Anterior Cruciate Ligament Function in Providing Rotational Stability Assessed by Medial and Lateral Tibiofemoral Compartment Translations and Subluxations

Abstract

Background: Rotational knee stability provided by the anterior cruciate ligament (ACL) in the pivot-shift phenomena involves analysis of more complex robotic testing profiles and resulting tibiofemoral compartment kinematics and subluxations. Hypotheses: Using anterior-posterior tibial forces along with internal and valgus tibial moments will produce a major anterior subluxation of both tibiofemoral compartments not obtained with internal and valgus moments alone. Increasing the internal torque in pivot-shift testing will constrain the anterior subluxations of the medial and central tibial compartments. Study Design: Controlled laboratory study. Methods: A 6 degrees of freedom robotic knee testing system applied anterior translation and rotational loading profiles in 10 cadaveric knees before and after ACL sectioning. Changes in knee motion limits were measured, and medial and lateral tibiofemoral compartment translations were determined by digitization of tibial plateau anatomic landmarks. Loading profiles simulated Lachman and tibial rotation tests as well as typical pivot-shift loading profiles from prior in vitro and in vivo studies. Results: After ACL sectioning, anterior tibial translation increased by 10.3 ± 3.7 mm at 25° of flexion ( P < .001). Internal tibial rotation increased by 1.6° ± 1.1° (5 N·m; P > .05). In pivot-shift tests (anterior translation, 100 N; internal rotation, 1 N·m; valgus, 7 N·m), the tibial rotation center shifted outside the medial tibial margin, with abnormal anterior translation of both compartments (medial, 12.9 ± 3.9 mm; lateral, 7.5 ± 3.7 mm; P < .001), with internal rotation decreasing by 4.1° ± 3.5° ( P < .05). A greater internal rotation torque (5 vs 1 N·m) in the pivot-shift test constrained and limited anterior tibial translation and prevented anterior subluxation of the medial compartment ( P < .001). Conclusion: Sectioning of the ACL produces major increases in tibiofemoral compartment translations and only small increases in internal tibial rotation. The simulation of the pivot shift requires a combined loading profile of anterior translation, internal rotation, and valgus, which produces the greatest anterior subluxation of the medial and lateral tibiofemoral compartments. This testing profile is recommended to be included along with other loading profiles for future ACL studies. The application of a high internal rotation torque in cadaveric pivot-shift tests constrains anterior tibial subluxation of the medial and center compartments and appears less ideal for analysis of ACL function and graft reconstructions. Clinical Relevance: Surgeons should be cautious in interpreting conclusions on ACL function and graft reconstructions without knowing the resulting tibiofemoral subluxations or loading conditions that may limit maximum anterior tibial femoral subluxations.