## 4 Discussion  
The null hypotheses were rejected as all groups showed significant differences. The present study digitized a set of actual dendiform models with an intraoral scanner to assess the accuracy of its subsequent additively manufactured casts over time and their comparisons with digitized intraoral scan data. It is difficult to find clear reference points for the measurement of the tooth and gingival surfaces. Accordingly, resin markers were designed and attached to gingival surfaces and used as reference points for measurement; these reference points allowed consistent results to be derived through repeated measures.  
In contrast to intraoral scan data and additively manufactured casts, in which 30 measured values were obtained individually for each set of intraoral scan data and each set of additively manufactured casts per each predetermined measurement distance, only a single true value can exist for each predetermined measurement distance. Therefore, deviations between the intraoral scan data and additively manufactured predetermined measurement distance were compared to a reference dendiform model set. Moreover, to eliminate the possibility of misinterpretation due to the presence of positive and negative values, we compared absolute deviation values.  
Several studies have reported the acceptable range of deviations in additively manufactured casts as 20-100 \(\upmu\)m [13, 20, 36, 37, 38, 39]. Accordingly, the deviations of groups PPIA, PP1D, and PP7D can be regarded as within the clinically acceptable range if intraoral scan data are to be used for the fabrication of dental prostheses and further additively manufactured for the verification process, because the difference between groups IOS and PP7D was 90.08 \(\upmu\)m. This result is partially in line with a previous study evaluating the aging of additively manufactured casts over a 4-week period, which concluded that 3D-printed additively manufactured casts should not be used in the fabrication of the definitive prosthesis after 3-4 weeks since their fabrication [40].  
Shrinkage over time is an indication that additively manufactured dental casts lack stability, and thus, it may not be advisable to use them after long-term storage. The present study verified this with a sufficient number of samples and from linear measurement aspects. The accuracy of dental casts can be measured with various approaches, among  
Figure 4: Linear deviation values of all groups. Data expressed as mean \(\pm\) standard deviations. Differences among groups with \(p<0.05\) expressed in different capital letters.  
which 3D deviation measurements and linear measurements are the most popular [41; 42; 43; 44]. While 3D analysis offers overall insights and can be expressed as color-coded maps for easy visualization, linear measurements enable direct evaluation with reference to specific points and provide an intuitive understanding of the amount of deviation by facilitating sub-analyses in different orientations [45; 46; 47; 48; 49].  
The present study has a few limitations. In addition to the in vitro nature of the study design, only a single type of intraoral scanner and 3D printer were used. Further studies employing various intraoral scanners and 3D printers are necessary. The timepoints of measurements were not at equal intervals; therefore, it was impossible to detect the critical timepoints for significant changes. Furthermore, complete arches were scanned, which may be less clinically relevant when fabricating short-span prostheses. A comparison of the outcomes in scans of partial-arches will be crucial to draw case-sensitive conclusions useful in clinical settings.