# Mechanical Assessment of CAD/CAM Fabricated Hybrid Ceramics: An In Vitro Study
## 5 Discussion  
The current study assessed the mechanical properties, specifically the flexural strength test, compressive strength test, and Vickers hardness test of CAD/CAM fabricated hybrid ceramic restoration materials manufactured by subtractive and additive techniques. The hypothesis of this study has been rejected. The results of the present study showed that there were significant differences between subtractively and additively manufactured CAD/CAM fabricated hybrid ceramic restorative materials. In the current study, a three-point bending flexure strength test was carried out for hybrid ceramic materials manufactured by additive and subtractive manufacturing techniques. The additive manufacturing technique showed significant differences among both the milled and the three different degrees (0\({}^{\circ}\), 45\({}^{\circ}\), and 90\({}^{\circ}\)) of 3D-printed groups in the flexural strength test. A similar test was also conducted in previous studies [36; 37]; nevertheless, the groups being compared were selected differently. [PERSON] et al. [37] chose six composite resin materials for subtractive manufacturing and three composite resin \begin{table}
\begin{tabular}{c c c} \hline \hline
**Variables (\(\mathbf{u=15}\))** & **Mean** & **SD** \\ \hline Milled unpolish & 84.7933 & 1.78064 \\ Milled polish & 85.0333 & 1.14434 \\
3D (0\({}^{\circ}\)) unpolish & 3.5267 & 0.18696 \\
3D (0\({}^{\circ}\)) polish & 6.680 & 0.08619 \\
3D (45\({}^{\circ}\)) unpolish & 3.6600 & 0.14541 \\
3D (45\({}^{\circ}\)) polish & 8.4733 & 0.24339 \\
3D (90\({}^{\circ}\)) unpolish & 7.4133 & 0.28502 \\
3D (90\({}^{\circ}\)) polish & 12.4333 & 0.37544 \\ \hline \hline \end{tabular} \(n\); number of specimens, SD; standard deviation, 3D; three-dimension, \({}^{\circ}\); degree.  
\end{table} 
Table 6: Descriptive statistics of comparison of different groups in the Vickers hardness test. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Variables** & **MD** & \multicolumn{2}{c}{**CI**} & \multicolumn{1}{c}{\(\mathbf{p}\)} \\  & & **Lower** & **Upper** & \\ \hline Milled vs. 3D-printed (0\({}^{\circ}\)) & \(-\)55.56 & \(-\)59.75 & \(-\)51.38 & 0.0001 * \\ Milled vs. 3D-printed (45\({}^{\circ}\)) & \(-\)10.14 & \(-\)14.32 & \(-\)5.95 & 0.0001 * \\ Milled vs. 3D-printed (90\({}^{\circ}\)) & \(-\)33.12 & \(-\)37.31 & \(-\)28.93 & 0.0001 * \\
3D-printed (0\({}^{\circ}\)) vs. 3D-printed (45\({}^{\circ}\)) & 45.42 & 41.24 & 49.61 & 0.0001 * \\
3D-printed (0\({}^{\circ}\)) vs. 3D-printed (90\({}^{\circ}\)) & 22.44 & 18.25 & 26.63 & 0.0001 * \\
3D-printed (45\({}^{\circ}\)) vs. 3D-printed (90\({}^{\circ}\)) & \(-\)22.98 & \(-\)27.17 & \(-\)18.79 & 0.0001 * \\ \hline \hline \end{tabular} (MD; mean differences, CI; confidence interval, \(p\); \(p\)-value, 3D; Three-dimension, vs.; versus, \({}^{\circ}\); degree, \({}^{\star}\); statistically significant (\(p<0.05\)).  
\end{table} 
Table 5: Post hoc comparison among the groups for compressive strength. materials for additive machine manufacturing. Furthermore, [PERSON] et al. [36] included two subtractive composite-based blocks and two additive composite-based resins used for two different polymerization methods. The current study included only one type of hybrid ceramic restorative material for additive and subtractive techniques. Nevertheless, even though there were differences in material selection, all studies, including the current study, showed significant differences when compared to the flexure strength among groups. The subtractive technique showed more flexure strength than the additive technique in this outcome. Flexure strength is termed to be capable to withstand the combination of tensile and compressive stresses [24; 38]. To determine the mechanical characteristics of hybrid ceramic restorative materials, flexure strength plays an imperative role [38]. In the current study, the flexure strength value of milled hybrid ceramic restorative material achieved the ISO specifications [34]. A similar outcome was also observed in a study by [PERSON] et al.