Knee replacements typically consist of three components: the lower end of the femur, the top of the tibia, and the back of the patella. Materials used include metals and plastics, which are strategically integrated together to result in surfaces that are easily mobile. Types of metals used include alloys of titanium and cobalt-chromium. Polyethylene is a highly wear-resistant plastic that is often placed around the joint area. Ceramic is another material that can be used in knee replacements, but is not as common.