## 4 Discussion
\begin{table}
\begin{tabular}{l c c c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**X Axis**} & \multicolumn{4}{c}{**Y-Axis**} & \multicolumn{4}{c}{**Z-Axis**} \\ \hline  & Bias & SD & Lower & Upper & Bias & SD & Lower & Upper & Bias & SD & Lower & Upper \\ EN\_L & \(-\)0.02 & 0.14 & \(-\)0.31 & 0.26 & \(-\)0.03 & 0.22 & \(-\)0.47 & 0.41 & \(-\)0.04 & 0.24 & \(-\)0.51 & 0.43 \\ EN\_R & \(-\)0.07 & 0.41 & \(-\)0.88 & 0.75 & 0.02 & 0.15 & \(-\)0.27 & 0.31 & \(-\)0.07 & 0.19 & \(-\)0.44 & 0.30 \\ MEN & \(-\)0.05 & 0.24 & \(-\)0.52 & 0.42 & \(-\)0.02 & 0.14 & \(-\)0.30 & 0.27 & \(-\)0.11 & 0.42 &subjects; hence, the analyses are reliable. In the study of [PERSON] et al., a new reference system for the analysis of infants was introduced [26]. This reference system was found to be clinically applicable for soft tissue analysis in growing individuals, and thus, it could be used to superimpose 3D soft tissue pictures of growing individuals. However, one of the limits of this system was the use of the mid-pupil point, which could easily be affected by pupil position at scanning time. Instead, in our study, by using the Mid-Endocanthion point, which has been shown to be the most stable point during growth in patients assessed by facial scans, it was possible to base the origin of the reference system on a more stable and reliable landmark [30]. Interestingly, although two different methods to construct the sagittal plane in the FS were used, the differences between the two methods were very small and not statistically significant. Both methods started by the same transversal plane (TR_R-TR_L-N) based on two bilateral points (TR_R and TR_L) and on the Nasion (N). When a good scan is performed, and an ear-to-ear image is present, the TR points and the N point present good repeatability, as shown also in this study, and then, the transversal plane is deemed reliable. Furthermore, the Nasion was the most repeatable sagittal landmark in the \(X\)-axis, and this also affected the accuracy and the reliability of the sagittal landmark of the FS with respect to the sagittal landmark of the CBCT scan. Indeed, the subsequent construction of the coronal plane first, or the sagittal plane first, did not affect the reliability of the reference system, as highlighted by the fact that the differences found with respect to CBCT used as the gold standard were not statistically significant. The coronal CBCT plane and the two coronal FS planes differed by almost 30\({}^{\circ}\) in both growing and non-growing subjects at both time points. On the other hand, this discrepancy was anticipated due to the different inclination of the CBCT and FS axial planes. The most important aspect was the stability of the differences in all of the variables assessed that confirmed good reliability of the reference system. The choice of the following reference system was based on the high reproducibility of the identification of N, EN, and TR and the high stability of MEN during growth [30; 31]. Stereophotogrammetry, or in general, three dimensional facial scanning, has been indicated to be as valid as CBCT, with the differences between the two acquisition methods being clinically insignificant for soft tissue analysis [32]; hence, the authors suggested stereophotogrammetry as a non-invasive, accurate, and reliable method for imaging and analyzing facial soft tissue [33]. Furthermore, using 3D imaging for cephalometric analysis could enhance the quality of the diagnosis in the orthodontic field [34]. Several authors have assessed the reliability and reproducibility of 3D landmarks on facial scans [35; 36; 37; 31], and due to the absence of ionizing radiation associated with this acquisition method, this has been considered ideal for growth studies [11; 30; 38].