Post-gel shrinkage with pulse activation and soft-start polymerization

This study investigated the influence of pulse activation and soft-start polymerization regimens on the post-gel shrinkage of a visible light-activated composite resin (Z100). A light-cure unit (BISCO VIP) that allowed for independent command over time and intensity was used. The six light-curing modes that were examined include: Control (C)-400 mW/cm2 [40 seconds]; Pulse Delay I (PDI)-100 mW/cm2 [3 seconds], delay [3 minutes], 500 mW/cm2 [30 seconds]; Pulse Delay II (PDII)-200 mW/cm2 [20 seconds], delay [3 minutes], 500 mW/cm2 [30 seconds]; Soft-start (SS)-200 mW/cm2 [10 seconds], 600 mW/cm2 [30 seconds]; Pulse Cure I (PCI)--two 400 mW/cm2 [10 seconds] and one 400 mW/cm2 [20 seconds] pulses with 10 seconds interval between; and Pulse Cure II (PCII)-two 400 mW/cm2 [20 seconds] pulses with 20 seconds interval between. A strain-monitoring device measured the linear polymerization shrinkage associated with the various cure modes during and post light polymerization up to 60 minutes. Five specimens were made for each cure mode. Data was analyzed using one-way ANOVA and Scheffe's post-hoc test at significance level 0.05. Post-gel shrinkage associated with PDI was significantly lower than with PDII, SS and PCI immediately post light-polymerization. At one-minute post light polymerization, PDI had significantly lower shrinkage compared to PDII and SS. Significant differences in shrinkage were observed between PDI and SS only at 10, 30 and 60 minutes. At all time intervals, no significance in post-gel shrinkage was observed between the control and all-pulse activation/soft-start polymerization regimens.