Impact of pH and application time of meta-phosphoric acid on resin-enamel and resin-dentin bonding

Abstract

Purpose: To evaluate the immediate microshear resin-enamel bond strength ( μ SBS) and the immediate and 6- month microtensile bond strength ( μ TBS) and nanoleakage (NL) of the adhesive interface performed by di ff erent pHs of 40% meta-phosphoric acid (MPA) were compared with conventional 37% ortho-phosphoric acid (OPA) under di ff erent application times. Additionally, the enamel etching patterns were evaluated and the chemical/ morphological changes induced by these di ff erents groups were evaluated. Materials and methods: One hundred and ninety-eight extracted human molars were randomly assigned into experimental groups according to the combination of independent variables: Acid [37% ortho-phosphoric acid (OPA), 40% meta-phosphoric acid (MPA) at pHs of: 0.5, 1 and 2] and Application Time [7, 15 and 30 s]. Enamel- bond specimens were prepared and tested under μ SBS. Resin-dentin beams were tested under μ TBS tested im- mediately or after 6-months of water storage. Nanoleakage was evaluated using bonded-beams of each tooth/ time-period. Enamel etching pattern and chemical and ultra-morphology analyses were also performed. The μ SBS (MPa) data were subjected to a two-way repeated measures ANOVA (Acid vs. Application time). For μ TBS, Acid vs application time vs storage time data were subjected to three-way ANOVA and Tukey's test ( α = 0.05). Results: MPA pH 0.5 showed μ TBS similar to OPA, independently of the application time on enamel (p > 0.05) or dentin (p > 0.05). OPA provided higher nanoleakage values than MPA (p = 0.003). Signi fi cant decreases in TBS and increases in NL were only observed for OPA after 6 months (p = 0.001). An increase in the application time resulted in a more pronounced etching pattern for MPA. Chemical analysis showed that dentin deminer- alized by MPA depicted peaks of brushite and octacalcium phosphate. MPA exposed less collagen than OPA. However, optimal results for MPA were dependent on pH/application time. Conclusion: The use of 40% meta-phosphoric acid with a pH of 0.5 is an alternative acid-etching agent for dentin and enamel bonding. Furthermore, the use of MPA preserves the resin-dentin interface over a 6-months period, due to presence of brushite and octacalcium phosphate and a reduced demineralization pattern.