Microleakage of Class II resin composite restorations restored with conventional, bulk-fill, and short fiber-reinforced composites with or without polyethylene fiber reinforcement: a comparative in vitro study

Microleakage remains a major concern in Class II resin composite restorations and is associated with postoperative sensitivity, marginal discoloration, secondary caries, and restoration failure. Various strategies, including the use of bulk-fill composites and fiber reinforcement, have been proposed to reduce polymerization shrinkage stress and improve marginal sealing. This study aimed to evaluate the effect of composite type and polyethylene fiber reinforcement on the microleakage of Class II resin composite restorations. This in vitro study was conducted on 40 extracted human premolars randomly assigned to five groups ( n = 8): conventional composite, conventional composite with polyethylene fiber, bulk-fill composite, bulk-fill composite with polyethylene fiber, and short fiber-reinforced composite (SFRC). Standardized Class II cavities were restored according to the respective protocols. Following thermocycling (500 cycles, 5–55 °C), microleakage was evaluated using 2% methylene blue dye penetration under a stereomicroscope (×10). Data were analyzed using the Kruskal–Wallis test followed by Dunn’s post hoc test (α = 0.05). A significant difference in microleakage scores was observed among the study groups ( P < 0.001). The bulk-fill composite group demonstrated the highest microleakage scores, whereas the SFRC group showed the lowest scores. Polyethylene fiber reinforcement reduced microleakage in both conventional and bulk-fill composites. Significantly higher microleakage was observed in the bulk-fill composite group than in the conventional composite reinforced with polyethylene fiber (CPF) and SFRC groups. No statistically significant difference was observed between SFRC and conventional composite reinforced with polyethylene fiber. Composite type significantly influenced microleakage in Class II restorations. Short fiber-reinforced composite demonstrated the most favorable marginal sealing performance among the evaluated materials. Polyethylene fiber reinforcement was associated with a reduction in microleakage, although the magnitude of improvement varied according to the restorative material.