Long-term release of monomers from modern dental ... - Dentsply

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Monomer release from composite materials 69 In various studies (4, 9) the degree of monomer– polymer conversion in dental-composite resins has been found to vary between approximately 35 and 77%. Several authors investigated a possible correlation between the degree of conversion from the monomer to polymer which determine the chemical stability and solubility. Pearson & Longman (10) showed that reduced irradiation significantly increases the solubility. Tanaka et al. (1) found that increasing the irradiation time from 30 to 50 s resulted in a significant decrease in residual monomer contents and the quantities that were released into water. In contrast to the above studies, Ferracane (4) found a very poor correlation between the degree of conversion and solubility. The elution of substances from composite materials has been widely studied (11–15). Geurtsen et al. (13) evaluated the release of substances from restorative materials and also the cytotoxicity of the released monomers on fibroblasts. In their study, substances were identified that caused severe or minor cytotoxic effects in all permanent and primary cell lines. Additionally, their results indicate that for most of the severely toxic composite components, few cytotoxic alternatives are available. Spahl et al. (16) identified (co)monomers, additives, and contaminants from the manufacturing process to be released from resin composites. Several in vitro studies (12, 17) have shown the cytotoxic, genotoxic, mutagenic or estrogenic effects and pulpal and gingival/oral mucosa reactions of some of the monomers released from the composite materials. However, most of the in vitro studies (14, 18) have used experimental composite materials to investigate the process of monomer elution, leading to useful information about commercial products. Several studies have been conducted in water and in oral/food-simulating liquids, among which 75% ethanol/water solution has been used. This solution is recommended by the Food and Drug Administration (FDA) Guidelines of the USA (1976, 1988) as a food simulator and might be considered clinically relevant (19). It simulates the influence of certain beverages (including alcoholic), vegetables, fruits, and syrup. Different studies (9, 12) indicate the important role of the type of the solvent, and that irreversible processes, such as leaching of components, may occur (e.g. in the presence of ethanol). A close match in the solubility parameter results in maximum softening of the resin (20, 21). This phenomenon may contribute to irreversible material degradation provoked by ethanol because it penetrates the matrix and expands the space between polymer chains into which soluble substances, such as unreacted monomers, may diffuse. A 75% ethanol solution has a solubility parameter which matches that of BisGMA (20, 21). Another point of concern about the elution process of monomers of the resin composites is the method used to determine the quality and the quantity of the eluted monomers. Analysis is usually performed with highperformance liquid chromatography (HPLC) (14, 22). In this case, pure components are used as external standards for the identification and quantification of the eluted components. However, as reported previously (12), this method has some disadvantages and it is not suitable to give an exact identification of the substances eluted. Liquid chromatography–mass spectrometry (LC–MS) is an analytical technique that combines the advantages of an LC instrument with those of a mass spectrometer. High-performance liquid chromatography is a means of separating components of mixtures by passing them through a chromatographic column so that they emerge sequentially. Because LC operates in the liquid phase but MS is a gas-phase method, it is not a simple matter to connect the two. In order to allow the separated components of a mixture to be passed sequentially from the LC into the MS, an interface is needed. Having interfaced the two, LC–MS is a very powerful technique (23). A combination of HPLC (or LC) with MS can be used in these cases for a very specific identification of the compounds eluted (15). The fact that most of the studies made in this field were performed without LC–MS highlights the need for further research in this field. Although the advantages of the light-cured composites cannot be questioned, the chemically cured composites still have important applications in contemporary restorative dentistry, especially in areas that are not easily penetrable by light (24). This low conversion rate (17–24) of these materials could result in an increased release of monomers. Core build-up materials may be in contact with the oral cavity for a long period of time before crown preparation is performed. No information on the elution of monomers from core build-up materials exists in the literature. Although much literature has been published on the release of monomers (3, 11–13, 15, 25), information is still lacking with respect to the elution of monomers from modern composite materials such as ormocers, nanohybrid composite resins, and core build-up materials that are used daily in the dental clinical practice. Therefore, new studies are necessary to evaluate these materials under clinically relevant conditions. Additionally, no information is available on the release of monomers over a post-cure period of longer than 3 months. A longer storage time period simulating clinical oral conditions would be very useful in order to evaluate the composite materials. However, to correspond to the period that the composite restorations are expected to remain in the mouth, a storage time of 5–10 yr would be necessary. This would not be particularly useful for studying composite materials because each year new composite materials become commercially available and therefore results concerning 10-yr-old composite materials would not be clinically relevant. Therefore, the aim of the present in vitro study was to identify and quantify the elution of monomers of three different resin composites using liquid chromatography tandem mass spectrometry (LC–MS/MS), after different curing times and storage periods up to 1 yr after curing. Additionally, the following hypothesis was examined: composite materials with new chemistry, such as ormocers, release fewer monomers than materials with traditional chemistry.
70 Polydorou et al. Table 1 Composite materials tested Material Type Main monomer composition Manufacturer Filtek Supreme XT Ceram X Clearfil Core Universal restorative, nanohybrid ÔOrmocerÕ, universal nano-ceramic restorative Self-curing composite core build-up material BisGMA, TEGDMA, UDMA, BisEMA Methacrylate modified polysiloxane, dimethacrylate resin TEGDMA, BisGMA 3M ESPE Dental Products, Seefeld, Germany Dentsply DeTrey, Konstanz, Germany Kuraray Europe, Du¨ sseldorf, Germany Material and methods Three different resin-composite materials were used: a nanohybrid (Filtek Supreme XT; 3M ESPE Dental Products, Seefeld, Germany), an ormocer (Ceram X; Dentsply DeTrey, Konstanz, Germany) and a chemically cured (Clearfil Core; Kuraray Europe, Du¨ sseldorf, Germany) resin composite. Detailed information about the composition of the composite materials and the manufacturers are given in Table 1. For curing the samples of the two light-cured resin composites (Filtek Supreme XT and Ceram X), a halogen unit (Elipar Highlight; 3M ESPE) was used. The light intensity of this unit was 780–800 mW cm )2 . The spectral irradiance was determined using a visible curing light meter (Cure Rite; Dentsply, York, USA). From each of the two light-cured composite materials (Filtek Supreme XT and Ceram X), four groups (n = 10) were prepared, one for each curing time tested (0, 20, 40, and 80 s). From the chemically cured resin composite (Clearfil Core) 10 specimens were fabricated. The specimens were prepared using forms, given from Dentsply DeTrey, allowing the production of standardized cylindrical specimens (4.5 mm in diameter and 2 mm thick). The forms were positioned on a transparent plastic matrix strip lying on a glass plate. Then, they were filled with the composite materials. The samples were built up in one increment (2 mm). After inserting the materials into the discs, a transparent plastic matrix strip (Kerr Hawe, Bioggio, Switzerland) was placed on top of them in order to avoid creating an oxygeninhibited superficial layer. In addition, a glass slide was used in order to flatten the surface. Every sample was then cured according to the group to which it belonged. Directly after curing, each sample was immediately immersed in 1 ml of 75% ethanol. The samples were stored in a dark box at room temperature (’21°C) and the storage medium (1 ml) was renewed 24 h, 7 d, 28 d, and 1 yr after the polymerization. From the storage medium removed, ethanol samples were prepared and stored until analysis at 4°C in the dark. Analytical method Information about the substances used as standards for the analysis are given in Table 2. The reference standards of BPA, BisGMA, one UDMA, and TEGDMA were purchased from Dentsply DeTrey. A second type of UDMA was obtained from Deltamed (Freidberg, Germany). The two types of UDMA differ in chemical composition and molecular weight. Usually, different types of UDMA are used for the production of composite materials. The analysis was performed as described in detail by Polydorou et al. (15) with the difference being that the limits of quantification were improved: 1 lg ml )1 for UDMA, 0.5 lg ml )1 for TEGDMA,1 lg ml )1 for BisGMA, and 0.5 lg ml )1 for BPA. Values lower than these levels could not be qualified. For the analysis, the ethanol samples were investigated using LC–MS/MS. A Bruker Esquire 3000 plus (Bruker- Franzen Analytik, Bremen, Germany) was used. Separation of the monomers was performed using a CC 125/4 Nucleodur 100-5 C18ec HLC-Column (Machery & Mayiel, Du¨ ren, Germany) and a solvent gradient of 0.1% formic acid and acetonitrile. For the analysis, external calibrations with ethanol standards (up to a 1,000 lg ml )1 ) were carried out with the help of the peak areas of the extracted ion chromatograms (EICs). Table 3 presents the identification masses that were used for the qualification and the quantification of the substances during LC–MS/MS. Statistical analysis The significance level of the statistical analysis was 0.05. For the statistical analysis, the repeated-measures analysis of variance (anova) was used with two between-factors (materials and monomers) and two within-factors (curing time and storage time). Because the data were not normally distributed, the logarithms were used for the statistical analysis. According to the Ôcorrelation procedureÕ, the log-transformed data were normally distributed (P = 0.27). Table 2 Monomers used as reference standards Substances Name Chemical type Mol. weight CAS-number BisGMA Bisphenol A glycidyl methacrylate C 29 H 36 O 8 513.0 1565-94-2 TEGDMA Triethylene glycol dimethacrylate C 14 H 22 O 6 286.32 109-16-0 UDMA 1 Urethane dimethacrylate Product C 26 H 42 O 8 N 2 498.0 – UDMA 2 Urethane dimethacrylate C 23 H 38 N 2 O 8 470.56 41137-60-4 or 72869-86-4 BPA Bisphenol A C 15 H 16 O 2 228.29 80-05-7
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