## 4 Discussion  
One-day prosthesis treatment is a comprehensive dental treatment that is carried out in the dental clinic in the following sequence: (1) scanning of the dental shape using digital intraoral scanners without taking an impression; (2) designing prostheses using CAD software; and (3) fabricating prostheses using CAM. Same-day prosthesis treatment has more advantages than traditional prosthesis treatment, and it is, therefore, becoming more widespread. Many studies have compared various scanners and investigated the marginal and internal fit of prostheses fabricated using CAD/CAM systems [17, 18, 19, 20, 21, 22, 23, 24]. However, few have compared different types of chairside CAD/CAM systems, and almost none have compared all-in-one type systems with combined-type systems. The all-in-one type system means that the device used in all processes from scanning to CAD/CAM is configured by one manufacturer. On the contrary, the combined-type system means that the device used in all processes by a combination of different manufacturers. In the present study, we used two all-in-one type systems (CEREC and EZIS) and one combined-type system (Carestream). With the all-in-one systems, the entire process from scanning to fabrication was performed within one construction based on a stable system wherein each component was optimized. However, the combined-type system required one system to be set by combining each component. This could be inconvenient because optimizing processes may need to be run in each step due to the system lacking a stable linkage between components. The following factors may influence the marginal and internal fit of ceramic crowns fabricated using CAD/CAM systems: the kind of prostheses [17, 18], materials for fabrication [5, 19], abutment tooth design [17], accuracy of the scanner [20, 21, 22, 23], design software [24], assigned input variables [20], and accuracy of the processing equipment [8, 19, 24]. In this experiment, the accuracy of the scanners, the function of the design software, the assigned input variables, and the accuracy of the processing equipment may have influenced the marginal and internal fit of the ceramic crowns. The Omnicam of the CEREC system is the first intraoral scanner that does not require use of titanium dioxide powder; it obtains images with confocal laser imaging-based rendering instead [25]. Likewise, the CS3500 of the Carestream system does not require the use of powder, whereas the PO of the EZIS system does, obtaining the image through triangulation technology-based stitching [25]. One study found that the trueness of the Omnicam was 13.8 \(\pm\) 1.4 \(\upmu\)m [26], whereas another showed that the trueness of the Omnicam and CS3500 were 31.8 \(\pm\) 5.4 \(\upmu\)m and 26.7 \(\pm\) 3.5 \(\upmu\)m, respectively [27]. In addition, the trueness of the EZIS system's PO is approximately 20 \(\upmu\)m, according to the manufacturer. Recently, intraoral scanners with higher performance and quality have been developed and have been widely distributed, contributing to diversity in the types of chairside systems available and greater accuracy in the fabrication of prostheses. [PERSON] et al. presented a marginal gap of \(<\)120 \(\upmu\)m in a crown fabricated using a CAD/CAM system; they reported that this difference may be due to the conversion of the digital model into a prosthesis [28]. [PERSON] et al. stated that a ceramic crown (IPS e.max CAD; Ivoclar Vivadent AG, Schaan, Liechtenstein) showed a marginal gap of 74-76 \(\upmu\)m, which is within the clinically acceptable range [29]. In this study, the cement space appropriate to each system was determined before the main experiment was conducted. In many previous studies involving prostheses fabricated using CAD/CAM systems, marginal and internal fit was evaluated with no detailed descriptions of the input variables.