## 4 Discussion  
The primary goals of this systematic review were (a) to evaluate the efficacy of laser-activated dental bleaching compared to conventional in-office bleaching methods, and (b) to critically appraise the laser bleaching protocols employed to date. This systematic literature review encompassed a total of 19 clinical and 20 in vitro studies, incorporating both qualitative and quantitative assessments. The results indicated that clinical studies predominantly focused on evaluating color outcome and sensitivity, while in vitro studies primarily emphasized color outcome and hard tissue morphology alterations. Within the pool of included studies, there were 39 investigations where the bleaching gel was activated by a laser device. Specifically, a \"hybrid\" LED and laser combination device (HL) was applied in 12 of the clinical and 2 of the in vitro studies. Additionally, 17 clinical and all the 20 in vitro studies assessed gels of high HP concentration (i.e., HP 30% (\(w/v\)) or higher), and only 2 of the included clinical studies examined low concentration gels (i.e., HP below 20% (\(w/v\))). In a recent systematic review ([PERSON] 2018) investigating whitening efficacy and tooth sensitivity (TS) in office vital bleaching, with or without light application, the authors concluded that bleaching efficacy and TS are not influenced by light activation [8]. The authors pointed out that this was an overall comparison without considering variations of protocols or the type of light applied. Specifically, out of the 21 included studies, ten applied a LED/laser combination, and only four applied a laser device to activate the bleaching gel, while the remaining seven applied another source of light activation. In our analysis, the focus was instead based on laser-activated protocols, and only such studies with at least one laser or hybrid LED/laser-activated group serving as the test group were included. Amongst the included 19 clinical and 20 in vitro studies, only three [54; 56; 71] and three respectively [81; 89; 91] revealed significantly worse results for the test groups. In contrast, nine clinical [53; 55; 57; 60; 63; 65; 66; 69; 70] and ten in vitro studies [72; 76; 77; 78; 84; 86; 88; 89; 90] could, at least in principle, provide significantly improved results for the laser and hybrid LED-laser groups. Nevertheless, the statistical analysis performed did not show a significant difference between these. \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameter Examined** & **Hard Tissue Alterations** \\ \hline Test group wavelength & ns \\ \hline Handpiece type & ns \\ \hline Gel thickness & ns \\ \hline Fluence test group & ns \\ \hline Difference in gel contact time & \(p\) = 0.088 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Parameters examined and the respective \(p\) values found regarding the morphology outcome. Abbreviations: ns, not significant (threshold significance level \(\alpha\) = 0.05). In another systematic review (Maran 2019) exploring the differences in efficacy (color outcome) between light-activated protocols and light-free in-office bleaching protocols, no significant difference in color change was found regardless of the HP concentration (Maran 2019). Amongst the 28 studies incorporated in the above analysis, a laser activation source was utilized in only four of them, while a LED/laser device was employed in 13 of the studies. In our study, through statistical analysis, it was revealed that the conventional (non-irradiated) control group requires a H\({}_{2}\)O\({}_{2}\) (HP) dose level which is, on average, 24.15% (\(w/w\)) higher in order to achieve a color status which is equivalent to that of the laser-activated bleaching group. This difference was highly significant (\(p\) = 0.0032).