Constant Dose Light Intensity Studies on BisGMA/TEGDMA Based Composites

Objectives: To study the effect of light intensities, under a constant dose, on polymerization shrinkage stress (PS), degree of conversion (DC) and exotherm of model dimethacrylate-based composites.
Methods: A 7:3 mixture of BisGMA:TEGDMA with micro-sized glass filler loading levels of 50 wt% and 75 wt% each were investigated. NIST SRI 6005 (standard reference instrument, a cantilever-beam based instrument) with an in situ near infrared spectrometer and a microprobe thermocouple was used for simultaneous measurement of PS, DC and temperature in real time. Light irradiance used for the photo-polymerization were 500mW/cm², 2000mW/cm² and 4000mW/cm² while maintaining a constant dose. Measurements were carried out under three instrumental compliances, 0.33µm/N, 6.31µm/N and 12.32µm/N, that cover possible compliances of clinically prepared tooth cavities.
Results: For all the specimens tested under the lowest compliance of the instrument, their final PS increased with the light intensity, more specifically, it was around 10.5% to 62.7% higher for the two higher intensities when compared to the lowest intensity. However, under higher compliances, their final PS were not significantly different for the intensities tested. The final DCs of the specimens tested were either similar or marginally higher, up to 6.5%, at the two higher intensities when compared to the lowest intensity. The peak rise in temperature increased significantly with intensity for all the specimens tested, more particularly, it was around 53.1% to 144.4% higher for the two higher intensities when compared to the lowest intensity. The argument on DC and temperature is independent of the instrument compliance.
Conclusions: In the limits of this study it can be concluded that the variation of light intensity in photo-polymerization, under a constant dose, can not only effect the DC but can significantly influence the PS (instrument compliance dependent) and exotherm.