## 2 Materials and Methods  
The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Ethics Committee of the University of Regensburg, Germany (date of approval: 20 October 2021; protocol code 21-2612-101). Prior to recruitment, the study was registered in the German Clinical Trials Registry DRKS (clinical trial registration number DRKS00027937). Participants were included if they were able to stand unassisted in an upright position with straightened leg(s), which is required for proper measurement, if they were able to understand and follow the examiner's instructions and if an acute bony, ligamentous, meniscal or cartilage injury could be excluded by clinical and radiographic anamnesis. Each participant has given written informed consent to participate in the study. Subjects were divided into two groups according to their BMI (group I: <30 kg/m\({}^{2}\); group II: \(\geq\)30 kg/m\({}^{2}\)). The maximum allowed time span between SPM and FLR was 4 months. In addition, those with an acute injury and a surgical intervention on the leg were excluded. A further exclusion criterion was an implanted hip joint replacement, as this makes sonographic detection of the femoral head impossible. In 13 of the total of 18 participants, both leg axis measurement methods were performed on the same day. The SPM method was performed under precisely standardized conditions. The participant, undressed but for his or her underwear, was positioned on level ground, standing at a distance of 3 m from the camera. The camera (Apple(r) iPad Air 3 rd generation, 8-megapixel front camera, Apple Inc, Cupertino, CA, USA) stood on a tripod (Tripod Stativ 106 3D by Hama(r) GmbH & Co. KG, Monheim, Germany), which was positioned at the participant's knee height in order to minimize the parallactic error. The participant was then asked to remain in a relaxed position with their weight equally distributed on both feet, which were positioned hip-width standing. The legs were then aligned to achieve the exact orthograde positioning of the patellae by the examiner, analogous to the FLR [19]. Next, three different joint points were defined to determine the mechanical femorotibial axis: (1) Hip joint: first, the femoral head was sonographically detected in the anterior-posterior direction with a portable ultrasound device (Butterfly iQ(r), Butterfly Network, Inc., Burlington, MA, USA) in a standing position (see Figure 1). The skin was marked with an adhesive electrode at the point that showed the highest point of convexity of the femoral head on ultrasound. (2) Knee joint: the next step was palpatory detection of the knee joint gap both medially and laterally and the application of an elastic band at this level. If palpation was difficult due to the soft tissue mantle, detection was also assisted sonographically. A line was then drawn on the digital image from the medial to the lateral intersection of the rubber band with the background where the rubber band rests in the knee joint gap, and the midpoint of this line was determined as the knee joint center. (3) Ankle joint: the center of the line connecting the bony prominences of the medial and lateral malleoli was determined, which quite accurately intersects the center of the talus (see Figure 2). One weight-bearing, full-leg anterior-posterior digital radiograph was taken in the identic standardized position, analogously to the positioning described above for the SPM, and recommended by [PERSON] et al. [19]. The HKA of the photograph and the corresponding full-leg radiograph (transferred anonymized as.jpeg file) were measured with an angle measuring application (Angle Meter 360, \(\copyright\)AK App Develop). The FLR's HKA was defined by the angle of the straight lines between the center of the femoral head/center of the femoral intercondylar notch and the center of the intercondylar notch/center of the talus.