## 4 Discussion  
The aim of the present ex vivo study was to evaluate and compare the discoloration potential of four different CSCs used in regenerative endodontic procedures, as well as to assess the role played by blood contamination on color variation severity when CSCs are applied. Theoretically, recently introduced biomaterials (Biodentine, TotalFill, and PCM) with a faster setting time when  
Figure 5: \(\Delta\)E of all experimental groups over time. \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline  & **MTA/Saline** & **Biodentine/Saline** & **TotalFill/Saline** & **PCMS/Saline** & **Blood** & **MTA/Blood** & **Biodentine/Blood** & **TotalFill/Blood** & **PCM/Blood** \\ \hline \hline
**Saline** & 0.010 \({}^{*}\) & 1.000 & 1.000 & 1.000 & 0.010\({}^{*}\) & 0.001 \({}^{*}\) & 1.000 & 0.707 & 0.006 \\
**MTA/Saline** & 0.001 \({}^{*}\) & 0.000\({}^{*}\) & 0.007\({}^{*}\) & 1.000 & 0.796 & 0.008 \({}^{*}\) & 0.218 & 0.996 \\
**Biodentine/Saline** & & 1.000 & 0.999 & 0.002\({}^{*}\) & -0.001 \({}^{*}\) & 0.899 & 0.544 & 0.012\({}^{*}\) \\
**TotalFill/Saline** & & & 1.000 & 0.010\({}^{*}\) & -0.001\({}^{*}\) & 1.000 & 0.918 & 0.009 \\
**TCD/Saline** & & & & 0.010\({}^{*}\) & -0.001\({}^{*}\) & 1.000 & 0.921 & 0.071 \\
**Blood** & & & & 1.000 & 0.012\({}^{*}\) & 0.174 & 0.943 \\
**MTA/Blood** & & & & & 0.001\({}^{*}\) & 0.028\({}^{*}\) & 0.750 \\
**Biodentine/Blood** & & & & & & 0.939 & 0.082 \\
**TotalFill/Blood** & & & & & & & & & \\ \hline \hline \end{tabular}
* Statistically significant difference (\(p<0.05\)).  
\end{table} 
Table 3: \(p\)-values obtained from group comparison regarding the interaction of factor material and factor treatment. compared to the gold standard MTA might present superior color stability. Moreover, it is crucial to unveil the possible role played by blood when in contact with CSCs since blood exposure is likely to occur in both REPs and vital pulp therapy. An insight into these matters would allow adequate selection of the biomaterial to use based on the clinical conditions, ultimately allowing the accomplishment of better esthetic results. In fact, it has been extensively reported that REPs lead to dental discoloration. This undesirable consequence of regenerative therapy is a major concern for both clinicians and patients, especially when an esthetic area is involved [4; 5; 6; 7]. REPs protocol involves the creation of a cervical plug by placing a 3-4 mm CSC layer in the coronal portion of the root canal. Endodontic materials should present chromatic stability and optic properties similar to dental structures and not cause tooth discoloration over time [14]. However, as previously mentioned, the association of CSCs with color variation over time is well-described within the literature. The obtained results show that there are statistically significant differences in \(\Delta\)E over time (\(p<0.001\)). In fact, statistical differences were found considering the factor material (\(p<0.001\)), the factor treatment (\(p=0.007\)) or the interaction of both factors (\(p=0.001\)), over time. Therefore, the null hypothesis, stating that there are no statistically significant differences between the experimental groups, has been rejected. Several possible mechanisms have been outlined to explain color alteration following CSC placement, one being that the composition of the biomaterial is a major contributing factor to CSCs' discoloration potential [26]. MTA formula contains bismuth oxide as a radiopacifier (Bi\({}_{2}\)O\({}_{3}\)), which has been proven to lead to tooth discoloration through its (1) reduction or (2) oxidation when in contact with strong oxidizing agents, such as dentin collagen and sodium hypochlorite, used as an irrigation solution [26].