Light curing method for reducing polymerization contraction stress of light-cured composite restorations

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Light-cured resin-based composites are widely used in clinical practice because of the esthetic advantages, ease of use, excellent bonding to tooth structure, and improved mechanical properties.' However, the polymerization reaction of light-cured resin composites is faster than that of self-cured composites, which creates more gaps between the resin and cavity wall/floor than self-cured resin composites.This projects was to evaluate the effect of light curing methods on the marginal sealing and cavity wall adaptation of resin composite restorations, and on the hardness at the top and bottom surfaces of a body of resin composite; Cylindrical cavities were prepared on flat bovine dentin surfaces. The teeth were restored with adhesive systems followed by light-cured composite. The resins were cured with NEW LIGHT VL-ll (VL-ll, GO), Co-bee (CBS GO) and experimental light curing unit (EP, GO); Group 1: VL-ll 700 mW/cm^2 s; Group 2: OB (Ramped curing) 210 mW/cm^2 to 700 mW/cm^2 5 s + 700 mW/cm^2 s; Group 3: EP (Stepped curing) 270 mW/cm^2 s + 5 s (interval) + 600 mW/cm^2 50 s. After thermocycling for 500 cycles between 500 and 5500 with a dwell time' 30 second, dye penetration tests around the cavity margin and cavity wall adaptation on cut surface were carried out. Dye penetration length was calculated as a percentage of the total cavity wall length.The ramped and stepped curing significantly improved marginal sealing and resin composite adaptation to the cavity wall. These slow-start curing methods caused accelerating the hardening at the bottom surfaces of the resin composite.