Systematic effects of femoral component rotation and tibial slope on the medial and lateral tibiofemoral flexion gaps in total knee arthroplasty

Purpose: To quantify the effects of the systematic internal and external femoral component rotations and tibial slope on the medial and lateral tibiofemoral gaps in total knee endoprostheses. Methods: Nineteen knee cadaver specimens with an intact ligament apparatus were fixed in a custom frame, facilitating physiological flexion motion. Virtual total knee arthroplasty (TKA) was performed on three-dimensional models obtained from computed tomography scans (0 degrees and 90 degrees flexions) with systematically altered femur rotations and tibial slopes. Results: Both the femur rotation and the tibial slope influenced the medial and lateral tibiofemoral flexion gaps (p < 0.001), and the effects differed between the medial and lateral sides (p < 0.001). The medial tibiofemoral flexion gap increased by 2.90 +/- 0.34 mm and decreased by 2.66 +/- 0.26 mm for 7 degrees external and internal femur component rotations, respectively (both with p < 0.001). The lateral tibiofemoral flexion gap decreased by 3.11 +/- 0.31 mm and increased by 3.29 +/- 0.33 mm for 7 degrees external and internal femur component rotations, respectively (both with p < 0.001). Conclusion For established surgical methods, we recommend a neutral femur rotation for a 0 degrees tibial slope and a 3 degrees external femur rotation for a tibial slope of 9-10 degrees. The translational potential of this research shows that while the rotation of the femoral component in extension has no effect on the gap size, owing to the axis of rotation being perpendicular to the gap, for a 90 degrees flexion, we not only observe differences in the gap size between the medial and lateral but also unequal differences on either side depending on the inward or outward rotation. The main reason for this is the position of the axis of rotation, which is not precisely half way between the lateral and medial contact points. The results show that rotation of the femoral component always creates an unbalanced flexion gap. The translational potential of this article: The article points out the differences in the tibiofemoral gap in total knee endoprostheses due to the systematic internal and external femoral component rotation. While in lower leg extension there are no differences seen, in 90 degrees knee-flexion there are unequal differences within the medial and lateral compartment that show a mathematical relationship towards the femoral compartment rotation which needs to be intraoperatively considered.