## 4 Discussion  
The aim of this study was to compare conventional impression and digital impression using intraoral scanners qualitatively; 3D surface analysis was a single measurement. Thus, the presence of random errors has been neglected in the present results. In the present results, the average deviations between each intraoral scan and the model scan were within 0.10 mm. The acceptance level of deviation is different on its use. For instance, a 0.12 mm margin discrepancy has been reported to be the limit for a clinically acceptable crown margin in prosthetics [32]. For orthodontics, [PERSON] et al. [33]. suggested that study models' accuracy should be about 0.30 mm, while [PERSON] and [PERSON] [34] reported \begin{table}
\begin{tabular}{c c c c} \hline \hline  & _iTero_ **vs.**_Model Scan_ & **TRIOS** **vs.**_Model Scan_ & **\(p\) Value** \\ \cline{2-4}  & _Mean_ \(\pm\) _SD_ & _Mean_ \(\pm\) _SD_ & **\(p\) Value** \\ \hline Maxilla & \(0.10\pm 0.03\) & \(0.09\pm 0.03\) & 0.206 \\ Mandible & \(0.09\pm 0.02\) & \(0.09\pm 0.03\) & 0.359 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Surface deviations between each intraoral scan and model scan (unit: mm). Figure 3: Color histograms showing the deviations between the intraoral scan and models after best-fit alignment. (**A**), Surface deviation between the iTero scan and model scan. (**B**), Surface deviation between the TRIOS scan and model scan. that a measurement difference of less than 0.20 mm was clinically acceptable. [PERSON] et al. [35] suggested that a measurement difference within 0.27 mm was clinically insignificant. However, in the case of fabricating orthodontic appliances, the value of 0.1 mm might be a problem of ill fitness of appliance. Fabricating orthodontic appliances using intraoral scans includes the 3D printing process of appliances. Thus, the potential errors might be increased. The clinically acceptable values reported in previous studies are necessary to reconsider in cases of fabricating and printing appliances using intraoral scans. After registration of the iTero scan onto the model, the color histogram showed local deviations in the mandibular posterior regions. This finding was consistent with the result of a previous study by [PERSON] et al. [24]. The authors reported that most data presented some deviations in the posterior areas [24]. [PERSON] et al. [31]. reported local deviations along the palatal surfaces of the molars and incisal edges of the anterior teeth less than 100 \(\upmu\)m. [PERSON] et al. [36]. found that conventional and digital impression methods differed significantly in the complete-arch precision, and digital impression systems had higher local deviations within the complete arch cast. [PERSON] et al. [37]. reported that conventional alginate impression showed the lowest precision compared with intraoral scanning systems (Cerec Omnicam Ortho; Ormco Lythos) with guided scanning procedures for higher precision. As digital impression systems continue to improve and develop rapidly, they may prove to be an equivalent or better alternative to conventional impression techniques. To evaluate accuracy, two factors are usually considered; trueness and precision. Trueness is the deviation of the impression geometry from the original geometry, while precision is the deviation between repeated impressions [37; 38]. In this study, precision between the conventional and intraoral scans was not evaluated; only trueness was evaluated. Regarding trueness, the model scan was considered a reference in this study because the models are still used in orthodontic clinics. Thus, it is necessary to evaluate the difference between alginate impression and the intraoral scanner. To be exact, this study investigated the agreement between the alginate impression and intraoral scans, not the accuracy thereof. In the results, the reason for the deviation between alginate impression and the intraoral scanner could be explained by the scanning process.