## 4 Discussion
One of the main advantages of stereophotogrammetry is the presence of facial texture on the 3D photo, which helps the clinician in landmark identification, increases the information available to the specialist regarding planned treatment, and makes it easier and more effective to communicate the findings between patients and clinicians [35]. On the other hand, there is still a lack of normative values, as present for 2D cephalometry, for assessing the soft tissues, and this often leaves soft tissue analysis on the side compared to skeletal analysis [39]. Another drawback of this new technology is the mare magnum of software and hardware available; this demotivates some clinicians in approaching new trails preferring the old lateral cephalogram [40]. Even though normative values of 3D facial soft tissue are not available, some proposals have been made [31], but certainly, more studies are needed to improve their reliability. The validity and reproducibility of 3D scans and 3D soft tissue landmarks were widely investigated in several studies. Some of them were more focused on the validity of the method with respect to landmarks obtained from a coordinate measuring machine on facial plaster casts [41], while others were more focused on the intra-operator and inter-operator reproducibility and repeatability of 3D soft tissue points [35; 37; 42]. The validity of 3D soft tissue analysis was investigated by [PERSON] et al. [41] and [PERSON] et al. [43] using a coordinate measuring machine compared with the 3D positions of land marks on the 3D images. The studies found an extremely low system error (\(\sim\)0.2-0.93 mm) and a high accuracy in landmark placing (\(\sim\)0.07-0.79 mm). Regarding the repeatability of 3D landmarks, few differences were found between 3D stereophotogrammetry and laser scans. Similarly, studies performed on 3D stereophotogrammetry and laser scans found, on average, an intra-operator and inter-operator reproducibility of around 1 mm [31; 35; 37; 42]. The most repeatable landmarks were the ones located on the midline plane [31] and landmarks with well-defined borders or edges [37; 42]. Indeed, also in this study, the landmark characterized by the best repeatability was Naison, a midline point, which displayed the best repeatability along the \(X\)-axis, and good repeatability along the \(Z\)-axis. In fact, usually, on face scans, it has to be considered that landmarks on the \(X\)- and \(Z\)-axis are found to have better repeatability than landmarks on the \(Y\)-axis [37]. On the other hand, bilateral points without any bone references (such as zygion or gonion) are more difficult to find and thus were excluded from our analysis [35; 44]. Some bilateral landmarks such as TR_L, TR_R, EN_L, and EN_R displayed particularly good intra-operator repeatability in this study. The good repeatability of EN_L and EN_R was probably related to the possibility to work with color images: thanks to this feature, it was easier to identify the eyes and the inner commisance of the eye fissure due to the contrast between the sclera and the skin. This simple identification would not have been so clear using monochromatic scans. TR_L and TR_R also presented exceptionally good repeatability because they present clear anatomical definition, and this improves the reliability of the reference systems that were introduced in this study. Establishing a new analysis technique for soft tissue changes could be useful for example to assess the effects of Class II and Class III orthopedic treatments or of extractive treatments on soft tissues. As with all orthodontic treatment modalities, the primary goals of growth modification are both to correct the skeletal discrepancy and to achieve optimal facial esthetics. The variation in facial soft tissues during orthodontic treatment has been the subject of numerous studies, mainly based on cephalometry [45; 46; 47; 48].