# Revisiting the Role of Knee External Rotation in Non-Contact ACL Mechanism of Injury
## 4 Discussion
In our study, the peak stress (11 MPa) and strain (4.8%) were obtained at the AM bundle with the combined knee valgus (50 Nm abduction moment), 20 degrees of knee flexion, during landing (4000 N axial load) and with tibial ER (15 Nm and 20 Nm). A study by [PERSON] et al. [73] showed that the ACL bundles develop higher load-related material properties than the posterior bundles. The anterior bundles developed significantly larger moduli, maximum stresses and strain energy densities to maximum stress than the posterior subunits. The average stresses at rupture were, respectively, 38 MPa vs 15 MPa for the different bundles, which is in accordance with the present work. The tibial rotation plays an indisputable role in the ACL tear mechanism, but there was not a significant effect of the magnitude of the tibial rotation momentum (15 Nm vs 20 Nm) on the stress and strain values. This suggests that 15 Nm tibial rotation may be enough to cause an ACL tear (especially in cases of low energy injury mechanism) and increasing to 20 Nm may not lead to much further damage. The anterior tibial load showed low sensitivity in peak ACL stress and strain values in the FEM simulation, which suggests that, although the ACL has a major role in restricting posteroanterior tibial translation [74; 75], an -anterior load seems to have no significant effect on the ACL peak stress and strain values and probably a lesser role (than previously thought) in the ACL tear. Our findings are clinically relevant and offer some directions for future studies. Upcoming cadaveric biomechanical studies should focus on ER when testing the ACL tear mechanism. The role of ER in ACL tears is undervalued and should be better understood as it may have important implications for the diagnosis, treatment and prevention of ACL injuries. Assuming that ER has an important role in ACL tears, orthopaedic surgeons also need to consider the anteromedial structures of the knee when performing ligament reconstructive or reparative surgery. Moreover, our model can be used in future studies to better understand the ACL and neighbouring structures (other knee ligament, meniscus and cartilage) and biomechanical behaviour under different kinematic and kinetic conditions [34; 76; 77]. Our ACL model and testing conditions can also serve as a base to simulate different orthopaedic procedures (such as ACL reconstruction [31; 78; 79; 80] or ACL repair) [81], investigate the ACL stress and strain after these procedures and assess the potential risk of re-rupture. The use of FEM modelling can provide a faster and cheaper testing method than cadaveric, biomechanical or clinical studies [28] and, therefore, can be used as a predicting tool for both prevention and simulation of the potential results after an ACL reconstruction or repair procedure. There are several limitations that need to be acknowledged. The FEM models simulate the behaviour of specific biomechanical conditions but are limited in their nature as it is not possible to simulate the true nature of real-world situations in which there are many factors that act in combination to predispose someone to an ACL injury [82]. Moreover, several potential confounders, such as interindividual varying anatomy and biomechanics, cannot be accounted for in FEM and may have influence on the results. However, we used average ACL footprint data to better adjust our model to the general population and thus increase the potential for external validity. Hence, FEM should be used to advance and complement experimental studies but not to override or replace them. The experimental data used in this study for validation purposes were not independently collected but extracted from a previous study [26] that used both cadaveric and computational testing.