The effect of phosphoric acid concentration on resin tag length and bond strength of a photo-cured resin to acid-etched enamel

Objectives: To determine the relationship between depth of penetration and tensile bond strength of a photo-cured resin to phosphoric acid etched enamel, and the efficacy of enamel etchants that are less aggressive than a concentration of 10% H3PO4. Methods: The tensile bond strength and length of tags produced by a photo-cured (20 s) resin consisting of pre-polymerized TMPT/silica in 3% HNPM–TEGDMA on acid-etched enamel was determined. The enamel etchants tested were various concentrations (3–65%) of phosphoric acid. The resin was applied to enamel samples that had been abraded with No. 600-grit SiC paper and acid etched (30 s) to create test specimens that were loaded to fracture on a testing device. The HCl-treated, then cut specimens, were examined under scanning electron micrcoscopy and light microscopy. Results: The tensile bond strength (10 MPa) of resin to enamel, pre-treated with various acid concentrations did not vary significantly. But resin tag length was found to decrease significantly from 22 μm for 35% H3PO4 to 12 μm for 20% H3PO4 to 9 μm for 5, 10 and 65% H3PO4 to 5 μm for 3% H3PO4. Significance: These findings suggest that the length of the tags created by the tested photo-cured resin on phosphoric acid-etched enamel contributes little to the bond strength of the test specimens, and that the adhesive strength of the resin to H3PO4 etched enamel is mainly attributable to the resinʹs ability to penetrate between the enamel crystallites and rods. Further, enamel pre-treatment by phosphoric acid etchants of concentrations lower than 10% may be satisfactorily employed. The use of less aggressive acid concentrations might minimize any potential adverse effects to enamel substrates.