Light-induced control of polymerisation shrinkage of dental composites by generating temporary hardness gradients

Irradiation of light-curing dental filling materials in a single direction results in a temporary hardness gradient in the direction of the irradiatio n. The photoactivated polymerisation process begins at the site of the high est light intensity. In the simplest possible model, the polymerising compo sites irradiated in a single direction shows three adjacent co-existing pha ses: an almost hardened, a gelled and a still plastic phase. As long as all three phases are present, any shrinking of the contracting phases can be c ompensated by the plastic phase. A knowledge of the distribution of these p hases and their spatial and temporal modulation by the selection of suitabl e curing light parameters provides simple techniques for reducing shrinkage gaps around voluminous fillings in large dental cavities.