The medial ligaments and the ACL restrain anteromedial laxity of the knee

Abstract

Purpose: The purpose of this study was to determine the contribution of each of the ACL and medial ligament structures in resisting anteromedial rotatory instability (AMRI) loads applied in vitro.

Methods: Twelve knees were tested using a robotic system. It imposed loads simulating clinical laxity tests at 0° to 90° flexion: ±90 N anterior-posterior force, ±8 Nm varus-valgus moment, and ±5 Nm internal-external rotation, and the tibial displacements were measured in the intact knee. The ACL and individual medial structures-retinaculum, superficial and deep medial collateral ligament (sMCL and dMCL), and posteromedial capsule with oblique ligament (POL + PMC)-were sectioned sequentially. The tibial displacements were reapplied after each cut and the reduced loads required allowed the contribution of each structure to be calculated.

Results: For anterior translation, the ACL was the primary restraint, resisting 63-77% of the drawer force across 0° to 90°, the sMCL contributing 4-7%. For posterior translation, the POL + PMC contributed 10% of the restraint in extension; other structures were not significant. For valgus load, the sMCL was the primary restraint (40-54%) across 0° to 90°, the dMCL 12%, and POL + PMC 16% in extension. For external rotation, the dMCL resisted 23-13% across 0° to 90°, the sMCL 13-22%, and the ACL 6-9%.

Conclusion: The dMCL is the largest medial restraint to tibial external rotation in extension. Therefore, following a combined ACL + MCL injury, AMRI may persist if there is inadequate healing of both the sMCL and dMCL, and MCL deficiency increases the risk of ACL graft failure.

Keywords: Anterior cruciate ligament; Anteromedial rotatory instability; Biomechanics; Medial collateral ligament; Posterior oblique ligament; Restraint of tibiofemoral joint laxity.

Fig. 1

Superficial MCL (sMCL) and posteromedial capsule, which includes the posterior oblique ligament (POL), have been removed to reveal the deep MCL (dMCL). The dMCL is taut and well-aligned to resist tibial external rotation in the extended knee with tibial external rotation. The black dot is the centre of the femoral attachment of the sMCL (medial aspect of a right knee, with the anterior to the left and proximal to the top)

Fig. 2

Robotic joint testing system with knee specimen in place at 0° flexion. The tibia is secured to the end of the robot arm, which has the load cell at its wrist (where the cable attaches). The femur is secured vertically in the fixed mounting on the base of the robot system

Fig. 3

Contributions (%) of each of the ACL, AMR, sMCL, dMCL, and POL + PMC to resisting 90 N tibial anterior translation force, across 0° to 90^o knee flexion (mean + SD, n = 12). Both the ACL and sMCL were significant restraints at all angles tested

Fig. 4

Contributions (%) of each of the ACL, AMR, sMCL, dMCL, and POL + PMC to resisting 5 Nm tibial external rotation torque, across 0° to 90^o knee flexion (mean + SD, n = 12)

Fig. 5

Contributions (%) of each of the ACL, AMR, sMCL, dMCL, and POL + PMC to resisting 8 Nm tibial valgus angulation moment, across 0° to 90^o knee flexion (mean + SD, n = 12)