Does the Medial or Lateral Tibial Slope Have a Greater Effect on ACL Reconstruction Mechanics? A Biomechanical Analysis

Background: Although the posterior tibial slope (PTS) has been identified as a key factor influencing anterior cruciate ligament (ACL) graft biomechanics and clinical outcomes, little is known regarding which plateau has the greater effect on mechanics of the ACL-reconstructed knee. There are differences in slopes of the medial and lateral tibial plateaus, but this difference has yet to be biomechanically investigated in an ACL-reconstructed setting. Purpose: To investigate the individual effects of medial and lateral PTS differential modifications on ACL graft force, anterior tibial translation (ATT), and internal tibial rotation in a cadaveric model. Study Design: Controlled laboratory study. Methods: Twelve fresh-frozen human cadaveric knees (mean age, 61.6 years) were tested. Native PTS was measured using computed tomography. ACL reconstruction was performed, followed by individual anterior closing-wedge osteotomies of the medial and lateral compartments to achieve a 0° slope. Each compartment was then adjusted in 5° increments with 3-dimensional printed wedges and secured with an external fixator. Biomechanical testing consisted of a 500-N axial load and a 1-N⋅m axial torque across 8 medial-lateral PTS states: native/native, 5°/10°, 5°/5°, 10°/5°, 10°/10°, 10°/15°, 15°/15°, and 15°/10°. ACL graft force, ATT, and internal tibial rotation at full extension were recorded. Linear mixed modeling was used to evaluate the effect of medial and lateral slopes on the outcome variables. Results: The lateral tibial slope contribution had a greater effect on ACL graft force and ATT than the medial tibial slope contribution. Each 1° increase in lateral slope was associated with a 4.9-N (11.1%) increase in graft force and a 0.63-mm (25.2%) increase in ATT ( P < .001). Each 1° increase in medial slope was associated with a 1.9-N (4.3%) increase in graft force and a 0.48-mm (19.2%) increase in ATT ( P < .001). Internal tibial rotation was not significantly affected by either lateral or medial PTS. Conclusion: Although both slopes significantly affected ACL graft force and ATT, lateral PTS had a greater effect on ACL graft force and ATT compared with the medial PTS and may warrant primary consideration for a slope-correcting osteotomy. Clinical Relevance: While correction of both medial and lateral slopes is important, the lateral PTS contributes more significantly to ACL graft forces and ATT.