## 4 Discussion
Although validity could be confirmed for subjects with a BMI lower than 25 kg/m\({}^{2}\), the measurement error increased for those with a BMI greater than 25 kg/m\({}^{2}\)[26]. These results clearly show that clinical measurement methods are sensitive to methodological errors if the BMI rises above a certain level. Reasons for this can be, on the one hand, the more difficult detection and marking of the femoral head, and on the other hand, the assessment of the rotation of the lower extremity, as determination of the orthograde alignment of the patella is difficult. The question now arises as to what extent the restriction of the BMI of subjects to \(<\)30 kg/m\({}^{2}\) represents a relevant limitation of the population that is eligible for the presented measurement technique. A broad epidemiological study of children and adolescents whose age-specific BMI was measured showed that in childhood and adolescence, the 97 th BMI percentile is 21.1 kg/m\({}^{2}\) for 8-year-olds and 28.8 kg/m\({}^{2}\) for 18-year-olds. This, in turn, means that limiting the BMI of subjects to below 30 kg/m\({}^{2}\) only affects far less than 3% of the total population and should, therefore, not be considered a relevant limitation [27]. Conversely, this means that the new technique can be applied to at least 97% of children and adolescents with accurate results. In the adult population, on the other hand, approximately one in five subjects (20.5%) would not be suitable for measurement due to a BMI \(>\) 30 kg/m\({}^{2}\), as the average BMI increases continuously with age and is significantly higher in adults than in children and adolescents [28; 29; 30]. [PERSON] et al. have already highlighted the effect of tibial rotation on vurus deformity measurement, showing that especially internal rotation and vurus angulation of the tibia are associated with a higher deviation of the measured frontal plane alignment compared to neutral controls [31]. [PERSON] et al. purposely measured lower limb alignment in different positions concerning leg rotation using a digital photography technique and showed that leg rotation has a big influence on the measured results [14]. This highlights the importance of establishing a precise, standardized measuring setup. The correct positioning of the lower limb and orthograde alignment of the patella, regardless of the type of measurement of the frontal leg axis, is especially important [9; 19]. The reliability of digital full-leg radiography--even if it is considered to be the gold standard so far [32; 33]--is currently not fully clear in clinical results. Although [PERSON] et al. show a high inter-rater and intra-rater reliability [34], other studies, e.g., the one conducted by [PERSON] et al., state mediocre intra-rater and quite poor inter-rater reliability [35]. [PERSON] et al. similarly suggest, after examining the inter- and intra-observer variability of radiographic measurements of knee alignment, that this variability must be considered in the interpretation of measurements [36]. Against this background, the inter-rater reliability of SPM (ICC 0.99; CI (95) 0.95-0.99, \(p<\) 0.001) seems to be quite comparable to that of the FLR. [PERSON] et al. estimate that measurement errors of radiographic full-leg radiographs range from 1.1 to 1.6\({}^{\circ}\), which represents a non-negligible factor that must be taken into account when making therapeutic decisions [14; 31; 37; 38; 39]. The HKA's mean value from the photograph measurement was 1.6\({}^{\circ}\) higher (more towards valgus) than the FLR-derived mean value. The degree of deviation does not allow this method to replace a full-leg radiograph for the more accurate diagnostics of frontal plane lower limb malalignment needed when planning a surgical treatment, but it can certainly pose as a reliable diagnostic alternative for known lower limb deformities during follow-up examinations, for screening methods or for further study purposes.