Patients frequently ask, “What exactly is a total knee replacement?” The simplest answer is that it is a replacement of the worn and arthritic surfaces of the knee joint. A total knee replacement resurfaces the worn parts of the joint. Unlike a hip replacement, that removes the ball of the hip joint, a total knee replacement takes away very little of the bone within the knee joint. It is better to think of a knee replacement as a refinishing of the knee surfaces. With arthritis, the cartilage covering the ends of the bone within the knee joint is badly worn. In a knee replacement this damaged cartilage, along with a very small amount of bone, is removed with very precise guides and instruments. The implants, which are made of metal and plastic in a variety of sizes, are then fitted to the bone to provide an artificial surface that causes no pain when the knee is in motion.
Partial Knee Replacement
Partial knee replacement (usually called a “unicompartmental” or “bicompartmental” replacement) is done if only part of the knee joint is damaged by arthritis. Think of the knee as having three compartments or sides: an inside compartment, an outside compartment and a front compartment for the kneecap. Most frequently, it is the inside compartment that becomes arthritic. The decision to perform a partial replacement that resurfaces just one compartment or side is made only if the opposite side and the kneecap have healthy normal cartilage at surgery.
Revision Total Knee Replacement
A third type of knee replacement is referred to as a revision total knee. Since it is repeat surgery, a revision is more complex and often requires a special type of implant used for those patients who have had a knee replacement that has failed. Approximately 1 in 10 implants will fail over a 10-year period and will require a revision of the prosthesis. The bone is not as strong when an implant is removed and the ligaments supporting the knee may be damaged. A revision prosthesis helps address these problems because the surgeon can fit a stem inside the canal of the bone to provide better support for the prosthesis.
If the bone is badly damaged, some knee replacement surgeries, usually revision total knee replacements, require a bone graft to reconstruct a deficiency in the patient’s bone. This is an unusual circumstance and will, in most cases, be discussed at the time of your initial office visit. The human-donor bone graft, which is obtained from a bone bank, has been tested for disease in a process even more stringent than the testing process for blood. Your surgeon and his assistants will be glad to answer your questions about bone grafting and will review the advantages and disadvantages with you.