Impact of different polishing and aging procedures on the surface characteristics of bulk-fill resin composites

The aim of this study was to investigate the effects of different polishing systems and artificial aging procedures on the surface roughness, gloss, and microhardness of conventional and bulk-fill resin composites. One nanohybrid resin composite [Clearfil Majesty Esthetic (CME)] and three bulk-fill resin composites [SonicFill (SF), Opus APS (O-APS), and FiltekTM Bulk-Fill (FBF)] were evaluated. Ninety specimens were prepared for each material and subjected to 3 polishing protocols [Mylar strip (control), OneGloss, and OptiDisc]. Following polishing, the specimens underwent 3 different aging procedures: distilled water storage, thermocycling, and ultraviolet (UV) aging. Measurements for surface roughness (Ra), gloss (GU), and Vickers microhardness (VHN) were recorded at baseline (T0), after polishing (T1), and post-aging (T2). Atomic force microscopy images of 2 randomly selected specimens from each test subgroup. Data were analyzed using three-way ANOVA and Spearman’s correlation analysis. Statistically significant differences were observed among the materials, polishing systems, and aging procedures for all measured parameters (p < 0.001). FBF consistently exhibited the highest gloss and hardness values and the lowest surface roughness, while SF showed the poorest surface characteristics. The interaction of control × distilled water resulted in superior surface properties. Significant two-way and three-way interactions were found among material, polishing, and aging variables (p < 0.05). Strong negative correlations were identified between surface roughness and gloss values at all time points (r = -0.717 to -0.850), whereas a very weak positive correlation was noted between surface roughness and hardness after aging (r = 0.173, p = 0.001). Material composition, polishing approach, and aging conditions significantly affected the surface behavior of the resin composites. Among the tested materials, FBF exhibited the most stable microhardness, gloss, and surface smoothness, while SF showed the greatest surface deterioration. Multi-step polishing (OptiDisc) maintained smoother and glossier surfaces than the single-step system (OneGloss). Thermocycling and UV aging produced the most adverse effects on surface quality. The proper combination of a stable bulk-fill composite and a multi-step polishing system is therefore recommended to achieve optimal long-term clinical performance.