Summary
The role of the ACL in the loaded knee was found to vary from knee to knee, and this may help explain the variation in injury patterns observed in video analyses.
Abstract
Introduction
The role of ligaments in stabilising the knee has mostly been analysed in the unloaded knee. However, the vast majority of ACL injuries occur when there is also considerable axial compression acting across the knee. The aim of this study was to determine the contributions of soft tissues to knee stability in the loaded knee.
Methods
Four cadaveric knees* (half female, age <65) were tested in a robotic system. The native knee was loaded with a nominal 50 N compression and flexed to 90° and extended back to zero with zero loading across all other axes. The flexion/extension arc was then repeated for six further load cases: 5 Nm internal rotation, 5 Nm external rotation, 8 Nm valgus, 8 Nm varus, 90 N anterior and 90 N posterior. All seven load cases were then repeated but with 400 N of tibiofemoral compression applied. The soft tissue contribution to knee stability was then analysed via sequential dissections, with kinematic play back and the principle of superposition. Fourteen structures were tested: skin & fat, anteromedial capsule, superficial MCL, deep MCL, posteromedial capsule, popliteofibular ligament, popliteal tendon, LCL, iliotibial band, anterolateral capsule (including the ALL), medial meniscus, lateral meniscus, ACL and PCL. Cutting order was randomised for each knee.
Results
For the unloaded knee, results corresponded with well-established understanding: for example, the ACL was found to be the primary restraint to anterior drawer, the collaterals restrained varus/valgus, etc. In the loaded knee, the roles of the menisci in stabilising the knee became more apparent: contributing up to 80% of varus/valgus restraint, and up to 42% of anterior/posterior restraint. The ACL was observed to restrain anterior drawer, as per established understanding, but it was also found to have unexpected contributions to varus/valgus and knee rotational torque. Rather than limit motion under the applied torques, the opposite was observed, it was found to positively contribute torque. In some knees the ACL applied a varus torque, in others a valgus torque, in some internal rotation torque, in others external rotation torque, with torques up to 6 Nm measured.
Discussion
In the loaded knee, anterior tibial translation due to posterior tibial slope led to significant tension in the ACL such that it contributed to, rather than restrained, varus/valgus and internal/external rotation torque. The direction of the torque depended on where the centre of rotation was located in relation to the line of action of the ACL and was found to vary between knees. This varying function of the ACL in the loaded knee may explain the results of video analyses that have found the ACL can rupture in both internally and external rotated knee positions, with the knee in valgus, and in rare cases even in varus.