## 3 Results  
Significant differences were found in the number of weeks of home whitening (Figure 5) until color stabilization was reached when \(\Delta\)E values were repeated at two consecutive evaluations (weekly evaluations) between Groups 1S and 2S (Mann-Whitney U test: \(p<0.001\)). The single in-office whitening session group (Group 1S) required a mean number of 11.6 weeks (95% CI 10.6-12.5) while the two-session group (Group 2S) required 8.2 weeks (95% CI 7.8-8.5). When the two groups (1S and 2S) were compared at the end of the in-office whitening phase, no significant differences in \(\Delta\)E were identified (Mann-Whitney U test \(p=0.827\)); nor were differences found at the end of home whitening when color had stabilized (Mann-Whitney U test \(p=0.120\)). When the groups were analyzed independently, it was found that each underwent significant changes in \(\Delta\)E between the end of in-office whitening and the end of home whitening (Table 2). No differences in the three color parameters were found between the two groups at the start of the trial. L*, a* and b* values all changed significantly from the initial evaluation to the end of in-office whitening and the end of home whitening (Table 3). After in-office whitening, no significant differences in L*, a* and b* were found between the groups, but after home whitening, significant differences were identified in a* (\(p=0.033\)) and b* (\(p=0.008\)), but not in L* (\(p=0.154\)) with equal modification of a* in Group 2S compared with Group 1S (1.4 versus 1.7) and slightly higher modification of b* in Group 2S than Group 1S (10.4 versus 9.4). So, it may be stated that all teeth exhibited an increase in luminosity as a result of whitening, a shift in the red-green axis towards green and a reduction in yellow. Figure 5: Mean values and confidence intervals for the number of weeks of home whitening until color stabilization was achieved in relation to number of in-office whitening sessions: 1 = Group 1S; 2 = Group 2S. Analyzing the number of weeks of home whitening by means of the Kaplan-Meier method and comparing survival curves between the two groups (Figure 6), significant differences between the curves were observed (log-rank test (Mantel-Cox) \(p<0.001\); Breslow test \(p<0.001\); Tarone-Ware test \(p<0.001\)). Figures 7 and 8 show photographs of one patient in each group taken before the start of treatment, at the end of the in-office whitening phase and at the end of the home whitening phase (Figures 7 and 8). \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Group**} & \multirow{2}{*}{**\(\Delta\)E after In-Office Whitening**} & \multirow{2}{*}{**\(\Delta\)E after Home Whitening**} \\  & & **Phase Media 95\% CI** & & **Phase Media 95\% CI** \\ \hline
**1S (one-session)** & 3.84 & 14.1 \\
**N = 57 teeth** & 3.20–4.73 & 12.9–15.3 \\ \hline
**2S (two-session)** & 3.79 & 12.8 \\
**N = 58 teeth** & 3.24–4.34 & 11.7–14.0 \\ \hline  & Mann–Whitney U test & Mann–Whitney U test \\  & \(p=0.827\) & \(p=0.120\) \\ \hline \hline \end{tabular}
* Significant difference \(p<0.05\).  
\end{table} 
Table 2: \(\Delta\)E after in-office and home whitening according to the number of in-office sessions (one or two).