Primary Knee Symposium: Biomechanical Considerations in the Knee

Measured Resection vs. Gap Balancing - Kenneth Krackow, MD

Dr. Krackow looked at the two different aspects of measured resection and gap balancing. He reviewed classic extension-flexion balancing at 0° and 90° which entails: minimum tibial resection, resection of the posterior femur to the same thickness as the components, then finishing at extension. The potential issue with this is that the gaps could end up loose or tight at flexion-extension other than 0° and 90°.

Measured resection was borne from the need for an accurate, reproducible way of preparing a femur for ingrowth fixation. With this technique, the tibial cut was made independently of the femur, according to the anticipated thickness of the replacement component. However, he cautions that the actual resection is rarely the exact same as the replacement component. In addition, measured resection is often different for CR and PS knees since PCL resection can increase the flexion space about 4-5mm.¹ PS intercondylar pegs are taller than the 4-5 mm difference, and gross instability in flexion is not typically seen, which may be partly the reason for better flexion with some PS knees.

Dr. Krackow then assessed anatomic versus classical rotation during flexion. During flexion gap balancing, the knee is flexed at 90 degrees, the tibia is distracted from the femur, and the anterior and posterior femoral resections are rotated to be parallel to the cut tibia. The question is, in this case, is the femur properly rotated with respect to the tibia (considering bony landmarks)? Dr. Krackow submits that there is no way to assure that the collaterals are balanced at 90 degrees of flexion, i.e. that the tibia is in neutral alignment, in terms of varus-valgus position, in flexion. In addition, if you do any moderate ligament releases in extension, varus, or valgus, there is a good chance that you may over-resect or over-release, leading to relative gapping in flexion. This is because, in flexion, the secondary stabilizers from posterior are relaxed.

Other issues may be introduced if the knee has deformities. In any event, inappropriate releases may lead to mal-rotated femoral components. PSlide A PSlide B PSlide C PSlide D PSlide E

Dr. Krackow concluded with his overall concerns regarding flexion gap balancing and appropriate anterior and posterior femoral cuts. PSlide F PSlide G