Effect of laser pulse energy and repair composite viscosity on the repair shear bond strength of aged bulk-fill resin composites

Background Repairing failed resin composite restorations remains clinically challenging, and the longevity of repaired restorations depends on achieving a strong interfacial bond. This study evaluated the influence of different surface treatments and repair composite types on the repair shear bond strength (RBS) of aged bulk-fill resin composites. Materials and methods One hundred and fifteen standardized Filtek™ One Bulk-Fill Restorative (3M) composite discs (4 mm × 10 mm) were thermocycled (5,000 cycles, 5–55 °C). Specimens were divided into five surface treatment groups: Er:YAG laser at 50 mJ (L50), 150 mJ (L150), and 250 mJ (L250), air abrasion (A), and diamond bur control (B). After surface treatment, a universal adhesive was applied, and three composite cylinders (2 mm × 2 mm) were built per specimen using either Filtek Z250 XT Nano-Hybrid (N), Filtek Bulk-Fill Flowable (F), or Filtek One Bulk-Fill Restorative (R) ( n = 10). Repaired samples were thermocycled again (×5,000) and tested for RBS. Surface roughness (Ra, Sa) was assessed by profilometry and AFM, and SEM micrographs were used to examine surface morphology. Data were analyzed using two-way and one-way ANOVA with post hoc tests ( α = 0.05). Results Surface treatment and its interaction with the repair composite type significantly influenced RBS ( p < 0.05), whereas the repair composite type did not significantly influence RBS ( p > 0.05). The highest RBS values were recorded for L250 and Air Abrasion, while L50 yielded the lowest across all composites. L250 showed the highest Ra and Sa values. AFM and SEM confirmed pronounced peaks, valleys, and microretentive pits in the L250 and Air Abrasion groups, whereas smoother surfaces were observed following bur and low-energy laser treatments. Conclusions High-energy Er:YAG laser (250 mJ) and air abrasion significantly enhanced repair bond strength by generating retentive surface topographies. The repair composite type had minimal influence, though low-viscosity materials performed better on highly ablated surfaces due to improved flow and adaptation.