Tears of the anterior cruciate ligament (ACL) are often accompanied by injuries to the anterolateral complex (Kaplan fibres (KF), anterolateral capsule including the anterolateral ligament (C/ALL)), plus posterior lateral meniscus root (PLMR) tear. These concomitant injuries have been separately associated with increased anterolateral instability, but their relative contributions to stability remain unknown. The aim was to investigate the relative contributions of the ACL, C/ALL, KF and PLMR to translational and rotatory stability of the knee. It was hypothesized that the KF are the primary restraint against tibial internal rotation.
10 fresh-frozen human cadaveric knees (aged 56 ± 4 years) were tested in a 6-degrees of freedom robot system (Stäubli TX90). Initially, the robot found the passive path by neutralizing loads/torques in each knee. Then, 90N anterior/posterior force, 5Nm internal/external and 8Nm valgus/varus torques were applied in 0°, 30°, 60° and 90° and the kinematics were recorded. The kinematics were replayed after sequentially cutting the ACL, C/ALL, KF and the PLMR (order varied) and the respective load/torque drop was measured. The decrease of load/torque reflected the respective contribution of the cut structure to restrain knee laxity. One-way analysis of variance (ANOVA) with repeated measures were used to find significance in the contribution of the anatomical structures across the cutting stages separately for each load/torque and flexion angle.
The ACL was the primary restraint for anterior translation and accounted for 71 ± 11% in 0° and 64 ± 8% in 30° knee flexion (p < 0.0001). Internal rotation was mainly constrained by the KF with growing contribution from lower to higher flexion, reaching: 44 ± 23% at 90° (p < 0.01) and followed by C/ALL: 14 ± 13% at 90° (p < 0.05)). The ACL was a restraint of internal rotation in 0° knee flexion: 16 ± 13% (p < 0.05), but insignificant in higher flexion. The PLMR was not a significant restraint of internal rotation or anterior translation. However, cutting the PLMR resulted in a considerable torque decrease for valgus: 33 ± 17% at 90° (p < 0.001), but was insignificant in resisting external rotation: max 5 ± 6% at 30° and posterior draw: 11 ± 9% at 30°.
The Kaplan fibres were a significant restraint against tibial internal rotation, especially in higher (60° and 90°) knee flexion, and the ACL to anterior draw across 0° to 90°, as hypothesised. The C/ALL complex contributed 10% to 14% resistance to internal rotation from 30° - 90° knee flexion. The PLMR resisted neither internal rotation nor anterior translation significantly, but had a large effect in valgus rotation. The findings with clinical implications are: 1: The KF on the lateral femoral condyle and shaft are a strong restraint of internal rotation (along with overlying ITB), and 2: The PLMR provided negligible resistance to anterolateral knee laxity within normal laxity limits (ACL intact/reconstructed).