Effect of Zein-Coated Magnesium Oxide Nanoparticle Incorporation on the Shear Bond Strength of Different Resin Composites After Thermocycling

Objectives: Studies have shown that nanoparticles can impart beneficial properties when integrated into resin composites; however, it remains unclear how this affects shear bond strength. This study aimed to evaluate the effects of composite formulation and zein-coated magnesium oxide nanoparticle (zMgO NP) concentration on shear bond strength (SBS) and failure mode following thermocycling. Methods: A total of 180 extracted human molars were randomly allocated among four composite formulations and three zMgO NP concentrations (0%, 0.5%, and 1%; n = 15). Composite cylinders were bonded to prepared dentin surfaces and thermocycled for 10,000 cycles between 5 °C and 55 °C. SBS was analyzed using two-way ANOVA and post hoc multiple comparisons with Tukey’s test. Failure modes were assessed stereomicroscopically and analyzed using the chi-square test. Results: The SBS of the modified composites in all groups significantly increased, with the 0.5% zMgO NP groups exhibiting the highest SBS. The 0.5% zMgO NPs N-Ceram group had the highest SBS overall. The unmodified Z350 composite exhibited the highest adhesive failure, while the control N-Flow composite demonstrated the highest cohesive failure. The modified conventional and flowable composites showed initially lower adhesive failure, which increased progressively as zMgO NPs increased. The bulk-fill composite initially showed higher adhesive failure, with the 0.5% group demonstrating lower adhesive failure and the 1% group exhibiting increased adhesive failure. Conclusions: Adding zMgO NPs to resin composites improved SBS after thermocycling, with material- and concentration-dependent differences in failure-mode analysis. Further validation studies are needed to verify clinical viability.