Hardness gradients of dual-polymerized flowable composite resins in simulated root canals

The Effect of Touch-Cure Polymerization on the Conversion and Hardness of Core Build-Up Resin Composites: A Laboratory Study

To improve the self-curing capacity and interfacial strength with dentine of dual-cured composite materials, touch-cure activators have been introduced. The aim of the study was to evaluate the effect of these activators on the hardness and conversion of dual-cured resin composite core build-up restoratives. The materials tested were Clearfil DC Core Plus (CF) and Gradia Core (GC) with the corresponding adhesives Clearfil S³ Bond Plus (for CF) and G-Premio Bond/G-Premio DCA activator (for GC). Disk-shaped specimens (n = 6/group) were prepared for the following groups: dual-cured, self-cured and self-cured in contact with the adhesive activators at the bottom surface. After a 3-week storage period (dark/dry/37 °C) the Martens hardness (HM) and degree of conversion (DC%) were determined for the previously mentioned groups and the top surfaces of groups in contact with the adhesives. A statistical analysis was performed by a one-way ANOVA and Holm–Sidak test per material and a Pearson’s correlation analysis (HM vs. DC%) at an α = 0.05. The self-cured specimens resulted in significantly lower HM and DC% values from the dual-cured group, as expected. However, in the presence of the adhesives with touch-cure activators, the conversion of the self-cured groups showed insignificant differences in HM and DC% from the dual-cured in both composite materials. The improvements on the bottom composite surfaces in contact with the adhesives did not extend to the entire specimen length. Nevertheless, improved interfacial curing may improve interfacial durability.

Effect of curing mode on the conversion and IIT-derived mechanical properties of core build-up resin composites

The aim of the study was to evaluate the degree of conversion and the mechanical properties of five composite core build-up materials polymerized in dual-curing and self-curing modes. The materials tested were: Clearfil DC Core Plus (CF), Gradia Core (GC), Luxacore-Z Dual Smartmix (LX), Multicore Flow (MC) and Paracore (PC). Disk-shaped specimens were prepared from each material; half the specimens were light-cured, whereas the rest were only self-cured. After a 3-week storage period (dark/dry/37 °C) the Martens Hardness, Indentation Modulus, and Elastic Index were determined by instrumented indentation testing (IIT), while the degree of conversion was assessed by ATR-FTIR spectroscopy. Statistical analysis was performed by 2-way ANOVA and post-hoc testing (α = 0.05). The dual-curing mode resulted in statistically higher Martens Hardness and Elastic Index than the self-curing mode in most materials but showed insignificant differences in Indentation Modulus. MC and PC demonstrated significantly higher degree of conversion in both curing modes. Overall, the self-curing mode was inferior to the dual-curing in conversion and mechanical properties for most products, despite their differences in monomer composition and filler loading.

Time-dependent Microhardness Gradients of Self-adhesive Resin Cements Under Dual- and Self-curing Modes

CLINICAL RELEVANCE

Acid-functional monomers in self-adhesive resin cements may decrease their self-curing polymerization ability. Light irradiation optimizes polymerization performance.

SUMMARY

Purpose: The aim of this study was to investigate Knoop microhardness of self-adhesive resin cements under dual- and self-curing modes in simulated canals for describing the polymerization behavior.Methods and Materials: Slots in lightproof silicone cylinders with one open end were filled with the following eight materials: a traditional resin cement (Duolink), a core build-up resin material (MultiCore Flow), and six self-adhesive resin cements (RelyX Unicem 2, G-Cem Automix, Maxcem, Biscem, Multilink Speed, and PermaCem 2.0). The resins were exposed to light through the open end and then stored in a lightproof box. The Knoop hardness gradient for each resin was measured after 1 hour and 120 hours. Surface readings were obtained at 1-mm intervals from 1 mm to 10 mm away from the open ends. The data were analyzed by two-way analysis of variance and the Student-Newman-Keuls test (α=0.05).Results: All the resin materials had stable Knoop hardness numbers (KHNs) at a certain depth; their KHNs in the self-curing mode did not change (p>0.05). The region above this certain depth was regarded as having undergone the dual-curing mode, and the KHN decreased gradually with depth (p<0.05). Between 1 and 120 hours postexposure, the ratio of the KHN at a 5-mm depth (self-cured) to that at a 1-mm depth (dual-cured) increased in Duolink and MultiCore Flow. However, the ratios of the six adhesive resin cements varied.Conclusion: Without light, most self-adhesive resin cements differed from traditional dual-cured resin materials in terms of Knoop micro-hardness, and they had a lesser capacity for chemical-induced curing.