Viscosity-Dependent Shrinkage Behavior of Flowable Resin Composites

Flowable resin composites are extensively used in restorative dentistry, where linear polymerization shrinkage and the resulting shrinkage stress are critical for clinical success. This study investigated the relationship between viscosity, linear polymerization shrinkage, and shrinkage stress in flowable resin composite materials. Two low-flow resin composites (Beautifil Flow Plus F00, Estelite Universal Flow SuperLow), two medium-flow resin composites (Tetric EvoFlow, Estelite Universal Flow Medium), and two high-flow resin composites (Beautifil Flow F10, Estelite Universal Flow High) were examined. Viscosity (n = 3) of the unset materials was determined using a cone–plate rheometer. The composites were photoactivated for 20 s at 1226 mW/cm², and linear polymerization shrinkage (n = 8) and shrinkage stress (n = 8) of 1.5 mm-thick specimens were recorded in real time for 5 min using a custom-made linometer and stress analyzer, respectively. Data were analyzed with Kruskal–Wallis rank tests followed by Conover post hoc tests, and Spearman correlation analyses were conducted to assess relationships between parameters (α = 0.05). A significant negative correlation was observed between viscosity and shrinkage stress (r = −0.943, p = 0.017). Beautifil Flow F10 exhibited the significantly lowest viscosity (14.60 ± 0.17 Pa·s) and the highest shrinkage stress (0.83 ± 0.14 MPa) among the materials, whereas low-flow composite Estelite Universal Flow SuperLow showed the lowest shrinkage stress (0.65 ± 0.10 MPa). Linear shrinkage ranged from 1.89 ± 0.13% to 3.18 ± 0.21%, but was not correlated with viscosity or stress (p > 0.05). In conclusion, viscosity critically influences polymerization-induced shrinkage stress development in flowable resin composites. Higher-viscosity flowable composites might be beneficial regarding stress build-up during polymerization compared with high-flow composites.