The Effect of Topical Fluoride Agents on Coated Nickel-Titanium Archwires

Abstract

This study examined the effect of exposure to topical fluoride agents on the mechanical properties and surface characteristics of esthetic coated nickel-titanium archwires. Two types of coated wires were tested: one with a polymer coating and one with a rhodium ion coating. Wires were divided into treatment groups based on type of fluoride exposure: no treatment (DI water), neutral sodium fluoride gel, neutral sodium fluoride rinse, or acidulated phosphate fluoride (APF) gel. A three-point bend test in DI water at 37±1°C was performed on specimens before and after test solution exposure. Unloading forces at 1, 1.5, and 2mm of deflection were reported, along with unloading elastic modulus and yield strength. Three representative specimens from each treatment group along with untested wires underwent qualitative scanning electron microscopy (SEM) surface topography analysis following test solution exposure. Results of the present study indicated no significant difference between treatment groups for any mechanical properties measured (p > 0.05) after fluoride exposure. However, significant differences were observed between wire types for all measures within all treatment groups (p < 0.05). Rhodium-coated wires exhibited significantly lower unloading forces, lower yield strength, and higher elastic modulus than polymer-coated wires. This may be due to effects of the coatings or due to differences in the underlying nickel-titanium. SEM analysis revealed the polymer coating peeled off in areas of contact with testing apparatus. With polymer-coated wires, exposed underlying nickel-titanium exhibited pitting corrosion after APF gel treatment, which did not occur with other fluoride groups or the DI water group. Similarly, with rhodium-coated wires exposed to APF gel, corrosion pitting appeared to go through the rhodium coating into the underlying NiTi wire, which did not occur with other fluoride groups or the DI water group. The lack of degradation of mechanical properties of wires exposed to fluoride in this study suggests a potential protective effect of coatings on nickel-titanium. However, polymer coating instability could pose a problem by allowing underlying nickel-titanium to come into contact with fluoride when used in the mouth. Further research is needed to determine whether this protective effect continues even after substantial coating degradation.