Digital image correlation study on polymerization shrinkage of resin-based composites

Objectives: The aim of the study was to analyze polymerization shrinkage of resin-based composites (RBCs) cured with different light-curing regimes using digital image correlation. Methods: Three RBCs were used in the study: low-shrink, microhybrid Filtek Silorane (3M ESPE), nanohybrid Tetric EvoCeram (Ivoclar Vivadent) and microhybrid Filtek Z250 (3M ESPE). Materials were placed in Teflon moulds, 5 mm in diameter and 2 mm thick, held in a fixating device. One surface was lightly sprayed with fine black paint to produce irregular speckles and the other was irradiated from 1 mm tip-to-surface distance with one of the three light-curing regimes: standard halogen at 500 mW/cm2, soft-start halogen and standard LED at 600 mW/cm2. Digital images of each sample were taken by the Aramis system (Aramis 2M, GOM, Braunschweig, Germany) based on two cameras at 25º angle, immediately prior and after light curing. Local shrinkage of the photographed surface was calculated as Mises strain. The data were statistically analyzed using two-way analysis of variance at alpha=0.05. Results: No significant difference in strain was observed between the three materials (p>0.05). Strain distribution was non-homogeneous, the outer or peripheral 1-2 mm showed significantly higher strains than central parts in each material (p<0.05). Peripheral strain was in the range of 2-4% whereas that in the central parts was between 1 and 2%. Soft-start curing regime did not reduce polymerization shrinkage compared to standard halogen or LED units (p>0.05). Conclusions: Polymerization shrinkage strain was non-homogeneously distributed across the surface of RBCs. Shrinkage was independent of the chemical composition of the three tested RBCs as well as of the applied light-curing regime. Digital image correlation based on two cameras allows qualitative and quantitative three-dimensional analysis of shrinkage patterns in RBCs.