## 2 Materials and Methods
### Crown Analysis  
All crowns were examined under optical magnification louupe (3.5\(\times\)) to ensure that they were free from any defects and no adjustments were made on the inner surfaces of the crowns [26]. Crowns were then kept in dry and lightproof boxes to scan within 48 h after the fabrication. First, the scanner was calibrated and then the printed and milled crowns were scanned by using an intraoral scanner (Medit 1500 v. 1.2.1, Medit, Seoul, Korea) by the same operator ([PERSON]) and the scan files were converted to STL files (test-scan STL). During the scan, the crowns were hold with small tweezers and as the intraoral scanner utilized has a filter for colors, the parts in contact with the tweezers were scanned afterwards. For the deviation analysis, root mean square (RMS) was used to indicate how far the deviations were from zero between the 2 different datasets [32]. A low RMS value indicated a high degree of 3D matching of the superimposed data, which translated to high trueness [33]. First, test-scan STL files and designed crown STL file (RS-STL) were imported into a software (Medit Link, Medit, Seoul, Korea). Compare tool (Medit Compare v1.1.1.61, Medit, Seoul, Korea) of the software, which has an automatic alignment tool was used for the superimposition, virtually separation of the surfaces, and RMS calculation. For the superimposition, RS-STL was moved to the reference data and test-scan STL was moved to the target data by using the alignment tool of the software. Then, manual alignment tool of the software was selected and 3 reference points; central fossa, mesial and distal triangular fossae were selected both in the reference and target data. Test-scan STL files were then superimposed over the RS-STL file (Figure 3). To generate color maps to represent the 3D deviation, deviation display mode of the software was used. The maximum/minimum critical (nominal) values were set at \(+50/-50\)\(\upmu\)m with a tolerance range of \(+10/-10\)\(\upmu\)m, respectively [34]. After the superimposition, color-difference maps were created to compare the test scan STL file and the RS-STL for the overall RMS, which includes all surfaces of the crowns. The software automatically calculated the RMS from the color-difference maps, without the need for an additional formula. For the RMS of external, intaglio, marginal area, and intaglio occlusal surfaces of the crowns, the test-scan STL files and the RS-STL file were imported again, and these surfaces were virtually separated both in test-scan STL files and RS-STL file [34] dividing crowns into 4 different parts by using the edit mode of the software. After separation, the superimposition was done once again for each surface (external, intaglio, marginal area, and intaglio occlusal surfaces) of each crown by using automatic alignment mode of the software, and the color-difference maps were generated for those surfaces and the RMS  
Figure 3: (**A**): Reference points determined for the superimposition of STL files (1: Central fossa; 2: Distal triangular fossa; 3: Mesial triangular fossa), (**B**): Superimposition of the Test-scan STL over the RS-STL by using these points. values were automatically calculated (Figure 4). The areas with deviations exceeding the scale utilized were presented as gray by the software. However, all areas were included in the RMS calculation. For margin quality comparison, each crown was randomly numbered by an independent individual, and then visually examined with an optical microscope (Zeiss) under \(\times\)60 magnification by a single operator ([PERSON]) who was blinded about the numbering, and a grading system from 1 to 3 was used as performed in a previous study [29]. Grade 1 crown margins indicated rough edges similar to layers. Grade 2 crown margins indicated slightly rough edges similar to waves. Grade 3 crown margins indicated smooth edges (Figure 5). Margin quality was examined at each margin location (buccal, lingual, mesial, and distal) of each crown, and the average was calculated for each printed or milled crown.