## 4 Discussion  
The use of digital technologies and 3D printing in restorative adhesive dentistry has numerous advantages. However, there is still controversy if it could be a better alternative to conventional impressions. [PERSON] et al. recently found, with a new in vivo protocol, that the highest precision belonged to the conventional impression extra orally digitized with a laboratory scanner, with an average deviation of 10 \(\upmu\)m [21, 34]. However, in a recent systematic review, [PERSON] et al. [35] concluded that the efficacy of a full digital workflow is unclear due to a dearth of evidence. Note that most of the studies in the literature are held in vitro with ideal conditions, therefore avoiding clinical problems such as blood and saliva control. The present ex vivo model is meant to deal with real conditions that might affect IOS and conventional impression taking, in order to assess their reliability. STL files were compared with a three-dimensional overlapping method that has already been widely used and described by previous studies in literature, showing reliable results [29, 32, 36]. Obtained results showed no significant differences in terms of surface trueness between tested scans and RS, so the first null hypothesis was accepted. This is in agreement with the study of [PERSON] et al. which evaluated the deviations in superimposed impressions, concluding that there was no superiority of any impression technique [37]. In a very recent systematic review on clinical trials, the majority of the selected studies followed similar materials and methods to compare digital and conventional procedures [38], ultimately concluding that conventional impressions are more accurate than digital impressions in vivo. However, it was also reported that significant evidence about digital technologies performances in vivo is not up to date, which might explain the discrepancy with the present study. By contrast, the present study data showed a statistically significant difference between the tested physical casts concerning both marginal and internal adaptation of milled adhesive restorations; thus, the second null hypothesis was rejected. Several studies already evaluated the internal and marginal fit of conventional fixed prosthesis restorations [39]. In the present study, interfacial fit was analyzed through micro-CT, which enables internal, non-destructive analysis of samples compared to traditional methods [40, 41]. Previous micro-CT studies involved linear measurements on two-dimensional images, which limited the number of points measured and hampered generation of a reproducible reference, possibly introducing operator bias. The use of an automatic software thresholding and 3D thickness analysis of a flowable, yet highly radiopaque, cement layer was conducted in order to reduce operator biases. According to the findings of the present study, 3 DC showed both marginal and internal interfacial fit values superior to those of gypsum casts. Indirect restorations were projected and milled starting from the same STL files from which 3 DC were printed, confirming that 3D printing and restoration's CAD project were correctly performed. Moreover, due to the high precision shown, it is assumable that a digital workflow could benefit from the use of 3 DC whenever a physical model is needed. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Type of Cast**} & **Marginal Adaptation** & **Internal Adaptation** \\  & **(\(\upmu\)m)** & **(\(\upmu\)m)** \\ \hline Gypsum & 135.78 \(\pm\) 30.85 & 212.31 \(\pm\) 28.35 \\
3 DC & 57.63 \(\pm\) 22.24 & 144.55 \(\pm\) 24.95 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mean marginal and internal interfacial adaptation data \(\pm\) standard deviation, expressed in \(\upmu\)m for conventional casts and 3 DC. The fit analysis yielded significantly higher internal gap values (144.55-212.31 \(\upmu\)m average, respectively, for 3 DC and gypsum casts) compared to marginal values (57.63-135.78 \(\upmu\)m average, respectively, for 3 DC and gypsum casts).