Total knee replacement has not been in widespread use quite as long as hip replacement, but it has a long track record and is now quite common. The first designs that resembled the modern knee replacements used today appeared in the 1970's and rapidly went through a number of evolutionary changes. It has now reached the status of a mature technology in the past couple of decades, and it is widely accepted in the orthopaedic literature that most patients undergoing total knee replacement can have an expectation of at least 95% success rate at 10 year follow-up or longer.
The basic concept of a total knee replacement (also known as total knee arthroplasty) is to replace the rough, irregular surfaces of the ends of the bones (the femur and tibia) with new surfaces. This eliminates the "bone on bone" changes from severe arthritis and allows the ends to glide smoothly over one another, with artificial surfaces that have no nerves in them. The undersurface of the patella (knee cap) may or may not be replaced also with a plastic button.
These new surfaces resemble a metallic cap that is affixed to the ends of the bone (most often with cement, although press fit cementless prostheses are sometimes used). For this reason, although "total knee replacement" has been the term used for several decades, "knee resurfacing" would probably be a more accurate description since it is usually half an inch or less that is actually removed from the ends of the bones and replaced. It is not unlike a dental procedure in which a bad tooth is capped. After the joint is replaced, there is no longer any arthritis in the joint, because the joint surface is entirely artificial.
Partial knee replacements also exist, most often as a unicondylar knee replacement, which replaces one side of the knee only. These are less invasive procedures and typically have a quicker recovery, with the advantage of retaining more "factory original" parts. However, only some patients are candidates for a partial knee replacement. It will only help the portion of the knee it replaces in most cases, and if both sides of the knee joint are worn out, it is often better to consider a total knee replacement. Some patients also have significant deformity or angulation, making it difficult or impossible to correct alignment and biomechanics without a total knee replacement.
At the time of surgery, the ends of the thigh bone (femur) and upper leg (tibia) are typically quite worn out. Frequently, the ends of the joint look very similar to two heads of cauliflower in a very worn out knee, covered with lumpy and bumpy osteophytes (spurs) and areas of exposed bone, grinding against each other.
Nuts & Bolts: Total Knee Replacement Procedure
Regardless of the surgical approach used, the same general steps have to be performed during the surgery. Some surgeons use a tourniquet for the procedure, and others prefer to identify transected blood vessels and ligate them at the time of surgery (rather than have them bleed into the joint after surgery when the tourniquet is released). Tourniquets can also be a source of soreness and circulation problems after surgery, and for that reason we typically do not use them in routine knee replacement surgeries in our practice.
After exposing the knee joint - usually with a vertical incision in the front of the knee - the irregular, arthritic ends of the femur and tibia are resected. These cuts are made in a way to keep the mechanical axis of the knee properly aligned, which usually requires keeping the perpendicular cut at about 5 to 7 degrees off of the vertical axis. Because the end of the femur is rounded (i.e., shaped like a cam mechanism), it is also necessary to make chamfer cuts. These are usually made in such a way that the new "cap" fits very tightly over the chamfer cuts. Remnants of the menisci and anterior cruciate ligament, if they are still present, are removed.
Next the upper end of the tibia is resected. It is important for the surgeon to cut and prepare this surface at the proper angles also; if the cut is tilted too much side to side, the knee will either be excessively bowed or knock-kneed. Similarly, if it is angled too far up or down when viewed from the side, knee flexion and extension may be adversely affected. It is also important for patients with severely bowed legs to understand that full correction of the deformity may not be possible at the time of surgery.
To learn more about the Knee and the services we provide or to make an appointment, please call us at 203.598.0700.