Marginal Gap Formation in Class II Micro-Hybrid and Bulk-Fill Composite Restorations Using Different Matrix Systems

Objectives: Composite resin restorations are widely used due to their favorable mechanical properties and aesthetics. However, polymerization shrinkage may compromise marginal adaptation, particularly in Class II cavities. This study evaluated the influence of Metal (MM) and Transparent Matrices (TM) on marginal gap formation in micro-hybrid and bulk-fill composite restorations. Materials and Methods: Forty caries-free extracted molars were randomly allocated into four groups (n = 10). Cavities were restored using either micro-hybrid composite with centripetal layering or bulk-fill composite with bulk-filling technique, combined with MM or TM (Tofflemire). Marginal adaptation was assessed using Scanning Electron Microscopy (SEM) after artificial aging and thermocycling. Statistical analysis was performed with paired and unpaired t-tests (α = 0.05), and effect sizes were calculated. Results: MM groups demonstrated superior marginal adaptation in dentin (ES = 0.93, p = 0.005) and enamel (ES = 0.76, p = 0.02) compared with TM. In bulk-fill composites, matrix type showed no significant effect (p = 0.16). Micro-hybrid restorations with MM exhibited a large effect size (ES = 1.48, p<0.001) compared to TM. No significant enamel margin differences were observed between composites (p = 0.89). Conclusion: Matrix choice significantly influenced marginal quality in micro-hybrid restorations, but not in bulk-fill composites. Bulk-fill composites demonstrated more consistent dentin adaptation, suggesting reduced matrix dependency.