الفهرس 
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Abstract
Introduction: Radiation caries is a serious complication to head and neck cancer (HNC) radiotherapy, for which the primary teeth are more susceptible to be affected. Preventive protocols are recommended to enhance dental structure resistance against the direct effects of radiotherapy. Aim: This in vitro study aims to evaluate the efficacy of diode laser, FV, and nHA on microhardness, micromorphology and mineral content of primary teeth enamel, before and after exposure to radiotherapy. Methodology: Twenty primary molars were sectioned into two halves in a mesiodistal direction, to obtain 40 specimens, which were then randomly allocated into five groups. group 1 (Control Negative) n = 5 was not subjected to any treatment or radiation. group 2 (Control positive) n = 5 was gamma irradiated with a dose of 60 Gray. For Groups 3, 4, and 5, specimens were divided into two subgroups: A and B (n = 5/subgroup). Subgroups A were gamma irradiated, then exposed to different surface treatments: 3A:10٪ nano-hydroxyapatite (nHA) paste, 4A: 5٪ sodium fluoride varnish (FV), and 5A: diode laser 980 nm. Subgroups B were exposed to surface treatments (3B: 10٪ nHA, 4 B: 5٪ FV, and 5B: diode laser 980 nm), then gamma irradiated. Surface micromorphology and microhardness were examined using environmental scanning electron microscope (ESEM), and Vickers microhardness tester, respectively. Environmental Electron Dispersive Analytical X-ray(EDAX) was used in this study to quantify the Calcium (Ca), Phosphorus (P) weight (wt٪) and the Ca/P ratio. Results: group 2 (G) specimens possessed the lowest mean microhardness, while nHA-G (3B), G-Fl (4A), and L-G (5B) had significantly higher values. ESEM analysis showed an alteration in group G and the obliteration of enamel micropores with remineralizing agents. The melting and fusion of enamel in laser subgroups were also observed.Regarding Ca (wt٪), there was a significant difference between different groups (P=0.032). The highest value was found in subgroup 4A while the lowest value was found in 5A. The highest value of P (wt٪), was found in subgroup 4A while the lowest was found in Group2. Considering Ca/P wt٪, there was insignificant difference between different groups (P=0.096). The highest value was found in Group1 while the lowest was found in subgroup 5B. The chemical structure of enamel in gamma irradiated teeth had been enhanced with the application of nHA and FV. Using the diode laser alone didn’t show a positive effect in improving the minerals of enamel. Conclusions: The findings indicated that using FV, nHA, or diode laser increased microhardness and maintained the integrity of the enamel microstructure. Therefore, applying preventive strategies should be considered in HNC.