Structural Integrity of Particulate and Fiber-Reinforced Bulk-Fill Resin Composites

Background: This study evaluated long-term water sorption, solubility, and hygroscopic expansion of bulk-fill resin composites (BRCs), with and without fiber reinforcement, at 37 °C, focusing on the influence of material composition on stability in aqueous environments. Methods: Five resin composites were tested: fiber-reinforced everX Flow (EVX); particle-reinforced SDR, Filtek Bulk-fill Flowable Restorative (FBF), and Filtek Bulk-fill Posterior Restorative (FBP); and conventional Filtek Z350 XT (FZ). Nine disk-shaped specimens per material were evaluated for water sorption and solubility according to ISO 4049. Hygroscopic dimensional changes were measured using laser scanning. Filler mass and degree of conversion were also assessed. Results: After 80 days, water sorption ranged from 13.79 to 30.13 μg/mm3. FZ showed the highest sorption and EVX the lowest. Flowable bulk-fill materials had lower sorption than packable materials. Solubility ranged from −3.10 to 0.73 μg/mm3. Hygroscopic expansion ranged from 0.64% to 1.51%, with EVX showing significantly lower expansion and FZ the highest. Conclusions: Short E-glass fiber-reinforced flowable composite showed lower hygroscopic expansion, yielding superior dimensional stability. Additionally, material solubility was highly matrix-dependent, with TEGDMA-free formulations demonstrating the lowest values. Conversely, applying the conventional composite beyond recommended increment depths severely impaired monomer conversion, which directly correlated with maximal water sorption and hygroscopic expansion.