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

INTRODUCTION

In an attempt to reduce polymerization shrinkage of composites, some light curing units (LCU) presented "soft-start polymerization" in addition to the classical high light intensity mode. This study investigated whether those soft-start polymerization modes delivered by those LCUs are suitable to minimize the internal stress by increasing the time available for the flow of the material and decreasing the marginal debonding.

MATERIAL AND METHODS

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 enabled us to have 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: Step polymerization: Exposure for 10 seconds at 100 mW/cm2 then 20 seconds at 700 mW/cm2. Group C: Pulse polymerization: Exposure for 20 cycles, each cycle consists in: 1 second at 700 mW/cm2 then 0.25 second at 0 mW/cm2. Group D: Ramp polymerization: Exposure for 10 seconds with a slow rise of the intensity to reach 700 mW/cm2 then 20 seconds at 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.

RESULTS

The main effect for the type of restoration (p = 0.185), and the interaction effect (p = 0.640) did not reach statistical significance. There was not a statistically significant main effect for the type of polymerization used (p = 0.082). There was a difference between the group A (M = 2.40, SD = 1.102), and group C (M = 1.67, SD = 1.348). However, when the Tukey HSD test was used, no statistically significant differences between group A and group C were found (p = 0.103).

CONCLUSIONS

There were no statistically significant differences in microleakage between the different soft-start polymerization modes tested, even between the nano-filled and micro-hybrid composite materials cured with different polymerization regimens.