Effect of fluoride prophylactic agents on the mechanical properties of nickel-titanium-based orthodontic wires

Background: Titanium-based alloys have high corrosion resistance because they form a thin, stable oxide layer. Nevertheless, fluoride prophylactic agents can cause corrosion and associated discoloration of titanium-based orthodontic wires. The purpose of this investigation was to study the effects of fluoride prophylactic agents on the mechanical properties of nickel-titanium (Ni-Ti) and copper-nickel-titanium (Cu-Ni-Ti) orthodontic archwires. Methods: Preformed rectangular Ni-Ti and Cu-Ni-Ti wires were immersed in either an acidulated fluoride agent, a neutral fluoride agent, or distilled water (control) for 1.5 hours at 37°C. After immersion, the loading and unloading elastic modulus and yield strength of the wires were measured with a 3-point bend test in a water bath at 37°C, in accordance with the criteria in the current American National Standard/American Dental Association Specification No. 32 for Orthodontic Wires (2000). Scanning electron microscopy was also used to characterize the effects of the fluoride treatment on the wire topography. Results: Unloading mechanical properties of Ni-Ti orthodontic wires were significantly decreased after exposure to both fluoride agents (1-way analysis of variance [ANOVA] and Dunnett's post hoc, α =.05); however, Cu-Ni-Ti wire mechanical properties were not significantly affected by either fluoride agent (1-way ANOVA, α =.05). Corrosive changes in surface topography were observed for both wires, with Cu-Ni-Ti appearing to be more severely affected. Conclusions: The results suggest that using topical fluoride agents with Ni-Ti wire could decrease the functional unloading mechanical properties of the wire and contribute to prolonged orthodontic treatment.