Indirect Restoration Thickness and Time after Light-Activation Effects on Degree of Conversion of Resin Cement

Abstract

This study evaluated the effects of indirect restorative materials, curing conditions and time on the degree of conversion (DC) of a dual-cured resin cement using infrared spectroscopy. The resin cement (RelyX Unicem 2, 3M ESPE) was applied to the diamond surface of a horizontal attenuated-total-reflectance unit and activated using one of following conditions: self-cure, direct light exposure, light exposure through indirect restorative materials (resin nano-ceramic: Lava Ultimate, 3M ESPE or feldspathic ceramic: Vita Blocks Mark II, Vita Zahnfabrik). Four thicknesses (0.5, 1.0, 1.5 or 2.0 mm) of each indirect material were analyzed, and the light-activation was performed using a blue LED light. Data (n=5) were analyzed by three-way ANOVA, Tukey's post hoc and Dunnett's tests (a=5%). No significant differences in DC were observed between indirect materials of similar thickness. All groups exhibited higher DC after 10 min than after 5 min. At both times points, the self-cure group exhibited significantly lower DC than all the light exposure groups. Only when the overlying indirect restoration had a thickness of 2 mm did DC decrease significantly. The presence of a thick, indirect restoration can decrease the DC of resin cement. DC after 10 min was higher than after 5 min. The self-cure mode yielded lower DC than the light-activating one.

Delayed Light Activation Improves Color Stability of Dual‐Cured Resin Cements

“…These data are consistent with other studies in the literature. 9,12,19,20 In the present study, the light activation protocols, as well as the different opacities of the interposed ceramics significantly influenced the color change after 8 days of storing in dry dark canisters (post-irradiation conversion). The delayed light activation caused higher color change than the immediate activation (Figs 1 and 2).…”

“…This continued polymerization consists in the conversion of monomers even after the light activation process and can happen for days after the cementation. These data are consistent with other studies in the literature …”

“…Besides the advantage of allowing better polymerization in deep areas, where the light intensity is attenuated, dual‐cured cements also show superior mechanical properties such as flexural strength, elasticity modulus, hardness, and DC when compared to other activation modes . For ceramic laminate veneers cementation, light‐cured resin cements are usually used due to the better time control during the clinical procedure; however, since the thickness and type of the ceramic can interfere with the DC of the resin cement, in several cases it is preferred to use a material that does not entirely depend on light activation…”

Light curing in dentistry and clinical implications: a literature review

“…The indirect materials commonly used are composite resins, glass and polycrystalline (or zirconia) ceramics, which present different optical and light absorption properties that influence light attenuation during light-activation of an underlying resin cement. [77][78][79][80] To overcome the effects of curing light attenuation, dual-cured cementing systems were developed and some of them are used in combination with adhesive systems, which contain co-initiators, such as sulfinic acid salts that produce free radicals and contribute to the polymerization reaction of the dual-cured resin cements. [81][82] However, when light activated the most of cementing systems generate higher degree of conversion values than autopolymerizing mode, which can compromise some properties of resin cement, such as flexural strength, modulus, hardness, solubility and water sorption.…”

“…This study did not find significant differences between materials when using the same thickness and reported that the light attenuation caused by 2.0-mm thick millable materials for CAD-CAM system resulted in significantly lower degree of conversion than those obtained with thin materials (0.5 and 1.0 mm thick), which did not differ from the direct light exposure of resin cement (without material interposition). 80 Effects of curing light on the temperature of tooth pulp and soft tissues…”