Prosthetic hip. Coloured X-ray of a prosthetic (artificial) hip joint in place. Prosthetic hip joints are used to replace diseased hip joints that have become stiff and painful as a result of arthritis. During a hip replacement operation the ball at the end of the thigh bone is cut off and its rounded socket in the pelvis is enlarged. Metal joint components are then cemented in place of the damaged bone, the shaft (bottom) fitting into a specially made hollow inside the thigh bone. Hip replacements can dramatically improve mobility but the long-term outlook is uncertain as the artificial components can loosen with time.