Total knee replacement employs specially designed components, or prostheses, made of high strength, biocompatible, metals and plastics, to replace the cartilage in your knee. The metal that is most commonly used is an alloy of cobalt, chromium and molybdenum. The plastic is ultra-high molecular weight polyethylene. These materials have been used in joint replacement for about 30 years and their behavior in the body is well-known. The components are very precisely manufactured and the surfaces are congruent, smooth and highly polished. In this manner, congruent, smooth, low-friction surfaces are restored to the knee.

In modern total knee replacement surgery, only the worn-out cartilage surfaces of the joint are replaced. The entire knee is not actually replaced. The operation is basically a "re-surfacing" (or "re-tread") procedure. Only a small amount of bone is removed, the collateral ligaments are left intact, and the muscles and tendons are left intact. Alignment abnormalities can usually be corrected during the operation by adjusting the direction of the cuts of the bones, removing bone spurs (osteophytes), and lengthening tight ligaments. Front and side views of a knee following total knee replacement are shown in Figures 3A and 3B respectively. Note that the smooth surfaces of the joint are restored. The joint space is now comprised of polyethylene. The operation only replaces the worn surfaces of the joint. The ligaments, tendons and muscles are retained.

 

3A	3B

 

Total knee replacement is a major surgical procedure. A tourniquet is usually placed up high around the thigh in order to limit bleeding during the operation. An incision is made in the front of the knee. The large quadriceps muscle covering the front of the femur and knee is moved, temporarily, to one side, in order to expose the surfaces of the knee joint. Specialized instruments are used to trim off the worn-out surfaces and shape the ends of the bones. The femoral component is metallic, and is similar in size and shape to the end of the femur bone (thigh bone). The tibial component, which goes on the top of the leg bone (or tibia), may have a metallic base, but the top surface is always polyethylene. The undersurface of the knee cap (patella) is cut flat and covered with another polyethylene component. Since metal covers the surface of the femur (thigh bone) and polyethylene covers the surfaces of both the tibia (leg bone) and patella (knee cap), total knee replacement involves metal-on-plastic articulation.

The components are attached to the bone with a specialized polymer (polymethylmethacrylate), commonly referred to as "bone cement". Alternatively, some components have a porous texture on their under-surface, into which the bone can grow. This method of attachment is referred to as "porous ingrowth."