Physical properties and gap formation of light-cured composites with and without 'softstart-polymerization'

Recently it has been pointed out that light-initiated prepolymerization at low intensity followed by a post-light-cure at full intensity ('softstart-polymerization') may lead to light-cured composite fillings with improved marginal adaptation. The aim of this study was to examine the influence of this procedure with different initial cure conditions on physical material properties like flexural modulus, flexural strength and Vickers microhardness of different composites. Additionally, an in vitro study was carried out to investigate the influence of different initial cure conditions on the marginal quality of composite restorations in cases of Class V cavities with a cemento-enamel junction.

Physical properties of composite materials Tetric (Vivadent, Liechtenstein) and Charisma (Kulzer, Germany) were evaluated. Three hundred specimens were made for the flexural tests performed similar to EN 24049:1993. Microhardness was measured with a Vickers indenter. For the in vitro study of Class V composite fillings, cavities of 32 extracted teeth were filled with Tetric and analysed by quantitative marginal gap analysis and dye penetration test before and after thermocycling.

The results indicate that initial cure with decreased light intensity followed by final cure with high light intensity has no influence on microhardness and increases flexural modulus and flexural strength. With the low-level initial cure/high-level final cure regime the marginal integrity was significantly better compared with the high light intensity curing system.

Therefore, initial cure with low light intensity followed by final cure with high light intensity significantly improves the marginal integrity of light-cured composite fillings and also the material properties.