The hip is a ball and socket joint comprised of two parts. The socket, or acetabulum, is part of the pelvis – and the ball, or femoral head, is the upper end of the thigh bone. As individuals age, the hip joint can begin to wear out due to the effects ofosteo arthritis, rheumatoid arthritis, traumatic injuries, and bone loss resulting from insufficient blood supply, a condition known asavascular necrosis.