## 1 Introduction
It should normally be positioned halfway between the medial compartment and the lateral part (Figure 1c), but due to the predisposition of the affected patient to a continuous wear of the intra-articular cartilage, an overcorrection is preferred in the sense that the point is positioned at 62.5% of the distance. There is also the possibility that the surgeon chooses to set this point between 50 and 70%, depending on the degree of cartilage wear [21]. * Determine the position of the \"hinge\" point B relative to the tibial plateau and to the lateral cortex of the tibia (Figure 1b). This is the point around which the angulation is made, i.e., the center of the rotation of the angulation (hereinafer, \"CORA\"). This point is important because one needs a stable lateral hinge, and a fracture line though the articular surface should be avoided. There are no specifically defined values on this matter in the operative technique, but rather ranges of values are taken into consideration: 15-20 mm relative to the articular plane and 5-10 mm relative to the lateral cortex of the tibia [24; 20; 25; 26]. * Afterwards, the point of initiation of the cut for performing the osteotomy, situated on the medial cortex, is established (depicted as \"point O\" in Figure 1b, hereinafter \"cutting point\"). There are no studies suggesting the accurate positioning of the cutting point. However, given the limitations generated by the surrounding anatomic structures, the cutting point should be placed at a minimum of 30 mm and at a maximum of 50-60 mm from the articular line [24; 25; 26; 20; 27]. The osteotomy line is depicted as BO line in Figure 1b. * Further on, the opening correction angle \(\alpha\) is determined (hereinafter, \"correction angle\") (Figure 1b). For this, a line is drawn from point B to point S, the center of the ankle joint, and then a circular arc is drawn with the center in point B of radius BS from point S to the intersection with the previously drawn orange line, resulting in point S'. Angle \(\alpha\), formed between line BS and line BS', is the correction angle. The line joining the center of the femoral head and point S' is the new corrected mechanical axis. To actually obtain the correction, a wedge osteotomy (depicted as \"OBO\" in Figure 1b) is performed, having at its tip the angle \(\alpha\), which displaces the mechanical axis to the desired position. The 3D rotation of the proximal fragment of the tibia around the hinge is presented in Section 2. From the geometric parameters presented in the above, we further focus our attention on the following (depicted also in Figure 1b):  
* The correction angle. * The position of the cutting point relative to the corresponding tibial plateau. The behaviour of the bone at the CORA point can generate fracture lines to the articular surface of the lateral plateau and can also generate fracture lines to the lateral cortex of the proximal tibia [29]. An important issue regarding this point is that it should allow the opening of the osteotomy, but it should not generate instability that would finally lead to a non-union of the osteotomy gap. There are few objective data in the literature that analyze the influence of the correction angle and of the positioning of the cutting point on the mechanical behaviour of the bone around the CORA point. The working hypothesis in our research is that the positions of the cutting point and the correction angle are influencing the mechanical properties of the lateral hinge. The final aim of this study is to determine the optimal positioning of the cutting point relative to the correction angle \(\alpha\) by analyzing the mechanical behaviour of the lateral hinge. The objectives of the research are the following:  
* To conduct a numerical research using the finite element method (hereinafter, \"FEM\") to study the intraoperative behavior of the CORA area for different positions of the cutting point and for different correction angles. The FEM analysis may predict the mechanical stresses developed at the lateral hinge and the risk of fracture.