There are many different models of replacement joints. Whether they are made for first-time replacements or for revisions, they all have basically the same elements.
A hip replacement consists of a cup component and a ball-and-stem component. The cup consists of a shell (usually metal) and a liner (usually plastic) and replaces the hip socket in the pelvis. The ball (usually metal) replaces the damaged head of the femur (thighbone) and fits into the replacement socket. The ball’s stem fits securely into a canal drilled into the center of the thighbone. In revision surgery, the cup component may be larger than the one used in the first surgery, and the stem attached to the ball is usually longer than the original stem. In both cases, this is to compensate for bone that is weaker than it was during the first surgery or for bone that is missing.
A knee replacement consists of a tibial component made up of a T-shaped tray made from metal and a plastic spacer; a metal femoral component; and a plastic patellar component. The tibial component replaces the damaged cartilage and bone that were removed from the top of the tibia (shinbone). The tray’s stem is fitted into a canal drilled into the center of the shinbone (the stem used in a revision is usually longer than the original). The plastic spacer on the tray moves smoothly against the metal of the femoral component, which replaces the damaged cartilage and bone that were removed from the end of the thighbone. Some models of femoral component have a stem, which is fitted into a canal drilled into the center of the thighbone (and in a revision, the femoral component usually has a long stem). The patellar component is attached to the back of the patella, or kneecap. The patellar component fits the groove down the center of the femoral component so the kneecap can move up and down smoothly.