## 4 Discussion  
At-home bleaching is becoming more popular for tooth bleaching since its introduction in 1989 by [PERSON] and [PERSON] [18; 22]. In the past, home bleaching was performed with trays that have reservoirs for the bleaching agent. Nowadays, no evidence supports that bleaching using reservoir trays is superior to that using non-reservoir bleaching trays [36; 38; 39; 40]. This would suggest that using clear aligners for tooth bleaching would effectively improve tooth shade when clear aligners are used as bleaching trays during the orthodontic treatment. In addition, tooth bleaching using clear aligners as bleaching trays has many advantages [35; 36; 37]. These include reducing the cost for the patient by eliminating the process required to make the bleaching trays, which involves taking new impressions for fabricating new trays. Additional advantages include reductions in the overall chairside time and improvements in the patient's oral hygiene [18; 30]. [PERSON] and [PERSON] (2020) used an Invasl aligner as a bleaching ray in two patients. They aimed to assess the feasibility of having bleaching performed at the same time as orthodontic treatment without having non-whitened spots in the areas where the attachment buttons are located [18]. They suggested that home bleaching using 10% carbamide peroxide could be performed concurrently with Invaslign orthodontic treatment because the bleaching material had small-molecular-size particles that were able to penetrate the dentine of the areas adjacent to the attachment buttons without the need for direct contact between the bleaching material and tooth surface to achieve efficient bleaching for these areas [18]. Another study suggested the use of lingual attachments to overcome the issue of obtaining a homogenous tooth whitening, although if the clinician's planned movements required anterior labial attachments, tooth whitening was suggested to be postponed to the end of the treatment by using the final aligner during finishing or using a post-treatment retainer [37]. Furthermore, [PERSON] et al. (2020) suggested that the use of bleaching agents with aligners at the beginning of orthodontic treatment was not recommended as the crowding of the teeth would affect the fit of the tray and the final result of tooth whitening [35]. From the authors' perspective, tooth bleaching during orthodontic treatment with clear aligners can be effectively performed and achieve a homogenous tooth shade and an absence of non-whitened spots. This is because of the ability of the small-molecular-size particles of the bleaching agent to penetrate the areas adjacent to the attachments. However, in the presence of severe crowding, tooth bleaching can be postponed until after the alignment of the teeth during the finishing step. Moreover, the effect of tooth bleaching on enamel mineral content has been evaluated by several studies, with some studies suggesting that bleaching materials could increase the enamel susceptibility to demineralisation [41; 42]. However, no macroscopic changes in the enamel were reported in the literature, and another study suggested that only high concentrations of carbamide peroxide and hydrogen peroxide were associated with increased enamel susceptibility to demineralisation [43; 44]. In addition, several studiesshowed that using demineralising agents could increase the microhardness of bleached enamel [45, 46, 47]. For example, [PERSON] et al. (2013) reported that using Remin Pro resulted in remineralisation and increased the microhardness of bleached enamel [45]. However, from the authors' perspective, tooth bleaching is still considered a conservative option for patients requesting white teeth compared to indirect restorations, including veneers or crowns. Furthermore, [PERSON] and his colleagues (2020) suggested that the application of 2 mm\({}^{3}\) of bleaching gel in the center of the Invisalign aligner provided the optimal gel distribution for efficient tooth whitening without gingival irritation [35].