الفهرس 
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Abstract
Dental caries is a microorganism infection that causes regional disintegration and destruction to the mineralized architecture. It is considered one of the most common dental biofilm-related diseases. Although oral biofilm can be composed of a variety of bacteria, oral streptococcus is considered the most important cause of dental caries. As it can quickly break down carbohydrates and produce a large amount of acid and induce tolerance to low pH environments. The objective of this study was to assess the antibacterial effect of diode laser versus Nanosilver fluoride, in-vitro, on Streptococcus Mutans count of oral biofilm of primary teeth. The sample consisted of 108 extracted human primary anterior teeth free from caries, developmental defects or restorations. Teeth were cleaned, polished, and stored in 0.1٪ Thymol solution. The crowns were separated from the roots and then they were embedded in acrylic resin blocks, exposing only the labial surface and labeled from the backside. Baseline readings were obtained by the environmental scanning electron microscope to evaluate enamel surface topography with the EDX (analytical technique used for the elemental analysis or chemical characterization of a sample) and by measuring the bacterial count and viability (the number of Colony Forming Units per milliliter (CFU/ml). Grouping of samples: After the preparation is completed, the specimens were randomly allocated into four groups using closed envelope randomization according to the test agents used. group I (Control group): included 60 teeth with no treatment This group was subdivided into two subdivisions -Control Negative: 12 teeth with no oral bacterial biofilm formation. -Control Positive: 12 teeth with oral bacterial biofilm formation for each of the following groups. group II (Nanosilver fluoride): included (12) teeth subjected to AgNPs fluoride varnish. For two minutes, the solution was in contact with the tooth surface. A micro brush was used to apply two drops of NSF solution to each specimen.Evaluation is carried out after 24 hours and stored in 2 ml of phosphate buffer. group III (Laser group): included 24 teeth irradiated with a diode laser (980 nm). This group was subdivided into 2 subdivisions A- High power diodelaser included (12) teeth irradiated with high power diode. The specimens were subjected to 2 irradiations. Using the following parameters: 980nm wavelength, 2W output power, 2 irradiations, each lasting 20 seconds, with a 60 seconds break, the tip diameter is 320 μm in sweeping motion and a continuous-wave laser generator. The laser beam was irradiated across 1 mm of the specimens and kept at the same distance from them at all times. B-Low power diode laser included 12 teeth irradiated with a low power diode laser. The specimens were subjected to a 980 nm diode laser with a power of 200mW, applied in a continuous-wave laser with a distance of 0.5 cm and a tip diameter of 320 μm in a sweeping motion for 3 minutes. group IV (Nanosilver fluoride and diode laser): included (12) teeth subjected to two drops of Nanosilver fluoride varnish for 2 minutes, applied with a micro brush, then irradiated with low power diode laser 980nm with power 200 mW applied for 3 minutes at 0.5 cm distance, 320 μm tip diameter in continuous-wave laser source and sweeping motion. After treatment, all samples were put back into the glass tube containing phosphate buffer and incubated for 24 hours. Then, the tube is sonicated to separate the biofilm formed on the sample’s enamel surface, and the number of colony-forming units per milliliter (CFU/ml) was determined. The results of the study showed: 1- Environmental Scanning Electron microscope The ESEM pictures showed the quality of the tested tooth surfaces in different test periods and research groups. Photomicrographs were taken under magnification (1000X-5000X) before and after treatment. A- Preoperative Assessment: The preoperative evaluation of all groups showed a relatively similar intact and smooth enamel surface, with few small scratches and few small pores. The enamel surface showed normal surface structure such as perikymata, and the normal rod ends are scattered on the smooth enamel surface B-Postoperative Assessment: - group II (Nanosilver fluoride): The enamel surface of deciduous teeth is nearly entirely covered by a NSF layer, according to postoperative ESEM photomicrographs. This layer can separates bacteria from the tooth enamel. The decrease in the number of bacteria in this group may be due to that, in addition to its antibacterial effect. -group III: (Diode laser) A- High power diode laser: The postoperative photomicrographs revealed a substantial increase in the number and accentuation of the keyhole enamel surface structure with irregular enamel surface. This resulted in the removal of the rodless enamel layer exposing the underlying normal structure (rod end keyhole). Hence, more irregularities which is less mineralized than surface enamel are observed and this may explain why this group has the higher bacterial accumulation. B- Low power diode laser: An apparent increase of surface irregularity of enamel was shown without removing the outer enamel surface. This may explain the superiority of the low-power diode laser on the bacterial count reduction over the high-power diode laser. -group IV (Nano silver fluoride followed by a low-power diode laser): Postoperative ESEM photomicrographs revealed that the enamel was clearly smooth and the enamel surface irregularity was minimal. The superiority in this group is due to the presence of layer of NSF which prevented laser beam from making surface irregularities and superior effect of low diode laser. 2-Energy Dispersive X-ray Calcium results: The results of the EDX measurements in our study showed that the calcium enamel content (wt٪) was preserved and increased after NSF and diode laser application in all groups (p<0.001). The NSF showed the highest increase in the calcium followed by the high and low power diode laser (no significant difference between them) and then the combination of NSF and low power diode laser showed the least calcium. Phosphorus results The phosphorus results showed that a phosphorus enamel content (wt٪) was preserved and increased in all treatment groups. However, this increase was not statistically significant between the control groups and the treatment groups and also in between the different treatment groups. 3-Bacterial count and viability: CFU results (Percentage of change): In group I (control positive), the bacterial count and viability increased and in groups II, III, and IV, bacterial counts and viability decreased. We found that NSF showed the highest antibacterial action against cariogenic Streptococcus mutansbiofilm and in reducing its count, followed by the combination of NSF and low power diode laser group and there was no significant difference in their result. The effectiveness of the diode laser alone was lower. There was no significant difference between high and low power diode although low power diode gave better results.