Effect of radical amplified photopolymerization (RAP) in resin-based composites

Diversity of short-term DC outcomes in bulk-fill RBCs subjected to a 3 s high-irradiance protocol

OBJECTIVES

To determine the short-term (5 min) initial effects of a high-irradiance light-curing (LC) protocol on light transmission (LT%), radiant exposure (RE) and degree of conversion (DC%) of different bulk-fill resin-based composites (RBCs).

MATERIALS AND METHODS

Six bulk-fill composites with different viscosities were investigated: OBF (One Bulk Fill, 3 M), EB (Estelite bulkfill,Tokuyama), PFill, PFlow, ECeram and EFlow (PowerFill, Poweflow, Tetric EvoCeram bulkfill, Tetric Evoflow bulkfill, Ivoclar), subjected to different LC protocols: one ultra-high-intensity (3 W/cm2 -3 s via PowerCure LCU) and two conventional (1.2 W/cm2 -10 s and 20 s via PowerCure and Elipar S10 LCUs). Specimens (n = 5) were polymerized within their molds (ϕ5 mm × 4 mm depth) to determine LT% and RE at 4 mm using a MARC-LC spectrometer. For real-time DC% measurements by FTIR, similar molds were utilized. Data were analyzed by one-way ANOVA and Tukey post-hoc tests at 5 % significance.

RESULTS

Regardless of the applied LC protocols, OBF and low-viscosity RBCs (EB, PFlow and EFlow) had the lowest and highest LT%, RE, DC% and RPmax, respectively. RE results of all RBCs were in the same sequence: Elipar-20 s > PCure-10 s > PCure-3 s. DC% of PFill and PFlow displayed no significant difference between the applied LC protocols (p > 0.05). The polymerization kinetic in all materials was well described by an exponential sum function (r2 varied between 0.85 and 0.98), showing a faster polymerization with the PCure-3 s protocol.

SIGNIFICANCE

The measurement of LT% and DC% at 5 min gave an insight into the developing polymerization process. The initial response of these bulk-fill composite to a high-irradiation protocol varied depending on their composition and viscosity, being faster for low viscosity materials. Nevertheless, even though multiple resin composites are designed to be efficient during photopolymerization, care should be taken when selecting materials/curing protocol.

Comparison of a Nanofiber-Reinforced Composite with Different Types of Composite Resins

The aim of this study was a comprehensive evaluation and comparison of the physical and mechanical properties of a newly developed nano-sized hydroxyapatite fiber-reinforced composite with other fiber-reinforced and particle-filled composites. Commercially available eight composite resins (3 fiber-reinforced and 5 particle-filled) were used: Fiber-reinforced composites: (1) NovaPro Fill (Nanova): newly developed nano-sized hydroxyapatite fiber-reinforced composite (nHAFC-NF); (2) Alert (Pentron): micrometer-scale glass fiber-reinforced composite (µmGFC-AL); (3) Ever X Posterior (GC Corp): millimeter-scale glass fiber-reinforced composite (mmGFC-EX); Particle-filled composites: (4) SDR Plus (Dentsply) low-viscosity bulk-fill (LVBF-SDR); (5) Estelite Bulk Fill (Tokuyama Corp.) low-viscosity bulk-fill (LVBF-EBF); (6) Filtek Bulk Fill Flow (3M ESPE) low-viscosity bulk-fill (LVBF-FBFF); (7) Filtek Bulk Fill (3M ESPE) high-viscosity bulk-fill (HVBF-FBF); and (8) Filtek Z250 (3M ESPE): microhybrid composite (µH-FZ). For Vickers microhardness, cylindrical-shaped specimens (diameter: 4 mm, height: 2 mm) were fabricated (n = 10). For the three-point bending test, bar-shaped (2 × 2 × 25 mm) specimens were fabricated (n = 10). Flexural strength and modulus elasticity were calculated. AcuVol, a video image device, was used for volumetric polymerization shrinkage (VPS) evaluations (n = 6). The polymerization degree of conversion (DC) was measured on the top and bottom surfaces with Fourier Transform Near-Infrared Spectroscopy (FTIR; n = 5). The data were statistically analyzed using one-way ANOVA, Tukey HSD, Welsch ANOVA, and Games-Howell tests (p < 0.05). Pearson coefficient correlation was used to determine the linear correlation. Group µH-FZ displayed the highest microhardness, flexural strength, and modulus elasticity, while Group HVBF-FBF exhibited significantly lower VPS than other composites. When comparing the fiber-reinforced composites, Group mmGFC-EX showed significantly higher microhardness, flexural strength, modulus elasticity, and lower VPS than Group nHAFC-NF but similar DC. A strong correlation was determined between microhardness, VPS and inorganic filler by wt% and vol% (r = 0.572-0.877). Fiber type and length could affect the physical and mechanical properties of fibers containing composite resins.

The Effect of Additional Finishing and Polishing Sequences on Hardness and Roughness of Two Different Dental Composites: An In Vitro Study

Aim:

The aim of this study was to evaluate the effect of additional finishing and polishing sequences on a new structural colored (Omnichroma) and a conventional dental composite (Estelite Sigma Quick).

Materials and Method:

Forty disk-shaped dental composite specimens were prepared from each dental composite and assigned to four groups in terms of additional finishing polishing sequences. Group 1: Mylar strip (control). Group 2: Abrasive disks. Group 3: After abrasive disks, application of a felt brush with abrasive paste. Group 4: After abrasive disks, application of a felt brush with abrasive paste. Then, a single-bottle adhesive was applied as a surface sealer. Hardness and surface roughness were measured using the Vickers hardness test and a profilometer, respectively. SEM images of one of each group were captured at 500x, 1500x, and 3500x magnifications. Two-way analysis of variance, Tukey HSD, and Shapiro–Wilk tests were used for the statistical analysis.

Results:

Mean hardness values differ in terms of finishing-polishing sequences ( P < .001), dental composites ( P < .001), and interaction of finishing-polishing sequences-dental composites ( P < .001). Mean roughness values differ in terms of finishing-polishing ( P = .002). The main effect of composites on roughness values was not found statistically significant ( P = .990). Also, there is no difference between the mean roughness values in terms of the interactions of finishing-polishing and composites ( P = .967).

Conclusion:

Finishing-polishing with abrasive disks and abrasive paste are important steps for these dental composites. However, the application of a single-bottle adhesive as a surface sealant on these composites decreases the hardness of the materials.

Cytotoxicity of Methacrylate Dental Resins to Human Gingival Fibroblasts

This study aimed to assess the acute and delayed cytotoxicity of three, popular light-cured methacrylate-based restorative resins (MRs): Charisma (C), Estelite (E), and Filtek (F), to human gingival fibroblasts in culture. Cells were grown for up to 24 h with light-cured (or pre-cured) resins. We evaluated resin cytotoxicity, redox imbalance, necrosis/apoptosis, miR-9, and heat shock protein 70 (HSP70). The role of resin-induced oxidative stress (damage) in HSP70-response (repair) was assessed using binary fluorescence labeling. All MRs decreased viable cell numbers and cell proliferation and damaged cell membranes, and their 24 h-delayed toxicity was lower (C), higher (F), or similar (E) to that induced by freshly-cured resins. Cell membrane damage induced by C and E decreased with time, while F produced a linear increase. All resins generated intracellular oxidative stress with the predominant necrotic outcome, and produced heterogeneous responses in miR-9 and HSP70. The double fluorescence (damage/repair) experiments pointed to common features of E and F but not C. In the subset of cells, the binary response induced by E and F was different from C, similar to each other, and positively interrelated. Experimental data show that selective MR cytotoxicity should be taken into account when considering repetitive use or massive reconstruction.

The Influence of Various Photoinitiators on the Properties of Commercial Dental Composites

The aim of this article was to compare the biomechanical properties of commercial composites containing different photoinitiators: Filtek Ultimate (3M ESPE) containing camphorquinone (CQ); Estelite Σ Quick (Tokuyama Dental) with CQ in RAP Technology^®; Tetric EvoCeram Bleach BLXL (Ivoclar Vivadent AG) with CQ and Lucirin TPO; and Tetric Evoceram Powerfill IVB (Ivoclar Vivadent AG) with CQ and Ivocerin TPO. All samples were cured with a polywave Valo Lamp (Ultradent Products Inc.) with 1450 mW/cm². The microhardness, hardness by Vicker's method, diametral tensile strength, flexural strength and contraction stress with photoelastic analysis were tested. The highest hardness and microhardness were observed for Filtek Ultimate (93.82 ± 17.44 HV), but other composites also displayed sufficient values (from 52 ± 3.92 to 58,82 ± 7.33 HV). Filtek Ultimate not only demonstrated the highest DTS (48.03 ± 5.97 MPa) and FS (87.32 ± 19.03 MPa) but also the highest contraction stress (13.7 ± 0.4 MPa) during polymerization. The TetricEvoCeram Powerfill has optimal microhardness (54.27 ± 4.1 HV), DTS (32.5 ± 5.29 MPa) and FS (79.3 ± 14.37 MPa) and the lowest contraction stress (7.4 ± 1 MPa) during photopolymerization. To summarize, Filtek Ultimate demonstrated the highest microhardness, FS and DTS values; however, composites with additional photoinitiators such as Lucirin TPO and Ivocerin have the lowest polymerization shrinkage. These composites also have higher FS and DTS and microhardness than material containing CQ in Rap Technology.

Post-irradiation surface viscoelastic integrity of photo-polymerized resin-based composites

OBJECTIVE

A class of ultra-rapid-cure resin-based composites (RBCs) exhibited immediate post-irradiation surface viscoelastic integrity using an indentation-creep/recovery procedure. The aim of this study was to determine whether such behavior is more generally characteristic of a wider range of RBCs.

METHODS

Eight representative RBCs were selected based on different clinical categories: three bulkfills (OBF, Filtek One Bulk Fill; VBF, Venus Bulkfill; EBF, Estelite Bulkfill), three conventional non-flowables (XTE, Filtek Supreme XTE; GSO, GrandioSo; HRZ, Harmonize) and conventional flowables (XTF, Filtek Supreme XTE Flow; GSF, GrandioSo Flow). Stainless steel split molds were used to fabricate cylindrical specimens (4mm (dia)×4mm). These were irradiated (1.2W/cm²) for 20s on the top surface. Post-irradiation specimens (n=3), within their molds, were centrally loaded with a flat-ended 1.5mm diameter indenter under 14MPa stress: either immediately (<2min) or after 24h delayed indentation. Stress was maintained for 2h, then - after removal - recovery measurements continued for a further 2h. Indentation depth (%) versus time was measured continuously to an accuracy of <0.1μm. Data were analyzed by One-way ANOVA and Tukey post-hoc tests (α=0.05).

RESULTS

Time-dependent viscoelastic indentation was observed for all RBCs. For immediate indentation, the maximum indentation range was 1.43-4.92%, versus 0.70-2.22% for 24h delayed indentation. Following 2h recovery, the residual indentation range was 0.86-3.58% after immediate indentation, reducing to 0.22-1.27% for delayed indentation. The greatest immediate indentation was shown by VBF followed by XTF and GSF. OBF, HRZ, XTE and GSO had significantly lower indentations (greater hardness). XTE showed a significantly reduced indentation maximum compared to OBF (p<0.05). Indentations delayed until 24h post-irradiation were reduced (p<0.05) for most materials.

SIGNIFICANCE

The indentation-creep methodology effectively characterized resin-based composites within several categories. Viscoelastic properties evaluated by the indentation-creep method confirmed that highly filled RBCs were more resistant to indentation. Indentations were reduced after 24h post-irradiation due to further matrix-network development.

Applicability of Exposure Reciprocity Law for Fast Polymerization of Restorative Composites Containing Various Photoinitiating Systems

OBJECTIVES

The exposure reciprocity law (ERL) has been used to calculate the optimal irradiation time of dental composites. This study examined the applicability of ERL for fast polymerization of restorative composites containing various photoinitiating systems using a high-power multi-peak light-emitting diode (LED) lamp.

METHODS

Three commercial composites differing in photoinitiating systems were tested: Filtek Ultimate Universal Restorative (FU) with a camphorquinone-amine (CQ-A) photoinitiating system, Tetric EvoCeram (TEC) with CQ-A and (2,4,6-trimethylbenzoyl)phosphine oxide (TPO), and Estelite Σ Quick (ESQ) with CQ and a radical amplified photopolymerization (RAP) initiator. Specimens 2-mm thick were polymerized using a high-power multipeak LED lamp (Valo) at 3 pairs of radiant exposures (referred to as low, moderate, and high) ranging from 15.8-26.7 J/cm2. They were achieved by different combinations of irradiation time (5-20 seconds) and irradiance (1300-2980 mW/cm2) as determined with a calibrated spectrometer. Knoop microhardness was measured 1, 24, and 168 hours after polymerization on specimen top (irradiated) and bottom surfaces to characterize the degree of polymerization. The results were statistically analyzed using a three-way analysis of variance and Tukey's post hoc tests, α = 0.05.

RESULTS

Microhardness increased with radiant exposure and except for ESQ, top-surface microhardness was significantly higher than that on bottom surfaces. Combinations of high irradiance and short irradiation time significantly increased the top-surface microhardness of TEC at low and moderate radiant exposures, and the bottom-surface microhardness of FU at a low radiant exposure. In contrast, the microhardness of ESQ on both surfaces at high radiant exposure increased significantly when low irradiance and long irradiation time were used. With all tested composites, bottom-surface microhardness obtained at low radiant exposure was below 80% of the maximum top-surface microhardness, indicating insufficient polymerization.

CONCLUSION

Combinations of irradiance and irradiation time had a significant effect on microhardness, which was affected by photoinitiators and the optical properties of composites as well as spectral characteristics of the polymerization lamp. Therefore, ERL cannot be universally applied for the calculation of optimal composite irradiation time. Despite high irradiance, fast polymerization led to insufficient bottom-surface microhardness, suggesting the necessity to also characterize the degree of polymerization on the bottom surfaces of composite increments when assessing the validity of ERL.

Effect of the Photo-initiator System Contained in Universal Adhesives on Radicular Dentin Bonding

CLINICAL RELEVANCE

The photo-initiator system based on an advanced polymerization system may be an alternative that can be used to overcome the disadvantages of radicular dentin, especially for the apical third.

SUMMARY

Objectives: The purpose of this study was to evaluate the effects of universal adhesives with different photo-initiator systems applied in etch-and-rinse (ER) and self-etch (SE) modes on dentin interaction (push-out bond strength [PBS], nanoleakage [NL], and degree of conversion [DC] within the hybrid layer) in the different root thirds after fiber post cementation.Methods and Materials: Roots of endodontically prepared human premolars were randomly divided into six groups according to one of three adhesive systems (Scotchbond Universal [SBU], Ambar Universal [AMB], and Ambar Universal APS [AMB-APS]) and two adhesive strategies (ER and SE) for each system. Posts were cemented, and PBS was tested at 0.5 mm/min. The NL was evaluated by scanning electron microscopy. DC was measured using micro-Raman spectroscopy. The data were analyzed by three-way analysis of variance and Tukey tests (α=0.05).Results: AMB-APS showed similar performance in all root thirds (p>0.05) and higher values of DC, especially in the apical third (p<0.0001). AMB and SBU showed the lowest values in the apical third (p<0.0001).Conclusions: The APS photo-initiator system contained in universal adhesives is a feasible alternative for improving radicular bonding procedure.

Effect of light-curing time on light-cure/post-cure volumetric polymerization shrinkage and regional ultimate tensile strength at different depths of bulk-fill resin composites

The aim of this study was to investigate volumetric polymerization shrinkage (VS), using swept-source optical coherence tomography (SS-OCT), of bulk-fill composites with different light-curing strategies; immediately after light-irradiation and after 24 h, and to evaluate their regional ultimate tensile strength (UTS) at different curing depths. The immediate VS after photo-polymerization decreased when the curing light-curing time was reduced from 20 to 10 to 5 s. On the other hand, their VS values after 24 h significantly increased due to the post-cure polymerization, resulting in similar VS values at all the light-curing times. Five seconds light-curing decreased the regional UTS of resin composites after the 24 h period compared with the 10 and 20 s light-curing regimes; therefore, the remarkable progress of post-cure polymerization after light-curing for a short time would not lead to an improvement in the mechanical properties of resin composites. The influence of the light-curing time on the curing depth was dependent upon the bulk-fill composite material.

Surface roughness and filler particles characterization of resin-based composites

The purpose of this study was to evaluate the surface roughness (Ra), and the morphology and composition of filler particles of different composites submitted to toothbrushing and water storage. Disc-shaped specimens (15 mm × 2 mm) were made from five composites: two conventional (Z100™, and Filtek™ Supreme Ultra Universal, 3M), one "quick-cure" (Estelite ∑ Quick, Tokuyama), one fluoride-releasing (Beautiful II, Shofu), and one self-adhering (Vertise Flow, Kerr) composite. Samples were finished/polished using aluminum oxide discs (Sof-Lex, 3M), and their surfaces were analyzed by profilometry (n = 5) and scanning electron microscopy (SEM; n = 3) at 1 week and after 30,000 toothbrushing cycles and 6-month water storage. Ra data were analyzed by two-way analysis of variance and Tukey's test (α = 0.05). Filler particles morphology and composition were analyzed by SEM and X-ray dispersive energy spectroscopy, respectively. Finishing/polishing resulted in similar Ra for all the composites, while toothbrushing and water storage increased the Ra of all the tested materials, also changing their surface morphology. Beautifil II and Vertise Flow presented the highest Ra after toothbrushing and water storage. Filler particles were mainly composed of silicon, zirconium, aluminum, barium, and ytterbium. Size and morphology of fillers, and composition of the tested composites influenced their Ra when samples were submitted to toothbrushing and water storage.

Does translucency influence cure efficiency and color stability of resin-based composites?

OBJECTIVE

To determine if material's translucency influences the cure efficiency and color stability of resin-based composites (RBCs).

METHODS

Four commercially-available RBCs indicated for aesthetic restorations were selected in different translucent/opaque shades: IPS Empress Direct (IED) A3 Dentin, A3 Enamel, Trans 20, and Trans 30; Filtek Z350 XT (FZX) A3D, A3B, A3E, and CT; Estelite ∑ Quick (EQ) OA3, A3, and CE; and Opallis (OP) DA3, EA3, and T-Neutral. Color was obtained in the L'C'h' system at three distinct periods: 24h after photoactivation (baseline), after 30d of water storage (WS), and after 30d of coffee storage (CS). The translucency parameter (TP) of each RBC was calculated at baseline. The degree of CC conversion (DC) was obtained by Fourier-transform infrared spectroscopy at 0.05mm (top) and 2mm (bottom) surfaces; the cure efficiency considered the bottom/top ratio. The CIEDE2000 color difference (ΔE₀₀) was calculated considering the WS-baseline and CS-baseline values. Data were submitted to two-way ANOVA and Tukey's post-hoc test (α=0.05). Pearson's tests were used to analyze the correlations between TP and DC, and between TP and ΔE₀₀.

RESULTS

For all RBCs, cure efficiency was not affected by materials' translucency. A positive, significant correlation was observed between TP and DC at the bottom surface for FZX only. For all RBCs, the higher the TP, the higher the ΔE₀₀. The ΔE₀₀ was higher after CS than after WS, except for EQ A3. Positive correlation between TP and ΔE₀₀ were observed for all materials.

CONCLUSION

The translucency did not influence the cure efficiency but affected the color stability for all RBCs.

CLINICAL SIGNIFICANCE

High-translucent RBCs presented lower color stability and should be used carefully.

UV-Cleavable Polyrotaxane Cross-Linker for Modulating Mechanical Strength of Photocurable Resin Plastics

A UV-cleavable supramolecular cross-linker was designed to effectively control the mechanical strength of photocurable resin plastics. The resin monomer-soluble polyrotaxane (PRX) cross-linker was synthesized by introducing a hydrophobic n-butyl group and a cross-linkable methacrylate group in α-cyclodextrin threading to a polyethylene glycol containing UV-cleavable end groups. The UV-cleavable PRX cross-linker was completely dissolved in 2-hydroxyethytl methacrylate (HEMA) and camphorquinone, representative photocurable resin components. The dumbbell-like stiff resin plastic was prepared by irradiating the mixture with 450 nm blue light. The stiffened resin plastic maintained its ultimate tensile strength (UTS) under visible light irradiation. However, the UTS of the resin plastic was remarkably decreased to 40% of the original value once the plastic was exposed to 254 nm UV light. This indicates that the suggested UV-cleavable PRX cross-linker is effective in modulating the mechanical strength of photocurable resin plastics.