Objectives
Polymerization shrinkage in restored teeth is implicated in restoration longevity. This study explored how development of shrinkage and elastic modulus of resin-based composites interact and discussed the level of residual stress they create.
Methods
Residual stress was assessed by cuspal flexure of typodont premolars with 3 × 3 mm mesio-occlusal-distal preparations. The teeth were restored with one of three bulk-fill flowable composites (everX Flow Short-fiber Reinforced, GC; Filtek Bulk Fill, 3 M; or SDR Flow+ Bulk Fill, Dentsply Sirona) placed in one bulk increment. Tooth surfaces were scanned before (‘baseline’) and after restoration (15 min and 24hrs). Cuspal flexure was determined by comparing scans of restored teeth with their baseline. Bond integrity was verified with dye. Post-gel shrinkage was measured using a strain gauge, and elastic modulus at 15 min and 24hrs using four-point bending. Sample size was 10. Results were analyzed with ANOVA statistics (α=0.05).
Results
Cuspal flexure differed significantly among composites (p = 0.001) but not between 15 min and 24hrs (p = 0.889). The fiber-reinforced composite caused significantly higher cuspal flexure compared to the conventional flowables (17μm versus 7μm) despite its lower post-gel shrinkage (0.53 versus 0.74 and 0.58 vol%). The elastic modulus of the fiber-reinforced composite was significantly higher at 15 min than the other composites (8 GPa versus 2 GPa), and increased significantly at 24hrs (12 GPa versus 8 GPa).
Significance
High elastic modulus development rate eliminated benefits associated with low post-gel shrinkage. Cuspal flexure and therefore residual shrinkage stress was largely determined by the early elastic modulus, when composite was still shrinking, not the mature (final) elastic modulus value.
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