Evaluation of resin composite polymerization by three dimensional micro-CT imaging and nanoindentation

Light-cured composites undergo shrinkage during polymerization. The aim of this study was to evaluate regional shrinkage within a light-cured composite during polymerization by microcomputed tomography and mechanical properties by nanoindentation in bonded or non-bonded class-I cavity.

Zirconium oxide spherical fillers (30 μm diameter) were added as markers to a composite resin, filled into a box-shaped class I cavity with or without a bonding agent. The marker fillers were traced in 3D scans obtained by micro-CT before and after polymerization using a software (TRI/3D-BON). The average hardness of the resin composites determined by nanoindentation at each 250 μm depth was plotted against depth.

In the bonded cavity, the filler particles at the top region moved toward the bottom of cavity, but at deeper depths, the direction of vertical movement changed toward the top of cavity (irradiated surface). A significant linear regression was found between filler displacement and composite depth (R(2)=0.9761). In the unbounded cavity, all the fillers moved toward the light curing source, and a significant power-law regression was found between filler displacement and composite depth (R(2)=0.849). In both groups, the data scattering increased at regions deeper than 3.5 mm, where the hardness, representing degree of conversion of composite, significantly decreased compared to the surface region.

The magnitude and direction of regional polymerization shrinkage depends on boundary conditions, depth and conversion degree. Polymerization shrinkage effect is most significant at the deepest part of the cavity. The application of micro-CT combined with sophisticated image analysis is a novel approach to investigate shrinkage mechanisms of dental composites.