The Lateral Meniscus Posterior Root and Meniscofemoral Ligaments are Stabilizing Structures in the ACL Deficient Knee: A Biomechanical Study

Abstract

The lateral meniscus posterior root was a significant primary stabilizer of the knee for internal rotation and anterior tibial translation during pivoting activities at lower flexion angles and internal rotation at higher flexion angles. The menisci have an important role in stabilizing the knee joint. The biomechanical effects of lateral meniscal posterior root tears with versus without meniscofemoral ligament (MFL) tears in anterior cruciate ligament (ACL) deficient knees have not been studied in detail. To determine the biomechanical effects of the lateral meniscus (LM) posterior root tear in anterior cruciate (ACL) - intact and ACL-deficient knees. In addition, the biomechanical effects of disrupting the meniscofemoral ligaments (MFLs) in ACL deficient knees with meniscal root tears was evaluated. It was hypothesized that disruption of the posterior horn lateral meniscal root attachment would cause a significant increase in internal rotation during simulated pivot shift and internal rotation tests in the ACL-deficient knee, both with and without intact meniscofemoral ligaments. In addition, it was hypothesized that disruption of the LM root and MFLs would significantly increase internal rotation at high flexion angles. Controlled laboratory study. Ten paired (n = 20) fresh-frozen cadaveric knees were mounted in a 6 degree-of-freedom robot for testing and divided into two sequential cutting groups. The sectioning order for group 1 was (1) ACL, (2) LM posterior root, (3) MFLs, and for group 2 was (1) LM posterior root, (2) ACL, and (3) MFLs. For each cutting state, displacements and rotations of the tibia were measured and compared to the intact state following a simulated pivot shift test (5 Nm internal rotation torque combined with a 10 Nm valgus torque) at 0°, 20°, 30°, 60° and 90° of knee flexion; an anterior translation load (88 N) at 0°, 30°, 60° and 90° of knee flexion; and internal rotation (5-Nm) at 0°, 30°, 60°, 75° and 90° of knee flexion. Cutting the LM root and MFLs significantly increased ATT during a simulated pivot shift test at 20° and 30° when compared to the ACL cut state (both p<0.05). Cutting the LM root in ACL intact knees significantly increased internal rotation by between 0.7° ± 0.3° and 1.3° ± 0.9° (all p<0.05), except at 0° (p=0.136). When the ACL + LM root cut state was compared to the ACL cut state, the increase in internal rotation was significant at higher flexion angles of 75° and 90° (both p<0.05) but not between 0°- 60° (all p>0.2). For an anterior translation load, cutting the LM root in ACL deficient knees significantly increased ATT only at 30° (p=0.007). The LM posterior root was a significant stabilizer of the knee for anterior tibial translation during a simulated pivot shift test at lower flexion angles, and internal rotation at higher flexion angles. Increased knee instability due to lateral meniscal root deficiency may contribute to increased functional limitations in patients and potentially to increased loads on ACL reconstruction grafts.