Background
Increased femoral antetorsion is one important risk factor for patellofemoral instability and anterior knee pain syndrome in teenagers and young adults [
1‐
4]. Correction osteotomy at the distal femur has been shown to be a reliable option for correction of torsional pathologies [
4‐
8]. Increased lateral facet pressure and increased medial retinaculum strain was shown to be correlated with increased femoral antetorsion [
9‐
11]. In the author’s clinical observation, valgus deformities often appear in conjunction with torsional deformities in cases of patellofemoral instability. Valgus malalignment is defined as a lateral deviation of the mechanical axis (line from the femoral head to the upper ankle joint), which is referred to the center of the knee joint on a frontal plane radiograph [
12]. For more accuracy and confirmability, the amount of valgus deviation is usually measured as the angle between the mechanical axis of the tibia versus the mechanical axis of the femur [
13,
14]. Regarding the femur, mechanical and anatomical axes differ on a frontal plane radiograph, whereas the mechanical anatomical angle (AMA) depends on the femoral torsion [
15]. While changes of the mechanical and anatomical axes are likely to occur in derotational osteotomy, unintended changes on axes should be minimized. According to previous publications, rotational osteotomies may result in unplanned three-dimensional (3D) effects, such as aggravation of valgus malalignment [
1,
16].
Thus, a reliable anatomical reference of the cutting plane is the most important surgical step to prevent these complications. Mathematical models, commonly used in robotics for calculating transformations for serial kinematics, allow for a prediction of the resulting 3D changes when rotation of a limb is performed with a defined angle of the rotation-joint [
17]. When these models are transferred to derotational osteotomies, prediction of the exact changes of axes can be made when there is a known angle of the cutting plane and the tubular bone is rotated on its axis. However, these calculations are not easily applicable in the OR. Therefore, easy to handle tools, such as pre-calculated tables and individual cutting guides, could help surgeons increase the precision of their surgical procedure.
The purpose of this study was to perform a derotational osteotomy at the distal femur, as it is done in cases of patellofemoral instability, and demonstrate the predictability of three-dimensional (3D) changes on axes in a cadaveric model. We hypothesized that a distal femoral derotational osteotomy with a defined inclination of the cutting plane can correct the frontal alignment due to correction of torsion according to the preoperative calculated values.
Discussion
The most important findings of the study were that when a pre-calculated inclined osteotomy is performed, which is referred to the virtual anatomical axis by use of a simple 3D printed cutting guide, a precise postoperative result on frontal and torsional angle can be achieved in distal femur rotational osteotomy. The objective of this study was to develop simplified charts for angle calculations based on precise mathematical models and provide a proof of concept in a cadaveric model to demonstrate the predictability of 3D correction in the clinical setting with the use of simple individual cutting guides.
Derotational osteotomy is the gold standard for the treatment of torsional deformities in the lower extremity, as Dickschas et al. postulate by regards of their clinical cases [
4]. Little is known about correlation of valgus malalignment and patellofemoral instability. According to the literature, combining a distal femoral lateral open-wedge or medial closing-wedge osteotomy to a derotational osteotomy is rarely done because of its complexity [
7]. But, several studies suggest that increased valgus deformity is considered to be an additional risk factor in such cases [
4,
7,
24]. Furthermore, different studies have shown the correlation of valgus alignment and increase of patellofemoral arthritis [
25‐
28]. Yet it is unknown if correction of valgus malalignment may necessarily correct tracking abnormalities at the patellofemoral joint [
28]. The proposed Q-angle by Brattström in 1964 and his annotations grounded the idea of alignment tracking [
29]. He suggested that muscular vectors might be improved due to derotation, although the Q-angle may not be reliable and very accurate in clinics [
30]. We believe that a combined correction of alignment, which is a derotation of an increased antetorsion and correction of valgus malalignment, improves patellar stability and patellar tracking. Therefore, our single cut approach in distal femoral derotational osteotomy can avoid aggravating frontal malalignment and can even be used for an intended change on axes.
Single-cut correction for torsional and angular deformity has been described many years ago by the French D’Aubigne in 1952 [
31]. There is even a European patent from 1996 held by a French inventor Du Toit for surgical instruments to “guide a saw while it makes an oblique cut in a long bone” (EP 0570187). Subsequently the planning of the osteotomy angle with mathematical models and tables was improved in the 80’s for example by Sangeorzan et al. [
32] Despite the mathematical calculations to achieve the correct osteotomy plane, Gürke and Strecker et al. showed a graphical approach to define the plane of the single-cut osteotomy in 1999 [
33]. However, our approach does not address the center of deformity and we aimed to achieve a reproducible technique for cases of patellofemoral instability and indicated derotational osteotomy. Therefore, only one plane is inclined, given by an easy-to-read table, from the lateral view versus the shaft, as this reflects the standard surgical approach.
It is known that any rotational osteotomy will have changes on frontal and sagittal axes as described by Paley et al. in principles of deformity corrections, and as shown by Kim et al. and Lee et al. in their computed mathematical articles [
15,
16,
34]. In order to receive reproducible results in a clinical cadaver model, a new mathematical approach for calculation of the cutting plane was chosen. Our findings show that a correct reference of the cutting plane versus the anatomical shaft is elementary. Using a cutting plane perpendicular to the virtual anatomic axis will lead to a slight increase of AMA and will not aggravate a valgus malalignment. Trigonometrical calculations from the robotics show that inclination of the cutting plane from the sagittal view will have the most change on axis on the coronal view, and only slight change of axis on the sagittal view. Based on the presented table (Table
3), change on varus (cutting plane: antero-proximal to postero-distal) or valgus (cutting plane: antero-distal to postero-proximal) alignment can be performed.
This presented “robotic formula” and the provided table (Table
3) with common values for clinical use show that an inclination of the cutting plane between 8.8° and 11.8° (3° difference) will have a change in the coronal change of axis from 3.5° to 4° (0.5° difference), when rotation by 20° is performed. In terms of clinical accuracy this may be acceptable; however, this can be improved by more exact surgical guides as it is shown in this study and has been described for distal femur open wedge osteotomy by Victor et al. in 2013 [
35].
The current study may help to explain why reference of the osteotomy plane to the shaft is one of the most important step in such procedures in order to avoid unintended changes on axes. Bowing of the femur can lead to increased or decreased mLDFA, dependent whether osteotomy is proximal or distal as Nelitz et al. showed in a computed model [
1]. In that study, the cutting plane was always set to be perpendicular to the actual anatomic shaft, which differs distally versus proximally. We believe that our approach is able to simplify derotational osteotomy and to avoid postoperative malalignment. Angulation of the cutting plane to the virtual anatomical axis is key to an exact reproducible result in our study.
It is likely that other factors lead to a certain margin of error when measuring angles. To improve accuracy in calculations, an exact frontal plane radiograph of the knee joint is important. As coronal CT reconstructions may help to improve accuracy regarding correct plane of views, we normally perform MRI images for torsion measurement to reduce radiation to young patients. Methods for torsion measurement are well investigated and show reliable results in terms of intraobserver and interobserver agreement as Kaiser et al. describe [
36]. But different threshold values depending on the measurement technique should be considered in clinical use. We used the method from Waidelich et al. for exact pre- and postoperative measurements because of the iatrogenic prepared proximal femur bones [
19]. These had passed through different femoral neck removals due to the THA workshop, but fully preserved femoral heads and trochanters. Therefore, this method was suitable for our model, and with regards to the virtual anatomical axis, this method supports our model even more.
The performed derotation of 20°, which is visually controlled by two k-wires, shows acceptable results (mean 19.69°). Regarding the mathematic formula, if cutting plane is 10° oblique, derotation between 19° and 21° results in a difference of change of coronal axis by only 0.4°. In order to gain perfection for rotational control, an electromagnetic tracking device processing in real-time as Geisbuesch et al. describe, could be added [
37]. The height of the osteotomy (7 cm from the distal joint line) which was chosen due to plate design should not affect the overall outcome, if planning of the osteotomy with its corrective angle is done at this same height. If the osteotomy is supposed to be more proximal, the corrective angle will increase and the inclination of the cut will have to be increased, as well. The mathematical model does not have an angular limitation. However, as shown in Table
3 (the robotic table), a derotation of 20° and an accompanied frontal alignment correction of 10° equals an oblique cutting angle of 30°, which has to be considered in practice.
The principal limitations of this work lie in the perfect anatomical overview from the frontal and lateral view in order to navigate the perfect osteotomy cut. The removed soft tissue helped to perfectly rotate and handle the osteotomy. In vivo, this can be very challenging regarding medial-lateral translation or missed hinge of the osteotomy. The biomechanical nature of the study contains iatrogenic prepared specimen, which showed common pathologic angle values (increased torsion and slight decreased mLDFA). We produced antetorsion artificially in the femoral neck after THA by k-wire fixation. However, the problem of antetorsion of the femur is not only located at the femoral neck. It can occur at the proximal, diaphyseal or distal side of the femur, which was shown by Seitlinger et al. [
18] Moreover, this simplified model does not involve any form of dysplasia of the condyles, trochlea, and shaft, which may be observed in patellofemoral instability cases. Another limitation is that surgery was performed by only one surgeon, in order to receive consistent data and proof the concept. But when the calculations and surgical approach are taken into clinics and performed by several surgeons, accuracy may be decreased. 3D-printed cutting guides are used in several publications, and even in osteotomy surgery [
35,
38,
39]. But, its practicality and understanding are not widespread. Therefore, stainless steel cutting guide assemblies with intraoperative adjustments of the inclination may help surgeons as well. For a consecutive study, we suggest to perform the analysis and surgery on total leg cadavers with soft tissue for proof of its feasibility in a clinical setup.