Reduced light energy density decreases post-gel contraction while maintaining degree of conversion in composites

The objective of the study was to evaluate the relationship between curing light intensity and (1) linear post-gel polymerization contraction strain, and (2) degree of conversion of a dental composite. Cylindrical specimens of a dental resin composite were cured from a distance of 7 mm for 40 s at four attenuated light intensities (71%, 49%, and 34% of control intensity and for 20 s at 71% plus 20 s at 100% intensity). A group cured at full intensity served as a control. Degree of conversion (DC) was measured at the top and bottom and linear contraction strain was measured at the bottom of the composite samples. DC at the sample top was significantly different (P < 0.05) between all groups except the 71% and 49% intensity groups. At the sample bottom, DC resulting from the two highest intensities (71% and 100%) were not significantly different from each other (P > 0.05). All other groups were significantly different from each other (P < 0.05). DC for the sample cured at two light intensities was not significantly different from those cured at the lower intensity or higher intensity for 40 s (P > 0.05). The sample cured with two intensities showed a 21.8% reduction from the contraction strain predicted by a light energy density calculation. Application of light at less than the maximum intensity of the curing light resulted in significant reduction of polymerization contraction strain without significantly affecting the degree of conversion.