This study’s purpose was to measure and compare the degree of conversion (DC) and the amount of elutable substances from modern resin-based composites (RBCs) as function of polymerization time. One nano-hybrid RBC based on tricyclodecane-(TCD)-urethane (Venus ® Diamond) and two conventionally formulated RBCs (TetricEvo Ceram ® , Filtek™ Supreme XTE) were considered.
DC ( n = 5) was investigated in real time for 5 min by Fourier transform infrared spectroscopy (ATR-FTIR) in a filling depth of 2 mm at varied irradiation times (5, 10, 20, 40 s). After storing the specimens in ethanol/water for 7 d at 37 °C the eluates were analyzed by gas chromatography/mass spectrometry. Results were statistically analyzed using a multivariate analysis ( α = 0.05) an independent t -test ( p < 0.05) and a Pearson correlation analysis.
In all groups increasing curing time resulted in a significant increase in DC. For TetricEvo Ceram ® a high significant inverse correlation was found between DC and the amount of eluted camphorquinone (CQ, Pearson correlation coefficient = −0.88), ethylene glycol dimethacrylate (EGDMA, −0.73), 4-N,N-dimethylaminobenzoic acid ethylester (DMABEE, −0.87), triethylene glycol dimethacrylate (TEGDMA, −0.68), Tinuvin P (−0.71) and bisphenol-A-polyetheylene glycol dimethacrylate (BisEMA, −0.84). Unexpectedly DC and the amount of eluted methyl acrylate (MAA) correlated directly (0.72). In the specimens of Venus ® Diamond a significant inverse correlation was found between DC and the amount of eluted CQ (−0.69) and TEGDMA (−0.50), whereas in the specimen of Filtek™ Supreme XTE DC correlated with CQ (−0.96), EGDMA (−0.70), DMABEE (−0.87), TEGDMA (−0.92) and MAA (−0.92).
This study demonstrated a strong inverse correlation between DC and elutable substances in RBCs. Both evaluation methods emphasis the importance of an adequate polymerization (20, 40 s), since short curing-times (5, 10 s) resulted in lower DC and higher amount of eluted substances with toxic potential.
Methacrylate based dental restoratives are widely used in practice. For dental use it is very important that methacrylate based monomers have at least two double bonds like the Bowen monomer bisphenol-A-glycidyldimethacrylate (BisGMA) or triethylene glycol dimethacrylate (TEGDMA) to form branched polymers. In contrast to (co)monomers with one double bond which can only build linear polymer chains, (co)monomers with two or more double bonds can build a three-dimensional polymer network . During the polymerization process new single bonds will be built from double bond. Apart from the formation of a three-dimensional network, the degree of conversion (DC) of the (co)monomers is also important in dentistry. The DC of (co)monomers to polymers has influence on the material properties as well as on the biocompatibility of a material (the lower the DC the higher the amount of uncured monomers and additives) .
The Fourier transform infrared spectroscopy (FTIR) is an established method to determine remaining double bonds . It must be emphasized that the remaining double bonds are not directly related to the amount of remaining (co)monomers ( Fig. 1 ). For measurements with FTIR both cases from Fig. 1 a and b are equal. In the first case ( Fig. 1 a) both (co)monomers are bound to the polymer chain with a single bond resulting from opening a double bond, the second double bond remains unreacted. In the second case ( Fig. 1 b) the (co)monomer is not linked to a polymer chain, its two double bonds are unreacted. With FTIR in both cases two unreacted double bond can be measured.
The second case is of toxicological interest because it could be shown that these (co)monomers and other substances can be eluted from polymerized dental methacrylate based materials . Bleaching can degrade the three-dimensional polymer network and therefore the amount of elutable monomers and additives increases . In vitro studies have shown that some of the elutable methacrylate based monomers like BisGMA, TEGDMA, 2-hydroxyethyl methacrylate (HEMA) and methyl methacrylate (MMA) as well as elutable additives like camphorquinone (CQ) can have estrogenic, mutagenic, teratogenic and genotoxic effects . From in vivo studies it is known that during the metabolism of BisGMA, TEGDMA and HEMA epoxides like 2,3-epoxymethacrylic acid will be formed . Furthermore elutable substances can cause allergic contact dermatitis and asthma in e.g. dentists and their personal .
It is discussed that the amount of elutable substances is depending on a lot of parameters like elution medium, polymerization time and the degree of cure. In earlier studies e.g. the relationship between leachability of polymerization initiator and degree of conversion of visible light-cured artificial resin systems were examined . For our best knowledge up to now, no study from different modern resin-based composites (RBCs) is available comparing the DC and the amount of elutable substances from the same specimen in dependence of the polymerization time.
Therefore the aim of our study was to test if the degree of C C double bond conversion measured with FTIR correlates with the amount of elutable substances from composites measured with gas chromatography/mass spectroscopy (GC/MS). Moreover the influence of polymerization time on the DC of C C double bonds and on the amount of elutable substances was measured. The null hypotheses tested in our study were both, DC and the amount of elutable substances would not be influenced by the polymerization time and would not correlate.
All solvents and reagent products were obtained from Merck, Darmstadt, Germany and of highest purity available.
Preparation of samples
From the light-curable nano-hybrid dental restorative materials Tetric Evo Ceram ® (Ivoclar Vivadent, Ellwangen, Germany), Venus ® Diamond (Heraeus Kulzer, Hanau, Germany), and Filtek™ Supreme XTE (3M ESPE, Seefeld, Germany, Table 1 ) specimens of approximately 30 mg (thickness of 2 mm, diameter of 3 mm, color A3, with a resulting surface of 32.99 mm 2 and volume of the cylinder of 14.13 mm 3 ) were prepared under photolaboratory conditions. The specimens ( n = 5) were covered with plastic matrix strips (Frasaco, Tettnang, Germany) and polymerized for 5, 10, 20 and 40 s by using an LED light source (Freelight2, 3M-ESPE, 1241 mW/cm 2 ). The curing unit was directly applied on samples’ surface. The irradiance of the curing unit (1241 mW/cm 2 ) was measured by means of a calibrated fiber optic spectrally resolving radiometer equipped with an integrating sphere (S2000, Ocean Optics, USA).
|Nano-hybrid RBCs||Manufacturer/batch||Resin matrix||Filler||Filler (w/v)|
|Tetric Evo Ceram ®||Ivoclar-Vivadent, N30018||BisGMA, UDMA, DMDMA||Ba-glass, YbF 3 , MO, PPF||76/54|
|Venus ® Diamond||Heraeus Kulzer, 10036||TCD-DI-HEA, UDMA||Ba–Al–F-glass||81/64|
|Filtek™ Supreme XTE||3M-ESPE, N163566||BisGMA, BisEMA, UDMA, TEGDMA, PEGDMA||ZrO 2 –SiO 2 cluster SiO 2 and ZrO 2 nanofiller||78.5/63.3|
After preparation specimens were incubated in a mixture of ethanol/water (3:1) at 37 °C for 7 d because after 7 d the major amount of elutable substances were eluted. Caffeine (CF; 0.1 mg/ml) was added to the eluates and each aliquot was analyzed by GC/MS.
Degree of cure
The measurements of the DC ( n = 5) were made in real time with an FTIR-Spectrometer with an attenuated total reflectance (ATR) accessory (Nexus, Thermo Nicolet, Madison, USA). Therefore, the un-polymerized composite paste was put directly on the diamond ATR crystal in a mold 2 mm high and with a diameter of 3 mm. The mold was filled in one increment and the curing unit was applied directly on the sample surface. The FTIR spectra were recorded in real time for 5 min on the lower surface of the samples with irradiation for 5, 10, 20 and 40 s. The diameter of the measured surface was 800 μm, the wave number range of the spectrum was 4000–650 cm −1 and the FTIR spectra were recorded with two scans/s at a resolution of 8 cm −1 .
To determine the percentage of the remained unreacted double bonds, the DC was assessed as the variation of the absorbance intensities peak area ratio of the methacrylate carbon double bond (peak at 1634 cm −1 ) and those of an internal standard (aromatic carbon double bond; peak at 1608 cm −1 ) during polymerization, in relation to the uncured material :
DC Peak height ( % ) = 1 − ( 1634 cm − 1 / 1608 cm − 1 ) Peak height after curing ( 1634 cm − 1 / 1608 cm − 1 ) Peak height before curing × 100