Interfacial gap formation of class II composite restorations with proximal box elevation using bulk-fill materials: a micro-CT study

To evaluate the effect of proximal box elevation (PBE) using different bulk-fill resin composites on interfacial gap formation of Class II direct restorations at the cervical region using micro-computed tomography (micro-CT). Standardized two separate box-shaped Class II cavities were prepared on both mesial and distal surfaces of 25 sound human lower molars. Cavities were divided into five groups (n = 10) based on different bulk-fill composites used for PBE: (1) Group SB: sonic-activated bulk-fill, (2) Group EB: low-viscosity bulk-fill, (3) Group VB: thermo-viscous bulk-fill, (4) Group AB: bioactive dual-cure bulk-fill, and (5) Group FZ: no PBE (control)-conventional resin composite. A dual-cure universal adhesive was applied in self-etch mode. In PBE groups, a 3 mm bulk-fill layer was applied before layering conventional composite, while no PBE group was restored using conventional composite. Specimens underwent thermocycling (10,000 cycles, 5-55 °C). Micro-CT analysis measured interfacial gap formation based on radiolucent areas at the gingival floor of tooth-restoration interface. Data were analyzed using Welch ANOVA and Games-Howell post hoc tests (p < 0.05). Groups SB and VB showed significantly higher gap formation than Groups EB and AB, as well as Group FZ(control/no box elevation) (p < 0.05). No significant differences in gap formation were found between Groups EB, AB, and FZ (p > 0.05). PBE with low-viscosity and bioactive dual-cure bulk-fill composites effectively minimized interfacial gaps at the cervical region, suggesting their suitability for deep Class II restorations. PBE using low-viscosity bulk-fill composite resin and bioactive dual-cure bulk-fill composite resin could be preferred due to superior cavity adaptation in the cervical region compared to no proximal elevation.