Technical Aspects of Total Knee Replacement
The knee joint is opened with an incision that is across the front of the knee. This incision runs from the top of the knee cap to the prominent bone on the tibia, which is named the tibial tubercle. Underneath the skin the incision is carried along the medial (inside) side of the patella bone, and the patella can then be moved to the side of the knee (outside or lateral side).
Exposure By Retractors
The patella can be held in that position with a tool called a retractor while the work is done on the ends of the femur and tibial bones. Before the patella is retracted, any spurs (which have the medical name of osteophyte) are removed to make the retraction easier.
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). Whether the posterior cruciate ligament is kept or sacrificed has not made a difference through 25 years of use in regard to the function of the knee or the longevity of the knee replacement (see the research section for an article we have published on this). 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. In patients under the age of 60, this replacement of the undersurface of the patella may not be done if the patella surface shows normal articular cartilage.
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 This type of fixation most commonly is only used in patients under the age of 60. It requires that the bone be very strong to be used. This type of fixation also has good longevity when used in appropriate bone.