Fluoride release from an orthodontic glass ionomer adhesive in vitro and enamel fluoride uptake in vivo

Abstract

The objective of this study was to investigate the in-vitro fluoride release from a glass ionomer orthodontic bonding system (Fuji I, GC Corporation, Tokyo, Japan) over a 2-month period and the in-vivo enamel fluoride uptake after 6 months. Ten metal brackets were bonded with either glass ionomer or composite resin (Transbond XT, 3M Unitek, Monrovia, Calif; Light Cure), which served as controls, to recently extracted molars. The bonded teeth, cut at the level of their roots, were stored in distilled water that was renewed after every fluoride measurement at 1, 3, 7, 30, and 60 days. The in-vitro fluoride release was measured by using a fluoride ion-selective electrode, connected to an ion analyzer. Fifteen pairs of premolars were bonded with metal brackets with either the Fuji or the Transbond adhesive. Six months later, the teeth were extracted for orthodontic purposes, embedded in resin, and cross-sectioned, and the fluoride compositions between the outer and bulk enamel surfaces were evaluated with scanning electron microscopy and energy dispersive analysis. The results were analyzed with nonparametric 1-way analysis of variance (ANOVA) on ranks for in-vitro fluoride release and nonparametric 2-way ANOVA on ranks for in-vivo fluoride enamel uptake; group differences were investigated with the Holm-Sidak test at the .05 level. The Spearman rank correlation coefficient test was used to investigate the association between fluoride and aluminum levels in the interfaces of the specimens bonded. The initial burst of fluoride release observed for the Fuji adhesive after the first day of the experiment had a significant decrease with time, and it persisted throughout the monitoring period (60 days) (P <0.05). Fluoride concentrations were found in both the outer and deeper enamel surfaces, with the outer sites having 4 times higher fluoride relative to the bulk for the glass ionomer (P <0.05), and higher fluoride was found in the outer layers for the glass ionomer bonded enamel specimens (P <0.05). However, the concurrent identification of aluminum and fluoride traces in the enamel implied that the source of this high fluoride concentration originated from cement particles and not from ionic uptake. The short-term fluoride release and the absence of documented enamel uptake suggest that the glass ionomer orthodontic adhesive tested might provide protective action only through the reservoir mechanism.