| BackgroundFluoride varnish is a widely used topical fluoride in western countries as a slow-releasing fluoride material for caries prevention, most for children and population in high-risk of dental caries. Fluoride varnish is mainly composed of fluoride, film-forming material and organic solvent. After applied to teeth surface, the varnish can stay for a period of time and during this time, it slowly releases fluoride ions that can react with enamel to create CaF2 deposition on enamel surface and FAP in enamel which can both decrease demineralization and increase remineralization. But the contact time for fluoride varnish on the enamel surface is still limited, usually less than 24 hours, which leaded to limited anti-demineralization property of the varnish. And all these properties is closely related to the film-forming material. Therefore, film-forming material still needs further study to improve the overall performance of fluoride varnish.ObjectivesTo study the influence of film-forming materials of fluoride varnishes on the fluoride uptake of enamel and the protective effect of fluoride varnish containing different film-forming materials on demineralization of bovine enamel by microhardness and morphology study. All the study can provide the selection basis of film-forming materials for the development of a new fluoride varnish.Materials and methods1. The assay of the fluoride uptake of bovine enamel treated with fluoride varnishes with different film-forming materialsFive fluoride varnishes containing the same fluoride and one film-forming material(osmotic acrylic resin, copal resin, polymerized rosin, polyvinyl butyral, ethylmethacrylate copolymer resin respectively) were prepared. Cylindrical enamel blocks were drilled from fresh bovine incisors and imbedded in one end of plastic rods. The surface of the embedded enamel was coated with one of the five fluoride varnish for 12 hours. Then the fluoride varnish coating was removed using acetone without any damage of enamel structure. The calcium fluoride precipitated on the enamel surface was dissolved using 1.0mol KOH solution and the fluoride in it was measured with a fluoride-ion selective electrode and the amount of the KOH-soluble fluoride per unit area on the enamel surface was calculated (μg/cm2). The samples of the enamel in surface layer(within 50μm and 100μm respectively) were obtained using a microdrill and was dissolved in perchloric acid solution for measuring the structurally bound fluoride (μg/cm) using a fluoride-ion selective electrode.2. Microhardness study on the anti-demineralization effect of fluoride varnishes with different film-forming materialsEnamel blocks (4mm×4mm×3mm) cut from freshly extracted bovine incisors using low speed saw, were embedded using PMMA with labial surface exposed, followed by polishing using water-proof sandpaper. Then the baseline surface microhardenss(SMH1) of each enamel block was determined and 35 blocks with SMH1 ranging from 320kg/mm2～370kg/mm2(Knoop hardness) were selected and randomly distributed into 7 groups with 5 blocks each.1-5 groups were varnish groups and the 6th group got no varnish treatment while the 7th group got no treantment as a sound enamel control group. The buccal surface of each block of 1-5 groups was treated with one of five fluoride varnishes(containing osmotic acrylic resin, copal resin, polymerized rosin, polyvinyl butyral, ethylmethacrylate copolymer resin as film-forming material respectively). The varnishes were applied onto the enamel surfaces in a thin layer and then kept in 100% humidity for 12h, which were then removed and then all blocks of 1-6 groups were subjected to a pH-cycling treatment for 7 days. After pH-cycling, surface microhardness(SMH2) of these blocks of 6 groups were measured again. And the percentage loss of surface microhardness (%SMHC) was calculated. Next, all the blocks were longitudinally sectioned through the center. Half of each block was used for cross-sectional microhardness(CSMH) determination and was embedded in acrylic resin so that the sectioned enamel block could be exposed and polished. The indentations were made at 10μm,30μm,50μm, 70μm,90μm, 110μm,220μm,330μm from the outer enamel surface.3. Morphology study on the protective effect of fluoride varnishes composing different film-forming materialsFollowing the above study, the other half of all the blocks were used for morphology study to assess the buccal surface change by SEM and colour 3-D laser scanning microscope. Finally surface roughness(Sa) was used to evaluate the protective capability of fluoride varnishes containing different film-forming materials for the surface structure of bovine enamel.Results1. The amounts of KOH-soluble fluoride of the enamel surfaces coated with the five fluoride varnishes were different and correlated with the film-forming materials. The structurally bound fluoride, both within 50μm and 100μm showed a similar results. And the fluoride varnish with osmotic acrylic resin and copal resin had more fluoride reaction products as CaF2 and structurally bound fluoride.2. All fluoride varnish groups containing different film-forming materials significantly reduced demineralization compared to the no varnish group (P<0.05). CSMH results showed that the fluoride ions in the varnish groups could be absorbed until 90μm depth from the outer enamel surface. And osmotic acrylic resin group had an obvious protective effect on %SMHC and CSMH until 50μm depth (P<0.05)3. The no varnish group showed severe demineralization under SEM with characteristic of badly damaged structure of enamel. Diffusion of enamel structure could be seen everywhere. And all the varnish groups showed a protective effect on the surface enamel and they had different etching patterns. Again the fluoride varnish with osmotic acrylic resin had a better protection for enamel surface structure than other varnish groups. Similar results were found from 3-D laser scanning microscope, and osmotic acrylic resin group presented a lowest Sa (P<0.05)ConclusionThe type of film-forming material of fluoride varnish does not only affects the fluoride uptake of enamel surface, but also affects the resistance of demineralization for enamel and surface protection. Osmotic acrylic resin is more suitable for being a film-forming material for fluoride varnish. |
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