The components of an artificial knee joint are made from some combination of hard polished metal, hard ceramic, and tough, slick plastic. The same materials are used for hip implants—the stem portions of most hip implants are made of metal alloys, the ball portions are made from metal alloys or polished ceramic, and the acetabular sockets are made from metal, polyethylene (plastic), a combination of polyethylene backed by metal, or ceramic. All materials that are used to construct artificial joints need to meet the following criteria:
- They must function without provoking a local or systemic immune response.
- They must be able to resist wear and corrosion in order to retain their strength and shape.
- They must mechanically duplicate the joint they are replacing. For example, they must be strong enough to withstand repetitive weight-bearing loads and flexible enough to bear stress without breaking, while allowing smooth movement.