Planning hip replacement operation. Computer model of a hip prosthesis (red and blue) in position on X-rays of the hip joint being replaced. The prosthetic (artificial) hip has two main parts. A hole is drilled in the shaft of the thigh bone (femur) for the femoral prosthesis (blue). This ends in a ball that fits into the cup socket (red) that will be glued to the pelvis. The femoral head, which will be removed, is the ball of the old ball-and-socket joint. Hip replacement operations are carried out when the old hip fails through injury or disease, most often arthritis.