## 3 Results
### Data Analysis  
#### 3.4.1 Clinical Studies  
(a) Analysis of aims of studies and respective evaluation methods  
The various aims of the studies and respective evaluation methods are provided below:  
* Color change: 13/16 studies used an objective (spectrophotometric) method.
* Sensitivity: 14/14 used a Visual Analogue rating Scale (VAS).
* Temperature on enamel: 1/1 used a thermocouple.
* DNA Damage Biomarkers: 1/1 applied a biochemical examination (GCF, saliva, serum).
* Proteolytic and ROS activities in dentin-pulp complex: 1/1 applied a spectrofluorometric evaluation. (b) Outcome assessment. Regarding the primary outcome (a), 30% of the studies presented a positive outcome, 62% showed no difference between test and control groups, while only 8% showed a negative outcome for the test group.  
Specifically, for each examined aim, the number of studies and the respective outcome are exhibited in Figure 2:  
(c) Evaluation of gel and laser application protocols for studies showing a positive outcome  
Regarding the primary outcome (b), the different laser bleaching protocols that exerted a beneficial effect are displayed in Table 5.  
In the majority of the studies mentioned above, a hybrid LED/laser device was used in conjunction with varying H\({}_{2}\)O\({}_{2}\) concentrations, resulting in observed inhomogeneity. However, the common irradiation parameters were 18/18 and 24/42 J/cm\({}^{2}\), repeated 3-4  
\begin{table}
\begin{tabular}{l l l l l l} \hline  & **Gel Thickness** & **Gel Contact** & **Spot** & **Irradiation** & **Fluence (J/cm\({}^{2}\))** & **Irradiation** \\  & **(mm)** & **Time (min)** & **Size/Handpiece** & **Cycle** & **per Cycle** & **Cycles** \\ \hline
810 nm + (34–38)\% H\({}_{2}\)O\({}_{2}\) & 1 mm & 16 min & 2.8 cm\({}^{2}\)/quadrant & 15 & 46 & 8 \\
[53,55] & 1 mm & 20 min & 400 \(\mu\)m/ & 30 & 88 & 2 \\  & & & single-tooth & & & \\ \hline Hybrid + & 1-2 mm & 24 min & Full arch & 60/60 & 18/18 & 3 \\
[35–38]\% H\({}_{2}\)O\({}_{2}\) & 1-2 mm & 22.5 min & Full arch & 120/120 & 24/42 & 3 \\
[59,60,63,69,70] & 1 mm & 24 min & Full arch & 180/180 & 36/63 & 3 \\  & Unknown & 32 min & Full arch & 60/60 & 18/18 & 4 \\ \hline Hybrid + 25\% H\({}_{2}\)O\({}_{2}\)[63] & 1-2 mm & 22.5 min & Full arch & 120/120 & 24/42 & 3 \\ \hline Hybrid + 15\% H\({}_{2}\)O\({}_{2}\) with TiO\({}_{2-}\) & 1–2 mm & 48 min & Full arch & 60/60 & 18/18 & 3 \\
[65] & & & & & & \\ \hline \end{tabular}
\end{table}
Table 5: The laser bleaching protocols that exerted a beneficial effect in the clinical studies conducted.  
Figure 2: Aims of clinical studies and their respective outcome significance. Codification: green: test group significantly better, yellow: no significant difference, red: test group significantly worse—note: the total number differs from the total number of studies, as some studies had more than one aim.  
times, with a total gel contact time of 24 min. For the lower concentrated bleaching gel, the duration extended up to 48 min. As for the sole use of one wavelength, there are two studies where an 810 nm laser was applied. However, these two protocols were completely different with regard to the spot size, irradiation time, and fluence parameters employed.