## 2 Materials and Methods  
Four intraoral scanners: Cerec PrimeScan, Medit i700, Cerec Omnicam and Planmeca PlanScan and one laboratory scanner (Medit T500) were used in this study. We considered IOSs released more than five years ago as being part of the old generation of IOSs. The two old generation scanners selected for this study were Planmeca PlanScan and Cerec Omnicam. The new generation includes recently launched IOSs that are considered by the manufacturer to be the most advanced and developed in technology and software, also being their current flagship product. Medit i700 and Cerec Primescan were selected as the new generation scanners to be tested in this study. A typodont (AG-3; Frasaco, Tettnang, Germany) was selected for the study. Full all ceramic crown preparations with deep chamfer, 1 mm of axial reduction and 1.5 mm of occlusal reduction were manually conducted on the left mandibular second premolar and on the left mandibular second molar of the typodont. An dedentulous area was present in place of the left mandibular first molar, obtained after removing the tooth from the typodont and filling the gap with polyvinyl siloxane (Elite HD+, [PERSON], Badia Polesine, Italy). Ten consecutive scans were performed by a single operator with each intraoral scanner in order to obtain the 3D meshes to be compared with the reference data. The scanning path was identical for all the IOS's starting on the occlusal surface of the distal abutment, coming to the buccal of both abutments and the dedentulous area, then, the occlusal surface of mesial abutment and dedentulous area and finishing with the lingual surfaces. Calibration was performed according to the manufacturer's instructions. All the scans were performed in the same day, around the same time, the light intensity being verified to be constant during all of the scans with the help of a lux-meter (GM1010, Benetech, Shenzhen, China). During the scanning procedure, the typodont was mounted inside the phantom (Phattom head PK-2 TSE; Frasaco) and the position of the assembly was not moved or altered in any way between the scans. Consequently, 40 STL files were obtained: 10 meshes for PrimeScan, 10 meshes for i700, 10 meshes for Omnicam and 10 meshes for PlanScan. The desktop dental scanner Medit T500 was used to obtain the reference scan in standard tessellation language (STL). The calibration of the desktop scanner was performed according to the manufacturer's instructions in order to obtain a highly accurate 3D model (the reference model). The scanned data were processed with the help of Geomagic Control X (Version:16.0.2.16496, 3D Systems, Wilsonville, OR, USA), a complete metrology grade, quality control software used for analyzing and comparing 3D meshes. The reference data were uploaded into the software and the area of interest including the two prepared teeth and the dedentulous area was isolated from the rest of the mesh. The isolating procedure was done manually inside the software, respecting the margins of the preparations and the boundary of the polyvinyl siloxane that simulated the dedentulous area. For consistency, the area of interest remained the same for all the following comparisons, not being necessary to retrace the margins of the area of interest as the reference data remain the same and only the measured data are being swapped. The intraoral scanner mesh was uploaded into the software as measured data to be compared with the reference data. Only the isolated area of interest was analyzed between the meshes, in order to focus on the scanning accuracy of the area for the fixed partial denture, excluding the remaining area of the typodont. The \"initial alignment\" function was executed to superimpose the intraoral scanner mesh over the reference mesh followed by the \"best fit alignment\" that concentrates on the alignment of the isolated area of interest in order to secure a precise overlapping of the two meshes.