Adaptation and Bonding of Bulk-Fill Composites in Deep Preparations

Polymerization shrinkage in resin-based composites can lead to gap formation at the tooth-restoration interface, potentially compromising the long-term success of restorations. Bulk-fill composites have been developed to reduce shrinkage stress, but their adaptation and bond strength-especially in deep cavities-remain areas of concern. This study investigated the adaptation and bond strength of a newly developed dual-cure bulk-fill composite in 4 mm deep preparations compared to light-cured and self-adhesive bulk-fill composites in six groups. Standard composite molds were used to observe and measure sealed floor area (SFA%) of the composite after the polymerization process under optical coherence tomography (OCT) imaging. Micro-tensile bond strength (MTBS) testing was conducted in extracted human teeth. OCT showed that the prototype dual-cure composites had the lowest gap formation during polymerization (SFA 91%), while the self-adhesive composite demonstrated the highest debonding from the cavity floor (SFA 26%, <i>p</i> < 0.001). For MTBS analysis, the lowest mean bond strength was recorded for the self-adhesive composite (~21 MPa) and the highest for a light-cured bulk-fill (~50 MPa, <i>p</i> < 0.05). Overall, the dual-cure bulk-fill composites exhibited less gap formation than the light-cured ones. The prototype dual-cure material with 90 s waiting before light-curing showed the best adaptation. However, these differences were not reflected in the bond strength values to the cavity floor dentin using the universal adhesive used in the current study, as the light-cured composite showed the highest bond strength values. The self-adhesive composite showed the poorest results in both experiments, indicating that the application of a bonding system is still necessary for better adaptation and bonding to the cavity floor dentin.