Shrinkage Stress of Conventional vs. Ion-Releasing Composites Using High-Intensity Photo-Activation

Objectives: This in-vitro study examined the effects of high-intensity and conventional light-curing on real-time shrinkage stress of conventional and ion-releasing resin composites.
Methods: Two conventional resin composites specifically designed for rapid high-intensity photo-polymerization (Tetric PowerFill, Tetric PowerFlow), two commercially available ion-releasing composites (ACTIVA Presto, Beautifil II), and four experimental ion-releasing composites were included. The experimental composites contained 10 or 20wt% bioactive glass (BG): either BG 45S5 (C-10, C-20) or a customized low-sodium fluoride-containing BG (F-10, F-20). Photo-activation was conducted using two protocols: 3s at 3000mW/cm² or 10s at 1200mW/cm². Real-time shrinkage stress of 1.5-mm-thick specimens (n=8 per group) was measured for 5min after the start of photo-activation using a semi-rigid stress analyzer (instrument compliance: 0.4µm/N), and the kinetic parameters maximum shrinkage stress rate R_max and time to reach R_max (t_Rmax) were determined. Statistical analysis included independent t-tests and one-way ANOVA followed by Tukey's post-hoc test (α=0.05).
Results: Shrinkage stress values at the end of the 5-min observation time ranged from 0.25±0.07MPa (C-20, 3s) to 0.81±0.07MPa (F-20, 10s). The experimental ion-releasing composites (F-10, F-20) exhibited the highest shrinkage stress, showing no significant differences between 3-s and 10-s light-curing, and reached the significantly highest R_max values. The two commercially available ion-releasing composites (ACTIVA Presto, Beautifil II) along with C-10/C-20 showed significantly higher shrinkage stress at 10-s vs. 3-s light-curing. For F-10, F-20, and Tetric PowerFlow, no significant differences in shrinkage stress were found between the light-curing protocols. C-10 and C-20 exhibited the significantly highest t_Rmax values with 10-s light-curing. The curing protocol did not affect t_Rmax in commercially available composites.
Conclusions: Composites designed for rapid high-intensity photo-polymerization and commercially available ion-releasing composites showed similar shrinkage stress values within the same curing protocol. The highest shrinkage stress was measured for the experimental composites with fluoride-containing bioactive glass (F-10, F-20).