Comparing the effectiveness of self-curing and light curing in polymerization of dual-cured core buildup materials

Chemical and mechanical properties of dual-polymerizing core build-up materials

OBJECTIVES

To investigate the chemical (degree of conversion (DC)) and mechanical properties (Martens hardness (HM), elastic indentation modulus (E_IT), and biaxial flexural strength (BFS)) of four dual-polymerizing resin composite core build-up materials after light- and self-polymerization.

MATERIALS AND METHODS

Round specimens with a diameter of 12 mm and a thickness of 1.5 mm were manufactured from CLEARFIL DC CORE PLUS (CLE; Kuraray), core·X flow (COR; Dentsply Sirona), MultiCore Flow (MUL; Ivoclar Vivadent), and Rebilda DC (REB; VOCO) (N = 96, n = 24/material). Half of the specimens were light-polymerized (Elipar DeepCure-S, 3 M), while the other half cured by self-polymerization (n = 12/group). Immediately after fabrication, the DC, HM, E_IT, and BFS were determined. Data was analyzed using Kolmogorov-Smirnov, Mann-Whitney U, and Kruskal-Wallis tests, Spearman's correlation, and Weibull statistics (p < 0.05).

RESULTS

Light-polymerization either led to similar E_IT (MUL; p = 0.119) and BFS (MUL and REB; p = 0.094-0.326) values or higher DC, HM, E_IT, and BFS results (all other groups; p < 0.001-0.009). When compared with the other materials, COR showed a high DC (p < 0.001) and HM (p < 0.001) after self-polymerization and the highest BFS (p = 0.020) and Weibull modulus after light-polymerization. Positive correlations between all four tested parameters (R = 0.527-0.963, p < 0.001) were found.

CONCLUSIONS

For the tested resin composite core build-up materials, light-polymerization led to similar or superior values for the degree of conversion, Martens hardness, elastic indentation modulus, and biaxial flexural strength than observed after self-polymerization. Among the tested materials, COR should represent the resin composite core build-up material of choice due to its high chemical (degree of conversion) and mechanical (Martens hardness, elastic indentation modulus, and biaxial flexural strength) properties and its high reliability after light-polymerization. The examined chemical and mechanical properties showed a positive correlation.

CLINICAL RELEVANCE

The chemical and mechanical performance of dual-polymerizing resin composite core build-up materials is significantly affected by the chosen polymerization mode.

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.

Hardness Development in Resin Composite Core Materials

PURPOSE

To investigate the hardness characteristics of 13 contemporary resin core materials.

MATERIALS AND METHODS

Specimens (n = 12) were fabricated using stainless steel molds with top surfaces of dual-cure products photopolymerized while additional groups were allowed to self-cure. Twelve Knoop hardness indentations 500 microns apart were obtained of photopolymerized top and bottom sample surfaces as well as the self-cured sample surface with the mean recorded as the representative sample hardness. Testing was completed at 10 minutes, 1 hour, and 24 hours. In addition, hardness values were compared to that obtained from polished coronal dentin samples. Mean data between groups were analyzed with Kruskal-Wallis/Dunn's, within groups with repeated measures ANOVA/Tukey's.

RESULTS

Hardness results were material dependent. All but two products demonstrated a 0.8 bottom/top Knoop hardness ratio at 10 minutes. Product's self-cure cure reaction did not attain hardness similarity with any photopolymerized top surfaces and while some materials were found to have similar dentin hardness to resin top surface ratio similarity, only one product had hardness equal to or greater than that of dentin during any time period.

CONCLUSIONS

Under this study's conditions, hardness development was material dependent and all but two products demonstrated adequate hardness-derived degree of cure assessment at 10 minutes after preparation. Self-cured samples demonstrated hardness increase; however, no self-cured material achieved hardness similarity to photopolymerized top surfaces. None of the materials achieved hardness similarity to dentin and only one product demonstrated hardness greater than that of dentin.

Repair Bond Strength of High-viscosity Glass-ionomer Cements Using Resin Composite Bonded with Light- and Self-cured Adhesive Systems

CLINICAL RELEVANCE

High-viscosity glass-ionomer cements (HVGICs) used with atraumatic restorative treatment can be repaired with light- or self-cured adhesive systems; however, the repair bond strength of two-step, self-etching and one-step adhesives in the light-cure mode surpass one-step self-cure adhesives. Working on a feasible self-cure approach in the absence of such in rural areas as well as in war zones is of prime importance.

SUMMARY

Fracture resistance, gap and void formation in root-filled mandibular molars restored with bulk-fill resin composites and glass-ionomer cement base

AIM

To evaluate fracture resistance and gap/void presence of root-filled mandibular molars restored with 2 bulk-fill and 1 conventional resin composites, with or without a glass-ionomer cement (GIC) base.

METHODS

Coronal access and mesio-occlusal (MO) cavities were prepared, then root canal treatment was performed on 30 mol/L. The teeth were randomly divided, according to the cavity volume, into 6 experimental groups (N = 5) and restored with conventional/light-cured (Ceram-X), bulk-fill/light-cured (SureFil SDR) or bulk-fill/dual-cured (Core-X Flow) with/without a 2-mm thick GIC base. Gaps and voids (%) were determined using microcomputed tomography. Intact teeth and unrestored teeth were used as negative and positive controls. Fracture load (N) was determined using a universal testing machine.

RESULTS

No significant difference in fracture resistance or gap/void formation was found among the 3 resin composites. GIC-base groups revealed significantly lower fracture strength than intact teeth, while fracture strengths of no GIC-base groups were not significantly different from intact teeth. GIC-base groups revealed significantly more gaps and voids in the area of the GIC than the resin composite.

CONCLUSION

Conventional and bulk-fill resin composites provided similar fracture resistance and gaps/voids in root-filled molars with MO cavities. Placing a GIC base decreased fracture resistance and increased gap/void formation.

Delayed Photoactivation of Dual-cure Composites: Effect on Cuspal Flexure, Depth-of-cure, and Mechanical Properties

Objectives:

This study tested whether delayed photoactivation could reduce shrinkage stresses in dual-cure composites and how it affected the depth-of-cure and mechanical properties.

Methods and Materials:

Two dual-cure composites (ACTIVA and Bulk EZ) were subjected to two polymerization protocols: photoactivation at 45 seconds (immediate) or 165 seconds (2 minutes delayed) after extrusion. Typodont premolars with standardized preparations were restored with the composites, and cuspal flexure caused by polymerization shrinkage was determined with three-dimensional scanning of the external tooth surfaces before restoration (baseline) and at 10 minutes and one hour after photoactivation. Bond integrity (intact interface) was verified with dye penetration. Depth-of-cure was determined by measuring Vickers hardness through the depth at 1-mm increments. Elastic modulus and maximum stress were determined by four-point bending tests (n=10). Results were analyzed with two- or three-way analysis of variance and pairwise comparisons (Bonferroni; α=0.05).

Results:

Delayed photoactivation significantly reduced cuspal flexure for both composites at 10 minutes and one hour (p≤0.003). Interface was &gt;99% intact in every group. Depth-of-cure, elastic modulus, and flexural strength were not significantly different between the immediate and delayed photoactivation (p&gt;0.05). The hardness of ACTIVA reduced significantly with depth (p&lt;0.001), whereas the hardness of Bulk EZ was constant throughout the depth (p=0.942).

Conclusions:

Delayed photoactivation of dual-cure restorative composites can reduce shrinkage stresses without negatively affecting the degree-of-cure or mechanical properties (elastic modulus and flexural strength).

Genotoxic potential of dental bulk-fill resin composites

OBJECTIVE

To investigate both genotoxicity and hardening of bulk-fill composite materials applied in 4-mm layer thickness and photo-activated for different exposure times.

METHODS

Three flowable bulk-fill materials and one conventional flowable composite were filled in molds (height: 4mm) and irradiated for 20 or 30s. The top (0mm) and bottom (4mm) specimen surface were mechanically scraped, and eluates (0.01g composite in 1.5ml RPMI 1640 cell culture media) prepared for each material, surface level and irradiation time. Genotoxicity was assessed in human leukocytes using both the alkaline comet assay and cytokinesis-blocked micronucleus assay, and Knoop hardness (KHN) was measured at the top and bottom specimen surface (n=8).

RESULTS

At both irradiation times, none of the bulk-fill composites significantly affected comet assay parameters used in primary DNA damage assessment or induced significant formation of any of the scored chromatin abnormalities (number of micronuclei, nuclear buds, nucleoplasmic bridges), whether eluates were obtained from the top or bottom surface. Furthermore, no decrease in KHN from the top to the bottom surface of the bulk-fill materials was observed. On the other hand, the conventional composite irradiated for 20s showed at 4-mm depth a significant increase in the percentage of DNA that migrated in the tail and a significant increase in the number of nuclear buds, as well as a significant decrease in KHN relative to the top surface.

SIGNIFICANCE

Bulk-fill resin composites, in contrast to conventional composite, applied in 4-mm thickness and photo-activated for at least 20s do not induce relevant genotoxic effects or mechanical instability.

Monomer conversion and shrinkage force kinetics of low-viscosity bulk-fill resin composites

OBJECTIVE

To investigate the subsurface degree of conversion (DC) and shrinkage force formation of low-viscosity (flowable) bulk-fill composite materials.

MATERIALS AND METHODS

Three flowable bulk-fill resin composites [SureFil SDR flow (SDR; Dentsply DeTrey), Venus Bulk Fill (VB; Heraeus Kulzer) and x-tra base (XB; VOCO)] and one conventional flowable control composite material [EsthetX flow (EX; Dentsply DeTrey)] were tested. The materials were photoactivated for 20 s at an irradiance of 1170 mW/cm2 and the DC (n=5) was recorded at 0.1-, 1.5- and 4-mm depth using Fourier transform infrared spectroscopy. Shrinkage forces (n=5) of 1.5-mm-thick specimens were continuously recorded for 15 min using a custom-made stress analyzer. Data were statistically analyzed by ANOVA, Tukey's HSD and Bonferroni's post-hoc tests (α=0.05).

RESULTS

SDR generated the significantly lowest shrinkage forces (22.9±1.4 N), but also attained the significantly lowest DC at 1.5-mm depth (67.5±0.8%). The conventional flowable composite EX generated the significantly highest shrinkage forces (40.7±0.7 N) and reached a significantly higher DC (74.4±1.3%) compared to SDR and XB at 1.5-mm depth. The shrinkage force values of VB (29.4±1.1 N) and XB (28.3±0.6 N) were similar (p>0.05). All materials attained significantly higher DC at 4-mm depth than at the near-surface.

CONCLUSION

The tested low-viscosity bulk-fill materials show lower shrinkage force formation than a conventional flowable resin composite at high levels of degree of conversion up to 4-mm incremental thickness.

Pre-heating of high-viscosity bulk-fill resin composites: effects on shrinkage force and monomer conversion

OBJECTIVES

To investigate the influence of pre-heating of high-viscosity bulk-fill composite materials on their degree of conversion and shrinkage force formation.

METHODS

Four bulk-fill composite materials (Tetric EvoCeram Bulk Fill-TECBF, x-tra fil-XF, QuixFil-QF, SonicFill-SF) and one conventional nano-hybrid resin composite (Tetric EvoCeram-TEC) were used. The test materials were either kept at room temperature or pre-heated to 68°C by means of a commercial heating device, before being photoactivated with a LED curing unit for 20s at 1170mW/cm(2). Shrinkage forces (n=5) of 1.5-mm-thick specimens were recorded in real-time for 15min inside a temperature-controlled chamber at 25°C (simulating intraoral temperature after rubber dam application) with a custom-made stress analyzer. Degree of conversion (n=5) was determined at the bottom of equally thick (1.5mm) specimens using Fourier transform infrared spectroscopy. Data were analyzed with Student's t-test, ANOVA and Tukey's HSD post-hoc test (α=0.05).

RESULTS

Composite pre-heating significantly increased the degree of conversion of TECBF, but had no effect on monomer conversion of the other materials investigated. For each of the test materials, pre-heated composite generated significantly lower shrinkage forces than room-temperature composite. At both temperature levels, TECBF created the significantly highest shrinkage forces, and QF caused significantly higher shrinkage forces than both XF and TEC.

CONCLUSIONS

Both the composite material and the pre-cure temperature affect shrinkage force formation. Pre-heating of bulk-fill and conventional restorative composites prior to photoactivation decreases polymerization-induced shrinkage forces without compromising the degree of conversion.

CLINICAL SIGNIFICANCE

Composite pre-heating significantly reduces shrinkage force formation of high-viscosity bulk-fill and conventional resin composites, while maintaining or increasing the degree of monomer conversion, dependent upon the specific composite material used.

Repair bond strength of dual-cured resin composite core buildup materials

The reparability of dual-cured resin composite core buildup materials using a light-cured one following one week or three months storage, prior to repair was evaluated. Two different dual-cured resin composites; Cosmecore™ DC automix and Clearfil™ DC automix core buildup materials and a light-cured nanofilled resin composite; Filtek™ Z350 XT were used. Substrate specimens were prepared (n = 12/each substrate material) and stored in artificial saliva at 37 °C either for one week or three months. Afterward, all specimens were ground flat, etched using Scotchbond™ phosphoric acid etchant and received Single Bond Universal adhesive system according to the manufacturers' instructions. The light-cured nanofilled resin composite (Filtek™ Z350 XT) was used as a repair material buildup. To determine the cohesive strength of each solid substrate material, additional specimens from each core material (n = 12) were prepared and stored for the same periods. Five sticks (0.8 ± 0.01 mm(2)) were obtained from each specimen (30 sticks/group) for microtensile bond strength (μTBS) testing. Modes of failure were also determined. Two-way ANOVA revealed a significant effect for the core materials but not for the storage periods or their interaction. After one week, dual-cured resin composite core buildup materials (Cosmecore™ DC and Clearfil™ DC) achieved significantly higher repair μTBS than the light-cured nanofilled resin composite (Filtek™ Z350 XT). However, Clearfil™ DC revealed the highest value, then Cosmecore™ DC and Filtek™ Z350 XT, following storage for 3-month. Repair strength values recovered 64-86% of the cohesive strengths of solid substrate materials. The predominant mode of failure was the mixed type. Dual-cured resin composite core buildup materials revealed acceptable repair bond strength values even after 3-month storage.

Effect of modulated photo-activation on polymerization shrinkage behavior of dental restorative resin composites

This study investigated the influence of modulated photo-activation on axial polymerization shrinkage, shrinkage force, and hardening of light- and dual-curing resin-based composites. Three light-curing resin composites (SDR bulk-fill, Esthet X flow, and Esthet X HD) and one dual-curing material (Rebilda DC) were subjected to different irradiation protocols with identical energy density (27 J cm(-2) ): high-intensity continuous light (HIC), low-intensity continuous light (LIC), soft-start (SS), and pulse-delay curing (PD). Axial shrinkage and shrinkage force of 1.5-mm-thick specimens were recorded in real time for 15 min using custom-made devices. Knoop hardness was determined at the end of the observation period. Statistical analysis revealed no significant differences among the curing protocols for both Knoop hardness and axial shrinkage, irrespective of the composite material. Pulse-delay curing generated the significantly lowest shrinkage forces within the three light-curing materials SDR bulk-fill, Esthet X flow, and Esthet X HD. High-intensity continuous light created the significantly highest shrinkage forces within Esthet X HD and Rebilda DC, and caused significantly higher forces than LIC within Esthet X flow. In conclusion, both the composite material and the applied curing protocol control shrinkage force formation. Pulse-delay curing decreases shrinkage forces compared with high-intensity continuous irradiation without affecting hardening and axial polymerization shrinkage.

Effect of luting agents on the tensile bond strength of glass fiber posts: An in vitro study

STATEMENT OF PROBLEM

Fiber posts can fail because of loss of retention; and it is unknown which luting agent provides the highest bond strength.

PURPOSE

The purpose of this study was to investigate the tensile bond strength of glass fiber posts luted to premolar teeth with 6 resin composite luting agents.

MATERIAL AND METHODS

Ninety-six single-rooted extracted human mandibular premolars were sectioned 2 mm coronal to the most incisal point of the cementoenamel junction. Root canals were instrumented and obturated with laterally condensed gutta percha and root canal sealer (AH26). Gutta percha was removed from the canals to a depth of 8 mm and diameter post spaces with a 1.5 mm were prepared. The specimens were divided into the following 6 groups according to the luting agent used (n=16): Group V, Variolink II; Group A, RelyX ARC; Group N, Multilink N; Group U, RelyX Unicem; Group P, ParaCore; Group F, MultiCore Flow. Each specimen was secured in a universal testing machine and a separating load was applied at a rate of 0.5 mm/min. The forces required to dislodge the posts were recorded. A 1-way analysis of variance (ANOVA) was applied to the mean retentive strengths of various cement materials (α=.05).

RESULTS

Significant differences were recorded among the 6 cement types (P<.001). Three materials provided statistically equivalent mean bond strengths (RelyX Unicem, Paracore, and MultiCore Flow) that were significantly greater than for the other 3 materials.

CONCLUSIONS

Fiber posts luted with RelyX Unicem, Paracore, and MultiCore Flow demonstrated significantly higher bond strengths.

Hydroxyapatite induces spontaneous polymerization of model self-etch dental adhesives

The objective of this study is to report for the first time the spontaneous polymerization phenomenon of self-etch dental adhesives induced by hydroxylapatite (HAp). Model self-etch adhesives were prepared by using a monomer mixture of bis[2-(methacryloyloxy)ethyl] phosphate (2MP) with 2-hydroxyethyl methacrylate (HEMA). The initiator system consisted of camphorquinone (CQ, 0.022 mmol/g) and ethyl 4-dimethylaminobenzoate (4E, 0.022-0.088 mmol/g). HAp (2-8 wt.%) was added to the neat model adhesive. In a dark environment, the polymerization was monitored in-situ using ATR/FT-IR, and the mechanical properties of the polymerized adhesives were evaluated using nanoindentation technique. Results indicated that spontaneous polymerization was not observed in the absence of HAp. However, as different amounts of HAp were incorporated into the adhesives, spontaneous polymerization was induced. Higher HAp content led to higher degree of conversion (DC), higher rate of polymerization (RP) and shorter induction period (IP). In addition, higher 4E content also elevated DC and RP and reduced IP of the adhesives. Nanoindentation result suggested that the Young's modulus of the polymerized adhesives showed similar dependence on HAp and 4E contents. In summary, interaction with HAp could induce spontaneous polymerization of the model self-etch adhesives. This result provides important information for understanding the initiation mechanism of the self-etch adhesives, and may be of clinical significance to strengthen the adhesive/dentin interface based on the finding.