the arthritis. At right is the same hip after a hip replacement operation. The head of the femur and socket in the pelvic bone have been replaced by prosthetic implants. This restores the mobility of the hip.