## 3 Results
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multirow{2}{*}{\begin{tabular}{c} **Ra (Sa)** \\ **Mean** \\ **(nm)** \\ \end{tabular} } & \multirow{2}{*}{**SD**} & \multirow{2}{*}{**N**} & \multirow{2}{*}{**SEM**} & \multicolumn{4}{c}{**vs.**} & \multicolumn{4}{c}{**vs.**} & \multicolumn{4}{c}{**vs.**} & \multicolumn{4}{c}{**vs.**} \\  & & & & & **Healthy** & **Healthy** & **Healthy** & **Predialysis** & **Predialysis** & **Predialysis** & **Predialysis** & **Predialysis** \\ \hline Healthy untreated & 178 & 24 & 4 & 17 & 1.000 & 0.000 * & 0.001 * & 0.019 * & 0.001 * & 0.002 * & & \\ \hline Healthy & 256 & 17 & 4 & 12 & 0.000 * & 1.000 & 0.012 * & 0.000 * & 0.005 * & 0.116 & 0.000 * & 159 \\ \hline Healthy & 37 & 2.3 & 4 & 1.3 & 0.001 * & 0.012 * & 1.000 & 0.000 * & 0.003 * & 0.660 & & \\ \hline \
\begin{tabular}{c} Predialysis \\ untreated \\ \end{tabular} & 55 & 8.6 & 4 & 6.2 & 0.019 * & 0.000 * & 0.000 * & 1.000 & 0.000 * & 0.000 * & & \\ \hline
\begin{tabular}{c} Predialysis \\ \end{tabular} 16\% & 40 & 8.2 & 4 & 5.9 & 0.001 * & 0.005 * & 0.003 * & 0.000 * & 1.000 & 0.068 & 0.000 * & 746 \\ \hline \multicolumn{11}{c}{\({}^{*}\) statistically significant.} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mean values and standard deviation for roughness. Figure 2: Enamel samples from predialysis patients. (**a**) Before bleaching; (**b**) after 16% gel bleaching: (**c**) after 40% gel bleaching. (**a1,b1,c1**) Optical image; (**a2,b2,c2**) AFM bidimensional image; (**a3,b3,c3**) AFM tridimensional image. Scanning parameters: scanned area for—20 \(\times\) 20 \(\upmu\)m\({}^{2}\), 556 \(\times\) 556 pts, scan frequency—1 Hz, FB 1, scan speed—97.85 \(\upmu\)m/s. Low roughness values were identified in the enamel from predialysis patients treated with Opalescence 40%, closely followed by the roughness in the enamel of healthy patients treated with the same gel. In this study, there were no statistically significant differences within the same test group, i.e., teeth from healthy patients and teeth from predialysis patients for Opalescence gel with a concentration of 16%. The ANOVA results indicate higher tooth enamel roughness in healthy patients before and after whitening treatment, but the enamel substrate in either predialysis or healthy patients reveals a significant effect on the interaction of the enamel with the whitening gel (\(p\) = 0.000). The images obtained to evaluate the possible effects of tooth whitening on tooth enamel by SEM, on two groups of teeth, extracted from predialysis patients and from healthy patients, are presented in Figures 3 and 4. The images were recorded before and after the whitening treatment with the two commercial whitening gels at several scanning magnifications. In the presented images, areas characterized by deep channels, generating a much more variable appearance of the enamel surface, suggestive of an increase in enamel porosity, can be observed (Figure 4(c1),(c2)). No difference was observed between the samples from healthy or predialysis patients, treated with whitening gel of 16% concentration. In the dental enamel samples from the predialysis patients after bleaching with 40% gel (Figure 4(c4)), important areas of dissolution were observed that involved either the center or the periphery of the enamel prisms, generating characteristic engraving patterns.