# Revisiting the Role of Knee External Rotation in Non-Contact ACL Mechanism of Injury
## 1 Introduction  
The anterior cruciate ligament (ACL) mechanism of injury is still debated and not well understood. The understanding of how the ACL is torn during a noncontact mechanism is crucial to determine which anatomical structures are ruptured and improve the knowledge on the tear mechanism for diagnostic and treatment purposes [1]. Most of the past clinical, experimental and computational studies highlight the importance of the tibial internal rotation (IR) to evaluate rotational instabilities of the knee joint. This framework arises from four main concepts that are well known in the literature: (1) theclassic pivot-shift test that is performed with the tibia in IR [2]; (2) biomechanical studies showing the shift of the tibial centre of rotation towards the medial plateau in ACL-deficient knees [3]; (3) the bone-bruise patterns in magnetic resonance imaging (MRI) exams [4]; and (4) the anatomy of the ACL, suggesting that the ligament is tighter in IR than in external rotation (ER) [5]. For these reasons, the scientific literature considers that the noncontact ACL tear mechanism occurs mainly under 20-30 degrees of knee flexion combined with a valgus force and IR of the tibia, associated with posteroanterior and axial loads. Research has consequently been focused on the anterolateral structures of the knee [6; 7; 8; 9; 10; 11; 12], and surgical procedures have been developed with the aim of preventing the anterolateral displacement of the lateral tibial plateau, reducing the pivot-shift phenomenon and restoring the IR knee stability [7; 9; 10; 11; 12; 13]. Despite the fact that IR plays an important role in the ACL noncontact mechanism of injury, the literature also suggests that the ER mechanism should be considered [14]. The ACL has a significant effect on isolated tibofemoral ER at 20-30 degrees of knee flexion [15; 16], and the tibial ER shows a significant increase after an ACL tear [16; 17], suggesting that the ACL is also an ER restrictor and, consequently, is demanded during ER movement. Some authors have previously suggested that ER of the tibia can produce an equal or greater pivot-shift sign on physical examination [16; 18; 19]. The pivot-shift phenomenon, when evaluated intra-operatively or in cadaveric testing, has shown that the tibial centre of rotation is not identical in all isolated ACL-deficient knees with a significant medial plateau translation even with the pivot-shift test performed only in IR [20]. Despite the early evidence on the role of tibial ER in knee rotational stability, most experimental and simulation studies have solely focused on the IR when testing models of ACL injury [21; 22; 23; 24; 25; 26]. The contribution of ER in the ACL tear mechanism has thus been undervalued and is still not well understood. The importance of the anteromedial knee structures and instability has been overlooked, and consequently, the potential of surgical procedures that are able to control the anteromedial abnormality has been neglected. Experimental procedures provide a good insight into the knee biomechanics and computational--based on the finite element method (FEM)--can serve as a complementary tool for a better understanding. The FEM modelling provides a fast and low-cost analysis of different biomechanical scenarios, especially when compared to cadaveric and clinical studies, which can be used to test and predict the behaviour of biological tissues under different conditions. The FEM allows for the simulation of the joint-kinetics behaviour under different boundary conditions of lower limb biomechanics. Indeed, the FEM can be a suitable tool to compute and simulate the cumulative damage to different tissues and materials [27; 28] that are time-consuming and expensive to obtain experimentally [29; 30]. The use of FEM simulations can test varying hypotheses and aid in understanding the complex cause-effect of the response of the soft tissues to different loading conditions [31].