[Light-induced control of polymerization 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 irradiation. The photoactivated polymerisation process begins at the site of the highest light intensity. In the simplest possible model, the polymerizing composites irradiated in a single direction shows three adjacent co-existing phases: 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 compensated by the plastic phase. A knowledge of the distribution of these phases and their spatial and temporal modulation by the selection of suitable curing light parameters provides simple techniques for reducing shrinkage gaps around voluminous fillings in large dental cavities.