## 1 Introduction  
High tibial osteotomy (hereinafter HTO) is a surgical strategy used successfully to treat osteoarthritis (hereinafter OA) that occurs in the medial compartment of the knee [1; 2; 3]. The efficiency of this procedure is given both by the fact that it eliminates the axial deviations that appear at the level of the lower limb as a result of OA and by the fact that it postpones more radical operations, such as total or unicompartmental knee replacements [4]. HTO is preferred for young patients because it allows a relatively quick recovery, with the possibility of a quick return to daily activities, including sports. As is well known, for the treatment of this condition, which occurs in the medial area of the knee, there are two established types of HTO: opening wedge high tibial osteotomy (hereinafter OWHTO), applied in the medial zone of the knee and closing wedge high tibial osteotomy, hereinafter CWHTO), applied in the lateral zone [2]. Previous research, as well as clinical trials, have shown the superiority of OWHTO as surgery, due to the complications that may occur in the case of the other type of HTO: limb shortening,neurological problems, detachment of the lateral muscle or proximal fibula osteotomy [5,6]. Even if these complications do not occur in the case of OWHTO, for the success of the intervention and a quick recovery, some aspects must be treated carefully such as: a correct geometric planning of the surgery, establishing a possible overcorrection of the axial deviation caused by OA or finding the best fastener system. All this ensures a good stability of the joint and a fast postoperative recovery. Additionally, some shortcomings are avoided such as: lateral hinge microfractures, loss reduction or instability of the knee joint [7, 8, 9]. The need for and importance of the research presented in this article is primarily due to the incidence of OA among adults worldwide [10, 11, 12, 13, 14]. Knee osteoarthritis is an increasing condition generating pain and functional disability in people aged over 55 years. Symptomatic knee osteoarthritis generates significant health and economic costs, with almost 85% of the money spent on replacement surgeries [15]. More than this, projections show that this incidence will increase due to the ageing population [16]. Considering the quality of life impairment and the economic impact of knee replacement, the benefits of conservative treatment and knee-preserving surgery should be further analyzed [17]. The biomechanical aspects characterized by significant loads of the tibiefemoral joint are also important, loads that exceed, at certain moments (descending-climbing stairs, jumping, walking, etc. ), by several times the body weight [10], and the medial compartment of the knee, which is affected by OA, is more loaded than the lateral one, the ratio being 60% medial and 40% lateral in the case of healthy patients [11, 12, 13]. Taking into account the elements presented above, the main purpose of this study is to optimize OWHTO in terms of geometric planning, addressing two main directions. A first direction concerns the intraoperative behavior of the tibia during the realization of the correction angle; therefore, the creation of the osteotomy wedge. For this, the main geometric parameters that characterize the geometric planning of the OWHTO will be taken into account, namely: the correction angle, the position of the initiation point of the osteotomy plane on the medial cortex of the tibia and the position of the \"hinge\" relative to the tibial plateau and to the lateral cortex of the tibia. This \"hinge\" is called the center of the rotation of the angulation (hereinafter, \"CORA\"). The purpose of this approach is to establish the optimum geometrical planning in order to avoid the microfractures on the lateral cortex of the proximal tibia and on the articular surface of the lateral plateau. The second direction is a postoperative evaluation of the flat tibia-osteosynthesis assembly.