When the knee joint is replaced the surgeon has two choices about the kneecap (patella). He or she can leave the patient's patella in place or remove it and replace it with an implant. Unresurfaced or nonresurfaced means the patient's patella is cleaned up but left alone. Any bits of uneven bone are usually smoothed over. Pits and dings in the cartilage are also evened out.
If the patella is removed and replaced, then a plastic backed implant is used most often. Early patella replacements were made of metal but bits of metal kept breaking off causing problems.