Effect of composite type and filling techniques on surface smoothness, cervical microleakage, and marginal adaptation in class II restorations

Abstract Background Given the prevalence of proximal caries and the challenge of achieving durable marginal seals in Class II restorations, this in vitro study aimed to compare the surface smoothness, cervical microleakage, and marginal adaptation of Class II restorations using several different composites and filling techniques under simulated clinical conditions. Methods Eighteen human molars were selected, and 36 standardized Class II cavities were prepared (two cavities per tooth). The teeth were randomly assigned into six groups ( n = 6 cavities/group), which contains three single-composite restoration groups and three hybrid-composite restoration groups: conventional paste type composite layered restoration (S-paste), sonic activated bulk-fill composite restoration (S-sonic), highly filled flowable resin composite layered restoration (S-flow), sandwich technique, glass-ionomer cement liner and paste-type composites (H-GIC), bulk-fill resin liner combined with paste type composite (H-SDR), highly filled flowable resin liner with paste-type composite layered restoration (H-flow). A novel biomechanical model simulating proximal contact and rubber dam isolation was used. After thermocycling, restorations were evaluated for clinical performance, including surface smoothness (FDI criteria), cervical microleakage (ISO scoring), and marginal gap area via scanning electron microscopy (SEM). Results For surface smoothness, the H-flow group demonstrated optimal FDI score (100% score 1, p < 0.05), while the S-paste and H-SDR groups exhibited noticeable porosity. For marginal adaptation (gap area), the S-paste group showed the smallest gaps (1.70 ± 2.16 μm²), which was significantly better than other groups. Cervical microleakage was lowest in S-paste (67% score 0) and H-GIC (50% score 0). No single technique was superior across all outcomes. Conclusions Conventional incremental layering (S-paste) provided the best marginal adaptation, while the highly filled flowable liner technique (H-flow) offered superior surface smoothness. The choice of technique should be guided by clinical priority (marginal seal vs. surface finish). Clinical validation is required before strong recommendations can be made.