Changes in varus and valgus slope caused by tibial cutting block rotation in total knee arthroplasty

Most surgeons attempt to cut their tibial slope to align with their intended tibial component rotation; however, there is variability in guide placement and rotational landmarks. Tibial slope varies between 5 and 15 ° , while tibial cutting guide rotation can vary up to 40 ° in either direction. Just as acetabular cup positioning can be altered based on pelvic tilt, this study sought to quantify the analogous change in varus and valgus slope for every degree of tibial cutting guide rotation. Post-rotational varus and valgus tilt was quantified by the expression 90 ° − cos − 1 ( − cos ⁡ ( θ ) sin ⁡ ( σ ) + sin ⁡ ( θ ) sin ⁡ ( ϕ ) ) , and post-rotational posterior tibial slope (PTS) was quantified by 90 ° − cos − 1 ( − sin ⁡ ( θ ) sin ⁡ ( σ ) − cos ⁡ ( θ ) sin ⁡ ( ϕ ) ) , where θ represents the tibial cutting block rotation, σ represents any preexisting varus/valgus slope, and ϕ represents the PTS before rotation. Rotation was calculated using a standard 5° slope for CR components, mean anatomic slope of 7°, and larger degree of anatomic slope, 10°. There was a non-linear relationship between tibial varus and valgus change and rotation. Mean change for neutral varus/valgus knees every 10° over a 40° range yielded 0.80° for 5° PTS, 1.12° for 7° PTS, and 1.60° for 10° PTS. Additionally, there was a non-linear decrease in mean PTS per 10° rotation of 0.29° for 5° PTS, 0.41° for 7° PTS, and 0.59° for 10° PTS. Newer kinematic methods of total knee arthroplasty aim to restore pre-arthritic native anatomy. Surgeons should be aware of the potential impact of changing the rotation of tibial slope on both sagittal and coronal plane balance.