## 3 Results  
The initial database searches revealed 59 studies; after removal of the duplicates (\(n=31\)), 28 studies remained. After reading the titles and the abstracts, three articles were selected for full-text screening based on the inclusion criteria, and were ultimately included in the present study. No additional articles were identified using bibliographic or manual searches. Among the included studies, two studies evaluated the effectiveness of using 10% carbamide peroxide using Invisalign [35; 36]; in both studies, patients were asked to bleach their teeth for 8 h. However, the duration differed from 10 days in one study [35] to 4 weeks in the other [36]. On the other hand, the third study [37] assessed the effectiveness of tooth bleaching using F22 clear aligners. In this study [37], different concentrations of carbamide and hydrogen peroxides, as well as different durations of bleaching (1 or 2 weeks), were assessed. [PERSON] et al. (2020) evaluated the treatment outcome after bleaching using 10% carbamide peroxide when performed during orthodontic treatment with clear aligners [35]. In this study, 10 patients were asked to apply the bleaching agent in the areas of teeth 32, 31, 41, and 42 for 8 h per day for 10 days. Bleaching trays with reservoirs were used for teeth 41 and 32, while teeth 31 and 42 received bleaching using Invisalign aligners. The CIE L*, a*, b* system was used to evaluate tooth shade and to represent changes in tooth shade (\(\Delta\)E) using the following equation: \(\Delta\)E = \(\sqrt{\left(\Delta\text{a}\right)^{2}+\left(\Delta\text{L}\right)^{2}+\left( \Delta\text{b}\right)^{2}}\). Both bleaching protocols, with and without reservoir (Invisalign), resulted in significant reductions in tooth shade and comparable whitening effects. This was demonstrated by the average values of \(\Delta\)E, which were 4.0 and 3.5 for bleaching trays with and without reservoir, respectively. These results might be attributed to the use of a higher volume of the bleaching agent in the reservoir trays, about 4 mm, compared to 2 mm applied in the non-reservoir trays (Invisalign aligners). These results showed a shade improvement that was greater by 0.5 shades when bleaching was performed using conventional bleaching trays rather than orthodontic Invisalign aligners, however, the difference was not statistically significant. The authors reported hypersensitivity and dysgeusia in some cases even though the bleaching material which was used in the study already contained potassium nitrate, which would decrease the risk of dentine hypersensitivity. However, these side effects did not require treatment interruption and recovered spontaneously. Furthermore, the authors reported that placement of 2 mm\({}^{3}\) of the bleaching material in the centre of the teeth within the aligners resulted in a homogenous distribution of the gel in the tray and preventing harmful damage to the gum, while placement of the same volume near the gum line of the tooth in \begin{table}
\begin{tabular}{l c c} \hline  & **[PERSON] et al., 2019** & **[PERSON] et al., 2020** & **[PERSON] et al., 2021** \\  & [37] & [35] & [36] \\ \hline Random sequence & & & \\ generation (selection bias) & & & \\ \hline Allocation concealment & & & \\ (selection bias) & & & \\ \hline Blinding of participants & & & \\ and personnel & & & \\ (performance bias) & & & \\ \hline Blinding of outcome & & & \\ assessment (detection bias) & & & \\ \hline Incomplete outcome data & & & \\ (attrition bias) & & & \\ \hline Selective reporting & & & \\ (reporting bias) & & & \\ \hline Other bias & & & \\ \hline \end{tabular}
\end{table} 
Table 2: Evaluation of the risk of bias in the included studies. the tray provided the worst scenario and the highest risk for inhomogeneous distribution and injury to the gingival tissues. [PERSON] et al. (2021) assessed the effectiveness of tooth bleaching using the gold standard 10% carbamide peroxide using Invisalign [36].