QTH Units Research Articles

Color Stability Assessment of Three Different Resin Composites After Polymerization with Two Different Light-Curing Units

Purpose: Despite improvements in resin composite structures, color stability remains a challenge, and esthetic problems are the most common factors in renewing restorations. This study aimed to evaluate the effects of two light-curing units on the color stability of three different resin composite materials during a six-month period.
 Materials & Methods: Three different resin composites (Filtek Z550, Filtek Ultimate Flowable, Filtek Bulk Fill Restorative) with an A2 color shade were prepared for each combination of resin composite-curing unit (5x2mm) for a total of 30 specimens. Specimens were cured with either QTH Hilux Ultraplus (700mW/cm2) for 40 seconds or Radii Plus LED Curing Light (1400mW/cm2) for 20 seconds. For standardization of the amount of transmitted energy to the composite, all specimens received 28J/cm2. Color parameters were measured with a colorimeter (Minolta CR-321, Konica Minolta Sensing Pte Ltd, Singapore) after polymerization and at 24 hours, first week, first month, third month, and sixth month. Color changes(∆E) were calculated for the different storage periods. One-factor repeated three-way analysis of variance (ANOVA) was used to examine the changes in ΔE measurements over time for different fillers and devices.
 Results: Significant differences between composites and periods were observed in the color stability with polymerization either QTH or LED. Independently of the composites, there was no significant difference at the end of six months between QTH and LED units with a fixed energy density. The Filtek Z550 material showed the smallest and the Filtek Ultimate Flowable material showed the highest ∆E values at the end of the six months among the materials used.
 Conclusion: Clinically acceptable color changes were observed for all the materials at the end of six months.

The effect of light sources and CAD/CAM monolithic blocks on degree of conversion of cement.

PURPOSETo assess the degree of conversion (DC) and light irradiance delivered to light-cured and dual-cured cements by application of different light sources through various types of monolithic computer-aided design and computer-aided manufacturing (CAD/CAM) materials.MATERIALS AND METHODSRelyX Ultimate Clicker light-cured and dual-cured resin cement specimens with 1.5-mm thicknesses (n=300, 10/group), were placed under four types of crystalline core structure (Vita Enamic, Vita Suprinity, GC Ceresmart, Degudent Prettau Anterior). The specimens were irradiated for 40 seconds with an LED Soft-Start or pulse-delay unit or 20 seconds with a QTH unit. DC ratios were determined by using Fourier transform infrared spectroscopy (FTIR) after curing the specimen at 1 day and 1 month. The data were analyzed using the Mann-Whitney U test (for paired comparison) and the Kruskal-Wallis H test (for multiple comparison), with a significance level of P<.05.RESULTSDC values were the highest for RelyX Ultimate Clicker light-cure specimens polymerized with the LED Soft-Start unit. The combination of the Vita Suprinity disc and RelyX Ultimate Clicker dual-cure resin cement yielded significantly higher values at both timepoints with all light units (all, P<.05).CONCLUSIONWithin the limitations of this study, we conclude that the DC of RelyX Ultimate Clicker dual-cure resin cement was improved significantly by the use of Vita Suprinity and the LED Soft-Start light unit. We strongly recommend the combined use of an LED light unit and dual-cure luting cement for monolithic ceramic restorations.

Comparison of Surface Hardness of Various Shades of Twinky Star Colored Compomer Light-cured with QTH and LED Units.

IntroductionColored compomers are a group of restorative materials that were introduced in 2002 to repair primary teeth, and they provide attractive color and ease of use in pediatric dentistry. The aim of this study was to evaluate the effect of QTH and LED light-curing units on the surface hardness of different colors of Twinky Star compomers.MethodsIn this experimental study, a composite resin (Z250, 3M, and USA), an ionosit compomer (DMG, Germany) with A3 shade and 8 different Twinky Star colored compomer (Voco, Germany) samples were used. In all, 100 samples were prepared with 10 samples in each group, i.e., 10 Z250 composite resin, 10 ionosit compomers, and 10 Twinky Star compomer samples of each color. The samples were prepared in a 4×4-mm Teflon mold. Half of the samples were light-cured with QTH and the other half with LED units. Then, the surface microhardness was measured by Vickers hardness test. The data were analyzed with IBM-SPSS version 22, using the t-test and ANOVA.ResultsTwo-way ANOVA showed that the mean surface hardness of the compomer samples cured with the QTH unit was significantly higher than that cured with the LED unit (p < 0.001). In each curing unit, surface hardness of some materials exhibited significant differences with the highest hardness being observed in the Z250 composite resin (650.35 ± 56.320) and the lowest hardness being detected in the ionosit compomers (461.10 ± 96.170). One-way ANOVA also showed that, among the different colors of the Twinky Star compomer, the lowest hardness with both units (QTH and LED) was observed in the gold color (214.32 ± 22.026 and 175.116 ± 15.918, respectively).ConclusionThe colored compomer and the type of light-curing unit affected the microhardnesses of the surfaces. Different colors of Twinky Star compomers exhibited different surface microhardnesses.

Residual monomer release from orthodontic adhesives cured with different light sources

Introduction: The aim of this study was to evaluate the residual monomer release from orthodontic adhesives cured with light-emitting diode (LED) and halogen light sources.Methods: Seven hundred and twenty stainless steel brackets were divided into 3 groups according to the adhesive system used (Transbond XT light-cure adhesive [TXT], Transbond LR capsule [LR], and Light Bond light-cure adhesive paste [LB]), and each group was divided into 2 subgroups according to light-curing procedure (LED or halogen). Brackets were bonded with adhesives onto tooth buccal surfaces and polymerized. Each specimen contained 24 brackets that simulated the oral environment (n = 5). The specimens were immersed in a 75% ethanol/water solution at 37 °C for 10 min, 1 h, 1 d, 7 d, 14 d, and 30 d, respectively. Eluted monomers (Bis-GMA, UDMA, and TEGDMA) were detected using HPLC.Results: There was residual monomer release at all time periods, and the highest amount of release was observed cumulatively on the 30th day. The cumulative Bis-GMA released from adhesives was not different (p > 0.05). The cumulative TEGDMA released from adhesives was statistically different (p < 0.05). There was no statistical difference between QTH and LED light-curing units for each adhesive (p > 0.05).Conclusions: The release of residual monomers stays at a high level for a long time after polymerization. The total leaching of residual monomers from the Light Bond light-cure sealant resin plus Light Bond light-cure adhesive paste was higher than that of other materials for both curing units. Different curing units (LED or QTH) did not affect the monomer release from the orthodontic adhesives.

Marginal integrity of resin composite restorations restored with PPD initiatorcontaining resin composite cured by QTH, monowave and polywave LED units.

This study evaluated the influence of curing devices on marginal adaptation of cavities restored with self-etching adhesive containing CQ and PPD initiators and hybrid composite. Twenty-four class V (3 groups, n=8) with margins located on enamel and dentin were restored with Clearfil S3 Bond and Clearfil APX PLT, light-cured with a monowave LED, multiwave LED and halogen light-curing unit (LCU). Marginal adaptation was evaluated with SEM before/after thermo-mechanical loading (TML). On enamel, significantly lower % continuous margins (74.5±12.6) were found in group cured by multiwave LED when compared to monowave LED (87.6±9.5) and halogen LCU (94.4±9.1). The presence of enamel and composite fractures was significantly higher in the group light-cured with multiwave LED, probably due to an increased materials' friability resulted from an improved degree of cure. The clinician should aware that due to a distinct activation of both initiators, marginal quality may be influenced on the long-term.

Curing efficiency of dual-cure resin cement under zirconia with two different light curing units.

Adequate polymerization is a crucial factor in obtaining optimal physical properties and a satisfying clinical performance from composite resin materials. The aim of this study was to evaluate the polymerization efficiency of dual-cure resin cement cured with two different light curing units under zirconia structures having differing thicknesses. 4 zirconia discs framework in 4 mm diameter and in 0.5 mm, 1 mm and 1.5 mm thickness were prepared using computer-aided design system. One of the 0.5 mm-thick substructures was left as mono-layered whereas others were layered with feldspathic porcelain of same thickness and ceramic samples with 4 different thicknesses (0.5, 1, 1.5 and 2.0 mm) were prepared. For each group (n=12) resin cement was light cured in polytetrafluoroethylene molds using Light Emitting Diode (LED) or Quartz-Tungsten Halogen (QHT) light curing units under each of 4 zirconia based discs (n=96). The values of depth of cure (in mm) and the Vickers Hardness Number values (VHN) were evaluated for each specimen. The use of LED curing unit produced a greater depth of cure compared to QTH under ceramic discs with 0.5 and 1 mm thickness (p<0.05).At 100μm and 300 μm depth, the LED unit produced significantly greater VHN values compared to the QTH unit (p<0.05). At 500 μm depth, the difference between the VHN values of LED and QTH groups were not statistically significant. Light curing may not result in adequate resin cement polymerization under thick zirconia structures. LED light sources should be preferred over QTH for curing dual-cure resin cements, especially for those under thicker zirconia restorations.

Does the light source affect the repairability of composite resins?

The aim of this study was to examine the effect of the light source on the microshear bond strength of different composite resins repaired with the same substrate. Thirty cylindrical specimens of each composite resin--Filtek Silorane, Filtek Z550 (3M ESPE), Gradia Direct Anterior (GC), and Aelite Posterior (BISCO)--were prepared and light-cured with a QTH light curing unit (LCU). The specimens were aged by thermal cycling and divided into three subgroups according to the light source used--QTH, LED, or PAC (n = 10). They were repaired with the same substrate and a Clearfil Repair Kit (Kuraray). The specimens were light-cured and aged for 1 week in distilled water at 37 °C. The microshear bond strength and failure modes were assessed. There was no significant difference in the microshear bond strength values among the composite resins, except for the Filtek Silorane group that showed significantly lower bond strength values when polymerized with the PAC unit compared to the QTH or LED unit. In conclusion, previously placed dimethacrylate-based composites can be repaired with different light sources; however, if the composite to be repaired is silorane-based, then using a QTH or LED device may be the best option.

Dynamic mechanical thermal analysis of composite resins with CQ and PPD as photo-initiators photoactivated by QTH and LED units

ObjectivesThe aim of this study was to evaluate the thermal and mechanical properties of the composite resins containing the photo-initiators camphorquinone (CQ) and/or phenyl-propanodione (PPD) when photoactivated with halogen lamp (XL2500/3M-ESPE), monowave (UltraBlueIS/DMC) and polywave (UltraLume5/Ultradent) LED units. Materials and methodsA blend of BisGMA, UDMA, BisEMA and TEGDMA was prepared with the same wt% of photo-initiators CQ and/or PPD and 65wt% of silaneted filler particles. Compression strength (CS), diametral tensile strength (DTS) and diametral modulus (DM) were tested. Thermogravimetric analysis (TGA) was made and the lost residual monomer were verified. Dynamic mechanical thermal analysis (DMTA) was used for to analyze the glass transition temperature (Tg) and the storage modulus in 37°C. Degree of conversion (DC) was accomplished in the same samples of DMA using middle-infrared spectroscopy (mid-IR). ResultsCQ, CQ/PPD and PPD obtained the same results for all mechanical properties (CS, DTS and DM), lost residual monomer and storage modulus in 37°C, regardless LCU used. The results of Tg showed that the combination PPD-UltraLume5 produced the highest values. DC showed that the combination CQ-UltraLume5 resulted in the highest values and PPD-XL2500 in the lowest DC values. ConclusionThe study shows that PPD is not only effective photosensitizers, but also photocrosslinking agents for dental composite resins with a similar efficiency to CQ.

Characterization of heat emission of light-curing units

This study was designed to analyze the heat emissions produced by light-curing units (LCUs) of different intensities during their operation. The null hypothesis was that the tested LCUs would show no differences in their temperature rises. FIVE COMMERCIALLY AVAILABLE LCUS WERE TESTED: a "Flipo" plasma arc, "Cromalux 100" quartz-tungsten-halogen, "L.E. Demetron 1" second-generation light-emitting diode (LED), and "Blue Phase C5" and "UltraLume 5" third-generation LED LCUs. The intensity of each LCU was measured with two radiometers. The temperature rise due to illumination was registered with a type-K thermocouple, which was connected to a computer-based data acquisition system. Temperature changes were recorded in continues 10 and 20s intervals up to 300s. The Flipo (ARC) light source revealed the highest mean heat emission while the L.E. Demetron 1 LED showing the lowest mean value at 10 and 20s exposure times. Moreover, Cromalux (QTH) recorded the second highest value for all intervals (12.71, 14.63, 14.60) of heat emission than Blue Phase C5 (LED) (12.25, 13.87, 13.69), interestingly at 20s illumination for all intervals the highest results (18.15, 19.27, 20.31) were also recorded with Flipo (PAC) LCU, and the lowest (6.71, 5.97, 5.55) with L.E. Demetron 1 LED, while Blue Phase C5 (LED) recorded the second highest value at the 1st and 2nd 20s intervals (14.12, 11.84, 10.18) of heat emission than Cromalux (QTH) (12.26, 11.43, 10.26). The speed of temperature or heat rise during the 10 and 20s depends on light intensity of emitted light. However, the QTH LCU was investigated resulted in a higher temperature rise than LED curing units of the same power density. The PAC curing unit induced a significantly higher heat emission and temperature increase in all periods, and data were statistically different than the other tested groups (p<.05). LED (Blue Phase C5) was not statistically significant (p<.05) (at 10s) than QTH units, also LED (Blue Phase C5, UltraLume 5) generates obvious heat emission and temperature rises than QTH units (at 20s) except for those which have lower power density of LED curing units (first generation). Thus, the null hypothesis was rejected.

A Comparative Evaluation of Curing Depth and Compressive Strength of Dental Composite Cured with Halogen Light Curing Unit and Blue Light Emitting Diode: An in vitro Study

To evaluate the curing depth and compressive strength of dental composite using halogen light curing unit and light emitting diode light curing unit. Eighty cylindrical composite specimens were prepared using posterior composite P60(3M). Forty specimens, out of which 20 samples (group A) cured with halogen light and 20 samples (group B) cured using light emitting diode (LED) light were checked for curing depth according to ISO 4049. Remaining 40 samples out of which 20 samples (group I) cured using halogen light and 20 samples (group II) cured using LED light were checked for compressive strength using Instron universal testing machine. Twenty samples (group A) cured with halogen light showed better curing depth than 20 samples (group B) cured with LED light. Twenty samples (group I) cured with halogen light showed almost similar results as 20 samples (group II) cured with LED light for compressive strength. Halogen light commonly used to cure composite resin have greater depth of cure, when compared to LED light, while both the lights produced compressive strength which is almost similar. Lower depth of cure with the LED unit, compared to the QTH unit, is associated with different light scattering due to differences in spectral emission. LED technology differs from QTH by the spectral emission that favorably matches the absorption spectrum of camphorquinone.

Microhardness of nanofilled composite resin light-cured by LED or QTH units with different times

Aim: To evaluate the influence of light-curing units and light-curing time on the microhardness of a nanofilled composite resin. Methods: Forty-five composite resin (Z350 - 3M) specimens were randomly prepared using Teflon ring molds (4.0 mm internal diameter and 2 mm depth) and divided into nine experimental groups (n=5): three polymerization units (conventional - 450 mW/ cm2 ; 2nd generation LED - 1100 mW/cm2 ; and 3rd generation LED - 700 mW/cm2 ) and three lightcuring times (20 s, 40 s, and 60 s). All specimens were polymerized with the light-curing tip positioned 8 mm far from the top surface of the specimen. After 24 h, Knoop microhardness measurements were made on the top and bottom surfaces of the specimen, with a load of 10 g for 10 s. Five indentations were made on each surface. All results were analyzed statistically by subdivided parcel ANOVA (Split-Plot) and Tukey’s tests (p<0.05). Results: There were no statistically significant differences for the polymerization unit and light-curing time factors in either top or bottom surface. For all experimental conditions, the top surfaces showed greater hardness than the bottom surfaces (p<0.0001). Conclusions: The mode of polymerization and the lightcuring time did not affect the hardness of the nanofilled composite resin, and increasing the lightcuring time did not improve the hardness of the bottom surface of the composite resin.

A survey of power density of clinical curing-light units used in Changchun

To investigate the power density and other relevant data of clinical curing-light units used in Changchun, and to provide practice recommendations to clinical dentists about maintaining of cuing-light units. Stomatology hospitals, departments of stomatology in general hospitals, and private dental offices in Changchun were randomly selected to participate in the Survey. The investigation analyzed 270 curing-light units. The following data of curing-light units were gathered: brand, type, operation ages, numbers and types of light guide, resin build-ups on light guides, damages of light guides, use frequency, monitor and maintenance of curing lights, and unit numbers/chair numbers. There were 174 QTH and 96 LED units. The distribution of power density was from 0 to 1702 mW/cm(2). The mean power density was 413.2 mW/cm(2). The power densities of 73 lights were less than 200 mW/cm(2) and could not polymerize resin composites adequately. The mean number of operation age of the light units was 4.74 years. Most of clinical dentists didn't monitor the light-curing units and the situation of build-up from composite resin or damages on light guides was very severe. Most of the light-curing units used in Changchun were QTH. Some QTH units degenerate severely and need to be replaced with the new ones. Most of the clinical doctors lack the knowledge of how to properly monitor and maintain the light-curing units.

Irradiance Differences in the Violet (405 nm) and Blue (460 nm) Spectral Ranges among Dental Light‐Curing Units

Previous studies identified nonuniformity in the irradiance at the tip end of a variety of dental light-curing units (LCUs) and correlated those differences with potential clinical implications, but the spectral dependence of the irradiance uniformity has not yet been addressed. This study examined the irradiance uniformity across emitting tips of LCUs at two emission wavelengths, 405 and 460 nm. Two broadband emission light units (quartz-tungsten-halogen [QTH] and plasma arc [PAC]), and four commercial light-emitting diode (LED)-type LCUs were examined. The spectral radiant power from six LCUs was measured using a laboratory grade spectroradiometer (Ocean Optics, Dunedin, FL, USA). The spatial and spectral characteristics of irradiance across the emitting tips of these light units were recorded through 10-nm wide bandpass filters (centered at 405 nm [violet] or 460 nm [blue]) using a laser beam analyzer (Ophir-Spiricon, Logan, UT, USA). Irradiance distributions were reported using two-dimensional contour and three-dimensional isometric color-coded images. Irradiance uniformity at the tip end was determined using the Top Hat Factor (THF) for each filtered wavelength. Irradiance distributions from the QTH and PAC units were uniformly distributed across the tip end of the light guide, and THF values, measured through the 405 and 460-nm filters, were not significantly different. However, the three polywave LED units delivered non-uniform irradiance distributions with THF values differing significantly between the 405 and 460-nm emission wavelengths for each unit. Areas of nonuniformity were attributed to the locations of the various types of LED chips within the LCUs. All three polywave LED units delivered a nonuniform irradiance distribution across their emitting tip ends at the two important emission wavelengths of 405 nm and 460 nm, whereas the broadband light sources (QTH and PAC) showed no evidence of spectral inhomogeneity at these wavelengths.

Bi-axial flexural strength of dual-polymerizing agents cemented to human dentin after photo-activation with different light-curing systems.

This study aimed to assess the bi-axial flexural strength of two dual-polymerizing resin luting agents cemented to human dentin when photo-activated with different light-curing units. Two dual-cured resin cements: choice (CH) and Variolink II (VL) were tested. Hybrid composite resin (Z-250) discs (12×1.5mm) were fabricated. Three types of light-curing units were used halogen-curing unit (QTH), light-emitting diode (LED) and plasma arc (PAC). Sixty dentin discs of 0.5mm thickness were prepared from extracted human teeth. A circular mold (2.5mm in height and 12mm diameter) was utilized to create supporting structure for dentin, resin cement complex. The resin luting cement (0.5mm) was placed on the previously prepared dentin discs and covered with the prefabricated composite discs. Photo-activation of cements was performed for 40s with QTH and LED units and for 3s with PAC. The specimens were divided into 12 groups (20 specimens for each light source). Six groups were kept in distilled water for 24h and the rest were stored for 6weeks. Bi-axial flexural strength was determined using Instron machine. The data was analyzed using two-way ANOVA and Tukey test for comparison. The findings indicated that the bi-axial flexural strength values for both cements CH and VL were higher for 24h over 6weeks but not statistically significant when cured with QTH. Meanwhile, when LED light was used for photo-activation the cements, the flexural strength values reported were statistically higher of 24h over 6weeks storage at P=0.4(E-6) However, PAC light did not record any statistically significant difference between two duration for the CH cement although when used for polymerization of VL the reported value for 6weeks were statistically significantly higher value than 24h duration at P=0.002. When high immediate flexural strength is preferred in clinical situation photo-activation the cements with LED reported the greatest value.

Influence of light-curing units and restorative materials on the micro hardness of resin cements

The aim of this study was to evaluate the effect of indirect restorative materials (IRMs) and light-curing units (LCUs) on the micro hardness of dual-cured resin cement. A total of 36 cylindrical samples (2 mm thick) were prepared with dual-cured resin cement (Relyx ARC) photo-activated with either a QTH (Optilight Plus) for 40s or a LED (Radii) light-curing unit for 65s. Photo-activation was performed through the 2-mm- thick IRMs and the samples were divided into six groups (n=6) according to the combination of veneering materials (without, ceramic and indirect resin) and LCUs (QTH and LED). In the control group, the samples were light-cured with a QTH unit without the interposition of any restorative material. Vickers micro hardness test was performed on the top and bottom surfaces of each sample (load of 50 g for 15 secs). The data were statistically analyzed using a three-way ANOVA followed by Tukey x s post-hoc test ( P < 0.05). There were no statistically significant differences on the top surface between the light curing-units ( P > 0.05); however, the LED provided greater hardness on the bottom surface when a ceramic material was used ( P < 0.05). The mean hardness in photo-activated samples, in which there was no interposition of indirect materials, was significantly greater ( P < 0.01). It may be concluded that the interposition of the restorative material decreased the micro hardness in the deeper cement layer. Such decrease, however, was lower when the ceramic was interposed and the cement light-cured with LED.

Curing depth of restorative composites achieved with QTH and LED units

Curing depth of restorative composites achieved with QTH and LED units

Influence of the Internal Conditioning of Indirect Restorations of Resin Composite in Relation to Microleakage Using LEDs and QTH Units

The current study evaluated in-vitro microleakage of indirect composite inlays fixed with flowable composite and resinous cement and cured by LEDs and QTH units, thus modifying the internal conditioning of the inlays. Thirty-two non-carious teeth were selected and 64 cavity preparations were performed in both enamel and cementum. These teeth were divided into four groups: I--Elipar FreeLight appliance and Filtek Flowable composite; II--Ultralux appliance and Filtek Flowable composite; III--Elipar FreeLight appliance and RelyX cement and IV--Ultralux appliance and RelyX cement. The inlays were internally sandblasted with aluminum oxide, etched with 37% phosphoric acid, washed and silanized. The dental etching was carried out with 37% phosphoric acid for 15 seconds in dentin and 30 seconds in enamel. After being washed and dried, Single Bond dentinal adhesive was then applied, and the inlays were fixed with their respective luting agents, cured in close contact with their surfaces for 60 seconds, thermocycled and immersed in 0.5% basic fuchsine solution. The teeth were then washed and sectioned through the center of the restoration in order for the microleakage readings to be performed using the Image Tool Software. Then, ANOVA and Tukey's statistical tests were applied. In enamel, there was no significant difference for both groups with regard to microleakage; in cementum, the significant difference was 5% (p > 0.005). In agreement with the results, the inlays can be fixed with RelyX and Filtek Flowable resins.

A clinical survey of the output intensity of 200 light curing units in dental offices across Maharashtra

Aim:The purpose of this study is to examine the intensity of light curing units and factors affecting it in dental offices.Materials and Methods:The output intensity of 200 light curing units in dental offices across Maharashtra were examined. The collection of related information (thenumber of months of use of curing unit, the approximate number of times used in a day, and presence or absence of composite build-ups) and measurement of the intensity was performed by two operators. L.E.D Radiometer (Kerr) was used for measuring the output intensity. The average output intensity was divided into three categories (<200 mW/cm2, 200-400 mW/ cm2and >400 mW/cm2).Results:Among the 200 curing units examined, 81 were LED units and 119 were QTH units. Only 10% LED machines and 2% QTH curing units had good intensities (>400 mW/cm 2).Conclusion:Most of the examined curing lights had low output intensity ranging from 200 to 400 mW/cm2, and most of the curing units had composite build-ups on them.

Influence of the light curing unit and thickness of residual dentin on generation of heat during composite photoactivation

The aim of this study was to determine the effect of different types of composites (Filtek Z250, Esthet X and Filtek Supreme) and dentin thicknesses (0.5 or 1.0 mm) on the generation of heat during photoactivation by QTH (conventional halogen light), LED (light emitting diode), and PAC (xenon plasma light) light-curing units. Temperature changes were recorded with a thermocouple type K connected to a digital thermometer. Twenty chemically polymerized acrylic resin bases were prepared in order to guide the thermocouple and to support the dentin disks. On the acrylic resin base, elastomer molds of 2.0-mm thickness were adapted. The temperature increase was measured after composite photoactivation and the matrix was stored at 37 degrees C. After 24 hours, photoactivation was performed again and the temperature increase was measured. Obtained data were analyzed by ANOVA and Tukey's test (alpha= 0.05). The mean temperature increase produced by QTH was significantly lower than that of the other photoactivating units (P < 0.05), due to its low radiant exposure. There were no significant differences among the samples with regard to dentin thickness and type of composites (P > 0.05). The immediate temperature rise was statistically higher when compared to the increase after additional polymerization (24 h), in all groups (P < 0.05). Light intensity and exposure time appeared to be the most important factors causing temperature change.

Effect of veneering materials and curing methods on resin cement knoop hardness

This study evaluated the Knoop hardness of Enforce resin cement activated by the either chemical/physical or physical mode, and light cured directly and through ceramic (HeraCeram) or composite resin (Artglass). Light curing were performed with either conventional halogen light (QTH; XL2500) for 40 s or xenon plasma arc (PAC; Apollo 95E) for 3 s. Bovine incisors had their buccal surfaces flattened and hybridized. On these surfaces a mold was seated and filled with cement. A 1.5-mm-thick disc of the veneering material was seated over this set for light curing. After storage (24 h/37 masculineC), specimens (n=10) were sectioned for hardness (KHN) measurements in a micro-hardness tester (50 gf load/ 15 s). Data were submitted to ANOVA and Tukey's test (alpha=0.05). It was observed that the dual cure mode yielded higher hardness compared to the physical mode alone, except for direct light curing with the QTH unit and through Artglass. Higher hardness was observed with QTH compared to PAC, except for Artglass/dual groups, in which similar hardness means were obtained. Low KHN means were obtained with PAC for both Artglass and HeraCeram. It may be concluded that the hardness of resin cements may be influenced by the presence of an indirect restorative material and the type of light-curing unit.