When a knee replacement » is performed, the bone and cartilage on the end of the thigh bone (femur) and top of the shin bone (tibia) are removed. This is performed using precise instruments to create exact surfaces to accommodate the implant. A metal and plastic knee replacement implant is then placed to function as a new knee ****. Depending on the condition of the cartilage on the undersurface of the kneecap, this may also be replaced.