In total knee replacement surgery, the parts of bones the rub together are resurfaced with metal and plastic implants. Using computers and special precision instruments, the damaged surfaces of the bones are removed and replacement surfaces fixed into place. The surface of the femur is replaced with a rounded metal component that comes very close to matching the curve of your natural bone. The surface of the tibia I leg bone is replaced with a smooth plastic component. This flat metal component holds a smooth plastic piece made of ultra high molecular - weight polyethylene plastic that serves as the cartilage. The undersurface of the knee cap may also be replaced with an implant made of the same polyethylene plastic.