Excretion of dental resin monomers and metabolic intermediates via urine in guinea pigs

ObjectiveMonomers like BisGMA (Bisphenol-A-glycidyldimethacrylate) and comonomers like TEGDMA (triethyleneglycoldimethacrylate) are used in dental restorative materials in order to build up the three-dimensional network of filling materials. Since earlier investigations revealed uptake and subsequent metabolism of unpolymerized remainders of (co)monomers, the present experiment investigates the metabolic urine pattern of guinea pigs (n = 4) after application of TEGDMA or BisGMA (each dose = 0.02 mmol/kg body weight = 100%), respectively.MethodsFor the investigations BisGMA was pre-dissolved in DMSO and subsequently diluted with 0.9% NaCl solution (final DMSO concentration 1%) and TEGDMA was dissolved in 0.9% NaCl solution. The solutions were administered with a gastric tube into the animals. Control animals received either 0.9% NaCl or 0.9% NaCl solution with 1% DMSO solution.ResultsAfter 24 h in collected urine the following metabolites were identified. After administration of TEGDMA (mean relative concentration of administered substances) ± s.d. [%]; n = 4): unchanged TEGDMA: 12 ± 1.5%, MA: 2.4 ± 0.8%, and triethyleneglycol: 35 ± 2.2%. After administration of BisGMA (mean ± s.d. [%]; n = 4): unchanged BisGMA: 11.4 ± 2.7%, MA: 2.2 ± 0.6%, and bisphenol-A-bis(2,3-dihydroxypropyl)ether: 60.1 ± 5.2%).ConclusionNo further metabolites like the previously identified intermediate 2,3-epoxymethacrylic acid and derived reaction products were identified in the urine, indicating that these metabolites must have reacted further.     

Effect of bleaching on the elution of monomers from modern dental composite materials

ObjectiveThe aim of this study was to evaluate the effect of bleaching on the elution of monomers from two modern composite materials.MethodsTwo different resin composites (a nanohybrid the Filtek Supreme? XT and an ormocer the Ceram X?) were bleached with two products (hydrogen peroxide 38% for 45 min and carbamide peroxide 15% for 56 h). Four groups (n = 10, diameter: 4.5 mm, thickness: 2 mm) of each material were fabricated, two for each bleaching product. One group was used as unbleached control and the other one was bleached. Then the samples were stored in 1 ml of 75 vol% ethanol at room temperature, and the storage medium was renewed after 24 h, 7 days, and 28 days. From the storage medium that was removed samples were prepared and analysed with LC–MS/MS.ResultsNone of the bleaching products had an effect on the amount of Bis-GMA and TEGDMA released from Ceram X?. The amount of Bisphenol A released from the bleached samples of Ceram X? was significantly lower compared to the control samples. Bleaching reduced significantly the amount of Bis-GMA released from Filtek Supreme? XT. The amount of TEGDMA released from Filtek Supreme? XT was not affected by bleaching.SignificanceThe bleaching agents tested in the present study reduced the amount of some of the monomers released from the two composite materials. Bleaching of modern composite materials does not increase the release of monomers.     

Cytotoxic effects of dental bonding substances as a function of degree of conversion

ObjectivesRecently, we found that dental bonding substances tested alone or in combination with composites are far more cytotoxic than composite materials alone. These data are in line with several cytotoxicity reports but contradict in vivo studies showing the beneficial effects of bonding materials. The aims of the present study were to develop a preparation method for composite specimens modelling conditions in the oral cavity and to analyse the influence of bonding substances on the cytotoxicity of six different composite materials.MethodsCylindrical composite specimens were prepared in polyethylene blocks containing 5 mm diameter cylindrical holes (cylinder height 2 mm), covered with a polyethylene foil and light cured from one end for 40 s. In a second series of experiments, composite specimens were combined with bonding materials. Bonding was applied onto the polyethylene foil in one or two layers and light cured according to the manufacturers’ instructions. Subsequently, polyethylene moulds were placed on top of the bonding materials and composites prepared as described above. After unilateral light curing from the top of the cylindrical holes, visual confirmation of adherence at the base was obtained. Specimens were added to the cultures immediately after production or after preincubation for 7 days under cell culture conditions. Specimens were incubated with L-929 fibroblasts for 72 h and cell numbers determined by flow cytometry. To evaluate the degree of conversion (DC) of bonding materials cured with and without air inhibition a third series of experiments was performed. FTIR spectroscopic measurements were made on thin-films of dentin-bonding agents, cured under both an-aerobic and aerobic conditions, to determine degree of conversion.ResultsCytotoxicities of all six tested composites were significantly different (p < 0.0001) and diminished after 7 days of preincubation (p < 0.0001). Bonding substances had no statistically significant influence on the cytotoxicity of composite materials (p = 0.159). A highly significant statistical reduction in the degree of conversion for each resin cured under air inhibition conditions was documented (p < 0.01).SignificanceOur study demonstrates that cell culture toxicity data are highly model dependent and that internationally standardized test protocols for toxicity screening of dental materials in line with the existing standards are clearly needed to obtain comparable results.     

Effect of the addition of thiourethane oligomers on the sol⿿gel composition of BisGMA/TEGDMA polymer networks

ObjectivesThiourethane oligomers have been shown to increase the fracture toughness and reduce the polymerization stress of methacrylate-based materials. However, network formation has not been elucidated in these materials yet. The aim of this study was to evaluate how the addition of a thiourethane oligomer (TU) influences the sol/gel composition and network structure of methacrylate-based materials using dynamic mechanical analysis and extraction methods.Materials and methodsBisGMA/TEGDMA at systematically varied mass ratios (20/80 to 80/20 wt%) were mixed with pre-polymerized thiourethane oligomers at 0 (control) or 20 wt%, synthesized by combining pentaerythritol tetra-3-mercaptopropionate with dicyclohexylmethane 4,4⿲-Diisocyanate, at 1:2 isocyanate:thiol. 0.1 wt% of 2,2-Dimethoxy-2-phenylacetophenone was added as the photoinitiator and 0.3 wt% of 2,6-di-tert-butyl-4-methylphenol was added as a free radical inhibitor. Disk specimens (0.8 ÿ 10 mm in diameter, n = 3) were photoactivated at 270 mW/ (320⿿500 nm) for 1 min. The degree of conversion (DC) was measured in near-IR (˿6165 cm^⿿1). Specimens were immersed in two different solvents (water for 7 days or dicholoromethane for 48 h). Water sorption (WS) and solubility (SL) were obtained according to ISO 4049. The leachates for both solutions were analyzed with ¹H-NMR (400 MHz, CDCL₃). Bar specimens (1 ÿ 3 ÿ 25 mm, photocured and then post-processed at 180 °C for 8 h to DC > 95%) were subjected to dynamic mechanical analysis (⿿30 to 230 °C) to obtain glass transition temperature (Tg), tan delta curves and crosslinking density (ν). Data was analyzed with two-way ANOVA/Tukey⿿s test (95%).ResultsIn general, the presence of TU increased the overall conversion. The WS was similar for all groups, but the SL decreased by 2-fold with the addition of the TU oligomer for all compositions, except BisGMA/TEGDMA 80/20. The BisGMA concentration of the leachates increased with increasing BisGMA in the initial mixture, and with the presence of thiourethane. This compositional drift of the gel with the presence of TU was attributed to the preferential dissolution of TEGDMA into the TU network. Tg and ν decreased with the addition of TU, as expected. The addition of TU produced more homogeneous networks, as evidenced by narrower breadth of the tan delta curve.ConclusionThe addition of TU affected the composition of the sol/gel in crosslinked networks, which were more homogeneous and presented 2-fold less potentially toxic leachates than the methacrylate controls.Clinical significanceThe addition of TU may produce less cytotoxic materials based on the increased conversion and reduced amount of unreacted extractables from its network after water storage.     

In vitro stability of methylmethacrylic acid,TEGDMA and HEMA exposed to esterases

ObjectivesComonomers used in dental restorative materials, e.g. triethylenglycoledimethacrylate (TEGDMA), hydroxyethylmethacrylate (HEMA) and methylmethacylic acid (MMA) are methacrylic acid esters. Because of the chemical structure of these esters in earlier studies two different in vitro pathways were suggested. The present study was performed in order to analyze which of these chemical pathways is preferred in vitro: (a) saponification of TEGDMA, HEMA, and MMA leading to free methacrylic acid (MA) as an ionic intermediate or (b) enzymatically epoxidation of mother compounds leading to lipophilic intermediates.MethodsExperiments have been performed in an isolated system using MMA, TEGDMA or HEMA, respectively exposed to pig liver esterase (PLE) in phosphate buffer. The reaction of tested comonomers was terminated by the use of sodium hydroxide solution, sulfuric acid or saturated NaCl solution, respectively. The samples were analyzed by the use of headspace-gas chromatography–mass spectrometry.ResultsIn all samples a decomposition of comonomers by the use of PLE was observed. Due to the high rate constant k of MMA (e.g. k_MMA > 1.3 mg/(l s)) epoxidation of non-cleaved molecules of this substance can be excluded. Compared to MMA the decomposition of TEGDMA and HEMA is significantly slower (e.g. k_{(TEGDMA, PLE = 7 units/ml)} = 0.004 mg/(l s) and k_{(HEMA, PLE = 7 units/ml)} = 0.00013 mg/(l s)). Thus in case of a low liver enzyme concentration the epoxidation of non-cleaved molecules of TEGDMA and HEMA cannot be excluded.SignificanceIn the metabolic pathway of TEGDMA and HEMA the probability of an auxiliary chemical pathway was demonstrated. In case of MMA the formation of epoxidated metabolites can be excluded. In contrast to this the chemical pathway for TEGDMA and HEMA might lead to lipophilic intermediates which can be accumulated in fatty tissue.     

Methacrylamide–methacrylate hybrid monomers for dental applications

ObjectivesThe susceptibility of methacrylates to hydrolytic and enzymatic degradation may be a contributing factor limiting the clinical lifespan of resin composite restorations. The elimination of labile ester bonds is a potential advantage of methacrylamides, which have been shown to produce more stable restorative interfaces. The rationale of this study is to design hydrolytically and enzymatically stable adhesive monomers, with the added benefit of being able to form crosslinked networks. The objective of this study was to synthesize difunctional, hybrid methacrylate-methacrylamide monomers, and evaluate them as potential monomers for dental adhesives.Materials and methodsHEMA, TEGDMA (controls) or secondary methacrylamides (HEMAM – commercially available, 2EM and 2dMM – newly synthesized) either bearing a hydroxyl group or a methacrylate functionality (Hybrids-Hy), were added at 40 mass% to bisGMA. The photoinitiator system consisted of 2-dimethoxyphenyl acetophenone (DMPA) and diphenyl iodonium hexafluorophosphate (DPI-PF6) at 0.2 and 0.4 mass%, respectively. Polymerization kinetics were followed in real-time by near-IR spectroscopy during light activation at 630 mW/cm² for 300 s. Water sorption and solubility (WS, SL) were measured according to ISO 4049. Storage modulus in shear (G′) for 300 s was obtained by oscillatory rheometry. For the microtensile bond strength (μTBS), fully formulated adhesives containing 40 vol% ethanol were used to restore caries-free human third molars. Bonded specimens with 1 mm² cross-sectional area were tested after 48 h and 6 months storage in water at 37 °C. Single bond (SB) was tested as a commercial control. Data were analysed with one-way ANOVA and Tukey's test and Student's t-test (α = 0.05).ResultsIn general, hybrid versions showed lower polymerization rate and degree of conversion, whereas the methacrylate controls, HEMA and TEGDMA, showed the highest values. The hybrid versions showed lower values of WS and SL than their monofunctional versions. HEMAM Hy showed the highest values of G′ and TEGDMA, 2EM, and 2dMM-Hy the lowest. The μTBS values between 48 h and 6 months were statistically reduced only for the HEMA and both 2dMM materials. The formulation containing the monofunctional methacrylamide (HEMAM) showed only 9% reduction in μTBS after 6 months of aging, while the other groups showed a decrease ranging between 18% and 33%.ConclusionOverall, hybrid monomers showed lower reactivity than their analogous monofunctional versions, but had markedly lower water sorption. Shear storage modulus was affected differently by the addition of the second functionality. HEMAM-containing systems were able to maintain stable long-term dentin bond strength, which demonstrates that bonding stability is a result of the complex interplay among the factors studied.Clinical significanceThe novel monomers showed here are potential alternatives to the current methacrylate adhesives, with selected formulations presenting greater bond stability.     

Osseointegration of fiber-reinforced composite implants: Histological and ultrastructural observations

ObjectivesThe aim of this study was to evaluate the bone tissue response to fiber-reinforced composite (FRC) in comparison with titanium (Ti) implants after 12 weeks of implantation in cancellous bone using histomorphometric and ultrastructural analysis.Materials and methodsThirty grit-blasted cylindrical FRC implants with BisGMA–TEGDMA polymer matrix were fabricated and divided into three groups: (1) 60 s light-cured FRC (FRC-L group), (2) 24 h polymerized FRC (FRC group), and (3) bioactive glass FRC (FRC–BAG group). Titanium implants were used as a control group. The surface analyses were performed with scanning electron microscopy and 3D SEM. The bone–implant contact (BIC) and bone area (BA) were determined using histomorphometry and SEM. Transmission electron microscopy (TEM) was performed on Focused Ion Beam prepared samples of the intact bone–implant interface.ResultsThe FRC, FRC–BAG and Ti implants were integrated into host bone. In contrast, FRC-L implants had a consistent fibrous capsule around the circumference of the entire implant separating the implant from direct bone contact. The highest values of BIC were obtained with FRC–BAG (58 ± 11%) and Ti implants (54 ± 13%), followed by FRC implants (48 ± 10%), but no significant differences in BIC or BA were observed (p = 0.07, p = 0.06, respectively). TEM images showed a direct contact between nanocrystalline hydroxyapatite of bone and both FRC and FRC–BAG surfaces.ConclusionFiber-reinforced composite implants are capable of establishing a close bone contact comparable with the osseointegration of titanium implants having similar surface roughness.     

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.     

Effect of residual solvent on performance of acrylamide-containing dental materials

ObjectiveMethacrylamide-based monomers are being pursued as novel, hydrolytically stable materials for use in dental adhesives. The impact of residual solvents, due to the chemical synthesis procedures or the need for solvated adhesives systems, on the kinetics of polymerization and mechanical properties was the aim of the present investigation.MethodsTwo base monomers (70 wt% BisGMA or HEMAM-BDI — newly synthesized secondary methacrylamide) were combined with 30 wt% N,N-dimethylacrylamide. Eethyl acetate (EtOAc), or 75 vol% ethanol/25 vol% water (EtOH/H₂O) were added as solvents in concentrations of 2, 5, 15 and 20 wt%. The resins were made polymerizable by the addition of 0.2 wt% 2,2-dimethoxy-2-phenyl acetophenone (DMPA) and 0.4 wt% diphenyliodonium hexafluorophosphate (DPI-PF6). Specimens (n = 3) were photoactivated with a mercury arc lamp (Acticure 4000, 320–500 nm, 250 mW/cm²) for 5 min. Degree of conversion (DC, %) was tracked in near-IR spectroscopy in real time and yield strength and modulus of elasticity were measured in three-point bending after dry and wet storage (n = 6). The data was subject to one-way ANOVA/Tukey’s Test (p ≤ 0.05), or Student’s t-test (p ≤ 0.001).ResultsIn all groups for both BisGMA and HEMAM-BDI-based materials, DC and DC at Rp_max increased and maximum rate of polymerization decreased as solvent concentration increased. Despite the increased DC, BisGMA mixtures showed a decrease in FS starting at 5 wt% EtOAc or 15 wt% EtOH/H₂O. Yield strength for the HEMAM-BDI groups was overall lower than that of the BisGMA groups, but the modulus of elasticity was significantly higher.SignificanceThe presence of residual solvent, from manufacturing or from practitioner’s handling, affects polymerization kinetics and mechanical properties of resins. Methacrylates appear to be more strongly influenced than methacrylamides.     

Influence of photoinitiator system and nanofiller size on the optical properties and cure efficiency of model composites

ObjectiveTo establish the relationship between photoinitiator system and nanofiller size on the optical properties and cure efficiency of model composites.MethodsModel composites based on BisGMA/TEGDMA (60:40 mol%) were loaded with 40 wt% of 7 nm or 16 nm-sized filler particles. One of the following photoinitiator systems was added: camphorquinone (CQ) associated with an amine (EDMAB), monoacylphosphine oxide (TPO), or bysacylphosphine oxide (BAPO). The optical properties of disk-shaped specimens were measured 24 h after curing and repeated after storage in water for 90 days and coffee for 15 days. A large spectrum LED unit (Bluephase G2, Ivoclar Vivadent) was used for photoactivation. CIE L*a*b* parameters, color difference (ΔE), and translucency parameter (TP) were calculated. Knoop hardness readings were taken at top and bottom composite surfaces. Cure efficiency was determined by bottom/top hardness ratio. Data were statistically analyzed at α = 0.05 significance level.ResultsComposites formulated with 16 nm particles had higher CIE L* than those with 7 nm particles in all storage conditions. BAPO-based composites generally had lower CIE a* than the other composites. The group TPO + 16 nm before storage and all groups with 16 nm-sized particles after storage had lower CIE b* (i.e. lower degree of yellowing) than the other groups. TPO-based materials had higher color stability. The cure efficiency was not significantly affected by photoinitiator system or particle size. CQ + 7 nm had the lowest and BAPO + 16 nm the highest hardness values.SignificanceCombination of photoinitiator system and filler particle size might affect the optical properties of composites, with low influence on cure efficiency.     

Effect of resin matrix composition on the translucency of experimental dental composite resins

ObjectivesThe purpose of this study was to investigate the effect of the resin matrix composition on the translucency of experimental dental composite resins.MethodsThree types of unfilled resin matrices (TEGDMA-, UDMA- and BisGMA-based) were formulated and light cured. In addition, six different experimental dental composite resins with constant filler loading but varying in the type of monomer and the content of BisGMA were fabricated. Discs of each test material with 15.5 mm diameter and 1.0 mm thickness were prepared (N = 3) and light cured. Total and diffuse transmittance values for each sample were measured using a UV/VIS spectrophotometer with the range of readings from 380 to 700 nm. Difference in color was measured using the CIE Lab system.ResultsStatistical analysis by one-way ANOVA followed by Tukey's test showed that there was no statistically significant difference in transmittance values between the three unfilled resins. However, with the addition of filler, BisGMA-containing composite resins showed significantly higher transmittance values than the UDMA- and TEGDMA-based composite resins. Regression analysis revealed that there was a linear correlation between the percentage of BisGMA in the resin matrix and the total and diffuse translucency.SignificanceThe amount of BisGMA used in the resin matrix has a significant effect on the translucency of silica filler-containing dental composite resins.     

Experimental composites of polyacrilonitrile-electrospun nanofibers containing nanocrystal cellulose

ObjectiveTo test the effects of addition of polyacrilonitrile (PAN) nanofibers and nanocrystal cellulose (NCC)-containing PAN nanofibers on flexural properties of experimental dental composites.Methods11 wt% PAN in dimethylformamide (DMF) solution was electrospun at 17.2 kVA and 20 cm from the collector drum. NCC was added to the solution at 3 wt%. Fiber mats were produced in triplicates and tested as-spun. Strips (5 cm × 0.5 cm) were cut from the mat in an orientation parallel and perpendicular to the rotational direction of the collector drum. Tensile tests were performed and ultimate tensile strength (UTS), elastic modulus (E) and elongation at maximum stress (%) were calculated from stress/strain plots. Fiber mats were then infiltrated by resin monomers (50/50 BisGMA/TEGDMA wt%), stacked in a mold (2 × 15 × 25) and light-cured. Beams (2 × 2 × 25 mm) were cut from the slabs and tested in a universal testing machine. Data were analyzed by multiple t-test and one-way ANOVA (α = 0.05).ResultsAddition of 3% NCC resulted in higher tensile properties of the fibers. Fibers presented anisotropic behavior with higher UTS and E when tested in perpendicular orientation. The incorporation of 3% NCC–PAN nanofibers resulted in significant increase in work of fracture and flexural strength of experimental dental composite beams.SignificanceNCC was found to be a suitable nanoparticle to reinforce experimental dental composites by incorporation via nanofiber. This fundamental study warrants future investigation in the use of electrospun nanofibres as a way to reinforce dental composites.     

The influence of light-curing time on the bacterial colonization of resin composite surfaces

ObjectivesBacterial colonization of composite surfaces represents the main factor in the etiology of secondary caries around adhesive restorations. The authors’ aim was to evaluate the influence of light-curing time on mutans streptococci colonization (MS) of a resin composite material.MethodsSpecimens obtained from a dental resin composite were divided into 12 groups and light-cured with the same light source respectively for 10, 20, 30, 40, 60 or 80 s using two different curing-power levels: 400 and 800 mW/cm². A wild strain of MS was isolated and a 24-h-monospecific biofilm, adherent to the surfaces of the samples, was obtained. A colorimetric technique (MTT assay), based on the reduction of a yellow tetrazolium salt to a purple formazan, was used to evaluate the biomass adherent to the specimen surfaces. ANOVA and Scheffé’s tests were used to statistically analyze the results.ResultsTwo-way ANOVA demonstrated there was no interaction between curing-time factor and curing-power factor (p = 0.970); one-way ANOVA was used to analyze separately the data obtained from each curing-power level. Both levels showed highly significant differences (p < 0.0001) among the different curing time groups. The non-parametric test for trend showed in both levels the existence of a highly significant trend (p < 0.0001) for bacterial colonization reduction as curing time increases.SignificanceA reduced curing time seems to be responsible for increased in vitro colonization of composite surfaces by MS; this phenomenon is likely to be related to the presence of unpolymerized monomers on the material surface.     

Effects of monomer ratios and highly radiopaque fillers on degree of conversion and shrinkage-strain of dental resin composites

ObjectivesThe degree of conversion (DC) and polymerization shrinkage of resin composites are closely related manifestations of the same process. Ideal dental composite would show an optimal degree of conversion and minimal polymerization shrinkage. These seem to be antagonistic goals, as an increase in monomer conversion leads to a high polymerization shrinkage. This paper aims to determine the effect of opaque mineral fillers and monomer ratios on the DC and the shrinkage-strain of experimental composites based on (BisGMA/TEGDMA) monomers (traditionally used monomers). A relationship between the shrinkage-strain and the degree of conversion values was also investigated. The radiopacity of these experimental composites has been investigated in a previous paper.MethodsExperimental resin composites were prepared by mixing different monomer ratios of (BisGMA/TEGDMA) with Camphoroquinone and dimethyl aminoethyl methacrylate (DMAEMA) as photo-initiator system. Five different radiopacifying filler agents: La₂O₃, BaO, BaSO₄, SrO and ZrO₂ at various volume fractions ranging from 0 to 80 wt.% were added to the mixture. The degree of conversion of experimental composites containing different opaque fillers contents was measured using FTIR/ATR spectroscopy. The shrinkage-strain of specimens, photopolymerized at circa 500 mW/cm², was measured using the bonded-disk technique at room temperature with respect to time.ResultsThe result revealed that the DC and the shrinkage-strain decrease slightly with the increasing of opaque fillers loadings, but this decrease is not significant. However, these two properties are closely related to the monomer concentration of the organic matrix. The results have also showed a linear correlation between the shrinkage-strain and DC of experimental composites investigated.SignificanceThe nature and the volume effects of the opaque fillers on the DC and shrinkage of the experimental composites investigated were not significant. However, this study has confirmed the importance of viscosity in the system and shrinkage behavior of dimethacrylate monomers studied. Then we confirmed that direct relationships linked the shrinkage and the DC of filled dental resin composites.     

Color change of some aesthetic dental materials: Effect of immersion solutions and finishing of their surfaces

ObjectivesThe purpose of this research was to evaluate the color change of five aesthetic dental materials, before and after immersion in distilled water and blue food color solution for 7 and 21 days, and to study the effect of finishing the surfaces on any color change.MethodsDisc shaped samples of five types of light curing composite (A2) (n = 10 samples/composite) were prepared and all samples were light-cured with a Plasma Arc light cure unit for ten seconds. One side of each sample disc was finished and polished with a Super-Snap system all samples. After 24 h, color measurements of each sample were conducted using a digital spectrophotometer. Five sample discs from each composite group were immersed in 30 ml of food color solution for 7 and 21 days, while the remaining five sample discs were immersed in 30 ml of distilled water as a control. Color measurements were repeated for all samples at 7 and 21 days after immersion. The color changes were statistically analyzed using t-tests within the same group. A result was considered statistically significant at α = 0.05.ResultsThe color differences (ΔE) ranged from 0.4 to 4.66 and statistically significant differences on the finished and unfinished surfaces were observed after immersion in the food color solution for 7 days. No significant differences were found in any group after immersion in the food color solution for 21 days. The Tetric EvoCeram and Arabesk groups showed less color differences after 7 and 21 days than other composites.ConclusionFinished composite surfaces showed less coloration than unfinished surfaces after 7 days, but all surfaces (finished and unfinished) were highly colored for all composite types after 21 days.     

Dental pulp stem cells’ secretome enhances pulp repair processes and compensates TEGDMA-induced cytotoxicity

ObjectivesAim of this study was to investigate the effects of dental pulp stem cells’ (DPSCs) secretome, expressed through their culture conditioned medium (CM), on biological endpoints related to pulp repair and on TEGDMA-induced cytotoxicity.MethodsDPSCs cultures were established and characterized for stem cell markers with flow cytometry. CM was collected from DPSCs under serum deprivation conditions (SDC) and normal serum conditions (NSC) at various time-points. CM effects on DPSCs viability, migration and mineralization potential were evaluated by MTT assay, transwell insert and in vitro scratch assay and Alizarin Red staining/quantification respectively. TEGDMA (0.25–2.0 mM) cytotoxicity regarding the same biological endpoints was tested in the presence/absence of CM. TGF-β1 and FGF-2 secretion in CM was measured by ELISA.ResultsCM collected under SDC (4 d) was able to increase cell viability by 20–25% and to reduce TEGDMA cytotoxicity by 20% (p < 0.05). CM positive effects were not obvious when collected under NSC. Transwell assay showed significant increase (26%, p < 0.05) of DPSCs’ migration after CM exposure, whereas both migration assays could not support a migration rate improvement in TEGDMA-treated cultures exposed to CM compared to TEGDMA alone. CM significantly (p < 0.01) increased DPSCs mineralization potential and completely counteracted TEGDMA cytotoxicity on this process. ELISA analysis showed a time-dependent increase of TGF-β1 and a TEGDMA concentration-dependent increase of both TGF-β1 and FGF-2 in CM.SignificanceThese findings suggest that DPSCs secretome increases their viability, migration and mineralization potential and counteracts TEGDMA-induced cytotoxicy, revealing a novel mechanism of DPSCs autocrine signaling on pulp repair processes.     

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.     

Detection of Bisphenol A in dental wastewater after grinding of dental resin composites

ObjectivesThis study evaluated the release of bisphenol A (BPA) in wastewater after grinding of resin composites and tested three filtration materials.MethodsThree resin composites (Ceram X, Filtek Supreme XTE and Core-X flow) were used. Samples (5 mm × 2 mm, n = 10) were prepared using a metal mold and were polymerized for 20 s according to manufacturers’ instructions. A dental unit was disconnected from wastewater circulation and composite samples were ground under standardized procedures (200,000 rpm; 90 s). Wastewater was collected in glass bottles. Water samples were collected as control by performing the same procedure without grinding resin composite. All samples were stored at 7 °C for 6 months to simulate storage. Then they were analyzed by HPLC–FLD. Three filtration materials (Zeosorb, Katalox Light and Catalytic Carbon) were used for water treatment to remove BPA. BPA-water solutions were prepared; corresponding to the highest amount released by the resin composites. These solutions were analyzed before and after filtration by HPLC–FLD and their efficacy (%) was calculated.ResultsBPA was detected in all composite solutions: Ceram X and Filtek Supreme XTE showed similar findings (p > 0.05) which were significantly higher than the control (p < 0.001) and Core-X flow (p = 0.001). The efficacy of the filtration materials was: Katalox Light (5.09%) < Zeosorb (7.91%) < Catalytic Carbon (99.38%). Only Catalytic Carbon caused a clinically significant reduction of BPA (p < 0.05).SignificanceBPA can be released in wastewater during dental procedures. This release seems to be material dependent. Among the filtration methods tested, Catalytic carbon seems to be effective for BPA reduction in water.     

Development and characterisation of dental composites containing anisotropic fluorapatite bundles and rods

ObjectivesTo develop dental composites incorporating fluorapatite (FA) crystals as a secondary filler and to characterise degree of conversion, key mechanical properties and fluoride release.MethodsFA rod-like crystals and bundles were hydrothermally synthesised and characterised by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD) and ¹⁹F MAS-NMR. Composites were formulated containing BisGMA/TEGDMA/BisEMA and barium-aluminium-silicate glass (0FA). FA crystals were incorporated at 10 (10FA), 20 (20FA), 30 (30FA) and 40 wt% (40FA) maintaining a filler content of 80 wt% (63–67 vol%). Degree of conversion (DC), flexural strength (FS), flexural modulus (FM), fracture toughness (K_1C), Vickers hardness (HV) and 2-body wear were measured. Fluoride release was measured in neutral and acidic buffers.ResultsXRD and ¹⁹F MAS-NMR confirmed that only FA was formed, whilst SEM revealed the presence of single rods and bundles of nano-rods. DC ranged between 56–60% (p > 0.05). FA composites showed lower FM and lower FS (p < 0.05), but comparable wear resistance and HV (p > 0.05) to 0FA. 30FA and 40FA showed similar K_1C to 0FA (p > 0.05), with SEM showing evidence of toughening mechanisms, whereas 10FA and 20FA showed lower K_1C (p < 0.05). FA containing composites released fluoride that was proportional to the amount of FA incorporated (p < 0.05) but only under acidic conditions.SignificanceThe addition of FA to the experimental composites reduced strength and stiffness but not the DC, hardness or wear rate. 30FA and 40FA had a higher K_1C compared to other FA groups. Fluoride release occurred under an accelerated acidic regime, suggesting potential as a bioactive ‘smart’ composite.     

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.