Specimen aspect ratio and light transmission in photoactive dental resins

Achieving property enhancements in dental resin composites via reduced concentrations of camphorquinone within a ternary initiator system

OBJECTIVES

The study aimed to assess the impact of diphenyliodonium hexafluorophosphate (DPI) on the physicochemical properties of experimental resin composites (ECRs) featuring reduced concentrations of camphorquinone (CQ)/amine.

METHODS

Five concentrations of CQ (0.125, 0.25, 0.5, 0.75, and 1 mol%) with dimethylaminoethyl amine benzoate (EDAB) in a 1:2 mol% ratio (CQ:EDAB) were incorporated into a 50:50 mass% monomer blend of bisphenol glycidyl methacrylate (BisGMA) and triethyleneglycol dimethacrylate (TEGDMA). An additional 5 groups with the same CQ:EDAB concentrations had 0.5 mol% DPI added. Each resin group contained 60 wt% of 0.7 µm barium-alumino-silicate glass. Light transmission (n = 3), real-time degree of polymerization (n = 3), temperature change during polymerization (n = 5), polymerization shrinkage strain (n = 3), flexural strength, and modulus (n = 12), as well as water sorption and solubility (n = 5), were evaluated. Data were analyzed using two-way ANOVA and Tukey's post-hoc test (α = 0.05).

RESULTS

Light transmission was reduced in groups containing 0.125 and 0.25 mol% of CQ without DPI. DPI increased temperature, degree and rate of polymerization, despite the reduction in CQ/amine concentration. Additionally, there was an increase in polymerization shrinkage strain, flexural strength and modulus, and a reduction in water sorption and solubility in ECRs with DPI, even with lower concentrations of CQ/EDAB.

SIGNIFICANCE

DPI improved the assessed properties of composites across various concentrations of CQ/EDAB, showing the benefit of reducing the quantity of CQ used without compromising the properties and curing of the resin composites.

The effect of photoinitiator type and filler load on physicochemical and mechanical properties of experimental light-cured resin cements through lithium disilicate ceramics of different shades and thicknesses

OBJECTIVE

This study investigated the influence of photoinitiator types on degree of conversion (DC), rate of polymerization (RP), flexural strength (FS), flexural modulus (FM), and light transmittance (LT) of filled and unfilled light-curable resin cements through different thicknesses and shades of lithium disilicate ceramics.

METHODS

Lithium disilicate ceramic discs (IPS Emax Press, background [0.0], 0.5, 1.0, 2.0, 3.0, and 4.0 mm, shades A1 and BL3) were prepared. Experimental resin-based cements [TEGDMA/BisGMA (50/50 mass%)] were prepared using either camphorquinone (CQ)/amine (0.44/1.85 mol%) or TPO (0.44 mol%)], and a micro and nanofiller loads of nil (unfilled); 40/10 mass%; and 50/10 mass%). Resin cements (0.2 mm thick) were placed on the lower surface of the ceramic specimens and light-activated for 30 s from the upper surface using a Bluephase Style curing light (exitance at tip: 1236 mW/cm2 ± 1.20). LT and distribution of irradiance through the ceramics were measured using a UV-vis spectrometer and a beam profile camera, respectively (n = 3). The DC and RP were measured in real-time using mid infrared spectroscopy in attenuated total reflectance (ATR) mode (n = 3). FS and FM were measured using a universal testing machine (n = 5). Statistical analyses were performed on LT, DC, RP, FS, and FM data using a general linear model, and supplementary ANOVA and post hoc Tukey multiple comparison test were also performed (α = .05).

RESULTS

Thicknesses, shades, photoinitiator type, and fillers load significantly influenced the optical and mechanical characteristics of the resin-based materials (p < 0.05). The BL3 shade ceramic provided higher values of DC, RP, FS, FM, and LT compared with the A1 shade (p < 0.05). Increasing ceramic thickness decreased the properties of the resin-based materials (p < 0.05). Generally, TPO improved mechanical properties of the resin cement compared with CQ (p < 0.05).

SIGNIFICANCE

The luting process of indirect restorations may be improved by using high molar absorptivity, more reactive, and more efficient photoinitiators such as TPO, as opposed to conventional CQ. The use of such initiator may allow the placement of thicker and more opaque indirect restorations.

Mechanical Properties of Experimental Composites with Different Photoinitiator

Objective  The effect of different photoinitiators on mechanical properties of experimental composites was evaluated.

Materials and Methods  Resin composites were formulated by using a blend of bisphenol A-glycidyl and triethylene glycol (50/50 wt%) dimethacrylate monomers, and 65 wt% of barium aluminium silicate and silica filler particles. Photoinitiators used were 0.2% camphorquinone (CQ) and 0.8% co-initiator (DMAEMA); 0.2% phenyl-propanedione and 0.8% DMAEMA; 0.1% CQ + 0.1% phenyl propanedione and 0.8% DMAEMA; 0.42% mono(acyl)phosphine oxide (MAPO); and 0.5% bis(acyl)phosphine oxide (BAPO). Specimens ( n = 10) were light cured by using a multiple-emission peak light-emitting diode for 20 seconds at 1,200 mW/cm ² of irradiance and Knoop hardness and plasticization, depth of cure, flexural strength, and elastic modulus were evaluated. Data were statiscally analyzed at significance level of α = 5%.

Results  Experimental composites containing MAPO and BAPO photoinitiators showed the highest values of flexural strength, elastic modulus, top surface hardness, and lower hardness reduction caused by alcohol compared with CQ. Composites containing CQ and PPD showed similar results, except for depth of cure and hardness of bottom surface.

Conclusion  BAPO and MAPO showed higher flexural strength, elastic modulus, hardness on top surface, and lower polymer plasticization to CQ.

On the inaccuracies of dental radiometers

This study investigated the accuracy of sixteen models of commercial dental radiometers (DR) in measuring the output of thirty-eight LED light curing units (LCUs) compared with a 'gold standard' laboratory-grade spectrometer integrating-sphere (IS) assembly. Nineteen Type I (fiber-bundle light guide) and nineteen Type II (light source in head) LED LCUs were tested, some using different output modes and light guides, resulting in 61 test subsets per radiometer. Gold standard (GS) output measurements (n = 3) were taken using the IS and confirmed with two types of laboratory-grade power meter (PowerMax-Pro 150 HD and PM10-19C; Coherent). One DR (Bluephase Meter II, Ivoclar; BM II) allowed power (mW) as well as irradiance (mW/cm²) recordings. Irradiance readings (n = 3) for each DR/LCU were compared with the IS derived irradiance. Individual LCU irradiance values were normalized against IS data. The GS method yielded reproducible data with a 0.4% pooled coefficient of variation for the LCUs. Mean power values ranged from 0.19 W to 2.40 W. Overall power values for the laboratory-grade power meters were within 5% of GS values. Individual LCU/DR normalized irradiance values ranged from 7% to 535% of the GS; an order of magnitude greater than previous reports. BM II was the only radiometer to average within 20% of normalized pooled GS irradiance values, whereas other radiometers differed by up to 85%. Ten radiometers failed to provide any reading for 1 LCU. When tested with the PowerMax-Pro in high speed (20 kHz) mode, eight LCUs demonstrated pulsing outputs undetectable at the standard (10 Hz) data acquisition rate. Sufficient light exposure is critical for the successful curing of dental resin-based materials. Substantial discrepancies may occur between actual and estimated radiometric data using current DRs. More accurate DRs need to be developed. Manufacturers' accuracy claims for DRs should specify compatible LCUs and testing parameters.

Vickers Hardness and Shrinkage Stress Evaluation of Low and High Viscosity Bulk-Fill Resin Composite

The aim of this in vitro study was to evaluate the hardness and shrinkage stress (SS) of six bulk-fill resin composites. To evaluate microhardness (MH), ten 6 mm specimens were prepared using a metal mold for each selected bulk-fill resin composite and irradiated from the top side for 40 s using an LED light. After 24 h of storage, Vickers MH was evaluated on the upper, lower and lateral sides of the specimens. SS evaluation was then performed with a universal machine, which evaluated the contraction force generated by a bulk-fill composite specimen placed between two metal cylinders during and after light curing. The results were evaluated with a one-way ANOVA test with a post-hoc Bonferroni test and linear regression analysis (p < 0.05). All materials showed a significant MH decrease between the top and bottom surfaces. However, the bulk-fill materials tested performed differently when considering lateral depth progression. ANOVA tests for SS evaluation showed that both SDR and Venus Bulk Fill had significantly lower stress during irradiation than other tested materials. Further, MH decrease became significantly lower from the top surface at different depths in each tested group. Among the different resins, Venus Bulk Fill and SDR showed not only inferior hardness, but also a significant reduction in SS.

Optical and mechanical factors in the temporal development of tooth-composite bond

OBJECTIVES

To obtain an accurate picture of the temporal development of bond strength between resin composites and tooth structures during cure for assessing debonding at the tooth-composite interface.

METHODS

An assembly of uncured composite sandwiched between a glass block and a dentin slab with a layer of pre-cured adhesive was used in this study. A conventional composite was compared against a bulk-fill composite. The rate of bond formation was determined by measuring the tensile bond strength of specimens of different thicknesses at different time points during cure. The changing light irradiance exiting the composite as it cured was also recorded. Mode of fracture was analyzed by examining the fracture surfaces.

RESULTS

Photo-bleaching occurred in both resin composites. The development of the dentin-composite bond strength was initially dictated by the developing cohesive strength of the resin composite, and its final value was capped by the strength of the preformed dentin-adhesive bond. The higher interfacial irradiance in the bulk-fill composite did not lead to faster development of the overall bond strength. This was caused by its slower rate of cohesive strength development as reflected in the longer time for its interfacial irradiance to plateau and the greater proportion of cohesive failure seen in the initial stage of polymerization. The law of reciprocity did not hold for the development of dentin bond strength.

SIGNIFICANCE

The results from this study, when compared with the development of shrinkage stress, can be used as a basis for ensuring the integrity of the dentin-composite interface during cure.

Light-emitting Diode Beam Profile and Spectral Output Influence on the Degree of Conversion of Bulk Fill Composites

Objectives:

To evaluate the beam profile and the spectral output of monowave and polywave light-emitting diodes (LEDs) and their influence on the degree of conversion (DC) of bulk fill composites.

Methods:

A monowave LED (Smartlite Focus, Dentsply) and a polywave LED (Valo Cordless, Ultradent) were characterized using a resin calibrator and a laser beam profile analyzer. Two bulk fill composites, Sonic Fill 2 (SF) containing camphorquinone (CQ) and Tetric EvoCeram Bulk Fill (TEB) containing CQ associated with alternative photoinitiators, were placed in custom-designed molds (n=3) and photoactivated by the monowave or polywave LED with 20 J/cm2. To map the DC, longitudinal cross sections (0.5 mm thick) from the center of the restoration were evaluated using FT-NIR microscopy. SF and TEB light transmittances (n=3) through 4-mm-thick specimens were evaluated during curing. Data were analyzed using a split-plot analysis of variance and Tukey test (α=0.05; β=0.2).

Results:

The monowave LED had a radiant emittance of 20 ± 0.5 J/cm2 over 420-495 nm, and the polywave LED had an emittance of 15.5 ± 0.4 J/cm2 over 420-495 nm and of 4.5 ± 0.2 J/cm2 over 380-420 nm. The total radiant exposure at the bottom of TEB was 2.2 ± 0.2 J/cm2 with the monowave LED and 1.6 ± 0.3 J/cm2 with the polywave LED, and for SF it was 0.4 ± 0.1 J/cm2 for both LEDs. There were no differences in the curing profiles produced either by the monowave or the polywave LED (p=0.9), according to the regions under influence of blue and/or violet emission at the same depth. There was no statistical difference in the DC for SF using the monowave or polywave LED at any depth (p=0.29). TEB had a higher DC at up to 2 mm in depth when the polywave LED was used (p&lt;0.004), but no differences were found when starting at 2.5 mm.

Conclusions:

Monowave and polywave LEDs emitted nonhomogeneous light beams, but this did not affect the DC homogeneity of bulk fill composites. For composites containing CQ associated with alternative photoinitiators, polywave LEDs had a higher DC, but only at the top part of the restoration; lower wavelength absorption photoinitiators were ineffective in deeper areas.

Light transmittance and micro-mechanical properties of bulk fill vs. conventional resin based composites

OBJECTIVES

The aim of this study was to quantify the blue light that passes through different incremental thicknesses of bulk fill in comparison to conventional resin-based composites (RBCs) and to relate it to the induced mechanical properties.

MATERIALS AND METHODS

Seven bulk fill, five nanohybrid and two flowable RBCs were analysed. Specimens (n = 5) of three incremental thicknesses (2, 4 and 6 mm) were cured from the top for 20 s, while at the bottom, a spectrometer monitored in real time the transmitted irradiance. Micro-mechanical properties (Vickers hardness, HV, and indentation modulus, E) were measured at the top and bottom after 24 h of storage in distilled water at 37 °C. Electron microscope images were taken for assessing the filler distribution and size.

RESULTS

Bulk fill RBCs (except SonicFill) were more translucent than conventional RBCs. Low-viscosity bulk fill materials showed the lowest mechanical properties. HV depends highly on the following parameters: material (ηp (2) = 0.952), incremental thickness (0.826), filler volume (0.747), filler weight (0.746) and transmitted irradiance (0.491). The bottom-to-top HV ratio (HVbt) was higher than 80 % in all materials in 2- and 4-mm increments (except for Premise), whereas in 6-mm increments, this is valid only in four bulk fill materials (Venus Bulk Fill, SDR, x-tra fil, Tetric EvoCeram Bulk Fill).

CONCLUSIONS

The depth of cure is dependent on the RBC's translucency. Low-viscosity bulk fill RBCs have lower mechanical properties than all other types of analysed materials. All bulk fill RBCs (except SonicFill) are more translucent for blue light than conventional RBCs.

SIGNIFICANCE

Although bulk fill RBCs are generally more translucent, the practitioner has to follow the manufacturer's recommendations on curing technique and maximum incremental thickness.