Step-cure polymerization: effect of initial light intensity on resin/dentin bond strength in class I cavities

This in vitro study assessed the effect of a step-cure light curing method on resin/dentin bond strength on the buccal wall of Class I cavities in human teeth. Occlusal enamel was removed to expose a flat dentin surface. Twenty four box-shaped cavities (C-factor = 4.5) were prepared in dentin. Prime&Bond 2.1 was applied and TPH Spectrum (Dentsply) was inserted using a bulk-filling increment. The composite was light-cured using either a step-cure photoactivation technique or a one-step continuous curing method. For step-cure polymerization, the initial cure intensity was varied by changing the distance between the light source and the resin surface. The light-cured resins were cured using four low light intensities: 150(G1), 200(G2), 250(G3) and 300(G4) mW/cm2. In the continuous exposure curing method, the samples were light-activated for 40 seconds at 740 mW/cm2 and irradiation was applied in a box-shaped cavity and a flat cavity (exposed buccal wall, C-factor = 0.22). Samples were prepared for TBS testing by creating bonded beams (of approximately 0.8 mm2) obtained from the buccal wall. The data were analyzed using one-way ANOVA, Tukey Test and Dunnett's Test at a significance level of 0.05. The mean TBS values for the continuous exposure group in the flat and box-shaped cavities were 24.31 and 10.23 MPa, respectively. The corresponding TBS for step-cure polymerization was 23.13 (G3), 18.83 (G2), 14.87 (G1) and 13.26 MPa (G4). Bond strength values to the cavity wall were lower in the three-dimensional cavities and dependent on the light curing method (p < 0.05). The use of a low initial light intensity (200-250 mW/cm2) for 10 seconds followed by high irradiation intensity provided the best bond strengths, similar to bonding in a flat cavity.