Correlation between light intensity and exposure time on the hardness of composite resin

Longer light-curing time decreases the effect of ageing on composite resin hardness used in root reinforcement

This study evaluated the hardness of a composite resin used for root reinforcement, considering the light-curing time, root canal region and ageing due to long-term storage. Twenty incisor roots were reinforced using composite resin, varying the photopolymerisation time (40 or 120 s). Following fibre post cementation, the roots were transversely sectioned into coronal, middle and apical regions. Composite hardness was measured initially and after 18 months of water storage. Data underwent repeated measures analysis of variance and Tukey's post hoc tests. The factors 'light-curing time', 'root region' and 'ageing' affected the hardness. Significant interactions were observed between 'light-curing time × root region' and 'ageing × light-curing time'. Regardless of time, resin hardness in the apical region was lower. After ageing, hardness in the coronal and middle regions decreased when the light-curing time was 40 s, while no significant effect on hardness was noted with a light-curing time of 120 s.

Effect of irradiance from curing units on the microhardness of composite - a systematic review

Purpose The systematic review aims to evaluate the effect of light irradiance from light-curing units on microhardness of composite.Materials and methods The protocol was registered in PROSPERO following which primary search was carried out via MedLine, Scopus and Cochrane Library. A customised tool was used to assess the risk of bias. Among the 303 records retrieved from the databases, only ten articles qualified for qualitative synthesis after meeting all the requirements of the eligibility criteria. Covidence software was used to record the decisions. The studies published until 31 March 2021 were taken up for the review. The articles showed a low-to-moderate risk of bias.Results From a total of 303 articles, ten articles were reviewed for full text. Ten in vitro studies were included for qualitative analysis. There was heterogeneity in sample size, curing time and outcome measured. Therefore, meta-analysis was not performed. Out of ten studies, seven studies reported higher microhardness value for higher intensity than lower intensity of light.Conclusion Despite the fact that the findings of the evaluated studies are quite variable, significant scientific evidence revealed that high light intensity can enhance the hardness of resin composites.

Evaluation of irradiance and radiant exposure on the polymerization and mechanical properties of a resin composite

The objective of the present study was to evaluate the effect of irradiance and radiant exposure on the chemical-mechanical properties of a resin composite. A micro-hybrid resin composite (Clearfil AP-X, Kuraray) was investigated under two different irradiances: low (300 mW/cm2) and high (800 mW/cm2) and radiant exposures: 8 and 16 J/cm2. Four groups, named Low 8 J/cm2, High 8 J/cm2, Low 16 J/cm2, and High 16 J/cm2 were tested, and their flexural strengths, elastic moduli, depths of cure, and degrees of conversion were evaluated. Data were analyzed using two-way ANOVA and Tukey's test. A multiple linear regression model was used to correlate the irradiance and radiant exposure with dependent variables (α = 0.05). Irradiance and radiant exposure were found statistically significant for all dependent variables. The interaction between the factors was statistically significant only for the degree of conversion and elastic modulus. Group Low 16 J/cm2 exhibited a significantly superior performance in all the evaluated properties. Barring the degree of conversion, no significant differences were observed among the properties evaluated between the Low 8 J/cm2 and High 8 J/cm2 groups. The adjusted R2 values were high for the depth of cure and degree of conversion (0.58 and 0.96, respectively). Both irradiance and radiant exposure parameters play an important role in establishing the final properties of a micro-hybrid resin composite. Irradiance has a greater influence under higher radiant exposures.

Impact of light-curing distance on the effectiveness of cure of bulk-fill resin-based composites

Objective

To investigate the effect of light-curing distance on the effectiveness of cure (EC) of bulk-fill resin-based composites (RBCs).

Materials and methods

Two bulk-fill RBCs (a Tetric N-Ceram Bulk Fill (TN) and a Filtek Bulk Fill (FK)) are evaluated. Specimens (4 mm high) are cured for 20 s at different distances (0 mm (D0), 2 mm (D2), 4 mm (D4), 6 mm (D6) and 8 mm (D8)) and stored for 24 h in 100% relative humidity at 37 °C. The top and bottom surface hardness (SH) (n = 12) are assessed using a Knoop microhardness tester and the EC is calculated. The EC is characterized by the hardness ratio (HR) (mean bottom: top SH). An HR of 0.8 is used as the benchmark for an effective/adequate cure. Data are analyzed using one-way analysis of variance and Tukey’s post hoc test (α = 0.05). Correlations between the top and bottom surfaces are examined using the Pearson correlation (α = 0.05).

Results

For the TN, the HR at D8 is significantly lower than all other light-curing distances, while for the FK, it is significantly lower than D0 only.

Conclusion

The effect of light-curing distance on the EC of bulk-fill RBCs is material dependent. Notwithstanding the light-curing distance, the EC of the FK and TN is below the threshold HR value of 0.8 when photopolymerized for 20 s in 4 mm increments in black opaque molds.

Effect of Additional Light Curing on Colour Stability of Composite Resins

The objective of this study was to evaluate the effect of additional light curing on the colour stability of composite resins. Materials and methods: Four different composite resins—a nanofill, a nanohybrid, a microhybrid, and a bulk-fill composite resin—were tested. Eighty disc-shaped specimens were prepared from each material using either a quartz tungsten halogen or a light-emitting diode light source and were randomly divided into 2 groups according to the surface treatment: no polishing (nonpolished) or polishing with aluminum oxide discs (polished). Then additional light curing was applied to half of the specimens in each group. All specimens were immersed in coffee solution for 1 week. Colour was measured with a spectrophotometer at baseline and after 1 week of storage in coffee solution. Results: Statistically significant differences in colour stability were observed in the restorative materials according to the composition of composite resin, the polishing protocol, and additional light curing, whilst there were no significant differences according to the light source. Additional light curing reduced discolouration in all groups tested. Conclusions: Additional light curing may be beneficial after finishing and polishing in order to maintain aesthetics and increase the resistance of composite resins to discolouration.

Knowledge and Attitude of Dental Clinicians towards Light-Curing Units: A Cross-Sectional Study

Objectives

Light curing is crucial when applying composite resin restorations. Complete polymerization of the resin depends on delivering adequate light energy to it. Dental clinicians may be unaware of the importance of proper light-curing techniques. This study aimed at evaluating and comparing the level of knowledge of general practitioners (GPs) and specialists (SPs) regarding light-curing units.

Materials and Methods

An electronic survey was conducted online among GPs and SPs of various specialties, working in the governmental sector in Riyadh, Saudi Arabia. Collected data were analyzed for statistical significance.

Results

310 dentists were included in the study. Nearly half of the GPs (45.9%) and more than half of SPs (56.8%) use light-emitting diode (LED) type light-curing units (LCUs). 36.9% of GPs and 29.6% of SPs were unsure about the type of LCUs they use in their dental clinics. 10.8% of GPs and 8.5% of SPs knew the proper term of the power output of LCU. 52.2% of the GPs and 55.7% of SPs were wrong about advancements in technology of LED LCUs. Regarding the use of radiometer, 48.2% of SPs and 35.1% of GPs had responded wrongly, and 37.7% of SPs and 52.3% of GPs were not familiar with the device, showing a statistical significance (p=0.040). There was no statistical significance observed in the responses pertaining to their years of experience, expected for two questions.

Conclusion

Both GPs and SPs displayed inadequate knowledge regarding the use of LCUs. Further educational programs are recommended to spread awareness about the handling of LCUs among dental clinicians.

Structural and mechanical analysis of three orthodontic adhesive composites cured with different light units

OBJECTIVE

To evaluate the effects of three different curing units on the physical and mechanical features of three different orthodontic adhesive resin materials.

MATERIAL AND METHODS

45 specimens (5 mm in diameter, and 2 mm in thickness) of each of the three different adhesive composite resin materials (Transbond XT, Grēngloo™ Adhesive and Light Bond Paste) were cured with three different light units (a polywave third generation (Valo), a monowave (DemiUltra), and a second-generation LED (Optima 10)). To quantify degree of conversion (DC), the Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy was used in transmission mode (ALPHA FT-IR Spectrometer, Bruker Optics, Germany). Vickers hardness value was recorded under constant load 100 g for 10 s with a microhardness tester (HMV M-1, Shimadzu Corp., Kyoto, Japan). The data were statistically analyzed using Kruskal-Wallis and chi-square tests. The level of significance was considered p < 0.05.

RESULTS

The highest DC values were obtained as a result of curing with Optima 10. This rate was followed by Demi Ultra and Valo, respectively. Transbond XT samples showed a lower level of conversion than the samples of Light Bond Paste and Grēngloo™ Adhesive. The top surfaces of each material showed higher hardness values than the bottom surfaces (p < 0.05). The Light Bond Paste showed the highest hardness values both on the top and bottom surfaces among the three materials, followed by Grēngloo™ Adhesive. While the hardness values of the top surfaces of the samples cured with Demi Ultra and Valo light units were similar, higher hardness values are recorded with Valo on the bottom surfaces (Valo; 85.200/75.200 (top/bottom) versus Demi Ultra; 86.100/66.000 (top/bottom)).

CONCLUSIONS

The different DC and the surface hardness properties were recorded for the resin as orthodontic adhesives depending on different light units. Shorter radiation time caused lower DC and surface hardness values.

A Comparative Study of Light Transmission by Various Dental Restorative Materials and the Tooth Structure

CLINICAL RELEVANCE

Light transmission through dental materials and tooth structure has direct clinical implication on such factors as selecting an appropriate curing technique during a restorative process.

SUMMARY

[Effect of preheating on the properties of resin composite]

Resin composite, which is commonly used as a dental filling material, has some problems, such as poor wear resistance, polymerization shrinkage, and poor dentin marginal adaptability. Preheating of resin composite improves its pro-perties. This paper reviewed the effects of resin composite preheating on its monomer conversion, marginal microleakage, mechanical properties, and irritation on dental pulp.

Efficacy of Modern Light Curing Units in Polymerizing Peripheral Zones in Simulated Large Bulk-fill Resin-composite Fillings

The variation in micro-hardness (HV) within simulated large cavities (10 × 6 mm) filled in one increment with three bulk-fill resin-based composites (BF-RBC) was assessed by means of a universal hardness device. Modern blue and violet-blue light curing units (LCUs) were applied in three different positions, by rotating the LCU in 120° steps. The exposure distance was 3 mm. One center and two peripheral (4-mm apart from the center) HV line profiles were measured in 0.5-mm steps at 24 hours postpolymerization to calculate the depth of cure (DOC). Incident light, irradiance, and spectral distribution were recorded. A multivariate analysis (general linear model) assessed the effect of the varied parameters as well as their interaction terms on HV and DOC. The effect of LCU rotation was not significant ( p=0.109). The DOC varied between 3.46 mm and 5.50 mm and was more strongly influenced by the BF-RBC ( p<0.001, η_P²=0.774), followed by the width of specimen ( p<0.001, η_P²=0.554), while the influence of the LCU was very low ( p<0.06, η_P²=0.070). Whether a BF-RBC filling is cured as well in the periphery as in the center depends more on the material than on the curing unit used.

Effects of delivering the same radiant exposures at 730, 1450, and 2920 mW/cm2 to two resin-based composites

Objective:

To evaluate the effects of curing two resin-based composites (RBC) with the same radiant exposures at 730, 1450, and 2920 mW/cm².

Materials and Methods:

Two types of RBC, Filtek Supreme Ultra and Tetric-EvoCeram-Bulk Fill, were light-cured to deliver the same radiant exposures for 5, 10, or 20 s by means of a modified Valo light emitted diode light-curing unit with the light tip placed directly over each specimen. The RBC was expressed into metal rings that were 2.0 and 4.0 mm in thickness, directly on an attenuated total reflectance Fourier transform infrared plate heated to 33°C, and the degree of conversion (DC) of the RBC was recorded. The specimens were then removed and the Knoop microhardness (KHN) was tested at both the bottom and the top of each specimen. The KHN was tested again after 24 h and 7 days of storage in the dark at 37°C and 100% humidity. The DC and KHN results were analyzed with Fisher's protected least significant difference at α = 0.05.

Results:

The DC values for the specimens cured at the three different irradiance levels were similar. However, at different depths, there were differences in the DC values. In general, there were no clear differences among the samples cured in the three different groups, and the KHN was always greater 24 h and 7 days later (P < 0.05).

Conclusions:

Despite the curing time, and as long as the samples were cured with the same radiant exposures, there were no significant effects on the DC and KHN of both RBCs.

Factors affecting polymerization of resin-based composites: A literature review

AIM

The aim of this review was to help clinicians improve their understanding of the polymerization process for resin-based composites (RBC), the effects of different factors on the process and the way in which, when controlled, the process leads to adequately cured RBC restorations.

METHODS

Ten factors and their possible effects on RBC polymerization are reviewed and discussed, with some recommendations to improve that process. These factors include RBC shades, their light curing duration, increment thickness, light unit system used, cavity diameter, cavity location, light curing tip distance from the curing RBC surface, substrate through which the light is cured, filler type, and resin/oral cavity temperature.

CONCLUSION

The results of the review will guide clinicians toward the best means of providing their patients with successfully cured RBC restorations.

Evaluation of the surface hardness of composite resins before and after polishing at different times

PURPOSE

The aim of this study was to evaluate the surface hardness of six composite resins: Revolution, Natural Flow, Fill Magic Flow, Flow-it! (flowables), Silux Plus (microfilled) and Z100 (minifilled) before and after polishing at different times.

MATERIALS AND METHODS

For this purpose, 240 specimens (5mm diameter, 1.4mm high) were prepared. Vickers hardness was determined before and after polishing at different times: immediately, 24h, 7 and 21 days after preparation of the samples. Statistical analysis was performed by ANOVA and Tukey test.

RESULTS

There was no difference in the hardness of flowable resins, which had lower hardness than the minifilled resin. The minifilled resin showed the highest surface hardness as compared to the other materials (p<0.01). All materials exhibited higher hardness after polishing, being more evident after 7 days.

CONCLUSION

It may be concluded that, regardless of the composite resin, surface hardness was considerably increased when polishing was delayed and performed 1 week after preparation of the samples.

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.

Cytotoxic effects of halogen- and light-emitting diode-cured compomers on human pulp fibroblasts

OBJECTIVE

The aim of this study was to determine the cytotoxic effects of three different compomers (Dyract AP, Compoglass, and Hytac) cured using a halogen light-curing unit (LCU) and a light-emitting diode (LED) LCU on human pulp fibroblasts.

METHODS

Specimens of three compomers were added to human pulp fibroblast cultures. Cytotoxicity was evaluated over 96 h using the agar overlay method.

RESULTS

All three compomers tested were found to be moderately cytotoxic to human pulp fibroblasts, regardless of whether they were cured using halogen or LED LCUs. The decolorization zone of Hytac was significantly larger than those of the other compomers tested (P < 0.05). Dyract AP and Compoglass specimens showed greater decolorization when cured with LED than with halogen LCUs (P < 0.05).

CONCLUSION

Compomers are potentially toxic to human pulp fibroblasts, and the type of curing unit may affect compomer toxicity.

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).

Describing Adequacy of Cure with Maximum Hardness Ratios and Non-linear Regression

Redefined hardness ratios, based on extended cure intervals and maximum hardness when used in conjunction with non-linear regression, provide a readily available and accurate characterization of the curing performance of LCU-composite combinations, which is superior to the use of traditional per-specimen hardness ratios. It is recommended that the light curing guidelines provided to clinicians should be based on this more accurate description of curing behavior.

Effect of photoactivation protocol and radiant exposure on monomer conversion and flexural strength of a resin composite after water and ethanol storage

The use of soft-start photoactivation to reduce shrinkage stress has become widespread in restorative dentistry. However, an increased susceptibility to ethanol degradation of polymers formed by pulse-delay photoactivation was reported. It was hypothesized that reductions in flexural strength after ethanol storage were related not only to low curing rates, but also to the radiant exposure employed. A commercial composite was subjected to different curing protocols (continuous at high irradiance, continuous at low irradiance, and pulse-delay) and radiant exposures (6, 12, and 24 J/cm2). After 48 h, differences in degree of conversion were minimal and no differences in strength were detected among specimens stored in water. Ethanol storage caused significant strength reductions in pulse-delay and low irradiance specimens that received 6 J/cm2. The results suggest that when low irradiances or pulse-delay methods are used, a relatively high radiant exposure is necessary to originate a polymer network structure similar to that obtained by continuous high irradiance photoactivation.

The influence of curing times and light curing methods on the polymerization shrinkage stress of a shrinkage-optimized composite with hybrid-type prepolymer fillers

OBJECTIVES

The aim of the study was to determine the influence of different light curing units (LCU) and regimes on the polymerization shrinkage stress (PSS) and the mechanical properties of a nano-hybrid composite.

MATERIAL AND METHODS

The polymerization shrinkage force (PSF) was measured continuously with compliance compensation for 300s after photo-initiating the composite, Tetric EvoCeram (Ivoclar Vivadent, Schaan, FL, Shade A3) in a Stress-Strain Analyser. Astralis 10, Bluephase and MiniL.E.D LCU with exposure times 10, 20 and 40s were used (C-factor=0.33, n=8 per group). Immediately after the PSF measurements, mechanical properties of the samples were measured at the top and the bottom using a Fischerscope H100C (Helmut Fischer GmbH, Sindelfingen, Germany). Statistical analyses were done using one-way ANOVA (p<0.05) and Tukey post hoc test.

RESULTS

Significant differences in the PSS for 10, 20 and 40s polymerization using Astralis 10 were found. The MiniL.E.D recorded low stress values. Modulus of elasticity is high after curing the composite with Astralis 10 at 10, 20 and 40s and for Bluephase 40s. Low moduli of elasticity were recorded for the MiniL.E.D and for the Bluephase 20 and 10s. The hardness values (HV) followed the same pattern as the modulus of elasticity. The Ramping mode of the MiniL.E.D had prolonged gel point.

CONCLUSIONS

High intensity LCU produce not just high HV but also high shrinkage, making it important to balance both the effects by choosing the appropriate curing time. Soft-start regimes have no paramount benefit in a LED regarding stresses in the clinical situation.

Effect of irradiation mode on polymerization of dental composite resins

Many researchers have sought to reduce the polymerization shrinkage in composite resins. Controlling the irradiation light seems practical because the polymerization process initiates with light by activating the photoinitiators. The present study evaluated the effect of irradiation mode on microhardness and polymerization shrinkage of composite resins. Three different irradiation modes, STD, EXP, and MED, were taken under 40-s exposure condition. As results, the irradiation mode significantly affected the microhardness difference. However, in all products, the microhardness difference between the STD and MED modes was less than 10%. In all irradiation modes, microhardness was linearly correlated with the filler content of the tested specimens. The effect of mode on the difference of the polymerization shrinkage was not uniform even though the shrinkage values decreased following the order of the STD > EXP > MED mode. An inverse correlation was found between filler content and polymerization shrinkage in all irradiation modes and specimen thicknesses. In spite of the statistical significance of the irradiation modes on the difference of the measured values, their numeric difference was not great. Especially, compared with the STD and MED mode, the exponentially increasing light (EXP mode) produced no noticeable difference except for a delayed shrinkage process.

Effect of the increase of energy density on knoop hardness of dental composites light-cured by conventional QTH, LED and xenon plasma arc

The aim of this study was to evaluate the effect of the increase of energy density on Knoop hardness of Z250 and Esthet-X composite resins. Cylindrical cavities (3 mm in diameter X 3 mm in depth) were prepared on the buccal surface of 144 bovine incisors. The composite resins were bulk-inserted and polymerized using different light-curing units and times: conventional QTH (quartz-tungsten-halogen; 700 mW/cm²; 20 s, 30 s and 40 s); LED (light-emitting diode; 440 mW/cm²; 20 s, 30 s and 40 s); PAC (xenon plasma arc; 1700 mW/cm²; 3 s, 4.5 s and 6 s). The specimens were stored at 37°C for 24 h prior to sectioning for Knoop hardness assessment. Three measurements were obtained for each depth: top surface, 1 mm and 2 mm. Data were analyzed statistically by ANOVA and Tukey's test (p<0.05). Regardless of the light source or energy density, Knoop hardness of Z250 was statistically significant higher than that of Esthet-X (p<0.05). Specimens cured with PAC had lower hardness than those cured with QTH and LED (p<0.05). Higher Knoop hardness was obtained when the energy density was increased for LED and PAC (p<0.05). No statistically significant differences (p>0.05) were found for QTH. Knoop hardness values decreased with the increase of depth. The increase of energy density produced composites with higher Knoop hardness means using LED and PAC.

Hardness and diametral tensile strength of a hybrid composite resin polymerized with different modes and immersed in ethanol or distilled water media

OBJECTIVES

The aim of this in vitro study was to evaluate the microhardness and diametral tensile strength of a hybrid composite resin (Z250, 3M ESPE) polymerized with four different modes of light exposure and immersed in two different media.

METHODS

Composite resin specimens were randomly polymerized according to the experimental groups (conventional, 550 mW/cm(2)/30 s; soft start, 300 mW/cm(2)/10 s + 550 mW/cm(2)/20 s; high intensity, 1060 mW/cm(2)/10 s; pulse delay, 550 mW/cm(2)/1 s + 60 s of waiting time + 550 mW/cm(2)/20 s) and immersed in one of two media (distilled water or absolute ethanol) for 24 h. After that, microhardness (M) and diametral tensile strength (DTS) tests were performed.

RESULTS

For DTS, there were no statistical differences among the polymerization modes, however, ethanol medium groups presented statistically lower DTS (p < 0.05) than water medium. For the M test, samples immersed in ethanol medium presented lower M for almost all groups. Conventional mode presented higher M values for the groups immersed in water medium. In ethanol medium, conventional and pulse delay groups presented higher M values, statistically different (p < 0.05) from the high intensity group. For all experimental conditions, the top surface showed higher M than the bottom surface.

SIGNIFICANCE

Different polymerization modes can be related to the different polymer structures formed, and consequently with different physical properties of resin composite. The immersion media can alter the physical properties of resin composites of different polymer structures.

The effect of different light-curing units on fatigue behavior and degree of conversion of a resin composite

OBJECTIVES

The aim of this study was to investigate the effect of different light-curing units and irradiation modes on the mechanical fatigue strength and degree of conversion of a restorative resin composite.

METHODS

Conventional halogen, plasma arc and blue LED light-curing units were used for polymerization of a resin composite (Tetric) Ceram, Ivoclar, Vivadent, Liechtenstein). Initial fracture strength (FS) and flexural fatigue limit (FFL) as well as degree of conversion (DC) were measured. The FFL was determined under cyclic loading for 10(5) cycles in terms of a staircase approach. The specimens were stored for 14 days in 37 degrees C distilled water prior to testing. The curing efficiency was observed with Fourier transform infrared micromultiple internal reflectance spectroscopy. The measurements were carried out at 0.5 and 2.5 mm distance from the directly irradiated surface after 14 days storage in dark and dry conditions at 37 degrees C.

RESULTS

The highest FS, FFL and DC were observed from high energy curing devices and from extended curing intervals. The conventional halogen light exhibited the most homogenous in-depth curing efficiency along with a low loss of mechanical resistance under cyclic fatigue.

SIGNIFICANCE

Evaluation of flexural fatigue limit and curing efficiency correlate in terms of decreased mechanical strength due to insufficient light-curing intervals or light intensities. Initial promising fracture strengths do not correlate with a clinically more relevant fatigue loading and with the in-depth degree of conversion, both accounting for a significantly reduced strength performance.

Effects of resin composite composition and irradiation distance on the performance of curing lights

This study determined the effect of using five resin composites and two irradiation distances to test the performance of dental curing lights. Three types of curing lights with similar spectral distributions, but each delivering a different power density, were used for irradiation times ranging from 3 to 60 s. Power densities were measured at 2 and 9 mm from the tip of the light guide. Five composites 1.6 mm thick and of the same shade were irradiated at 2 and 9 mm from the light guide with energy densities of 1.2-38.0 J/cm(2). The Knoop hardness at the top and bottom of the composite specimens was measured 15 min after irradiation and again after immersion in water at 37 degrees C for 24 h. There was a linear relationship between the hardness and the logarithm of the energy density received by the composite (r2 > 0.81). The analysis of variance showed that the composite, the side tested, the distance from the light guide, and the curing light/irradiation time combination all had a significant effect on the hardness (p < 0.01). Plots of the hardness at the bottom 15 min after irradiation by each light were generated for all the composites. These plots illustrated that the effects of the different curing light/irradiation time combinations on hardness were not the same for each composite. The effects of each curing light/time combination on hardness were also different at 2 and 9 mm from the light guide. In conclusion, when comparing the effects of different light sources on resin polymerization, several different composites should be irradiated at clinically relevant distances from the light guide. Using high-powered curing lights for 3 or 5 s did not deliver sufficient energy to cure the 1.6-mm thick specimens of composites used in this study.

How light irradiance and curing time affect monomer conversion in light-cured resin composites

We tested the hypothesis that the degree of conversion of a light-cured dental composite relates to the calculated (s x mW cm-2 = mJ cm-2) rather than to the irradiance value (mW cm-2) of the light source. Two light-curable composite resins were cured with three different light irradiance values over different curing times. The specimens tested were 2, 4 or 6 mm thick, and the degree of conversion values were measured with Raman spectroscopy on the top and the bottom surfaces of the specimens. The highest conversion value of one of the materials was just below 60%, while the maximal conversion value of the other material was just below 65%. That difference in conversion values could be related to differences in monomer systems used in the two composites. By considering light energy per square centimeter (J cm-2) rather than light irradiance (mW cm-2), we found that equivalent energy values gave similar conversion values for a certain sample thickness. From these findings, we conclude that our experimental results support our hypothesis.

Effect of light power density variations on bulk curing properties of dental composites

OBJECTIVE

The hypothesis that low light intensity and long but sufficient curing time can produce composites with volumetric shrinkage, degree of conversion (DC%) and Young's modulus (E-modulus) comparable to those of high light intensity cured composite was tested, when the contraction strain and heat generation were lower with low light intensity curing.

METHODS

Dental composites (Z100 and Z250, 3M ESPE) were investigated. Specimens were cured with light intensities of 200, 450 and 800 mW/cm(2) for 140, 60 and 35 s from a distance of 7 mm. Strain-gages were used for contraction strain measurements. DC% was measured at the top and the bottom of 4 mm thick samples using FT-Raman spectroscopy. Volumetric polymerization shrinkage was determined using a water displacement method. E-modulus was determined in tension on composite specimens.

RESULTS

The results were analyzed using ANOVA and Duncan's multiple range tests and regular t-test. Polymerization stress level decreased significantly (p<0.05) when cured with 200 mW/cm(2) rather than with 800 mW/cm(2). Temperature rises were significantly different (p<0.05) for different composites and light intensity values. Reduction in light intensity did not decrease the DC% values significantly at the top surfaces. The most dramatic differences existed between top and bottom surfaces (p<0.05) rather than among curing groups. Measured E-modulus and volumetric shrinkage values were not significantly different (p>0.05) between different light intensity groups.

CONCLUSION

DC%, E-modulus and the volumetric shrinkage values in cured composites were not affected by low light intensity, however, the contraction strain and polymerization's exotherm were decreased. Thus our results support the proposed hypothesis.

High power light emitting diode (LED) arrays versus halogen light polymerization of oral biomaterials: Barcol hardness, compressive strength and radiometric properties

The clinical performance of light polymerized dental composites is greatly influenced by the quality of the light curing unit (LCU) used. Commonly used halogen LCUs have some specific drawbacks such as decreasing light output with time. This may result in a low degree of monomer conversion of the composites with negative clinical implications. Previous studies have shown that blue light emitting diode (LED) LCUs have the potential to polymerize dental composites without having the drawbacks of halogen LCUs. Since these studies were carried out LED technology has advanced significantly and commercial LED LCUs are now becoming available. This study investigates the Barcol hardness as a function of depth, and the compressive strength of dental composites that had been polymerized for 40 or 20s with two high power LED LCU prototypes, a commercial LED LCU, and a commercial halogen LCU. In addition the radiometric properties of the LCUs were characterized. The two high power prototype LED LCUs and the halogen LCU showed a satisfactory and similar hardness-depth performance whereas the hardness of the materials polymerized with the commercial LED LCU rapidly decreased with sample depth and reduced polymerization time (20 s). There were statistically significant differences in the overall compressive strengths of composites polymerized with different LCUs at the 95% significance level (p = 0.0016) with the two high power LED LCU prototypes and the halogen LCU forming a statistically homogenous group. In conclusion, LED LCU polymerization technology can reach the performance level of halogen LCUs. One of the first commercial LED LCUs however lacked the power reserves of the high power LED LCU prototypes.

Effect of Distance on the Power Density from Two Light Guides

PURPOSE

This study determined the effect of distance on the power density from standard and Turbo light guides (Demetron/Kerr, Danbury, Connecticut).

MATERIALS AND METHODS

Power density was measured from 0 to 10 mm away from the tip of standard 8-mm curved light guides and 13/8-mm Turbo curved light guides. To determine the effect of distance on power density, a polynomial regression line was fitted. The Kolmogorov-Smirnov (K-S) statistic and the Wilcoxon rank sum (WR) tests were used to determine if there was a difference in the rate at which the power density decreased for the standard and Turbo light guides as the distance from the tip increased. Photographs of the light dispersion from each tip were also taken.

RESULTS

At 0 mm, the mean (+/- SD) power density from the two standard light guides was 743 +/- 6.1 mW/cm2 and from the four Turbo light guides was 1128 +/- 22.1 mW/cm2. As the distance from the tip of the light-guide tip increased, the power density decreased, but the rate of decrease was greater from the Turbo light guides than from the standard light guides. At 6 mm the power density from the standard light guides fell to 372 mW/cm2 (50% of the original value) and the power density from the Turbo light guides fell to 263 mW/cm2 (23% of the original value). Both the K-S statistic and the WR sum test indicated that the distribution of light intensities was significantly different from the two light guides (WR p-value = .0246, K-S p-value < .0001). The two estimated polynomials intersected at 3.66 mm, and the 95% prediction intervals intersected at about 2.8 and 4.8 mm. Therefore, beyond 5 mm away from the tip of the light guide, the standard light guides gave higher power density readings than the Turbo light guides. Photographs showed that the light dispersed at a wider angle from the Turbo light guides than from the standard light guide.

CLINICAL SIGNIFICANCE

The design of the light guide of a light curing unit affects light dispersion, power density, and ultimately the dentist's ability to properly cure composite. For these reasons, manufacturers should report the power density at the tip of the light guide and 6 mm from the tip of the light guide, since significant differences exist between light guide designs.