## 4 Discussion  
Dental staining emerges as a frequent concern presented by individuals seeking aesthetic enhancements. Alterations in tooth hue emerge from a blend of internal and external influences, encompassing factors as diverse as the assimilation of substances to the consumption of pigmented edibles, thus leading to discolouration [27]. The market offers a wide range of products that can be used directly by patients or through in-office treatments, and all claim to achieve the endpoint of a whiter, more attractive smile [28]. The final result's accomplishment is the human perception of smile brightness after the treatment. However, this personal visual scale is not easy to replicate and \"translated\" into an actual numerical value. The optical density spectrophotometric analysis before and after  
Figure 7: Hardness changes (**A**) and Young’s modulus (**B**) after each bleaching treatment, compared to the untreated control. *** symbol indicates an extremely statistically significant difference (one-way ANOVA test followed by the Tukey HSD test) respect to Control; Blancone Ultra\({}^{+}\) and Pola Office. bleaching treatment has been described as a supportive method to obtain a reliable value of the bleaching treatment effectiveness [29]. [PERSON] et al. [30] showed spectrophotometric analysis is more accurate and reproducible than human assessment. [PERSON] et al. [31] demonstrated that colourimeters are significantly less reliable than spectrophotometers and digital cameras. Indeed, [PERSON] et al. [32] concluded that spectrophotometers achieved 80% reproducibility of the tooth shade than the human eye, which showed only 65% reproducibility. In our work, the spectrophotometrically evaluated data describe statistically significant differences for each bleaching treatment analysed. The treatments, therefore, appear effective in improving the brightness of the teeth, although there are differences in the quality and intensity of the final result. In fact, the comparison of each bleaching agent with BlancOne's ULTRA\({}^{+}\) highlights the higher performance of this product. Statistical analysis suggests a ranking of product efficacy with Opalescence Extraboost after BlancOne's ULTRA\({}^{+}\), and Pola Office and Zoom WhiteSpeed as the least reliable. Basically, the bleaching efficacy ranking is BlancOne's ULTRA\({}^{+}\) > Opalescence Extraboost > Zoom WhiteSpeed = Pola Office. Noteworthy is BlancOne's ULTRA\({}^{+}\) product, which allows the best results with a lower percentage of HP than Opalescence Extraboost, and with HP similar to Pola Office. Furthermore, BlancOne's ULTRA\({}^{+}\) requires the shortest application times on the tooth surfaces compared to the other three products analysed and reduced number of treatments. ULTRA\({}^{+}\) by BlancOne works through a process of chemical photoactivation of the gel using specific wavelengths of light, which in turn can accelerate the release of free radicals and singlet oxygen [27]. However, as suggested by the lower bleaching effectiveness of Zoom WhiteSpeed, photoactivation per se is not enough, and a good aesthetic result occurs through an efficient synergy between a specific wavelength of light energy and the ability of the photoactivator to absorb it. Therefore, despite more specific analyses having to be conducted, BlancOne's ULTRA\({}^{+}\) formulation containing three natural photoactivators in synergy with the 430-490 nm wavelength range could cause a more efficient release of free radicals and singlet oxygen to allow faster and effective bleaching results. On the other hand, Zoom WhiteSpeed is based on Photo-Fenton photoactivation [33]. Hydrogen peroxide bleaching products are well established for cosmetic tooth bleaching, but the use of these substances in home cosmetic products is quite limited [34]. Conversely, in professional treatments, high concentrations of HP are used, with possible negative consequences on soft and hard tissues [35]. Indeed, a fundamental aspect is the effect of the bleaching agent on the structure of the enamel.