## 4 Discussion  
The effect of whitening agents on the enamel surface is a controversial issue and the results of experiments are influenced by a multitude of variables. The use of in vitro studies allows valuable information to be obtained regarding the safety of whitening products, their effects on dental tissues, the differences between the concentrations used or between different action times. In this study, a cyclic model was used, with periods of teeth whitening followed by periods of remineralization in artificial saliva, in order to simulate the physiological conditions present during teeth whitening. The quantities of whitening gels chosen were sufficient to completely cover the tooth surface subject to whitening. Additionally, to simulate human saliva, the artificial saliva in which the teeth were kept contained electrolytes of sodium, potassium, calcium and magnesium. Spectrophotometers are extremely sensitive devices that can detect even minimal color changes that are not clinically detectable. For this reason, spectrophotometric analysis was used in the current study to determine and assess tooth color change, using the CIE parameters L*a*b* and \(\Delta\)E, as well as the closest shades. A precise time interval for clinically noticeable color change differences between bleached and unbleached enamel surfaces has not been determined yet. The time required for the discoloration to be clinically observed depends on many factors, including the layer thickness, the nature of the gels and their concentration, and the quality and quantity of the seal [11; 12; 13]. The differences between the results of previous and current studies could be attributed to the methodologies used. In the present study, the effect of differentiated discoloration between enamel surfaces from different patients treated with whitening gels of different concentrations was determined by in vitro means using the most standardized protocols and using a spectrophotometer that allows a detailed, pointed and synchronized analysis of two identical images made at different spatial moments, while previous studies in the literature use spectrophotometers that do not allow this synchronization. Considering the results of the present study, \(\Delta\)E variations are higher than the general average variations of \(\Delta\)L in the case of the Opalescence gel with a concentration of 40%, but smaller for Opalescence 16%. Using a combination of in-office and at-home whitening techniques leads to the most effective result in the shortest time. Teeth whitening after in-office whitening treatments can be further maintained and enhanced by using at-home whitening treatments. The AFM analysis after the cyclical period of 14 days of whitening treatment with two commercial products on the healthy patients' teeth demonstrated the presence of some microporosities on the enamel surface. These observations are consistent with those presented by [PERSON] et al. [14], who concluded that whitening products are capable of altering the surface of the enamel. The hypothesis that peroxide bleaching agents affect the organic component of the enamel is mentioned. Additionally, peroxides can affect not only the surface but also the internal structure of the enamel, because they have a low molecular weight that allows them to penetrate this dental layer [15; 16]. On the other hand, analyzing the AFM images of tooth enamel extracted from patients with renal insufficiency, predialysis, a lower roughness was noticed, which indicates a resistance to demineralization. An explanation can be given in terms of changes in calcium and phosphorus metabolism caused by chronic kidney disease. Phosphate retention can cause an increase in its level in saliva and thus, along with the composition of saliva (oftenmodified) and its alkaline pH, tooth enamel is more resistant to acid demineralization as well, as has previously been demonstrated [17].