## 3 Results  
An increasing trend was identified in the average values of \(\Delta E\) and \(\Delta L\) within the groups of predialysis teeth between the samples whitened with 40% concentration gel and those whitened with 16% concentration gel. Regarding the groups of teeth from healthy patients, the trend is reversed (Table 1). The statistical analysis showed no significant clinical chromatic modification before and after whitening in the case of the teeth from predialysis patients bleached with 40% gel (\(\Delta E<3\)), while in the teeth from healthy patients and bleached with 40% gel, perceptible color changes were identified (\(\Delta E>3.5\)). When comparing the effects of different concentrations of bleaching gels, significant changes have been observed between the two materials (\(p<0.05\)). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Group** & & \(\Delta E\) & \(p\) & \(\Delta L\) & \(p\) \\ \hline \multirow{4}{*}{Uremic groups} & & _B. 40\%_ & _B. 16\%_ & & _B. 40\%_ & _B. 16\%_ & \\ \cline{2-6}  & Mean & 2.042706 & 4.934480 & & 0.92 & 4.14 & \\ \cline{2-6}  & variance & 0.909187 & 3.678622 & & 4.052 & 7.093 & \\ \hline Healthy & Mean & 11.27015 & 4.105249 & & 1.4 & 3.54 & \\ \cline{2-6}  & variance & 4.279464 & 4.863659 & & 25.245 & 4.108 & \\ \hline \(p\) & & 0.001007 * & 0.410696 & & 0.31959 & 0.37451 & \\ \hline \hline \end{tabular}
* statistically significant.  
\end{table} 
Table 1: Mean values, variance and statistical significance in \(\Delta\)E and \(\Delta\)L values between groups. The highest variation in color change before and after bleaching was observed in teeth from healthy patients bleached with 40% gel, while the lowest difference was identified in teeth from predialysis patients bleached with the same material. AFM investigation allowed the analysis of surface modifications before and after bleaching in every group. The images obtained from healthy teeth before bleaching showed the enamel surface with homogenous aspect. After bleaching with 40% gel, the enamel presented heterogenic surface with microporosity in depth (Figure 1). In the case of enamel samples of teeth from the predialysis patients, the micromorphology AFM images show small enamel rods, with light surface depressions, more superficial, and longitudinal channels between the enamel rods. After 40% gel bleaching, in the enamel of teeth from the predialysis patients a fairly uniform structure was observed, without significant depressions (Figure 2). Figure 1: Enamel samples form healthy 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—20 \(\times\) 20 \(\upmu\)m\({}^{2}\), 556 \(\times\) 556 pts, scan frequency—1 Hz, FB 1, scan speed—97.85 \(\upmu\)m/s. The results recorded after the Ra (Sa) arithmetic mean roughness test, shown in Table 2, indicate values between 28 and 256 nm. The results of the ANOVA test for enamel roughness before and after bleaching reveal that between the average values of the two bleaching materials tested on healthy and predialysis teeth, there are statistically significant differences (\(p<0.05\)). The Scheffe PostHoc test indicates the pairs between which the differences of the mean values are statistically significant.