## 1 Introduction  
Knee osteoarthritis is a fairly common condition today and is characterized by the progressive wear of the articular cartilage and the degradation of the knee joint [1; 2; 3; 4]. The disease predominantly affects women and usually occurs in people aged over 40 [1; 5; 6; 7; 8]. The main factors involved in knee osteoarthritis aetiology are mechanical and inflammatory. Mechanical factors generate an increase of focal pressure on cartilage, while inflammatory conditions determine a decrease of cartilage mechanical strength [1; 9]. Any of these conditions lead to an overstress and, as a consequence, to the rupture of the cartilage tissue. Mechanical factors may be intraarticular, such as fractures consolidated with chondral surface step-offs, and meniscal lesions, traumatic chondral lesions, or extraarticular, such as axial deviations of the limb in the frontal or the sagittal plane [10]. Frequently, the cartilage damage, usually involving initially one knee compartment (medial or lateral), generates axial deviations of the limb, generating a positive feed-back loop, which leads to an increase in focal pressure on the already damaged articular cartilage [11]. One of the common situations that can occur in the above described scenarios is medial knee osteoarthritis with varus knee. An important biomechanical effect of this condition is the increase of the load in the knee joint. Hence, if on a healthy knee, the maximum forces acting on the knee during gait can reach up to three times the body weight, there are studies [12; 13] showing that a deviation angle of only 10 degrees varus causes a stress on the medial compartment of up to 7.4 times body weight and a fourfold increase of the risk of ingravescence of the medial osteoarthritis. The medial opening wedge high tibial osteotomy (hereinafter, \"medial opening wedge HTO\") is one of the procedures used to treat this condition. This is a surgical procedure used especially in the case of young adults with uncomputamental knee osteoarthritis that restores or even overcorrects the mechanical limb axis, immediately improving the knee function, conferring a long time cartilage protection, and relieving the pain [12; 14; 15; 16; 17; 18]. The main purpose of this research is to optimize the operative technique for treating knee osteoarthritis and for correcting the related axial deviations through medial opening wedge HTO. In order to highlight the geometrical parameters that are studied in our research, Figure 1 shows the geometric planning of the surgical intervention using one of the most frequently used methods in this sense, i.e., the Miniaci method [19; 20]. Figure 1a shows the the mechanical axis of the lower limb in the case of a healthy knee. As can be observed, this axis passes through the center of the femoral head through the middle of the knee and through the center of the ankle joint. Figure 1: Medial open wedge high tibial osteotomy (HTO)—mechanical axis of the lower limb (**a**), main steps (**b**) and Fujisawa point (**c**). As shown in Figure 1b, in the case of a patient with knee osteoarthritis, this axis does not pass through the middle of the knee but intersects the medial compartment, overstressing it. The purpose of the medial opening wedge HTO is that of correcting the axial deviation, i.e., of bringing the mechaning axis back to the correct position. The main steps for correcting the axial deviation using the Miniaci method [19; 20] are the following:  
* Draw a line (orange) that passes through the center of the femoral head and through a point known in the scientific literature as the \"FUJISAWA point\" (Figure 1b) [19; 21; 22; 23]. As can be observed from Figure 1b,c the Fujisawa point is a point positioned at 62.5% on the medial to lateral transverse knee line (hereinafter, \"the FUJISAWA point\"). The precise positioning of this point is still a controversial topic among specialists.