## 3 Discussion and Conclusions  
The authors presented a simple technique to match the intraoral scans with the facial scans of the patient in an easy, affordable, and accurate way. This method can be used in daily orthodontic practice, and it is totally safe for patients. The base of this technique is to superimpose the area of the dental arches acquired by a face scanner with the same area obtained by an intraoral scanner. The accuracy of the face scanner is important: good achievement of this technique is correlated to the ability of the face scanner to provide sufficient precision when scanning tooth surfaces in comparison with an intraoral scanner. Old-generation face scanners are used to scan a patient's head, but they need some external system of scan bodies to facilitate the superimposition of the face with the intraoral scan, resulting in a more difficult, complicated, and operator-dependent technique [16; 17; 18; 19; 20]. Another method consists of aligning the intraoral and facial scans using volumetric data from CBCT even if this exposes the patients to X-rays [1; 6]. However, this method could not be performed if a CBCT is not available or necessary for orthodontic diagnosis, according to the concept \"as low as diagnostically acceptable\" (ALADA), especially in orthodontics and pediatric dentistry [12; 13; 21; 22]. The accuracy of a CBCT is strictly related to the voxel dimension, FOV (field of view), and presence of soft tissue: high-resolution CBCT scans (voxel size 0.2 mm) have higher diagnostic accuracy than low-resolution CBCT (voxel size 0.5 mm) but expose the patient to a higher dose of X-ray [23]; a voxel size of 0.3 to 0.4 mm is adequate to provide CBCT images of acceptable diagnostic quality for treatment planning, but the voxel size and FOV must be taken into account to minimize patient radiation dose [24]; the soft tissue presence seems to affect the accuracy of the 3D hard tissue model obtained from a CBCT, below a generally accepted level of clinical significance of 1 mm [25]. Nowadays, the new generation of hybrid LED and infrared light source facial scanners based on structured light technology improves the accuracy and precision of the scan, making human face scanning more comfortable [9; 26; 27]. The Einscan H scanner has an accuracy of up to 0.05 mm, and future technological upgrades may still increase the precision and facilitate this technique. Figure 14: 3D deviation analysis between face scan with lip-and-cheek retractor and face scan with smile. RGB-colored scale bar (millimeters) is reported on the right. Histogram chart is reported at the bottom. However, to improve the acquired face scan quality, the authors suggest using a lip-and-cheek retractor in order to scan the dental surface as better as possible. It is also mandatory to acquire the tooth surfaces with a facial scanner at least from the upper right first premolar to upper left first premolar to perform a three-dimensional alignment between the facial and intraoral scans, and not only from the frontal perspective. This can resolve problems of misfit and error in the alignment of the occlusal plane, rotation, and palatal inclination of the teeth, which are very common in orthodontic cases. Despite this facial scanner presenting a depth of field of 1500 mm, enough to scan the tooth surfaces from incisors to premolars and molars, in some patients with small opening of the mouth, the superimposition of the lateral cephalogram can be useful to align the intraoral scan to facial scan according to the occlusal plane [28]. This integration provides also information about the virtual faceblow, position of the condyles, and cephalometric landmarks [10] (Figure 11). The accuracy of this technique was evaluated using 3D reverse modeling software Geomagic Control X. Deviations between scans were calculated and shown with a color-coded map. The tolerance range in green of the 3D deviation analysis was set to 0.1 mm for each superimposition, and the histogram chart showed the mean value of deviations.