Heat transfer properties and thermal cure of glass-ionomer dental cements.

Under clinical conditions, conventional glass-ionomer dental cements can be cured by application of heat from dental cure lamps, which causes acceleration in the setting. In order for this to be successful, such heat must be able to spread sufficiently through the cement to enhance cure, but not transmit heat so effectively that the underlying dental pulp of the tooth is damaged. The current study was aimed at measuring heat transfer properties of modern restorative glass-ionomers to determine the extent to which they meet these twin requirements. Three commercial glass ionomer cements (Ionofil Molar, Ketac Molar and Equia™ Fill) were used in association with three different light emitting diode cure lamps designed for clinical use. In addition, for each cement, one set of specimens was allowed to cure without application of a lamp. Temperature changes were measured at three different depths (2, 3 and 4 mm) after cure times of 20, 40 and 60 s. The difference among the tested groups was evaluated by ANOVA (P < 0.05) and post hoc Newman-Keuls test. All brands of glass-ionomer showed a small inherent setting exotherm in the absence of heat irradiation, but much greater temperature increases when exposed to the cure lamp. However, temperature rises did not exceed 12.9 °C. Application of the cure lamp led to the establishment of a temperature gradient throughout each specimen. Differences were typically significant (P < 0.05) and did not reflect the nominal power of the lamps, because those lamps have variable cooling systems, and are designed to optimize light output, not heating effect. Because the thermal conductivity of glass-ionomers is low, temperature rises at 4 mm depths were much lower than at 2 mm. At no time did the temperature rise sufficiently to cause concern about potential damage to the pulp.

Effects of plasma-emulating light-emitting diode (LED) versus conventional LED on cytotoxic effects and polymerization capacity of orthodontic composites.

OBJECTIVES: The aim of this study was to evaluate, the cytotoxicity of orthodontic composites in vitro as a function of degree of conversion (DC) and the light curing units (LCU) employed on mouse fibroblast (L929). MATERIALS AND METHODS: Cured samples of the composites Light bond (Reliance Orthodontic Products, Itasca, Illinois, USA), Ortho bracket paste (Bisco, Schaumburg, Illinois, USA), Opal bond MV (OPAL, South Jordan, Utah, USA), and Transbond XT (3M, Monrovia, California, USA) were prepared. Polymerization was performed with two LCUs: VALO Ortho (Ultradent, South Jordan, Utah, USA) is a third-generation LCU and Elipar S10 (3M, USA) is a second-generation LCU. Four samples were immersed in cell culture medium to obtain composite extracts. After incubation of L929 cell cultures with the extracts obtained, cytotoxicity was determined using the methyl tetrazolium test. Fourier transform infrared spectroscopy (FTIR) was used to evaluate DC for five samples. A multivariate analysis of variance (ANOVA), two-way ANOVA, and Tukey's honestly significant difference test were utilized for statistical analyses. RESULTS: Cytotoxicity and DC of all tested composites (p < 0.001) and the interaction between composites and LCUs (p < 0.01) were significantly different. LCUs had no significant influence on the cytotoxicity and DC of composite materials (p > 0.05). The correlations between cell viability and DC were positive for three composites but statistically insignificant. CONCLUSION: Composites and LCUs must be matched with one another to result in satisfactory maximal biocompatibility and DC. Opal Bond plasma light-emitting diode combination was a better choice for cell viability. Three composites showed a positive correlation between cytotoxicity and DC. Therefore high-intensity LCUs can be said to efficiently affect polymerization, and so, higher DC rates may achieve higher cell viability rates.

Effects of plasma-emulating light emitting diode (LED) versus conventional LED on cytotoxic effects of orthodontic cements as a function of polymerization capacity.

OBJECTIVES: The study was aimed at evaluating, in vitro, cytotoxicity of four resin-based orthodontic cements (RBOC) as a function of degree of conversion (DC) and the light curing unit (LCU) employed on mouse fibroblast (L929). MATERIALS AND METHODS: Nine samples were manufactured for each group of cements using plasma-emulating light-emitting diode (LED) and conventional LED. Toxicity was assessed by immersing four specimens to culture medium (24 h/37°C) for extracting residual monomer or cytotoxic substance. Cell mitochondrial activity of L929 cell was evaluated using methyl tetrazolium (MTT) test. DC was evaluated by Fourier transform infrared spectroscopy for five samples. RESULTS: Cements, LCUs, and interaction between cements and LCUs were found to play a statistically significant role in cytotoxicity (p < 0.0001). Opal band cement (OPAL) plasma LED was found noncytotoxic (90-100% cell viability). The other RBOC-LCU combinations were slightly cytotoxic (60-90% cell viability). Cements (p < 0.01) and LCUs (p < 0.05) had a statistically significant effect on DC. Conversely, interaction between cement and LCU had no statistically significant role on DC (p > 0.05). OPAL plasma LED displayed the highest levels of DC. The correlations between cell viability and DC were positive for three RBOCs. CONCLUSION: Therefore, high-intensity LCUs can be said to efficiently affect polymerization, so higher DC rates may achieve higher cell viability rates. CLINICAL RELEVANCE: Cements and LCUs must be matched to each another to result in higher DC and maximal biocompatibility. Dual cure systems presented relatively high cell survival and higher DC, thus expressing superior to single-cure systems with plasma LED.

Cytotoxicity evaluation of three light-cured dentin adhesive materials on human gingival fibroblasts, ex vivo.

PURPOSE: To evaluate the cytotoxic effects of three current light-cured dentin adhesives, in both uncured and post-cured conditions, on human gingival fibroblasts. MATERIAL AND METHODS: The materials tested were Heliobond, Adper Single Bond 2 and Xeno V, which are characterized by various compositions and application procedures. Each agent, in volumes of 5 and 10 µL, was tested after polymerization, and those unpolymerized were diluted in DMEM to 10-3 and 10-5. The cytotoxicity of the adhesives was assessed on the basis of a test of cell viability in a culture of human gingival fibroblasts, with the use of tetrazolic salt (MTT assay). RESULTS: The results showed that, among the adhesive/bonding systems tested, Xeno V was the least cytotoxic. There were statistically significant differences in cell survival between polymerized Xeno V, Adper Single Bond 2 and Heliobond in the amount of 5 µL as well as between the Xeno V and Adper Single Bond 2 in 10-5 dilutions. The tested adhesives were more toxic in the polymerized form than in the dilutions. Samples of 10 µL resulted in a lower survival percentage of fibroblasts compared to 5 µL. CONCLUSION: All the tested adhesives demonstrated cytopathic effects towards human gingival fibroblasts, but varied in their cytotoxicity. This has clinical implications. Dentists should follow the rules of adhesive application, precisely dose them and not allow direct contact with the gums as, even after polymerization, adhesive agents exhibit potential cytotoxic activity.

Dental light curing and its effects on color perception.

INTRODUCTION: Light curing has become increasingly popular for orthodontic bonding, partly as a result of improvements in light-curing unit technology and higher light intensities. The aim of this study was to determine orthodontists' knowledge of dental light-curing units, their safety aspects, and the possible effects on color perception. METHODS: Questionnaires were administered to 120 specialists or trainees to assess their knowledge of light curing and safety issues. In addition, 15 orthodontists and 15 nonorthodontists were asked to complete the Farnsworth Munsell 100 hue test to assess color perception. RESULTS: One hundred four questionnaires were returned, giving a response rate of 86.6%. Light-emitting diode lights were the most popular (73.4%), followed by quartz-halogen (9.2%) and plasma lights (5.5%); 11.9% were unsure of the type of light used, 84% did not know the intensity, and 67% did not know the wavelength of the lights. Although most used safety equipment-eg, paddles-7% used no safety measures. Seventy-six percent were either unsure or took no precautions during light curing for staff or patients who had previous cataract surgery, and up to 99% were either unsure or took no precautions during light curing for staff or patients taking photosensitizing medications. With the Farnsworth Munsell test, 28 participants had average color discrimination, with 2 demonstrating superior discrimination. There were no differences between the orthodontists and the controls, or between men and women. CONCLUSIONS: Orthodontists' knowledge of dental light-curing units and hazards is poor. Although potential risks are associated with the long-term use of these light-curing units, no effect on color discrimination was detected.

Effect of composite insertion technique on cuspal deflection using an in vitro simulation model.

OBJECTIVE: The objective of this study was to investigate, by simulation, the effect of conventional composite resin insertion techniques on cuspal deflection using bonded typodont artificial teeth. The deflection produced by a new low-shrinkage composite was also determined. MATERIALS AND METHODS: Sixty standardized MOD preparations on ivorine maxillary premolars were prepared: group A at 4 mm depth and group B at 6 mm depth. Each group was further subdivided according to composite insertion technique (n=6), as follows: 1) bulk insertion, 2) horizontal increments, 3) tangential increments, and 4) a modified tangential technique. Preparations were microetched, acid-cleaned, and bonded with adhesive resin to provide micromechanical attachment before restoration with a conventional composite (Spectrum TPH( 3 ), Dentsply). Two additional subgroups at 4 mm and 6 mm depth (n=6) were restored in bulk using low-shrinkage composite (Filtek LS, 3M/ESPE). All groups received the same total photo-polymerization time. Cuspal deflection was measured during the restorative procedure using two Linear Variable Differential Transformers attached to a data acquisition system. RESULTS: The average cuspal deflections for group A were 1) 40.17 ± 1.18 µm, 2) 25.80 ± 4.98 µm, 3) 28.27 ± 5.12 µm, and 4) 27.33 ± 2.42 µm. The deflections in group B were 1) 38.82 ± 3.64 µm, 2) 50.39 ± 9.17 µm, 3) 55.62 ± 8.16 µm, and 4) 49.61 ± 8.01 µm. Cuspal flexure for the low-shrinkage composite was 11.14 ± 1.67 µm (group A: 4 mm depth) and 16.53 ± 2.79 µm (group B: 6 mm depth). CONCLUSIONS: All insertion techniques using conventional composite caused cuspal deformation. In general, deeper preparations showed increased cuspal deflection-except in the case of bulk insertion, which was likely affected by decreased depth of cure. Cuspal movement using low-shrinkage composite was significantly reduced.

Temperature changes caused by light curing units on dentine of primary teeth.

AIM: This was to determine the temperature changes produced in dentine discs of primary teeth placed below a glass ionomer, microhybrid flow resin or microhybrid resin during the photocuring process with conventional halogen lamps and LEDs at different distances. STUDY DESIGN: Experimental design. MATERIALS AND METHODS: This in vitro study was carried out in the research laboratory of the Universitat International de Catalunya. We cut 1 mm thick dentine discs with the IsoMet 1000 cutting machine. Thereafter, we cut stainless steel rings of different heights. Subsequently, to facilitate the temperature measurement, we prepared silicone moulds, in which the dentine disc, stainless steel ring and the digital thermometer/ thermocouple were positioned. Once the silicone mould was finished, a 2 mm thick layer of the restorative material was placed on the dentine disc. Finally, the polymerisation process was conducted according to the times recommended by the manufacturers, and the temperature produced was recorded at the end of the procedure. STATISTICAL EVALUATION: Replies were analyzed using the STATGRAPHICS® Plus Version 5.0 statistics software system, in order to obtain comparative diagrams and graphs using the ANOVA multifactorial system. RESULTS: The photocuring lamps used on the restorative materials produced statistically significant differences in temperature, with p = 0.00001. CONCLUSION: Halogen lamps cause a greater temperature rise in materials than LEDs lamps, and the greatest rise is produced when microhybrid flow resin is photocured with the Optilux 501 halogen lamp.

Prepolymerized vs. in situ-polymerized fiber-reinforced composite implants--a pilot study.

The aim of this study was to investigate bone response to bioactive fiber-reinforced composite (FRC) implants under two polymerization conditions. Glass-fiber-dimethacrylate composite was tested as prepolymerized cylinder-shaped FRC implants and as cylindrical FRC implants polymerized in situ with blue light transmitted and scattered by the glass fibers. Ten FRC implants (6 prepolymerized and 4 in situ-polymerized implants) were placed in the right tibias of 3 pigs by means of a press-fit technique. After 12 weeks, light microscopy revealed only mild foreign-body reaction, with no accumulation of inflammatory cells on both the prepolymerized and the in situ-polymerized implants. The prepolymerized implants appeared to be fully integrated, whereas the in situ-polymerized implants were almost completely surrounded by a fibrous capsule. The present study suggests that in situ polymerization of FRC implants results in fibrous capsule formation and prevents integration with bone.

The effect of light curing units and modes on cytotoxicity of resin-core systems

Objective: The aim of this study was to compare the cytotoxic effects of various resin-based core materials that were cured with three light curing units (LCUs) in different modes on L–929 mouse fibroblast cells over 24 h and72 h periods.Study design: Eighty-four cylindrical discs (2 mm in thickness and 6 mm in diameter) of each material (Rebilda,Voco; Build-It FR, Pentron; Clearfil DC Core, Kuraray and Bis-core, Bisco) were cured by QTH LCU (soft-up and high-power modes), LED LCU (exponential and standard modes) and PAC LCU (normal and ramp-curing modes).Then the samples were aged for 24 and 72 hours in Dulbecco’s Modified Eagle Medium/Ham’s F12 (DMEM/F12).After each ageing interval, cytotoxicity of the extracts to cultured fibroblasts (L 929) was measured by MTT assay.The degree of cytotoxicity for each sample was determined according to the reference value represented by the cells with a pure culture medium. Statistical significance was determined by a multifactorial analysis of variance followed by one-way ANOVA and Tukey’s post-hoc test or independent t tests. Results: Data showed that there are significant differences among resin based core materials, LCUs, curing modes and time factors (p < 0.001). The test materials cured with the QTH and LED demonstrated statistically higher cellsurvival rates when compared with those cured with the PAC (p < 0.001).Conclusions: Differential toxic effects of the resin-based core materials cured with QTH, LED and PAC on the fibroblast cells may prove to be very important when suitable resin-core systems, LCUs and curing modes are used for operative restorations (AU)

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In-vitro assessment of temperature rise in the pulp during orthodontic bonding.

INTRODUCTION: In this in-vitro study, we evaluated the temperature changes in the pulp chamber during bracket bonding using 4 different light sources. METHODS: Eighty intact extracted maxillary central incisors were used. The teeth were divided into 4 groups of 20 teeth each. Brackets (Mini Twin, Dentaurum, Ispringen, Germany) were bonded with Transbond XT (3M Unitek, Monrovia, Calif) adhesive and light cured with low-intensity halogen light for 40 seconds, high-intensity halogen light for 40 seconds, light-emitting diode (LED) light for 20 seconds, and plasma arc light (PAC) for 6 seconds. Light curing was performed 5 mm from tooth surfaces. A J-type thermocouple wire was positioned in the center of the pulp chamber. The results were analyzed with analysis of variance (ANOVA) and the Tukey HSD test. RESULTS: ANOVA and the Tukey HSD test showed that pulp chamber temperature changes were influenced by the type of light source. All groups showed significant differences between each other (P <0.001). The intrapulpal temperature changes induced by different light sources were the following: high-intensity halogen (6.84 degrees C +/- 2.44 degrees C), low-intensity halogen (4.71 degrees C +/- 0.96 degrees C), LED (2.95 degrees C +/- 1.12 degrees C), and PAC (0.96 degrees C +/- 0.83 degrees C). CONCLUSIONS: High- and low-intensity halogen light induced significantly higher intrapulpal temperature changes than did the LED and PAC. Except for the high intensity halogen light, orthodontic bonding with light-curing units did not exceed the critical 5.5 degrees C rise in temperature reported to produce pulpal damage.

Effect of diode-pumped solid state laser on polymerization shrinkage and color change in composite resins.

A diode-pumped solid state (DPSS) laser emitting at 473 nm was used to test its influence on the degree of polymerization of composite resins. Eight composite resins were chosen and light cured with three different light-curing systems [a quartz-tungsten-halogen (QTH) lamp-based unit, a light-emitting diode (LED) unit, and a DPSS laser]. Polymerization shrinkage and color change in specimens were measured. According to the statistical analysis, each light-curing system produced a significantly different value of maximum polymerization shrinkage. In most specimens, the DPSS laser induced the least polymerization shrinkage. After being immersed in distilled water for 10 days, specimens light-cured by the DPSS laser had undergone less color change than those cured by the other units. In conclusion, the DPSS laser induced better or similar polymerization in terms of polymerization shrinkage and color change in composite resins compared with those of the QTH lamp-based and LED units.

Influence of curing mode intensities on cell culture cytotoxicity/genotoxicity.

PURPOSE: This study determined the cytotoxic/genotoxic effect of different curing modes on cell culture. METHODS: A thin layer of lymphocyte cultures was cured applying three different curing modes of Bluephase C8 LED curing unit. Cultures were exposed to light directly or through a layer (2 mm) of polymerized resin composite sample. Cells were analyzed using trypan blue exclusion test, acridine orange/ethidium bromide dyeing technique, and alkaline comet assay. RESULTS: Only low intensity mode after direct exposure significantly increased the number of nonviable lymphocytes detected using trypan blue. All curing procedures significantly increased the number of apoptotic lymphocytes regardless whether the exposure occurred directly or through the composite. Low intensity mode in direct exposure significantly elevated DNA migration compared to other curing modes. 1 hour after exposure significant increase in tail length and intensity for all modes and procedures was detected. However, DNA damage measured for cultures cured by low intensity mode was higher compared to the other two modes; thus, despite of curing light intensity, longer curing time leads to greater cytotoxicity/genotoxicity in cell culture.

Microleakage under orthodontic brackets using high-intensity curing lights.

OBJECTIVE: To compare the microleakage of the enamel-adhesive-bracket complex at the occlusal and gingival margins of brackets bonded with high-intensity light curing lights and conventional halogen lights. MATERIALS AND METHODS: Forty-five freshly extracted human maxillary premolar teeth were randomly separated into three groups of 15 teeth each. Stainless steel brackets were bonded in all groups according to the manufacturer's recommendations. Specimens (15 per group) were cured for 40 seconds with a conventional halogen light, 20 seconds with light-emitting diode (LED), and 6 seconds with plasma arc curing light (PAC). After curing, the specimens were further sealed with nail varnish, stained with 0.5% basic-fuchsine for 24 hours, sectioned and examined under a stereomicroscope, and scored for microleakage for the enamel-adhesive and bracket-adhesive interfaces from both the occlusal and gingival margins. Statistical analyses were performed using Kruskal-Wallis and Mann-Whitney U-tests with a Bonferroni correction. RESULTS: The type of light curing unit did not significantly affect the amount of microleakage at the gingival or occlusal margins of investigated interfaces (P >.05). The gingival sides in the LED and PAC groups exhibited higher microleakage scores compared with those observed on occlusal sides for the enamel-adhesive and adhesive-bracket interfaces. The halogen light source showed similar microleakage at the gingival and occlusal sides between both adhesive interfaces. CONCLUSIONS: High-intensity curing units did not cause more microleakage than conventional halogen lights. This supports the use of all these curing units in routine orthodontic practice.

Changes in shear bond strength of ceramic and stainless steel brackets with different visible light curing times and directions.

BACKGROUND: Selection of the appropriate curing time and light direction may enable the appropriate shear bond strength to be obtained and avoid enamel fracture during debonding. AIMS: To determine the effects of different curing times and light directions on the shear bond strengths of ceramic and stainless steel brackets. METHOD: Ninety-two recently extracted, upper premolars were randomly assigned to six groups. Either stainless steel or ceramic brackets were bonded to the buccal surfaces of the teeth. Group I, stainless steel brackets cured for 40 seconds from buccal surface; Group II, stainless steel brackets cured for 40 seconds from palatal surface; Group III, stainless steel brackets cured for 80 seconds from palatal surface; Group IV, ceramic brackets cured for 40 seconds from the buccal surface; Group V, ceramic brackets cured for 40 seconds from the palatal surface; Group VI, ceramic brackets cured for 80 seconds from the palatal surface. The shear bond strength was measured with a universal testing machine and the resin remaining after debonding scored with the Adhesive Remnant Index (ARI). The data were analysed with the oneway ANOVA, Tukey's HSD test and the Kruskal-Wallis test. Associations between the ARI and shear bond strength were determined with Pearson's correlation coefficient. RESULTS: Group IV (ceramic brackets cured for 40 seconds from buccal surface) had the highest shear bond strength (21.26 MPa) and Group II (metal brackets cured for 40 seconds from palatal surface) had the lowest shear bond strength (6.95 MPa).There was no significant difference in ARI scores among the groups. The association between the ARI scores and bond strength values was not statistically significant. CONCLUSIONS: Curing from the buccal surface for 40 seconds gave unacceptably high shear bond strength values for both stainless steel and ceramic brackets. Lower shear bond strengths occurred when the light was directed from the palatal surface, but a shorter curing time is more likely to be preferred by clinicians. Future studies should investigate the possibility of reducing the curing time for both brackets.

Calculation of contraction stresses in dental composites by analysis of crack propagation in the matrix surrounding a cavity.

OBJECTIVES: Polymerization contraction of dental composite produces a stress field in the bonded surrounding substrate that may be capable of propagating cracks from pre-existing flaws. The objectives of this study were to assess the extent of crack propagation from flaws in the surrounding ceramic substrate caused by composite contraction stresses, and to propose a method to calculate the contraction stress in the ceramic using indentation fracture. METHODS: Initial cracks were introduced with a Vickers indenter near a cylindrical hole drilled into a glass-ceramic simulating enamel. Lengths of the radial indentation cracks were measured. Three composites having different contraction stresses were cured within the hole using one- or two-step light-activation methods and the crack lengths were measured. The contraction stress in the ceramic was calculated from the crack length and the fracture toughness of the glass-ceramic. Interfacial gaps between the composite and the ceramic were expressed as the ratio of the gap length to the hole perimeter, as well as the maximum gap width. RESULTS: All groups revealed crack propagation and the formation of contraction gaps. The calculated contraction stresses ranged from 4.2 MPa to 7.0 MPa. There was no correlation between the stress values and the contraction gaps. SIGNIFICANCE: This method for calculating the stresses produced by composites is a relatively simple technique requiring a conventional hardness tester. The method can investigate two clinical phenomena that may occur during the placement of composite restorations, i.e. simulated enamel cracking near the margins and the formation of contraction gaps.

Temperature excursions at the pulp-dentin junction during the curing of light-activated dental restorations.

OBJECTIVES: Excessive heat produced during the curing of light-activated dental restorations may injure the dental pulp. The maximum temperature excursion at the pulp-dentin junction provides a means to assess the risk of thermal injury. In this investigation we develop and evaluate a model to simulate temperature increases during light-curing of dental restorations and use it to investigate the influence of several factors on the maximum temperature excursion along the pulp-dentin junction. METHODS: Finite element method modeling, using COMSOL 3.3a, was employed to simulate temperature distributions in a 2D, axisymmetric model tooth. The necessary parameters were determined from a combination of literature reports and our measurements of enthalpy of polymerization, heat capacity, density, thermal conductivity and reflectance for several dental composites. Results of the model were validated using in vitro experiments. RESULTS: Comparisons with in vitro experiments indicate that the model provides a good approximation of the actual temperature increases. The intensity of the curing light, the curing time and the enthalpy of polymerization of the resin composite were the most important factors. The composite is a good insulator and the greatest risk occurs when using the light to cure the thin layer of bonding resin or in deep restorations that do not have a liner to act as a thermal barrier. SIGNIFICANCE: The results show the importance of considering temperature increases when developing curing protocols. Furthermore, we suggest methods to minimize the temperature increase and hence the risk of thermal injury. The physical properties measured for several commercial composites may be useful in other studies.