## 4 Discussion
Nano-hydroxyapatite is the most frequently studied biomaterial in the medical field due to its biocompatibility and role as the main mineral component of bones and teeth. Studies have reported that nano-hydroxyapatite forms a protective layer on the intact enamel surface, providing the demineralized enamel with the necessary minerals to recover [23]. One in vitro study evaluated the remineralization efficiency of toothpastes on primary teeth and reported that toothpaste containing only nano-hydroxyapatite provided more mineral gain than toothpaste groups containing only fluoride and those containing fluoride and nano-hydroxyapatite. However, the differences between these groups were not statistically significant [45]. Similar results were also reported in other in vitro studies on primary and permanent teeth [25; 46]. In our study, according to the demineralization stage, a numerical increase was observed in the microhardness values of the Splat group. Splat was used as a hydroxyapatite-containing toothpaste, but this increase was not statistically significant. As reported in other studies, it may be stated that the property of hydroxyapatite to provide mineral deposition on the outer surface of lesions was in parallel with the findings of the SEM images of our study which presented the prisms becoming more prominent and the filling of the prism body compared to the demineralized enamel in the samples that were subjected to the Splat toothpaste [25; 46]. There are limited studies on the remineralization efficiency of sea salt. Since trace elements can affect the physicochemical properties of enamel by incorporating it into its structure, the remineralization effect of sea salt has been associated with the elements it contains [47; 48]. It has been shown to reduce dental caries due to its fluoride content [49]. In the group in which Logodent, containing sea salt, was applied, we observed a numerical increase in the microhardness value in the remineralization stage compared to that in the demineralization stage, but this increase was not statistically significant. Although we did not know the type of sea salt in Logodent and how much fluoride it contained, we observed in the SEM images that the prism bodies emptied in the demineralized enamel were filled after remineralization. Therefore, sea salt can be used as a remineralization agent thanks to the elements it contains. Although Theodent toothpaste was used in studies on theobromine-containing toothpaste, the cost of this toothpaste was high, and its availability in Turkey was limited [50; 51]. Therefore, the \"Natural Chocolate and Mint Extract Toothpaste for Kids\" introduced by the Eyup Sabri Tuncer Company, which was the only toothpaste for children containing theobromine in Turkey, was chosen for our study, even though it was for children aged 7-16 years. Due to the use of natural peppermint oil in its content for cosmetic purposes, care should be taken against the risk of swallowing by children. In our study, despite the numerical increase in the microhardness values in the remineralization stage compared to those in the demineralization stage, this increase was not statistically significant in the group in which this toothpaste was applied. Theobromine was reported to prevent the dissolution of apatite on the enamel surface by increasing the crystal size, and its effect increases in direct proportion to its concentration [22]. In their 28-day study, [PERSON] et al. [52] observed a higher increase in surface microhardness in the 1.1 Mmol theobromine group than in the 1100 ppm fluoride-containing toothpaste group. On the other hand, [PERSON] [53] conducted a 5-day study and found that theobromine did not increase the surface hardness more than the groups containing 226 ppm and 452 ppm fluoride despite using the same amount of theobromine. These results may have been influenced by the duration of the application.