Quantifying potential leg axis alteration from extramedullary tibial guide malalignment in total knee arthroplasty: a computer simulation

BACKGROUND: Despite technological advancements, the added benefit of navigated total knee arthroplasty has recently been questioned, and conventional instrumentation remains very common. Rotational errors during tibial resection in total knee arthroplasty (TKA) may influence postoperative alignment. Although previous studies have shown changes in coronal projection under these conditions, the geometric relationship between axial rotation and the effective posterior tibial slope remains insufficiently defined. The aim of this study was to quantify how axial malrotation of the tibial resection plane interacts with different posterior tibial slope targets and how these interactions affect both projected coronal alignment and the effective sagittal slope. METHODS: Three-dimensional (3D) surface models of the lower extremity of 20 different patients were generated from computed tomography data. Tibial cuts were simulated, beginning with an osteotomy plane perpendicular to the tibial mechanical axis, which served as a baseline plane. For each predefined posterior tibial slope (3°, 5°, and 7°), axial malrotation of the tibial resection plane was systematically varied (± 5°, ± 10°, ± 20°, ± 40°), and the resulting changes in projected coronal alignment and effective posterior tibial slope were recorded. RESULTS: Coronal deviation increased proportionally with both the magnitude of axial rotation and the planned posterior tibial slope. At a slope of 5°, ± 10° of malrotation resulted in a coronal deviation of 0.9° ± 0.01, increasing to ± 1.7° ± 0.01 at ± 20°. At a slope of 7°, ± 20° of rotation produced a coronal deviation of 2.4° ± 0.03. In contrast, the effective posterior tibial slope was minimally affected by axial rotation, changing by ≤ 0.3° even at ± 20° of malrotation. CONCLUSION: This study demonstrates that axial malrotation of a sloped tibial resection primarily affects the projected coronal mechanical alignment, while the effective posterior tibial slope itself remains largely preserved. The magnitude of coronal deviation increases with higher planned slope angles. In standard mechanically aligned scenarios, the isolated geometric effect appears limited; however, in slope-sensitive or individualized alignment strategies, rotational errors may contribute to measurable coronal projection changes.