# Geometrical Planning of the Medial Opening Wedge High Tibial Osteotomy--An Experimental Approach
## 1 Introduction  
Osteoarthritis (hereinafter OA) is a fairly common condition that occurs among adults worldwide [1; 2; 3; 4; 5]. Most often, this disease affects the medial area of the knee by progressively wearing off the articular cartilage in this compartment [6; 7] and the consequences are the appearance of pain and loss of joint function. This type of wear causes a varus-type axial deviation [8; 9; 10] which increases the load in the same medial area [11] of the knee which is already damaged. The existing mechanical loads at the tibiofemoral joint are quite high, exceeding the body weight several times [1]. When the body weight is supported by one leg while walking, 55-75% of these loads are applied to the medial compartment of the knee [2; 3; 4]. Due to these loads, the wear of the intra-articular cartilage and the increase in the varus-type axial deviation progress quite fast, being later amplified by the axial deviations that are likely to appear [12]. From a geometric point of view, as can be seen in Figure 1b, osteoarthritis involves decreasing the intra-articular space in the medial area and implicitly increasing it in the lateral area, moving the point of intersection between the medial-lateral articular line and the mechanical axis of the leg towards the medial area. In the case of a healthy knee, the mechanical axis of the leg (the blue line shown in Figure 1a), which joins the center of thefemoral head with the center of the tibiotalar (ankle) joint, passes right through the center of the knee [13]. The wear of the cartilage in the medial area causes the mechanical axis to move towards this area (blue line in Figure 1b), a displacement which is proportional to the degree of wear of the cartilage. Figure 1: Intersection between the medial-lateral articular line and the mechanical axis of the leg for a healthy knee (**a**); an ostheoartritic knee (**b**); and geometrical planning for the correction using the Miniacci method (**c**). As described above, the knee loading will be defective, with a high probability that the cartilage wear will progress and the disease will worsen. One of the most effective surgical strategies meant to stop this process and restore a proper knee loading is the open wedge high tibial osteotomy (hereinafter OWHTO) [13; 14]. This intervention is especially suitable for young and active patients who desire a speedy recovery in order to return to physical activities conducted prior to the disease and who seek the postponement of more complex surgeries, such as total or unicompartmental knee replacements [15]. The purpose of the OWHTO surgery consists in realigning the mechanical axis of the leg so that it no longer crosses the worn medial compartment [16; 17], implicitly obtaining a correct loading of the joint. Figure 1c presents the steps that are to be taken in order to correct this axial deviation, using one of the most frequently used methods for geometrically planning the OWHTO, the Minniaci method [17]. The first step consists in drawing a line (orange) that passes through the center of the femoral head and through the point situated on the knee joint line, through which the mechanical axis of the leg should pass after the correction. It is to be noted that the orange line does not intersect the center of the tibo-talar joint (the S point on the blue line) at the tibia level. The following steps have, as a final objective, the transfer of the center of the ankle in the point S' situated on the orange line (Figure 1c). The second step consists in establishing two points: the hinge point B called CORA (center of rotation of the angulation) and the O point, situated on the medial cortical of the tibia, and called the initiation point of the cut of the osteotomy plane. The position of these points results from the recommendations that have been made on the matter in literature [18]. The osteotomy plane follows the BO line in the transversal plane and is perpendicular to the limb frontal plane (Figure 1c).