## 4 Discussion
In this way, it is normal for the changes found in the enamel after whitening to be less obvious compared to those present in the tooth enamel of healthy patients. From the literature, it can be noticed that bleaching treatments induce changes in surface roughness and, therefore, influence the formation and retention of sub- and supragingual bacterial plaque. Therefore, the adhesion of Streptococcus mutans to the enamel is increased [18]. The same is true of Streptococcus sobrinus, but, surprisingly, not of Actinomyces viscosus [19]. Such an undesirable effect may have some implications for the development of a future carious lesions. However, there are currently no clinical reports available on the possible exiogenicity of whitening treatments. Therefore, the majority of bleaching agents induce mild to moderate changes in the enamel surface, with a decrease in its microhardness. However, these microlesions are less intense than those resulting from phosphoric acid etching, but may interfere with the adhesive properties of restorative materials. After a while, the porosity gradually decreases. Attempts to reduce mineral loss and microdefects on the enamel surface were performed with fluoride-containing products [20,21]. The results of the AFM analysis in the present study are similar to those reported by [PERSON] et al. [4] on the micromorphology and roughness of the dental enamel of hemodialysed patients when 16% carbamide peroxide was used for teeth bleaching. They concluded that the effects of whitening are less harmful to the teeth of uremic patients. According to SEM image analysis, the enamel surfaces of the teeth extracted from healthy patients show changes in morphology. The results are consistent with those obtained by [PERSON] et al. [22]. They evaluated the effects of three whitening agents with a concentration of 10% carbamide peroxide on the morphology of tooth enamel, also using SEM analysis. Based on their observations, they concluded that there are significant changes to the surface of the enamel after teeth whitening. Previous studies indicated that bleaching products with concentrations of hydrogen peroxide of 30-35% cause superficial alterations and reduce the calcium-phosphorus ratio [23,24]. As hydrogen peroxide with a concentration of 40% is a strong oxidizing agent, it is only indicated for professional use in the dental office. Carbamide peroxide is the most common product used for teeth whitening. When in contact with the outer surface of the enamel, it decomposes into water and oxygen which diffuses through the organic component of the enamel. This process causes the oxidation of organic pigments, located mostly in the dentin, resulting in a reduction in or even disappearance of dental staining [4]. [PERSON] et al. [25] studied the effects of whitening on healthy teeth and restorative treatments (dental amalgam, gold, ceramic, glass ionomer cement and composites), focusing on surface, hardness and morphology. Their results confirmed that, regardless of the whitening gel used, with hydrogen peroxide or carbamide peroxide, these systems do not produce changes in the surface morphology or microresistance of the restorative dental materials. These results support the clinical safety of commercial bleaching systems in the oral environment. In addition, there are no clinical studies or case reports in the literature, macroscopic or clinical documents on visible damage due to the destruction of vital tissue after bleaching. Investigative studies on external whitening therapies are often applied and, usually, microresistance of structural defects of the enamel is tested. However, there are inconsistencies in the results of these studies.