The LM Posterior Root Stabilizes the ACL Deficient Knee

OBJECTIVES: The menisci have an important role in stabilizing the knee joint. The biomechanical effects of lateral meniscal posterior root tears with or without meniscofemoral ligament (MFL) tears in anterior cruciate ligament (ACL) deficient knees have not been studied in detail. The aim of this study was to determine the biomechanical effects of disruption of the lateral meniscus posterior root attachment in ACL-intact and ACL-deficient knees. It was hypothesized that disruption of the posterior horn lateral meniscal root attachment would cause a significant increase in both anterior tibial translation (ATT) and internal rotation during a simulated pivot shift and also in internal rotation tests in the ACL-deficient knee, both with and without intact meniscofemoral ligaments. METHODS: 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) lateral meniscus posterior root and (3) MFL. The sectioning order for group 2 was (1) lateral meniscus posterior root, (2) ACL, and (3) MFL. 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 and external rotation (5-Nm) at 0°, 30°, 60°, 75° and 90° of knee flexion. RESULTS: 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 (p<0.05) (Figure 1). Cutting the LM root in ACL intact knees significantly increased internal rotation by between 0.4° ± 0.4° and 1.3° ± 0.9° (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 (p<0.05) at higher flexion angles (75° and 90°) but not between 0°- 60°. For an anterior translation load, cutting the LM root in ACL deficient knees significantly increased ATT at 30° (p<0.05). CONCLUSION: The LM 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. Lateral meniscal posterior root tears should be repaired concurrently with ACL reconstruction. Increased knee instability due to lateral meniscal root deficiency may contribute to increased functional limitations in patients and potentially to ACL reconstruction graft failure.