Materials is a very broad subject. They are all around us and, in some cases, even in us. Several different materials are chosen to work together inside your hip to enable joint motion just like your normal hip joint. The prosthesis is composed of two parts. One of these is a metal ball attached to a metal rod. These pieces are generally made from stainless steel, alloys of cobalt and chrome, and titanium. The other piece is a plastic cup that the metal ball fits snuggly into. This cup is glued into the hip socket with a plastic bone cement. This piece needs to be durable and wear resistant so it is made out of polyethylene. The metal rod is fitted into the femur bone in your leg to make the complete hip replacement. This is one aspect of materials in the medical world and IN our lives. The American Academy of Orthopaedic Surgeons provides additional information.