Analysis of local shrinkage patterns of self-adhering and flowable composites using 3D digital image correlation

Objective: To analyze local shrinkage patterns in terms of surface shrinkage strains and z-axis displacements in a novel self-adhering composite and a conventional flowable composite using three-dimensional digital image correlation. Method and Materials: Seven samples of each material were prepared in cylindrical Teflon molds 5 mm in diameter and 2 mm thick. The surface of the composites facing the cameras was sprayed with a fine layer of black paint. The unsprayed surface of each sample, opposite the one facing the cameras, was light cured for 40 seconds using a light-emitting diode unit. Digital images were taken immediately before and after light curing. Shrinkage was calculated as von Mises strains, and z-axis displacements were measured in microns. The data were statistically analyzed using two-way ANOVA at α = .05. Results: No significant difference in strain was observed between the two materials (P > .05). Strain distribution was nonhomogenous- the outer segments showed significantly higher strains than central parts in each material (P .05). The opposite was observed for z-axis displacements-significantly greater displacements were found in central parts compared to the outer segments (P .05). Conclusion: Different shrinkage vectors across the surface of the tested flowable composites showed predominant in-plane shrinkage of the outer surface segments and out-of-plane shrinkage of the inner segments. These complex local deformation patterns in composite materials indicate zones of different types of forces exerted on the tooth-restoration interface in situ. Keywords: 3D measurement, digital image correlation, flowable composite, polymerization, shrinkage, strain