Effect of light power density on development of elastic modulus of a model light-activated composite during polymerization

PURPOSE: Elastic modulus development during polymerization of a composite is a measure of the polymerization maturity and the restoration's ability to transfer stress to enamel and dentin. The characteristics of elastic modulus development in real time during cure are largely unknown. The purpose of this study was to evaluate the effect of light power density and total energy density on the early development of elastic modulus for a light-activated composite. METHODS: Cylindrical specimens of a model hybrid composite were tested in flexure in a dynamic mechanical analyzer (DMA). Specimens were light-activated (Variable Intensity Polymerizer, Bisco, Itasca, Illinois) for 60 seconds. Elastic modulus was measured continuously for 5 minutes from the start of light activation. Development of elastic modulus was assessed for six different light power densities and two reduced power density levels given at longer exposure duration to provide similar energy density values. One-way analysis of variance with Tukey's post hoc comparison test was used to evaluate significant differences of elastic modulus at p = .05. RESULTS: The rates of elastic modulus development and final moduli were dependent on the light power density applied. Composite specimens cured by equivalent energy densities using short times and high power density or long times and low power density produced equivalent elastic moduli. Elastic moduli for emitted power densities between 400 and 600 mW/cm2 (160-260 mW/cm2 measured at the specimen surface) were not significantly different (p > .05). CONCLUSIONS: Light power densities greater than 160 mW/cm2 measured at the specimen surface resulted in elastic moduli that were not significantly different. Equivalent energy densities produced comparable elastic moduli.