Impact of Time, Intensity and Total Energy on Composite Photopolymerization

Objectives: Photocuring dental composites faster and deeper are of great interest to clinicians. Some research suggests that using increased irradiance and shorter cure times based on the assumption that the total irradiance energy (TE) is the primary factor for achieving both attributes.  This study investigates the impact of TE on the degree of monomer conversion (DC) versus cure depth of dental composites.

Methods:  An experimental dental composite was placed into a 6x15mm hemicylindrical mold and photopolymerized with a variable intensity blue LED light in contact with the composite. Nine distinct time and irradiance combinations were generated from three TE levels (5, 10, 20 Joules) that were independently generated by three combinations of irradiance (400, 800, 1600 mw/cm²) and inversely related irradiance times. Cured samples were removed from the mold and evaluated for DC as a function of depth utilizing a Raman microscope.

Results: The DC data was plotted versus depth of cure for each condition and fit to a 2 constant model: y=A-Bx² where y = DC_{specified depth}, A=DC_top, x= specified depth from top of sample, and B=constant. All but one data set exhibited R² >/= 0.92. The table summarizes experimental conditions, the quadratic coefficients (A,B), R² values, standard error, DC at the surface and 3mm within the sample. DC_3mm/DC_top illustrates relative conversion 3mm within the sample compared to the surface.

 

Conclusions : The quadratic model y=A-Bx² provides a simple predictive tool for DC versus cure depth for a given set of irradiance conditions. The data also supports that high irradiance and short exposure times may result in reduced DC compared to lower irradiance and longer exposure times.