24-Hour Shrinkage Behavior of Ten Resin-Based Composites

To quantify the degree of conversion (DC), shrinkage strain, shrinkage stress, and polymerization kinetics of ten commercial resin-based composites (RBCs).

Ten RBCs were tested. Four rapid-cure Tetric formulations (Tetric plus Fill, PowerFill, Tetric plus Flow, and PowerFlow) were photocured for 3 s at 3.0 W/cm² for DC and strain measurements and the corresponding stress specimens received 2.5 W/cm² for 3 s. Six comparator RBCs were photocured for 10 or 20 s at 1.2 W/cm² in accordance with their manufacturers' instructions. The DC was measured using ATR-FTIR on 0.2-mm-thick specimens for 720 s, whereas axial shrinkage strain and stress were recorded on 1.0-mm-thick specimens for 24 h. Because material and protocol were not varied independently, across-material comparisons were interpreted as intended-use comparisons rather than isolated formulation effects.

Tetric plus Fill and Tetric PowerFill produced comparable DC and 24 h shrinkage strain and stress. Among the regular-viscosity RBCs, Admira Fusion x-tra produced the lowest 24 h strain and stress. Tetric PowerFlow produced lower 24 h strain and stress than Tetric plus Flow. The rapid-cure Tetric material/protocol combinations exhibited earlier and larger DC-, strain-, and stress-rate peaks. Estelite Flow had a lower maximum DC rate and a broad, lower strain-rate peak, but it achieved the highest DC at 720 s and had a smaller late-time strain slope. In a regression constrained through the origin, the 24 h shrinkage stress increased with 24 h shrinkage strain under the tensometer geometry and compliance used in this study.

The 24 h measurements show that low early shrinkage values may not predict the final material values and ranking.