Effect of power density of curing unit, exposure duration, and light guide distance on composite depth of cure

Effect of Different Light-tip Distances on Shear Bond Strength of Orthodontic Brackets Cured with Light-emitting Diode and High-intensity Light-emitting Diode

AIM

The aim was to find out whether the light-tip distance affected the shear bond strength of orthodontic brackets when cured with light-emitting diode (LED) and high-intensity LED at four different light-tip distances.

MATERIALS AND METHODS

Extracted human premolars were divided into eight groups. Each tooth was embedded in the self-cure acrylic resin block, and brackets were bonded and cured with different lights and different distances. Shear bond strength tests were performed in vitro using the universal testing machine. Data were analyzed using one-way ANOVA test.

RESULTS

The descriptive statistics for shear bond strength of orthodontic brackets cured with LED light at 0 mm was 8.49 ± 1.08 MPa, at 3 mm was 8.13 ± 0.85 MPa, 6 mm was 6.42 ± 0.42 MPa, and at 9 mm was 5.24 ± 0.92 MPa, and those cured with high-intensity light at 0 mm was 19.23 ± 4.83 MPa, at 3 mm was 17.65 ± 3.28 MPa, at 6 mm was 13.04 ± 2.36 MPa, and at 9 mm was 11.74 ± 1.4 MPa. Mean shear bond strength was found to decrease as the light-tip distance increased with both light sources.

CONCLUSION

Shear bond strength is higher when the light source is close to the surface to be cured, and it decreases as the distance increases. The highest shear bond strength was achieved with high-intensity light.

CLINICAL SIGNIFICANCE

Light-emitting diode or high-intensity units can be used for bonding orthodontic brackets without compromising the shear bond strength of the brackets, and that shear bond strength is stronger when the light source is close to the surface to be cured, and it decreases as the distance increases between the light source and the surface.

Effect of fiber post space irrigation with different peracetic acid formulations on the bond strength and penetration into the dentinal tubules of self-etching resin cement

STATEMENT OF PROBLEM

The post space must be irrigated with solutions that do not interfere with the bond strength and enhance penetration into the dentinal tubules of self-etching resin cement. Which solution is best is unclear. Peracetic acid with different formulations appears to be a good option.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of irrigating the fiber post space by using 1% peracetic acid (PA) at low (LH) or high (HH) hydrogen peroxide concentration on the bond strength and penetration into the dentinal tubules of RelyX U200 self-etching resin cement.

MATERIAL AND METHODS

After intraroot preparation for fiber posts in 60 endodontically treated teeth, the specimens were divided into 4 groups (n=15) according to the irrigation protocol: CG (control), distilled water; NA (NaOCl), 2.5% sodium hypochlorite; LHPA, PA with a low concentration of hydrogen peroxide; and HHPA, PA with a high concentration of hydrogen peroxide. The fiber posts were then cemented with self-etching resin cement labeled with rhodamine. Cross sections of the thirds were obtained by confocal scanning microscopy and submitted to the push-out and cement penetration tests. The data obtained in the push-out tests were evaluated by using ANOVA and the Tukey post hoc test, and those data obtained in the confocal screening were evaluated by using the Kruskal-Wallis and Dunn (Bonferroni correction) tests (α=.05).

RESULTS

Bond strength values in the cervical third of the CG were higher than those in NA (P=.035) but similar to those of the other groups (P=.05). In the other thirds, CG and LHPA values were similar to one another (P=.05) but higher than those of NA and HHPA (P<.001). The penetration rate assessed by confocal scanning microscopy revealed similar results in the cervical third for all groups (P=.075), except for NA which provided the lowest penetration. In the remaining thirds, CG and LHPA provided the highest penetration rate (P=.015).

CONCLUSIONS

The LHPA group had no negative effects on the bond strength and penetration into the dentinal tubules of the self-etching resin cement.

Effect of Irradiance and Exposure Duration on Temperature and Degree of Conversion of Dual-Cure Resin Cement for Ceramic Restorations

This study investigated the effects of irradiance and exposure duration on dual-cured resin cements irradiated through ceramic restorative materials. A single light-curing unit was calibrated to three different irradiances (500, 1000, and 1500 mW/cm²) and irradiated to three different attenuating materials (transparent acryl, lithium disilicate, zirconia) with 1-mm thicknesses for 20 or 60 seconds. The changes in irradiance and temperature were measured with a radiometer (or digital thermometer) under the attenuating materials. The degree of conversion (DC) of dual-cure resin cement after irradiation at different irradiances and exposure durations was measured with Fourier transform near infrared spectroscopy. Two-way analysis of variance revealed that irradiance ( p<0.001) and exposure duration ( p<0.001) significantly affected temperature and DC. All groups showed higher DCs with increased exposure times ( p<0.05), but there were no statistically significant differences between the groups irradiated with 1000 mW/cm² and 1500 mW/cm² ( p>0.05). Higher-intensity irradiances yielded higher temperatures ( p<0.05), but exposure time did not affect temperature when materials were irradiated at 500 mW/cm² ( p>0.05).

Mechanical and Thermal Properties of Dental Composites Cured with CAD/CAM Assisted Solid-State Laser

Over the last three decades, it has been frequently reported that the properties of dental restorative composites cured with argon laser are similar or superior to those achieved with conventional halogen and light emitting diode (LED) curing units. Whereas laser curing is not dependent on the distance between the curing unit and the material, such distance represents a drawback for conventional curing units. However, a widespread clinical application of this kind of laser remains difficult due to cost, heavy weight, and bulky size. Recently, with regard to the radiation in the blue region of the spectrum, powerful solid-state lasers have been commercialized. In the current research, CAD (computer-aided design)/CAM (computer-aided manufacturing) assisted solid-state lasers were employed for curing of different dental restorative composites consisting of micro- and nanoparticle-reinforced materials based on acrylic resins. Commercial LED curing units were used as a control. Temperature rise during the photopolymerisation process and bending properties were measured. By providing similar light energy dose, no significant difference in temperature rise was observed when the two light sources provided similar intensity. In addition, after 7 days since curing, bending properties of composites cured with laser and LED were similar. The results suggested that this kind of laser would be suitable for curing dental composites, and the curing process does not suffer from the tip-to-tooth distance.

Effect of Light-Curing Exposure Time, Shade, and Thickness on the Depth of Cure of Bulk Fill Composites

Purpose:

To investigate the effect of different light exposure times, shades, and thicknesses on the depth of cure (DOC) of bulk fill composites.

Methods and Materials:

Two bulk fill composites, Tetric EvoCeram Bulk Fill (TBF) and Sonic Fill (SF), and a conventional composite, Filtek Supreme Ultra (FSU), were evaluated. Samples (n=10) were made using two different shades (light and dark), thicknesses (2 and 4 mm) and exposure times (20 and 40 seconds). A Tukon 2100B-testing machine was used to obtain three Knoop hardness numbers (KHNs) measured at the top and bottom of each sample, and DOC was calculated as the bottom/top ratio. Statistical analysis was done using a Student t-test for comparisons between groups with a Bonferroni correction of p &lt; 0.004.

Results:

Top hardness values ranged from 79.79 to 85.07 for FSU, 69.49 to 91.65 for SF, and 51.01 to 57.82 for TBF. Bottom KHNs ranged from 23.54 to 73.25 for FSU, 45.74 to 77.12 for SF, and 36.95 to 52.51 for TBF. TBF had the lowest overall KHNs. Light-curing exposure time, shade, and material thickness influenced the DOC in most groups, especially at 4-mm depths. A higher bottom/top ratio was achieved when a 40-second cure was compared to a 20-second cure, when light shades were compared to dark shades, and when 2-mm increments were compared to 4-mm increments.

Influence of Emission Spectrum and Irradiance on Light Curing of Resin-Based Composites

PURPOSE

This study examined the influence of different emission spectra (single-peak and broad-spectrum) light-curing units (LCUs) delivering the same radiant exposures at irradiance values of 1200 or 3600 mW/cm² on the polymerization and light transmission of four resin-based composites (RBCs).

METHODS AND MATERIALS

Two prototype LCUs that used the same light tip, but were either a single-peak blue or a broad-spectrum LED, were used to deliver the same radiant exposures to the top surfaces of the RBCs using either standard (1200 mW/cm²) or high irradiance (3600 mW/cm²) settings. The emission spectrum and radiant power from the LCUs were measured with a laboratory-grade integrating sphere coupled to a spectrometer, and the light beam was assessed with a beam profiler camera. Four RBCs (Filtek Supreme Ultra A2, Tetric EvoCeram A2, Tetric EvoCeram T, and TPH Spectra High Viscosity A2) were photoactivated using four different light conditions: single-peak blue/standard irradiance, single-peak blue/high irradiance, broad-spectrum/standard irradiance, and broad-spectrum/high irradiance. The degree of conversion (N=5) and microhardness at the top and bottom of 2.3-mm-diameter by 2.5-mm-thick specimens (N=5) were analyzed with analysis of variance and Tukey tests. The real-time light transmission through the RBCs was also measured.

RESULTS

For all light conditions, the 2.3-mm-diameter specimens received a homogeneous irradiance and spectral distribution. Although similar radiant exposures were delivered to the top surfaces of the RBCs, the amount of light energy emitted from the bottom surfaces was different among the four RBCs, and was also greater for the single-peak lights. Very little violet light (wavelengths below 420 nm) reached the bottom of the 2.5-mm-thick specimens. The degree of conversion and microhardness results varied according to the RBC (p<0.05). The RBCs that included alternative photoinitiators had greater microhardness values at the top when cured with broad-spectrum lights, while at the bottom, where little violet light was observed, the results were equal or higher when they were photoactivated with single-peak blue lights. With the exception of the microhardness at the top of TPH, equivalent or higher microhardness and degree-of-conversion values were achieved at the bottom surface when the standard (1200 mW/cm²) irradiance levels were used compared to when high irradiance levels were used.

CONCLUSIONS

Considering the different behaviors of the tested RBCs, the emission spectrum and irradiance level influenced the polymerization of some RBCs. The RBCs that included alternative photoinitiators produced greater values at the top when cured with broad-spectrum lights, while at the bottom, results were equal or higher for the RBCs photoactivated with single-peak blue lights.

Bulk-filled posterior resin restorations based on stress-decreasing resin technology: a randomized, controlled 6-year evaluation

This randomized study evaluated a flowable resin composite bulk-fill technique in posterior restorations and compared it intraindividually with a conventional 2-mm resin composite layering technique over a 6-yr follow-up period. Thirty-eight pairs of Class II restorations and 15 pairs of Class I restorations were placed in 38 adults. In all cavities a single-step self-etch adhesive (Xeno V) was applied. In the first cavity of each pair, the flowable resin composite (SDR) was placed, in bulk increments of up to 4 mm. The occlusal part was completed with a layer of nanohybrid resin composite (Ceram X mono). In the second cavity of each pair, the hybrid resin composite was placed in 2-mm increments. The restorations were evaluated using slightly modified US Public Health Service (USPHS) criteria at baseline and then annually for a time period of 6 yr. After 6 yr, 72 Class II restorations and 26 Class I restorations could be evaluated. Six failed Class II molar restorations, three in each group, were observed, resulting in a success rate of 93.9% for all restorations and an annual failure rate (AFR) of 1.0% for both groups. The AFR for Class II and Class I restorations in both groups was 1.4% and 0%, respectively. The main reason for failure was resin composite fracture.

Wear Resistance of Bulk-fill Composite Resin Restorative Materials Polymerized under different Curing Intensities

INTRODUCTION

The aim of this study was to assess the wear resistance of four bulk-fill composite resin restorative materials cured using high- and low-intensity lights.

MATERIALS AND METHODS

Twenty-four samples were prepared from each composite resin material (Tetric N-Ceram, SonicFill, Smart Dentin Replacement, Filtek Bulk-Fill) resulting in a total of 96 samples; they were placed into a mold in a single increment. All of the 96 samples were cured using the Bluephase N light curing unit for 20 seconds. Half of the total specimens (n = 48) were light cured using high-intensity output (1,200 mW/cm²), while the remaining half (n = 48) were light cured using low-intensity output (650 mW/cm²). Wear was analyzed by a three-dimensional (3D) noncontact optical profilometer (Contour GT-I, Bruker, Germany). Mean and standard deviation (SD) of surface loss (depth) after 120,000 cycles for each test material was calculated and analyzed using one-way analysis of variance (ANOVA) with a significance level at p < 0.05.

RESULTS

The least mean surface loss was observed for SonicFill (186.52 urn) cured using low-intensity light. No significant difference in the mean surface loss was observed when comparing the four tested materials with each other without taking the curing light intensity into consideration (p = 0.352). A significant difference in the mean surface loss was observed between SonicFill cured using high-intensity light compared with that cured using low-intensity light (p < 0.001).

CONCLUSION

A higher curing light intensity (1,200 mW/cm²) had no positive influence on the wear resistance of the four bulk-fill composite resin restorative materials tested compared with lower curing light intensity (650 mW/cm²). Furthermore, SonicFill cured using low-intensity light was the most wear-resistant material tested, whereas Tetric N-Ceram cured using high-intensity light was the least wear resistant.

CLINICAL SIGNIFICANCE

The wear resistance was better with the newly introduced bulk-fill composite resins under low-intensity light curing.

Microhardness of composite resin cured through different primary tooth thicknesses with different light intensities and curing times: In vitro study

Objective:

The aim of this study was to evaluate the effect of increased exposure time and light intensity on microhardness of cured composite through different thicknesses of tooth structure in primary teeth.

Materials and Methods:

One hundred and seventy cylindrical resin composite specimens were prepared. All specimens were divided into 17 experimental and control groups. “Light-emitting diode” light curing unit (LCU) applied directly or through 1, 2, and 3 mm thicknesses tooth slices for experimental groups. The irradiation protocols were 25 and 50 s at 650 mW/cm² and 15 and 30 s at 1100 mW/cm². The “quartz-tungsten-halogen” LCU (400 mW/cm²) for 40 s was used in control group. Microhardness was measured by the Vickers hardness test.

Results:

Indirectly cured specimens and those cured through a 1 mm thick tooth structure, an increase in intensity caused hardness drop. In the specimens cured through 2 and 3 mm thick tooth structures, increased intensity and/or exposure time did not show any appropriate changes on microhardness.

Conclusion:

Irradiation through a 1.0 mm thick tooth slice resulted in reduced microhardness although it was still within the clinically acceptable level. The hardness values of the specimens cured through 2 or 3 mm thick tooth slices fell below the clinically acceptable level even after doubling the exposure time and/or light intensity.

Physical Properties of an Ag-Doped Bioactive Flowable Composite Resin

The aim of this work was to study the physical and antibacterial properties of a flowable resin composite incorporating a sol-gel derived silver doped bioactive glass (Ag-BGCOMP). The depth of the cure was calculated by measuring the surface micro-hardness for the top and bottom surfaces. The volumetric polymerization shrinkage was measured by recording the linear shrinkage as change in length, while the biaxial flexural strength was studied measuring the load at failure. The antibacterial properties of the samples were tested against Streptococcus mutans (S. mutans) and Lactobacillus casei (L. casei). The measured values were slightly decreased for all tested physical properties compared to those of control group (flowable resin composite without Ag-BG), however enhanced bacteria inhibition was observed for Ag-BGCOMP. Ag-BGCOMP could find an application in low stress-bearing areas as well as in small cavity preparations to decrease secondary caries. This work provides a good foundation for future studies on evaluating the effects of Ag-BG addition into packable composites for applications in larger cavity preparations where enhanced mechanical properties are needed.

Post-cure depth of cure of bulk fill dental resin-composites

OBJECTIVES

To determine the post-cure depth of cure of bulk fill resin composites through using Vickers hardness profiles (VHN).

METHODS

Five bulk fill composite materials were examined: Tetric EvoCeram(®) Bulk Fill, X-tra base, Venus(®) Bulk Fill, Filtek™ Bulk Fill, SonicFill™. Three specimens of each material type were prepared in stainless steel molds which contained a slot of dimensions (15 mm × 4 mm × 2 mm), and a top plate. The molds were irradiated from one end. All specimens were stored at 37°C for 24h, before measurement. The Vickers hardness was measured as a function of depth of material, at 0.3mm intervals. Data were analysed by one-way ANOVA using Tukey post hoc tests (α=0.05).

RESULTS

The maximum VHN ranged from 37.8 to 77.4, whilst the VHN at 80% of max.VHN ranged from 30.4 to 61.9. The depth corresponding to 80% of max.VHN, ranged from 4.14 to 5.03 mm. One-way ANOVA showed statistically significant differences between materials for all parameters tested. SonicFill exhibited the highest VHN (p<0.001) while Venus Bulk Fill the lowest (p≤0.001). SonicFill and Tetric EvoCeram Bulk Fill had the greatest depth of cure (5.03 and 4.47 mm, respectively) and was significant's different from X-tra base, Venus Bulk Fill and Filtek Bulk Fill (p≤0.016). Linear regression confirmed a positive regression between max.VHN and filler loading (r(2)=0.94).

SIGNIFICANCE

Bulk fill resin composites can be cured to an acceptable post-cure depth, according to the manufacturers' claims. SonicFill and Tetric EvoCeram Bulk Fill had the greatest depth of cure among the composites examined.

Distance and protective barrier effects on the composite resin degree of conversion

Context:

The food wrap films are used to cover the tip of curing light units in order to avoid contamination and prevent damage to the light guide. However, their effects on resin polymerization are not fully known.

Aims:

We investigated the effects on restoration efficiency of a food wrap protective barrier used on the tip of curing light units.

Materials and Methods:

For each treatment, five replications were performed, a total of 60 bovine incisor. The degree of conversion (%DC) of restorations with the composite resin Opallis EA2 was evaluated using 3 curing light devices (Optilux 501, Optilight and Ultra LED) and 2 curing distances (0 and 5 mm). The composite resin was tested for restoration of cavities in bovine crowns. %DC values were measured by the Fourier transform infrared spectroscopy-attenuated total reflectance technique.

Statistical Analysis Used:

The data were analyzed using 3-way ANOVA and Tukey's test.

Results:

Use of the protective film lowered %DC (F = 4.13; P = 0.05), and the effects of curing distance were associated to the curing light device (F = 3.61; P = 0.03).

Conclusions:

The distance from the light curing tip and use of a translucent protective barrier on the light-cure device can both impair composite resin %DC.

Post-curing conversion kinetics as functions of the irradiation time and increment thickness

Objective:

This study evaluated the variation of conversion degree (DC) in the 12 hours following initial photoactivation of a low-shrinkage composite resin (Venus Diamond).

Material and Methods:

The conversion degree was monitored for 12 hours using Attenuated Total Reflection (ATR) F-TIR Spectroscopy. The composite was placed in 1 or 2 mm rings and cured for 10 or 20 seconds with a LED lamp. ATR spectra were acquired from the bottom surface of each sample immediately after the initial photoactivation (P=0), 30 minutes (P=0.5) and 12 hours after photoactivation (P=12) in order to obtain the DC progression during the post-curing period. Interactions between thickness (T), irradiation time (I) and post-curing (P) on the DC were calculated through ANOVA testing.

Results:

All the first order interactions were statistically significant, with the exception of the T-P interaction. Furthermore, the shift from P=0 to P=0.5 had a statistically higher influence than the shift from P=0.5 to P=12. The post-curing period played a fundamental role in reaching higher DC values with the low-shrinkage composite resin tested in this study. Moreover, both the irradiation time and the composite thickness strongly influenced the DC.

Conclusions:

Increased irradiation time may be useful in obtaining a high conversion degree (DC) with a low-shrinkage nano-hybrid composite resin, particularly with 2 mm composite layers.

Mechanical and morphological evaluation of the bond-dentin interface in direct resin core build-up method

OBJECTIVES

The purpose of this study was to evaluate the interfacial adhesion between resin and root canal dentin in the direct resin core build-up method in terms of microtensile bond strength (μTBS) and dentin micro morphology.

METHODS

Single-rooted human teeth were decoronated at the cementoenamel junction and endodontically treated. Post spaces were prepared in the roots to a depth of 10mm. The spaces were then treated with a dual-cure bonding system, and filled with dual-cure resin composite. After 24-h storage in water at 37 °C, they were trimmed into approximately 1.0-mm(2) beams for μTBS. Bond strength was analyzed with one-way ANOVA and Tukey's test. The fractured surfaces were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDX). Sectioned specimens were observed by ultra-high-voltage transmission electron microscopy.

RESULTS

The bond strength of root dentin decreased gradually from the coronal to apical side, and the bond strength of the coronal section was significantly higher than that of the radicular section. Moreover, the failure modes in the coronal and apical sides of the specimens differed. The apical specimens fractured within the core material, while the coronal specimens fractured at the bonding layer. SEM and EDX analyses revealed that the core material penetrated into dentinal tubules in the apical region.

SIGNIFICANCE

In the direct resin core build-up method, the interfacial adhesion of resin to root canal dentin may be insufficient in the apical region of the root canal due to poor polymerization.

Resin-composite cytotoxicity varies with shade and irradiance

OBJECTIVES

The study was aimed at investigating the cytotoxicity of different composites as a function of composite shade and the light curing unit (LCU) employed.

METHODS

Non-polymerized and polymerized samples of the composites Grandio(®) (VOCO, Cuxhaven), Solitaire(®) (Heraeus Kulzer, Hanau) and Filtek Z 250(®) (3M/Espe, Seefeld) in two markedly differing shades (A2, C2) were prepared. Polymerization was performed with two LCUs: Heliolux II (Ivoclar/Vivadent, Ellwangen) and Swiss Master Light (EMS, Nyon, Switzerland). To obtain composite extracts, the samples were immersed in cell culture medium (DMEM--Dulbecco's Modified Eagle Medium), which was replaced daily up to the 7th day of the experiment, and then on the 14th, 21st and 28th day. After incubation of human gingival fibroblasts (HGF) with the extracts obtained, cytotoxicity was determined using the MTT test.

RESULTS

With the non-polymerized samples, essentially no influence of the composite shades investigated on HGF viability was detected, with the exception of the Solitaire material, where a higher cytotoxicity of the shade C2 in the non-polymerized state was found at the end of the observation period. After polymerization of the different composites, the cytotoxic reaction observed for the extracts of shade C2 was stronger than that observed for A2. After polymerization with the Heliolux II (HLX) LCU, the extracts of composites Grandio and Solitaire C2 were significantly more toxic than those of the A2 shade (p<0.01). Polymerization with the Swiss Master Light (SML) reduces this cytotoxic effect. The extracts of the Grandio composite showed the least cytotoxic effect throughout the observation period, irrespective of the LCU used. For the extracts of the Z250 specimens, the cytotoxicity observed was generally higher.

SIGNIFICANCE

The results show that the shade of the composite has an influence on its cytotoxicity and that this cytotoxicity is also influenced by the light curing unit used. It was observed that composites of the darker shade (C2) had a higher cytotoxicity, which varied with the LCU employed.

New insight into the "depth of cure" of dimethacrylate-based dental composites

OBJECTIVES

To demonstrate that determination of the depth of cure of resin-based composites needs to take into account the depth at which the transition between glassy and rubbery states of the resin matrix occurs.

METHODS

A commercially available nano-hybrid composite (Grandio) in a thick layer was light cured from one side for 10 or 40 s. Samples were analyzed by Vickers indentation, Raman spectroscopy, atomic force microscopy, electron paramagnetic imaging and differential scanning calorimetry to measure the evolution of the following properties with depth: microhardness, degree of conversion, elastic modulus of the resin matrix, trapped free radical concentration and glass transition temperature. These measurements were compared to the composite thickness remaining after scraping off the uncured, soft composite.

RESULTS

There was a progressive decrease in the degree of conversion and microhardness with depth as both properties still exhibited 80% of their upper surface values at 4 and 3.8 mm, respectively, for 10 s samples, and 5.6 and 4.8 mm, respectively, for 40 s samples. In contrast, there was a rapid decrease in elastic modulus at around 2.4 mm for the 10 s samples and 3.0 mm for the 40 s samples. A similar decrease was observed for concentrations of propagating radicals at 2 mm, but not for concentrations of allylic radicals, which decreased progressively. Whereas the upper composite layers presented a glass transition temperature - for 10 s, 55°C (±4) at 1 mm, 56.3°C (±2.3) at 2 mm; for 40 s, 62.3°C (±0.6) at 1 mm, 62°C (±1) at 2 mm, 62°C (±1.7) at 3 mm - the deeper layers did not display any glass transition. The thickness remaining after scraping off the soft composite was 7.01 (±0.07 mm) for 10 s samples and 9.48 (±0.22 mm) for 40 s samples.

SIGNIFICANCE

Appropriate methods show that the organic matrix of resin-based composite shifts from a glassy to a gel state at a certain depth. Hence, we propose a new definition for the "depth of cure" as the depth at which the resin matrix switches from a glassy to a rubbery state. Properties currently used to evaluate depth of cure (microhardness, degree of conversion or scraping methods) fail to detect this transition, which results in overestimation of the depth of cure.

Microhardness of composite materials with different organic phases in deep class II cavities: an in vitro study

OBJECTIVE

This study compared the microhardness of three composite resins with different organic matrices in deep class II cavities.

MATERIALS AND METHOD

A total of 36 extracted molars were randomly assigned to six groups and standardized class II cavities were prepared. The cavity design comprised three steps in a mesiodistal direction with an increasing depth (2, 4, and 6 mm). Twelve cavities each were restored using Filtek Supreme (FS), Quixfil (QF), and Filtek Silorane (SI). The materials were applied in incremental layers of 2 mm and cured either with Halogen Translux Energy (HTE) (n=18) or LED Bluephase C8 (LED) (n=18). Subsequently, the specimens were cross-sectioned, and microhardness was determined in various depths and at two different distances from the matrix.

RESULTS

QF yielded the highest KHN microhardness values (92.67 ± 12.77), followed by FS (65.53 ± 19.52) and SI (57.67 ± 8.33). Composites cured with LED achieved higher KHN values. All materials showed the highest microhardness values within the superficial increments and at a distance of 1000 μm from the matrix.

Influence of the interaction of light- and self-polymerization on subsurface hardening of a dual-cured core build-up resin composite

OBJECTIVE

To investigate the influence of time delay and duration of photo-activation on subsurface microhardness of a dual-cured resin composite.

MATERIAL AND METHODS

A commercially available dual-cured core build-up resin composite (Rebilda DC) was filled in cavities (diameter: 4.0 mm; height: 6.0 mm) of polystyrene molds and light-cured for 20 or 60 s either immediately after the filling procedure (time delay 0 s) or after a time delay of 30, 90, 180 or 300 s. Non-irradiated self-cured specimens served as a control group (n = 15). Specimens were stored in complete darkness and at 100% relative humidity at 37°C for 2 weeks and cross-sectioned. Knoop Hardness Numbers (KHNs) were measured six times per depth and averaged at distances of 0.25, 0.50, 1.00, 2.00, 3.50 and 5.50 mm from the light-exposed surface. Data were statistically analyzed using one- and two-way ANOVA followed by Scheffé's post-hoc test at a level of significance of 0.05.

RESULTS

Mean hardness values in all experimental groups ranged between 54.3 ± 2.1 and 58.1 ± 2.3 KHN. Light-curing did not significantly increase composite KHN at any depth measured. Delaying light exposure had no influence on KHN, irrespective of depth. A longer light-exposure time (60 versus 20 s) resulted in significantly higher KHN only at depths of 3.50 and 5.50 mm.

CONCLUSION

Photo-activation of the tested dual-cured resin composite provided no clinically relevant benefit compared to self-curing regarding the degree of hardening.

Effect of the insertion and polymerization technique in composite resin restorations: analysis of marginal gap by atomic force microscopy

This in vitro study evaluated the marginal gap at the composite tooth/resin interface in class V cavities under the influence of two insertion techniques and a curing system by means of atomic force microscopy (AFM). Forty enamel and dentin cavities were prepared on the buccal surface in bovine teeth with quadratic forms measuring 2 mm × 2 mm and depth of 1.5 mm. The teeth were then divided into four groups: group A, 10 cavities were restored in one increment, light cured by halogen light; group B, 10 cavities filled with bulk filling, light cured by the light emitting diodes (LED); group C, 10 cavities were restored by the incremental technique, light cured by halogen light; group D, 10 cavities were restored by the incremental technique, light cured by the LED. The teeth underwent the polishing procedure and were analyzed by AFM for tooth/restoration interface evaluation. The data were compared between groups using the nonparametric Kruskall-Wallis and Mann-Whitney tests (p < 0.05). The results showed a statistically significant difference between groups A and B and groups A and C. It was concluded that no insertion and polymerization technique was able to completely seal the cavity.

Curing efficiency of high-intensity light-emitting diode (LED) devices

We evaluated the curing efficiency of 4 high-intensity light-emitting diode (LED) devices by assessing percentage of residual C=C (%RDB), surface microhardness (SM), depth of cure (DC), percentage of linear shrinkage-strain (%LS), and percentage of wall-to-wall contraction (%WWC). The light-curing units tested were a QTH light, the Elipar TriLight (3M/ESPE), and 4 LED devices - the Allegro (Denmat), the Bluephase (Ivoclar/Vivadent), the FreeLight2 (3M/ESPE), and The Cure TC-01 (Spring Health Products). The %RDB was measured by microFTIR spectroscopy. Microhardness measurements (Vickers) were performed at the surface (H0) and at depths of 3 mm (H3) and 5 mm (H5) of cylindrical specimens. Depth of cure was expressed as the ratio of microhardness at each depth, relative to the corresponding surface value (H3/H0 and H5/H0). The bonded disc method was used to evaluate %LS. For the %WWC evaluation, cylindrical resin restorations were imaged by high resolution micro-CT and the %WWC was calculated at depths of 0 mm and 2 mm. There were no statistical differences among the LEDs in %RDB or %LS. The Bluephase and Allegro had the highest SM values. As compared with the other LEDs, the Bluephase and The Cure TC-01 had lower values for depth of cure at depths of 3 mm and 5 mm. There were no significant differences in %WWC among the LEDs at either depth, and the QTH had the lowest %WWC at both depths.

Bond strength of fiber posts to weakened roots after resin restoration with different light-curing times

INTRODUCTION

This study evaluated the bond strength of translucent fiber posts to experimentally weakened radicular dentin restored with composite resin and polymerized with different light-exposure time.

METHODS

Roots of 60 maxillary incisors were used. Twenty-four hours after obturation, the filling materials of root canals were removed to a depth of 12 mm, and 4 groups were randomly formed. In 3 groups, root dentin was flared to produce a space between fiber post and canal walls. In the control group, the roots were not experimentally weakened. The flared roots were bulk restored with composite resin, which was light-activated through the translucent post for 40, 80, or 120 seconds. Posts were cemented, and after 24 hours, all roots were sectioned transversely in the coronal, middle, and apical regions, producing 1-mm-thick slices. Push-out test was performed, and failure modes were observed.

RESULTS

The quantitative analysis showed significant statistical difference only among groups (P < .001). Comparing the weakened/restored groups, composite light-exposure time did not influence the results. Overall, adhesive failures occurred more frequently than other types of failures. Cohesive failures occurred only in the weakened/restored roots.

CONCLUSIONS

Intracanal root restoration with composite resin and translucent fiber posts provided similar or higher bond strength to dentin than the control group, regardless of the light-exposure time used for polymerization.

Effects of photocuring strategy on bonding of dual-cure one-step self-etch adhesive to root canal dentin

This study evaluated the effects of light power density and light exposure time on regional bond strength of Clearfil DC Bond to root canal dentin. Post spaces were prepared in extracted premolars. Root canal dentin was treated with a dual-cure bonding system, Clearfil DC Bond, and light-cured for 10, 20, or 30 seconds using two halogen light curing units: Optilux 501 (830 mW/cm2) and Hyperlightel (1350 mW/cm2). Following which, all post spaces were filled with a dual-cure resin composite. After 24-hour storage, microtensile bond strengths (microTBS) at the coronal and apical regions were measured. At the coronal region, microTBS values were similar among all the experimental groups. At the apical region, bond strength improved when the curing time was extended to 30 seconds with Optilux 501, and likewise with Hyperlightel when curing time was extended to 20 or 30 seconds. In addition, significant differences in microTBS between the coronal and apical regions disappeared with prolonged curing times.

Curing efficacy of light emitting diodes of dental curing units

Background and aims

The aim of the present study was to compare the efficacy of quartz tungsten halogen (QTH) and light emitting diode (LED) curing lights on polymerization of resin composite.

Materials and methods

A hybrid resin composite was used to prepare samples which were cured using two QTH and ten LED light curing sources. Twelve groups, each containing ten samples, were prepared using each light source. The cured depth of the resin was determined using ISO 4049 method and Vickers hardness values were determined at 1.0 mm intervals. Data was analyzed by ANOVA and Tukey test.

Results

Data analysis demonstrated a significant difference between light sources for depth of cure. At 1.0 mm below the surface all the tested light sources and at 2.0-mm intervals all light sources except two (Optilux 501 and LEDemetron I) and at 3.0-mm intervals only two light sources (PenCure and LEDemetron II) could produce hardness values higher than 80% of superficial layer values.

Conclusion

This study showed that a variety of LED light sources used in the present study are as effective as the high-intensity QTH lights in polymerization of resin composite.

Impact of refrigeration on the surface hardness of hybrid and microfilled composite resins

This in vitro study evaluated the Knoop hardness of the composite resins Charisma® (C) and Durafill VS® (D) polymerized in 3 different conditions: at room temperature (A) (23 ± 1°C); refrigerated at 4 ± 1°C and immediately photo-activated after removal from the refrigerator (0); and, refrigerated at 4 ± 1°C and photo-activated after a bench time of 15 min at room temperature (15). One hundred and twenty specimens (4 mm diameter and 2 mm depth) were made using a stainless steel mold and following manufacturer's instructions. All specimens were tested immediately after polymerization (I) and after 7 days of water storage in the dark at room temperature (7d). The data were subjected to ANOVA and post-hoc Tukey's test (a=0.05). On the top surface, CAI was statistically similar to C15I and DAI to D15I (p>0.05). On the bottom surface, CAI presented higher hardness values when compared to COI and C15I (p<0.05). The D groups showed no significant differences (p>0.05) on the bottom surfaces for any tested polymerization condition. After 7 days of storage, the Knoop hardness decreased significantly (p<0.05) for groups C7d and D7d except for C07d, which was not different from COI at either surface (p>0.05). D07d showed higher Knoop hardness (p<0.05) values on the top surface when compared to the other groups.

Kinetics of light-cure bracket bonding: power density vs exposure duration

INTRODUCTION

Recent technologic advances make it possible to increase the light power density to reduce the necessary exposure duration. The kinetics of polymerization are complex. The special case of indirectly curing the thin layler of composite below the metallic bracket base further increases this complexity. It was hypothesized that the concept of "total energy,"--the reciprocity between power density and exposure duration--does not hold for orthodontic light-cure bracket bonding.

METHODS

Stainless steel brackets were bonded on deciduous bovine incisors with a standard light-cured composite. A calibrated, powerful halogen lamp allowed for modification of power density from 300 to 3000 mW/cm2. Metallic brackets were bonded in 8 groups of 20 incisors each with various combinations of power densities and exposure durations to obtain 3 levels of energy density (6000, 12000, and 24000 mJ/cm2). Another group of 20 incisors served as the positive control with a conventional powerful halogen lamp (1000 mW/cm2) for 40 seconds. After storage for 24 hours at 37 degrees C in water, the bracket shear bond strength (SBS) and the adhesive remnant index (ARI) were measured.

RESULTS

It was confirmed that bracket SBS mainly depends on the energy density of the light cure. All groups with an energy density of 6000 mJ/cm2 had significantly lower SBS than the groups with higher energy densities (P <0.01). The dependence of SBS on exposure duration for the same energy density followed an exponential model of nonlinear regression (r2 = 0.98).

CONCLUSIONS

The concept of "total energy" does not hold for orthodontic light-cure bracket bonding. An exposure time of less than 4 seconds, irrespective of the power density, cannot guarantee sufficient bracket bond strength. There seems to be an advantage of power density over exposure duration in the context of metallic bracket bonding. These results show that, for an efficient light-cure bracket bonding, there is an absolute lower limit of exposure duration (4 seconds) and an upper limit of useful power density (3000 mW/cm2).

Effect of three types of light-curing units on 5-year colour changes of light-cured composite

The purpose of this study was to determine colour changes in a composite cured with tungsten-halogen, light-emitting diode (LED) or a plasma arc after 5 years. Five specimens 10 mm in diameter and 2 mm in height were prepared using Hybrid (Clearfil AP-X) composite for each test group. The corresponding specimens were cured with a tungsten-halogen curing light, a LED unit or with a plasma arc. Specimens were stored in light-proof boxes for 5 years after the curing procedure to avoid further exposure to light and stored in 37 degrees C in 100% humidity. Colorimetric values of the specimens immediately after curing and after 5 years were measured using colorimeter. The DeltaE*( ab ) values varied significantly depending on the curing unit used (p < 0.001). Curing time did not affect the colour changes of the specimens (p = 0.4). The results of this study suggest that composite materials undergo measurable changes due to the curing unit exposure.

Effects of light exposure time on composite resin hardness after root reinforcement using translucent fibre post

OBJECTIVES

The purpose of this in vitro study was to evaluate the Vickers hardness (VHN) of a Light Core (Bisco) composite resin after root reinforcement, according to the light exposure time, region of intracanal reinforcement and lateral distance from the light-transmitting fibre post.

METHODS

Forty-five 17-mm long roots were used. Twenty-four hours after obturation, the root canals were emptied to a depth of 12 mm and the root dentine was artificially flared to produce a 1mm space between the fibre post and the canal walls. The roots were bulk restored with the composite resin, which was photoactivated through the post for 40s (G1, control), 80 s (G2) or 120 s (G3). Twenty-four hours after post-cementation, the specimens were sectioned transversely into three slices at depths of 2, 6 and 10mm, corresponding to the coronal, middle and apical regions of the reinforced root. Composite VHN was measured as the average of three indentations (100g/15 s) in each region at lateral distances of 50, 200 and 350 microm from the cement/post-interface.

RESULTS

Three-way analysis of variance (alpha=0.05) indicated that the factors time, region and distance influenced the hardness and that the interaction timexregion was statistically significant (p=0.0193). Tukey's test showed that the mean VHN values for G1 (76.37+/-8.58) and G2 (74.89+/-6.28) differed significantly from that for G3 (79.55+/-5.18).

CONCLUSIONS

Composite resin hardness was significantly lower in deeper regions of root reinforcement and in lateral areas distant from the post. Overall, a light exposure time of 120 s provided higher composite hardness than the shorter times (40 and 80s).

Direct fiber-reinforced composite restoration in an endodontically-treated molar: a three-year case report

The reconstruction of structurally compromised non-vital posterior teeth may represent one of the most challenging adhesive-based restorative procedures. Several factors may influence the longevity of direct fiber-reinforced resin composite restorations: endodontic procedures prior to post cementation, dentin and/or post surface treatments, selection of the appropriate post design and architecture, resin composite polymerization and layering techniques. Thus, different specialties, such as endodontics and restorative dentistry, should work as a team to improve the longevity of restorations. This article presents three-year clinical results following reconstruction of a severely damaged endodontically-treated molar using direct fiber reinforced resin composite systems.

Monomer conversion and cytotoxicity of dental composites irradiated with different modes of photoactivated curing

The aim of the study was to compare the monomer conversion and cytotoxicity of two resin composites, Tetric Ceram (TC) and Heliomolar (HM), irradiated with a halogen light-curing unit (Optilux 501) using a number of curing modes, boost (OB), ramp (OR), and conventional (OC), as well as a light-emitting diode (LED) curing unit (LEDemetron; LEDe). The effects of irradiation times (10, 20, 30, and 40 s) were also investigated. The monomer conversion of resin composites was measured using Fourier transform infrared (FTIR) analysis. Resin samples were preimmersed in Dulbeco's modified engles medium (DMEM) for 24 h, and the periodontal ligament cells (PDL) were cultured with aged medium for 72 h to evaluate the cytotoxic effects of resin samples using the microtiter-tetrazolium (MTT) method. The spectral outputs of the curing units were also compared. After irradiation using the same curing mode, the TC monomer conversion was higher than the HM analog. The TC conversion value was lower after OC and LEDemetron irradiation for 10 s compared to exposures of 30 and 40 s. The HM conversion value was lowest for the 10-s OC irradiation, and highest for the 40-s OC exposure. In cytotoxicity tests, the negative control group was without irradiation. The succinate dehydrogenase (SDH) activities of the OB and OR-irradiated TC samples were significantly lower than analogs irradiated with LEDemetron for 40 s; however, no significant differences were demonstrated between OB, OR, and control groups. Further, no significant differences in SDH activity were demonstrated for OB, OR, and control groups of the HM.

Depth of Cure of Dental Resin Composites: ISO 4049 Depth and Microhardness of Types of Materials and Shades

Achieving a high degree of cure throughout a 2 mm thickness of light-activated resin composite did not occur for many types and shades of resin composite. Clinicians should check the depth of cure by using the scraping method.

Evaluation of the curing depth of two translucent composite materials using a halogen and two LED curing units

This in vitro study evaluated the influence of one halogen and two light-emitting diode (LED) curing units on the curing depth of a conventional hybrid and two translucent resin composites by measuring the Knoop microhardness. In the first part of the study, a conventional hybrid resin composite and three curing units (one halogen: 40 s polymerization time, two LEDs: 10 and 20 s) were used. Ten cylindrical resin composite samples were prepared for each curing unit and each polymerization time tested. After polymerization, the soft part of the samples was removed. The samples were embedded in a polyacrylic resin and separated in the middle towards the direction, top-bottom. On the section plane, Knoop microhardness measurements were performed every 1 mm, starting at 0.5 mm under the surface. In the second part of the study, two translucent resin composites and a conventional hybrid composite resin were cured with the three curing units, and the microhardness was measured as mentioned above. The difference between the curing units tested was found statistically significant (p = 0.0009), as well as the difference between the materials concerning curing depth (p = 0.0001). Both translucent materials achieved microhardness values equal to the 80% of the surface values, in depths 3.5-5.5 mm, depending on the curing units used.

Influence of shade and irradiation time on the hardness of composite resins

This study tested the following hypotheses: 1. increasing light irradiation time (IT) produces greater values of superficial hardness on different depths (0 and 3 mm); and 2. a dark shade composite (A3) needs longer IT than a light shade composite (A1) to produce similar hardness. Disk-shaped specimens (n=24 per shade) were fabricated using a 3-mm-thick increment of composite resin (Z100). Specimens were randomly assigned to 3 groups (n=8) according to the IT (400 mW/cm2) at the upper (U) surface: A1-10 and A3-10: 10 s; A1-20 and A3-20: 20 s; A1-40 and A3-40: 40 s. Specimens were stored in black lightproof containers at 37ºC for 24 h before indentation in a hardness tester. Three Vickers indentations were performed on the U and lower (L) surfaces of each specimen. The indent diagonals were measured and the hardness value calculated. The results were analyzed statistically by ANOVA and Tukey's test (alpha=0.05). Statistically significant differences were found between U and L surfaces of each composite shade-IT combination (p=0.0001) and among the ITs of same shade-surface combination (p=0.0001), except between groups A1-20U and A1-40U, confirming the study hypothesis 1 and partially rejecting the hypothesis 2.

Effects on microstrain and conversion of flowable resin composite using different curing modes and units

The flowable resin composite, Tetric Flow, was used to measure microstrain and degree of conversion after hardening with each of three curing machines: XL3000(XL) for 10, 20, 30, and 40 s; Optilux 501 using conventional mode (OC) for 10, 20, 30, and 40 s, as well as Optilux boost (OB, 10 s) and ramp modes (OR, 20 s); and LEDemetron (LEDe) for 10, 20, 30, and 40 s. The emitted power density and spectral distribution of the three light curing units were also measured. The LEDe output energy spectrum was centralized between 425 and 490 nm, which encompasses the excited wavelength of camphorquinone. The microstrain produced by the curing process is as a second-degree polynomial for each light source. The OB microstrain was highest, while the OR microstrain was lower. The ranking in order of degree of monomer conversion was as follows: XL10 <or=<or= OC10 <or=<or= LED 10 = OR = XL 20 = OC 20 = XL 30 <or=<or= LED 20 <or=<or= OC30 = LED 40 = XL 40 = OC40 = LED 30 <or= OB. The degree of conversion cured with OB was significant higher than other curing modes except OC30, OC40, LEDe30, LEDe40, and XL40. The conversion value of XL10 was the lowest. The LEDe produced higher conversion for the same emitted energy compared to the two halogen units.