Previous studies showed contradictory results regarding the effect of C-factor on composite SB1518 resin restorations. Laboratory studies showed that high C-factor increases the rate and amount of stresses resulting from polymerization shrinkage of resin composite restorations.19,29 Santini et al30 found no difference in the amount of microleakage between box-shaped cavities and V-shaped cavities at both enamel and gingival margins. Using bovine incisors, a difference in microleakage has been demonstrated between two cylindrical class V cavities of different dimensions, but of the same C-factor.15 Therefore, it was concluded that microleakage is more closely related to the volume of the restoration rather than to the C-factor.

14 Our results were very interesting, as class V cavities with higher C-factor had more microleakage than class V cavities with lower C-factor only when the fast curing mode was used. On the other hand, there was no difference in the amount of microleakage when the soft-start curing mode was used, regardless of the value of the C-factor. In all groups, the volume of the restorations was the same. These results can be explained by the fact that fast curing mode produces higher stresses at the adhesive system, and these stresses have the worst effect in case of unfavorable cavity design (i.e. high C-factor). One could speculate that the variation between the results of different studies can be attributed to variations in methodology, for example, type of cavity prepared in each study (class I vs. class II vs. class V), type of teeth used (human vs.

bovine vs. models), restorative materials used, the curing protocols employed in addition to the type of adhesive system and the way it has been manipulated. Another important factor is the way the investigators change the C-factor of the cavity, i.e., by increasing the depth or the width of the cavity, as using cavities of different depths results in different dentinal properties, which can affect microleakage. In our study, we purposely changed the C-factor by changing the shape of the cavities, keeping the volume and the depth of the cavities constant in all the tested groups. One LED curing light was used in this study, but with two curing modes. Although the curing time was different between the two curing modes used, the total energy delivered was the same (16.5 J/cm2).

Previous studies demonstrated that soft-start curing delivers low levels of energy initially, allowing the resin composite to flow. This releases the stresses of polymerization shrinkage, resulting in reducing microleakage.7,31,32 High polymerization stresses have been shown to increase Cilengitide leakage in class V cavities.12 On the contrary, Hofmann and Hunecke6 showed no difference between high intensity curing lights with soft-start curing, with regard to margin quality and marginal seal of class II resin composite restorations.