## Materials and Methods
### Samples Preparation
With the three different scanners, 20 sample scans were performed: 10 without and 10 with saliva. Before the scans, a retraction thread 00 (Ultrapak, Ultradent, South Jordan, UT, USA) was inserted apically at the finishing margin. The trueness and precision of the models were evaluated valuing the different previous IOSs studied [9; 10]. The scans were all carried out inside a box made of Plexiglas at a temperature of 37\({}^{\circ}\) and a humidity of 90%. The sample was placed during the scan in the middle of the box. In scans with saliva, the artificial saliva used (Biotene mouthwash moisturizing, GlaxoSmithKline, Mississauga, Canada) was injected into the artificial sulcus through a syringe with a fine blunt-tipped needle at a temperature of 37\({}^{\circ}\), until it reached the free margin of the artificial gingival. The intraoral scans were carried out by a single expert dentist. The reference scans and STL files of each IOS system were imported into a reverse-engineering software (Geomagic Studio 2015, Morrisville, NC, USA) and compared. In order to reduce potentially important factors for task effects on driving performance, the scans were carried out sequentially with a timer to break down work into intervals. Starting from the occlusal surface, the left, rear, right, and front surfaces scans were carried out. The \"Mesh doctor\" function was used to remove the independent polygons. The test scans were then cut, with these in occlusal vision (OVA). The finishing line of the reference samples was used as a common cutting model. The cut models were then saved in specific folders. Before starting the overlapping for surfaces, the validity of the method was tested: each reference model introduced into the software was duplicated and moved in space and then superimposed. Such a test was repeated five times to certify the reliability of the procedure. Once the validation tests were completed, the 3D test models were overlapped with those of reference. Such overlapping was obtained firstly by using the \"3-point registration\" function, and then, to optimize the alignment, the \"best fit\" superposition algorithm of the reverse-engineering software was applied. The congruence between the specific corresponding structures was calculated. Finally, a colorimetric map was developed for the immediate 3D visualization of the distances between the models, using the \"3D deviation\" function. The color scale ranged from a maximum deviation of +100 and -100 \(\upmu\)m. For the descriptive study, the fifth file of each IOS system of both samples was selected, both with and without saliva. Chromatic map was elaborated for expected distance computation between the closest point-to surfaces of the meshes composing the models (Figure 4). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**System** & **Company** & **Software** & **Source of Light** & **Acquisition** \\ \hline
**CS 3600** & Carestream Dental & 3 & LED & VIDEO \\ \hline
**TRIOS® 3** & 3 Shape & 1.4.7.5 & LED & VIDEO \\ \hline
**CEREC® Omnicam** & Sirona & 4.5.2 & LED & VIDEO \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Features of the intraoral scanners. Only on all the five files of each IOS system of both samples with and without saliva, 5 sections in the lingual vestibule sense and 5 in the distal mesial sense were made. The matching between the sections and the finishing lines have made it possible to identify 20 points along the perimeter of the reference finish line and the same number on the test line. Measurements of the distances between the 20 predetermined points were carried out and the averages calculated.