Effects of bonding materials on the mechanical properties of enamel around orthodontic brackets

Objective: To determine if the enamel around orthodontic brackets is significantly altered after demineralization followed by application of adhesives with and without fluoride-releasing ability. Materials and Methods: One hundred eight noncarious human premolars were divided into six groups of 18 each and exposed to a demineralization solution. Stainless steel brackets were bonded using two conventional composite resin etch-and-rinse systems, three self-etching primer (SEP) composite resin systems, and one resin-modified glass ionomer cement (RMGIC) system. One conventional and one SEP composite resin adhesive did not have fluoride-releasing ability, which was claimed for the other four adhesives. The elastic modulus and hardness of the enamel were determined with a nanoindenter at 10 equidistant depths ranging from 1-46 μm and at four regions: control (not exposed) enamel surface, under the adhesive, and at 50 μm and 100 mm from the bracket edges. Using the Kruskal-Wallis and Mann-Whitney U-tests (P <.0125 for statistical significance), these properties were compared at different regions. Results: The same behavior was observed for values of elastic modulus and hardness. Significant differences were found within approximately 21 μm of the enamel surface for etching with 35% phosphoric acid or priming with SEP, but only minimal changes occurred for the SEP adhesive. Increases in near-surface elastic modulus and hardness of enamel were found with the SEP adhesive and RMGIC with fluoride-releasing ability. Conclusions: Clinical use of the fluoride-releasing adhesives investigated may prevent demineralization of enamel around brackets during orthodontic treatment.