Thermal changes and cure depths associated with a high intensity light activation unit

Effect of translucency and absorbance of composite on temperature change during photopolymerization

This study investigated the effect of translucency and absorbance of conventional (FiltekTM-Z350-XT) and bulk-fill (Tetric®-N-Ceram) composites on temperature change during photopolymerization, using a non-contact infrared sensor. Three shades from each composite were selected to prepare disk-shaped specimens (n=3), which then photopolymerized with LED-light for 20 s. A second light exposure was performed on the photopolymerized specimens. The first peak temperature rise during composite photopolymerization (ΔTtotal), second peak temperature rise by the light (ΔTlight), and net peak temperature rise by composite curing heat (ΔTcomposite) were obtained from the temperature change vs. time curve. The changes in ΔTtotal and ΔTlight with varying the composite shade were greater than those in ΔTcomposite. The conventional composite showed higher ΔTtotal and ΔTlight than bulk-fill composite. ΔTtotal and ΔTlight increased as translucency parameter decreased, and absorbance increased. The potential risk for heat-induced pulpal damage should be considered when selecting a composite shade, especially for deep cavities.

Temperature Changes in Primary and Permanent Teeth Dentine of Varying Thicknesses Following Irradiation by Two Light Curing Units

AIM

This study was performed to determine the temperature rise under human dentin discs of different thicknesses from primary and permanent teeth during the photo-curing process using quartz tungsten halogen (QTH) or light-emitting diode (LED).

MATERIALS AND METHODS

The current experimental study sample consisted of 160 dentin discs of different thicknesses (0.5, 1, 1.5, and 2 mm), of which 80 dentin discs were prepared from sound lower second primary molars, and the remaining 80 dentin discs were prepared from sound lower third permanent molars extracted surgically for various reasons. A "K" type of thermal tentacle was placed in the center of an acrylic resin base, followed by the placement of a dentin disc. Then, the thermal changes were measured during the photo-curing of the composite using a second LED or QTH light curing unit for 20 s. Statistical evaluation was performed using the IBM SPSS Statistics® Version 20.0 software system (SPSS Inc., Chicago, IL, USA).

RESULTS

The current study found that the temperature rise in primary teeth (1.17-2.96°C) is significantly lower compared to the rise in permanent teeth (1.55-3.33°C), regardless of the dentin disc thickness or light curing unit used. The temperature rise decreases significantly when the thickness of dentin discs increases, regardless of the type of teeth or light curing unit used (P<0.05). Furthermore, QTH causes less temperature rise (1.17-2.65°C) compared to LED (1.61-3.33°C).

CONCLUSIONS

The temperature rise during polymerization of the resin composite with the second-generation LED appeared to be below 5.5°C. Hence, it appears to be safe for use during the restoration of primary teeth. Primary teeth dentin might be more effective than permanent teeth dentin in protecting the dental pulp.

Study on New Dental Materials Containing Quinoxaline-Based Photoinitiators in Terms of Exothermicity of the Photopolymerization Process

Modern dentistry places great demands on the dental composites used for filling tooth cavities or treating cavitated tooth decay. The aim of the work was to modify the properties of composites by changing the initiators and co-initiators. This was achieved by using initiators based on a quinoxaline skeleton and co-initiators that are derivatives of acetic acid, which is an advantage of these photoinitiating systems due to the elimination of aromatic amines from the photocurable composition. The composites also differed in dental fillers. The effect of the compounds on the exothermicity of the photopolymerization process, the surface morphology of the obtained materials and the maximum compressive strength were determined. The photoinitiating capacity of the two-component systems was tested by the microcalorimetric method using the multifunctional monomer TMPTA, typical for dental filler compositions. The new photoinitiating systems show particularly good efficiency of free radical polymerization initiation, which occurs by the photoinduced intermolecular electron transfer (PET) mechanism. The comparison of the tested systems with camphorquinone, a photoinitiator traditionally used in dentistry, made it possible to observe a decrease in temperature during photopolymerization without a significant decrease in the polymerization rate or increase in photocuring time, as well as a better homogeneity of the surface of the obtained polymeric materials. This indicates that dye-acetic acid derivative systems may be useful in dental applications.

Assessment of Heat Hazard during the Polymerization of Selected Light-Sensitive Dental Materials

Introduction. Polymerization of light-cured dental materials used for restoration of hard tooth tissue may lead to an increase in temperature that may have negative consequence for pulp vitality. Aim. The aim of this study was to determine maximum temperatures reached during the polymerization of selected dental materials, as well as the time that is needed for samples of sizes similar to those used in clinical practice to reach these temperatures. Materials and Methods. The study involved four composite restorative materials, one lining material and a dentine bonding agent. The polymerization was conducted with the use of a diode light-curing unit. The measurements of the external surface temperature of the samples were carried out using the Thermovision®550 thermal camera. Results. The examined materials significantly differed in terms of the maximum temperatures values they reached, as well as the time required for reaching the temperatures. A statistically significant positive correlation of the maximum temperature and the sample weight was observed. Conclusions. In clinical practice, it is crucial to bear in mind the risk of thermal damage involved in the application of light-cured materials. It can be reduced by using thin increments of composite materials.

Conversion and temperature rise of remineralizing composites reinforced with inert fillers

OBJECTIVES

Remineralizing experimental composites based on amorphous calcium phosphate (ACP) were investigated. The impact of curing time (20 and 40s), curing depth (1, 2, 3 and 4mm) and addition of inert fillers (barium glass and silica) on the conversion and temperature rise during curing were examined.

METHODS

Five ACP-composites and two control composites were prepared based on the light-curable EBPADMA-TEGDMA-HEMA resin. For temperature measurements, a commercial composite was used as an additional control. Conversion was assessed using FT-Raman spectroscopy by comparing the relative change of the band at 1640 cm(-1) before and after polymerization. The temperature rise during curing was recorded in real-time using a T-type thermocouple.

RESULTS

At 1mm depth, the ACP-composites attained significantly higher conversion (77.8-87.3%) than the control composites based on the same resin (60.5-66.3%). The addition of inert fillers resulted in approximately 5% lower conversion at clinically relevant depths (up to 2mm) for the curing time of 40s. Conversion decline through depths depended on the added inert fillers. Conversion values higher than 80% of the maximum conversion were observed for all of the ACP-composites at depths up to 3mm, when cured for 40s. Significantly higher total temperature rise for the ACP-composites (11.5-13.1 °C) was measured compared to the control composites (8.6-10.8 °C) and the commercial control (8.7 °C).

CONCLUSIONS

The admixture of inert fillers represents a promising strategy for further development of ACP-composites, as it reduced the temperature rise while negligibly impairing the conversion.

CLINICAL SIGNIFICANCE

High conversions of ACP-composites are favorable in terms of mechanical properties and biocompatibility. However, high conversions were accompanied with high temperature rise, which might present a pulpal hazard.

Characterizing the output settings of dental curing lights

OBJECTIVES

For improved inter-study reproducibility and ultimately improved patient care, researchers and dentists need to know what electromagnetic radiation (light) is emitted from the light-curing unit (LCU) they are using and what is received by the resin. This information cannot be obtained from a dental radiometer, even though many studies have used a dental radiometer.

METHODS

The light outputs from six LCUs (two QTH and four broad-spectrum LED units) were collected in real-time using an integrating sphere connected to a fiberoptic spectrometer during different light exposures.

RESULTS

It was found that the spectral emissions were unique to each LCU, and there was no standardization in what was emitted on the various ramp (soft-start) settings. Relative to the normal use setting, using the ramp setting reduced the radiant energy (J) delivered from each LCU. For one of the four broad-spectrum LED LCUs, the spectral emissions in the violet range did not increase when the overall radiant power output was increased. In addition, this broad-spectrum LED LCU emitted no light from the violet LED chip for the first 5s and only emitted violet light when the ramp phase finished.

CONCLUSIONS

A single irradiance value derived from a dental radiometer or from a laboratory grade power meter cannot adequately describe the output from the LCU. Manufacturers should provide more information about the light output from their LCUs. Ideally, future assessments and research publications that include resin photopolymerization should report the spectral radiant power delivered from the LCU throughout the entire exposure cycle.

Real-Time Analysis of Temperature Changes in Composite Increments and Pulp Chamber during Photopolymerization

Objective. The aim of this study was to evaluate the temperature change at various sites within the composite and on the pulpal side of dentin during polymerization of two composite increments. Materials and Methods. Class I cavities prepared in third molars were restored in two composite increments (n = 5). Temperatures were measured for 110 s using eight thermocouples: bottom center of cavity (BC), top center of 1st increment (MC), top center of 2nd increment (TC), bottom corner of cavity (BE), top corner of 1st increment (ME), top corner of 2nd increment (TE), pulpal side of dentin (PD), and center of curing light guide tip (CL). Results. Maximum temperature values (°C) measured during polymerization of 1st increment were MC (59.8); BC (52.8); ME (51.3); CL (50.7); BE (48.4); and PD (39.8). Maximum temperature values during polymerization of 2nd increment were TC 58.5; TE (52.6); MC (51.7); CL (50.0); ME (48.0); BC (46.7); BE (44.5); and PD (38.8). Conclusion. Temperature at the floor of the cavity was significantly higher during polymerization of 1st increment compared to 2nd increment. Temperature rise was higher at the center than at the corner and at the top surface than at the bottom surface of each increment.

Thermal analysis of bulk filled composite resin polymerization using various light curing modes according to the curing depth and approximation to the cavity wall

OBJECTIVE

The purpose of this study was to investigate the polymerization temperature of a bulk filled composite resin light-activated with various light curing modes using infrared thermography according to the curing depth and approximation to the cavity wall.

MATERIAL AND METHODS

Composite resin (AeliteFlo, Bisco, Schaumburg, IL, USA) was inserted into a Class II cavity prepared in the Teflon blocks and was cured with a LED light curing unit (Dr's Light, GoodDoctors Co., Seoul, Korea) using various light curing modes for 20 s. Polymerization temperature was measured with an infrared thermographic camera (Thermovision 900 SW/TE, Agema Infra-red Systems AB, Danderyd, Sweden) for 40 s at measurement spots adjacent to the cavity wall and in the middle of the cavity from the surface to a 4 mm depth. Data were analyzed according to the light curing modes with one-way ANOVA, and according to curing depth and approximation to the cavity wall with two-way ANOVA.

RESULTS

The peak polymerization temperature of the composite resin was not affected by the light curing modes. According to the curing depth, the peak polymerization temperature at the depth of 1 mm to 3 mm was significantly higher than that at the depth of 4 mm, and on the surface. The peak polymerization temperature of the spots in the middle of the cavity was higher than that measured in spots adjacent to the cavity wall.

CONCLUSION

In the photopolymerization of the composite resin, the temperature was higher in the middle of the cavity compared to the outer surface or at the internal walls of the prepared cavity.

Temperature increase during orthodontic bonding with different curing units using an infrared camera

AIM

To evaluate the effects of different curing units and light-tip tooth surface distances on the temperature increase generated during orthodontic bonding, using an infrared camera (IR) and artificial neural networks (ANN).

MATERIALS AND METHODS

Fifty-two freshly extracted human premolar teeth were used. Metallic orthodontic brackets were bonded to the buccal surfaces of the teeth and thermal records were taken using an IR camera and ANN. Brackets were cured with a light-emitting diode (LED) and high intensity halogen (HQTH). Teeth were divided into four groups according to the curing units (LED and HQTH) and curing distances (from tooth surface and 10 mm away from tooth surface). The results were analyzed with analysis of variance (ANOVA) and the Tukey HSD test.

RESULTS

The ANOVA and Tukey HSD tests revealed that temperature changes were influenced by the type of light source and exposure times. All groups revealed significant differences between each other (p < 0.001). The highest surface temperature increase was gained from curing with a LED unit from the tooth surface (11.35°C ± 0.91°C). The lowest surface temperature increase was gained from curing with a HQTH unit 10 mm away from the tooth surface (2.57°C ± 0.6°C).

CONCLUSION

The LED unit induced significantly higher temperature changes than did the HQTH. The temperature increase during orthodontic bonding was increased with long exposure time. A shorter light-tip tooth surface distance leads to greater increases in temperature.

Contemporary issues in light curing

This review article will help clinicians understand the important role of the light curing unit (LCU) in their offices. The importance of irradiance uniformity, spectral emission, monitoring the LCU, infection control methods, recommended light exposure times, and learning the correct light curing technique are reviewed. Additionally, the consequences of delivering too little or too much light energy, the concern over leachates from undercured resins, and the ocular hazards are discussed. Practical recommendations are provided to help clinicians improve their use of the LCU so that their patients can receive safe and potentially longer lasting resin restorations.

Temperature Rise during Primer, Adhesive, and Composite Resin Photopolymerization of a Low-Shrinkage Composite Resin under Caries-Like Dentin Lesions

Objective. This study evaluated temperature rise of low-shrinkage (LS) self-etch primer (P), LS self-etch adhesive (A), and P90 silorane-based composite resin systems, photopolymerized under normal and artificially demineralized dentin. Methods. Forty 1.5 mm-thick dentin discs were prepared from sound human molars, half of which were demineralized. Temperature rise was measured during photopolymerization using a K-type thermocouple under the discs: 10 s and 40 s irradiation of the discs (controls/groups 1 and 2); 10 s irradiation of primer (P), 10 s irradiation of adhesive (A), 40 s irradiation of P90 without P and A, and 40 s irradiation of P90 with P and A (groups 3 to 6, resp.). The samples were photopolymerized using an LED unit under 550 mW/cm² light intensity. Data was analyzed using repeated measures ANOVA and paired-sample t-test (α = 0.05). Results. There were no significant differences in temperature rise means between the two dentin samples for each irradiation duration (P > 0.0001), with significant differences between the two irradiation durations (P > 0.0001). Temperature rise measured with 40 s irradiation was significantly higher than that of 10 s duration for undemineralized and demineralized dentin P < 0.0001). Conclusions. Light polymerization of P90 low-shrinkage composite resin resulted in temperature rise approaching threshold value under artificially demineralized and undemineralized dentin.

Pulpal-temperature Rise and Polymerization Efficiency of LED Curing Lights

This paper provides practitioners with useful information on the importance of aligning the spectra of the LCU and the material in terms of polymerization efficiency and temperature rise in the pulp chamber.

Effect of various visible light photoinitiators on the polymerization and color of light-activated resins

The purpose of this study was to investigate effects of various visible light photoinitiators on the polymerization efficiency and color of the light-activated resins. Four photoinitiators, including camphorquinone, phenylpropanedione, monoacrylphosphine oxide (TPO), and bisacrylphosphine oxide (Ir819), were used. Each photoinitiator was dissolved in a Bis-GMA and TEGDMA monomer mixture. Materials were polymerized using dental quartz-tungsten halogen lamp (QTH), plasma-ark lamp and blue LED light-curing units, and a custom-made violet LED light unit. The degree of monomer conversion and CIE L*a*b* color values of the resins were measured using a FTIR and spectral transmittance meter. The degree of monomer conversions of TPO- and Ir819-containing resins polymerized with the violet-LED unit were higher than camphorquinone-containing resin polymerized with the QTH light-curing unit. The lowest color values were observed for the TPO-containing resin. Our results indicate that the TPO photoinitiator and the violet-LED light unit may provide a useful and improved photopolymerization system for dental light-activated resins.

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.

Refractive Index Mismatch and Monomer Reactivity Influence Composite Curing Depth

Limited cure depth is a drawback of light-activated composites. We hypothesize that curing light transmission and cure depth are influenced by monomer reactivity and filler/resin refractive index mismatch. Light transmission throughout cure was recorded for composites based on strontium (refractive index 1.51) or barium (refractive index 1.53) glass fillers. Fillers were mixed (70 wt%) with 4 bisphenol-A diglycidyl-ether-dimethacrylate (bis-GMA):triethylene glycol dimethacrylate (TEGDMA) formulations with refractive indices ranging from 1.4703 to 1.5370. Following polymerization, cure depth and pre- and post-cure translucency parameters were determined. Transmission changes and cure depths related to monomer reactivity and filler/resin refractive index mismatch with significant interaction. Composites became more opaque or translucent on curing. Optimizing filler/resin refractive index mismatch provides increased curing depth and assists shade-matching.

Temperature change, dentinal fluid flow and cuspal displacement during resin composite restoration

Dentin-bonding agents and resin composite materials typically require light activation for polymerization. Light curing generates heat, which may influence dentinal fluid flow (DFF) and cuspal displacement. This study investigated the relationship among temperature increase, DFF and cuspal displacement in extracted human maxillary premolars with a mesial occlusal distal (MOD) cavity preparation. Two types of curing light were compared. Temperature changes were measured using thermocouples located on the occlusal cavity floor and at the pulp-dentine junction, during polymerization of bonding agent and resin composite material. DFF and cuspal displacement were measured simultaneously using automated flow measurement apparatus and direct current differential transformers respectively. Temperature increases of up to 15 degrees C were recorded during the restoration procedures. A quartz tungsten halogen (QTH) unit produced a significantly greater temperature increase than a light-emitting diode unit and curing of the bonding agent generated less temperature increase than curing of the resin composite. Heating due to exothermic reaction during polymerization of bonding agent and resin was not significantly different between light sources or between bonding and curing (P > 0.05). The QTH unit produced both greater inward fluid flow and cuspal displacement during the irradiation of bonding agent and resin composite than the light-emitting diode unit. There was not a simple relationship between temperature increase, fluid movement and cuspal displacement. From a clinical point of view, the light-emitting diode unit can be considered preferable to the QTH light, because it caused significantly smaller temperature increase, fluid shift and cuspal displacement.

Photoinitiation chemistry affects light transmission and degree of conversion of curing experimental dental resin composites

INTRODUCTION

The effect of photoinitiator and co-initiator chemistry on the setting reaction and degree of conversion of dental resin-based composites (RBCs) has rarely been determined explicitly. This work examines the effect of type and concentration of photoinitiator and co-initiator on the rate of change of light transmission throughout polymerisation and degree of conversion of model RBC formulations.

METHODS

Bisphenol-A diglycidyl ether dimethacrylate (bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) (6:4 molar ratio) resins filled with silanized glass filler (74wt.%) and containing various photoinitiators (camphorquinone; CQ, 1-phenyl-1,2-propanedione; PPD, benzil; BZ), co-initiator types (N,N-dimethyl-p-amino benzoic acid ethyl ester; DABE, N,N-cyanoethyl methylaniline; CEMA, N,N-diethanol-p-toluidine; DEPT) and concentration (0.0-0.3% DABE) were polymerised using a halogen or LED light curing-unit (LCU) for 10, 20 and 40s. The setting reaction was monitored in real-time by measuring the light transmittance through the curing specimen and bulk degree of conversion (DC) evaluated using Fourier transform infra-red spectroscopy.

RESULTS

Specimens containing CQ and PPD cured with the halogen LCU did not have a significant effect on DC or changes in light transmission, although a significant increase in DC was observed for CQ compared with PPD specimens cured with the LED LCU. DABE and CEMA were more effective co-initiators than DEPT. Although DC was not limited by co-initiator concentration, the absence of a co-initiator resulted in marked differences in light transmission and decreased DC throughout 40s irradiation with each LCU type.

CONCLUSIONS

The spectral range emitted from different types of LCU and absorption characteristics of the photoinitiator chemistry of light-activated resin-based composites play a critical role in the efficiency of polymerisation.

Effect of composite shade, increment thickness and curing light on temperature rise during photocuring

OBJECTIVES

To examine the effect of composite shade, increment thickness and curing light characteristics on the temperature rise associated with composite photocuring.

METHODS

Four shades (C2, A4, B1 and B3), four sample thicknesses (2, 3, 4 and 5 mm) of a hybrid resin composite and two curing units, one with two modes of curing, were investigated. The composite samples were packed in polytetrafluoroethylene (PTFE) moulds and cured for 40 s. Samples cured with the ramp curing mode were irradiated for only 20 s. Temperature rises on the undersurface of the curing resin composite were measured using an infrared scanning system.

RESULTS

Shade C2 produced the highest maximum temperature of all shades (56.7 degrees C). Thinner samples produced greater temperature rises (2mm induced 60.9 degrees C, 5 mm induced 45.7 degrees C). Samples cured with Optilux 501 unit produced greater temperature rises (60.9 degrees C) than those cured with Dentsply unit (56.2 degrees C).

CONCLUSIONS

There was a quantifiable amount of heat generated during visible light curing of resin composite. The amount of heat generated was influenced by shade selected, thickness of material and characteristics of the light curing unit.

Heat shock induces the synthesis of the inflammatory mediator leukotriene B4 in human pulp cells

AIM

To measure the synthesis of leukotriene B4 (LTB4) in cultures of human dental pulp cells induced by heat shock.

METHODOLOGY

Primary pulp cells (PC) and dental pulp stem cells (DPSC) were cultivated under appropriate conditions. For the characterization of PC the expression of dentine sialophosphoprotein (DSPP) was evaluated by reverse transcription-polymerase chain reaction. Thermal stimulation of cell cultures was performed at temperatures of 37, 38, 39, 40, 42 and 45 degrees C for stimulation times of 5 and 30 s. LTB4 was quantified by reversed-phase high-performance chromatography and differences between the LTB4 concentrations of controls and heat stimulated cells were analysed with Friedman analysis of variances by ranks and multiple comparisons (P < 0.05).

RESULTS

Both cell cultures expressed DSPP under the conditions of the present experiment. The analysis revealed significantly enhanced LTB4 synthesis following thermal stimulations at 38, 39, 40, 42 and 45 degrees C compared with unstimulated controls for both PC and DPSC.

CONCLUSION

The present study demonstrated the capability of pulp cells to synthesize the arachidonic acid mediator LTB4 in response to heat shock. LTB4 has the capacity to induce inflammatory reactions and to sensitise afferent nociceptive nerve endings. LTB4 synthesis is induced by minor temperature changes, which are relevant for various clinical situations.

Halogen and LED light curing of composite: temperature increase and Knoop hardness

This study assessed the Knoop hardness and temperature increase provided by three light curing units when using (1) the manufacturers' recommended times of photo-activation and (2) standardizing total energy density. One halogen--XL2500 (3M/ESPE)--and two light-emitting diode (LED) curing units--Freelight (3M/ESPE) and Ultrablue IS (DMC)--were used. A type-K thermocouple registered the temperature change produced by the composite photo-activation in a mold. Twenty-four hours after the photo-activation procedures, the composite specimens were submitted to a hardness test. Both temperature increase and hardness data were submitted to ANOVA and Tukey's test (5% significance). Using the first set of photo-activation conditions, the halogen unit produced a statistically higher temperature increase than did both LED units, and the Freelight LED resulted in a lower hardness than did the other curing units. When applying the second set of photo-activation conditions, the two LED units produced statistically greater temperature increase than did the halogen unit, whereas there were no statistical differences in hardness among the curing units.