The effect of different power densities and method of exposure on the marginal adaptation of four light-cured dental restorative materials

Purpose: To determine whether marginal adhesion is sensitive to different irradiation parameters, we investigated the in vitro adhesion values of four dental resins on metal surfaces. Methods: Four groups of eight specimens each of Z250, Filtek flow, Dyract AP and Dyract flow were placed in pre-treated stainless steel cavities and irradiated using different methods of exposure. The curing lights used were a Spectrum 800 halogen curing light at settings of 800 and 450 mW/cm2 and an Optilux 501 ramping light. The maximum amount of push-out force required to displace the resin from the metal cavity was equated with adhesive value (shear bond strength). Comparisons (ANOVA, p<0.0001) were made within the same material and between the different materials when using different curing protocols. Results: Significant lower bond strengths were recorded when curing was done by gradually increasing the intensity (ramping method) compared to curing with the fixed intensities (p>0.0001) Comparing the fixed intensities, significant lower bond strength values were obtained at 800 mW/cm2 compared to 450 mW/cm2 (p<0.0001). For all exposures, the two flowable materials demonstrated weaker values when compared to the higher filled materials. Clinical significance: The advantage of initial slow polymerization (more elasticity and less tension) obtained by the so-called “soft start” method, was offset by a rise in total polymerization shrinkage, when final curing was completed at 1130 mW/cm2. These tests demonstrated that using halogen units, exposure for 40 s with an intensity of 450 mW/cm2 appeared to be the most promising for light-curing dental resin composites.