Total knee arthroplasty procedure is not an entirely accurate term. The knee is not totally replaced rather the joint is resurfaced. The cartilage surfaces are removed and replaced with metal and plastic components The overall shape and structure of the bones are maintained. In a normal knee joint the end of the femur (thigh bone), which consists of knuckle-shaped femoral condyles, slides against the upper part of the tibia (shin bone), or tibial plateau. During a total knee replacement surgery the femoral condyles are re-placed with metal, and the tibial plateau is replaced with a component that is a combination of plastic and metal. In addition, the patella (kneecap), which slides over the front part of the femur, is resurfaced with a round plastic "button." Critical to knee mechanics are the surrounding tendons, ligaments, and muscles; these are all preserved during the procedure.
Several types of prostheses are available for the surgeon to use. A surgeon chooses a prosthesis based on his or her preference and experience. The most common type used for a primary (first-time) total knee arthoplasty procedure is an unconstrained artificial joint on the next page). In essence, the end portion of the femur is resected (surgically re-moved) and "capped" with a metal surface. The upper portion of the tibia is removed and replaced with a metal tray upon which a plastic insert is placed. The metal is com-posed of a cobalt-chromium alloy, and the plastic is a large density large molecular weight polyethylene. The preserved tendons and ligaments are responsible for maintaining proper alignment of the joint surfaces.
A relatively new, second type of pros-thesis is the mobile bearing knee arthroplasty. This type of prosthesis can be described as a moving, polyethylene (plastic) post and insert, which separates the femoral condyles from the tibial tray. The plastic insert moves along the metal tibial tray in conjunction with normal knee motion. The mobile bearing knee arthroplasty is gaining popularity for use with younger patients because it theoretically causes less wear on the plastic cushion and less stress at the juncture of theprosthesis/bone connection. It is hoped that this design will be longer lasting than the unconstrained TKA prosthesis.
Another prosthetic design is the posterior stabilized total knee arthroplasty In this design, a plastic post extends from the plastic tibial tray and inserts into a housing on the femoral component be-tween the femoral condyles. The posterior stabilized TKA provides more stability for the knee and is used primarily with knees that have poor ligamentous or muscular support. It is most commonly used on elderly patients.
It is not important for you to be concerned with the "brand name" or exact design of the prosthesis. What is important is that the surgeon is comfortable with the model, that it yields the best results in his or her hands, and that it is a design appropriate for you. There is a significant learning curve involved in getting to know the finer points of any specific brand or make of TKA pros-thesis. Surgical mastery of a prosthetic system is the key to excellent results.
Another variable in the technique for TKA is the fixation of the prosthetic components—that is, how to attach the prosthesis to the bone. Fixation of the prosthetic components to the bone is done either with or without cement. The cemented technique involves using methyl methacrylate cement to "glue" the metal to the bone. Ce-mentless fixation is a method that requires precise bone cuts to achieve maximum con-tact of metal to bone. The metal components used with this technique have a porous surface to allow bone to grow into the prosthesis, thus achieving solid fixation.
The advantage of cement fixation is that the prosthetic components are instantly fixed to the bone, allowing the patient to bear full weight on the leg immediately after surgery. On the other hand, cement is a dis-advantage if the prosthesis ever needs to be replaced. It is difficult to chip out all the cement during removal of the old prosthesis. Therefore, cement is typically used in patients over sixty-five. Their bone tends to be more osteoporotic (softer), with less potential to grow into the metal. In addition, there is less likelihood of a revision being needed due to the lower demands on the joint and the overall relatively shorter remaining life expectancy when compared to younger patients. The early days of cement fixation had some problems, but recent results have improved dramatically due to newer pressurizing cement techniques.
Prosthetic cementless fixation is used for younger bone. The younger the patient, the better the bone quality and the better the ingrowth potential. The downside is that it typically takes six weeks for bone to grow into the porous metal backing of the prosthesis. This means that any weight-bearing activity is usually restricted for six weeks postoperatively. The advantage with the cementless technique is that fixation is achieved more naturally, and the metal is fixed directly to the bone without a cement intermediary. With a younger patient, there is a greater chance that future revision sur-gery will be needed. A cementless prosthesis is easier to revise than its cemented counterpart, as the surgeon does not have to deal with cement removal.
A patient cannot feel the difference be-tween the two types of fixation. The fixation technique is chosen by the surgeon based on the bone quality, age, and demands of the patient.