## 1 Introduction  
The assessment of soft tissues of the face has a fundamental role in the analysis and description of different patterns of craniofacial growth and growth modification. As such, orthodontists and maxillofacial surgeons have always been interested in facial changes during growth and after treatment [1]. Indeed, the main objectives of orthodontics are the correction of the occlusion as well as the improvement in the smile and facial aesthetics. Similarly, the primary concern of patients is the improvement in facial appearance, which is considered an important factor of well-being and social success [2, 3]. [PERSON] and co-workers have described a paradigm shift from the hard tissues toward the soft tissues; according to this revised point of view, the crucial aspect for successful treatment is the soft tissue position, which needs a reliable method for the evaluation of the effects that various orthodontic treatments and growth have on the face [4]. One method for the assessment of craniofacial changes is craniofacial anthropometry; however, albeit reliable and inexpensive, it is limited to direct measurements on subjects, it is time consuming, and it requires the patient's compliance, which is difficult to achieve in children [5]. On the other hand, the most common technique used to evaluate craniofacial growth and changes after treatment in orthodontics is cephalometry [6]. Although cephalometric analysis is an important diagnostic tool in orthodontics, it has some disadvantages in evaluating facial soft tissues [7]. First, the two-dimensionality creates some limitations in assessing the three-dimensional (3D) morphology and growth changes in the face. Second, the regular exposition to ionizing radiation of patients or healthy subjects for longitudinal studies is ethically questionable [8; 9]. Hence, the use of alternative non-invasive and radiation-free techniques seems to be the best way to understand how the face develops and changes with time and/or after treatment [10; 11; 12]. Furthermore, it has been hypothesized that almost 50% of the variability in the shape of soft tissues might be an effect of changes in the dental and skeletal tissues underneath [13]. Three-dimensional imaging systems have benefitted from tremendous and rapid improvements over the last 20 years and now allow for detailed diagnoses of facial soft tissue, opening new opportunities in orthodontic and maxillofacial treatment planning. Among these three-dimensional techniques, such as cone-beam computed tomography (CBCT), facial surface laser scanning [14], and 3D stereophotogrammetry [15], the use of stereophotogrammetry is largely spreading in research and clinical fields because of its radiation-free characteristic. Three-dimensional facial scanning seems to be a good alternative to record and analyze facial soft tissues because it is totally free of risks, fast to use, and relatively easy to analyze [15; 16]. Stereophotogrammetry involves determining the 3D coordinates of points on an object by taking measurements from two or more photographs combined with a speckle projection that are captured from different positions, possibly taken at the same time. This technique allows the system to work as a stereo pair, generating a 3D composite surface model where the texture is superimposed to the collection of points plotted on an X, Y, and Z coordinate system to create the final 3D image [17; 18; 19]. Yet, standardization of analysis and norms are needed. In the literature, longitudinal changes in soft tissues during growth have been analyzed mainly on facial photographs or on lateral cephalograms [20]. Recently, some studies have evaluated 3D facial soft tissue morphology among different populations [1; 21] and cross-sectional growth changes [22]. Moreover, 3D changes after surgical [19] and nonsurgical treatments [18; 23] were also analyzed.