# Revisiting the Role of Knee External Rotation in Non-Contact ACL Mechanism of Injury
## 4 Discussion  
The main finding of this study is that the location of peak stress and strain at the ACL is consistently at the AM bundle and always at the midportion of the ACL when the knee is under combined knee valgus, slight knee flexion, high axial load and with tibial ER. Our findings highlight the importance of ER in ACL injury, which is currently overlooked. Moreover, the finding that the ACL most probably begins to rupture at the AM bundle is also a novel finding. Our results were obtained using a knee model that uses a more realistic ACL model (two bundles) and is adjusted to the average individual (exact morphometry of ACL insertion points to the average population), which tested a multitude of different combinations of knee biomechanical kinetics to investigate which combinations would place the ACL at a higher risk of rupture. Our model and respective findings can be used for other different simulations and serve as the base model to further test the ACL behaviour under different conditions (for example, after ACL reconstruction or repair procedures). The peak stress and strain at the ACL (either at the AM or PL bundles) were consistently higher when ER was applied as compared with the IR mechanism (under the same loading conditions). This finding goes against the common knowledge in testing the ACL tear with IR (because the PL bundle controls IR). Indeed, most cadaveric and simulation studies that evaluate the ACL tear are performed using a stress mechanism with IR and not ER [21; 22; 23; 24; 25; 26]. There are several real-world examples of video analysis studies that support the ACL tear mechanism under ER. Many systematic video analysis studies have examined the moment of ACL injury in different sports and found that the tibia was most commonly externally rotated during the ACL noncontact mechanism of injury [47; 48; 49; 52; 57]. [PERSON] et al. [53] analysed the noncontact ACL injuries in 39 professional male football players and reported that the foot at the impact contact moment was in IR in six cases, in ER in six cases and neutral in 11 cases. [PERSON] et al. [52] also analysed videos from 134 consecutive ACL injuries in football and found that the foot was externally rotated in 59% and 66% of the athletes at the initial contact and at the injury frame, respectively. Another video analysis study evaluated 69 noncontact ACL tears in professional American football athletes and found that the foot was externally rotated in 90% of the cases [47]. Similarly, [PERSON] et al. [48] reported the systematic video analysis of 15 cases of noncontact ACL injuries in a professional rugby union with 11 cases showing an externally rotated foot at the initial contact. In team handball, [PERSON] et al. [49] evaluated the mechanisms for ACL injuries, and the tibia was either externally or internally rotated but most commonly externally rotated. Also, in skiing, where ACL injuries are frequent, the most common tear mechanism involves a combination of valgus and ER [57]. The tibiofemoral bone-bruise pattern after an ACL injury can also disclose the injury mechanism of an ACL tear. Although tibial IR is presented in most cases of injuries, there is large heterogeneity regarding the location of the tibial and femoral bone bruises [58; 59; 60] with approximately 25% of all bone bruises located exclusively in the medial tibial plateau and medial femoral condyle [60], suggesting that ER might be involved in an important proportion of the ACL mechanism of injuries. However, some caution is needed when interpreting the bone-bruise pattern to understand ACL injury dynamics because they can reflect the late phase of noncontact ACL injury rather than the injury mechanism leading to the ligament tear [61]. Some biomechanical studies also support an important role of ER in ACL injury. [PERSON] et al.