Evaluation of MDP and NaF in two-step self-etch adhesives on enamel microshear bond strength and morphology of the adhesive-enamel interface

Purpose: To evaluate the roles of MDP and NaF in the bonding resin of a two-step self-etch adhesive (2-SEA) on enamel bonding performance. Materials and Methods: The experimental 2-SEAs were composed of an MDP-containing self-etching primer and four different bonding resins which were MDP- and NaF-free (00), MDP-containing and NaF-free (M0), MDP-free and NaF-containing (0F), and MDP- and NaF-containing (MF) adhesives. Microshear bond strength (μSBS) to enamel and ultimate tensile strength (UTS) of the adhesive resin were determined. The morphological characteristics of the adhesive- enamel interface after acid-base challenge were observed using SEM to evaluate the acid-base resistant zone (ABRZ). Results: No decrease in μSBS was observed even after thermocycling (TC) in 00, 0F and MF. However, the μSBS of M0 significantly decreased after TC. The UTS of M0 significantly decreased after 1- and 180-day storage in water. SEM observations indicated that ABRZ was present in all groups. However, formation of erosion beneath the ABRZ was observed in M0 and 00, whereas formation of a slope without erosion was observed at the bottom of the ABRZ in MF. Conclusion: In the experimental 2-SEAs, the addition of MDP alone to the adhesive resin did not contribute to an improvement in enamel bonding durability and interfacial morphology, whereas the addition of NaF in the adhesive resin demonstrated an enhancement of the bonding durability and improvement of acid resistance at the bonded interface.