Howard J Kapp, Orthopedic Surgeon

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Bone Preparation and Implants

There are special instruments that are used to shape the end of the femur bone and the top of the tibial bone. The purpose of these instruments are to remove just the amount of bone necessary to give room to place the new surface. The surface placed on the end of the femur is a metal component that 'caps' the end of the femur bone and has curvatures which are very close to the curvatures of the normal knee joint. For the tibial bone, the top of the tibia is removed to form a flat surface and then a flat metal tray is placed onto the tibia into which a plastic component is snapped into place. This plastic becomes the new 'fake' cartilage for the tibia. This plastic also has curvatures that simulate the normal knee. When this surface is mated against the metal surface on the end of the femur, this helps to provide some intrinsic stability for the knee joint. Additional stability for the knee joint is provided by the ligaments which are maintained. The collateral ligaments (on the sides of the knee) are maintained, and is some knees, the posterior cruciate ligament will be kept. The anterior cruciate ligament is always sacrificed with total knee replacement. This is necessary to be able to correctly place the component and to fix them well. The posterior cruciate ligament is removed with some types of knee designs (posterior cruciate sacrificing designs). With the tibial metal tray, there is a short stem that goes into the tibial bone to provide rotational stability for this metal tray. The undersurface of the patella is also replaced with a plastic component in most knees.

The fixation of the components to the bone is most commonly bone with a bone cement, which effectively 'glues' the components to the bone. This use of bone cement has been very effective in the knee and does not have the history of failure in the knee as it has had in hip replacement. Cemented knee replacements have a longevity of more than 20 years, even with the earliest designs of knee replacement. A second form of fixation is what is named bone ingrowth fixation. Just as in the hip, the bone grows to the metal implants in the knee and fixes these metal implants to the bone