effects of sequential versus continuous irradiation of a light-cured resin composite on shrinkage, viscosity, adhesion, and degree of polymerization

Objective : This in vitro study compar ed different properties of a resin composite using a two‐step light‐curing method and a conventional continuous curing approach. Materials and Methods : Five different tests were performed on specimens of resin composite. Fifty‐five specimens of 3.5‐mm thickness were divided among 11 different test groups that varied in light‐curing protocols. Top and bottom surface hardness were measured after curing. Adhesion of composite to pretreated metal surfaces was tested using 36 specimens irradiated with four different curing methods. Linear and volumetric shrinkage of adhesion specimens were measured using a linometer. Measurements of viscosity were performed on 10 specimens during polymerization. Finally, the dynamics of polymerization during two‐step versus continuous light‐curing were evaluated by measuring the residual monomer concentration in each specimen after different irradiation. Results : Surface hardness depended on the total dose of light delivered, but not on how the light was delivered. Concerning adhesion, the results indicated that marginal adhesion was weakened by fast polymerization and was improved with a low‐intensity pre‐polymerization period. The results of the shrinkage test indicated less shrinkage a t the beginning of the process with the lower intensity light. However, at completion of polymerization, the total shrinkage did not differ significantly between groups. Also, during the curing process, the same viscosity changes occurred in the resin composite regardless of the irradiation method used. Finally, by the 40‐second completion point, specimens made with either two‐step or continuous light‐curing had reached a similar 90% degree of polymerization. Conclusions : With an adequate total irradiation dose, the properties of resin composite cured with a two‐step approach were as good as or better than those obtained using a conventional continuous curing method. Moreover, marginal adaptation was improved by two‐step polymerization, which could be explained by the ability of the material to flow during the early stages of the curing process.