Effect of different modes of light curing and resin composites on microleakage of Class II restorations--Part II

To reduce the polymerization shrinkage of the composite resins and after the introduction of the "slow and gradual polymerization" by GORACCI et al. in 1992, many light curing units (LCU) presented the "soft-start polymerization" in addition to the classical high light intensity mode. This study investigated whether this slow and gradual polymerization has the ability of reducing the marginal debonding, by minimizing the internal stress of the composite, knowing that rare are the studies which compared this mode of polymerization.

One hundred twenty Class II cavities were prepared at the mesial and the distal side of human extracted premolars. The teeth were randomly divided into four groups (n = 15). In each tooth the mesial Class II cavities were restored with the micro-hybrid resin composite Filtek Z250 (3M ESPE) and the distal Class II cavities were restored with the nano-filled resin composite Filtek Supreme (3M ESPE). The light curing unit used in this study was a conventional quartz tungsten halogen (QTH) curing light, the QHL 75 (Dentsply) modified by adding a regulating electronic device, controlled by a special software that gives 4 different modes of polymerization as follows: Group A: Standard polymerization: Exposure for 20 seconds at 700 mW/cm2, this group was used as control. Group B: Modified pulse-delay polymerization: Exposure for 2 seconds at 700 mW/cm2, 5 seconds at 0 mW/cm2, then 20 seconds at 700mW/cm2. Group C: Modified ramp polymerization: Exposure for 20 seconds with a slow rise of the intensity to reach the 700 mW/cm2 then 20 seconds at 700 mW/cm2. Group D: Slow and Gradual polymerization: Exposure for 4 minutes with a slow rise of the intensity to reach 700 mW/cm2. The teeth were stored in water at 37 degrees C for 48 hours, then finished and subjected to thermocycling (3000 cycles between 5 degrees C and 55 degrees C). All the teeth were immersed in 2% methylene blue solution for 12 hours at 37 degrees C, sectioned, and evaluated at the gingival margins. Data were statistically analyzed by two-way analysis of variance followed by Tukey HSD multiple comparisons.

The main effect for the type of composites (p = 0.682), and the interaction effect (p = 0.678) did not reach statistical significance. There was a statistically significant main effect for the type of polymerization used (p = 0.014). Post-hoc comparisons using the Tukey HSD test indicated that the mean score for the Group A (M = 2.40, SD = 1.102) was significantly different from the group D (M = 1.30, SD = 1.393, p = 0.010). The group D did not differ significantly from group B (M = 1.63, SD = 1.351, p = 0.767), and group C (M = 1.60, SD = 1.404, p = 0.819). There was a difference between the group A (M = 2.40, SD = 1.102), group B (M = 1.63, SD = 1.351) and group C (M = 1.60, SD = 1.404). However, when the Tukey HSD was used, no statistically significant differences between Group A, B and C were found (p = 0.122 and p = 0.098).

There was a statistically significant difference in microleakage between the "slow and gradual polymerization" and the standard polymerization, however no significant differences were found when using the nano-filled or the micro-hybrid composite materials cured with different polymerization regimens.