Seven Cuts to the Perfect Total Knee

There are a total of 7 bone cuts in a typical total knee replacement (TKR): distal femur, anterior femur, posterior femur, anterior chamfer, posterior chamfer, tibia, and patella. Each of these cuts has its own special science, and each cut can affect the other cuts and potentially the outcome of the TKR. The distal femoral cut starts the overall alignment of the leg. Five degrees of valgus is cosmetically appealing, avoids excessive valgus, and prevents thighs from rubbing together. The anterior femoral cut sets femoral component rotation, which has effects on patellar tracking and gap balancing. In most knees, correct rotation is approximately 3° of external rotation compared to the posterior condylar axis. An important exception is in valgus knees, where this could lead to accidental internal rotation. The posterior condyle cuts, with the tibial cut, determine the flexion gap. Injury to the medial collateral and posterior cruciate ligaments should be avoided. Anterior and posterior chamfer cuts must avoid these ligaments as well. The tibial cut is challenging. A 3° posterior slope is most typical, and rotation is crucial. Internal rotation is a common error, affecting patellar tracking. Changing rotation on a sloped cut also adds varus or valgus. The patella cut should not be too deep. Component placement should tend medial and superior. If a lateral release is necessary, it should be done from inside-out, with preservation of the blood supply.