## 4 Discussion
The fact that the margins of the prepared tooth were not uniform at all aspects is a limitation of the present study. Future studies are necessary to determine the effect of layer thickness on trueness and margin quality. The reference data set can be recorded with a laboratory scanner, or more precisely with a high-precision optical or tactile industrial scanner [40]. However, because such scanners cannot be used intraorally, the reference data set was taken with an intraoral scanner. The intraoral scanner used has a software that enables immediate scanning of the fabricated restoration and the analysis of the trueness of the restoration comparing with the design STL. Adequate scan accuracy with the intraoral scanner used has been demonstrated in previous studies on single crowns [41]. Generally, intraoral scanners have demonstrated sufficient accuracy especially when scanning partial arches, as executed in the present study [41]. Since the reference data set was used to fabricate all crowns, potential errors during scanning would be expected to affect all groups similarly and therefore, can be considered negligible for its effect on the comparisons aimed. The trueness was analyzed by the calculation of RMS differences between the test and reference datasets. Calculating RMS differences is the most commonly applied trueness analysis in digital dentistry [42]. However, the effect of different software programs to calculate RMS values between corresponding datasets is not clear [43; 44; 45]. The influence of the build angle on the fabrication of 3D-printed crowns has been shown in various studies [14; 18; 25; 26; 29]. The build angle of 45\({}^{\circ}\) was used to print the crowns based on the recommendation by the manufacturer. A 45\({}^{\circ}\) build angle is similar to 135\({}^{\circ}\) build angle in terms of crown orientation during the fabrication, except that the 45\({}^{\circ}\) angle assumes a buccal crown orientation while 135\({}^{\circ}\) angle assumes a lingual crown orientation. The effect of build angle on crown accuracy when DLP technology is used was previously investigated and the highest accuracy was achieved when 135\({}^{\circ}\) angle was used [18]. Due to the pilot nature of the present study, no sample size calculation could be performed. The sample size was based on earlier studies, focusing on the 3D-printing accuracy [14; 46] and enabled the detection of statistically significant differences in terms of trueness and marginal quality. One of the main limitations of the present study is that only one material was used for the fabrication of milled and 3D-printed crowns. Since previous studies have shown that the material has a significant influence on crown accuracy [47], the present study findings are not directly applicable to other materials. Nevertheless, the present study is the first of its kind indicating the influence of layer thickness on the accuracy of 3D printed resin crowns. The quantification of the marginal gap size was not performed in the present study. Since there was no difference between the milled and 3D-printed crowns when a layer thickness of 20 \(\upmu\)m or 50 \(\upmu\)m was used, a major difference in terms of marginal gap size would not be expected between these groups, however, marginal gap measurements can be performed in future studies to see how milling/printing trueness translates to marginal quality. The fact that the margin quality was only assessed by one observer is another weak point. Since the observer was blinded during the evaluation, it can at least be assumed that the risk of bias (e.g., by personal preferences) was minimized [48].