Measurement of the full-field polymerization shrinkage and depth of cure of dental composites using digital image correlation

ObjectivesThe aim of this study was to measure the full-field polymerization shrinkage of dental composites using optical image correlation method.MethodsBar specimens of cross-section 4 mm × 2 mm and length 10 mm approximately were light cured with two irradiances, 450 mW/cm² and 180 mW/cm², respectively. The curing light was generated with Optilux 501 (Kerr) and the two different irradiances were achieved by adjusting the distance between the light tip and the specimen. A single-camera 2D measuring system was used to record the deformation of the composite specimen for 30 min at a frequency of 0.1 Hz. The specimen surface under observation was sprayed with paint to produce sufficient contrast to allow tracking of individual points on the surface. The curing light was applied to one end of the specimen for 40 s during which the painted surface was fully covered. After curing, the cover was removed immediately so that deformation of the painted surface could be recorded by the camera. The images were then analyzed with specialist software and the volumetric shrinkage determined along the beam length.ResultsA typical shrinkage strain field obtained on a specimen surface was highly non-uniform, even at positions of constant distance from the irradiation surface, indicating possible heterogeneity in material composition and shrinkage behavior in the composite. The maximum volumetric shrinkage strain of ～1.5% occurred at a subsurface distance of about 1 mm, instead of at the irradiation surface. After reaching its peak value, the shrinkage strain then gradually decreased with increasing distance along the beam length, before leveling off to a value of approximately 0.2% at a distance of 4–5 mm. The maximum volumetric shrinkage obtained agreed well with the value of 1.6% reported by the manufacturer for the composite examined in this work. Using irradiance of 180 mW/cm² resulted in only slightly less polymerization shrinkage than using irradiance of 450 mW/cm².SignificanceCompared to the other measurement methods, the image correlation method is capable of producing full-field information about the polymerization shrinkage behavior of dental composites.     

Clinical performance and chemical-physical properties of bulk fill composites resin —a systematic review and meta-analysis

Objectives: The objective of this study was to perform a meta-analysis of clinical and laboratory studies to compare the performance of bulk-fill and conventional composite resins in terms of polymerization shrinkage, polymerization stress, cusp deflection, marginal quality, degree of conversion, microhardness, flexural strength, fracture strength and clinical performance. Data: One hundred three articles were included in this study, and the Peto method was used to compare the bulk-fill and conventional composites using the RevMan software. Sources: Searches were performed in the PubMed and Scopus databases. Study selection: Laboratory studies and randomized clinical trials comparing one of the previous detailed outcomes between bulk-fill and control composites were included. Conclusions: The bulk-fill composite resins showed less shrinkage, polymerization stress, cusp deflection and microhardness than conventional composites, while both materials presented a similar marginal quality, flexural strength and fracture strength. Also, bulk-fill materials with regular viscosity showed similar shrinkage. The conversion of bulk-fill materials with flowable consistency were similar to conventional composite resins with a thickness of up to 2 mm and greater than conventional composites with a thickness greater than 2 mm. Despite these in vitro differences, the clinical performance of bulk-fill and conventional composite resins was similar in randomized clinical trials, with one to ten years of follow up. In conclusion, the bulk-fill materials show better or similar performance to the conventional materials in clinical trials and laboratory studies in terms of volumetric shrinkage, polymerization stress, cusps deflection and marginal quality, with the only exception being the lower level of microhardness observed for bulk-fill composites with thickness up to 2 mm.     

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

ObjectiveTo provide conditions for the validity of the exposure reciprocity law as it pertains to the photopolymerization of dimethacrylate-based dental composites.MethodsComposites 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 0.01 W/cm² to 4 W/cm² at a peak wavelength of 460 nm (blue light). Vickers hardness of the composites was also measured.ResultsFor 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).SignificanceThe 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.     

The effect of resin composite pre-heating on monomer conversion and polymerization shrinkage

ObjectivesTo determine monomer conversion and polymerization shrinkage of a resin composite after different pre-heating procedures and storage intervals.MethodsFor a commercial resin-based composite the immediate (5 min) and final (24 h) degree of conversion was measured on top and bottom surfaces utilizing FTIR spectroscopy. Composite pre-heating temperatures were selected between 10 and 68 °C. Polymerization shrinkage was measured according to Archimedes’ principles of buoyancy after 5 min at respective pre-heating temperatures and after 24 h dark and wet storage at 37 °C. Intra-cavity temperature development was monitored using a K-type thermocouple.ResultsNo significant increase in immediate as well as in final degree of conversion were measured from composite pre-heating at 68 °C compared to 54 and 39 °C. Linear correlations were detected immediately after photo-polymerization and on the top surface after 24 h storage. Polymerization shrinkage as a function of pre-heating temperatures exhibited a linear correlation after 5 min, but no statistically different behavior after 24 h.SignificancePre-heating of resin composites does not increase degree of conversion over time. It can be clinically beneficial, due to a superior marginal adaptation. This advantageous effect of reduced material paste viscosity has to be clinically addressed, since temperature rapidly drops to the physiological level upon removal from the pre-heating device.     

The relationship between Shore hardness of elastomeric dental materials and Young's modulus

ObjectivesHardness of elastomers can be directly related to Young's modulus, a relationship that was investigated in detail by Gent in a paper in 1958. The aim of this study was to test this relationship for 13 dental elastomers (12 silicone and 1 polyether) using the equation derived by Gent and one from BS 903 (1950) that accounts for departures at low values.MethodsThe dental elastomers were subjected to tensile testing and Shore A scale hardness measurements. Young's moduli were calculated from the hardness values using the Gent equation and the BS 903 equation. These calculated values were then compared with values derived experimentally from the tensile tests.ResultsHardness values were in the range 30.2 (±0.5)–62.9 (±0.8) with the corresponding calculated modulus values in the range 1.1–4.1 MPa and 0.9–4.3 MPa for the Gent and modified equations, respectively. Young's modulus values derived from the tensile data were in the range 0.8 (±0.3)–4.1 (±0.3) MPa, showing good agreement with those calculated from the hardness values. Providing viscoelastic creep is minimal during the duration of the test, there is a reasonably well-defined relationship between Shore hardness and Young's modulus in the hardness range studied.SignificanceSimple, non-destructive hardness measurements can be used to determine Young's modulus values. Such values are needed in any calculations of stress distributions in soft lining materials, e.g. by FEA.     

Spatial and cure-time distribution of dynamic-mechanical properties of a dimethacrylate nano-composite

ObjectiveThe purpose of this study was to evaluate a nano-filled dental composite, with varying cure irradiation-time, in terms of the spatial distribution of dynamic-mechanical properties determined at nanometre scale and the resultant distinction between filler, matrix and inter-phase regions.Materials and methodsSpecimen groups (n = 5) of the composite Filtek Supreme XT were cured in 2 mm deep molds for 5, 10, 20 and 40 s, and stored for 24 h in distilled water at 37 °C. Properties were measured at 2 mm depth, on the lower specimen surfaces. Nano-dynamic-mechanical parameters (complex, storage and loss modulus, tan δ) were determined at an array of 65,000 locations in a 5 μm × 5 μm area. Micro-mechanical properties (hardness, modulus of elasticity, creep and elastic/plastic deformation) were also measured and additionally the real-time degree of cure, by ATR-FTIR, for 10 min after photo-initiation and after storage.ResultsThe spatial distribution of nano-dynamic-mechanical properties varied significantly enabling four distinguishable matrix, filler-cluster and inter-phase regions to be identified. Proceeding from matrix to filler-cluster locations, complex-moduli increased linearly and loss-factors decreased linearly, consistent with visco-elastic composite theory. Curing time strongly affected all measured properties at 2 mm depth. The organic matrix was shown to be inhomogeneous for all curing times. By increasing cure-time, the proportion of less well polymerized area decreased from 37.7 to 1.1%, resulting in a more homogeneous organic matrix.SignificanceThe experimentally observed graduated transition, in complex modulus and related dynamic-mechanical properties, across the matrix – inter-phases – filler-cluster regions is conducive to low internal stresses, in contrast to the abrupt modulus transitions anticipated or observed in many other particulate composite structures. The identification of these phase-regions provides a realistic basis for accurate nano- and micro-mechanical computational modelling.     

Microhardness of resin composite materials light-cured through fiber reinforced composite

ObjectivesTo compare polymerization efficiency of resin composite basing materials when light-cured through resin composite and fiber reinforced composite (FRC) by testing microhardness.MethodsSimulated indirect restorations were prepared by application of resin composite (Clearfil AP-X) or FRC (EverStick) to nylon rings with 1.5 mm thickness and 8 mm diameter, followed by light-curing. Resin composite basing material (Clearfil Majesty Flow or Clearfil AP-X) was applied to identical rings and light-cured through the simulated indirect restorations with exposure times of 20, 40, or 60 s. Light-curing though a ring without resin material (=no indirect restoration) served as control. For each combination of basing material and indirect restoration 10 specimens were prepared for each exposure time. Top and bottom surface Vickers microhardness numbers (VHNs) of basing materials were recorded after 24 h.ResultsAfter 60 s exposure time, VHNs with indirect FRC were not different from control VHNs, while VHNs with indirect resin composite were significantly lower (p < 0.001). Linear regression analysis revealed that resin composite basing material used had the greatest effect on top and bottom VHNs (p < 0.001). The presence of an indirect restoration resulted in decreased VHNs (p < 0.001), with resin composite resulting in lower VHNs when compared to FRC. Moreover, a longer exposure time resulted in increased VHNs (p < 0.001).SignificanceResults suggest that polymerization of resin composite basing materials is more effective when light-curing through an FRC than through a resin composite indirect restoration. Prolonging of exposure time, however, is necessary when compared to light-curing without presence of indirect restoration material.     

Alternative monomer for BisGMA-free resin composites formulations

ObjectiveWater sorption, high volumetric shrinkage, polymerization stress, and potential estrogenic effects triggered by leached compounds are some of the major concerns related to BisGMA-TEGDMA co-monomer systems used in dental composites. These deficiencies call for the development of alternative organic matrices in order to maximize the clinical lifespan of resin composite dental restorations. This study proposes BisGMA-free systems based on the combination of UDMA and a newly synthesized diurethane dimethacrylate, and evaluates key mechanical and physical properties of the resulting materials.Methods2EMATE-BDI (2-hydroxy-1-ethyl methacrylate) was synthesized by the reaction between 2-hydroxy-1-ethyl methacrylate with a difunctional isocyanate (1.3-bis (1- isocyanato-1-methylethylbenzene) – BDI). The compound was copolymerized with UDMA (urethane dimethacrylate) at 40 and 60 wt%. UDMA copolymerizations with 40 and 60 wt% TEGDMA (triethylene glycol dimethacrylate) were tested as controls, as well as a formulation based in BisGMA (bisphenol A-glycidyl methacrylate)-TEGDMA 60:40% (BT). The organic matrices were made polymerizable by the addition of DMPA (2.2-dimethoxyphenoxy acetophenone) and DPI-PF6 (diphenyliodonium hexafluorophosphate) at 0.2 and 0.4 wt%, respectively. Formulations were tested as composite with the addition of 70 wt% inorganic content consisting of barium borosilicate glass (0.7 μm) and fumed silica mixed in 95 and 5 wt%, respectively. All photocuring procedures were carried out by a mercury arc lamp filtered to 320–500 nm at 800 mW/cm². The experimental resin composites were tested for kinetics of polymerization and polymerization stress in real time. Flexural strength, elastic modulus, water sorption, and solubility were assessed according to ISO 4049. Biofilm formation was analyzed after 24 h by luciferase assay. Data were statistically analyzed by one-way ANOVA and Tukey's test (α ≤ 0.05).ResultsIn general, the addition of 2EMATE-BDI into the formulations decreased the maximum rate of polymerization (RP_MAX), the degree of conversion at RP_MAX (DC at RP_MAX), and the final degree of conversion (final DC). However, these reductions did not compromise mechanical properties, which were comparable to the BT controls, especially after 7-day water incubation. The incorporation of 60 wt% 2EMATE-BDI reduced water sorption of the composite. 2EMATE-BDI containing formulations showed reduction in polymerization stress of 30% and 50% in comparison to BT control and TEGDMA copolymerizations, respectively. Biofilm formation was similar among the tested groups.SignificanceThe use of the newly synthesized diurethane dimethacrylate as co-monomer in dental resin composite formulations seems to be a promising option to develop polymers with low-shrinkage and potentially decreased water degradation.     

Volumetric polymerization shrinkage and its comparison to internal adaptation in bulk fill and conventional composites: A μCT and OCT in vitro analysis

ObjectiveTo quantify the volumetric polymerization shrinkage (VPS) of different conventional and bulk fill resin composites, through micro-computed tomography (μCT), and qualitative comparison of gap formation through optical coherence tomography (OCT).MethodsBox-shaped class I cavities were prepared in 30 third-molars and divided into 5 groups (n = 6): G1- Filtek Z100 (Z100); G2- Tetric Evoceram Bulk Fill (TEC); G3- Tetric EvoFlow Bulk fill (TEF); G4- Filtek Bulk fill (FBU); and G5- Filtek Bulk fill Flowable (FBF). All groups were treated with Adper Single Bond Plus adhesive and light cured (Bluephase 20i). Each tooth was scanned three times using a μCT apparatus: after cavity preparation (empty scan); after cavity filling (uncured scan) and after light curing of the restorations (cured scan). The μCT images were imported into a three-dimensional rendering software, and volumetric polymerization shrinkage percentage was calculated (%) for each sample. In the same images, interfacial gaps in the pulpal floor were qualitatively evaluated. After μCT evaluation, the pulpal floor from each tooth was polished until a thin tooth structure was obtained and OCT images were obtained by scanning the pulpal portion. Gap formation was observed and qualitatively compared to the μCT images.ResultsVPS means ranged from 2.31 to 3.96% for the studied resin composites. The bulk fill materials, either high viscosity or flowable, were not statistically different from each other (p > 0.05). The conventional resin composite Z100 presented statistically higher VPS than both high viscosity bulk fill materials studied (p < 0.05), although it was statistically similar to the flowable bulk fill materials studied (p > 0.05). Both μCT and OCT methodologies enabled gap formation visualization, and images from both technologies could be associated. Gap formation was mostly observed for G1-Z100, G4-FBU, and G5-FBF. VPS% and pulpal gap formation could not be completely associated with each other for all groups and samples. Voids were observed in most of the resin composite fillings, and most VPS were observed in the occlusal area of the samples.SignificanceVolumetric polymerization shrinkage was material-dependent, although bulk fill materials did not differ from each other. Both μCT and OCT enabled interfacial pulpal gap formation visualization. VPS and gap formation cannot be completely associated with one another.     

Thermal shrinkage reveals the feasibility of pulse-delay photocuring technique

ObjectivesTo resolve the feasibility of the pulse-delay photocuring technique as a clinical strategy for reducing the detrimental polymerization stress induced in dental composites during the photocuring process.MethodsModel dental composites with high and low-filler contents were cured with the pulse-delay photocuring technique using different combinations of photocuring variables (irradiance, exposure time, and delay time). Irradiance used ranged from 0.1 W/cm² to 4 W/cm². The exposure time of the first pulse varied from 0.2 s to 27.2 s and the delay times ranged from 10 s to 120 s. The radiant exposure was varied from 4 J/cm² to 20 J/cm². A cantilever-beam based instrument (NIST Standards Reference Instrument 6005) was used to implement the photocuring technique for the measurement of the polymerization properties (the degree of monomer conversion, polymerization stress induced due to shrinkage, and temperature change due to the reaction exotherm and curing light absorbance) simultaneously in real-time. These properties were compared with those obtained using the conventional photocuring technique (i.e., using a constant irradiance for a fixed exposure time, a uniform exposure).ResultsThere exists a minimum radiant exposure, such that a reduction in the polymerization stress can be achieved without sacrificing the degree of monomer conversion by using the pulse-delay over the conventional photocuring technique. More specifically, stress reductions of up to 19% and 32% was observed with the pulse-delay when compared with the conventional photocuring technique at an irradiance of 0.5 W/cm² and 4 W/cm², respectively. The reduction occurred when the exposure time of the first pulse was greater than, but closer to, the gelation time (i.e., lower than the vitrification time) of the composite, regardless of the delay time used. Lower thermal shrinkage (contraction) during the post-curing time, rather than the stress relaxation during the delay time or lower degree of monomer conversion as claimed in the literature, is the cause of the reduction in the polymerization stress.SignificanceThe study clarifies a long-standing confusion and controversy on the applicability of the pulse-delay photocuring technique for reducing the polymerization stress and promotes its potential clinical success for dental restorative composites.     

Flowability of composites is no guarantee for contraction stress reduction

ObjectivesThe purpose of this study was to measure the contraction stress development of three flowable resin-composite materials (Grandio Flow, VOCO GmbH, Cuxhaven, Germany; Tetric Flow, Ivoclar Vivadent, Schaan, Liechtenstein; Filtek Supreme XT Flowable Restorative, 3 M ESPE, ST. Paul, MN, USA) and an universal micro-hybrid composite resin (Filtek Z250, 3 M ESPE, St. Paul, MN, USA) during photopolymerization with a halogen curing light, using a novel stress-measuring gauge.MethodsCuring shrinkage stress was measured using a stress-analyzer. Composites were polymerized with a halogen curing unit (VIP, Bisco Inc., Schaumburg, IL, USA) for 40 s. The contraction force (N) generated during polymerization was continuously recorded for 180 s after photo-initiation. Contraction stress (MPa) was calculated at 20 s, 40 s, 60 s, 120 s and 180 s. Data were statistically analyzed.ResultsFiltek Supreme XT Flowable Restorative exhibited the highest stress values compared to other materials (p < 0.05), while the lowest values were recorded with Tetric Flow (p < 0.05). Tetric Flow was also the only flowable composite showing stress values lower than the conventional composite Filtek Z250 (p < 0.05).SignificanceFlowable composites investigated with this experimental setup showed shrinkage stress comparable to conventional resin restorative materials, thus supporting the hypothesis that the use of flowable materials do not lead to marked stress reduction and the risk of debonding at the adhesive interface as a result of polymerization contraction is similar for both type of materials.     

Influence of methyl mercaptan at early setting stages on the properties of self curing addition silicone resilient denture lining materials

PurposeThe aim of the present study was to clarify the influence of early setting conditions on the properties of self curing addition silicone resilient denture lining materials.MethodsFour auto-mix types of commercial self curing addition silicone resilient denture lining materials were used. Agar plates containing various concentrations of methyl mercaptan (MMP) were prepared and cylindrical specimens (10 mm in height and 10 mm in diameter) of silicone liners were set on the agar plate. The Shore A hardness of the agar-contacted surface of the specimen was measured at 0, 1, 7 and 28 days after stored in 37 °C water. The viscoelastic properties (instantaneous elastic modulus, coefficient of viscosity and permanent deformation) of two silicone liners, which showed a great decrease in the Shore A hardness by agar contact, were determined by a creep meter.ResultsThe Shore A hardness, coefficient of viscosity, and instantaneous elastic modulus deceased when the specimen was set on the MMP containing agar plate, but the permanent deformation of these specimens increased.ConclusionThe exposure to MMP at the early setting stage influenced the properties of self curing addition silicone resilient denture lining materials.     

Effect of delivering light in specific narrow bandwidths from 394 to 515 nm on the micro-hardness of resin composites

ObjectivesThis study investigated the wavelength-dependent photosensitivity of eleven resin composites (Admira A2, Heliomolar A2, Herculite XRV A2, Pyramid Dentin A2, Solitaire 2 A2, Z250 A2, Ælite LS A2, Vit-l-escence A2, Tetric Ceram Bleach XL, Tetric Ceram A2, Pyramid Enamel Neutral).MethodsResin composites 1.6 mm thick were exposed to narrow bandwidths of light at the following peak wavelengths: 394, 400, 405, 410, 415, 420, 430, 436, 442, 450, 455, 458, 467, 470, 480, 486, 493, 500, 505, and 515 ± 5 nm. A spectroradiometer was used to ensure that the same irradiance (mW/cm²) and total energy density (J/cm²) was delivered through each filter. For each resin composite, three specimens were exposed through each filter. The Knoop micro-hardness at the top and bottom of the composites was then measured. The wavelength-dependent photosensitivity of each resin composite was analyzed by plotting the mean hardness achieved at each wavelength.ResultsThe composites responded variably when they received light through the narrow bandpass filters. Six resin composites had a single peak of wavelength-dependent photosensitivity at ～470 nm. Four resin composites had two peaks of wavelength-dependent photosensitivity at ～470 and ～405 nm. One resin composite had a single peak of wavelength-dependent photosensitivity at ～405 nm and was only sensitive to light below 436 nm.SignificanceUsing light delivered through narrow bandpass filters is a convenient method to determine the wavelength-dependent photosensitivity of resins and can be used to predict the performance of dental curing lights.     

Vertical and horizontal polymerization shrinkage in composite restorations

ObjectivePolymerization shrinkage developed in vertical and horizontal directions after light activation of light-curing composite restorative materials. The purpose of this study was to examine the effects of vertical and horizontal polymerization shrinkage on: (a) dimensional changes of resin composites in tooth cavities; (b) shear bond strengths to enamel and dentin; and (c) marginal gap width in a non-reacting Teflon mold.MethodsVertical and horizontal polymerization shrinkage in tooth cavities were measured immediately (3 min) after light activation. With the same time lapse, shear bond strengths to enamel and dentin and marginal gap widths in Teflon mold were also measured.ResultsThere was a significant correlation between vertical and horizontal polymerization shrinkage (r = 0.647, p = 0.043) in the tooth cavity. Composite materials which produced small vertical shrinkage also produced smaller horizontal shrinkage. Composite materials which produced small vertical shrinkage in the tooth cavity exhibited greater shear bond strengths to both enamel (r = −0.697, p = 0.025) and dentin (r = −0.752, p = 0.012). Composite materials which produced smaller horizontal shrinkage produced smaller marginal gap widths in the Teflon mold (r = 0.829, p = 0.003). No relationships were observed between horizontal shrinkage in the tooth cavity and shear bond strengths to both enamel and dentin (p > 0.05).SignificanceDuring the early stage of setting (<3 min) in tooth cavities, the vertical shrinkage of light-activated composite restorative materials was correlated with horizontal shrinkage.     

Effect of time on the post-irradiation curing of six resin-based composites

ObjectivesTo evaluate the effect of time on the Vickers microhardness (VH) at the top and bottom surfaces of six conventional resin-based composites (RBCs) up to twelve weeks after light curing.MethodsFive specimens of Filtek Supreme Ultra, Herculite Ultra, Mosaic Ultra, Tetric EvoCeram, TPH Spectra HV, and Venus Pearl were packed into opaque molds that were 2.3 mm in diameter and 2.5 mm deep. The uncured RBC specimens were covered by a polyester strip and photo-cured with an Elipar DeepCure-S light-curing unit (LCU) according to the manufacturer's instructions. After irradiation, the polyester strip was removed, and the Vickers microhardness was measured immediately at top and bottom surfaces. The hardness measurements were repeated after 30 min, 1 h, 2 h, 4 h, 24 h, 1 week, 4 weeks, and 12 weeks. In between, the specimens were stored in dry and dark conditions at 37 °C. Two-way ANOVA (α = 0.05) followed by Tukey–Kramer post hoc multiple comparison tests were used to determine where statistically significant differences existed.ResultsThe micro-hardness values at the top surface always exceeded those at the bottom surface. A significant logarithmic increase of the micro-hardness due to post-irradiation curing took place between 30 min and 24 h (p < 0.05). There was no significant increase in the VH after 24 h. Depending on the RBC, compared to the immediate values the hardness 24 h post-irradiation had increased by 11–27% at the top surface and by 21–58% at the bottom.SignificanceEven after 12 weeks, the bottom hardness values never reached the top microhardness values. The results of studies that wait 24 h or longer before measuring the properties of RBC specimens will be significantly enhanced by the impact of post-irradiation curing. Especially within the first 4 h, the time when specimens are measured is critical information and should be reported.     

Mathematical model for assessing true irradiance received by luting materials while curing through modern CAD/CAM resin composites

Statement of problemMeasurement of irradiance passing through a dental restoration for properly curing a dual- or light-polymerized luting composite is imprecise due to surface reflection.ObjectiveTo provide a mathematical correction of measured transmitted irradiance for predicting true transmitted light intensity through CAD/CAM restorations.MethodsA total of 432 specimens were fabricated. Seven modern CAD/CAM resin-based composites (RBCs) and one CAD/CAM glass-ceramic (control group) were sectioned and polished into specimens of 0.5–5 mm thickness (in 0.5 mm steps, n = 6). Irradiance of a violet-blue LED light curing unit (LCU) (power modes: Standard, High and Plasma) was measured after passing through each specimen with a spectrometer. Data was compared based on 95% confidence intervals and using univariate ANOVA followed by Tukey HSD (α = 0.05).ResultsThe measured transmitted irradiance passing through the specimens decreased exponentially. Significantly highest values of transmitted irradiance were measured for 0.5 mm thick specimens for all materials (p < 0.05). The decadic absorption coefficient for CAD/CAM-RBCs ranged from 0.292 mm⁻¹ to 0.387 mm⁻¹ while the control group (glass-ceramic) reached a significantly lower value of 0.283 mm⁻¹. The reflection ratio for all materials ranged from 12.6% to 18.5%.SignificanceA correction can be implemented to predict the true transmitted irradiance after passing through a dental restoration as function of initial irradiance, specimen thickness and material specific parameters. For a practitioner, this model may be applied depending on the specific treatment conditions, the individual LCU's radiant emittance and restoration thickness for the tested materials.     

Mechanical properties of dental resin/composite containing urchin-like hydroxyapatite

ObjectivesTo investigate the reinforcing effect of urchin-like hydroxyapatite (UHA) in bisphenol A glycidyl methacrylate (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA) dental resin (without silica nanoparticles) and dental composites (with silica nanoparticles), and explore the effect of HA filler morphologies and loadings on the mechanical properties.MethodsUHA was synthesized by a facile method of microwave irradiation and studied by X-ray diffraction (XRD), scanning electron microscope (SEM), and thermogravimetric analysis (TGA). Mechanical properties of the dental resin composites containing silanized UHA were tested by a universal mechanical testing machine. Analysis of variance was used for the statistical analysis of the acquired data. The fracture morphologies of tested composites were observed by SEM. Composites with silanized irregular particulate hydroxyapatite (IPHA) and hydroxyapatite whisker (HW) were prepared for comparative studies.ResultsImpregnation of lower loadings (5 wt% and 10 wt%) of silanized UHA into dental resin (without silica nanoparticles) substantially improved the mechanical properties; higher UHA loadings (20 wt% and 30 wt%) of impregnation continuously improved the flexural modulus and microhardness, while the strength would no longer be increased. Compared with silanized IPHA and HW, silanized UHA consisting of rods extending radially from center were embedded into the matrix closely and well dispersed in the composite, increasing filler-matrix interfacial contact area and combination. At higher filler loadings, UHA interlaced together tightly without affecting the mobility of monomer inside, which might bear higher loads during fracture of the composite, leading to higher strengths than those of dental resins with IPHA and HW. Besides, impregnation of silanized UHA into dental composites (with silica nanoparticles) significantly improved the strength and modulus.SignificanceUHA could serve as novel reinforcing HA filler to improve the mechanical properties of dental resin and dental composite.     

The viscoelastic behavior of dental adhesives: A nanoindentation study

ObjectivesIn order to predict the long-term performance of dental adhesives, it is necessary to understand their mechanical properties. The objective of this study was to use a new nanoindentation technique to characterize the in-plane linear viscoelastic properties of dental adhesive layers.MethodsThe dental adhesives used were Clearfil SE Bond (CSE) and Clearfil Tri-S Bond (CTS) by Kuraray Medical and Single Bond (SIB) and Single Bond Plus (SBP) by 3M ESPE. A thin film of each adhesive was made on a micro-glass slide, and was then tested on a nanoindenter system (ENT 1100, Elionix) with a Berkovich indenter at a constant loading rate of 0.1 mN/s up to a maximum load of 1.8 mN. The load–displacement data of the loading segment were fitted to a curve to find best fit parameters for a generalized Kelvin viscoelastic model, from which creep compliance and Young's modulus were calculated. The modulus results were compared to the values calculated by the nanoindentation device.ResultsThe experimental data fitted well to the viscoelastic model for all materials (R > 0.9999). SIB and CTS showed higher creep compliance compared to SBP and CSE. The modulus values obtained using the model were 4.0, 2.6, 2.4 and 4.2 GPa for CSE, CTS, SIB and SBP, respectively. The nanoindentation default software designed for time-independent materials significantly overestimated the modulus values up to 2.5 times.ConclusionAs generally expected for polymer materials, the adhesives tested showed time-dependent viscoelastic behavior. The mechanical evaluation techniques developed for time-independent materials ignore this behavior and may not be appropriate for dental adhesives.     

Influence of curing modes on the degree of conversion and mechanical parameters of dual-cured luting agents

PurposeTo investigate the effects of different curing modes, including tack cure, on the degree of conversion (DC) and mechanical parameters of dual-cured luting agents for all-ceramic restorations.MethodsImmediate light curing, intermittent light curing (2-s tack cure and a 1-min interval before the main cure), delayed light cuing (2-min delay) and chemical or no light curing were used to cure two dual-cured luting agents, RelyX Unicem and PermaCem 2.0, through a 1.5-mm thick lithium disilicate ceramic slide. DC (n = 3), micro-hardness (n = 5), shrinkage strain (n = 4) and shrinkage stress (n = 3) were measured under the aforementioned curing modes. The data were analyzed using two-way ANOVA and post-hoc Tukey HSD test, with the level of significance set at α = 0.05.ResultsFor both luting agents, all the light-curing modes produced similar final DC, but using chemical cure only could significantly reduce the DC. The mechanical parameters followed a similar pattern. There were positive but nonlinear correlations between DC and the other mechanical parameters, with the increase in these parameters with DC being slower initially.ConclusionsProvided adequate light curing is applied to a dual-cured luting agent, delaying the light curing or using a tack cure first to facilitate seating of a restoration may not have a significant impact on the luting agent’s final degree of conversion. However, using chemical cure only may result in inadequate cure of the luting agent and is recommended only for highly opaque restorations.     

Effect of different curing condition on material properties of acrylic resin for orthodontic appliances

The aim of this study was to evaluate the influence of different curing conditions on degree of conversion, glass transition temperature (Tg), microhardness and impact strength of an autopolymerizing acrylic resin. Twenty-one acrylic resin baseplates with wire clasps simulating removable appliances were produced according to three curing conditions and allocated in three groups (n = 7): Group A—curing in air during 1 h at room temperature and pressure; Group M—treated as in Group A plus additional heating by microwave energy (500 W; 3 min); Group TP—curing in water at 50 °C, during 15 min under 20 psi pressure. Microhardness was measured using a Knoop microhardness indenter, and the degree of conversion and Tg were determined by differential scanning calorimetry (DSC). For impact strength test, 12 non-notched specimens were fabricated per group according to ASTM D256 standard. Data were analyzed statistically by one-way ANOVA and Tukey's test (p < 0.05). Group TP presented the highest microhardness means (p < 0.05), followed by Groups M and A. Group TP showed statistically significant differences on impact strength means (p < 0.05) compared to Groups M and A, which, in turn, did not differ significantly to each other (p > 0.05). No significant differences were found among the Tg values of the three groups (p > 0.05). A better performance may be expected when autopolymerizing acrylic resins are cured under controlled heat and pressure conditions.