## 3 Results and Discussion
The purpose of this study was to compare several aspects of Y-TZP ceramics prepared via conventional SM and new SLA techniques. The combined results showed that the two materials had relatively similar microstructures (Figures 1-4) and thus statistically equivalent flexural strength (Table 2) and fracture toughness (Table 3). Therefore, the null hypothesis that there would be no difference in the microstructure, flexural strength, and fracture toughness between the SM and SLA Y-TZP ceramics was partially rejected. However, one should be cautious when generalizing the results directly to clinical situations. First of all, only one 3D-printing technique (SLA) was tested in this study. Further, only one commercial ceramic product per manufacturing technique was used, and their compositions were not completely identical. In addition, only 15 specimens for each ceramic, which is the minimum specified by the standard [20], were prepared and tested for flexural strength measurements and Weibull analysis. For these analyses, a larger number of samples (at least 30) is preferred [15]. Finally, a marginal fit evaluation of the crowns prepared by the two digital techniques was not included in the present study. In addition, the building orientation can influence the flexural strength data of dental ceramics. When zirconia was fabricated with the DLP printing technique, vertically built (0\({}^{\circ}\)) samples achieved higher strength values than samples fabricated with oblique (45\({}^{\circ}\)) or horizontal (90\({}^{\circ}\)) build angles on the platform [5]. The lower flexural strengths of 45-degree and 90-degree samples may have been attributable to more structural defects generated during the printing process. Therefore, for the appropriate calculation and comparison of the flexural strength of printed ceramics, considering the printing direction seems to be necessary. AM technologies are a powerful alternative to conventional SM procedures, with several definite advantages, including the ability to produce complex geometries, the  
Figure 7: SEM images of the SM (**a**) and SLA (**b**) Y-TZP ceramic surfaces showing Vickers indentations and cracks. The right is a magnification of the crack marked with a red square in the left image (left: magnification: 300\(\times\), scale bar = 20 \(\upmu\)m; right: magnification: 10,000\(\times\), scale bar = 1 \(\upmu\)m). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Y-TZP** & **Vickers Hardness (GPa) 1** & **Fracture Toughness (MPa-m\({}^{1/2}\))** \\ \hline SM & 11.6 \(\pm\) 0.6 a & 4.9 \(\pm\) 0.3 a \\ SLA & 11.5 \(\pm\) 0.4 a & 4.7 \(\pm\) 0.4 a \\ \hline \hline \end{tabular} 1 Within each column, the same lowercase superscript letters show no significant difference between the two different zirconia ceramics (Student’s \(t\)-test, \(p\) > 0.05).  
\end{table} 
Table 3: Vickers hardness/fracturing toughness of the SM- and SLA-fabricated Y-TZP ceramics. conservation of material, and the ability to form a body from multiple materials [31]. In addition, these approaches can manufacture both fully sintered (solid) and partially sintered (more porous) ceramic structures [31]. To confirm our findings, the dynamic mechanical properties, including the fatigue properties, of the newly developed SLA Y-TZP ceramic require scrutiny. In addition, the printing accuracy, esthetic performance, degeneration behavior, and biocompatibility of 3D-printed zirconia should be comprehensively studied to confirm successful applications of the material in clinical practice. To apply the new technology for the commercialization and utilization of 3D-printed ceramics, their quality, reliability, and clinical performance are still major issues to be solved.