Effect of Simulated Oral Aging on Surface Roughness and Microhardness of Bulk-Fill Composite Resins

Introduction: The goal of this study was to evaluate the influence of combined artificial aging protocols on the surface roughness and Vickers microhardness of bulk-fill resin composites, compared with a nanofilled composite used as a reference. Materials and Methods: A total of 120 cylindrical specimens were prepared from three bulk-fill composites (Tetric EvoCeram Bulk Fill, Filtek One Bulk Fill, Venus Bulk Fill) and one nanofilled composite (Filtek Supreme Ultra). Specimens were allocated into three aging conditions: mechanical wear (A), mechanical wear combined with pH-cycling (B), and mechanical wear combined with thermocycling (C). Surface roughness (Ra) and Vickers microhardness (VHN) were evaluated at two time points (T1: 120,000 cycles; T2: 240,000 cycles). Non-parametric statistical tests were applied (α = 0.05). Results: Aging protocols significantly influenced both Ra and VHN (p < 0.05). Overall, higher surface roughness and lower Vickers microhardness values were observed after cumulative aging, with material-dependent variations between T1 and T2. The greatest post-aging differences were observed under combined mechanical wear and pH-cycling (subgroup B), whereas mechanical wear alone showed the lowest changes. Filtek One Bulk Fill and Filtek Supreme Ultra showed more favorable post-aging Ra and VHN values, whereas Venus Bulk Fill showed less favorable post-aging surface properties. No significant correlation was found between Ra and VHN (rho = -0.009; p = 0.958). Conclusions: Combined aging conditions significantly affected the surface roughness and Vickers microhardness of resin composites, with the greatest post-aging differences observed under acidic challenges. Bulk-fill materials exhibit variable resistance depending on composition, emphasizing the importance of material selection for long-term clinical performance. Clinical relevance: Composite restorations exposed to combined mechanical and acidic challenges may show altered surface roughness and microhardness, highlighting the need for materials with enhanced resistance in high-risk oral environments.