To analyse the influence of irradiation time, aging before bleaching, and bleaching technique (home bleaching vs. in-office bleaching) on the amount of elutable substances from modern nano-hybrid resin-based composites (RBCs).
Specimens ( n = 5) of three nano-hybrid RBCs (Venus ® Diamond, Tetric EvoCeram ® and Filtek™ Supreme XTE) were irradiated for 5, 10, 20 and 40 s. The degree of conversion (DC) was measured in real-time with Fourier transform infrared spectroscopy (FTIR). Specimens were bleached either directly after irradiation or after aging (1.5 or 6 month in distilled water at 37 °C) with Opalescence ® PF15% for 6 h (simulation of home bleaching) or PF35% for 0.5 h (simulation of in-office bleaching) and incubated in ethanol/water (3:1) at 37 °C for 7 d. The eluates were analyzed by gas chromatography/mass spectrometry. Unbleached specimens at the above mentioned irradiation times were used as controls.
Bleaching increases the amount of elutable substances. This amount is generally stronger influenced by aging than by polymerization time or concentration of the gel. 2-Hydroxyethyl methacrylate was found in amounts up to 334.14 (106.91) μmol/l (Tetric EvoCeram ® , irradiation time 5 s; bleaching with 15% CP) as a destruction product. Diethoyxdimethylsilane was found in all eluates from bleached specimens, but not in the control groups. This substance may be formed by oxidation of 3-methacryloxy-propyltrimethoxysilane, indicating that the bond between inorganic filler and organic matrix might be weakened after bleaching.
Bleaching gels might alter the physical properties of resin-based composites, especially at low irradiation times and fresh placed restorations.
During the last decades the desire for white teeth has become more and more important all around the world. Besides discoloration of the tooth caused by food, smoking, tea, coffee or red wine, poor oral hygiene, diseases of enamel or dentin, pulpal necrosis, the use of pharmaceuticals like tetracycline or doxycycline, aging and genetic considerations, the main reason for bleaching teeth are of cosmetic matter . Hydrogen peroxide (HP) in the form of its precursor, carbamide peroxide (CP), an HP releasing agent, is widely used for tooth whitening (bleaching) both, in professionally and self-administered products . Various bleaching techniques are offered to patients including in-office bleaching, with usually 25–35% HP, and home bleaching, usually with 6.5–22% CP .
Different studies confirmed that a predictable tooth whitening can be achieved by using a 10% CP gel in a bleaching tray at night (the night-guard vital bleaching technique) , HP strips or 35% HP with or without light and/or heat activation (‘power bleaching’) . It could be shown that HP diffusion through dental tissues and its cytotoxic effects was proportional to the contact time of the bleaching gel with enamel, although shorter bleaching times reduced the results of bleaching . While many studies and case reports focus on the influence of HP/CP on enamel, dentin or pulp tissue, less data is available regarding the reactions of HP with the three-dimensional polymer network of resin-based composites (RBCs). Nevertheless few studies pointed out the need to enlighten patients with dental restorations that changes may occur during whitening, and their bleached restorations may need to be polished or replaced at the end of the treatment . One reason therefore is that the polymer network of RBCs consists of carbon-carbon (C C)-single or C C-double bonds, which may react with oxidants like HP . Durner et al. supposed that bleaching with HP could degrade the three-dimensional polymer network of RBCs, leading to an increase in unpolymerized monomers, additives and unspecific oxidative products release, when compared to unbleached controls.
It is common knowledge that elutable substances like monomers, co-monomers, initiators, stabilizers, decomposition-products or contaminants can lead in vitro to cytotoxic, teratogenic, estrogenic, mutagenic and/or genotoxic effects as well as to allergic reactions in vivo , like allergic contact dermatitis and asthma in humans . The release of these substances are shown in a lot of studies .
The oxidative interaction with the polymer network of RBCs during bleaching influences not only the amount of elutable substances, but also the physical properties of the filling. It could be proved that the color of RBCs changes significantly after bleaching, however with no clinical relevance . Another research group showed that the color-stability depends on the volume of resin matrices and filler type . A significant decrease in surface hardness (Knoop hardness test) of bleached RBCs, on superficial surfaces and in deeper layers was reported as well . It was also demonstrated that treating RBCs with HP/CP increased their surface roughness, however without clinical relevance .
Material properties as well as the amount of elutable substances can be influenced by several parameters like the irradiation time, the degree of conversion (DC), the storing condition of the specimens or the concentration of CP. Therefore the aim of our study was to investigate if 5, 10, 20 or 40 s polymerized specimens (while measuring the DC at the same time) of three different modern nano-hybrid RBCs aged under three different storing conditions before bleaching (no storage; 1.5 months; 6 months) and bleached by using two different bleaching techniques (0.5 h with 35% CP-gel simulating in-office bleaching and 6 h with 15% CP-gel simulating home bleaching) release higher amount of eluted substances compared to unbleached samples, polymerized under same conditions (control group).
The null hypothesis tested was that the amount of elutable substances would not be influenced by the parameters irradiation time and DC, nor by the parameters bleaching technique and storing time of the RBCs samples before bleaching. Furthermore the effect strength of the analyzed parameter on the amount of elutable substances was calculated.
Materials and methods
All solvents and reagent products were obtained from Merck, Darmstadt, Germany and of highest purity available.
Preparation of samples and irradiation time
From the light-curable nano-hybrid dental restorative materials Tetric EvoCeram ® , Venus ® Diamond, and Filtek™ Supreme XTE ( 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 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 irradiated 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 sample’s surface. The irradiance of the curing unit 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 wt./vol.|
|Tetric EvoCeram ®||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|
Degree of conversion (DC)
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 per second at a resolution of 8 cm −1 . The FTIR-spectrometry based on an idea from A. Michelson. Two rays were brought to interference. By variation of the wavelength of the rays, the resulting intensity of the interference was shown in an interferogram. The Fourier Transformation, a mathematical algorithm, is making an IR spectrum from the interferogram. Different chemical groups show a special sign (transmission) at a defined wavelength.
To determine the percentage of the remained unreacted double bonds, the DC was assessed as the variation of the absorbance intensities peak height 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 : <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='DCpeak height(%)=1−(1634cm−1/1608cm−1)peak height after curing(1634cm−1/1608cm−1)peak height before curing×100′>DCpeak height(%)=(1−(1634cm−1/1608cm−1)peak height after curing(1634cm−1/1608cm−1)peak height before curing)×100DCpeak height(%)=1−(1634cm−1/1608cm−1)peak height after curing(1634cm−1/1608cm−1)peak height before curing×100
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
Storing of the samples
After preparation, two groups of specimens were stored in distilled water at 37 °C for different times: 6 months ( n = 120), and 1.5 months ( n = 120) respectively. The storage medium was changed once a week. A third group ( n = 120) was bleached immediately (5 min after measuring the DC, thus 10 min after starting the photo-polymerization). For each material and irradiation time, unbleached specimens were used as control.
Bleaching of the samples
Every specimen was weighed (average weight 30 mg) and polished for 8 s with abrasive paper of 2500 grit, cleaned and dried with a cotton tissue. A 35% CP-gel (Opalescence ® PF 35%; Ultradent, South Jordan, Utah, USA, LOT U032), or 15% CP-gel (Opalescence ® PF 15; Ultradent, LOT Z022; Tooth Whitening Systems) was used as bleaching-gel.
For each irradiation time (5, 10, 20, 40 s), material (Tetric EvoCeram ® , Venus ® Diamond, Filtek™ Supreme XTE) and storage condition (no storage, 1.5 month, 6 months), five specimens were bleached by applying an approx. 1–1.5 mm thick gel layer. The bleaching-gel was removed carefully with a cotton tissue and the specimens were incubated in an elution medium of ethanol/water (3:1) at 37 °C for 7 d. Caffeine (CF; 0.1 mg/ml) was added to the eluates and each aliquot was analyzed by GC/MS. 1 μl of the elution media per 10 mg weight of the specimens was used.
With GC/MS analysis it is possible to separate volatile substances from sample mixtures in the gas phase. An aliquot of the sample was brought in the heatable injection system of the GC/MS. The volatile substances were vaporized and separated by interaction with the material of the column of the GC. A gas transports the volatile substance through the column of the GC into the MS where it was identified, e.g. by comparison with databases/reference substances.
The analysis of the eluates was performed on a Finnigan Trace GC ultra gas chromatograph connected to a DSQ mass spectrometer (Thermo Electron, Dreieich, Germany). A FactorFour ® capillary column (length 25 m, inner diameter 0.25 mm, coating 0.25 μm; Varian, Darmstadt, Germany) was used as capillary column for GC. The GC oven was heated from 50 °C (2 min isotherm) to 300 °C (5 min isotherm) with a rate of 10 °C/min and 1 μl of the solution was injected with a split of 1:30. Helium was used as carrier gas at a constant flow rate of 1 ml/min. The temperature of the split-splitless injector as well as of the direct coupling to the mass spectrometer was 250 °C. The MS was operated in electron ionization mode (EI, 70 eV), ion source temperature was 200 °C; only positive ions were scanned. Scan ran over the range m / z 50–500 at a scan rate of 1 scan/s for scans operated in full scan mode to qualify analytes. The results were referred to an internal CF standard (0.1 mg/ml CF = 100%), which allows determining the relative quantities of substances released from various resin-based materials. For absolute quantification of the elutable substances calibration curves were used by integration of the mass signal with the best signal to noise ratio. All eluates were analysed five times. The integration of the chromatograms was carried out over the base peak of the compounds, and the results were normalized by means of the internal CF standard. The identification of the various substances was achieved by comparison of their mass spectra with those of reference compounds/spectra, spectra from the NIST/EPA library, literature data and by chemical analysis of their fragmentation patterns . Reference retention times, reference spectra and calibration curves for each substance were obtained by using the pure substances from Sigma–Aldrich (Steinheim, Germany). For methodical reasons, monomers with a higher molecular weight like BisGMA and UDMA cannot be quantified with GC/MS.
Calculations and statistics
The results are presented as means (standard deviation, SD). A multivariate analysis (general linear model with partial eta-squared statistics) calculated within each tested material the effect of the parameters “irradiation time”, “time of storage before bleaching (aging)” and “bleaching technique” as well as their combined effect on the amount of eluted substances. The level of significance was set at p < 0.05. (SPSS Inc., Chicago, IL, USA, Version 20.0).