A knee replacement (also called knee Arthroplasty) might be more accurately termed a knee "resurfacing" because only the surfaces of the bones are actually replaced.
There are four basic steps to a knee replacement procedure.
Prepare the bone: The damaged cartilage surfaces at the ends of the femur and tibia
are removed along with a small amount of underlying bone.
Position the metal implants: The removed cartilage and bone is replaced with metal components that recreate the surface of the joint. These metal parts may be cemented or "press-fit" into the bone.
Resurface the patella: The undersurface of the patella (kneecap) is cut and resurfaced with a plastic button. Some surgeons do not resurface the patella, depending upon the case.
Insert a spacer: A medical-grade plastic spacer is inserted between the metal components to create a smooth gliding surface.
(Left) Severe osteoarthritis. (Right) The arthritic cartilage and underlying bone has been removed and resurfaced with metal implants on the femur and tibia. A plastic spacer has been placed in between the implants. The patellar component is not shown for clarity.