Degree of conversion of bulk-fill compared to conventional resin-composites at two time intervals

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) e213–e217

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Degree of conversion of bulk-fill compared to conventional resin-composites at two time intervals Ruwaida Z. Alshali a,b , Nick Silikas a,∗ , Julian D. Satterthwaite a a b

School of Dentistry, The University of Manchester, Higher Cambridge Street, Manchester M15 6FH, UK Department of Oral and Maxillofacial Rehabilitation, King Abdulaziz University, Jeddah, Saudi Arabia

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objective. The purpose of this study was to assess the degree of conversion (DC) over time,

Received 20 February 2013

using FTIR spectroscopy for bulk-fill flowable resin composite materials compared to con-

Received in revised form

ventional flowable and regular resin composite materials.

16 May 2013

Methods. Eight resin composites were investigated including flowable bulk-fill materials

Accepted 23 May 2013

SureFil SDR (SDR), Venus bulk-fill (VBF), x-tra base (XB), and Filtek Bulk Fill (FBF). Conventional flowable and regular composite materials included: Venus Diamond flow (VDF), Grandioso flow (GRF), Venus Diamond (VD), and Grandioso (GR). Degree of conversion (DC)

Keywords:

was assessed by Fourier transform infrared spectroscopy using attenuated total reflectance

Resin composite

technique. DC was measured for samples immediately post-cure (n = 3), and after 24 h stor-

Bulk-fill

age period at 37 ◦ C (n = 3). Results were analysed using one-way analysis of variance (ANOVA),

Degree of conversion

Bonferroni post hoc test, and independent-samples t-test at ˛ = 0.05 significance level.

FTIR spectroscopy

Results. Immediately post-cure, the mean DC values of the different materials were in the following order: GRF > VDF > SDR > VBF > XB > GR > FBF < VD and ranged from 34.7 to 77.1%. 24 h post-cure, DC values were in the following order: GRF > VBF > VD > SDR > VDF > GR > XB < FBF and ranged from 50.9 to 93.1%. GRF showed significantly higher DC values than all other materials at both time intervals while XB and FBF showed significantly lower values at 24 h post-cure. Significance. The 24 h post-cure DC values of the bulk-fill composites SDR and VBF are generally comparable to those of conventional composites studied; however, the 24 h post-cure DC values of XB and FBF were lower compared to the other materials. © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The degree of conversion (DC) of a resin composite is crucial in determining the mechanical performance of the material and its biocompatibility. Strength, modulus, hardness

and solubility have been shown to be directly related to the degree of monomer conversion [1–3]. In addition, assessment of changes of DC during polymerization is considered a useful tool in characterizing and understanding polymerization kinetics using different resin composite formulations and curing techniques.

∗ Corresponding author at: Coupland Building III, School of Dentistry, The University of Manchester, Manchester M15 6FH, UK. Tel.: +44 1612756747. E-mail address: [email protected] (N. Silikas). 0109-5641/$ – see front matter © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dental.2013.05.011

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Bis-GMA, TEGDMA, HEDMA 1048014/A3 VOCO GR Grandioso

UDMA: urethane dimethacrylate; EBPADMA: ethoxylated Bisphenol A dimethacrylate; TEGDMA: ttriethyleneglycol dimethacrylate; Bis-EMA: Bisphenol A polyethylene glycol diether dimethacrylate; Bis-GMA: Bisphenol A dimethacrylate; HEDMA: hydroxyethyl dimethacrylate; TCD-DI-HEA: Bis-(acryloyloxymethyl)tricyclo[5.2.1.02,6]decane.

89 wt%

81 wt% VD Venus Diamond Conventional regular composites

Low shrinkage nanohybrid nanohybrid

Heraeus

010021/AM

Bis-GMA, TEGDMA, HEDMA TCD-DI-HEA, UDMA 1104372/A2 Flowable GRF

VOCO

010027/A3 Flowable

Venous Diamond Flow Grandioso Flow Conventional flowable composites

VDF

Heraeus

Ba-Al-F silicate glass, YbF3 , SiO2 –

81 wt%

65 wt%

Ba-Al-F silicate glass, YbF3 , SiO2 –

64.5 wt% N377465/universal 3M ESPE FBF Filtek Bulk Fill

Flowable bulk fill

XB x-tra base

Flowable bulk fill

VOCO

1208392/universal

Aliphatic di-methacrylate (UDMA), Bis-EMA Bis-GMA, UDMA, Bis-EMA, Procrylat resins UDMA, EBPADMA

zirconia/silica, ytterbium trifluoride

75 wt%

65 wt% 010028/universal Heraeus Flowable bulk fill

Filler loading Filler

Ba-Al-F-B silicate glass, Sr-A-F silicate glass Ba-Al-F silicate glass, YbF3 , SiO2 – Modified UDMA, EBPADMA, TEGDMA UDMA, EBPADMA

Organic matrix Lot no./shade

1000830/universal DENTSPLY

Manufacturer Type

Flowable bulk fill

VBF

Uncured composite material was placed on the ATR crystal making sure that the crystal was completely covered by the material (n = 3), the FTIR spectra of the uncured samples were then collected. Each material sample was then cured at room temperature for 20 s (as recommended by the manufacturer) using a halogen curing light (Optilux 501, Kerr Corporation, USA) with an output irradiance of 600 mW/cm2 and standard curing mode. The light tip was kept as close as possible to the sample. The FTIR spectra of the cured samples were then collected immediately.

Venus Bulk Fill

Measurement of immediate post-cure DC

SDR

2.2.

Code

Eight resin composite materials were investigated including three flowable bulk-fill composites (Table 1). The DC was measured using FTIR (Avatar 360, Nicolet Analytical Instruments, UK) equipped with a single reflection horizontal attenuated total reflectance (ATR) accessory (MIRacle ATR, PIKE Technologies, 6125 Cottonwood Drive, Madison). The FTIR spectrometer was operated under the following conditions: 4000–500 cm−1 wavelength, 6 cm−1 resolution, and 32 scans.

Material

Device used for assessment of DC

SureFil SDR

2.1.

Group

Materials and methods Table 1 – Materials, manufactures, lot numbers, and composition.

2.

Bulk fill composites

The final DC depends mainly on intrinsic factors such as the chemical structure of the dimethacrylate monomer and photo-initiator concentration and extrinsic factors such as the polymerization conditions [4]. The DC of several Bis-GMA based resin-composites has been evaluated previously using infrared techniques. The reported DC values were in the range of 52–75%, with most of the materials in the 55–60% range [5,6]. The DC for adequate clinical performance has not yet been established. However, a negative correlation of in vivo abrasive wear depth with DC has been established for DC values in the range of 55–65% [7]. Accordingly, at least for occlusal restorative layers, DC values below 55% are not recommended [8]. Many studies have investigated the effect of filler load, size, and geometry on DC of the resin-composite [9–11]. DC was found to progressively decrease linearly with increasing opaque filler content [11]. Differences in filler geometry did not seem to influence DC of experimental composites. However, DC decreased in composites whose filler particles size approached the output wavelength of the curing unit (470 nm). This was explained by the scattering effect of fillers of this size on penetrating light during photoactivation [9]. The aim of this study was to assess the DC of some bulk-fill composite materials compared to that of conventional flowable and regular composites using FTIR spectroscopy at two time intervals: immediately post-cure, and 24 h post-cure. Two null hypotheses were investigated: (i) there is no difference in the DC values of the low stress bulk-fill composites SDR, VBF, and XB in comparison to those of conventional composite materials, and (ii) there is no difference between DC values immediately post-cure and 24 h post-cure for all materials.

68 wt%

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) e213–e217

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d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) e213–e217

2.3.

Measurement of 24 h post-cure DC

To assess the DC of the materials after 24 h storage at 37 ◦ C, thin samples (n = 3) were prepared by applying a small amount of each material on a piece of glossy transparent polyester film set on a glass microscope slide. The material was then covered with another piece of polyester film and pressed with another glass slide until a thin film was created. Each sample was checked to ensure consistent thickness. The sample was then cured with the curing light exit window against the glass slide. The samples were then stored for 24 h at 37 ◦ C in a lightproof oven within a sealed glass container with silica gel (to prevent water adsorption onto the surface of the samples and avoid the potential source of noise in an FTIR). After 24 h, each sample was carefully placed on the ATR crystal plate and pressed with a clamp to maintain good contact between the sample and the ATR crystal. An FTIR spectrum of the sample was then collected.

2.4.

DC calculation

For all samples, DC was measured by assessing the variation in the ratio of the absorbance intensities of aliphatic C C peak at 1638 cm−1 and that of an internal standard peak of aromatic C C at 1608 cm−1 of the uncured and cured samples. Due to the lack of aromatic C C, internal standard peaks at 1600 cm−1 and 1720 cm−1 were used in the case of SDR and VD respectively. The percentage DC was calculated for each sample using the following equation:



DC% = 1 −



Material

DC% (standard deviation) immediately post-cure 58.4 (0.5)a,b 55.7 (0.6)a,b,c 53.9 (1.0)a,c 49.5 (1.9)c 62.0 (3.3)b 77.1 (3.5) 34.7 (2.6) 50.0 (0.5)a,b,c

SDR VBF XB FBF VDF GRF VD GR

DC% (standard deviation) 24 h post-cure 76.1 (0.7)a 79.2 (1.8)a 62.1 (0.4) 50.9 (1.5) 70.6 (2.1)b 93.1 (0.2) 79.0 (1.3)a 69.1 (0.6)b

flowable composite GRF has statistically significant higher DC value (77.1%) than all other materials (p < 0.001), while VD had a statistically significant lower DC value of 34.7% than all other materials (p < 0.001). Within the flowable bulk-fill materials group, there was no significant difference in the DC values when materials were compared to each other except between SDR and FBF (p = 0.002). After 24 h storage at 37 ◦ C, XB and FBF showed statistically significant lower DC (62.1 and 50.9% respectively), while GRF had a statistically significant higher DC value (93.1%) than other materials. Both bulk-fill SDR and VBF showed DC comparable to the rest of the materials.

(1638 cm−1 /interrnal standard) peak area after curing) (1638 cm−1 /interrnal standard) peak area before curing

In the immediately post-cure investigation, DC was calculated using the pre-cure and post-cure absorption values of the same sample. For the 24 h post-cure investigation, DC was calculated using the 24 h post-cure absorption values of each sample against the absorption values of a separate single pre-cure sample of each material.

2.5.

Table 2 – Mean DC of materials immediately post-cure and 24 h post-cure (same superscript letters indicate non significant differences within the same column).

 × 100

Independent-sample t-test to assess the difference between the immediate post-cure and 24 h post-cure DC values for each material, showed that the mean DC of all the materials (excluding FBF) after 24 h storage at 37 ◦ C were statistically higher than those obtained immediately post-cure

Statistical analysis

Data were entered in to statistical software (SPSS 16.0, Chicago, IL, USA). For the data from samples used to assess the immediate post-cure and 24 h post-cure DC, values of each material were compared using a one-way analysis of variance (ANOVA) and Bonferroni post hoc test at ˛ = 0.05 level. Data obtained from the samples to assess immediate post-cure DC were compared to those of the 24 h post-cure DC using an independent T-test.

3.

Results

DC values and standard deviations (SD) of the materials are presented in Table 2 and Fig. 1. Immediately post-cure, the mean DC values of the different materials were in the folorder: GRF > VDF > SDR > VBF > XB > GR > FBF > VD. lowing 24 h post-cure, they were in the following order: Immediately GRF > VBF > VD > SDR > VDF > GR > XB > FBF. post-cure, Post hoc analysis showed that the conventional

Fig. 1 – Mean degree of conversion (DC) of materials immediately post-cure and 24 h post-cure at 37 ◦ C. The bars represent the standard deviation.

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(˛ = 0.05) with an average of 36% increase for each material. P values for the Independent sample t-test for the two different time intervals were: <0.001 for SDR, VBF, XB, VD, and GR, 0.018 for VDF, 0.001 for GRF, and 0.544 for FBF.

4.

Discussion

4.1.

Methodology

In this study, the DC of different commercially available dental composites was assessed using FTIR spectroscopy. Unlike indirect techniques which rely on measuring changes in the mechanical performance of the material to assess relative DC, FTIR spectroscopy allows the direct detection of the amount of unreacted C C in the resin matrix [12,13]. DC was assessed at two-time intervals (immediately postcure and 24 h post-cure). Different samples were used in the two experiments. Samples used for the immediate post-cure assessment could not be used for assessing the 24 h post-cure DC for two reasons. Firstly, removal of cured samples from the ATR crystal plate to be reassessed results in damage of the samples which consequently results in very poor spectra, collected from these samples. Secondly, when these samples were left undisturbed on the ATR crystal for 24 h and then measured, the loss of contact between the samples and the ATR crystal due to polymerization shrinkage results in highly distorted spectra with high level of noise.

4.2.

DC changes over 24 h

Most of post-irradiation polymerization has been shown to take place in the first few minutes or one hour after removal of the irradiation source [14,15], with subsequent slower increase up to a maximum of 24 h post-irradiation [16]. In this study, all the materials (except FBF), showed significant increase of DC at 24 h post-irradiation and storage at 37 ◦ C when compared to those obtained immediately post-cure. The DC values of some of the materials have been previously assessed using spectroscopy techniques. Values of 58.9% (±2.9) and 65.0% (±1.9) have been reported for SDR and VBF respectively. These were obtained 5 min after curing using a curing light of an irradiance of 1226 mW/cm2 for 10 s [17]. When compared to the values obtained immediately post-cure in this study, 58.4% (±0.5) and 55.7% (±0.6) for SDR and VBF respectively, these values are comparable considering differences in light irradiance and post-irradiation time. DC of VDF immediately post-cure has been assessed using Raman spectroscopy with a DC value of 68% (±1.0) when specimens were cured for 40 s using an LED light [18]. This is comparable to the immediate post-cure DC value of VDF, 62% (±3.2), obtained in this investigation.

4.3.

Effect of chemical composition on ultimate DC

Since polymerization conditions were kept standardized in this study, differences in the DC of the materials can be attributed to variations in the chemistry of their resin matrix. The main two features of a monomer that affect the degree of conversion and reactivity are the initial monomer viscosity

and flexibility of its chemical structure [19] and the ultimate degree of conversion of different monomer systems increases in the following order: Bis-GMA < Bis-EMA < UDMA < TEGDMA [20]. Bis-GMA is considered the most viscous and least flexible monomer due to the strong intramolecular hydrogen bonding via its pendant hydroxyl groups OH and the presence of rigid aromatic nuclei in its structure. UDMA is also a viscous monomer due to the hydrogen bond intramolecular interaction between its imino ( NH ) and carbonyl groups ( C O). However, the viscosity of UDMA is much lower than that of Bis-GMA because of the weak hydrogen bond of its imino ( NH ) group compared to that of hydroxyl groups ( OH) [21]. Also, the presence of imino groups ( NH ) in the urethane structure of UDMA monomer is responsible for the characteristic chain transfer reactions that provide an alternative path for the continuation of polymerization. These reactions result in increased mobility of radical sites on the network and consequently enhanced polymerization and monomer conversion [20]. This explains the high reactivity and ultimate DC of UDMA based monomer systems (SDR, VBF, VDF, and VD) when compared to that of Bis-GMA based composites (GR, XB, and FBF). However, when Bis-GMA is diluted with the low viscosity TEGDMA monomer, a synergistic effect on the rate of polymerization, network plasticization, and DC has been observed [20]. This might explain the significantly higher DC of GRF than that of other materials since it may contain a higher TEGDMA/Bis-GMA ratio to enhance its flowable consistency. The significantly lower 24 h post cure DC of XB and FBF materials than all other tested composites may be due to the presence of the high molecular weight Bis-EMA monomer mixed with UDMA. Although this bulky monomer with its stiff central phenyl ring core might enhance the mechanical performance of the material, it can significantly restrict the mobility of UDMA monomers and decrease their reactivity and ultimate conversion value. Also, for XB, its high filler loading might contribute toward a lower DC.

4.4.

Immediate post-cure DC

Polymerization shrinkage stress can be effectively reduced by decreasing the rate of polymerization either by controlling the initial curing light intensity or by modifying the polymerization inhibitor concentration, which manifests as low DC values at early stages of polymerization [22–24]. The relatively low immediate post-cure DC values of the bulk-fill materials SDR, VBF, FBF and XB when compared to those of conventional flowable materials GRF and VDF may be the effect of altered polymerization behavior of the former and may contribute to reduced shrinkage stress upon polymerization as was shown for SDR [25]. However, more studies are required to kinetically assess the immediate DC values of bulk-fill composites and effect on their shrinkage stress values compared to conventional materials.

5.

Conclusions

Within the limitation of this study the following can be concluded:

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) e213–e217

(1) The ultimate DC values of the bulk-fill composites SDR and VBF were generally comparable to those of conventional composites studied. The resin chemistry does not seem to negatively influence the polymerization process in these materials. (2) Although clinically acceptable (>55%), the ultimate DC of the bulk-fill composite XB was significantly lower than the other materials. (3) The conventional flowable composite GRF showed a significantly higher DC than that of other materials that may be attributed to its high concentration of TEGDMA monomer.

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