## 4 Discussion  
Significant differences in terms of trueness were found comparing the 100 \(\upmu\)m printing layer thickness to other groups. Therefore, the first null hypothesis was rejected. The best margin quality was found in milled crowns, followed by the 50 \(\upmu\)m layer thickness group. Accordingly, the second null hypothesis was rejected. In terms of trueness, the present study showed that 3D-printed interim crowns, printed with a layer thickness of 20 or 50 \(\upmu\)m, are similar to the milled interim crowns, which are currently used routinely. Similar results have been reported also for 3D-printed ceramic crowns [15; 35]. The effect of the layer thickness on the trueness of 3D printed casts and complete dentures has been demonstrated in previous studies [30; 36]. Those studies demonstrated the highest trueness when a layer thickness of 100 \(\upmu\)m was used, whereas the 100 \(\upmu\)m layer thickness resulted in the lowest trueness in the present study. The main difference between those and the present study may be due to the nature of the objects printed. Neither casts nor dentures have as thin and tapered margins as crowns have, instead, they have large plain and round surfaces [36]. When the layer thickness is too small, discrepancies can occur, especially with such thin margins [30]. The combination of thin layer thickness and thin margins could also be an explanation for the fact that the trueness seems to be slightly higher in the 50 \(\upmu\)m group compared to the 20 \(\upmu\)m group, although this difference was not statistically significant. It should be noted that the aforementioned scoping review on complete denture fabrication included various studies [36], which applied varying settings for the 3D-printing, and therefore, the deviations in trueness may not be attributed to the layer thickness alone. A study, which showed that layer thickness did not affect the trueness of printed models when 100 \(\upmu\)m was applied, still recommended the use of 100 \(\upmu\)m layer thicknesses due to the economic advantages as the printing time is shortened [37]. However, the time factor may be less important when small restorations such as crowns are fabricated. Considering the effect of the layer thickness on trueness at thin surfaces such as crown margins [30], high layer thickness may be expected to result in low trueness, which was confirmed by lower trueness and marginal quality found with  
Figure 8: Margin quality of groups according to margin location, evaluated by using a 3-point scale that ranged from 1 (worst marginal quality) to 3 (best marginal quality). the 100 \(\upmu\)m group in the present study. Although a previous study has shown that the marginal fit of 3D-printed crowns is within a clinically acceptable range, and that there is no difference between 3D-printed and milled crowns [38], significant differences in the margin quality were found in the present study. The marginal quality in the present study was evaluated by a 3-point scale instead of absolute values, indicating worse margin quality of the 3D-printed crowns irrespective of the layer thickness. Interestingly, the margin quality was not influenced by margin location in 20 \(\upmu\)m and 50 \(\upmu\)m groups, but in milled and 100 \(\upmu\)m groups. The significantly worse margin quality at the buccal aspect of the milled group may be due to the slightly narrower preparation margin on the distobuccal side. During the milling process, defects may occur at thin crown margins [39]. Hypothetically, the preparation margin could also be the reason for the better marginal quality at the lingual crown margin in the 100 \(\upmu\)m group. The lingual aspect of the margin preparation was the most uniform, which might have allowed the crown to be fabricated accurately even with the largest layer thickness of 100 \(\upmu\)m. A large layer thickness might have a negative effect on the margin quality, especially with varying crown margin thicknesses.