Mechanical manifestation of the C-factor in relation to photopolymerization of dental resin composites

Abstract

Objective: This study aims to assess the validity of a recent theory which proposes that (1) the magnitude of the shrinkage stress of resin composites depends on the thickness of the boundary layer under triaxial constraints relative to the total thickness of the specimen and (2) the boundary-layer thickness is proportional to the diameter of the specimen.

Methods: Cylindrical specimens of three commercially available resin composites, three diameters (4, 5 and 6.3mm) and four thicknesses (2, 3, 5 and 6.5mm) were tested. Curing was applied using a LED light for 40s. Microscopic images (32×) of the specimens before and after curing were analyzed to determine the lateral shrinkage profile along the vertical axis. Boundary-layer thickness was determined from the point where lateral shrinkage displacement first reached the maximum value found at mid-thickness.

Results: Lateral shrinkage displacement at mid-thickness was close to the theoretical value based on published shrinkage-strain data, with the ratio between experimental and theoretical values being 1.04±0.06. The boundary-layer thickness was found to be proportional to specimen diameter only, independent of material, C-factor, and specimen thickness. The proportionality constant was 0.64±0.07, which was approximately 3 times that of the effective value indicated by shrinkage strain/stress calculations.

Significance: This study validates the assumption made in the shrinkage-stress theory recently proposed and provides a more precise and mechanistic interpretation for the C-factor, i.e. the C-factor, as a measure of a specimen's constraint, is the ratio between the boundary-layer thickness and the total thickness of the specimen.

Keywords: Boundary layer; C-factor; Composite resins; Shrinkage strain.