## 4 Discussion

Table 3: Comparison of Knoop hardness (KHN) within and between the study groups. demonstrated changes of negligible quantity or no significant changes in enamel surface roughness following bleaching [49; 50; 51; 52]. The differences in the results across different studies could be attributed to the study design, composition of the bleaching materials, relationship established between concentration of the bleaching agent and duration of use, and also the bleaching agent diffusion capacity and pH [53]. The active ingredients in the whitening products used in this study are varied. Group A contains 10% carbamide peroxide, equivalent to 3.62% H\({}_{2}\)O\({}_{2}\); iWhite instant Dark Stains whitening kit and iWhite instant Dark Stains toothpaste are activated by hydrated silica, sodium hexametaphosphate, and charcoal powder; and the whitening pen is activated by sodium bicarbonate and organic coconut charcoal. However, in recent years it has been well-established that polishing, fluoride application, and remineralizing solutions can significantly reduce the roughness of bleached enamel [47]. Surface hardness measurement is a reasonably easy way of determining the mechanical property of enamel and dentin to withstand plastic deformation from a reference sample, and it is strongly related to mineral component loss or gain [22]. The enamel microhardness measurement results showed no statistically significant difference between groups before experimental treatment. On the other hand, there was a highly statistically significant decrease in KHN between groups after whitening, except for the Active Wow whitening pen group, where the decrease was negligible and nonsignificant. The distilled water group showed an increased hardness, which was nonsignificant. The contact time of bleaching agent with enamel, the active ingredients of bleaching gels and their concentration, pH, activators, and thickeners are related to the decrease in hardness [46; 47]. In addition to affecting the bleaching efficiency, the pH of the whitening agent can also alter the morphology of the enamel surface and the hardness of teeth [46]. The pH of the vast majority of bleaching products on the market is neutral or slightly acidic. Nonetheless, the medium becomes acidic over time following application [54], which causes hardness reduction. Based on the outcome of this study, all whitening agents demonstrated reduced enamel hardness following whitening, which is in agreement with previous studies [38; 46; 48; 55; 56; 57]. The concentration of citric acid for effective whitening is reported to be at 5% [9]. In the current study, only 0.25% of the citric acid was used for whitening, but still there was highly significant decrease in hardness compared to other groups (335.4 to 227.9 KHN). This study is in disagreement with previous studies that reported no changes in hardness following bleaching [21; 28]. Exposing teeth to saliva after bleaching, on the other hand, promotes remineralization and stabilization of enamel hardness [58]. This was further confirmed in a review that used microhardness testing for structural enamel defects after whitening found that those studies that used human saliva and fluoride to closely simulate an intraoral environment and a post-treatment evaluation phase had a lower risk of enamel microhardness reduction than the others [20]. In the present study, 2 mm-thick enamel specimens were used to determine the whitening products' effect on enamel microhardness and surface roughness. One of the important factors contributing to the effectiveness of whitening products on the tooth surface is the enamel and dentine thickness. Randomized clinical investigations have found that tooth sensitivity after bleaching treatments is only present in the anterior teeth, with the severity of this side effect being directly correlated with the thickness of the enamel and dentin [59; 60]. The diffusion of chemicals generated by bleaching gels through enamel and dentin may also be influenced by variations in the thickness of enamel and dentin.