Comparison of polymerization contraction stresses between self- and light-curing composites

Objectives: The objective of this study was to examine the distributions and the magnitudes of the internal stresses in self- and a light-curing composite restorations resulting from polymerization shrinkage. Methods: Butt-joint box-shaped cavities (5.0×2.0 mm2, 2.0 mm in depth) prepared in composite molds were filled with either a self- or light-curing transparent resin composite. The restorations were cross-sectioned perpendicular to the longitudinal axes of the cavities and observed using polarizing microscopes. The principal stresses in the restorations, normal and shear stresses at the cavity wall were evaluated by photoelastic analysis. Results: The distributions of the principal stresses and the stresses generated at the cavity wall in both the self- and the light-curing composite restorations were similar. The maximum stress generated at the cavity wall in the light-curing composite restorations was twice as large as that seen in the self-curing restorations. Conclusions: The results of this study indicated that the difference in the magnitude of the internal stresses between self- and light-curing composites was not related to the distribution of the stresses. The velocity of polymerization appeared to be the most important factor contributing to the magnitude of the internal stresses generated in the composite restorations in this study.