| Background:NiTi archwires have been widely used for initial alignment of malocclusions mainly because of their unique properties of superelasticity and shape memory. NiTi archwires also have excellent biocompatibility and corrosion resistance. However, white spot lesions of teeth around brackets occurred during orthodontic treatment could result in caries. The prevalence of white spot lesions in patients who underwent fixed orthodontic treatment is about 50%. Therefore, it is considered as a main problem for application of fixed orthodontic treatment. Researches have shown that some fluoride products (gels, tooth paste and mouth rinse) can significantly prevent the development of enamel demineralization and white spot lesions of teeth around brackets.Objective:This study is to investigate the effect of fluoride on the mechanical properties of NiTi archwires and the change of surface morphology, unloading force and microhardness in artificial saliva (AS) of different pH value, concentration of NaF and immersing time.Methods:Straight NiTi wire specimens were obtained by cutting out 20 mm sections from the posterior segments of 0.016'heat-activated NiTi wires. According to the pH value of the AS, the concentration of NaF and immersing time, all the 72 specimens were randomly divided into 9 groups which include 1 control group and 8 experiment groups. Each group contains 8 specimens. Control group:specimens were kept out of AS.Experiment groups:Group A:specimens were immersed in AS of pH 4 and Owt% NaF for 1 day.Group B:specimens were immersed in AS of pH 4 and 0.05wt% NaF for 1 day.Group C:specimens were immersed in AS of pH 4 and Owt% NaF for 3 days.Group D:specimens were immersed in AS of pH 4 and 0.05wt% NaF for 3 daysGroup E:specimens were immersed in AS of pH 6 and Owt% NaF for 1 day.Group F:specimens were immersed in AS of pH 6 and 0.05wt% NaF for 1 day.Group G:specimens were immersed in AS of pH 6 and Owt% NaF for 3 days.Group H:specimens were immersed in AS of pH 6 and 0.05wt% NaF for 3 days.The surface morphology, microhardness and unloading force of all the species were observed by SEM, vickers hardness tester and three-point bending test.Results:1. Surface morphology:There were many small dots in the surface of NiTi wires which were immersed in the AS with Owt% NaF (group A, C, E, G) indicating that pit corrosion occur on the surface of NiTi wires. Besides pit corrosion, general corrosion was observed on the surface of species which were immersed in the AS with 0.05wt% NaF (group B, D, F, H). The corrosion was more seriously in pH=4 solution (group A, B, C, D) than in pH=6 solution (group E, F, G, H).2. Unloading force:All the experimental groups have significant difference with the control group except group E and group F. PH value is the main factor that can significant effect the unloading force of NiTi wires (p<0.001). There is also interactive between pH value and immersing time. The interactive can also effect the unloading course of the NiTi wires (P< 0.001). The unloading force of the NiTi wires increase with the pH value when they were immersed in the AS of the same dose of NaF and immersing time (P< 0.05).3. Microhardness:The microhardness of the experimental groups was significantly increased comparing to the control group (p<0.05). The pH value is the main factor that can significant effect the microhardness of NiTi wires (p<0.001). There is also interactive between the concentration of NaF and immersing time. The interactive can also effect the microhardness of the NiTi wires (P< 0.001). The microhardness of the NiTi wires increase with the pH value decreasing when they were immersed in the AS of the same dose of NaF and immersing time (P< 0.05).Since all the data were distributed normally by Kolmogorov-Smirnov test, the data from the microhardness test and three-point bending test were analyzed by Univariate Analysis of Variance factor, Post Hoc Tests multiple comparison tests followed LSD test at a confidence level of 95%.Conclusion:The surface morphology of NiTi wires change significantly in pH=4 groups. There were pit corrosion and general corrosion on the surface of 0.05wt% NaF groups (group B, D, F, H), while there was only pit corrosion on the surface of on the species of 0wt% NaF (group A, C, E, G). The unloading force of the NiTi wires increased with the raise of pH value, while, the microhardness of the NiTi wires increased with the pH value decreasing.The pH value is the main factor that can significantly effect the unloading force and microhardness of NiTi wires (p<0.001). Fluoride can exist in the form of NaF or HF in acid environment and destroy the oxide layer.
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