Five second photoactivation? A microhardness and marginal adaptation in vitro study in composite resin restorations

Abstract

Studies defining the characteristics of light curing units and photoactivation methods are necessary to allow the correct choices to be made in daily practice. This study aimed to determine whether different photoactivation protocols for composite resins [periodic level shifting (PLS) - 5 second and soft-start] are able to maintain or enhance the mechanical properties and marginal adaptation of restorations. Restorations were placed in bovine teeth using the following photoactivation methods: continuous light for 20 seconds (control group); PLS technology (PLS - 5 second group); and continuous light and a light guide tip distance of 6 mm after which the tip was placed at the surface of the restoration (soft-start group). The teeth were transversely sectioned in the incisal-cervical direction. Thirty halves were randomly selected for Knoop microhardness testing (n = 10). The other 30 halves were subjected to scanning electron microscopy analysis. The images obtained were measured to identify the highest marginal gap, and statistical tests for variance analysis were conducted. Microhardness tests showed no statistically significant difference between the photoactivation methods analysed (P ≥ 0.01). The tests showed a difference among depths (P < 0.01), with the deeper layers being the hardest. In analysing marginal adaptation, no significant difference was identified between the higher marginal gap values in the continuous (mean = 10.36) and PLS - 5 second (mean = 10.62) groups, and the soft-start group (mean = 5.83) presented the lowest values (P < 0.01). The PLS - 5 second and soft-start protocols did not alter the hardness of the restorations. Moreover, the PLS - 5 second protocol did not alter the marginal adaptation, whereas the soft-start protocol improved marginal adaptation.