## 3 Results
### Descriptive Analysis
#### 3.1.2 Facial Skeleton + Soft Tissue  
[PERSON] et al. merged stereophotogrammetry and conventional CT data of human faces by a point-based method [23; 24]: DICOM and Di3D software (Di3D, Dimensional Imaging, Hillington Park, Glasgow, UK) files were converted into a VRML (virtual reality modelling language) common language by Amira (Thermo Fisher Scientific, Berlin, Germany) and superimposed with a two-step procedure: firstly, Procrustes registration was applied in order to normalize information of size, shape and space, then face and skeleton were matched by means of 10 corresponding landmarks distributed on the entire face area of the human head. Finally, iterative closest point algorithm (ICP) was applied. The registration error was calculated to be \(\pm\)1.5 mm using the closest point distance from the transformed Di3D skin surface to the transformed CT skin surface. In 2009 [PERSON] et al. introduced a new approach to acquire a three-dimensional virtual skull model appropriate for orthognathic surgery planning without the use of plaster dental models and without deformation of the facial soft tissue mask named \"triple cone-beam computed tomography scan procedure\" [25]. This technique, validated on 10 orthognathic patients, required the acquisition of three CBCT scans: a CBCT of the patient with a wax bite wafer in place, a second, low dose, CBCT of the patient with a triple Tray AlgiNot impression in place and third high resolution scan of the impression previously done. The three DICOM files were pre-aligned manually and then overlapped by a triple voxel-base registration (N-1). In order to investigate the accuracy of the procedure, a synthetic skull was used; a supplementary CBCT scan in extended field modus was performed on the synthetic skull with the triple tray in position. The superimposition between this and the scan of the impression has been assumed as a gold standard (N-2); the mean distance between the registered impression scan N-1 and registered impression scan N-2 was \(0.08\pm 0.03\) mm. [PERSON] et al. evaluated the feasibility of integrating 3D pictures and CBCT images on 29 orthognathic patients [26]. DICOM files of CBCT were converted into the.stl binary format; 3D images of bone and skin were segmented from CBCT scans. Then, 3D data were superimposed using a surface-based method with 3 dMDpatient software (3 dMD LLC, Atlanta, USA). The root mean square (RMS) between the measured distances was applied to calculate the error of the procedure (0.74 mm for the whole face). Areas surrounding lips and eyes resulted to be the most variable regions in the face during registration, as shown by the color maps. [PERSON] et al. tried to establish how much the CBCT's field of view (FOV) can influence the superimposition accuracy between scans and full-face stereophotogrammetric images [27]. For 30 patients, 3D facial images and CBCT scans were acquired; the 13 cm FOV (small FOV) scans were simulated from 22 cm FOV (large FOV) ones by deleting the excess files. For each patient, the large FOV scan and the corresponding 3 dMD image were imported into VRMesh software and then merged; the same procedure was performed with the small FOV. The study showed the absence of any clinically significant difference in terms of accuracy of superimposition between a large or small FOV.  
[PERSON] et al. introduced a new 3D facial scanner that is combined with a CBCT apparatus to minimize the time lapse between the two registration; consequently, this technique may reduce any bias due to facial mimics or postural changes [28]. An extraoral scan and CBCT were initially superimposed, considering face skin as a reference; hence, the 3D facial image was voxelized, face skin files were extracted from the CBCT by a thresholding process and the two data sets were merged together using Morpheus 3D Software (Morpheus, Gyoung-gi Korea). The registration accuracy of \(0.60\pm 0.12\) mm was established by the average surface distance of soft tissue surfaces of the CBCT and 3D facial images.