Hip replacement. Artwork of a total hip replacement. The prosthetic hip joint consists of a metal shaft (grey) and ball (white) attached to the top of the femur (thigh bone) and a plastic socket (blue) that is inserted into the pelvis. The majority of hip replacements are a result of osteoarthritis, a degenerative joint disease that results in the loss of cartilage between the joint and the growth of bone in place of the cartilage. This causes pain, stiffness and loss of mobility.