# Geometrical Planning of the Medial Opening Wedge High Tibial Osteotomy--An Experimental Approach
## 1 Introduction
The third step consists in performing the correction of the deviation by creating an osteotomy wedge OBO', with an angle \(\alpha\) at its tip (Figure 1c). Performing this wedge causes the center of the tibo-talar joint to move in the point S' from the orange line, and thus, the geometrical planning of the OWHTO is finalized. As can be observed from the above, the most important step--on which the following steps are dependent--is the first one, when the intersection point of the orange line (Figure 1c) with the knee joint line is established. Strictly from a geometric point of view, as stated above, this mechanical axis is considered aligned when it intersects the medial-lateral articular line at a point located in the middle of the joint--the red line in Figure 1b. In fact, there is a consensus in the surgical practice [18; 19; 20; 21; 22] regarding the requirement to make an overcorrection through which the corrected mechanical axis would intersect the joint not through the middle, but through points located closer to the lateral area of the knee (the yellow and orange lines in Figure 1b). This overcorrection will generate an offload on the worn medial compartment and a transfer of the load to the lateral area, thus stopping the progress of the osteoarthritis [23; 24]. Regarding the optimal positioning of the point on the medial-lateral articular line through which the mechanical axis of the leg will pass after the OWHTO, there is no unitary opinion. There are, however, a few established approaches [18; 19; 20; 21; 22; 23; 24; 25; 26]. One of the widely used over-corrections in surgical practice is the one suggested by [PERSON], [PERSON] et al. [18; 19], indicating a position located at approximately 62-66% of the distance measured from the medial to the lateral area, more specifically 62.5%--the orange line in Figure 1b, a position known as the Fujisawa point. Other research highlights the need for a correlation of the corrected mechanical axis position with the degree of cartilage wear. Thus, if one-third of the cartilage is lost, the mechanical axis should pass at 55-57.5% of the distance measured from the medial; if two-thirds of the cartilage is lost, the axis should pass at 60-62.5% of that distance, while in the case of an almost total cartilage wear and loss, the axis should pass at 65-67.5% [20]. Following a study conducted on 116 patients [21] with medial compartment knee osteoarthritis undergoing treatment with OWHTO, it was concluded that an optimal overcorrection is one where the mechanical axis passes through the tibial plateau through a point located at 71.93%, measured from the medial to the lateral area. Even though there were authors [22] who recommended even higher values--75% (yellow line in Figure 1b)--an excessive overcorrection is not recommended, as it can lead to degeneration and wear in the lateral compartment which will cause other dysfunctions [18; 25; 26]. This happens because it is proven that with each 1 mm transfer of the position of the intersection point of the mechanical axis with the medial-lateral articular line, a load transfer of up to 41 N from the medial compartment to the lateral compartment takes place [11]. This will result in considerable loads on the lateral compartment and implicitly to the appearance of pain, decreased knee mobility, and the need to opt for a prosthetic, which can be inserted with a total or partial knee arthroplasty [25; 26]. The working hypothesis of this study consists in experimentally verifying the fact that using an overcorrection for the position of the leg's mechanical axis when performing OWHTO leads to a better biomechanical stability of the joint. Consequently, the main purpose of this research is the optimization of the geometrical planning of the OWHTO, by taking into account the optimal position of the intersection point between the medial-lateral articular line and the corrected mechanical axis.