Depth-dependent Photopolymerization Reaction Kinetics in Dental Composites

Objective: While many studies have examined depth of cure in composites, no information is available on real-time reaction kinetics, including dynamic temperature change, as a function of depth during composite photopolymerization.

Methods: A 1 cm vertical column of Bis-GMA/TEGDMA-barium glass composite between masked glass slides was positioned in a near-infrared (NIR) spectrometer beam used to assess real-time reaction kinetics and conversion.  An aperture restricted the NIR signal to a 1 mm horizontal slice at predetermined depths of 1, 3, 5, 7 and 9 mm within the composite.  An embedded thermocouple positioned just below the NIR sampling level provided dynamic localized temperature sensing.  Photopolymerization was triggered by a 90-120 s exposure of the open upper composite surface with a dental curing light at either 50 or 500 mW/cm² incident irradiance (n=3).

Results: The dynamic conversion data analyzed at 30, 60 and 90 s time-points showed progressive exponential declines in conversion with increasing depth: r² values >0.96.  At all depths, the lower irradiance exposure produced significantly lower conversion (p < 0.05) which decreased more rapidly with respect to depth compared with the higher irradiance.  The rate of polymerization assessed at 10% conversion also showed exponential declines with depth in the composite: r² values >0.92.  The coupled conversion and temperature data demonstrate that internal temperature and reaction rate both decrease with increasing depth within the specimen but the thermal front consistently precedes the reaction front.  At the 9 mm sampling depth, delays of >10 s and >40 s were observed between the initial light exposure and the localized onset of polymerization at the high and low light intensities, respectively.

Conclusions: This novel analytical approach provides comprehensive, site-specific kinetic data and clearly shows the significant effects of attenuated light transmission on photopolymerization in composite materials. 

Supported by NIH/NIDCR U24DE016502/2R01DE14227 and a gift from Septodont.