# Revisiting the Role of Knee External Rotation in Non-Contact ACL Mechanism of Injury
## 4 Discussion
[62] analysed the passive knee biomechanical properties in tibial rotation and suggested that there may be a greater risk of ACL injury resulting from impingement against the lateral wall of the intercondylar notch, which has been shown to be associated with tibial ER and knee abduction. Similarly, [PERSON] and [PERSON] [63] reported a qualitative difference between placing the knee in external or internal rotation. When the knee was externally rotated past 15 degrees and flexed to between 30 and 40 degrees, the impingement occurred at the midportion of the ACL. No similar impingement was noted with internal rotation of the tibia on the fixed femur. The tibiofemoral axial loads contributed significantly to the ACL tear, which was expected based on previous data from the literature [47; 48; 49; 50; 51; 52; 53; 54]. This finding highlights the reliability of our model, which shows that higher axial load increases the peak stress and strain at the ACL. Axial loads often result from a landing situation [64; 65] or when pivoting the foot at the ground (which has a higher axial load as compared to a walking load situation without impact). Indeed, video analysis studies show that the ACL often tears when there is an impact of the foot against the ground (for example, when landing) [49; 51; 66; 67]. The simulations with the higher axial load (4000 N for landing [55] vs. 700 N for walking [56]) showed that the ACL stress and strain values were consistently higher when the 4000 N was applied. The highest peak strain and stress values were located at the midportion of the AM and PL bundles, which suggests that the tear will probably occur at the midportion of the ACL. Combined data from 31 studies and 2088 individuals show that ACL tears occur mostly at the proximal third, but one third of these studies only included proximal ACL tears [68; 69; 70]. When more recent data are carefully analysed, the mid-substance tears account for half of the incidence of ACL tears [68]. Cadaveric testing also points to a tear mechanism starting at the mid-substance of the ACL [71]. A recent FEM study on the injury mechanism of ACL also tested the stress distribution at the ACL under three different loading conditions (134 N anterior tibial load, 5 Nm external tibial torque, 5 Nm internal tibial torque) [34]. They found that the peak stress was localized at the femoral insertion when under the anterior tibial load (14.9 MPa), at the mid-substance when under ER (0.8 MPa) and near the tibial insertion site when under IR (0.9 MPa). Their findings are similar to the present study when under ER, where we found that the peak stress was also at the mid-substance of the AM bundle. However, the findings for anterior tibial load and internal rotation, showing peak stress at the femoral and tibial insertions, were not corroborated in our study. One of the reasons for the diverging results might have arisen from the fact that the authors of the previous study tested the movement isolated, whereas we combined the knee movement under combining kinetic scenarios, which emulates better real-world conditions as shown by the video-analysis studies [47; 48; 49; 52; 57]. Moreover, it was not possible to compare which bundle was most affected, as the previous study modelled the ACL as a single structure. Side-to-side comparison of peak strain was also not possible because the previous study had not tested the ACL strain. The native resistance of the ACL is described by the literature to be approximately 2160 \(\pm\) 157 N among the young population. These values were tested using INSTROM in cadaveric knees submitted to axial and posteroanterior forces with the knee joint placed in a fixed 30\({}^{\circ}\) semiflexion [72]. The mechanism produced with the INSTROM device does not correspond to the reality of an ACL tear as it does not take into account the forces related to tibial rotation.