Effects of Topical Fluoride Prophylactic Agents on the Mechanical Properties of Orthodontic Nickel-Titanium Closed Coil Springs and Stainless Steel Closed Coil Springs

Abstract

The purpose of this study was to investigate the effects of topical fluoride prophylactic agents on the mechanical unloading properties of nickel-titanium (NiTi) and stainless steel (SS) closed coil springs. Spring were stored at 37˚C under static load in phosphate buffered saline (PBS) and treated with either neutral sodium fluoride (NaF) or acidulated phosphate fluoride (APF) five days per week for two minutes. Mechanical testing was done in a dH₂O bath at 37˚C at 0-, 1-, 4-, 8-, and 12 weeks. Unloading forces for NiTi and SS springs were measured at 9-, 6-, and 3 mm and 2-, 1.5-, and 1 mm, respectively. Scanning electron microscopy was used to evaluate surface topography of selected springs after 12 weeks. Based on a 1-Factor ANOVA and Dunnett’s post hoc, 3M NiTi springs showed a significant decrease (p <0.01) in the unloading force at each extension following exposure to both fluoride treatments, but only after 12 weeks. The AO NiTi springs showed a significant decrease in unloading force at each extension after 12 weeks following exposure to NaF. However, with SS springs, there was no significant effect of either fluoride treatment on the SS springs at any extension or time point. SS also springs showed no significant surface topography changes, irrespective of storage conditions, which correlates with the lack of fluoride effects on SS mechanical property effects. In contrast, while there were NiTi surface topography changes (pitting and mottling) following PBS+APF exposure, those changes could not be directly linked to the observed changes in mechanical properties. Results suggest topical fluoride used with NiTi springs could potentially lead to prolonged treatment time due to decreased unloading properties. However, topical fluoride used with SS springs should not affect treatment duration.