Effect of light exposure on fracture toughness and flexural strength of light-cured composites

Temperature Rise at the Pulp-Dentin Junction for a Multi-Layered Composite Restoration using the Finite Element Method

Objectives:

During the light-curing process of composite restoration, excessive heat can be produced, which can potentially lead to pulp necrosis (death). In this study, we aimed, based on the Finite Element Method (FEM), to assess the risk of pulp damage during the light-curing process by investigating the influence of light-curing devices, under various irradiation regimes, on the temperature increase at the pulp-dentin junction, during a one-layer or multi-layered deep composite restoration.

Methods:

A Three-dimensional finite element method model of typical geometry and material properties, as commonly reported in the literature, was employed in COMSOL Multiphysics simulations in order to determine the temperature increase in the pulp. Various combinations of light intensities, durations, and irradiation regimes were investigated for the two cases, of shallow and deep multi-layered composite restoration.

Results:

Results of light-curing composite resins within enamel; indicate that the temperature rise during the curing process was within the safety margins. Results of light-curing composite resin restorations closer to the pulp with thin remaining dentin, indicate a temperature increase that could be sufficient to cause thermal injury in the pulp. Modulating the light output marginally, reduced the temperature rise while reducing the intensity and increasing the curing duration which was consistently more effective in this respect.

Conclusion:

The results clearly demonstrate that with currently adopted standard procedures, there exists a risk of thermal injury during multi-layered composite restorations with thin remaining dentin; it is thus important to establish appropriate curing regimes that would lead to minimal temperature increase during deep composite restorations and hence reduce the risk of thermal injury to the pulp.

Fracture Toughness Comparison of Three Indirect Composite Resins Using 4-Point Flexural Strength Method

Objectives  The advantages of indirect composite restorations such as less crack formation during their computer-aided design/computer-aided manufacturing process, compared with ceramic restorations, have resulted in their growing popularity. However, restoration failure is a major concern with regard to the long-term clinical success of restorations and may occur as the result of propagation of a crack originated from an internal flaw in the restoration. This study aimed to compare the fracture toughness of three indirect composite resins.

Materials and Methods In this in vitro experimental study, 10 specimens measuring 3 × 3 × 18 mm were fabricated of Gradia, Crios, and high impact polymer composite indirect composites. A single edge notch with a diameter < 0.3 mm and 0.3 mm length was created in the 9 mm longitudinal dimension of specimens using a no. 11 surgical scalpel. The specimens were then subjected to 4-point flexural strength test in a universal testing machine with a crosshead speed of 0.1 mm/s until failure.

Statistical Analysis  Data were analyzed using IBM SPSS Statistics via one-way analysis of variance (ANOVA) and Tukey’s HSD (honestly significant difference) test. The statistical power was set at p ˂ 0.05.

Results  One-way ANOVA showed a significant difference in fracture toughness of the three composite groups ( p = 0.000). According to the Tukey HSD analysis, the fracture toughness of HIPC was significantly higher than that of the other two composites. The fracture toughness of Gradia was significantly lower among all.

Conclusions  Within the limitations of this study, the results showed that high temperature-pressure polymerization can increase resistance to crack propagation and subsequently improve the clinical service of indirect composite restorations. Although we do not know the filler volume percentage of HIPC, it seems that filler volume percentage of the composite is inversely correlated with fracture toughness.

Mechanical properties of nanocomposite biomaterials improved by extrusion during direct ink writing

In this study, single filaments of acrylated epoxidized soybean oil (AESO)/polyethylene glycol diacrylate (PEGDA)/nanohydroxyapatite (nHA)-based nanocomposites intended for bone defect repair have displayed significant improvement of their mechanical properties when extruded through smaller needle gauges before UV curing. These nanocomposite inks can be deposited layer-by-layer during direct ink writing (DIW) - a form of additive manufacturing. Single filaments were prepared by extruding the nanocomposite ink through needles with varying diameters from 0.21 mm to 0.84 mm and then UV cured. Filaments and cast specimens were tensile tested to determine elastic modulus, strength and toughness. The cured nanocomposite filaments were further characterized using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). SEM confirmed that the hydroxyapatite nanoparticles were well dispersed in the polymer matrices. The ultimate tensile strength and moduli increased as the diameter of the extrusion needle was decreased. These correlated with increased matrix crystallinity and fewer defects. For instance, filaments extruded through 0.84 mm diameter needles had ultimate tensile stress and modulus of 26.3 ± 2.8 MPa and 885 ± 100 MPa, respectively, whereas, filaments extruded through 0.21 mm needles had ultimate tensile stress and modulus of 48.9 ± 4.0 MPa and 1696 ± 172 MPa, respectively. This study has demonstrated enhanced mechanical properties resulting from extrusion-based direct ink writing of a new AESO-PEGDA-nHA nanocomposite biomaterial intended for biomedical applications. These enhanced properties are the result of fewer defects and increased crystallinity. A means of achieving mechanical properties suitable for repairing bone defects is apparent.

Resin viscosity determines the condition for a valid exposure reciprocity law in dental composites

OBJECTIVE

To provide conditions for the validity of the exposure reciprocity law as it pertains to the photopolymerization of dimethacrylate-based dental composites.

METHODS

Composites made from different mass ratios of resin blends (Bis-GMA/TEGDMA and UDMA/TEGDMA) and silanized micro-sized glass fillers were used. All the composites used camphorquinone and ethyl 4-dimethylaminobenzoate as the photo initiator system. A cantilever beam-based instrument (NIST SRI 6005) coupled with NIR spectroscopy and a microprobe thermocouple was used to simultaneously measure the degree of conversion (DC), the polymerization stress (PS) due to the shrinkage, and the temperature change (TC) in real time during the photocuring process. The instrument has an integrated LED light curing unit providing irradiances ranging from 0.01W/cm² to 4W/cm² at a peak wavelength of 460nm (blue light). Vickers hardness of the composites was also measured.

RESULTS

For every dental composite there exists a minimum radiant exposure required for an adequate polymerization (i.e., insignificant increase in polymerization with any further increase in the radiant exposure). This minimum predominantly depends on the resin viscosity of composite and can be predicted using an empirical equation established based on the test results. If the radiant exposure is above this minimum, the exposure reciprocity law is valid with respect to DC for high-fill composites (filler contents >50% by mass) while invalid for low-fill composites (that are clinically irrelevant).

SIGNIFICANCE

The study promotes better understanding on the applicability of the exposure reciprocity law for dental composites. It also provides a guidance for altering the radiant exposure, with the clinically available curing light unit, needed to adequately cure the dental composite in question.

Characterization of Ultraviolet-Cured Methacrylate Networks: From Photopolymerization to Ultimate Mechanical Properties

In this study, the effect of different process conditions on the material properties of a single UV-cured layer of methacrylate resin, typically used in the stereolithography (SLA) process, is assessed. This simplified approach of the SLA process gives the opportunity to study the link between process conditions and mechanical properties without complicated interactions between different layers. Fourier-transform infrared analysis is performed to study the effect of light intensity, curing time, and initiator concentration on the monomer conversion. A model is developed based on the reaction kinetics of photopolymerization that describes and predicts the experimental data. The effect of curing time and light intensity on the glass-transition temperature is studied. A unique relation exists between conversion and glass-transition temperature, independent of the light intensity and curing time. Tensile tests on UV-cured resin show an increase in yield stress with increasing curing time and a linear relation between glass-transition temperature and yield stress. However, a lower light intensity leads to a different network structure characterized by a lower yield stress and glass-transition temperature. The correlations between process conditions and the mechanical properties of UV-cured methacrylate systems are established to better understand the role of the processing parameters involved in the SLA process.

Intracoronal stress transfer through enamel following RBC photopolymerisation: A synchrotron X-ray study

OBJECTIVES

To measure the spatial distribution of crystallographic strain in tooth enamel induced by the photo-polymerisation of a dimethacrylate resin based composite cavity restoration.

METHODS

Six sound first premolar teeth, allocated into two groups (n=3), were prepared with mesio-occlusal distal cavities. The enamel was machined at the point of maximum convexity on the outer tooth to create a vertical fin of thickness 100μm and 0.5mm depth to allow for synchrotron X-ray diffraction measurements. 2D diffraction patterns were used to determine crystallite orientation and quantify changes in the hydroxyapatite crystal lattice parameters, before and after photo-polymerisation of a composite material placed in the cavity, to calculate strain in the respective axis. The composite was photo-polymerised with either relatively high (1200mWcm^-2, group 1) or low (480mWcm^-2, group 2) irradiances using LED or quartz halogen light sources, respectively. A paired t-test was used to determine significant differences in strain between irradiance protocols at ɑ=0.001.

RESULTS

Photo-polymerisation of the composite in the adjacent cavity induced significant changes in both the crystallographic c and a axes of the enamel measurement area. However the magnitude of strain was low with ∼0.1% difference before and after composite photo-polymerisation. Strain in enamel was not uniformly distributed and varied spatially as a function of crystallite orientation. Increased alignment of crystallites perpendicular to the cavity wall was associated with higher c axis strain. Additionally, strain was significantly greater in the c (p<0.001) and a axis (p<0.001) when using a high irradiance photo-polymerisation protocol.

SIGNIFICANCE

Although cuspal deflection is routinely measured to indirectly assess the 'global' effect of composite shrinkage on the tooth-restoration complex, here we show that absolute strains generated in enamel are low, indicating strain relief mechanisms may be operative. The use of low irradiance protocols for photo-polymerisation resulted in reduced strain.

Stress-reduced Direct Composites for the Restoration of Structurally Compromised Teeth: Fiber Design According to the "Wallpapering" Technique

PURPOSE

The purpose of this work was to present a restoration technique based on an understanding of the biomechanical properties of the dentinoenamel complex (DEC) and the physical-mechanical properties of the resin-based composite including the stress generated from both polymerization shrinkage and occlusal forces. Technique Summary: The DEC is a functional interphase that provides crack tip shielding; the DEC should be preserved during restorative procedures. Dentists can design the strategic placement of restorative materials into the cavity to both resist the mode of failure and mimic the performance characteristics of the intact natural tooth. The term "wallpapering" describes a concept of covering the cavity walls with overlapping closely adapted pieces of Leno weaved ultra-high-molecular-weight polyethylene (LWUHMWPE) ribbons. The key for success is that the ribbons are adapted and polymerized as closely as possible against the contours of residual tooth substrate. The resulting thin bond line between the fibers and the tooth structure creates a "bond zone" that is more resistant to failing due to the intrinsic stress and energy absorbing mechanism of the LWUHMWPE ribbons. The formation of defects and voids, from which crack propagation may start, is also reduced. The fibers' tight adaptation to tooth structure allows a dramatic decrease of the composite volume between the tooth structure and the fiber, thus protecting the residual weakened walls from both the stress from polymerization shrinkage and the occlusal load.

CONCLUSION

By using a similar approach, fiber-reinforced stress-reduced direct composite restorations may be performed in the restoration of structurally compromised vital and nonvital teeth.

Polymerization shrinkage stress of resin-based dental materials: A systematic review and meta-analyses of technique protocol and photo-activation strategies

PURPOSE

A systematic review was conducted to determine whether there were any alternative technique or additional step strategies available to reduce and control polymerization shrinkage stress development in dental resin-based restorative materials.

DATA SOURCES

This report followed the PRISMA Statement. A total of 36 studies were included in this review. Two reviewers performed a literature search up to December 2016, without restriction of the year of publication, in seven databases: PubMed, Web of Science, Scopus, SciELO, LILACS, IBECS, and BBO.

STUDY SELECTION

Only in vitro studies that evaluated polymerization shrinkage stress by direct testing were included. Pilot studies, reviews and in vitro studies that evaluated polymerization shrinkage stress by indirect methods (e.g., microleakage or cuspal deflection measurements), finite elemental analysis or mathematical models were excluded. Of the 6.113 eligible articles, 36 studies were included in the qualitative analysis, and the meta-analysis was performed with 25 studies. A global comparison was performed with random-effects models (α = 0.05). The strategies were subdivided as follows: the use of an alternative technique protocol of placing the material inside the tooth cavity; the modification of the irradiation intensity or total energy delivered to the material; the use of an alternative light-curing source; or the use of an alternative photo-activation mode. All alternative strategies showed statistically significant differences when compared with their respective controls (p < 0.05).

CONCLUSION

The use of alternative light-curing sources contributed more to minimizing stress development than placing the material by means of an alternative technique protocol or by modifying the irradiant intensity or total energy delivered to the material during photo-activation. Moreover, the use of an alternative photo-activation mode (intermittent light, exponential, soft-start or pulse delay modes) was shown to be an effective strategy for reducing and controlling stress development in resin-based dental materials.

Flexural strength and microhardness of anterior composites after accelerated aging

BACKGROUND

This study aimed to evaluate the flexural strength and microhardness of three different anterior composites after 10 000 thermocycles.

MATERIAL AND METHODS

The mechanical properties of a nano-fill composite (Filtek Ultimate Universal Restorative (FUR) (Enamel)), a nano-hybrid composite (Clearfil Majesty ES2 (ES2) (Enamel)), and a micro-hybrid composite (G Aenial Anterior (GAA)) were investigated in this study. For the microhardness test, 8-mm diameter and 2-mm thickness composite discs were used (n = 10), and for the flexural strength test, 25x2x2 mm bar-shaped specimens were prepared (n = 13). The specimens were tested at 24 h and after 10 000 thermocycles. Data were analyzed using two-way analysis of variance and the post-hoc Tukey test (p < .05). Correlations between hardness and flexural strength were calculated using Pearson's correlation analysis.

RESULTS

There was a significant difference in the microhardness values of the materials (p < .05). FUR exhibited significantly higher microhardness than ES2 and GAA. However, the flexural strength of three composites was statistically similar at 24 h (p > .05). Pearson correlation analysis revealed that there was a negative relationship between the mean hardness and flexural strength values (correlation coefficient = -0.367, p = .043). After 10 000 thermocycles, microhardness values of each material and flexural strength of ES2 and GAA decreased significantly according to 24 h.

CONCLUSIONS

The nano-fill composite FUR displayed significantly higher microhardness values. However, each resin composite was statistically similar for flexural strength values. Ten thousand thermocycles significantly affected microhardness and flexural strength. Key words:Flexural strength, microhardness, anterior composites.

Resin Composite Properties and Energy Density of Light Cure

According to the ‘total energy concept’, properties of light-cured resin composites are determined only by energy density because of reciprocity between power density and exposure duration. The kinetics of polymerization is complex, and it was hypothesized that degree of cure, flexural strength, and flexural modulus were influenced not only by energy density, but also by power density per se. A conventional resin composite was cured at 3 energy densities (4, 8, and 16 J/cm2) by 6 combinations of power density (50, 100, 200, 400, 800, and 1000 mW/cm2) and exposure durations. Degree of cure, flexural strength, and flexural modulus increased with increasing energy density. For each energy density, degree of cure decreased with increasing power density. Flexural strength and modulus showed a maximum at intermediate power density. Within clinically relevant power densities, not only energy density but also power density per se had significant influence on resin composite properties.

Bisphenol A release from orthodontic adhesives and its correlation with the degree of conversion

INTRODUCTION

Our objective was to quantitatively assess and compare the bisphenol A (BPA) released from an orthodontic adhesive using a light-emitting diode device (LED) or a halogen light-curing unit (HLC) at 3 tip-to-bracket distances (0, 5, and 10 mm) and varying curing times using high-performance liquid chromatography. BPA release with self-etching and moisture-insensitive primers with light-cured and chemically cured composites was also evaluated. BPA release was correlated to the corresponding degree of conversion.

METHODS

Our sample consisted of 598 stainless steel first premolar brackets. Of these, 520 were used for assessing BPA release and divided into 13 groups of 40 each. In groups I, II, and III, the composite was cured with the LED for 20 seconds at distances of 0, 5, and 10 mm, respectively. Groups IV, V, and VI were cured with the HLC for 40 seconds at the same 3 distances. Groups VII and VIII were cured for 5 and 10 seconds with the LED, and groups IX and X were cured for 10 and 20 seconds with the HLC at 0-mm distance. Groups XI, XII, and XIII consisted of brackets bonded with a self-etching primer and Transbond (3M Unitek, Monrovia, Calif), with a moisture-insensitive primer and Transbond, and with a chemically cured composite. The remaining 78 brackets were also divided into 13 groups and used for assessing the degree of conversion.

RESULTS

The LED devices demonstrated significantly less BPA release and greater degrees of conversion (P <0.05). For both units, BPA release increased and the degree of conversion decreased as the tip distance increased and curing time decreased. The chemically cured group showed significantly less BPA release (P <0.05). Among the light-cured composites, those cured according to the manufacturers' recommendations (40 seconds and 0-mm distance for the HLC unit) released less BPA than did the self-etching primer and the moisture-insensitive primer. The degree of conversion was greatest for the chemically cured composite, whereas it was similar for the conventional, self-etching primer, and moisture-insensitive primer groups. However, correlations ranged from strongly negative to weakly positive between BPA release and degree of conversion.

CONCLUSIONS

Clinicians should consider using LEDs in clinical practice and should keep the light-cure tip as close to the bracket as clinically possible. Curing time should be according to the manufacturer's recommendations. These steps will ensure less BPA release and a greater degree of conversion. Since chemically cured composites had less BPA release and a greater degree of conversion, they can be considered superior to light-cured composites in this aspect.

Effect of Light Sources on the Bond Strength of Resin Material to Thin-walled Roots

The aim of this study was to evaluate the bond strength to the dentin of an adhesive material used for root reinforcement light activated with different sources. Roots were divided into 4 groups (n=15) according to the light source used to activate the resin reinforcement: GI, non-weakened roots (control); GII, halogen light (H) 600 mW/cm2; GIII, LED 800 mW/cm2 and GIV, LED 1500 mW/cm2. The reinforcement was done with adhesive, composite resin and fiberglass posts. After 24 h, the specimens were sectioned and the first slice of each post region was used in the push out test in a universal testing machine with a crosshead speed of 0.5 mm/min. Failure modes of the debonded specimens were examined. Data (MPa) were analyzed by ANOVA and Holm-Sidak test (=0.05). The second slice from each region was analyzed by scanning electron microscopy (SEM). LED-1500 (4.69 ± 1.74) provided bond strength similar to the control group (5.05 ± 2.63) and statistically different from H-600 (1.96 ± 0.94) and LED-800 (2.75 ± 1.90), which were similar to each other (p<0.05). Cervical (4.16 ± 2.32) and middle (4.43 ± 2.32) regions showed higher bond strength than the apical (2.25 ± 1.50) (p<0.05). There was a prevalence of adhesive failures in H-600 and LED-800 and cohesive failures in LED-1500. SEM showed the formation of long, numerous and fine tags. It was concluded that LED-1500 provided higher bond strength of resin reinforcement to the dentin.

The reciprocity law concerning light dose relationships applied to BisGMA/TEGDMA photopolymers: theoretical analysis and experimental characterization

OBJECTIVES

A model BisGMA/TEGDMA unfilled resin was utilized to investigate the effect of varied irradiation intensity on the photopolymerization kinetics and shrinkage stress evolution, as a means for evaluation of the reciprocity relationship.

METHODS

Functional group conversion was determined by FTIR spectroscopy and polymerization shrinkage stress was obtained by a tensometer. Samples were polymerized with UV light from an EXFO Acticure with 0.1wt% photoinitiator. A one-dimensional kinetic model was utilized to predict the conversion-dose relationship.

RESULTS

As irradiation intensity increased, conversion decreased at a constant irradiation dose and the overall dose required to achieve full conversion increased. Methacrylate conversion ranged from 64±2% at 3mW/cm(2) to 78±1% at 24mW/cm(2) while the final shrinkage stress varied from 2.4±0.1MPa to 3.0±0.1MPa. The ultimate conversion and shrinkage stress levels achieved were dependent not only upon dose but also the irradiation intensity, in contrast to an idealized reciprocity relationship. A kinetic model was utilized to analyze this behavior and provide theoretical conversion profiles versus irradiation time and dose.

SIGNIFICANCE

Analysis of the experimental and modeling results demonstrated that the polymerization kinetics do not and should not be expected to follow the reciprocity law behavior. As irradiation intensity is increased, the overall dose required to achieve full conversion also increased. Further, the ultimate conversion and shrinkage stress that are achieved are not dependent only upon dose but rather upon the irradiation intensity and corresponding polymerization rate.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

SIGNIFICANCE

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

Functional and Aesthetic Guidelines for Stress-Reduced Direct Posterior Composite Restorations

Amalgam has been used in the restoration of structurally compromised posterior teeth for many years. When placing large amalgam restorations, replacement of weak cusps with restorative material is recommended to prevent tooth fracture. This recommendation can be modified with new guidelines using modern adhesive techniques. Semidirect and indirect inlay/onlay composite restorations have progressively replaced amalgam restorations over the past 20 years. Lately, single visit direct resin-bonded composite (RBC) restorations have also been used as a viable alternative to conventional indirect restorations. This paper is intended to introduce a step-by-step protocol for the direct restoration of structurally compromised posterior teeth using RBCs with stress-reducing protocols.

Influence of prolonged light-curing time on the shear bonding strength of resin to bleached enamel

This study evaluated the effect of prolonged light-curing time using a light-emitting diode unit (LED) on the shear bond strength of a resin composite to enamel immediately after bleaching. The enamel surfaces of human molars were divided into four groups: one control and three bleaching groups. One bleaching group (CP) was exposed to a 10% carbamide peroxide bleaching agent and bonded after 24 hours. The other two bleaching groups (HP) were bleached with a 38% hydrogen peroxide bleaching agent, then bonded either within one hour (HPA) or after 24 hours (HPB). All groups were subdivided into two subgroups and cured for two different times (20 or 40 seconds) with an LED unit. Shear bond strength (SBS) was tested with a universal-testing machine and the data were analyzed by ANOVA and post-hoc tests. Scanning electron micrographs of representative specimens were taken. A significant difference was seen between the control and HPA groups for both curing times (p = 0.000). However, neither the CP nor HPB groups showed any significant differences compared with the control groups (p > 0.05). Two-way ANOVA showed that a significant effect of the curing time factor was recorded for all groups (p = 0.000). Prolonged curing time, using an LED unit with a light intensity of 500 mW/cm2, increased resin-enamel bonding strengths for the control and bleached groups when bonding was performed after 24 hours of immersion in deionized water. However, the SBS was still compromised when bonding was performed immediately to enamel bleached with 38% HP.

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

OBJECTIVES

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.

MATERIALS AND METHODS

Two dual-cured resin cements: choice (CH) and Variolink II (VL) were tested. Hybrid composite resin (Z-250) discs (12 × 1.5 mm) 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 0.5 mm thickness were prepared from extracted human teeth. A circular mold (2.5 mm in height and 12 mm diameter) was utilized to create supporting structure for dentin, resin cement complex. The resin luting cement (0.5 mm) was placed on the previously prepared dentin discs and covered with the prefabricated composite discs. Photo-activation of cements was performed for 40 s with QTH and LED units and for 3 s with PAC. The specimens were divided into 12 groups (20 specimens for each light source). Six groups were kept in distilled water for 24 h and the rest were stored for 6 weeks. Bi-axial flexural strength was determined using Instron machine. The data was analyzed using two-way ANOVA and Tukey test for comparison.

RESULTS

The findings indicated that the bi-axial flexural strength values for both cements CH and VL were higher for 24 h over 6 weeks 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 24 h over 6 weeks 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 6 weeks were statistically significantly higher value than 24 h duration at P = 0.002.

CONCLUSION

When high immediate flexural strength is preferred in clinical situation photo-activation the cements with LED reported the greatest value.

Evaluation of depth of cure and knoop hardness in a dental composite, photo-activated using different methods

The study aimed at evaluating the depth of cure and knoop hardness of a microfine-hybrid composite resin that was photo-activated using different methods. A bipartite brass mold was filled with composite resin and photo-activation was performed using four methods: (1) Intermittent method using quartz tungsten halogen (QTH) light curing unit (LCU) (2) Continuous method (QTH) (3) Exponential method (QTH) (4) Continuous method using light-emitting diode (LED). Depth of cure was measured at the unexposed bottom surface of the specimen using microtester as a penetrometer. The surfaces exposed to light were subjected to knoop hardness testing, using a digital microhardness tester. Knoop hardness measurements were obtained at the top surface and at depths of 1, 2, 3, 4 and 5 mm. The data was analyzed using anova and Tukey's test (5%). Results showed that the depth of cure was higher with the intermittent method (QTH), followed by the continuous method (QTH), the exponential method and the continuous method (LED). At the top surface and up to 1 mm, continuous method (LED) demonstrated the highest knoop hardness number (KHN). At 2 mm and up to 5 mm, intermittent method (QTH) presented the highest KHN and continuous method (LED) showed the lowest KHN. At all depths, continuous method (QTH) showed higher KHN, as compared to the exponential method (QTH), except at 2 mm where both showed no significant difference.

Compressive strength of esthetic restorative materials polymerized with quartz-tungsten-halogen light and blue LED

This study compared the compressive strength of a composite resin and compomer photoactivated with a conventional quartz-tungsten halogen-light (XL 3000, 3M/SPE) and a blue light-emitting diode (LED) (SmartLite PS; Dentsply/De Trey). Forty disc-shaped specimens were prepared using a split polytetrafluoroethylene matrix (4.0 mm diameter x 8.0 mm hight) in which the materials were inserted incrementally. The curing time of each increment was of 40 s with the QTH and 10 s with the LED. The specimens were randomly assigned to 4 groups (n=10), according to the light source and the restorative material. After storage in distilled water at 37 degrees C +/- 2 degrees C for 24 h, the specimens was tested in compressive strength in a universal testing machine with load cell of 500 kgf running at a crosshead speed of 0.5 mm/min. Data (in MPa) were analyzed statistically by ANOVA and Student-Newman-Keuls test (p<0.05). For the composite resin, light curing with the QTH source did not produce statistically significant difference (p>0.05) in the compressive strength when compared to light curing with the LED source. However, light curing of the compomer with the QTH source resulted in significantly higher compressive strength than the use of the LED unit (p>0.05). The composite resin presented significantly higher (p>0.05) compressive strength than the compomer, regardless of the light source. In conclusion, the compressive strength of the tested materials photoactivated with a QTH and a LED light source was influenced by the energy density employed and the chemical composition of the esthetic restorative materials.

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

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

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.

Quantum yield of conversion of the photoinitiator camphorquinone

The primary absorber in dental resins is the photoinitiator, which starts the photo polymerization process. We studied the quantum yield of conversion of camphorquinone (CQ), a blue light photoinitiator, in dental resin composites using a LED lamp (3M FreeLight) and a Quartz Tungsten Halogen (QTH) lamp (VIP) as the light curing units at five different irradiances. The molar extinction coefficient, epsilon(469), of CQ was 46+/-2 cm(-1)/(mol/L) at 469 nm. The reciprocity of irradiance and exposure time holds for changes of CQ absorption coefficient, that is, irradiance x exposure time (=radiant exposure)=constant. Both LED and QTH lamps yielded the same curing threshold (the radiant exposure when CQ absorption drops to 1/e) and the same quantum yield conversion under different irradiances. In our dental resin formulation (0.7 wt.% CQ with reducing agents 0.35 wt.% dimethylaminoethyl methacrylate (DMAEMA) and 0.05 wt.% butylated hydroxytoluene (BHT)) the quantum yield was measured as 0.07+/-0.01 CQ conversion per absorbed photon.

Cuspal movement and microleakage in premolar teeth restored with resin-based filling materials cured using a ‘soft-start’ polymerisation protocol

OBJECTIVES

To investigate the effect of polymerisation shrinkage strain of four posterior filling materials on cuspal movement, degree of conversion (DC) and cervical gingival microleakage of mesio-occlusal-distal (MOD) restorations placed incrementally in maxillary premolar teeth using a 'soft-start' polymerisation protocol.

METHODS

Forty sound extracted upper premolar teeth were subjected to standardised preparation of a large MOD cavity before restoration. A 'soft-start' polymerisation curing regimen was used and each posterior filling material was placed in eight increments with the appropriate bonding system. A twin channel deflection measuring gauge allowed a measurement of individual cusp deflections at each stage of polymerisation. Restored teeth were thermocycled before immersion in a 0.2% basic fuchsin dye for 24h. After sagittal sectioning of the restored teeth in a mesio-distal plane, the sectioned restorations were examined to assess cervical microleakage. The DC was also assessed using a diffuse-reflectance accessory on a Fourier transform infra-red spectrophotometer.

RESULTS

A significant increase in cuspal movement recorded for Z100 (20.06+/-4.71) compared with Filtek Z250 (16.52+/-3.26), P60 (14.23+/-3.71) and Admira (11.11+/-2.47). No significant reduction in cuspal movement was identified when compared with a previous study [6] where a full-intensity standard polymerisation protocol was employed. No significant differences were also identified between the materials when the cervical gingival microleakage scores or DC were examined for the 'soft-start' compared with the standard polymerisation protocol.

CONCLUSIONS

Although material type remained a significant factor, the use of a 'soft-start' polymerization compared with a standard curing regime did not offer any significant reduction in associated cuspal movement, DC or gingival microleakage at the cervical dentine cavosurface margin of the cavities restored with the resin-based filling materials.

Investigation of polymerisation shrinkage strain, associated cuspal movement and microleakage of MOD cavities restored incrementally with resin-based composite using an LED light curing unit

OBJECTIVES

To investigate the polymerisation shrinkage strain, associated cuspal movement, degree of conversion (DC) and cervical gingival microleakage of mesio-occlusal-distal (MOD) cavities restored with four resin-based composite (RBC) filling materials placed incrementally using a light emitting diode (LED) light curing unit (LCU).

METHODS

Standardised extensive MOD cavity preparations on extracted teeth were performed on 40 sound upper premolar teeth. Restoration of the teeth involved the placement of RBCs in eight increments with the appropriate bonding system before irradiation using an LED LCU. Buccal and palatal cusp deflections at each stage of polymerisation were recorded using a twin channel deflection measuring gauge. Following restoration, the teeth were thermocycled, immersed in a 0.2% basic fuchsin dye for 24 h, sagittally sectioned and examined for cervical microleakage. The DC was determined using a Fourier transform infra-red (FT-IR) spectrometer.

RESULTS

No significantly difference (P=0.677) in cuspal movement was recorded for Z100 (13.1+/-3.2 microm) compared with Filtek Z250 (8.4+/-3.5 microm), P60 (7.3+/-3.8 microm) and Admira (6.7+/-2.7 microm). The LED LCU deflections were compared with a halogen LCU used in a conventional (Fleming GJP, Hall D, Shorthall ACC, Burke FJT. Cuspal movement and microleakage in premolar teeth restored with posterior filling materials of varying reported volumetric shrinkage values. Journal of Dentistry, 2005;33:139-146) and soft-start mode (Fleming GJP, Cara RR, Palin WM, Burke FJT. Cuspal movement and microleakage in premolar teeth restored with posterior filling materials cured using 'soft-start' polymerization. Dental Materials, 2006, , in press) and a significant reduction in cuspal movement was identified for curing type and material type (P<0.001 and P=0.002, respectively). No significant differences were noted between the four RBC materials investigated when the DC or microleakage scores were examined for the LED LCU.

SIGNIFICANCE

It would appear that irradiation of RBCs using the LED LCU offered a significant reduction in associated cuspal movement in large MOD cavities. However, the microleakage scores following polymerisation were significantly increased with dye penetration into the pulp chamber from the axial wall evident in teeth restored with the LED LCU.

Comparison of the Flexural Strength of Five Adhesive Resin Cements

STATEMENT OF PROBLEM

The increased use of adhesive resin cements in bonded prosthetic restorations has led to restorations debonding under function.

PURPOSE

This investigation evaluated the differences in the flexural strength of new adhesive resin cements as a function of specimen age and storage condition.

MATERIALS AND METHODS

Four new dual-cure cements were compared to C/B Metabond. Twenty specimens of each of the five cements were prepared in a rectangular glass mold (25 x 2 x 2 mm). The new cements were light-activated with a 550 mW/cm(2) lamp for 80 seconds on both the top and bottom surfaces. The auto-cured cement was allowed to set according to manufacturer's directions. Half the specimens were tested immediately after curing while the other half were stored in distilled water at 37 degrees C for 30 days. A three-point bending test was performed using an Instron at a crosshead speed of 1 mm/min. The results were analyzed by analysis of variance and Scheffé tests ( p < 0.05) to examine the effect of specimen age and storage condition.

RESULTS

RelyX ARC exhibited a significantly higher flexural strength compared with Calibra and Panavia F when tested immediately. The standard cement, C/B Metabond, deformed and did not fracture at the immediate test time. After storage, the flexural strength had significantly improved from the immediate test time for Calibra, Cement-It, Panavia F, and C/B Metabond. However, there were no significant differences in the flexural strength among the cements when tested after 30 days in water at 37 degrees C.

CONCLUSION

Immediately after curing, these new adhesive resin cements are not equivalent, as evidenced by the significant variability in the measured flexural strength. The distinctions among the cements diminish after aging in water, which may be due to residual polymerization or a plasticizing effect from water absorption.

CLINICAL SIGNIFICANCE

When light-cured, all the new adhesive resin cements have greater early strengths than the auto-cured cement; however, the wide variation in immediate bending strength suggests that some cements may be more appropriate for use in high-stress clinical situations such as resin-bonded fixed partial dentures.

Variation of depth of cure and intensity with distance using LED curing lights

OBJECTIVES

The purpose of the study was to determine the correlation between intensity of light-emitting diode (LED) and tungsten-halogen light sources, and depth of cure of a resin composite at different distances.

METHODS

Four LED curing lights (Flashlite 1001, Freelight 2, Smartlite IQ and Ultralume 5) and one tungsten halogen (Optilux 501, with 8 and 11 mm tips) were evaluated. Intensity was measured according a modified ISO Standard 10650 at distances of 0, 2, 4, 6, 8, 10 mm between the light tip and detector. Depth of cure (DOC) of TPH Spectrum shade A2 was measured according to the international standard ISO 4049 at the same distances.

RESULTS

For all lights, intensity decreased as distance increased. The authors documented a logarithmic correlation between intensity and distance for all lights except the Smartlite IQ, Ultralume 5 and the Optilux 501 with the 11 mm tip, which showed a linear relationship between intensity and distance. All lights demonstrated a logarithmic correlation between intensity and DOC, and a linear correlation between DOC and distance. Smartlite IQ and Optilux 501 (11 mm tip) also had the least reduction in intensity and DOC at 10 mm.

SIGNIFICANCE

Clinicians often an experience difficulty placing the light tip close to the resin surface when curing resin composites. While both intensity and DOC decrease with increasing distance, the relationship between these factors and distance may not be similar for all lights and may depend on the characteristics of individual lights.

Effect of time and polymerization cycle on the degree of conversion of a resin composite

The aim of this investigation was to verify the influence of 3 light curing units on the degree of conversion, using different irradiation conditions: 1) manufacturers' recommended times of photo-activation, 2) standardizing total energy density among the units and 3) standardizing energy density at the 450-490 nm wavelength range among the units and the effect of these irradiation conditions on the post-cure. Three light curing units were used: halogen, light emitting diodes (LED) and xenon plasma. Seven groups were tested (n=6). Twenty-four hours after the photo-activation procedures, half of the composite specimens were submitted to Fourier Transformed Infrared Spectroscopy analysis. The other half was analyzed after 1 month. The results were submitted to 2-way ANOVA and Tukey's test (5%). Twenty-four hour analysis revealed that the second set of irradiation conditions produced a similar degree of conversion among the LCUs. After 1 month, the conversion values were statistically higher for 20 seconds of halogen exposure (increased from 46.78 to 49.66%), 20 seconds of LED exposure (from 46.20 to 51.15%), 30 seconds of LED exposure (from 48.29% to 50.68%) and 3 seconds of PAC exposure (from 42.57 to 51.39%). The initial degree of conversion and post-cure depended on the photo-activation condition applied.

Direct cuspal-coverage posterior resin composite restorations: A case report

The clinical success of direct composite restorations is the result of the correct use and performance of adhesive systems, resin composites and light curing systems. Total-etch adhesive systems and microhybrid resin composites have seen continuous improvement; various clinical techniques have been introduced to address polymerization shrinkage. Manufacturers have introduced sophisticated light-curing devices with the hope of improving performance. Direct resin bonded composites (RBCs) are becoming the first choice in many clinical situations. This article presents an experimental clinical technique that outlines the reconstruction of severely damaged posterior teeth missing multiple cusps; particular attention to incremental and curing techniques is adopted to complete each restoration.

Effect of light-activation methods and water storage on the flexural strength of two composite resins and a compomer

The present study evaluated the flexural strength of three composite resins recommended for direct esthetic restorations: a polyacid modified composite (Dyract AP), a unimodal composite resin (Filtek Z250) and a hybrid composite resin (Point 4). The variation factors, apart from the type of composite resin, were the light activation method and the water storage period. The composite resins were light-cured in continuous mode (40 s, 500 mW/cm²) or in ramp mode (0-800 mW/cm² for 10 s followed by 30 s at 800 mW/cm²) and stored for 24 hours or 30 days in distilled water at 37°C. The data were analyzed by ANOVA and Tukey test for multiple comparisons (alpha = 0.05). The composite resin Z250 presented the highest mean flexural strength (166.74 MPa) and Dyract AP presented the lowest one (129.76 MPa). The storage for 30 days decreased the flexural strength in ramp mode (24 h: 156.64 MPa; 30 days: 135.58 MPa). The light activation method alone did not lead to different flexural strength values.

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.

Evaluation of mechanical properties of Z250 composite resin light-cured by different methods

This study evaluated some mechanical parameters of Z250 composite resin using different light-curing methods. Ten specimens were prepared for each mechanical test group with different dimensions according to the test. Light-curing was performed by: a). continuous light (800mW/cm²-40s); b). exponential light (0-800mW/cm²-40s); c). intermittent light (2s-600mW/cm²; 2s without light-80s); d). stepped light (10s-150mW/cm²; 30s-650mW/cm²); e). PAC (1320mW/cm²-3s); f). LED (350mW/cm²-40s). After 24 ± 1 h, the specimens were loaded at a crosshead speed of 0.5 mm/min until fracture. The mechanical properties were calculated and analyzed by ANOVA and Tukey test (5%). The results showed that the highest compressive strength values were found for the continuous, exponential, intermittent and stepped light methods, whereas PAC and LED obtained the lowest values. LED, stepped light, PAC, exponential and continuous light presented the highest values for diametral tensile strength. The intermittent light showed the lowest value, which was significantly lower than the value obtained for LED only. Flexural strength results were not significantly different between all light-curing methods. Finally, the highest modulus of elasticity values were obtained for LED, exponential, continuous and intermittent light, whereas PAC and stepped light showed the lowest values. The mechanical properties were affected by light-curing methods employed.

A pilot study of a simple photon migration model for predicting depth of cure in dental composite

OBJECTIVES

The purpose of this study was to build a photo migration model to calculate the radiant exposure (irradiancextime) in dental composite and to relate the radiant exposure with extent of cure using polymer kinetics models.

METHODS

A composite (Z100, Shade A2) cylinder (21 mm diameter by 15 mm deep) was cured with a tungsten-halogen lamp emitting 600 mW/cm2, 1 mm above the composite for 60s. For each of the 2x1 mm grids along the longitudinal cross section (diameter versus depth), the degree of conversion (DC) and hardness (KHN) were measured to construct the curing extent distribution. The inverse adding-doubling method was used to characterize the optical properties of the composite for the Monte Carlo model simulating the photon propagation within the composite cylinder. The calculated radiant exposure (H) distribution along the cross section was related to the curing extent DC/DC(max) distribution and fitted with two polymer curing kinetics models, the exponential model DC=DC(max)[1-exp((ln0.5)H/H(dc)(50%))] and Racz's model [Formula: see text] , where H(dc)(50%) is a fitting parameter representing the threshold for 50% of the maximum curing level.

RESULTS AND SIGNIFICANCE

The absorption and scattering coefficients of uncured composite were higher than that of cured composite at wavelengths between 420 and 520 nm. A roughly hemi-spheric distribution of radiant exposure in the Monte Carlo simulation result was comparable with the curing profiles determined by both DC and KHN. The relationship between DC (or KHN) and H agreed with the Racz model (r2=0.95) and the exponential model (r2=0.93). The H(dc)(50%) was 1.5(0.1), equal for the two models (P<0.05). The estimated radiant exposure threshold for 80% of the maximum curing level was between 3.8 and 8.8 J/cm2. The simulation results verify that the radiant exposure extends to a greater depth and width for composite with lower absorption and scattering coefficients. Given the optical properties and the geometry of the composite, and the spectrum and the geometry of the light source, the Monte Carlo simulation can accurately describe the radiant exposure distribution in a composite material to predict the extent of cure.

Two-Year Clinical Evaluation of Nonvital Tooth Whitening and Resin Composite Restorations

BACKGROUND

Adhesive systems, resin composites, and light curing systems underwent continuous improvement in the past decade. The number of patients asking for ultraconservative treatments is increasing; clinicians are starting to reevaluate the dogma of traditional restorative dentistry and look for alternative methods to build up severely destroyed teeth.

PURPOSE

The purpose of this study was to evaluate the efficacy of nonvital tooth whitening and the clinical performance of direct composite restorations used to reconstruct extensive restorations on endodontically bleached teeth.

MATERIALS AND METHODS

Twenty-one patients 18 years or older were included in this clinical trial, and 26 endodontically treated and bleached maxillary and mandibular teeth were restored using a microhybrid resin composite. Patients with severe internal (tetracycline stains) and external discoloration (fluorosis), smokers, and pregnant and nursing women were excluded from the study. Only patients with A3 or darker shades were included. Teeth having endodontic access opening only to be restored were excluded; conversely, teeth having a combination of endodontic access and Class III/IV cavities were included in the study. A Vita shade guide (Vita Zahnfabrik, Bad Säckingen, Germany) arranged by value order was used to record the shade for each patient. Temporary or existing restorations were removed, along with a 1 mm gutta-percha below the cementoenamel junction (CEJ), and a resin-modified glass ionomer barrier was placed at the CEJ. Bleaching treatment was performed using a combination of in-office (OpalescenceXtra, Ultradent Products, South Jordan, UT, USA) and at-home (Opalescence 10% PF, Ultradent Products) applications. Two weeks after completion of the bleaching, the teeth were restored using a combination of PQ1 adhesive system and Vit-l-escence microhybrid resin composite (Ultradent Products). Wedge-shaped increments were placed and cured using the VIP Light (Bisco, Inc Schaumburg, IL, USA) through a combination of pulse and progressive curing techniques.

RESULTS

All but one restoration were evaluated by two independent evaluators every 6 months during a 2-year period using modified US Public Health Service criteria. No restoration failed and "alpha" scores were recorded for all parameters but color stability, which was scored "bravo." Analysis of variance showed a significant shade change between baseline (mean=14.4+/-1.9) versus 2 weeks (mean=1.6+/-0.7) and 2 years (mean=2.8+/-1.7) (p<.0001). Although a significant shade change was observed between 2 weeks and the 2-year follow-up (p=.008), no significant difference was reported between the baseline and 2 weeks (12.9+/-2) versus baseline and 2 years (11.9+/-2.3).

CONCLUSIONS

Significant tooth lightening was reported after the completion of whitening therapy on devitalized teeth; shade rebound was reported in less than 50% of the treated teeth and was limited to a maximum of four shades. A microhybrid resin composite demonstrated excellent clinical performance in the restoration of all endodontically treated and bleached teeth after a 2-year evaluation period.

CLINICAL SIGNIFICANCE

Nonvital tooth whitening is responsible for a significant change in color of endodontically stained teeth. Successful nonvital tooth-whitening therapy allows for conservative tooth preparation, preserving and reinforcing sound tooth structure. The proper use of modern adhesive systems along with resin composite restorations precludes the use of more extensive restorative treatment, delaying expensive crown and bridge procedures.

Influence of photoactivation method on conversion, mechanical properties, degradation in ethanol and contraction stress of resin-based materials

OBJECTIVES

To investigate the influence of photoactivation method on degree of conversion (DC), flexural strength (FS), flexural modulus (FM) and Knoop hardness (KHN) of a composite and an unfilled resin (Filtek Z250 and Scotchbond multi-purpose plus, 3M ESPE) after storage in water or ethanol, and on composite contraction stress (CS).

METHODS

Specimens 1x2x10 mm were prepared for FS test, photoactivated by 600 mW/cm2 x 40 s (A), 200 mW/cm2 x 120 s (B), or 600 mW/cm2 x1 s +3 min delay +600 mW/cm2 x 39 s (C), and tested after 24 h in water or ethanol. Load and displacement values were used to calculate FM. Specimen fragments were used to measure KHN. DC was determined by FT-Raman spectroscopy. CS was determined by mechanical testing. Data were submitted to ANOVA/Tukey's test (alpha=0.05).

RESULTS

Composite DC was not affected by photoactivation (A: 65+/-1.8%; B: 66+/-3.4%; C: 65+/-2.9%). Unfilled resin DC was statistically higher using method A (79+/-0.3%) than B (74+/-1.0%) and C (73+/-0.9%). Photoactivation did not influence composite properties, regardless of the storage medium (p>0.05). After ethanol storage, FS of the unfilled resin was lower for specimens irradiated by method B (p<0.001). Pulse-delay curing (C) significantly reduced CS (7.7+/-1.3 MPa), compared to A (10.7+/-1.2 MPa) and B (10.1+/-1.3 MPa).

SIGNIFICANCE

Photoactivation method did not affect composite properties or susceptibility to ethanol degradation. For the unfilled resin, DC was lower with the use of low intensity and pulse-curing, while FS after ethanol storage was reduced by low intensity curing. Pulse-delay curing significantly reduced CS.

The effect of specimen dimensions on the flexural strength of a composite resin

PUORPOSE: The aim of this study was to evaluate the effect of specimen dimensions on the flexural strength of a composite resin (Heliomolar RO). METHODS: The different dimensions tested - length x width x height (mm) were: 25x2x4; 25x2x2 (ISO 4049); 15x2x2; 10x2x2 and 10x2x1. Light-curing was performed at 600 mW/cm² for 40s, three times along the 25mm specimens, twice along 15mm specimen and once for the 10mm specimens. Specimens of all dimensions were light-cured on both surfaces, or only on one side. In the latter, the load was applied on the irradiated side or on the opposite one. RESULTS: It was shown that the flexural strength was not affected by specimen length. When light-curing was performed on both surfaces, similar flexural strength values were obtained for any dimension. Despite the number of irradiated surfaces, specimens with a height of 1mm also obtained similar values. On the contrary, specimens with a height of 4mm, exposed only on one surface, reached the lowest strength. CONCLUSION: The use of specimens with lower dimensions can lead to flexural strength values similar to the ones obtained with standardized specimen (ISO 4049), with the advantage of demanding less amount of material and being less time consuming.

Evaluation of depth of cure and Knoop hardness in a dental composite photo-activated using different methods

The aim of this study was to evaluate the depth of cure and Knoop hardness in the P60 composite resin photo-activated using different methods. A bipartite brass matrix (3 mm in diameter X 11 mm in height) was filled with the composite and photo-activation was performed using continuous light, exponential light, intermittent light, plasma arc curing (PAC) or light-emitting diodes (LED). After opening the matrix, the uncured material was removed with a steel spatula and the polymerized composite was measured using a pachymeter. The specimens were then included in self-curing acrylic resin and worn longitudinally and the hardness was measured on the surface and at depths of 1, 2, 3, 4 and 5 mm. The data were analyzed by ANOVA and Tukey's test (5%). The results showed that the depth of cure was higher with the intermittent light, followed by continuous light, exponential light, PAC and LED methods. Up to a depth of 2 mm, all methods revealed similar hardness values, but there were differences between them at other depths, at which LED demonstrated the lowest values followed by PAC.

Plasma arc versus halogen light curing of orthodontic brackets: a 12-month clinical study of bond failures

The purpose of this randomized clinical trial was to evaluate the clinical performance of brackets cured with 2 different light-curing units (conventional halogen light and plasma arc light); 83 patients treated with fixed appliances were included in the study. With the "split-mouth" design, each patient's mouth was divided into 4 quadrants. In 42 randomly selected patients, the maxillary left and mandibular right quadrants were cured with the halogen light, and the remaining quadrants were cured with the plasma arc light. In the other 41 patients, the quadrants were inverted. A total of 1434 stainless steel brackets were examined: 717 were cured with a conventional halogen light for 20 seconds; the remaining 717 were cured with the plasma arc light for 5 seconds. The number, cause, and date of bracket failures were recorded for each light-curing unit over 12 months. Statistical analysis was performed with the Fisher exact test, the Kaplan-Meier survival estimates, and the log-rank test. No statistically significant differences were found between the total bond failure rates of the brackets cured with the halogen light and those cured with the plasma arc light. Neither were significant differences found when the clinical performances of the maxillary versus mandibular arches or the anterior versus posterior segments were compared. These findings demonstrate that plasma arc lights are an advantageous alternative to conventional light curing, because they significantly reduce the curing time of orthodontic brackets without affecting the bond failure rate.

The effect of light-curing modes on the microleakage of cervical resin composite restorations

OBJECTIVE

This study investigated the effect of light-curing modes on the microleakage of cervical resin composite restorations.

METHODS

Eighty wedge-shaped cervical cavities, C-factor approximately 1.3, were prepared on the labial surface of bovine incisors. The cavities were treated with a two-step experimental self-etching primer adhesive system containing an antibacterial monomer (ABF), according to the manufacturer's instructions, and filled with a single increment of a hybrid resin composite. The restorations were allocated randomly into four groups of 20: Group 1-the restorations were light-cured for 40 s with a conventional light-curing unit (New Light VL-II); Group 2-the restorations were cured with Candelux using a two-step mode; Group 3-the restorations were cured with Astralis 7 using a ramping mode; Group 4-the restorations were light-cured for 3 s with a plasma arc light-curing unit (Wave Light). Half of the specimens from each group were subjected to thermocycling (5-60 degrees C, 15 s dwell time, 5000 cycles) prior to immersion in 0.5% basic fuchsin. The teeth were sectioned and the degrees of dye penetration were scored. The data were analyzed with either the Kruskal-Wallis test or the Mann-Whitney U-test (p < 0.05)

RESULTS

No significant differences were found in the microleakage scores among the light-curing modes used. The restorations tended to display more microleakage when they were subjected to thermocycling, but the differences were not statistically significant.

CONCLUSIONS

These findings suggest that light-curing modes may have no effect on the microleakage of cervical cavities with a small C-factor, as is the case in this study.

Polymerization of resin composite restorative materials: exposure reciprocity

OBJECTIVE

To examine whether there is reciprocity between irradiation time and irradiance with regard to the mechanical properties of filled, resin composite restorative materials (RCs).

METHODS

Four visible light-cured RCs, all of shade A3, were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein), Filtek Z250 (FZ) (3M, St Paul, MN, USA) and Prodigy condensable (PR) (Kerr, Orange, CA, USA). Bar specimens (1.0 x 1.5 x 16.0 mm(3)) were cured at irradiances (I) ranging from 25-1500 mW/cm(2) and irradiation times (t) of 1-3000 s. Six specimens at 250 combinations of t and I were prepared and stored in artificial saliva of pH 6, at 37 degrees C for 7d before performing three-point bend tests for flexural strength (F), flexural modulus (E) and total energy to failure (W). Contour plots of property value vs. t and I on log scales were prepared.

RESULTS

The contour plots showed three regions: unset at low I.t, a plateau corresponding to more or less full property development, and connecting ramp. The boundary between the plateau and the ramp suggests the minimum acceptable exposure. No practical lower limit to irradiance was detected, but there may be no benefit from increasing I beyond about 1,000 mW/cm(2). The slopes of the contours in the log-log plots provided a test of the hypothesis of reciprocity. These slopes were approximately -1.5 for HR, TC and PR; and approximately -1 for FZ, compared with the expected value of -1. The general hypothesis therefore fails. The existence of localized maxima in property values is further evidence of that failure, even for FZ.

SIGNIFICANCE

Dentists may use any lamp, including LED sources, and attain satisfactory results providing irradiation time is long enough. Manufacturers ought to supply a graph indicating the minimum acceptable exposure for each product for specified curing lamps. Calculations based on total energy delivered to guide irradiation protocols are invalid and do not recognize product behavior.