## 3 Results  
Figure 1 graphically illustrates the surface roughness (R\({}_{\rm a}\)) values of the study groups before and after the bleaching application. Both bleaching systems exhibited a deleterious effect on the surface roughness. However, the deleterious effect was observed to be significant only in the nanohybrid composite using the Opalescence Regular bleaching system. The highest increase in surface roughness was observed in the nanohybrid composite using Opalescence Regular (0.58 \(\upmu\)m in control and 0.76 \(\upmu\)m in experimental; \(p\) = 0.000), while the lowest increase in surface roughness was observed in the microhybrid composite using  
HiSmileTM (0.66 um in control and 0.68 um in experimental; \(p\) = 0.192). The \(p\)-value of the control and experimental groups in the microhybrid composite using Opalescence Regular and in the nanohybrid composite using HiSmileTM were also observed as insignificant, i.e., 0.071 and 0.092, respectively. Table 2 displays the nanohardness data of the study groups. The highest nanohardness (in GPa) was observed in the control group of the microhybrid composite using Opalescence Regular (0.24 \(\pm\) 0.02 GPa), while the lowest nanohardness was witnessed in the experimental group of the nanohybrid composite using HiSmileTM (0.20 \(\pm\) 0.02 GPa). The paired \(t\)-test outcome suggests a statistical difference between the control and experimental groups of the microhybrid composite using Opalescence Regular (\(p\) = 0.041). However, the bleaching application caused a reduction in nanohardness in other remaining groups. However, the difference was observed as insignificant (\(p\) > 0.05). Table 3 displays the elastic modulus data of the study groups. The highest elastic modulus (in GPa) was observed in the control group of the microhybrid composite using Opalescence Regular (9.67 \(\pm\) 1.46 GPa), while the lowest elastic modulus was witnessed in the experimental group of the microhybrid composite using Opalescence Regular (7.7 \(\pm\) 1.2 GPa). The paired \(t\)-test suggests the statistical difference between the control and experimental groups of the microhybrid composite using Opalescence Regular (\(p\) = 0.023). \begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**Bleaching Gel** & **Composite** & **Treatment** & **Mean** & **Std. Dev.** & \(p\)** \\ \hline \multirow{4}{*}{Opalescence Regular} & Microhybrid & control & 0.24 & 0.02 & \multirow{4}{*}{0.041 \({}^{*}\)} \\  & & experimental & 0.23 & 0.07 & \\ \cline{2-6}  & Nanohybrid & control & 0.21 & 0.02 & \multirow{4}{*}{0.080} \\  & & experimental & 0.21 & 0.02 & \\ \hline \multirow{4}{*}{HismileTM} & Microhybrid & control & 0.23 & 0.03 & \multirow{4}{*}{0.064} \\  & & experimental & 0.22 & 0.02 & \\ \cline{1-1} \cline{2-6}  & Nanohybrid & control & 0.21 & 0.02 & \\ \cline{1-1} \cline{2-6}  & & experimental & 0.20 & 0.02 & \\ \hline \hline \end{tabular} Note: \(p\) < 0.05, \({}^{*}\) = significance between two variables, GPa = gigapascal.  
\end{table} 
Table 2: Mean nanohardness (in GPa) of the two composite materials distributed according to two different treatments. Figure 1: Graphical representation of surface roughness of the control and experimental groups: (**A**) surface roughness using Opalescence Regular bleaching gel on microhybrid and nanohybrid restorative composites. (**B**) depicts the surface roughness using HiSmileTM bleaching gel on microhybrid and nanohybrid restorative composites. Figure 2 shows the load-displacement nanoindentation behavior exhibited by the resin of the nanohybrid composite after the bleaching regime at a loading and unloading rate of 0.5 mN/s at 23 \(\pm\) 1 \({}^{\circ}\)C.  
Table 4 displays the flexural strength of the study groups. The highest flexural strength (in MPa) was observed in the control group of the microhybrid composite using HiSmile(tm) (82.18 \(\pm\) 5.22 MPa), while the lowest flexural strength was witnessed in the experimental group of the nanohybrid composite using Opalescence Regular (71.75 \(\pm\) 4.75 MPa).