The knee replacement implants are generally made of titanium or cobalt-chromium-based alloys, along with a plastic spacer made of medical polyethylene in between. Some implants are also made of oxidized zirconium. Certain criteria that have to be fulfilled by all implants are:-They should be biocompatible, i.e., should not cause an allergic or immunologic reaction within the body.They should be able to perform all the functions of the tissue they are replacing, i.e., they should be strong and flexible enough to allow the joint to move as if it was normal.They should be long-lasting, i.e., they should be able to maintain their size and shape without eroding for a very long time.