In knee replacement surgery, the damaged bone and cartilage are replaced with metal and plastic surfaces that are shaped to restore knee movement and function. The new artificial knee is called a prosthesis.The prosthesis is generally composed of two metal pieces fitted onto the ends of the tibia (shin bone) and the femur (thigh bone) and a plastic piece inserted between them to act as a bearing. Stainless steel, cobalt or chrome alloys or titanium may be used for these components. Durable, wear resistant polyethylene (plastic) is used for the bearing. A plastic bone cement may be used to anchor the prosthesis into the bone. Some joint replacements also can be implanted without cement when the prosthesis is designed to fit and lock onto the bone directly.