JJOD 2405 1–9 journal of dentistry xxx (2015) xxx–xxx

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Curing profile of bulk-fill resin-based composites Q1

Xin Li, Pong Pongprueksa, Bart Van Meerbeek, Jan De Munck * KU Leuven-BIOMAT, Department of Oral Health Sciences, KU Leuven (University of Leuven) & Dentistry, University Hospitals Leuven, Kapucijnenvoer 7, Blok A – Box 7001, BE-3000 Leuven, Belgium

article info

abstract

Article history:

Objective: To evaluate the curing profile of bulk-fill resin-based composites (RBC) using

Received 26 August 2014

micro-Raman spectroscopy (mRaman).

Received in revised form

Methods: Four bulk-fill RBCs were compared to a conventional RBC. RBC blocks were light-

7 January 2015

cured using a polywave LED light-curing unit. The 24-h degree of conversion (DC) was

Accepted 9 January 2015

mapped along a longitudinal cross-section using mRaman. Curing profiles were constructed

Available online xxx

and ‘effective’ (>90% of maximum DC) curing parameters were calculated. A statistical linear mixed effects model was constructed to analyze the relative effect of the different

Keywords:

curing parameters.

Bulk-fill composites

Results: Curing efficiency differed widely with the flowable bulk-fill RBCs presenting a

Degree of conversion

significantly larger ‘effective’ curing area than the fibre-reinforced RBC, which on its turn

Curing profile

revealed a significantly larger ‘effective’ curing area than the full-depth bulk-fill and

Micro-Raman

conventional (control) RBC. A decrease in ‘effective’ curing depth within the light beam was found in the same order. Only the flowable bulk-fill RBCs were able to cure ‘effectively’ at a 4-mm depth for the whole specimen width (up to 4 mm outside the light beam). All curing parameters were found to statistically influence the statistical model and thus the curing profile, except for the beam inhomogeneity (regarding the position of the 410-nm versus that of 470-nm LEDs) that did not significantly affect the model for all RBCs tested. Conclusions: Most of the bulk-fill RBCs could be cured up to at least a 4-mm depth, thereby validating the respective manufacturer’s recommendations. Clinical significance: According to the curing profiles, the orientation and position of the light guide is less critical for the bulk-fill RBCs than for the conventional RBC. # 2015 Published by Elsevier Ltd.

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1.

Introduction

Q2 An incremental layering technique to apply photo-curable

resin-based composites (RBCs) is widely recommended to Q3 overcome problems related to polymerization shrinkage and

to ensure adequate polymerization. However, disadvantages associated with this technique, such as incorporation of voids, a greater risk on contamination between layers and an extended chair time, still remain a practical discomfort.1

Recent advances in RBC technology, especially with regard to new monomers, translucency, initiator systems2,3 and filler technology,4 address these shortcomings at least partially and have led to the introduction of bulk-fill RBCs. Bulk-fill RBCs can be defined as composites that can be properly cured in a single layer of (mostly) a 4-mm thickness. To directly assess polymerization efficiency of light-cure RBCs, the main properties measured in laboratory are the degree of conversion (DC) and the depth of cure (DoC).2,4–11 The latter is officially defined by ISO standard 4049.12 However,

* Corresponding author. Tel.: +32 16 33 27 90; fax: +32 16 33 27 52. E-mail address: [email protected] (J. De Munck). http://dx.doi.org/10.1016/j.jdent.2015.01.002 0300-5712/# 2015 Published by Elsevier Ltd.

Please cite this article in press as: Li X, et al. Curing profile of bulk-fill resin-based composites. Journal of Dentistry (2015), http://dx.doi.org/ 10.1016/j.jdent.2015.01.002

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a recent study correlating DoC with micro-mechanical properties revealed that this method overestimated the curing efficiency of bulk-fill RBCs.7 As DC is a direct evaluation method of curing efficiency, a DC profile over a larger crosssection may be better suited to determine the maximum increment thickness for clinical use. Because of the increased volume cured at once, it is more likely in clinical practice that the light guide is misaligned or positioned further away from the restoration surface. To assess the effect of such positioning faults, a curing profile of an area larger than the diameter of the light guide can be helpful. Such a curing profile can be constructed by mapping DC in function of depth and the relative position of the light guide along a cross-section of a cured RBC block that is larger than the light guide of the light-polymerization unit. A recent development in dental resin technology is the use of different initiator systems within one material.10 Apart from the commonly used photo-initiator system camphorquinone (CQ), in combination with a tertiary amine as coinitiator, other photo-initiators such as trimethylbenzoyldiphenylphosphine oxide (TPO)13 and dibenzoyl germanium derivative (Ivocerin)14 have been employed. However, the emission spectrum of common commercially available lightemitting diode (LED) light-curing units is optimized for the

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Q6

peak-absorption wavelength of the CQ photoinitiator15 at 430– 480 nm, whereas the absorption range of TPO is 350–425 nm15 and 370–460 nm for Ivocerin.14 To overcome this absorptionwavelength mismatch, LED chips with different spectral outputs have been inserted in so-called polywave LED lightcuring units. The resultant light beam of such polywave LED light-curing units may, however, be inhomogeneous, this due to the out of centre position of the different LEDs.16–18 The effect of this beam inhomogeneity on DC at deeper areas can be visualized by constructing a complete curing profile. The purpose of this study was to evaluate the curing profile of bulk-fill RBCs as compared to that of a conventional nanohybrid RBC. The null hypotheses tested were1 that there is no difference in the curing profiles of the RBCs tested, and2 that the curing profile is not affected by the position of the different wavelength LEDs within the light-curing unit.

2.

Materials and methods

2.1.

Specimen preparation

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This study evaluated the curing profiles of four bulk-fill RBCs and one conventional nano-hybrid RBC (Table 1). Per group,

Table 1 – List of the RBCs tested in this study. RBC

Manufacturer

everX Posterior

GC

Filtek Bulk Fill Flowable

3M ESPE

SDR

Dentsply

Type

Shadea

Fibre-reinforced bulk-fill base

U

Flowable bulk-fill base

U

Flowable bulk-fill base

U

Tetric EvoCeram Bulk Fill

Ivoclar Vivadent

Full-depth bulk-fill nano-hybrid

IVA

Herculite XRV Ultra

Kerr

Conventional nano-hybrid (control)

A2

54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69

Batch no. LOT1308052

N387550

LOT1306112

LOT S38401

LOT4856756

Matrix compositionb,c Bis-GMA, TEGDMA, PMMA

Bis-GMA, Bis-EMA, UDMA

Modified UDMA, TEGDMA, EBPADMA Bis-GMA, Bis-EMA, UDMA

Ethoxylated Bis-GMA, TEGDMA

Filler content (filler size)

Photo-initiatorsc CQ, DMAEMA

74.2 wt%; E-glass fibre (1–2 mm length), barium borosilicate glass filler (0.1–2.2 mm) CQ, EDMAB 64.5 wt%; ytterbium trifluoride (0.1– 5.0 mm), zirconia silica (0.01–3.5 mm) CQ 68 wt%; barium and strontium aluminosilicate glass (NAd) 80 wt%; barium glass filler (0.04–3.0 mm)

CQ, TPO, Ivocerin

CQ 78 wt%; quartz, barium glass filler (0.4 mm), colloidal silicondioxide (0.02–0.05 mm)

a

Shade U = universal; Shade IVA = universal A shade. According to technical information provided by the manufacturer. c Abbreviation of chemicals: Bis-GMA = bisphenol A diglycidyl dimethacrylate; Bis-EMA/EBPADMA = ethoxylated bisphenol A diglycidyl dimethacrylate; CQ = camphorquinone; DMAEMA = N,N-dimethylaminoethyl methacrylate; EDMAB = ethyl 4-dimethyl aminobenzoate; Ivocerin = dibenzoyl germanium derivative; PMMA = polymethylmethacrylate; TEGDMA = triethylene glycol dimethacrylate; TPO = 2,4,6trimethylbenzoyl diphenylphosphine oxide; UDMA = urethane dimethacrylate. d NA = Not available. b

Please cite this article in press as: Li X, et al. Curing profile of bulk-fill resin-based composites. Journal of Dentistry (2015), http://dx.doi.org/ 10.1016/j.jdent.2015.01.002

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three rectangular RBC blocks (length: 16 mm, width: 6 mm, thickness: 12 mm) were prepared using a custom-designed transparent polymethylmethacrylate (PMMA) mould (Fig. 1a). The top surface of each block was covered by a polyester film that was flattened using a thin microscope cover glass. A polywave LED light-curing unit (Bluephase 20i, Ivoclar Vivadent, Schaan, Liechtenstein) was positioned at the exact middle of each specimen using a custom-made PMMA jig. The specimen was then light-cured for 20 s using the Bluephase 20i (Ivoclar Vivadent) light-curing unit in ‘high’ power mode. The light guide of the light-curing unit was always positioned in the same way, so that the 410-nm and 470-nm LEDs were always likewise oriented onto the composite.

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2.2.

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Micro-Raman (mRaman) mapping

After 24 h of dark, dry storage at 37 8C, the block was sectioned in half, perpendicular to the top surface and parallel to the long axis of the block (Fig. 1), this using an automated watercooled precision diamond saw (Accutom-50, Struers, Ballerup, Denmark). DC was mapped along the cross-section (width: 16 mm, depth: 12 mm) using mRaman (Senterra, Bruker, Billerica, MA, USA). Every 500 mm, a mRaman spectrum was collected, resulting in 744 measuring points (representing an area of 15 mm width and 11.5 mm depth, or 172.5 mm2) for each cross-section. To eliminate issues of oxygen inhibition and surface irregularities, measurements were started 200 mm below the top surface. At each measurement position, the specimen was focused using the built-in auto-focus function. A near-infrared (785 nm) laser with a power output of 50 mW,

3

a 50 objective and a 50  1000 aperture were used to generate the spectrum. The integration time was set to 10 s with 2 coadditions. The CCD detector possessed a 1024  256 pixel resolution, and was cooled down thermo-electrically to a temperature of 65 8C. Spectra of uncured specimens (n = 9) measured with the same settings were used as reference.

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2.3.

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Data analysis

The spectra were post-processed using the Opus Spectroscopy Software version 7.0 (Bruker Optik, Ettlingen, Germany), including baseline correction and integration of the relevant peaks. DC was calculated as the ratio of peak intensities of the aliphatic 1640 cm1 and aromatic 1610 cm1 peaks in the cured and uncured materials. The following formula was used:   1  Rcured DC ¼  100 Runcured

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where R is the ratio of intensities of the 1640 cm1 and 1610 cm1 peaks in the spectra of the cured or uncured specimens.

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All results were imported into a software package (R3.01, R Foundation for Statistical Computing, Vienna, Austria). An automated script eliminated invalid data, gathered at points with air bubbles or where the autofocus did not function properly. Doing so, 6% of the data points, of which the peak intensity at 1610 cm1 was smaller than 500 a.u. or the peak intensity at 1640 cm1 was smaller than 200 a.u., were omitted. The mean DC of all values at points within 1 mm of the centre of the light guide and with a depth smaller than

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Fig. 1 – Schematic explaining the experimental setup. (a) A PMMA mould was filled with RBC. (b) Using a custom-made jig, the light tip of the curing unit was positioned at the exact middle of the specimen with the 410-nm LED (pink/grey coloured) positioned at the left side of the specimen. (c) After 24-h storage, the specimen was cross-sectioned along the long side of the specimen. (d) Using mRaman, DC was mapped along the cross-section. Please cite this article in press as: Li X, et al. Curing profile of bulk-fill resin-based composites. Journal of Dentistry (2015), http://dx.doi.org/ 10.1016/j.jdent.2015.01.002

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1 mm was calculated, and regarded as ‘optimally’ cured (maximum DC). Points with a DC higher than 90% of this value were regarded as ‘effectively’ cured.9,19 For every specimen, the curing profile was graphically presented in a contour plot as a percentage of maximum DC (‘optimally’ cured). For the purpose of this graphical representation only, missing/omitted data points were replaced by linear approximation. To statistically assess the curing profile of every RBC, a linear mixed effects model was constructed, taking into account five parameters, as there are curing depth (being further referred to as the parameter ‘depth’), composite located outside versus inside the light beam (referred to as ‘beam’), relative distance to the curing centre (‘distance’), position of the 410-nm LED (‘410-nm LED’) and that of the 470nm LEDs (‘470-nm LEDs’). For a better fit with the linear mixed effects model, the curing-depth factor was transformed by taking the square curing depth and incorporated as such in the model. First order interactions were included in the model as well. Different specimens of the same RBCs were added to the model as a random factor. To compare curing efficiency of the different RBCs, the mean maximum DC (%) was determined, as well as the number of points that were cured ‘effectively’ (>90% of maximum DC) in terms of the ‘effective’ curing area (in percentage of the whole specimen area), the ‘effective’ curing depth within the light beam and at the specimen middle (in mm), and the ‘effective’ curing width outside the light beam’ (in mm). These data were statistically analyzed using 1-way ANOVA and Tukey multiple comparisons test. All tests were performed at a significance level of a equal to 0.05.

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2.4.

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Rectangular RBC blocks (length: 16 mm, width: 6 mm) with different thicknesses of 1, 2, 3, and 4 mm, were prepared using a PMMA mould, and light-cured using the polywave LED lightcuring unit (Bluephase 20i) for 20 s. Light irradiance through these blocks was measured in triplicate to assess the amount of light reaching the different curing depths. Therefore, the polywave LED light-curing unit was aligned to a calibrated spectrometer (USB4000, Ocean Optics, Dunedin, FL, USA). The anterior sensor of a MARC Patient Simulator (Bluelight Analytic, Halifax, NS, Canada) was removed from the central incisor in the dental manikin head, and placed on a bench. RBC blocks were positioned in between the light guide of the lightcuring unit and the sensor, after which the irradiance was measured in function of composite depth. The lower detection limit of the anterior sensor of the MARC Patient Simulator (Bluelight Analytic) is 11 mW/cm2.

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3.

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Representative curing profiles of the tested RBCs are shown in Fig. 2. The highest mean maximum DC was 80.0% for the flowable bulk-fill RBC Filtek Bulk Fill Flowable (3M ESPE) and for the fibre-reinforced RBC everX Posterior (GC) (Table 2). A slightly lower mean maximum DC of 77.3% was recorded the flowable bulk-fill RBC SDR (Dentsply), while mean maximum DCs of 70.8% and 73.0% were measured for the

Transmitted light irradiance

Results

full-depth bulk-fill RBC Tetric EvoCeram Bulk Fill (Ivoclar Vivadent) and the conventional (control) RBC Herculite XRV Ultra (Kerr), respectively. As part of the whole specimen area, approximately 75% and 65% of the measuring points were considered to have been cured ‘effectively’ (>90% of maximum DC; delimited by the green/light grey line in Fig. 2) for the bulkfill flowable RBCs SDR (Dentsply) and Filtek Bulk Fill Flowable (3M ESPE) (Table 2). Only 30% of the area appeared cured ‘effectively’ for the fibre-reinforced RBC everX Posterior (GC) (Table 2). These three bulk-fill RBCs were the only ones, for which it was visually clear that all composite in the mould was cured. The full-depth bulk-fill RBC Tetric EvoCeram Bulk Fill (Ivoclar Vivadent) and the conventional (control) RBC Herculite XRV Ultra (Kerr) only cured ‘effectively’ for 19% and 14% of the area, respectively. Visually, about half of these RBC blocks remained soft and was thus hardly cured (Table 2). These area percentages of ‘effective’ cure corresponded very well to the ‘effective’ curing depth as calculated by the statistical model (Table 2). The bulk-fill RBCs SDR (Dentsply), Filtek Bulk Fill Flowable (3M ESPE) and everX Posterior (GC) were found to have been cured ‘effectively’ within the light beam up to a depth of 9.4 mm, 7.8 mm and 5.5 mm, respectively, while the ‘effective’ curing depths for Tetric EvoCeram Bulk Fill (Ivoclar Vivadent) and Herculite XRV Ultra (Kerr) were 3.1 mm and 1.9 mm, respectively (Table 2). The linear mixed effects model revealed that all five parameters statistically influenced the model and thus the curing profile (Table 3), except for the unilateral position of the 410-nm LED versus that of the three 470-nm LEDs; this did not have any negative effect in case of the flowable bulk-fill RBC SDR (Dentsply) and the full-depth bulk-fill RBC Tetric EvoCeram Bulk Fill (Ivoclar Vivadent). This beam inhomogeneity had only a small negative effect on the curing profile of everX Posterior (GC) and Filtek Bulk Fill Flowable (3M ESPE), while a somewhat larger effect was recorded for the conventional (control) RBC Herculite XRV Ultra (Kerr). The rather minor effect of beam inhomogeneity also appeared from the curing profiles, in which no visual differences between the left and right side were detected (Fig. 2). The irradiance at the light-guide tip of the polywave LED light-curing unit used in high power mode was 1174 mW/cm2. Interposition of the RBC blocks with increasing thickness decreased the light irradiance (Fig. 3). The light irradiance at 4mm depth was highest for the flowable bulk-fill RBC’s SDR (Dentsply) and Filtek Bulk Fill Flowable (3M ESPE), being 109 and 106 mW/cm2, respectively. A 4-mm depth light irradiance of 86 mW/cm2 was measured for the fibre-reinforced RBC everX Posterior (GC), while much smaller irradiances of 47 and 11 mW/cm2 were recorded for the full-depth bulk-fill composite Tetric EvoCeram Bulk Fill (Dentsply) and the conventional RBC Herculite XRV Ultra (Kerr), respectively. These results correlated very well with the ‘effective’ curing depth within the light beam (Table 2) measured on basis of the curing profiles (r2 = 0.9115, p = 0.0115).

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4.

238

Discussion

Although filling cavities with composite in bulk facilitates and fastens the restorative procedure, two major conditions

Please cite this article in press as: Li X, et al. Curing profile of bulk-fill resin-based composites. Journal of Dentistry (2015), http://dx.doi.org/ 10.1016/j.jdent.2015.01.002

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Fig. 2 – Curing profiles of a representative RBC block for the flowable bulk-fill RBCs SDR (Dentsply) and Filtek Bulk Fill Flowable (Dentsply), the fibre-reinforced bulk-fill RBC everX Posterior (GC), the full-depth bulk-fill RBC Tetric EvoCeram Bulk Fill (Ivoclar Vivadent), and the control nano-hybrid RBC Herculite XRV Ultra (Kerr). Data are presented as a percentage of maximum DC. The green/light grey line delimits the area, where composite was ‘effectively’ cured (>90% of maximum DC) as calculated by the respective statistical model. The discontinuous blue vertical line represents the specimen middle, while the distance between the continuous blue lines represent the diameter of the light guide of the light-curing device. The light guide was always positioned in the same way, so that the 410-nm and 470-nm LEDs were always likewise oriented onto the composite.

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should be met to be successful. Polymerization shrinkage and its associated stress increase in a thicker composite bulk, which may in turn affect marginal integrity and clinical performance.20 Otherwise, the composite should cure effectively at deeper parts, considering in particular that light transmission through a thicker composite layer will be substantially attenuated.21 With the introduction of many new bulk-fill composites, there exists a great need to examine if they fulfil both primary requirements. Previous data have been shown that polymerization shrinkage stress did not

weaken the bond to cavity-bottom dentine of high C-factor Class-I restorations for the flowable bulk-fill composite SDR (Dentsply), this in contrast to a significantly reduced bonding effectiveness recorded for a conventional flowable and pastelike composite.22 Some more recent preliminary data using a similar study setup23 confirmed the favourable data recorded for SDR (Dentsply), but however also revealed a significant bond-strength reduction to cavity-bottom dentine in Class-I restorations for the flowable bulk-filling composite Filtek Bulk Fill Flowable (3M ESPE) and the full-depth bulk-fill composite

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Table 2 – The ‘effective’ curing (>90% of maximum DC) of composite in terms of area, depth and width outside the light beam.a RBC

a

Mean maximum DC (%)

‘Effective’ curing depth (mm)

‘Effective’ curing area (%)b

Light beamc

Middled

‘Effective’ curing width outside the light beam (mm)e

everX Posterior Filtek Bulk Fill Flowable SDR Tetric EvoCeram Bulk Fill

80.0 80.0

30% b 65% a

5.47 c 7.84 b

6.66 c 9.25 b

77.3 70.8

75% a 19% c

9.45 a 3.14 d

10.05 a 4.19 d

4 0

Herculite XRV Ultra

73.0

14% c

1.86 e

3.00 e

0

1.08 4

Values with the same superscript are not significantly different (Tukey multiple comparisons test). Percentage of area (points) of the profile cured to more than 90% of maximum DC. c The depth up to which the composite was cured ‘effectively’ within the path of light beam, meaning reaching a DC of more than 90% of maximum DC. d The depth up to which the composite was cured ‘effectively’ at the middle of specimen, meaning reaching a DC of more than 90% of maximum DC. e The width of the area outside the light beam at a 4-mm depth, where the composite was cured ‘effectively’ (>90% of maximum DC). b

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Tetric EvoCeram Bulk Fill (Ivoclar Vivadent). This difference in performance for bulk-fill composites clearly necessitates further in-depth research. With this study, we focused on the second requirement to achieve adequate polymerization when bulk-filling cavities up to a 4-mm depth. Therefore, DC was mapped along a crosssection of a composite block using mRaman. Compared with the commonly used Fourier transform infrared spectroscopy (FTIR), mRaman is an easier and more adapted technique.24 This chemical analyzer enables to measure DC with a spatial resolution of about 2 mm (corresponding to a 50 objective). Considerable differences in the curing profiles of the different RBCs tested were observed, by which hypothesis1 was rejected. The hypothesis that the position of the different LEDs within the light-curing unit does not affect the curing profile, was partially rejected, as this factor was found to be

significant for some RBCs (Table 3). However, the effect was very small, as hardly any difference between the parameters ‘410-nm LED’ and ‘470-nm LEDs’ were recorded for each RBC. In clinical practice, this effect may even be negligible, since then many more clinical variables are involved at the same time. In general, manufacturers of bulk-fill RBCs were able to improve polymerization depth by the use of potent photoinitiator systems along with an increased translucency.1,7,25,26 The favourable results on polymerization efficiency found in our study for the flowable bulk-fill composites SDR (Dentsply) and Filtek Bulk Fill Flowable (3M ESPE) are in line with results reported in literature.3,27,28 The polymerization efficiency at greater depth of SDR (Denstply) should probably be ascribed Q4 primarily to its high translucency, which enhances the transmission of light through the material. This also appeared

Table 3 – Statistical analysis of the different parametersa (and their mutual interaction) influencing the curing profiles using a linear mixed effects model. RBC

Depthb

Beamc

Distanced

410-nm LEDe

470-nm LEDsf

Beam  distance

everX Posterior Filtek Bulk Fill Flowable SDR Tetric EvoCeram Bulk Fill

0.202* 0.115*

9.035* 7.257*

0.368* 0.090*

0.283* 0.976*

0.310* 0.859*

2.372* 1.703*

0.022* 0.011*

0.013* 0.012*

0.075* 0.513*

3.625* 8.626*

0.145* 0.802*

1.200 1.250

1.226 1.549

0.888* 2.341*

0.005 0.005*

0.002 0.024

1.473*

66.388*

2.380*

3.669*

4.033*

14.194*

0.654*

0.005

Herculite XRV Ultra

Depth  beam

Depth  distance

a

The higher the value, the more the parameter (or the interaction of two parameters) influences the model. This refers to curing depth, which was square transformed in the model. c This parameter refers to a difference in DC of the composite located outside the light beam versus DC of the composite located inside the light beam. d This parameter refers to the relative distance to the curing centre. e This parameter refers to the unilateral position of the 410-nm LED at the left side of the specimen (see Fig. 1b). f This refers to the position of the three 470-nm LEDs at the right side of the specimen (see Fig. 1b). * Factors that significantly affected DC according to the statistical model. b

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Fig. 3 – Light irradiance in mW/cm2 in function of the composite thickness the light was transmitted through. Using least squares regression, a reciprocal function was fitted to the data.

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from our measurement of transmitted light irradiance, showing that up to a depth of 4 mm the transmitted light irradiance was higher for SDR (Dentsply) than for the other RBCs tested, except for Filtek Bulk Fill Flowable (3M ESPE). In addition, according to the manufacturer’s technical information, a special photo-active group is embedded in the urethane-based methacrylate monomer of SDR (Dentsply); this group is claimed to interact with CQ, thereby boosting the polymerization process. The flowable bulk-fill composite Filtek Bulk Fill Flowable (3M ESPE) contains the least amount of filler of all materials investigated. As translucency is correlated with filler amount ,29 this lower filler loading must also have contributed to the recorded higher transmitted light irradiance and thus the higher polymerization efficiency measured. The curing profile of Filtek Bulk Fill Flowable (3M ESPE) appeared only slightly less deep and wide than that of SDR (Dentsply). The bulk-fill fibre-reinforced composite everX Posterior (GC) consists of a resin matrix filled with randomly orientated E-glass fibres and inorganic particulate filler.4 The fibres may conduct and scatter the light over longer distances, explaining the relatively deep and wide polymerization observed,5,6 nevertheless less extended as compared to that of SDR (Dentsply) and Filtek Bulk Fill Flowable (3M ESPE). Compared to the three abovementioned bulk-fill RBCs, the ‘effective’ curing depth within the light beam of Tetric EvoCeram Bulk Fill (Ivoclar Vivadent) is about half as high (Table 2: 3.14 mm). At the specimen middle, however, the ‘effective’ curing depth was 4.2 mm (Table 2). Important to note is that Tetric EvoCeram Bulk Fill (Ivoclar Vivadent) is the only bulk-fill RBC we tested that has similar optical and mechanical properties as a conventional RBC.26 It is therefore according to the manufacturer indicated to fill the entire cavity (up to 4 mm in bulk), whereas all other bulk-fill RBCs tested require a final cover layer of a conventional hybrid RBC. Compared to the conventional (control) RBC (Herculite XRV Ultra, Kerr), the ‘effective’ curing depth within the light beam and at the specimen middle is still significantly higher (Table 2); the latter confirms that the conventional RBC should be applied in layers not thicker than 2 mm. The improved

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polymerization efficiency of Tetric EvoCeram Bulk Fill (Ivoclar Vivadent) should be attributed to a slightly increased translucency, as measured in this study in terms of transmitted light irradiance and in comparison to that of the conventional (control) RBC. In addition, the manufacturer claims to also have included an optimized initiator system in the composite formulation, more specifically TPO and a socalled ‘‘initiator booster’’ (Ivocerin); both photo-initiators require light irradiated by a polywave light-curing unit that besides the classical 470-nm LED is also equipped with the lower wavelength 410-nm LED.9 Our study set-up providing curing profiles did not only enable to measure the ‘effective’ curing depth, also the ‘effective’ curing width outside the light beam could be determined (Table 2). Besides increased curing depth, the bulk-fill RBCs also presented with an increased curing width. DC was at least 90% of maximum DC (Fig. 2: green/light grey line) up to 4 mm outside of the light beam (or up to 8 mm sideways from the specimen centre), this up to almost a 7-mm depth for the flowable bulk-fill RBC SDR (Dentsply) and almost a 5-mm depth for the Filtek Bulk Fill Flowable (3M ESPE), both thus much beyond the maximum layer thickness of 4 mm recommended by the respective manufacturers. This implies that internal light scattering within the RBC significantly contributes to the local light intensity and the resultant local polymerization efficiency. Clinically, this finding should be regarded as very favourable, as the improved curing efficiency in depth and width offers some safety margin to the clinician in case the light guide of the curing unit is not (or cannot be) positioned optimally towards the restoration surface. The curing profiles indeed showed that even areas outside the direct beam path were properly cured. In our study set-up, light energy was not absorbed by the wall of themould, by which light was conducted deeper (and wider) within the specimen thanks to internal scattering. This largely explains the high ‘effective’ curing depths recorded in this study (Table 2), in particular as compared to other studies. Commonly used stainless steel2,4,7 and teflon moulds9 block internal scattering more than the transparent PMMA mould we used. Non-transparent moulds reduce the total light energy that can reach the deepest parts.30,31 For example, an ‘effective’ curing depth of 4.1 mm was determined using Vickers micro-hardness for Filtek Bulk Fill Flowable (3M ESPE) in another study.2 This is approximately half of the curing depth (within the light beam) at 90% of maximum DC reported in our study (Table 2: 7.8 mm). This difference must thus largely be explained by the use of a transparent PMMA mould in our study versus an opaque stainless steel mould employed in the other study.2 As in the latter study the mould cross-section size was only 2  4 mm2 (versus 50.24 mm2 as area of the light guide), 84% of the surface light energy was blocked by themould before it could enter the RBC. Given the importance of internal scattering observed in our curing profiles, the reduced internal scattering in the stainless steel mould must have considerably reduced the depth of cure in that study.2 Moreover, if the light guide is misaligned and/or moved outside the specimen centre towards the mould wall, the resultant curing depth will be further reduced. Also, if the light beam irradiated by the curing light is not homogenous, correct positioning of the light guide becomes more critical. Within the polywave curing light we used, the 410-nm LED is placed unilateral at one side (Fig. 1b). When only a small area of that light guide is used, it may

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greatly affect the irradiance received by the specimen.17,18 All these effects combined may explain the very different curing depths reported in literature, even despite a similar light output was employed and/or when the same RBC was tested. Nevertheless, many low-viscosity composites were reported to be optimally polymerized up to a depth of 4 mm,32 hereby corroborating our data. That study however also reported a decrease in DC with 12% at a 4-mm depth for Filtek Bulk Fill Flowable (3M ESPE). This may be related to the relatively narrow and long mould cylinder the authors used (diameter: 3 mm, depth: 6 mm). To determine curing depth clinically is difficult, but could for instance be estimated using finite element modelling. We used the polywave light-curing unit Bluephase 20i (Ivoclar Vivadent), which is equipped with three LED chips emitting light at a spectrum ranging between 430–500 nm with a peak maximum at 470 nm, and one chip emitting light at a range between 390 and 430 nm with a peak maximum at 410 nm, this as measured by our spectrophotometer. Statistical analysis showed that the unilateral position of the 410-nm LED chip significantly affected DC of everX Posterior (GC), Filtek Bulk Fill Flowable (3M ESPE) and Herculite XRV Ultra (Kerr), but also that this effect was very small and in fact can be considered as clinically insignificant, being well below the variability in DC measured. Moreover, the 410-nm LED positioning did not affect DC of SDR (Denstply) and Tetric EvoCeram Bulk Fill (Ivoclar Vivadent). Data on beam homogeneity of the polywave LED light-curing device we used was not found, but can probably be expected to be low.17,18 Such beam inhomogeneity has been reported to result in insufficiently polymerized areas at the top surface of a thin composite layer.16 In this study, however, no local differences in DC were observed, despite that we fixed the light guide with a jig and also controlled the position of the 410-nm LED (always positioned at the left side). In our study, the wide and deep blocks allowed lots of light scattering, distributing the light energy with different wavelengths effectively within the composite. Combined with the fact that in dental practice it is very hard to fix the curing light in a single position for 10–20 s (always involving small movement), beam inhomogeneity, at least when using the polywave Bluephase 20i (Ivoclar Vivadent), could be less an issue in the clinical situation. As light transmission is very important to obtain effective polymerization at deeper areas, we also measured light transmission through cured composite discs. Our results correlated very well (r2 = 0.91) with the ‘effective’ curing depth within the light beam, as measured on basis of our curing profile. This suggests that translucency might be the main parameter controlling curing efficiency at depth, as was also concluded in another study that analyzed micro-mechanical properties in function of curing depth.26 By combining the curing profile with the measured transmitted light irradiance, the minimum irradiance required to obtain optimal cure at a depth of 4 mm could be estimated. For Herculite XRV Ultra (Kerr), irradiation for 20 s at 76.9 mW/cm2 resulted in adequate polymerization, while for Tetric EvoCeram Bulk Fill (Dentsply) about 70.2 mW/cm2 was required (Fig. 3). For the other bulk-fill RBCs, the irradiance at a 4-mm depth was less than 10% of the irradiance at the surface. Nevertheless, at a 4-mm depth DC was found to have scarcely decreased, suggesting initiator saturation.

5.

Conclusion

In agreement with the manufacturers’ claims, the results of this study confirm that the bulk-fill resin-based composites tested can be cured ‘effectively’ to at least a 4-mm depth (at the specimen middle). No local differences in DC were observed, by which the effect of beam inhomogeneity appeared minor. Moreover, the more translucent bulk-fill composites did cure also outside the direct light path thanks to internal scattering, so that proper positioning and orientation of the light guide may be less an issue, this as long as enough light energy can enter the restoration.

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Acknowledgements

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Dr. Li’s study at KU Leuven is supported by the China Q5 Scholarship Council (File No.201206270126). Dr. Pongprueksa received a scholarship from the Royal Thai Government supported by Ministry of Sciences and Technology (MOST).

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references

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Curing profile of bulk-fill resin-based composites.

To evaluate the curing profile of bulk-fill resin-based composites (RBC) using micro-Raman spectroscopy (μRaman)...
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