Influence of tibial slope on extension and flexion gaps in total knee arthroplasty: increasing the tibial slope affects both gaps

PURPOSE: Increasing the tibial slope is often performed if the flexion gap is narrower than the extension gap. The main hypothesis of this study is that increasing the tibial slope coincidentally enlarges the extension gap. METHODS: Twenty formalin-fixed cadaveric knees were obtained for study. After CT in full extension and 90 degrees flexion, the data of each specimen were entered into a standardized coordinate system and virtual bone cuts were performed with incrementally increasing the posterior slope. Gaps were measured at tibiofemoral contact points in 90 degrees -flexion and full extension in the medial and lateral compartment. RESULTS: Increasing the tibial slope did significantly widen both the extension and the flexion gaps (p > 0.001). In extension, the opening rates, i.e. the gap increase per degree of slope increase, were equal medially and laterally (0.5 mm +/-0.1) medial vs 0.6 mm (+/-0.0) lateral), whereas in flexion the lateral gap did open significantly more than the medial one (0.6 mm +/-0.1) medial vs 0.9 mm (+/-0.1) lateral (p > 0.001), resulting in a significantly greater flexion gap laterally. CONCLUSIONS: Increasing the tibial slope beyond the pre-operative planning in order to widen a tight flexion gap intra-operatively is not recommended as doing so will increase the extension gap simultaneously and will make the medial and lateral flexion gaps unequal.