Hip joint replacement. Coloured X-ray of the pelvis (front view) of a 73 year old man, showing an artificial or prosthetic hip joint implant (dark blue, lower right). The hip joint is a ball- and-socket joint. The ball part of the implant is attached by a long peg into the femur bone, with surgical cement. The socket part of the joint, located in the pelvis bone, has also been replaced. The new hip joint is made of a composite metal alloy. Hip joint replacement may be needed due to disease such as arthritis, or due to an injury.