Loosened hip replacement. Coloured X-ray of a man's hip showing a prosthetic hip implant (black) in a loosened ball joint. The implant is embedded in the femur (thigh bone, lower centre) & replaces the head of the femur. Its articulation (joint) is with the pelvis (upper left) where it forms a ball-and-socket joint. Here an artificial socket (pink) has been fitted into the old pelvic socket, but it has loosened and become displaced, leaving a dark space. The cement holding it in place has failed, requiring surgical correction. A hip prosthesis mimics the old ball-and-socket joint, which is often replaced due to arthritis or severe injury, to restore the mobility that was lost.