Planning hip replacement operation. Computer model of the bones (blue) of a hip joint and the planned hip prosthesis (red and blue) intended to replace it. The bone model will have been generated using medical scans. 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.