الفهرس 
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Abstract
Background: The aim of this study was to evaluate the effect of combining fractional CO2 LASER and nanohydroxy apatite on surface microhardness and color of enamel with initial defects. Material and methods: Two types of nano hydroxyapatite (nHAP) was prepared; Pure hydroxyapatite (nHA) and Fluoro hydroxyapatite (nFHA), Sixty extracted premolar teeth without visible caries or structural defects on enamel surface were used, immersed in 10 ml of a demineralizing solution for 2 weeks to create artificial white spot lesions, they were randomly allocated into two groups; group 1: nHA, group 2: nFHA, each group is then subdivided into 2 subgroups (A and B) where two different in vitro remineralization procedures have been performed, the first procedure utilizes a 10 wt% nHA aqueous slurries only, the second was first exposed to irradiation from a fractional CO2 laser then (nHAP) was applied then laser was applied once again . Microhardness and color were measured using a micro-Vickers hardness tester and spectrophotometer respectively, then the surface topography was assessed using scanning electron microscope. Results: Specimens that were irradiated with fractional CO2LASER then treated with nanohydroxyapatite aqueous slurries showed the highest microhardness value, lowest color difference where ΔE was less than 3.3 units in the two groups, and SEM revealed a relatively smooth, more homogeneous surface compared to those in the groups treated with nanohydroxyapatite alone. Laser application after applying nanohydroxyapatite significantly decreased the surface microhardness and did not have a significant effect on the color. In all tests the pure type of nanohydroxyapatite gave better results than the nanofluroapatite type. Conclusion: Nano-hydroxyapatite has remarkable remineralizing effects on initial lesions of enamel, certainly higher when combined with laser application.