## 4 Discussion  
The present study analyzed the accuracy of two types of abutments (premolar and molar), using three types of intraoral scanners (CS3600, i500, and EZIS PO) and one type of laboratory scanner (3 shape E1), and analyzed the impacts of the volume and area of the abutments on the scanning accuracy. Between the four types of scanners, there were significant differences in the accuracy between premolars and molars, and the accuracy was excellent for premolars (\(p\) < 0.001; Table 1). In addition, there were significant positive correlations between the volume and area of the abutments and the accuracy of the four types of scanner (\(p\) < 0.001; Table 3). Therefore, all the null hypotheses of the present study were dismissed (\(p\) < 0.001). [PERSON] et al. evaluated the scanning accuracy of an abutment for a single crown using two types of intraoral scanner (CEREC Bluecam and CEREC Omnicam) and reported a scanning accuracy similar to the results of the present study (17.5 \(\pm\) 1.8 \(\upmu\)m) (Table 1) [32]. Several studies that evaluated the scanning accuracy of an abutment for a single crown showed different positions and shapes of the abutment  
Figure 5: Comparison of RMS values for premolars and molars according to scanner. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & **E1** & **CS3600** & **i500** & **EZIS PO** \\ \hline \multirow{2}{*}{**Volume**} & PCC & 0.300 & 0.625 & 0.717 & 0.743 \\ \cline{2-6}  & \(p\) & \textless{}0.001 * & \textless{}0.001 * & \textless{}0.001 * & \textless{}0.001 * \\ \hline \multirow{2}{*}{**Area**} & PCC & 0.349 & 0.631 & 0.725 & 0.742 \\ \cline{2-6}  & \(p\) & \textless{}0.001 * & \textless{}0.001 * & \textless{}0.001 * & \textless{}0.001 * \\ \hline \hline \end{tabular}  
* Significance determined by Pearson correlation analysis, \(p\) < 0.00625, with the Bonferroni correction method. PCC, Pearson correlation coefficient.  
\end{table} 
Table 3: Correlation coefficient values for volume, area, and accuracy. and did not present the volume and area. In addition, no previous research has been conducted on the impacts of the volume and area of abutments on the scanning accuracy. The results of the present study suggest that the volume and area of the abutments had significant positive correlations with the scanning accuracy (\(p<0.001\); Table 3). In an actual clinical environment, the preparation of various abutments is performed to produce a single crown [33], and on the basis of the results of the present study, it can be inferred that the greater the volume and area of an abutment, the poorer the scanning accuracy. There were significant differences between the accuracies of the three types of intraoral scanner and one type of laboratory scanner used in the present study. The accuracy was highest with the laboratory scanner, and in the intraoral scanner group, the accuracy was highest with the CS3600. Previous studies have noted that if the scanning accuracy of virtual casts exceeds 100 \(\upmu\)m, the fixed prostheses might fit incorrectly, and an acceptable range of scanning accuracy of less than 100 \(\upmu\)m has been proposed, based on the acceptable cement space of the fixed prostheses [24; 25; 26]. Thus, there were significant differences between the scanners used in the present study; however, all four types of scanner lay in the acceptable range of a scanning accuracy of less than 100 \(\upmu\)m.  
[PERSON] et al. compared the scanning accuracy of nine types of intraoral scanners and reported significant differences depending on the scanners' image acquisition method [34]. The intraoral scanners used in the present study can be divided into a scanner using the individual image acquisition method (EZIS PO) and scanners using a video sequence acquisition method (CS3600 and i500). Previous studies reported that the individual image acquisition method took more scan time compared to the video sequence acquisition method and had a tendency toward poor scanning accuracy [35; 36].