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DOI 10.1007/s10337-014-2647-3Chromatographia

OrIgInal

Quantification of Conversion Degree and Monomer Elution from Dental Composite Using HPLC and Micro‑Raman Spectroscopy

Edina Lempel · Zsuzsanna Czibulya · Sándor Kunsági‑Máté · József Szalma · Balázs Sümegi · Katalin Böddi

received: 30 September 2013 / revised: 30 January 2014 / accepted: 21 February 2014 © Springer-Verlag Berlin Heidelberg 2014

layer increment up to 3 mm from the top led to 10 % decrease in DC and 30–35 % increase in monomer elution. Further increase in depth from 3 to 4 mm caused 30 % drop in DC and 55 % increase in the amount of leached mono-mers. The overall result of the findings indicates that direct correlation exists between DC of composite and the elution of unreacted monomers.

Keywords High-performance liquid chromatography · Micro-raman spectroscopy · Dental composite · Degree of conversion · Monomer elution

Introduction

The polymerization of dimethacrylates produces densely crosslinked networks, resins, which find wide applications in dentistry such as dental composites. resin-based com-posite (rBC) materials are widely accepted among dental clinicians for restoration of tooth cavities. These resins polymerize when exposed to light, otherwise known as light-activated rBCs. The term degree of conversion (DC), for methacrylate-based materials, generally means the per-centage of C=C bonds of the matrix monomers reacted with polymer. The DC is conventionally calculated as the ratio of C=C double bonds in cured and uncured materi-als related to an internal standard. Ideally, the rBC would have all of its monomer converted to polymer when the DC is 100 %. However, one of the most common drawbacks of all rBCs is inadequate polymerization; not all of the dimethacrylate monomer is converted to a polymer and considerable residual monomers remain in the final product [1]. The DC ranges from 50 to 70 % under conventional irradiation conditions [2, 3]. Many factors affect the polym-erization rate of the resin, such as composition, type, and

Abstract The purpose of the study was to analyze the correlation between the quantity of eluted monomers from dental resin-based composite using reverse-phase HPlC and the degree of conversion (DC) using micro-raman spectroscopy, and to evaluate the influence of the energy of polymerization delivered on the composite material and the applied resin layer thickness on these properties. There was direct proportion in degree of conversion and inverse proportion in monomer elution when the energy of light polymerization was increased from 20 to 40 J cm−2; how-ever, further increase in energy density did not influence significantly the DC and the elution of monomers. Inves-tigating the depth of cure significant differences could be measured both in DC and the elution of monomers. 1 mm

Published in the special paper collection 9th Balaton Symposium on High-Performance Separations Methods with guest editor attila Felinger.

E. lempel (*) Department of restorative Dentistry and Periodontology, University of Pécs, 5 Dischka gy Street, Pecs 7621, Hungarye-mail: lempel.edina@pte.hu

Z. Czibulya · S. Kunsági-Máté Department of general and Physical Chemistry, University of Pécs, 6 Ifjúság Street, Pecs 7624, Hungary

Z. Czibulya · S. Kunsági-Máté Janos Szentagothai research Center, 20 Ifjúsag Street, Pecs 7624, Hungary

J. Szalma Department of Oral and Maxillofacial Surgery, University of Pécs, 5 Dischka gy Street, Pecs 7621, Hungary

B. Sümegi · K. Böddi Department of Biochemistry and Medical Chemistry, University of Pécs, 12. Szigeti Street, Pecs 7624, Hungary

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polarity of the monomer molecules contained; the filler volume fraction and the type; concentration of photoinitia-tors; and the light and energy properties delivered on the material and the applied resin layer thickness so the depth of cure [4, 5]. It has been shown that the same degree of conversion is produced by a fixed energy amount (energy density: J cm−2), independent of variations in light irra-diance [6]. Thus, a recommendation of 21 and 24 J cm−2 energy density has been made for the adequate polymeri-zation (not more than 50–70 %) of 2 mm thick composite specimen [6]. light from the curing source must be able to adequately polymerize deeper composite regions than just the top, irradiated surface. However, as light passes through the composite, it is absorbed and scattered, reducing its effectiveness to initiate polymerization, and consequently resulting in variation of cure with depth [5].

The DC is generally accepted to correlate with the elu-tion of unreacted monomers; however, it is not known how big a difference in DC is required to produce a notice-able change in monomer elution. The amount of leachable unreacted monomers reflects the solubility of the rBCs. Solubility influences the resistance to degradation in the oral environment, and from this point of view, it is crucial for the biocompatibility of resin composite restorations. leaching of monomers from a rBC may lead to cytotoxic reaction of the soft tissues [7, 8]. The amount of molecules that leach from rBCs, besides the DC, can be affected by the specimen thickness, the size and chemical composition of the leachable species, the polymeric matrix composi-tion, the filler particle type and content, the porosity and the solvent [4]. The elution of monomers and DC of rBCs has been extensively investigated [4, 9–11]. To determine the quality and the quantity of the eluted monomers high-performance liquid chromatography (HPlC) is the most common analytic method [12, 13]. By measuring the DC, the amount of pendant methacrylate groups as well as unre-acted monomers in the composite can be evaluated using micro-raman spectroscopy [11, 14].

as mentioned above, the DC of rBC and thus elution of unreacted monomers depend on the sufficient energy density of delivered light and thickness of the increment. nevertheless, the relative contributions of these variables to the degree of conversion are still unknown; this informa-tion might be regarded as essential for the development of new strategies to increase DC and to reduce the quantity of leached unreacted monomers. Furthermore, the effects of these variables on DC and elution still need to be properly determined.

The objective of this in vitro study was to analyze the correlation between the quantity of eluted monomers using reverse-phase HPlC and the DC of rBC using micro-raman spectroscopy. Experiments were done to evaluate how the energy of the light used for illumination influences

the polymerization and how the polymerization performed in layers with different thickness and depth is affected by the illumination.

Materials and Methods

BisgMa (bisphenol A diglycidil ether dimethacrylate, 98 %), UDMa (urethane-dimethacrylate, ≥97 %) and TEg-DMa (triethylene glycol dimethacrylate, 95 %) (Sigma-aldrich, Steinheim, germany) were used as standard mate-rials for the identification of the monomer peaks in the chromatograms. BisgMa/UDMa/BisEMa(6)/TEgDMa-based Filtek Z250 (3M ESPE, St. Paul, Mn, USa, a2 shade) submicron, hybrid composite resin models were pre-pared, washed and stored in a 75 % ethanol/water solution (Spektrum-3D, Debrecen, Hungary). acetonitrile (aCn) (VWr International, leuven, Belgium) was used for the preparation of the mobile phase for the HPlC separation.

Preparation of the Composite resin Specimens for Evaluation of the Effect of light Exposure Time

The composite resins were poured into a stainless steel mold with a size of 2 mm in diameter × 2 mm in thickness (n = 20) and positioned on a glass slide. The top and the bottom of the resin composite were covered with a polyes-ter (Mylar) strip to avoid contact with oxygen, which is an inhibitor of the polymerization. The specimens were irradi-ated with light emitting diode (lED) curing unit (λ = 420–480 nm; lED.C, Woodpecker, guilin, China) with 20 s (n = 15) and 40 s (n = 5) exposure time at light intensity of 1,000 mW cm−2 with an irradiated diameter of 10 mm. Cur-ing time recommendation from the manufacturer for 2.5 mm thickness and most of shades is 20 s. The light intensity of the lED light source was monitored before and after cur-ing with a radiometer (SDS, Kerr, Danbury, CT, USa). The curing light guide was positioned directly above the com-posite specimen to ensure that the tip of the light guide was parallel to the sample. During the cure treatment, the Mylar was removed. Five specimens, cured with 20 s exposure time, were post-cured in a xenon polymerization furnace (Dentacolor XS, Heraeus Kulzer gMBH, Hanau, germany) for 90 s and further 5 specimens for 180 s. The light inten-sity of the furnace was 250 mW cm−2, and the wavelength was 320–520 nm. after polymerization, the specimens were analyzed with micro-raman spectroscopy and afterward stored in 1 ml of 75 % ethanol/water solution for 72 h in darkness at room temperature for further dissolution of the unreacted monomers. The resins were removed after 72 h, and the ethanol solutions containing dissolved unpolymer-ized monomers were analyzed with reverse-phase high-per-formance liquid chromatography (rP-HPlC).

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Preparation of the Composite resin Specimens for Evaluation of the Depth of Polymerization

Uncured Filtek Z250 composite was placed in a cylindrical sil-icon mold made of heavy body vinyl polysiloxane impression material (Express, 3M, St Paul, Mn, USa): 4 mm in diameter, 4 mm in height. The material was covered with Mylar strip and pressed with a glass slab to force the composite to adapt to the confines of the mold. Photoactivation was performed using lED unit (with the above-mentioned parameters) with 20 s exposure time. after photoactivation, the cylinder of cured composite was fixed to an acrylic base with soft wax, and transversally cut with a rotary saw (model 650, South Bay Technology Inc., San Clemente, Ca, USa), using a dia-mond disc under water spray. Sections provided specimens representing the 1–4 mm layer thicknesses. after sectioning, the specimens were analyzed with the same procedures as described in “Preparation of the Composite resin Specimens for Evaluation of the Effect of light Exposure Time”.

Micro-raman Spectroscopy Measurement

The polymerized composite samples were examined using labram Hr 800 Confocal raman spectrometer (HOrIBa Jobin–Yvon S.a.S., longjumeau Cedex, France). The fol-lowing sets of parameters were applied during the micro-raman measurements: 20 mW He–ne laser with 632.817 nm wavelength, spatial resolution ~1.5 μm, spectral resolu-tion ~2.5 cm−1, magnification × 100 (Olympus UK ltd., london, UK). Three point spectra were taken on the top sur-face of the composite specimens at random locations with 10 s integration time and six acquisitions were averaged for each geometrical point. Spectra of uncured composite were taken as reference. Post-processing of spectra was performed using the dedicated software labSpec 5.0 (HOrIBa Jobin–Yvon S.a.S., longjumeau Cedex, France) and including the band fitting procedure using the levenberg–Marquardt method of non-linear peak fitting for the best fit [15, 16]. The ratio of double-bond content of monomer to polymer in the composite was calculated according to the following equation:

where R is the ratio of peak intensities at 1,639 and 1,609 cm−1 associated with the aliphatic and aromatic (unconjugated and conjugated) C–C stretching in cured and uncured composites, respectively [16].

rP-HPlC Measurements

The rP-HPlC instrument consists of a Dionex P680 gra-dient pump, rheodyne 8125 injection valve, and a Dionex UVD 170U UV–Vis detector (Dionex gmbH, germering,

DC % =

(

1 −

(

Rcured/

Runcured

))

× 100,

germany). Data acquisition was completed using Chrome-leon software (version: 6.60 SP3 Build 1485). The sepa-rations were performed on a Kovasil (particle size: 6 μm, pore size: 11 nm, Zeochem, Uetikon, Switzerland) ODS (C18) column with gradient elution. Eluent “a” was 40 % v/v aCn in bidistilled water, whereas eluent “B” contained 95 % v/v aCn. The applied gradient required 30 min, dur-ing which time the “B” eluent content increased from 20 to 100 %. The flow rate was 1.2 ml min−1. The separa-tion was followed by regeneration. after 30–31 min, the “B” eluent content decreased from 100 to 20 %, and after 31–46 min, the system was washed with 100 % “a”.

Chromatograms were monitored at 205, 215, 227, and 254 nm, and among these, 205 nm was found to be opti-mal; therefore, the evaluation relied on the data collected at this wavelength.

The residual monomers of BisgMa, UDMa and TEgDMa were calculated from the calibration curve (R2 ≈ 0.998) using the areas under the curve of peaks produced by BisgMa, UDMa and TEgDMa. The TEg-DMa, UDMa and BisgMa standard solutions had reten-tion times of 7.59, 11.34 and 13.50 min respectively, whereas the peaks were well separated from each other. all measurements of residual monomers were performed at room temperature.

Validation of the Monomer Determination. Limit of Detection (LOD), Limit of Quantification (LOQ)

The detection limit of the method (defined as the amount of monomers giving a peak height five times higher than the noise level) was low: TEgDMa: 2.2 pmol (0.65 ng), UDMa 3.3 pmol (1.57 ng), BisgMa 1 pmol (0.52 ng). The quantification limit of the method (defined as the amount of monomers giving a peak height 10 times higher than the noise level) was low for TEgDMa: 4.4 pmol (1.3 ng), for UDMa 6.7 pmol (3.1 ng) and for BisgMa 2 pmol (1.0 ng). Calibration was carried out in the concen-tration range of 2.5–100.0 μg × ml−1 monomers, respec-tively. a calibration curve was plotted by the measurement of standard solutions at 205 nm (R2) 0.9988 for TEgDMa, 0.9998 for UDMa and 0.9996 for BisgMa, respectively. all injections were repeated three times.

Statistical analysis

The statistical analysis was performed using SPSS (Statisti-cal Package for Social Science, SPSS Inc., Chicago, USa) software for Windows. The values for degree of conver-sion and for residual monomers between the studied test groups were compared by a one-way analysis of variance (anOVa) test followed by post hoc tests (Tukey’s and Dunnett’s T3).

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Results

Degree of Conversion—Micro-raman Spectroscopy

Table 1 summarizes the mean and standard deviations of the degree of conversion for the composite material after illumination at 20 and 40 s with lED and at 20 s with lED followed by 90 and 180 s polymerizations in the furnace. Figure 1 shows the micro-raman spectra of resin com-posite obtained after light curing with the different expo-sure times. The polymerization effect can be followed by decreasing the intensity of the raman peak at 1,639 cm−1, while considerable decrease of the raman peak located at 1,609 cm−1 can be observed. This spectral change is associ-ated with the aliphatic to aromatic conversion of the mate-rial and reflects the formation of polymeric structure. The frequency of the raman peak associated with the stretching of the aliphatic C–C bond slightly decreases as the molecu-lar environment changes by the progress of polymerization. Investigating the effect of exposure time on rate of polym-erization, the obtained raman peak intensity at 1,609 cm−1 increases suddenly, while the intensity of the other peaks decreases stepwise.

The difference in DC % was significant (p < 0.05) between 20 and 40 s exposure times; however, post-polymerization in the furnace did not provide significantly increased DC %.

Table 2 shows the mean and standard deviations of the degree of conversion for four increments of composite after illumination at 20 s with lED curing unit. Figure 2 shows micro-raman spectra of the four layer thicknesses of resin composite obtained after light curing. Evaluating the depth of cure, the change of obtained raman peaks can be fol-lowed continuously. The degree of polymerization is indi-cated by the applied colors of the curves (from black to red, respectively), and the rate of polymerization by the size of the represented arrows.

Statistically significant differences in DC % (p < 0.05) associated with the different layer thicknesses were obtained.

Monomer Elution—HPlC

Figure 3 shows the amount of eluted BisgMa, UDMa and TEgDMa from Filtek Z250 resin composite mate-rial according to the duration of exposure. The amount of

Table 1 Degree of conversion (DC %) at different exposure time

The same superscript letters indicate statistical significance in DC % (one-way anOVa; p < 0.05)

Exposure time Mean (%) SD

20 s lEDa 46.28 3.96

40 s lEDa 53.99 3.00

20 s lED + 90 s furnace 54.87 1.88

20 s lED + 180 s furnace 55.83 3.95

Fig. 1 The change of raman spectra due to different time and type of applied polymerization method. The inset shows the fitted gauss curves by Origin 8.5.1 on the range of spectra, which were used for DC % calculations

Table 2 Degree of conversion (DC %) in different depth

The same superscript letters indicate statistical significance in DC % (one-way anOVa; p < 0.05)

Depth Mean (%) SD

1 mma 56.68 2.74

2 mma,B 50.42 1.62

3 mmB,C 45.13 2.17

4 mmC 17.78 1.44

Fig. 2 The change of raman spectra of the differently polymerized sample layers from the top to the bottom. The inset shows the fitted gauss curves using Origin 8.5.1 on the range of spectra, which were used for DC % calculations

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eluted UDMa was the highest in case of each exposure time. Half amount of BisgMa was leached than UDMa and the amount of eluted TEgDMa was the least. There was statistically significant difference in the amount of eluted UDMa and BisgMa between the 20 and 40 s exposure times (p < 0.05). Further increase in curing time did not affect significantly the elution of monomers. The release of unreacted TEgDMa monomer was not influ-enced by the time of polymerization.

Figures 4 and 5 present the amount of eluted mono-mers from different depths. 50 % more amount of leached UDMa monomer was detected than BisgMa in each depth; meanwhile, very low amount of released TEgDMa was observed. The difference in the amount of each mono-mer was statistically significant among the investigated depths and was approximately two times higher between each depth (p < 0.05).

Discussion

In the present study, monomer elution from rBC was quan-tified using HPlC and correlated with the DC determined by micro-raman spectroscopy. Further aim was to evaluate the influence of the setup of light illumination delivery on the material and the depth of cure on these properties. In the present study, Filtek Z250 commercial resin composite samples were used to investigate the correlation between the elution of monomers and DC. The Filtek Z250 is based on BisgMa/UDMa/TEgDMa matrix system. BisgMa, UDMa and TEgDMa were identified by their retention times in HPlC (determined during calibration measure-ments). This is a standard method used for the determina-tion of monomer elution from rBCs [17].

The release of monomers has a potential effect on the structural stability and wear rate as well as the biocompat-ibility of the material. The chemistry of the solvent, the size and chemical nature of the released components and the extent of polymerization are the most significant factors for monomer elution [1, 18].

In the present study, the total quantity of eluted residual UDMa was considerably greater than quantity of residual BisgMa and TEgDMa. The possible explanation is the difference in molecular weight of the monomers (Bis-gMa > UDMa > TEgDMa) and the total content of the monomer in the composite (BisgMa > UDMa > TEg-DMa). In Filtek Z250, the TEgDMa content is very low; the matrix is based on mostly BisgMa and UDMa. Tan-aka et al. [19] discovered that small molecular weight mon-omers could be extracted in considerably higher quantities than large molecular weight monomers. Small molecular weight monomers such as TEgDMa have higher mobil-ity and will be eluted faster than large molecules such as BisgMa and UDMa, despite their low content in the com-posite resin [19, 20]. Comparing the molecular weight of BisgMa and UDMa, BisgMa has higher weight, thus the release of UDMa is faster and more in a certain time inter-val. Furthermore, both of these resins are of higher molecu-lar weight and, therefore, have fewer double bonds per unit of weight which permits to reach a higher DC value.

To minimize the amount of residual monomer elution, resin-based restorative materials have to be polymerized to a high degree [21]. Characteristics related to the light source, such as energy density and spectral flux, alter the final DC [22–24]. according to rueggeberg et al. [23] and Emami et al. [6], there is a close correlation between energy density and the DC. Energy density (J cm−2 or mW s cm−2) is the product of light intensity (mW cm−2) and irradiation time (s). It is suggested that the same degree of conversion is produced by a fixed amount of energy, independent of

Fig. 3 amount of eluted monomers from composite cured with dif-ferent exposure time

Fig. 4 amount of eluted monomers from different layer depth of composite

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variations in light irradiance [6]. In their studies, with dif-ferent light energy densities with the same light source, Silikas et al. [25] showed that when light energy was decreased, DC decreased dramatically too. Peutzfeldt et al. [24] also suggested that the higher the energy density, the higher is the DC of a monomer. The above findings might further indicate that the total energy amount delivered was a contributing factor in DC efficiency of a light-curing unit. However, according to our findings, there is a limit in total energy where further rise does not increase significantly the rate of polymerization. as the material characteristics, such as matrix composition, concentration of photoinitiator, filler particle size and distribution, are factors which affect the polymerization kinetics and rate of conversion, the results might be different from the present findings when the investigated composite material is different.

In the present study, lED curing unit was used to polymerize the composite samples with 20 and 40 s exposure times. The intensity of this lED unit is 1,000 mW cm−2, thus the delivered energy was 20 and 40 J cm−2. To increase the applied energy, further polymeri-zation was performed on the samples in a polymerization furnace (xenon light intensity: 250 mW cm−2). The irradi-ation time in the furnace was 90 and 180 s, thus the deliv-ered total energy was 42.5 and 65 J cm−2. The DC values (46.28 and 53.99 %) were in accordance with the deliv-ered energy up to 40 J cm−2; however, above this energy density the difference between the DC (53.99, 54.87 and 55.83 %) was not significant. It means that the DC % of specimens cured with 40 s exposure time was 7.71 % higher than DC % of samples cured for 20 s; however, there was no significant increase in DC % of specimens polymerized further for 90 or 180 s in the furnace in spite of the higher energy density delivered on these samples. a significant amount of heat is transferred to the composite by the light curing unit during photoactivation, especially in the polymerization furnace, where the heat production is notable (60–90 °C) [26]. Several studies have demon-strated that the final conversion increases at relatively high

curing temperatures (i.e., 40–60 °C) due to the enhanced mobility of the reaction media [27, 28]. In contrast with these findings, present results did not indicate significant increase in DC of the samples after polymerization in the furnace (60–80 °C).

In accordance with the DC % of each groups, the UDMa and BisgMa monomer release showed the same pattern; however, the amount of eluted TEgDMa was not significantly influenced by the energy density, although a tendency was noticeable when the delivered energy den-sity increased. The possible explanation is the very low content of TEgDMa of the resin matrix. The higher the content, the more is the detectable change in the amount of released monomers. according to the DC values, the high-est amount of UDMa and BisgMa was released from the samples cured with 20 s exposure time, and decreased elu-tion was observed from specimens polymerized with 40 s exposure time. Quantity of monomer elution from sam-ples is post-cured with 90 and 180 s in the furnace does not differ significantly from each other and there was no significant difference in the elution of monomers between these groups and the group cured with 40 s exposure time. The saturation of monomer conversion in Filtek Z250 at this energy density might be caused by the limited con-centration of photoinitiator and/or the filler loading of the material.

Comparison among the four layer thicknesses showed a significant reduction in DC between 56.68 and 17.78 % from the top to the bottom of the specimens. This decrease is generally explained by light attenuation in the specimen due to reflection, absorption, and scattering of light [29].

nomoto and Hirasawa [30] stated that the depth of cure is dependent on the light permeability of the filler as well as the monomer composition and type and concen-tration of initiator, inhibitor, and accelerator in the resin materials. according to Obici et al. [5] and Yap et al. [31], the light is not significantly reduced in intensity in more shallow composites (~1 mm layer thickness); there-fore, the maintained energy is sufficient to initiate the

Fig. 5 HPlC chromatogram with eluted monomers from dif-ferent layer depth. 1 (0–1 mm), 2 (1–2 mm), 3 (2–3 mm), 4 (3–4 mm) chromatograms represent the eluted monomers from the different composite layer depth

Quantification of Conversion Degree and Monomer Elution

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reaction. Conversely, in deeper regions (3–4 mm), as the light passes through the bulk of the restorative material, the light absorption and scattering by the resin compos-ite greatly decrease the light intensity, thereby decreas-ing the potential for curing and increasing the elution of leachable monomers [5]. according to our results, the DC % of the top 1 mm is 56.68 %, and in 2 and 3 mm layer thickness the DC % is 50.42 and 45.13 % which means ~5 % decrease in each mm from the top toward the bottom. However, in the bottom layer (4 mm), the DC % is only 17.78 % which is nearly 30 % drop in DC. The poor polymerization was perceptible during the sample preparation, it was soft and sticky. Musanje and Darwell [32] qualified the depth of cure—defined by the depth at which hardness is equal to an arbitrary 80 % of the hardness at the surface—as an ill-defined concept, driven only by convenience. Instead, these authors rec-ommend that depth of cure should be defined as the depth at which maximum conversion occurs for a given irradi-ance and irradiation time. In the first part of our investi-gation, an increased DC % (from 49.28 to 53.99 %) of the 2 mm samples was observed when the light energy was increased from 20 to 40 J cm−2; however, further increase did not affect significantly the DC. Furthermore, in the second part, when the depth of cure was investi-gated, our results represent higher DC % in the 1 mm samples (56.68 %) cured for 20 s compared with the DC % of the 2 mm samples (50.42 %). Consequently, in the present study, 1 mm thick Filtek Z250 sample cured with 40 J cm−2 light energy would be acceptable clinically if the recommended depth of cure is defined as the depth at which the maximum DC is reached for a given irradiance energy.

In contrast with our finding, Obici et al. [5] observed almost 10 % higher DC value in Filtek Z250 cured with lED curing unit despite the lower energy density in each layer thicknesses even in the 4 mm (58.77 %) and 5 mm (54.73 %) sample. The possible explanation for this dif-ference may be that Fourier transform infrared spectro-photometer was used to analyze the DC of rBC samples. according to their results, there was no significant differ-ence between DC of the 1 and 2 mm samples, while in our study 1 mm samples polymerized at a significantly higher degree than the 2 mm samples.

reporting on the elution of monomers with regard to the depth of cure each monomer had significantly higher elu-tion when the depth was increased. When the depth was increased from 1 to 2 mm and from 2 to 3 mm, the elu-tion rate of each monomer was approximately 30–35 %. There was an increased elution rate (55 %) of BisgMa and UDMa when the depth was increased from 3 to 4 mm; however, the elution rate of TEgDMa was only 37 % simi-larly to the shallower depths.

Conclusion

The influence of energy of polymerization delivered on Filtek Z250 and the influence of curing depth on DC and elution of monomers and correlation between these two parameters were evaluated in the present study.

Within the limitations of this study, the following con-clusions can be drawn:

1. There was a significant increase in DC and decrease in monomer elution when the energy was increased from 20 to 40 J cm−2; however, further increase in energy density did not influence significantly the DC and the elution of monomers. For this composite material, the energy density which polymerizes the material at the highest degree is 40 J cm−2.

2. The applied resin layer thickness significantly influ-ences both DC and elution of monomers. 1 mm layer increment up to 3 mm led to 10 % decrease in DC and 30–35 % increase in monomer elution for all BisgMa, UDMa, TEgDMa. Further increase in depth from 3 to 4 mm caused 30 % drop in DC and 55 % increase in the amount of leached BisgMa and UDMa. The ratio between DC and quantity of monomer elution is approximately 1:3, if the variable is the depth of polymerization, thus 1 % increase in DC provides 3 % decrease in monomer (BisgMa, UDMa) elution in Filtek Z250.

3. The overall result of the study indicates that direct cor-relation exists between DC of composite and the elu-tion of unreacted monomers.

4. In conclusion, for Filtek Z250 composite, the depth of cure at which maximum conversion occurs is 1 mm instead of the recommended 2.5 mm. For the manufac-turer, it would be more adequate to recommend energy density instead of exposure time.

Acknowledgments This work was supported by PTE ÁOK-Ka-2013/5 research grant and the Hungarian research project no. TÁMOP-4.2.2.a-11/1/KOnV-2012-0065.

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