Effect of fluoride on nickel titanium orthodontic wires
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Fluoride concentration and pH level affect corrosion resistance of NiTi orthodontic wires, but it was not determined which factor is responsible for lowering the resistance to corrosion. Most studies examined the effect of fluoride in low pH conditions, neglecting the fact that both factors contribute to the corrosion of NiTi orthodontic wires. Therefore, all experimental groups in our study had the same fluoride concentration and a different pH levels. Moreover, a control group composed of both components; sodium fluoride and lactic acid was added to the experiment. Thus, an attempt to establish a better understanding of the isolated effect of fluoride and pH level on the corrosion resistance and metal ion release of NiTi orthodontic wires. We have also compared two commercially available mouth rinses; ACT for children and ACT for adults. Both mouth rinses contain the same fluoride concentration and different pH levels. Different approaches were used to evaluate the effect of fluoride on NiTi orthodontic wires. The first approach was to obtain Young's modulus (E) using a universal testing machine. The results showed that natural saliva (pH=8.13), sodium fluoride (pH=6.21) and ACT for children (pH=6.05) groups were significantly different from the acid (pH=2.50), fluoride and acid (pH=3.08) and ACT for adults (pH=4.42) groups. It is also indicated that, in acidic environment, the ability of NiTi orthodontic archwires to transfer the applied load is higher. In other words, the differences in the pH levels among the groups had influenced the stiffness of the wires. The groups with low pH levels; acid (pH=2.50), fluoride and acid (pH=3.08) and ACT for adults (pH=4.42) groups showed stiffer wires than the groups with high pH levels; natural saliva (pH=8.13), sodium fluoride (pH=6.21) and ACT for children (pH=6.05) groups. The next approach was used for the analysis of metal ion concentration in the immersion medium using an inductively coupled plasma mass spectrometer (lCP-MS). The results suggest that except for fluoride & acid group (pH=3.08) that showed traces of Ni & Ti, all groups showed absence of ion metal release in term of Ni or Ti. Both Ni, Ti ions were detected above 0.4 ppb detection limit but below the lowest concentration in the calibration curve,1ppb.