## 4 Discussion
\begin{table}
\begin{tabular}{l r r r r r r} \hline \hline  & **Mean** & **SD** & **Min** & **Mdn** & **Max** & **Significance \({}^{*}\)** \\ \hline Control T0 & \(-0.54\) & \(0.28\) & \(-0.80\) & \(-0.60\) & \(-0.20\) & A \\ Control T1 & \(0.74\) & \(0.88\) & \(-0.40\) & \(0.50\) & \(2.00\) & A \\ Control T2 & \(-0.92\) & \(0.82\) & \(-2.10\) & \(-1.00\) & \(0.00\) & A \\ \hline Ozonized Gel T0 & \(-0.70\) & \(0.54\) & \(-1.40\) & \(-0.60\) & \(-0.10\) & A \\ Ozonized Gel T1 & \(0.40\) & \(1.26\) & \(-1.60\) & \(1.00\) & \(1.60\) & A \\ Ozonized Gel T2 & \(-1.34\) & \(0.35\) & \(-1.70\) & \(-1.50\) & \(-0.80\) & A \\ \hline Ozonized Oil T0 & \(-1.42\) & \(0.75\) & \(-2.50\) & \(-1.60\) & \(-0.70\) & A \\ Ozonized Oil T1 & \(0.72\) & \(0.63\) & \(0.00\) & \(0.70\) & \(1.40\) & A \\ Ozonized Oil T2 & \(-1.42\) & \(0.79\) & \(-2.50\) & \(-1.30\) & \(-0.50\) & A \\ \hline Ozonized Oil + Regen T1 & \(-1.50\) & \(0.57\) & \(-1.80\) & \(-1.80\) & \(-0.50\) & A \\ Ozonized Oil + Regen T2 & \(0.58\) & \(0.65\) & \(-0.10\) & \(0.30\) & \(1.50\) & A \\ Ozonized Oil + Regen T3 & \(-1.24\) & \(0.48\) & \(-1.90\) & \(-1.00\) & \(-0.80\) & A \\ \hline \hline \end{tabular} \({}^{*}\) Different letters show a statistically significant difference between the groups (\(p<0.05\)).  
\end{table} 
Table 5: Median (min, max) for the value B for the two groups at each experimental time. tooth erosion, tooth mineral degradation, increased susceptibility to demineralization, and pulpal damage [31]. The efficacy of ozone therapy on the bleaching of a tooth may depend on the application time, gas flow rate, and on the synergic effect with conventional bleaching agents. A recent systematic review was conducted by [PERSON] et al. [32] to determine whether O3 can improve the clinical performance of tooth bleaching in vital teeth. The authors found that the bleaching effectiveness for the combination of O3 + hydrogen peroxide (HP) compared to HP was similar and, based on the published literature, concluded that ozone is not superior to the conventional technique using HP on the change of tooth color. Moreover, they also found that ozone did not cause sensitivity when used alone. When it is used in combination with HP, patients tend to report hypersensitivity only when ozone is applied before HP, whereas no sensitivity is perceived when ozone is applied subsequently to it. Despite the bleaching effect of ozone on the tooth that has been investigated in the literature, to the best of our knowledge, no research has been conducted until now to determine a potential bleaching effect towards composite resins. These dental materials suffer from the risk of internal and external pigmentation, the latter caused by agents such as fruit juices, tea, coffee, and cola. Previous in vitro studies have been conducted to assess the lightening effects of both home-applied and office-applied tooth-bleaching products and polishing procedures on stained composite resins [33]. [PERSON] and colleagues [34] determined that bleaching improved the color of red wine- and coffee-stained composite resin to a level that was analog to the baseline. Studies comparing the effects of polishing to bleaching stained composite resins have had different results. [PERSON] and colleagues [35] found that repolishing was more effective than an in-office bleaching product (40% hydrogen peroxide bleaching gel) for color improvement. Conversely, [PERSON] and [PERSON] [8] determined that another in-office bleaching protocol was more effective than simulated office polishing. The color recovery effect on discolored composite resins has been investigated until now only for bleaching and polishing systems, as reported in the studies mentioned above. On the contrary, no studies have been conducted to assess the bleaching effect of ozone on restorative composite resins. In the present study, a positive but moderate influence was assessed for ozone when evaluating its action on the color change of discolored resins, with a change limited to E and L colorimetric values.