Restoring Marginal Integrity in Subgingival Class II Composite Restorations: Overcoming Polymerization Shrinkage Stress Through Incremental Stratification With Bulk-Fill Flowable Liners

Abstract

Polymerization shrinkage stress in posterior composite restorations remains a primary etiological factor in marginal gap formation, secondary caries, and postoperative sensitivity, particularly in subgingival Class II cavities where moisture control and bulk insertion are practically constrained. This study evaluated the efficacy of an incremental stratification protocol utilizing low-viscosity bulk-fill flowable liners in reducing cervical marginal microleakage compared to conventional oblique layering. Standardized Class II cavities were prepared in 60 extracted human premolars, assigned to four restorative protocols, and subjected to thermocycling, dye penetration, and micro-computed tomography (micro-CT) analysis. Results demonstrated that the flowable liner-based incremental protocol yielded statistically significant reductions in marginal gap width (p < 0.01) and dye penetration depth, establishing a clinically actionable approach for managing C-factor-driven interfacial stress in deep posterior restorations.