4 Basic principles of osteotomies around the knee
After detailed examination of the patient, correct planning is the key to a successful osteotomy. A variety of approaches can be used to achieve a good result. The choice of surgical technique might be influenced by personal preference and experience of the surgeon.
Computer-aided surgery, especially computer-guided navigation, is of increasing interest for osteotomy procedures. It may become a common tool in performing osteotomies around the knee in the future  (see chapter 19 “Computer-assisted navigation in proximal tibial osteotomy”). This would of course change the entire planning process. Nevertheless, computer- aided surgery is not standard in all clinics and nowadays, the majority of osteotomies are still planned and performed using conventional techniques.
Understanding normal anatomy of the lower extremity and its physiological angles and axes is essential for planning. The anatomical and mechanical axes and angles are described in detail in chapter 1 “Physiological axes of the lower limb”.
The weight-bearing line (Mikulicz line, mechanical axis) of the leg is the connecting line between the center of the femoral head and the center of the ankle joint (see chapter 1 “Physiological axes of the lower limb”, Fig 1-3 ). The point where it crosses the joint line of the knee is of special interest. The distance from this line to the middle of the knee identifies and quantifies mechanical axis deviation (see chapter 1 “Physiological axes of the lower limb”, Fig 1-6 ). Under physiological conditions the crossing point should be located at the center or slightly medial in the knee joint. A deviation can be defined in different ways:
It can be measured in millimeters from the center of the knee [2–4].
Fujisawa  scales of each compartment separately, from the center of the knee as 0%, to the medial or lateral border as 100% ( Fig 4-1 ). The devation is defined in percentage.
Another possibility is to scale the entire width of the tibial plateau from the medial border (0%) to the lateral border (100 %) (see chapter 9 “High-tibial open-wedge valgization osteotomy with plate fixator”, Fig 9-15 ).
2 Localization of deformity
The nature of the deformity should be understood before planning is done. The malalignment test described in chapter 1 “Physiological axes of the lower limb”, subchapter 5 “Systematic analysis of axial deformities” helps to localize the level of the deformity (see chapter 1 “Physiological axes of the lower limb”, Fig 1-5 ). Axial deformities can exist due to isolated deformity of either femur or tibia, or due to combined deviations of the long bones. It is important to identify patients with complex deformities, as they need more detailed analysis and planning (see chapter 14 “Double osteotomies of the femur and the tibia”) [2–4]. However, the majority of patients who are candidates for a correction osteotomy present with minor deformities localized around the knee joint. Even with a mechanical axis deviation within the physiological range, an unloading osteotomy might be indicated in order to prevent further development of osteoarthritis.
3 Planning step A
Prerequisites for the planning process are:
A good quality weight-bearing x-ray of the entire lower extremity
Definition of the type and localization of the deformity
Knowledge of any associated ligamenteous instability
3.1 Level of the osteotomy
Determination of the level and kind of correction is the first step in planning an osteotomy procedure. Several planning methods with different advantages and disadvantages exist. The choice might be guided by the technique the surgeon is most comfortable with. However, the following principles must be considered in all methods:
The osteotomy should be performed at the apex of the deformity. This will result in an optimal correction. Performing an osteotomy at a different level will not restore the physiological axes but create a new deformity.
The metaphysis of a long bone is the region of best healing capacity. Bone healing is significantly decreased at the diaphyseal bone. The anatomy of the distal femur needs for osteotomy at the metadiaphyseal junction, whereas tibial osteotomy can easily be performed in the metaphysis. Therefore, healing time favors tibial osteotomy. Especially open-wedge osteotomy of the distal femur can result in delayed union or nonunion. The most favorable location of hinge points for different types of osteotomies around the knee are displayed in Table 4-1 .
Open-wedge osteotomies are generally easier and more precise to perform than closed-wedge osteotomies. Furthermore, the opening procedure allows for intraoperative “fine-tuning” by adjusting the opening with a spreader. In most cases bone grafting is unnecessary when angular stable implants are used [7, 8].
A closed-wedge osteotomy at the lateral proximal tibia has been the classic procedure for treatment of varus osteoarthritis. Long-term results are good to excellent [9–13]. Nevertheless, the necessity of an osteotomy of the proximal fibula might damage the peroneal nerve. The incidence of postoperative lesions ranges from 2-16% [14–16].
Sagittal instability can be influenced favorably by alteration of the tibial slope (see below and in chapter 11 “Osteotomy and ligament instabilitiy: tibial slope corrections and combined procedures around the knee joint”).
Restoring or preserving the horizontal joint line (midjoint line) is mandatory for achieving a good result .
3.2 Correction of the sagittal plane
If anterior knee instability (ie, ACL insufficiency) is present, the tibial slope should be decreased, whereas in posterior cruciate ligament (PCL) insufficiency the slope should be increased [18–20]. If the slope exceeds 8-10° in case of chronic anterior knee instability, it is advisable to decrease the slope to 5° in order to minimize the anterior force vector, provided the sagittal correction does not cause hyperextension of the knee joint.
Chronic PCL instability will improve at a tibial slope of about 12° since the increased slope creates an anterior force vector. The effect of tibial slope correction is described in detail in chapter 11 “Osteotomy and ligament instabilitiy: tibial slope corrections and combined procedures around the knee joint”.
3.3 Correction of the transversal plane
Transversal plane or rotation deformities of the lower limb should be corrected at the level where the deformity is present. As patellar tracking may be changed significantly after corrections around the knee, it is important to analyze patellofemoral alignment. Evaluation, planning, and correction are described in detail in chapter 15 “Rotational osteotomies of the femur and the tibia”.
* DFO = distal femur osteotomy
** HTO = high-tibial osteotomy
3.4 Amount of correction
Under physiologic conditions, the mechanical axis passes through the center of the knee or slightly medial of it (see chapter 1 “Physiological axes of the lower limb”). However, in a well-aligned knee, load distribution is not well-balanced but physiologically 60% in the medial and 40% in the lateral compartment . Therefore it is not sufficient to restore a physiological alignment in cases of medial osteoarthritis. Instead, overcorrection by shifting the weight-bearing line slightly to the lateral compartment is recommended. The authors define the corrected axis between 10% and 35% laterally on the Fujisawa-scale  ( Fig 4-2 ), whereby higher corrections are chosen for cases with more severe osteoarthritis. The aim of the mechanical axis is at 10-15% of the scale in the lateral compartment for patients who have lost one third of their medial compartment cartilage, 20-25% if two thirds of medial cartilage is lost, and at 30-35 % if the medial cartilage is completely lost .
In patients with valgus deformity together with lateral compartment osteoarthritis, the corrected mechanical axis can be planned at 0-20% medially on the scale depending on cartilage loss. Overcorrection to the opposite compartment does not seem to be as important as in case of varus osteoarthritis, as the load distribution in the knee is not symmetric ( Fig 4-2 ) .
4 Planning step B (Miniaci method)
Once the localization and kind of osteotomy is defined the preoperative drawing can be done. This can be done either on the weight-bearing x-ray of the leg or at a digital work station. Several methods of planning an osteotomy are described in the literature [5, 22–24]. Based on a study by Fujisawa et al  and the planning method described by Miniaci , the authors have developed a technique to define the correction angle. The planning procedure includes three steps [23, 25–27] and is demonstrated in this chapter for a varus osteoarthritis knee.
The first step is only necessary in cases of lateral knee instability in a valgization osteotomy: To avoid overcorrection, instability or laxity of the lateral collateral ligament must be taken into account. A virtual “push view” image is drawn ( Fig 4-3a-c ). Valgus and varus stress x-rays are required. First, the bony contour of the tibia is traced on a transparent paper. Then the height of the joint spaces of the stressed compartments, as seen in the stress views, is added to the drawing. The tibia is now superimposed on the weight-bearing x-ray of the leg in the corrected position, whereby the distracted medial and lateral joint space are included. The corrected x-ray is the scaffold for further planning. This method helps to avoid overcorrection due to lateral instability.
In a valgization osteotomy, the mechanical axis is planned between 10 and 35% in the lateral compartment on the Fujisawa scale  (see Fig 4-2 ) depending on the severity of medial cartilage loss and between 0 and 20% in the medial compartment for varization osteotomy.
The correction angle of the osteotomy and the height of the osteotomy gap are defined using the planning technique described by Miniaci . The new weight-bearing line is drawn from the center of the femoral head, passing the knee at the point defined in step 2 to the height of the ankle joint line. The hinge of the osteotomy (H) is defined at the lateral cortex of the tibia and connected distally with the new and the old center of the ankle joint. The correction angle α can be measured between the two lines A and B ( Fig 4-3d-f ). The angle of correction at the proximal tibia corresponds to angle α between lines A and B.
Hernigou  developed a trigonometric chart which allows for determination of the opening of the osteotomy gap based on the mediolateral diameter of the osteotomy (D, Fig 4-3e ) and the desired correction angle (see Table 4-1 ). Nevertheless, this should not replace preoperative radiographic planning.
Alternative planning algorithms for high-tibial osteotomy are described in the following chapter (see chapter 5 “Detailed planning algorithm for high-tibial osteotomies”). For planning of a supracondylar femoral varization osteotomy see chapter 13 “Supracondylar varization osteotomy of the femur with plate fixation”.