The effects of light intensity, temperature, and comonomer composition on the polymerization behavior of dimethacrylate dental resins.

One of the most common combinations for the organic phase of dental restorative materials is BisGMA (2,2bis[4-(2-hydroxy-3-methacryloyloxypropoxy) phenyl]propane) and TEGDMA (triethylene glycol dimethacrylate). However, this copolymer has some drawbacks, such as volume shrinkage during cure and lack of complete double-bond conversion. If the properties of this system are to be improved, an attempt must be made to understand the underlying kinetics of the reaction. This work examines the effects of light intensity, temperature, and composition on the polymerization behavior of BisGMA/TEGDMA copolymerizations. Using differential scanning calorimetry, we monitored the rates of photopolymerization for various experimental conditions. The BisGMA/TEGDMA copolymerization behaved similarly to other dimethacrylate systems and exhibited diffusion-controlled kinetics. It was found that the maximum rate of polymerization was significantly affected by the intensity of the light, and the temperature of the polymerization affected the conversion at which the maximum rate occurred. When the composition of the mixture was varied, it was discovered that the viscosity of the system played a significant role in the polymerization rate and the onset of reaction-diffusion-controlled termination. Mixtures which contained from 50 wt% to 75 wt% BisGMA displayed the highest maximum rate. This feature suggests that TEGDMA is an excellent diluent, since it increases the mobility of the reacting medium; however, the high reactivity is due to the presence of BisGMA. Therefore, based on compositional dependence, we conclude that the BisGMA portion of the mixture largely controls the polymerization mechanisms and kinetics.     

Bonding of light-cured glass ionomer cement to polycarbonate resin treated with experimental primers

The effect of experimental primers on the shear bond strength of polycarbonate composite resin with light-cured glass ionomer cements was investigated. Mixtures of methylmethacrylate (MMA) with the comonomers 2-hydroxyethyl methacrylate (HEMA), triethyleneglycol methacrylate (TEGDMA), and bisphenol-A-glycidymethacrylate (bisGMA) were used as primers. Polycarbonate composite resin rods of circular cross section and plates were bonded, with and without precured and nonprecured primers, using 2 light-cured glass ionomer cements (commercially available [LC] and experimental [EX]). In addition, commercial polycarbonate composite resin brackets with precured 50% TEGDMA/MMA primer were bonded to etched human enamel with both cements. Shear bond strengths were measured. Results were compared by ANOVA and Scheffe's tests at P = .05. The 30% HEMA/MMA, 50% TEGDMA/ MMA, 10% bisGMA/MMA, and 30% bisGMA/MMA primers produced the higher shear bond strengths (9.5 to 20.8 MPa) with LC and EX to polycarbonate composite resin. The 50% TEGDMA/MMA primer was most effective in improving the shear bond strengths of both LC and EX. Precured 50% TEGDMA/ MMA primer on a commercial resin bracket was effective in providing good shear bond strength to enamel.     

Onium salt improves the polymerization kinetics in an experimental dental adhesive resin

The aim of this study was to evaluate the influence of an onium salt in the polymerization kinetics of a dental adhesive model resin. A monomer mixture, based on Bis-GMA, TEGDMA and HEMA, was used as a model dental adhesive resin, which was polymerized using camphorquinone (CQ) as a photo-initiator in addition to either binary or ternary photo-initiator systems. The binary systems were formulated with different concentrations of diphenyliodonium hexafluorphosphate or ethyl 4-dimethylaminobenzoate in relation to the monomer and 1mol% of CQ. The ternary system was a blend of 1mol% of CQ, 2mol% of ethyl 4-dimethylamino benzoate and 0.25, 0.5, 1, 2 or 4mol% of onium salt. Real time Fourier Transform infrared spectroscopy was used to investigate the polymerization reaction over the photo-activation time. When CQ was used as photo-initiator, a slow polymerization reaction was observed and a lower monomer conversion. The addition of a second component (onium salt or amine) increases the polymerization rate and conversion independent on the co-initiator concentration. Ternary photo-initiator system showed an improvement on the polymerization rate of the dental adhesive model resin leading to high conversion in short photo-activation time. Also, a mechanism for initiating polymerization using an amine or onium salt as a co-initiator component is proposed.     

The effect of cure rate on the mechanical properties of dental resins

OBJECTIVE: This study investigates the effect of cure rate on the mechanical properties of a common dimethacrylate dental resin formulation (75/25 wt% bis-GMA/TEGDMA). METHODS: The polymerization rate and final conversion of the exact specimens subsequently used for mechanical testing were monitored by near-infrared (near-IR) spectroscopy. The glass transition temperature (T(g)) and modulus, as a function of temperature, were determined by dynamic mechanical analysis (DMA). Iniferter initiating systems were used to create partially cured networks that did not contain any trapped radicals. By the elimination of trapped radicals from the system, the formed networks can be characterized as a function of both temperature and double bond conversion without inducing additional thermal cure during testing. RESULTS: Copolymer specimens were cured with UV and visible light initiating systems, UV light intensities that varied by over four orders of magnitude, and cure temperatures that differed by 60 degrees C. Even though the polymerization rates for these resins were vastly different, similar T(g) and modulus were measured for specimens cured to the same final double bond conversion. SIGNIFICANCE: This study shows that highly cross-linked dimethacrylate systems, such as bis-GMA/TEGDMA, exhibit similar network structure and properties as a function of double bond conversion, regardless of the method or rate of cure.     

Antioxidative vitamins decrease cytotoxicity of HEMA and TEGDMA in cultured cell lines

OBJECTIVES AND METHODS: In a previous study it was postulated that toxicity of 2-hydroxyethylmethacrylate (HEMA) and triethleneglycoldimethacrylate (TEGDMA) is based on oxidative metabolites. In this study the influence of antioxidative vitamins (including uric acid) on the toxicity of HEMA or TEGDMA was tested. Toxicity of HEMA and TEGDMA was determined in rat alveolar epithelial L2, human malignant A549, and human fibroblast-like 11Lu cells by inhibition of methionine incorporation (as a marker of protein synthesis inhibition) and by determination of glutathione depletion, as well as by measurement of GSSG increase. RESULTS: Toxicity of the composite components HEMA and TEGDMA was demonstrated by GSH depletion as the most sensitive method. Five hundred micromoles per litre Vitamin C or 250 micromol/l Vitamin E were mostly able to decrease toxicity of HEMA and TEGDMA in the cell lines tested. In addition, 250 micromol/l Vitamin A was only effective in L2 cells impairing HEMA toxicity and 250 micromol/l uric acid impairing TEGDMA toxicity as assessed by decreased GSH depletion. In A549 cells only methionine incorporation inhibition but not GSH depletion was significantly affected. By contrast, in 11Lu cells methionine incorporation inhibition was not significantly changed, but GSH depletion was. CONCLUSIONS: The postulated mechanism of HEMA or TEGDMA toxicity based on radical metabolites is supported by the effectivity of the antioxidative substances tested in mitigating toxicity and by the greater susceptibility of the glutathione redox system as compared to protein synthesis inhibition in assessing toxicity.     

Development of a new photocurable composite resin with reduced curing shrinkage.

OBJECTIVE: This study was conducted to evaluate the potential of a novel trifunctional methacrylate as a component of a photocurable composite resin with reduced curing shrinkage. METHODS: Tris[4-(2'-hydroxy-3'-methacryloyloxypropoxy)phenyl]methane (TTEMA) was synthesized by reacting triphenylolmethane triglycidyl ether (TTE) with methacrylic acid in the presence of 4-(dimethylamino)pyridine. Photopolymerization reactivity and volumetric shrinkage of unfilled resins based on TTEMA were investigated by Fourier transform infrared spectroscopy and density measurements, respectively, and the results were compared with those for conventional dental monomers. A three-point bending test of the TTEMA-containing composite resin was carried out. RESULTS: TTEMA was easily prepared in a good yield of 89%. Unfilled resins of TTEMA and bis-GMA, each containing 40% TEGDMA, showed similar photopolymerization reactivity. TTEMA exhibited a very low photopolymerization shrinkage of 2.09%, and 3:2 TTEMA-TEGDMA unfilled resin revealed 10% lower shrinkage than a conventional bis-GMA system containing the same amount of TEGDMA. The flexural strength of a light-activated composite resin formulated with TTEMA is comparable to that of a bis-GMA composite resin under the same conditions. SIGNIFICANCE: TTEMA is promising for application as a photocurable dental monomer due to ease of synthesis, good polymerization reactivity, and relatively low curing shrinkage.     

Cytotoxicity of dental composite components and mercury compounds in lung cells.

OBJECTIVE: The effect of dental composite components triethyleneglycoldimethacrylate (TEGDMA) and hydroxyethylmethacrylate (HEMA), as well as mercuric chloride (HgCl2) and methylmercury chloride (MeHgCl) was investigated on the release of lactatedehydrogenase (LDH) from alveolar epithelial lung cell lines in vitro. METHODS: The confluent cell layers from the A549 (human, malignant) and the L2 cells (rat) were incubated with various concentrations of HEMA, TEGDMA, MeHgCl and HgCl2 at 37 degrees C in 2% (v/v) CO2 atmosphere for 8h. In further experiments the L2 cells were incubated with the same compounds for 6-48 h. LDH release was measured and the values were expressed as percentage of the LDH content. The values were plotted on a concentration log-scale and the substance concentration at the maximum slope was assessed as effective concentration (EC50). RESULTS: A significant (p<0.05) increase in the LDH release was found in the L2 cells after 8-h incubation with HEMA (4 mmol/l), TEGDMA (2 mmol/l), MeHgCl (0.01 mmol/l) and HgCl2 (0.015 mmol/l), and in A549 cells with HEMA (14 mmol/l), TEGDMA (15 mmol/l), MeHgCl (0.15 mmol/l) and HgCl2 (0.05 mmol/l), compared to controls. The EC50 values from compounds in the L2 cells are shown in the following table (mean; sem in parentheses; n=3-6; #n=1): [see text]. SIGNIFICANCE: The toxic effect of HgCl2 and MeHgCl from the L2 cells was about 100-700-fold higher than of the dental composite components. A significant (p<0.05) time dependent increase of toxicity was observed with TEGDMA, HEMA and MeHgCl.     

Relocation and reinforcement of the adhesive/composite interface with spontaneous amine-peroxide interfacial polymerization

ObjectivesThis study demonstrates a spontaneous redox polymerization process located at the adhesive-composite interface that precedes photocure of the composite with the intent to improve bonding.MethodsAn aromatic amine and benzoyl peroxide redox initiator system was partitioned between BAPO-photoinitiated BisGMA/HEMA adhesive and BisGMA/TEGDMA resin-composites. The composite was placed on the photocured adhesive layer with a brief delay before photopolymerization of the composite layer. Micro-tensile bond strength between the adhesive and composite was assessed in comparison with the non-redox active control materials.ResultsThe presence of amine or peroxide in these resins without the redox initiation contribution enhanced both the rate and the final conversion of the BAPO-based photopolymerizations. Control formulations using redox-only initiation showed active polymer formation starting at approximately 30 s when physical mixing of the redox components was involved; however, simply by waiting 60 s between composite placement and photocure provided adequate time for passive interfacial diffusion of benzoyl peroxide from the pre-cured adhesive into the overlaid aromatic amine-containing composite such that a sufficient degree of redox initiated interfacial polymerization occurred prior to the composite photocure. The result was a significant increase in the adhesive to composite micro-tensile bond strength with the failure site moved away from the mainly interfacial failure noted for the control.SignificanceThe stress-free autonomous pre-conversion of a redox-initiated thin film of composite that then provides a compositionally homogeneous interface for composite photopolymerization offers a means to enhance at least short-term bond strength between the adhesive and composite phases during restorative placement.     

Influence of polymerization initiator for base monomer on microwave curing of composite resin inlays

Microwave polymerization was used to make composite resin inlays and the effect examined of the concentration of polymerization initiator for the base monomer. The monomers used were 2,2-bis [4-(3-methacriloxy-2-hydroxypropoxy) phenyl] propane (Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA). Bis-GMA and TEGDMA were mixed in a ratio of 6:4 by weight and were separated into five groups. To each group was added benzoyl peroxide (BPO) in the ratios of 0.1, 0.3, 0.5, 0.7 and 0.9 wt% as the polymerization initiator. These were used as the base monomers. The results showed that the degree of conversion of the cured sample increased with increasing concentration of BPO from 0.1 to 0.5 wt%, however there was no significant difference at 0.5, 0.7 and 0.9 wt% (P> 0.01). Compression strength, diametral tensile strength and the Knoop hardness showed a similar tendency as the degree of conversion. No significant difference was recorded in the Knoop hardness between the top and the bottom surfaces (P> 0.01), which suggested a uniform polymerization in the cured sample. Thus, microwave polymerization would be an efficacious method for making resin inlays with the addition of BPO to the base monomer (Bis-GMA:TEGDMA, 6:4). The maximum conversion was found at a concentration of 0.5 wt%.     

Photopolymerization of N,N-dimethylaminobenzyl alcohol as amine co-initiator for light-cured dental resins.

OBJECTIVE: The present study was carried out in order to assess the suitability of N,N-dimethylaminobenzyl alcohol (DMOH) as co-initiator of camphorquinone (CQ) and 1-phenyl-1,2-propanedione (PPD) in light-cured dental resins. METHODS: DMOH was synthesized and used as co-initiator for the photopolymerization of a model resin based on {2,2-bis[4-(2-hydroxy-3-methacryloxyprop-1-oxy)phenyl]propane} (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA). Experimental formulations containing CQ or PPD in combination with DMOH at different concentrations were studied. The photopolymerization was carried out by means of a commercial light-emitting diode (LED) curing unit. The evolution of double bonds consumption versus irradiation time was followed by near-infrared spectroscopy (NIR). The photon absorption efficiency (PAE) of the photopolymerization process was calculated from the spectral distribution of the LED unit and the molar absorption coefficient distributions of PPD and CQ. RESULTS: DMOH is an efficient photoreducer of CQ and PPD resulting in higher polymerization rate and higher double bond conversion compared with dimethylaminoethylmethacrylate. The PAE for PPD was higher than that for CQ. However, the polymerization initiated by PPD progressed at a lower rate and exhibited lower values of final conversion compared with the resins containing CQ. This observation indicates that the lower polymerization rate of the PPD/amine system should be explained in terms of the mechanism of generating primary radicals by PPD, which is less efficient compared with CQ. SIGNIFICANCE: The DMOH/benzoyl peroxide redox system, has recently been proposed as a more biocompatible accelerator for the polymerization of bone cements based on poly(methyl methacrylate), because cytotoxity tests have demonstrated that DMOH possesses better biocompatibility properties compared with traditional tertiary amines. The results obtained in the present study reveal the suitability of the CQ/DMOH initiator system for the polymerization of light-cured dental composites.     

Dimethacrylate based on cycloaliphatic epoxide for dental composite.

OBJECTIVES: The aim of this study was to investigate the kinetics and mechanical properties of dimethacrylate monomer based on cycloaliphatic epoxide for dental restorative composite. METHODS: Dimethacrylate based on cycloaliphatic epoxide (EPCDMA) was copolymeirzed with TEGDMA by varying the curing conditions: monomer composition and light intensity. A real-time near FTIR technique was employed to monitor the double bond conversion and the rate of polymerization. Dynamic mechanical analysis was performed on a dynamic mechanical analyzer, and volume shrinkage of the cured samples was determined by pycnometric method. RESULTS: The results of kinetics showed that, two peaks of maximum rate of polymerization (Rpmax) occurred when the amount of TEGDMA was more than 30wt%. Increasing the amount of TEGDMA, the final double bond conversion and polymerization shrinkage both increased, while the glass transition temperature (Tg) decreased. All EPCDMA/TEGDMA mixtures had slightly higher storage modulus at body temperature (37 degrees C). When compared with Bis-GMA/TEGDMA (70/30), the EPCDMA/TEGDMA (70/30) system assumed similar reactivity and volume shrinkage but higher Tg. SIGNIFICANCE: EPCDMA had comparable mechanical properties to those of Bis-GMA.     

Acrylamide monomers in universal adhesives

ObjectivesThe mono-functional monomer 2-hydroxyethyl methacrylate (HEMA) is often added to universal adhesives (UAs) to improve surface wetting and prevent phase separation. Nevertheless, HEMA promotes water sorption and hydrolysis at adhesive interfaces, hereby affecting long-term bonding to dentin. This study investigated if two acrylamide monomers could replace HEMA in an UA formulation applied in etch-and-rinse (2E&R) and self-etch (1SE) bonding mode.MethodsFour experimental UAs were bonded to bur-cut dentin. In addition to 12 wt% 10-MDP, 25 wt% Bis-GMA and 10 wt% TEGDMA as common monomer composition, 20 %wt ethanol and 15 %wt water as solvent, and 3 wt% polymerization-related additives, the four formulations solely differed for either the acrylamide cross-linker monomer ‘FAM-201′ as TEGDMA alternative and HEMA replacement, the hydroxyethyl acrylamide monomer ‘HEAA’ as HEMA alternative, HEMA (‘HEMA+’), or extra TEGDMA in a HEMA-free control (‘HEMA-’), all added in a 15 wt% concentration. The split-tooth study design involved application in 2E&R mode on one tooth half versus 1SE mode on the corresponding half. Micro-tensile bond strength of half of the micro-specimens was measured upon 1-week distilled water storage (‘immediate’ 1w μTBS), with the other half measured after additional 6-month storage (‘aged’ 6 m μTBS). Statistics involved linear mixed-effects (LME) modelling (p < .05). Additionally, interfacial TEM characterization, thin-film (TF) XRD surface analysis, LogP determination, and a cytotoxicity assay were carried out.ResultsFAM-201 revealed significantly higher μTBS than HEMA+ at 1w and 6 m when applied both in E&R and SE bonding modes. HEAA’s μTBS was significantly lower than that of HEMA+ at 1w when applied in SE mode. TF-XRD and TEM revealed similar chemical and ultrastructural interfacial characterization, including stable 10-MDP_Ca salt nano-layering. FAM-201 was least cytotoxic and presented with an intermediary LogP, while HEAA presented with the highest LogP, indicating high hydrophilicity and water-sorption sensitivity.SignificanceThe acrylamide co-monomer FAM-201 could replace HEMA in an UA formulation, while HEAA not.     

Primary cyclization in the polymerization of bis-GMA and TEGDMA: a modeling approach to understanding the cure of dental resins.

An optimal dental restorative polymeric material would have a homogeneous cross-linking density giving it consistent mechanical strength throughout the material. When multifunctional monomers are polymerized, a pendant double bond can react intramolecularly with the radical on its propagating chain to form a loop, which results in a primary cyclization reaction. Primary cyclization does not contribute to overall network structure, causes microgel formation, and leads to heterogeneity in the polymer. Knowledge of how cure conditions control the degree of primary cyclization and cross-linking in the polymer is important in developing better dental materials. To gain more understanding about the evolving polymer network, the photopolymerization of a typical dental resin (75/25 wt% bis-GMA/TEGDMA) is modeled using a first principals approach. The overall polymerization rate behavior of 75/25 wt% bis-GMA/TEGDMA is predicted using experimentally obtained propagation and termination kinetic rate constants. The effect of chain stiffness and light intensity on the polymerization kinetics is also explored. Furthermore, the model predicts the extent of cross-linking and primary cyclization in the growing polymer network. At 45% conversion, the fraction of bis-GMA and TEGDMA pendant double bonds created that have cycled is 11 and 33%, respectively. The model shows that using a stiff monomer, like bis-GMA, in dental resins diminishes the extent of cyclization and increases the cross-linking density of the polymer. Therefore, better mechanical properties are obtained than if more flexible monomers were used.     

Effect of the photoinitiator system on the polymerization of secondary methacrylamides of systematically varied structure for dental adhesive applications

ObjectiveThe aim of this study was to investigate the influence of the photoinitiator system on the polymerization kinetics of methacrylamide-based monomers as alternatives to methacrylates in adhesives dental-based materials.MethodsIn total, 16 groups were tested. Monofunctional monomers (2-hydroxyethyl methacrylate) – HEMA; (2-hydroxy-1-ethyl methacrylate) −2EMATE, (2-hydroxyethyl methacrylamide) – HEMAM; and (N-(1-hydroxybutan-2-yl) methacrylamide) −2EM; were combined with bifunctional monomers containing the same polymerizing moieties as the monofunctional counterparts (HEMA-BDI; 2EMATE-BDI; HEMAM-BDI; and 2EM-BDI) at 50/50 M ratios. BHT was used as inhibitor (0.1 wt%) and the photoinitiators used were: CQ + EDMAB (0.2/0.8), BAPO (0.2), IVOCERIN (0.2), and DMPA (0.2), in wt%. The polymerization kinetics were monitored using Near-IR spectroscopy (∼6165 cm⁻¹) in real-time while the specimens were photoactivated with a mercury arc lamp (Acticure 2; 320−500 nm, 300 mW/cm²) for 5 min, and maximum rate of polymerization (Rp_max, in %.sˉ̄¹), degree of conversion at Rp_max (DC@Rp_max, in %), and the final degree of conversion (Final DC, in %) were calculated (n = 3). Initial viscosity was measured with an oscillating rheometer (n = 3). Data were analyzed using Two-way ANOVA for the polymerization kinetics and one–way ANOVA for the viscosity. Multiple comparisons were made using the Tukey’s test (∝ = 0.05).ResultsThere was statistically significant interaction between monomer and photoinitiator (p < 0.001). For the methacrylates groups, the highest Rp_max was observed for HEMA + DMPA and 2EMATE + BAPO. For methacrylamides groups, the highest Rp_max were observed for HEMAM and 2EM, both with DMPA. Final DC was higher for the methacrylate groups, in comparison with methacrylamide groups, independent of the photoinitiators. However, for the methacrylamide groups, the association with BAPO led to the lowest values of DC. In terms of DC@Rp_max, methacrylate-based systems showed significantly higher values than methacrylamide formulations. DMPA and Ivocerin led to higher values than CQ/EDMAB and BAPO in methacrylamide-based compounds. BAPO systems showed de lowest values for both HEMA and HEMAM formulations. For the viscosity (Pa.s), only 2EM had higher values (1.60 ± 0.15) in comparison with all monomers. In conclusion, polymerization kinetics was affected by the photoinitiators for both monomers. Viscosity was significantly increased with the use of secondary methacrylamide.Significancethis work demonstrated the feasibility of using newly-synthesized methacrylamide monomers in conjunction with a series of initiator systems already used in commercial materials.     

Alterations of energy metabolism and glutathione levels of HL-60 cells induced by methacrylates present in composite resins

OBJECTIVES: Methacrylic compounds such as 2-hydroxyethyl methacrylate (HEMA), triethylene glycol dimethacrylate (TEGDMA) and bisphenol A glycerolate (1 glycerol/phenol) dimethacrylate (Bis-GMA) are largely present in auto- or photopolymerizable composite resins. Since the polymerization reaction is never complete, these molecules are released into the oral cavity tissues and biological fluids where they could cause local adverse effects. The aim of this work was to verify the hypothesis that the biological effects of HEMA, TEGDMA and Bis-GMA - at a non-cytotoxic concentration - depend on the interaction with mitochondria and exert consequent alterations of energy metabolism, GSH levels and the related pathways in human promyelocytic cell line (HL-60). METHODS: The biological effects of methacrylic monomers were determined by analyzing the following parameters: GSH concentration, glucose-6-phosphate dehydrogenase (G6PDH) and glutathione reductase (GR) activity, oxygen and glucose consumption and lactate production along with cell differentiation and proliferation. RESULTS: All monomers induced both cellular differentiation and decrease in oxygen consumption. Cells treated with TEGDMA and Bis-GMA showed a significant enhancement of glucose consumption and lactate production. TEGDMA and HEMA induced GSH depletion stimulating G6PDH and GR activity. CONCLUSIONS: All the monomers under study affect the metabolism of HL-60 cells and show differentiating activity. Since alterations in cellular metabolism occurred at compound concentrations well below cytotoxic levels, the changes in energy metabolism and glutathione redox balance could be considered as potential mechanisms for inducing clinical and sub-clinical adverse effects and thus providing useful parameters when testing biocompatibility of dental materials.     

Apoptosis induced by the monomers HEMA and TEGDMA involves formation of ROS and differential activation of the MAP-kinases p38, JNK and ERK.

OBJECTIVES: Cytotoxic methacrylate monomers have been identified in aqueous extracts of freshly cured compomers. Some of these compounds, including HEMA and TEGDMA, induce apoptosis and necrosis in vitro. The aim of the present study was to elucidate possible signaling pathways involved in apoptosis following exposure to HEMA or TEGDMA in a salivary gland cell line. METHODS: The cells were exposed to various concentrations of HEMA or TEGDMA. ROS formation was determined by dichlorofluorescein assay. Phosphorylated MAP-kinases ERK1/2, p38 and JNK, as well as specific caspases were identified by Western blotting. Apoptosis was assayed by fluorescence microscopy. RESULTS: HEMA or TEGDMA exposure resulted in ROS formation and concentration-dependent apoptosis as well as phosphorylation of ERK. Phosphorylation of JNK and p38 was induced by HEMA. Selective inhibitors of ERK and JNK modified the apoptotic response after HEMA and TEGDMA exposure, whereas p38 inhibition modified the apoptotic response only after HEMA exposure. Vitamin C reduced HEMA-induced apoptosis. SIGNIFICANCE: ROS formation and differential MAP kinase activation appear to be involved in the apoptotic response following exposure to HEMA and TEGDMA.     

Effect of TEGDMA/BisGMA ratio on stress development and viscoelastic properties of experimental two-paste composites

In this study, we explored the reduction of shrinkage stresses in restored teeth by stimulating viscous flow of adhesive restoratives during curing, by increasing the TEGDMA/BisGMA ratio in the resin of composite restoratives. We studied a series of experimental two-paste composites with different amounts of TEGDMA (30, 50, 70 wt%, respectively) in the resin by mechanical testing, infrared spectroscopy, and dilatometry, to determine how comonomer composition affects the mechanical and chemical properties of the composite during curing. It was found that the polymerization rate of BisGMA-TEGDMA composites is indicative of the viscoelastic behavior during curing: The higher the reactivity, the higher the stiffness and viscosity development. Composites with 50 wt% TEGDMA in the resin displayed the highest maximum polymerization rate. High amounts of TEGDMA in the resin only modestly increased the pre-gel viscous flow (= lowered viscosity) property of composites. Of these composites, high post-gel shrinkage is the decisive factor in high shrinkage stress development.     

Inhibition of TEGDMA and HEMA-induced genotoxicity and cell cycle arrest by N-acetylcysteine.

OBJECTIVES: Dental resin monomers like triethylene glycol dimethacrylate (TEGDMA) and 2-hydroxyethyl methacrylate (HEMA) are able to cause an imbalance of the redox state in mammalian cells. The resulting oxidative stress originating from reactive oxygen species (ROS) has been associated with cytotoxicity. We hypothesized that ROS might contribute to the generation of genotoxicity by TEGDMA and HEMA as well. Therefore, we examined the formation of micronuclei in V79 cells by both resin monomers in the presence of the antioxidant N-acetylcysteine (NAC), which scavenges ROS. In addition, we analyzed the effects of TEGDMA and HEMA on the normal cell cycle in the presence of NAC. METHODS: V79 fibroblasts were exposed to increasing concentrations of TEGDMA and HEMA in the presence and absence of NAC for 24h. Genotoxicity was indicated by the formation of micronuclei. The modification of the normal cell cycle was analyzed by flow cytometry (FACS). RESULTS: A dose-related increase in the number of micronuclei in V79 cells-induced by TEGDMA and HEMA indicated genotoxicity of both chemicals. However, the formation of micronuclei was reduced in the presence of 10 mmol/L NAC, indicating its protective role. A cell cycle delay in G2 phase caused by TEGDMA was absent when cells were co-treated with NAC. Similarly, the presence of NAC led to a reversion of the cell cycle delay in HEMA-treated cell cultures. SIGNIFICANCE: Our results suggest that genotoxic effects and the modification of the cell cycle caused by TEGDMA and HEMA are mediated, at least in part, by oxidative stress.     

Investigations of step-growth thiol-ene polymerizations for novel dental restoratives

OBJECTIVES: The goal of this work was to investigate the feasibility of formulating novel dental restorative materials that utilize a step-growth thiol-ene photopolymerization. Particularly, we are aiming to significantly reduce the polymerization shrinkage and shrinkage stress while retaining adequate physical properties as compared to current dimethacrylatre-based systems. METHODS: The thiol-ene system is composed of a 4:3 molar mixture of triallyl-1,3,5-triazine-2,4,6-trione (TATATO) and pentaerythritol tetramercaptopropionate (PETMP). The simultaneous measurement of shrinkage stress and functional group conversion was performed. Solvent extraction of unreacted monomers and dynamic mechanical analysis on the polymer networks that were formed were also studied. Flexural strength was measured for both filled and unfilled PETMP/TATATO and Bis-GMA/TEGDMA systems. RESULTS: Photopolymerization of PETMP/TATATO occurs at a much higher rate, with the maximum polymerization rate six times faster, than Bis-GMA/TEGDMA cured under the identical conditions. The results from the simultaneous measurement of shrinkage stress and conversion showed that the onset of shrinkage stress coincides with the delayed gel point conversion, which is predicted to be 41% for the 3:4 stoichiometric PETMP/TATATO resin composition. The maximum shrinkage stress developed for PETMP/TATATO was about 0.4 MPa, which was only approximately 14% of the maximum shrinkage stress of the Bis-GMA/TEGDMA system. Adequate flexural strength and flexural modulus values were obtained for both filled and unfilled PETMP/TATATO systems. SIGNIFICANCE: The dramatically reduced shrinkage stress, increased polymerization rate, significance increased functional group conversion, and decreased leachable species are all benefits for the use-of thiol-ene systems as potential dental restorative materials.     

Strengthening effect of aluminum fluoride added to resin composites based on polyacid-containing polymer.

OBJECTIVES: The study investigates the possible strengthening effect of aluminum ions liberated during long-time water storage of experimental resin composites containing a polymer with carboxylic acid groups. METHODS: The organic part of the resin composites was formed by photopolymerization of methacrylate monomers. The monomer mixture was composed of UEDMA, HEMA or TEGDMA, and 0-40 mol% HEMAN (the adduct of HEMA and maleic anhydride). The filler part of the resin composites consisted of a conventional silanated glass filler and 1 or 5 wt% of AlF3 x 3H2O. The rupture strength and bending modulus were measured without water storage and after 1 month, 1 year, and 3 years of water storage at 37 degrees C of the resin composite. RESULTS: The experimental composites that had been stored in water had lower rupture strength and modulus than the composites that were tested without water storage. Specimens containing 30 or 40 mol% HEMAN increased in strength, and specimens containing 20, 30, or 40 mol% HEMAN increased in modulus during the period of water storage. Depending on composition, values for strength and modulus obtained after 3 years of water storage was up to 50% higher than the values obtained after 1 month. SIGNIFICANCE: The strengthening effect of aluminum fluoride may be interpreted as the result of increased crosslinking by aluminum ions and carboxylate groups. The results may serve as a basis for the improvement of the mechanical properties of polyacid-modified resin composites.