## 4 Discussion
Further technical innovations should be developed to eliminate conventional dental laboratory procedures as much as possible to reduce errors in digital models. This study found that none of the methods led to a digital model that was a significantly accurate reproduction of the true dental arch. However, the digital model produced using the extraoral scans showed less deviation than the other digital models produced with an intraoral scanner and CBCT scans. Currently, intraoral optical scanner, the only method to scan a patient's mouth directly, isrecommended for scanning area within a single quarter of the arch to obtain accurate scan data [3,7]. However, in this study the intraoral scanner continuously stitched captured images to scan the full dental arch, and this process might increase the systematic error resulting in the greatest deviation of intraoral scanner model. Meanwhile, the deviation on the Z-axis was greater than that on the X- and Y-axes in all the digital models. This is consistent with a previous report that showed large deviations in crown length measurements [18]. It can be assumed that the Z-axis expansion of the plaster model was greater than the expansion on the X- and Y-axes, probably because the impression material or rigid tray we used for impression was prevented from expanding on the X- and Y-axes. Moreover, due to the limited vertical height of the tooth crown, smaller reference balls were attached to the Z-axis than to the X- and Y-axes. This possible source of experimental error may explain these results. Additionally, due to relatively low CBCT scanning resolution, along with material-induced causes such as impression distortion or plaster expansion [28,29], errors are accumulated throughout the process of digital model acquisition, resulting in the reduced accuracy of cast CBCT scanning data. As reported in previous studies, the digital models mostly showed smaller measurement values than the true measurements [14,30]. Researchers have suggested that this tendency of \"downsizing\" in digital models might be due to the partial volume effect during scanning or algorithms used in the conversion software [30, 31, 32]. Clinicians should be aware of the possibility of downsized models when using digital models generated using object scanning devices. Notably, the current study found that the CBCT-generated data were more accurate than the intraoral data, contrary to a previous study [33]. The CBCT scanning protocol conducted in this study includes software that converts image data (DICOM) into digital model data (STL). The software analyzes the impression material and plaster density in the CBCT image data and used a specific algorithm to convert them into more accurate STL data, in order to minimize conversion errors and artifacts. This innovation in the conversion software might have contributed to the enhanced accuracy of the CBCT-produced models. Impression-scanned models were only produced using CBCT in this study due to inadequacies of the extraoral optical scanner. This unit could not produce a digital surface when scanning the impression material because the light source could not reach the undercut area and therefore did not reflect back to the detector [8]. However, CBCT uses an X-ray beam, which penetrates materials and then reaches the detector, so this scanner functioned well for undercut areas. A further study comparing the accuracy of CBCT and extraoral scanners for dental impressions is needed, as well as the development of a desktop extraoral scanner feasible for impression scans. In conclusion, digital model accuracy should be assessed based on true dental arch measurements consisting of width, length, and height variables, considering the complex steps involved in digitization. Additionally, the use of a CMM and software-based measurements using reference balls is suggested for minimizing measurement errors. The enhanced accuracy of the CBCT-produced digitized models compared to previous studies suggests the feasibility of further development of CBCT-based models.