## 4 Discussion
In fact, in this study, the authors showed that there was no effect of pressure applied to the tray on the gel distribution. Instead, the gel distribution was affected by the chemical and physical properties of the gel, gel volume, gel thickness, and the point of application. Moreover, [PERSON] et al. (2019) showed a uniform distribution of blue-dyed whitening sprays across the maxillary anterior tooth surfaces of F22 clear aligners (Sweden & Martina SpA, Due Carrare, Italy) when the whitening spray was sprayed onto the internal labial surface of the aligner, with one spray at the central incisors and one spray in each of the canine regions [37]. In the same study, the authors emphasised that there was no effect of using a reservoir tray on tooth whitening effectiveness [37]. Furthermore, other studies which used aligners for dental bleaching showed no adverse effects on the gingiva or on tooth sensitivity that would result in disruption of treatment [36, 37]. Therefore, it can be concluded that tooth bleaching using clear aligners can be a safe treatment modality that would result in no harm to the gingivae when performed under professional supervision. Moreover, [PERSON] et al. (2019) evaluated the effect of tooth whitening on the microstructural properties and the translucency of F22 clear aligners. After assessment with a scanning electron microscope (SEM) and double-beam spectrophotometer, the authors suggested that different concentrations of different bleaching agents had no effect on the microstructural properties and translucency of F22 clear aligners [37]. The authors only reported the presence of striations which were the result of normal cleaning and the presence of some surface irregularities associated with the heat used for gliding the aligners rather than being a chemical degradation. Even though only a few studies evaluated the effect of the bleaching materials on the clear aligner material, the current evidence suggests that tooth bleaching can be performed effectively using clear aligners without adversely affecting the physical properties of the clear aligner and the goals of orthodontic therapy. The present systematic review was conducted to evaluate the effectiveness of home bleaching using clear aligners and to compare this treatment protocol to tooth bleaching using conventional bleaching trays. Three studies assessed the effectiveness of bleaching using clear aligners, only one of them compared the bleaching using clear aligners to that using standard bleaching trays [35]. In this study, patients in both groups showed improvements in tooth shade with no statistically significant differences in tooth shade between the two groups. The other two studies also showed improvements in tooth shade in patients receiving dental bleaching using clear aligners, however, they did not have a control group in which patients received bleaching using standard bleaching trays for comparison [36, 37]. Furthermore, there were some limitations in the included studies which respect to the clarity of the randomization process, the allocation sequence concealment, as well as the blindness of the treatment providers and outcome assessors. This puts the available evidence at \"some concerns\" to \"high risk\" of bias when evaluated using the Cochrane risk of bias tool. However, based on the available evidence it can be concluded that home bleaching during orthodontic treatment with clear aligners is a valid treatment option that can reduce overall treatment cost and time. The limitations of the present study include the low number of evaluated studies, which already had a small sample size and had some inaccuracies in the randomisation process. Therefore, further clinical studies with larger sample sizes, more precise protocols for randomisation, and a longer follow-up period evaluating the efficiency of bleaching during clear aligner treatment are required to confirm the validity of using clear aligners for tooth bleaching alongside orthodontic treatment.