The Role of Fibers Within the Tibial Attachment of the Anterior Cruciate Ligament in Restraining Tibial Displacement

Purpose: To evaluate the load-bearing functions of the fibers of the anterior cruciate ligament (ACL) tibial attachment in restraining tibial anterior translation, internal rotation, and combined anterior and internal rotation laxities in a simulated pivot-shift test. Methods: Twelve knees were tested using a robot. Laxities tested were: anterior tibial translation (ATT), internal rotation (IR), and coupled translations and rotations during a simulated pivot-shift. The kinematics of the intact knee was replayed after sequentially transecting 9 segments of the ACL attachment and fibers entering the lateral gutter, measuring their contributions to restraining laxity. The center of effort (COE) of the ACL force transmitted to the tibia was calculated. A blinded anatomic analysis identified the densest fiber area in the attachment of the ACL and thus its centroid (center of area). This centroid was compared with the biomechanical COE. Results: The anteromedial tibial fibers were the primary restraint of ATT (84% across 0 degrees to 90 degrees flexion) and IR (61%) during isolated and coupled displacements, except for the pivot-shift and ATT in extension. The lateral gutter resisted 28% of IR at 90 degrees flexion. The anteromedial fibers showed significantly greater restraint of simulated pivot-shift rotations than the central and posterior fibers (P < .05). No significant differences (all < 2 mm) were found between the anatomic centroid of the C-shaped attachment and the COE under most loadings. Conclusions: The peripheral anteromedial fibers were the most important area of the ACL tibial attachment in the restraint of tibial anterior translation and internal rotation during isolated and coupled displacements. These mechanical results matched the C-shaped anteromedial attachment of the dense collagen fibers of the ACL.