Degree of Conversion Contributes to Dentin Bonding Durability of Contemporary Universal Adhesives

Purpose: The objectives of this study were to evaluate the micro-tensile bond strength (mu TBS) of five contemporary universal adhesives to dentin after 24 hours and thermocycling (TC), to measure their degrees of conversion (DC) and to test the correlation between mu TBS and DC. Methods and Materials: Four commercially available universal adhesives, Prime&Bond universal (PBU), Ecosite Bond (EB), G-Premio Bond (GPB), and Clearfil Universal Bond Quick (UBQ), and one experimental adhesive, UBQ without an amide monomer (UBQ-A), were used in this study. For the mu TBS test, midcoronal dentin of 50 human molars was exposed, ground using 600-grit SiC paper, and the adhesives were applied according to the manufacturers' instructions. After resin-composite buildup and 24-hour water storage, onehalf of the specimens were subjected to 15,000 thermal cycles. The specimens were sectioned into beams and stressed in tension at a crosshead speed of 1 mm/min until failure. The DC of adhesives applied to dentin was evaluated using attenuated total reflectance Fourier-transform infrared spectroscopy immediately after light-curing. All data were statistically analyzed at a significance level of 0.05. Results: The highest mu TBSs were obtained with UBQ, UBQ-A, and PBU, which were not significantly different from each other both after 24 hours and TC. The mu TBS of GPB was lower compared with the aforementioned adhesives, but significantly only after TC, and the lowest mu TBSs were obtained with EB. TC did not affect the mu TBSs of UBQ, UBQ-A, and PBU significantly, but a significant decrease was observed with GPB and EB. The highest DC was obtained with PBU and UBQ, followed by 2-hydroxyethyl methacrylate-rich adhesives UBQ-A and EB, which exhibited significantly lower DCs. The DC of GPB could not be determined because the reference peak at 1608 cm(-1) was not detected in its spectra. A significant positive correlation was shown between mu TBS and DC after 24 hours (r=0.716) and TC (r=0.856). Conclusion: mu TBS and DC were positively correlated, more markedly after TC, which suggests that DC may be an important factor for bond durability.