1. Introduction
reating deciduous teeth in children, especially at a young age when their cooperation in receiving treatment is poor, is always a challenge for dentists. Due to the different structure of deciduous teeth, compared to permanent teeth and the rapid progression of caries as a result of pulp involvement, always early and non-invasive treatment methods are of high priority to prevent caries or stop the process of caries in the early stages (Early/White spot lesions) in children [12]. Numerous laboratory studies compared the effects of different substances on the enhancement of enamel microhardness; however, most studies were performed on permanent human or animal teeth. Deciduous enamel is different from permanent teeth concerning microscopic structure. Thus, the present study aimed to compare the effects of three fluoride-containing substances on microhardness changes in primary decay enamel of deciduous teeth artificially and under laboratory conditions.
2. Materials and Methods
In this study, 45 extracted human deciduous molars were used. The maximum storage time of the samples was less than 3 months. The collected samples were immersed in 0.1% thymol solution for 24 hours for disinfection. Using a brush with a slow handpiece, they were cleaned of any debris and washed with distilled water, and remained at room temperature until studied in normal saline solution. Using a diamond cutting disc, enamel blocks with dimensions of 1 × 4 × 4 mm were prepared from the buccal surface and mounted on Acropars self-healing acrylic. The obtained samples were divided into 3 groups of 15 (first group: treated with fluoride varnish, second group: treated with fluoride mouthwash, & the third group was treated with fluoride toothpaste) and the microhardness of each group was taken in 3 steps, as follows:  
Step 1: Measure the initial microhardness of the samples
Step 2: Measure the microhardness of the samples of all three groups after causing artificial caries
Step 3: Measure the microhardness of the samples in each group after exposure to fluoride-containing materials
3. Results
Based on the present study findings, the mean microhardness (in all 3 groups), significantly decreased after artificial caries, compared to healthy enamel (P<0.05).
The mean microhardness of healthy enamel in the first group was measured to be 347 VH89 and VH37/257 after artificial rot. It was found to indicate an increase in microhardness in this group. There was a significant difference between the mean microhardness of the first and second stages of measurement as well as the second and third stages of measurement; however, there was no significant difference between the mean value of the first and third stages of measurement, highlighting the positive effect of fluoride varnish in increasing enamel remineralization. Artificial decay was to such an extent that it was not significantly different from healthy enamel (P=0.126).
In the second group, the mean microhardness of healthy enamel equaled 313.89 VH, which decreased to 254.69 VH after artificial caries; after receiving mouthwash twice a week for 3 weeks, the mean microhardness of samples increased to 268/67VH (Tables 1, 2, 3). 






There was a significant difference between the microhardness measured in the first and second, as well as first and third stages; however, despite the increase in microhardness after the effect of Oral B 0.12% mouthwash, no significant difference was observed between the microhardness measured in the second and third stages. In the third study group, measurements were performed as per the previous two groups and the average microhardness of healthy enamel was computed as 331.45 VH, which decreased to 255.90 VH after artificial caries; after using toothpaste, the average microhardness of the study samples reached 263/27VH daily for 20 days. In this group, as in the second group, there was a significant difference between the microhardness measured in the first and second, as well as first and third stages; however, despite the increase in microhardness after the effect of 0.2% GC MI Paste plus 0.2% toothpaste between the measured microhardness, there were no significant differences between the second and third stages. The effect of V-Varnish fluoride 5% on increasing the microhardness of enamel with artificial decay was significant i.e., higher than Oral B mouthwash 0.12% and GC MI Paste plus toothpaste 0.2%. Microhardness changes between toothpaste and mouthwash groups were not statistically significant.
4. Discussion
In this laboratory study, microhardness changes in the primary enamel of deciduous teeth (artificially created) were investigated after the application of V-varnish fluoride varnish, Oral B Complete mouthwash, and GC MI Paste plus toothpaste. The purpose of the demineralization and remineralization cycle in vitro is to evaluate the effectiveness of fluoride-containing compounds to protect the demineralized enamel against acid [14]. The process used in this study was a standard formula to simulate the condition of the mouth that acidic substances and the activity of cariogenic bacteria that cause the demineralization of tooth enamel and primary caries (White Spot Lesion).
Due to the desirable properties of fluoride varnish, including its simple use for young children (even before the age of one year), the small volume of the material used, reducing the possibility of swallowing and no need to maintain isolation after application on the teeth and due to the higher concentration of fluoride in the varnish than toothpaste and mouthwash, its use is recommended three times a year for children at high risk of caries or a three-course diet with 10-day intervals within a month. Also, due to the increased microhardness of enamel with primary decay after using mouthwash, it is recommended to use mouthwashes containing fluoride after the age of 6 years. According to the present study, the least improvement in microhardness was observed in the group of toothpaste. However, due to the increase in microhardness of enamel with primary decay and the use of a toothbrush with toothpaste is the most common method of plaque control and oral hygiene at home. It is also recommended to use a regular fluoride toothpaste in children after 2-3 years of age.

Ethical Considerations
Compliance with ethical guidelines
This study was approved by the Ethics Committee of the Arak University of Medical Science (Code: IR.ARAKMU.REC.1397.264).

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors.

Authors' contributions
All authors equally contributed to preparing this article.

Conflicts of interest
The authors declared no conflicts of interest.


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