## 4 Discussion  
Determining lower limb alignment is essential to detect malalignment of the lower extremities, as it is considered to be a critical factor for developing degeneration of the knee joint [1,2]. As ionizing radiation is needed in standardized methods right now, there is a need for a non-radiographic measurement. Admittedly, it has to be said that several studies have already shown that the dose of radiographic radiation from even \"large-area\" images such as FLRs could be reduced through technical developments and better filters, without any loss of relevant diagnostic information. A standardized whole leg radiograph is estimated to have an effective dose of approximately 10 mSv [23,24]. For comparison, the German Federal Office for Radiation Protection BBS estimates that a German citizen is exposed to an average of an effective dose of 2.1 mSv of natural radiation exposure [25]. Against this background, the dose of such a radiological examination as an FLR seems quite relevant, especially in the field of children and adolescents. One of the aims of the development of the presented photographic method was the establishment of a new diagnostic method in the field of children, not only for study purposes but also in clinical practice to reduce radiation exposure. Secondly, there is a need for quick and easy measurement in a clinical setting that could easily be used as a screening method in a broad, extraclinical setting. To avoid radiation, several research teams have already been introduced to find alternatives to the FLR, and have already tested them for their validity in comparison to the FLR [10, 11, 12, 13, 14, 15, 16]. [PERSON] et al. have shown that digital photographs can be used to determine lower limb alignment properly. They defined a PA (photographic alignment) angle, measured from a pelvis-to-floor photograph, which is the angle between the estimated center of the hip, knee and ankle joints. The estimation of the joint centers follows specific landmarks. Figure 3: Scatterplot depicting the relationship between the sonographic-assisted photographic measurement (SPM) and full-leg radiographs (FLRs) of hip–knee–ankle angles (HKAs) for subjects with BMI \(<\) 30 kg/m\({}^{2}\) (\(n\) = 24). [PERSON] et al. also showed that the PA angle may be used in place of the HKA of the FLR if a bias of 4.5\({}^{\circ}\) is added [15]. [PERSON] et al. described another way of determining lower limb alignment by precising the anatomical and mechanical axis using a digital photograph. They also focused on factors that influence the measured results most, like leg rotation (interclass correlation coefficient (ICC): 0.658), changes in radiograph-object distance or standing position with feet apart or together. [PERSON] et al. concluded that measurement using their method is highly reliable and suitable for routine clinical use, inter alia for cross-sectional analysis of groups wherein legs are under particular stress [14]. What distinguishes the presented method from the studies just mentioned is, among other things, the approach of using sonographic detection of the femoral head and the measurement of the HKA in both SPM and FLRs for better comparison. The present study shows that measurement of lower limb alignment in the frontal plane using a digital photograph can provide reliable and accurate results. SPM adds 1.6\({}^{\circ}\) more valgus, in average, to the FLRs' HKA in the subpopulation with a BMI lower than 30 kg/m\({}^{2}\). The SPM's mean deviation rises significantly if BMI exceeds 30 kg/m\({}^{2}\) (\(p<\) 0.001), with negative effects on precision rates in comparison to the radiographic method. The effect of BMI on the non-radiographic measurement of lower limb alignment has already been stated by other studies. [PERSON] et al. tried to determine frontal plane knee alignment using a three-dimensional reconstruction model based on movement analysis and also compared their results to full-leg radiographs.