Introduction: This study aimed to investigate the effect of different surface treatments on the bond strength of resin cement to nickel-chrome (Ni-Cr) alloy. Methods: Forty disk-shaped specimens of Ni-Cr alloy were prepared and divided into 4 groups. In the first group, the specimens’ surface was sandblasted with 50 µ Al2 O3 particles. In the second group, the specimens were prepared with the Er:YAG laser. In the third group, the specimens were prepared using the Er:YAG laser after sandblasting. In the fourth group, the specimens’ surface was covered with a thin layer of MKZ metal primer after sandblasting. Then the cylinders of composite resin were bonded to the treated metal surfaces using Panavia F2.0 resin cement. All of the samples were subjected to 2000 thermal cycles. The shear bond strength was tested using a universal testing machine at the crosshead speed of 0.5 mm/min. The failure mode was also observed by a stereomicroscope. Data were analyzed using the one-way ANOVA and the Tukey HSD test at a significance level of 0.05. Results: The shear bond strength from the highest to the lowest were as follows: the Er:YAG laser group, the sandblast and MKZ primer combination group, the sandblast group, and the sandblast and Er:YAG laser combination group. The mean differences of shear bond strength between the Er:YAG laser group and the sandblast group (P=0.047) and also between the Er:YAG laser group and the sandblast and Er:YAG laser combination group (P=0.015) were statistically significant. Conclusion: Among the different surface treatments employed in this study, Er:YAG laser treatment increased the shear bond strength between the metal alloy and the resin cement (Pavnavia F2).

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Background: Color change is one major drawback of tooth-colored resin-based restorations. Objectives: This study aimed to assess the color stability of three commonly used resin-based restorative materials upon exposure to tea and coffee. Materials and Methods: Discs were fabricated from Spectrum TPH (Dentsply/Caulk), Denfil (Vericom), and Filtek Z250 (3 M) microhybrid composites and immersed in coffee and tea solutions for two hours on the first day and the whole of the second, third, and fourth days. The color was assessed visually and recorded using the Lobene Stain Index after each period of immersion. The color change of the three composite resins was compared using the Kruskal-Wallis test, Mann-Whitney U test, and Friedman test. The level of significance was set at 0.05. The Cohen's Kappa was also calculated to assess inter-rater agreement. Results: The three composite resins showed statistically significant color changes after four days of immersion in a coffee solution (P = 0.014), but their color change in the tea solution was not significant (P > 0.05). A comparison of color changes in the composites after one (two hours) and four days of immersion in tea and coffee solutions revealed a significant difference in color changes between Spectrum TPH and the other two composites (P < 0.001). Conclusions:The three microhybrid composites used in this study showed variable color stability upon exposure to a coffee solution. The color stability of Spectrum TPH was inferior to that of Denfil and Filtek Z250.