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Chemical stability of rhodium coated NiTi orthodontic wire in artificial saliva and commercial fluoride agents

Aims: Evaluation of chemical stability of rhodium coated nickel-titanium (NiTi) orthodontic wire in artificial saliva and commercial fluoride agents. Methods: Ten cm long specimen of rhodium coated (RhNiTi) wire was immersed in 10 ml of artificial saliva (1.5 g/l KCl, 1.5 g/l NaHCO3, 0.5 g/l, NaH2PO4–H2O, 0.5 g/l KSCN, 0.9 g/l lactic acid, pH=4.8) incubated at 37 °C. All experiments were performed in triplicate. Artificial saliva samples were collected after 3, 7, 14, 21, and 28 days of immersion. Amount of eluted Ni2+ and Ti4+ ions in artificial saliva from tested wires were measured by means of inductively coupled plasma- optical emission spectroscopy (ICP-OES), an axially viewed Thermo Elemental ‘‘IRIS Intrepid II” XSP, Duo, (Thermo Electron Corporation). In between collection of saliva, wires were immersed for 5 minutes at 37 °C to following commercial fluoride agents as imitation of regular weekly treatment: Elmex gelée (7500/5000 ppm of fluoride in form of amine fluorides/sodium fluoride), MI Paste Plus (900 ppm of fluoride in form casein phosphopeptide-amorphous calcium phosphate fluoride (CPP-ACPF)), and Mirafluor-k- gel (6150 ppm of sodium fluoride). After fluoridation, the wires were rinsed with distilled water and immersed to fresh artificial saliva. Also, chemical stability of RhNiTi wire was determined by immersion only to artificial saliva. The fluoride agents were chosen because of their commercial availability, identical methods of application, various fluoride ion concentrations, various chemical formulations, and differences in pH (Mirafluor-k-gel 5.1 ; Elmex gelée 5.5 ; MI Paste Plus 6.6). Results: Release of Ni2+ ions from Rh NiTi orthodontic wires decreases in following order: artificial saliva > MiPaste > Elmex > Mirafluor. Release of Ti4+ ions from Rh NiTi orthodontic wires decreases in following order: MiPaste > Mirafluor > Elmex > artificial saliva. Conclusion: Chemical stability of RhNiTi wires increases with increased fluoride concentrations because rhodium and fluoride form protective layer.

NiTi; corrosion; chemical stability; ICP-OES

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