## 4 Discussion
### Comparison between In-Office and At-Home Dental Bleaching  
The use of low bleaching agent concentrations (10-16% CP or 6% HP) applied for at least two weeks is indicated for at-home bleaching [41]. For in-office procedures, bleaching agents (HP 25-40% or CP 35%) are applied for shorter amounts of time with or without light activation [41]. It can be performed at-home or in-office with a wide range of techniques. At-home-bleaching treatment ray-based system uses trays built on an impression and prepared in the laboratory, the bleaching gel is placed inside the trays and used for a few hours during the day or overnight for a few days (from two to three weeks) [42,43] (Figure 4). Figure 4: Home bleaching, trays built. \begin{table}
\begin{tabular}{c c c c c} \hline
**Authors** & **Type of the Study** & **Aim of the Study** & **Materials** & **Results** \\ \hline [PERSON] 2017 [38] & Randomized controlled clinical trial & Compared the bleaching efficacy of two different at-home bleaching systems (10\% CP and 6\% HP). & Ninety-two patients were chosen and randomly divided into three groups: negative control, 10\% CP, and 6\% HP. & Although both bleaching agents were clinically effective, 10\% CP was more successful. \\ \hline [PERSON] et al., 2018 [41] & Single-blinded randomized parallel clinical trial & Evaluate TS in patients undergoing in-office tooth bleaching with 37\% CP and 35\% HP. & Furry patients were randomly assigned to receive two sessions of in-office tooth whitening using either 35\% HP or 37\% CP resulted in CT. The degree of TS for each patient was assessed before, and up to 24 h after bleaching. \\ \hline [PERSON] et al., 2018 [42] & Randomized clinical study & Randomized clinical study & Evaluate the psychosocial impact and aesthetic perceptions of 42 patients undergoing non-vital-tooth bleaching with HP (35\% and CP (37\%)). & Forty-seven patients were chosen and divided into two bleaching agent used: group 1 (35\% HP) and group 2 (37\% CP). & Both gels were highly effective, and the color change lasts at least three months. \\ \hline [PERSON] et al., 2018 [42] & Clinical trial & Evaluate dental color stability after various bleaching techniques. & Sixty patients were divided into four groups: 35\% HP in-office bleaching, in-office application of 3\% HP followed by in-office bleaching using 35\% HP, 3\% HP in-office bleaching, and 10\% CP at-home bleaching. \\ \hline [PERSON] et al., 2018 [39] & Clinical trial & Clinical trial & Evaluate dental color stability after various bleaching techniques. & Sixty patients were divided into four groups: 35\% HP in-office bleaching, in-office application of 3\% HP followed by in-office bleaching using 35\% HP, 3\% HP in-office bleaching, and 10\% CP at-home bleaching. \\ \hline \end{tabular}
\end{table} 
Table 5: Reported studies about comparison between the use of carbamide peroxide and hydrogen peroxide [CP: carbamide peroxide, HP: hydrogen peroxide, TS: tooth sensitivity]. On the other hand, in-office bleaching is performed chairside and uses a bleaching gel with higher concentrations of hydroxide placed on the tooth for minutes, activated by an LED light [18] (Figures 5 and 6). Different bleaching protocols based on HP and CP concentrations have been proposed in the literature [44].The classic at home-bleaching was described by [PERSON] in 2018. He used a bleaching gel containing 10% calcium hydroxide inside trays using for 30 min, twice daily for 2 weeks. After one month of follow-up, the bleaching was significant [45]. [PERSON] and [PERSON] proposed an at-home bleaching protocol with a 10% HP gel containing a desensitizing gel (3% potassium nitrate and 0.2% sodium fluoride) inside it. The authors compared it with a gel of the same HP concentration but without desensitizer. Both were used for 3 h a day for 21 days. There was no difference reported in either TS or color [46; 47]. In addition, [PERSON] stated that the presence of a reservoir in a bleaching trap did not influence the effectiveness of the treatment, TS, and gingival irritation [48].