Total hip joint prosthesis. Illustration of the design of a total hip joint endoprosthesis. This shows the structure of the prosthesis which has a acetabular implant (socket, top), femoral head (ball, blue), inlay, and hip stem (grey, bottom). The socket gets fitted into the pelvis, and the hip stem and femoral head is fitted where the top of the femur bone would be. A hip prosthesis mimics the old ball-and-socket joint, which is often replaced due to osteoarthritis or severe injury, to restore the full mobility and functionality of the hip joint.