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 Table of Contents  
ORIGINAL RESEARCH
Year : 2020  |  Volume : 11  |  Issue : 1  |  Page : 4-10

Polymerization Shrinkage, Shrinkage Stress, and Degree of Conversion in Bulk-Fill Resin Composites After Different Photo-Activation Methods


1 Department of Restorative and Cosmetic Dentistry, School of Dentistry, Mashhad University of Medical Sciences, Mashhad, Iran
2 Dental Materials Research Center, Mashhad University of Medical Sciences, Mashhad, Iran
3 Dental Research Center, Mashhad University of Medical Sciences, Mashhad, Iran

Date of Submission18-May-2019
Date of Decision14-Jun-2019
Date of Acceptance23-Oct-2019
Date of Web Publication11-Apr-2020

Correspondence Address:
Fatemeh Namdar
Department of Operative Dentistry, School of Dentistry, Mashhad University of Medical Sciences, Vakil-Abad Boulevard, Mashhad
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/denthyp.denthyp_41_19

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  Abstract 


Introduction: The aim of this study was to evaluate the effect of photo-activation methods on the shrinkage behavior of bulk-fill resin composites. Materials and Methods: Three bulk-fill resin composites (Tetric N-Ceram bulk-fill, Xtra fill, and Filtek flowable) were compared in terms of polymerization shrinkage, shrinkage stress, and degree of conversion (DC). Two curing methods were used: 1) conventional and 2) soft-start curing. Five disk-shaped specimens of each bulk-fill composite were fabricated. The deflection disk method and custom-made linear variable differential transformer (LVDT) device evaluated the dimensional changes during polymerization. Universal testing machine was used to measure shrinkage stress. To evaluate DC, the absorbance peaks were obtained using Fourier Transformation Infrared Spectroscopy (FTIR). Data were analyzed by one-way ANOVA and Tukey test. Results: Tetric N-Ceram bulk-fill (conventional mode) showed the highest polymerization shrinkage rate. Filtek flowable resin composite showed least amount of shrinkage stress and highest DC, while Xtra fill (with conventional curing) showed the highest shrinkage stress. Conclusion: Photo-activation method had no effect on decreasing the polymerization shrinkage except for Tetric N-Ceram; also, polymerization shrinkage stress in Filtek flowable composites with both curing methods was less than other groups. DC was product dependent.

Keywords: Bulk-fill resin composite, degree of conversion, polymerization shrinkage, shrinkage stress


How to cite this article:
Majidinia S, Bagheri H, Ramezani S, Giv MJ, Vatanparast N, Namdar F. Polymerization Shrinkage, Shrinkage Stress, and Degree of Conversion in Bulk-Fill Resin Composites After Different Photo-Activation Methods. Dent Hypotheses 2020;11:4-10

How to cite this URL:
Majidinia S, Bagheri H, Ramezani S, Giv MJ, Vatanparast N, Namdar F. Polymerization Shrinkage, Shrinkage Stress, and Degree of Conversion in Bulk-Fill Resin Composites After Different Photo-Activation Methods. Dent Hypotheses [serial online] 2020 [cited 2020 May 29];11:4-10. Available from: http://www.dentalhypotheses.com/text.asp?2020/11/1/4/282246




  Introduction Top


Making a perfect direct resin composite restoration at a reasonable time is worthwhile for both clinician and patient. During light curing procedure, polymerization shrinkage and contraction stress occur within the material. Incremental layering technique has long been accepted because it allows penetrating enough light through resin composite restoration and reduces polymerization shrinkage stress, although this technique is time consuming and most clinicians prefer simple approaches.[1],[2] Recently innovative bulk-fill resin composites have been marketed claiming that they have greater depth of cure up to 4 mm, allow bulk placement, and minimize polymerization shrinkage.[3]

During light exposure, the internal stress is transferred to all bonded surfaces. If shrinkage stress exceeds the bond strength, cuspal deflection, de-bonding, recurrent caries, post-operative sensitivity, and failure of restoration occur.[4] Based on Hooke’s law, polymerization shrinkage stress is equal to the elastic modulus multiplied by the strain. Therefore enhanced elastic modulus and shrinkage increase the shrinkage stress in the resin composite structure.[5] Bulk-fill resin composites with more flowability have shown lower polymerization shrinkage stress due to their lower elastic modulus,[6] although the volumetric polymerization shrinkage of these composites is still between 2% and 3%.[7] There are different methods to achieve deeper curing and reduced shrinkage stresses among the manufacturers of bulk-fill resin composites. These include using additional photoinitiators and enhanced light transmission through the resin composite, reduced filler content, use of monomers and fillers with similar refractive index.[8]

Soft-start curing has been introduced as a technique to reduce the polymerization shrinkage by relieving stress while achieving proper degree of conversion (DC). This technique is initiated with 10 s low light intensity (650 mw/cm2) to delay vitrification of resin composite and increases the ability of a material to relieve shrinkage stresses with prolonged period so that resin can flow. Soft-start curing is followed with increased light intensity for the remaining time of photo-activation.[9] Several studies have reported the efficacy of soft-start curing method to reduce shrinkage stress when compared to conventional light curing. Nalçaci et al.[10] reported that soft-start irradiation mode of halogen lights or LED units produced significant lower microleakage at both cervical and occlusal margins as compared to traditional light curing methods. Piccioni et al.[11] explained that soft-start light activation reduced the clinical consequences of shrinkage stress although cuspal deflection was not completely prevented

Since that the polymerization shrinkage is still a serious and critical challenge in the tooth-colored restorations and due to increased usage of bulk-fill composites, we evaluated the shrinkage behavior of bulk-fill composites using soft-start photo-activation method. The aim of this in vitro study was to evaluate the effect of soft-start photo-activation on the polymerization shrinkage behavior of three different bulk-fill resin composites. Our null hypotheses were:
  1. There are no significant differences in the polymerization shrinkage, shrinkage stresses, and DC between the different bulk-fill resin composites.
  2. There are no significant differences in the polymerization shrinkage, shrinkage stress, and DC between the different photo-activation methods.



  Materials and Methods Top


In this in vitro study, three bulk-fill resin composites were compared in terms of polymerization shrinkage, shrinkage stress, and DC. The materials tested in this study are listed in [Table 1]. Tetric N-Ceram bulk-fill (Ivoclar Vivadent, Schaan, Liechtenstein) and Xtra fill (Voco Gmbh, Cuxhaven, Germany) are high-viscose and Filtek flowable (3M Espe, Germany) is low-viscose bulk-fill resin composites. For preventing the effects of confounding variables, shade A2 was selected for each composite except for Tetric N-Ceram that used IVA shade. An LED light-curing unit (Bluephase N, Ivoclar Vivadent) was used in two photo-activation methods: 1) an irradiance of 1200 mW/cm2 for 40 s (Conventional photo-activation); 2) light-curing was initiated with an irradiance of 650mw/cm2for 10 s, then was followed with an irradiance of 1200 mW/cm2for 30 s (soft-start curing).
Table 1 Resin composites materials and manufacturers’ information

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Measurement of the polymerization shrinkage

Polymerization shrinkage was measured using a custom-made linear variable differential transformer device (LVDT) (Nemo, Iran). Glycerin gel was applied on the glass slide to allow the resin to shrinkage and to prevent surface adhesion. Uncured bulk-fill resin composites were dispensed in the glass slides. Then glass slides were transferred in the bottom of a Teflon ring. Five disk-shaped specimens of each bulk-fill composite material were fabricated with dimensions of 4 mm in diameter and 2 mm thickness. The upper surface of the ring was covered by the glass cover slip. An LED light-curing unit was fixed with a plastic mold, 1 mm under the glass slide, and the specimens were light cured with two different light-curing protocols. The deflection disk method and LVDT device were used to evaluate the dimensional changes during polymerization. When the disk-shaped sample was curing, polymerization shrinkage caused disk bending. The bending was monitored by an accuracy of 0.001 µm. The polymerization shrinkage values were recorded in a computer at each 0.5 s by the software. This was measured ten times for each material and each photo-activation protocol.

Measurement of the polymerization shrinkage stress

Silane (Pulp dent, USA) was applied on the one surface of each glass slide. Then a thin layer of bonding agent (Adper Single bond, 3M ESPE) was applied on the glass slide and light cured for 20 s. The bonded surfaces of two glass plates were positioned against each other and fixed on the measuring instrument with 1 mm vertically apart. The space between the glass slides was filled with one of the three bulk-fill resin composites. Uncured resin composite was placed between two slides to form a disk-shaped specimen of 10 mm diameter and 1 mm thickness. The composite material was irradiated using an LED curing device and one of the two different protocols. Universal testing machine (STM-20, Santam, Iran) was used with a speed of 0.5 mm/min to measure shrinkage stress of three bulk-fill resin composites. This machine measures shrinkage stress using a load cell attached to the curing resin composite. During light curing, shrinkage stresses established in the resin composite and between glass slides were monitored continuously. These stresses caused slight movement of load cell end and were recorded in computer using the software. Shrinkage stress curves were produced five times for each group.

Measurement of the degree of conversion

Uncured bulk-fill resin composites were placed into a disc and the absorbance peaks were obtained before polymerization using Fourier Transformation Infrared Spectroscopy (FTIR) (EQUINOX 55, Bruker, Germany). Plastic molds with 4 mm diameter and 4 mm depths were used to evaluate the DC. A glass slide was placed at the bottom of the molds. The plastic molds were then bulk filled with the different bulk-fill resin composites in a single increment, and their surfaces were covered with another glass slide to extrude excess material by pressure. Resin composites were then light cured from the top surfaces using an LED curing device with two different protocols. DC in the bottom surfaces was evaluated immediately by FTIR spectrometer using attenuated total reflectance (ATR) technique. DC was calculated by the ratio of aliphatic C<td:glyph name=“dbnd”/>C at 1636 cm−1 in cured composite to aromatic C&9552;C at 1608 cm−1 in uncured composite.

Statistical analysis

The normality of distribution was initially evaluated using the Shapiro-Wick test. As the data were normally distributed, this state was considered in the selection of statistical analyses. The differences in measurements were analyzed with ANOVA and Turkey test using SPSS software (statistical package for social sciences, version 22.0, SPSS Inc., Chicago, USA) with the significant level set at 0.05.


  Results Top


Polymerization shrinkage

Different groups were compared descriptively to evaluate the polymerization shrinkage pattern [Figure 1]. The values of polymerization shrinkage were analyzed by comparing the area below the curve graph.
Figure 1 Polymerization shrinkage pattern of different resin composites cured with two photo-activation methods as a function of time

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One-way ANOVA showed a significant difference between polymerization shrinkage of different groups (P = 0.00) and multiple comparison Tukey test showed that Tetric N-Ceram with conventional mode had significant differences with others (P < 0.05). The lowest polymerization shrinkage was related to Filtek flowable (with soft-start curing) [Table 2].
Table 2 Mean of degree of conversion (%) and polymerization shrinkage (at 120 s) for three bulk-fill resin composites after different curing protocols

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Polymerization shrinkage stress

Shrinkage stress pattern of different bulk-fill resin composites versus time was shown in [Figure 2]b. Two-way ANOVA and Tukey analyses exhibited that there were significant differences between shrinkage stress of Filtek flowable composite (with both curing mode) and others (P = 0.00). The lowest shrinkage stress was related to Filtek flowable (with both curing protocols), while Xtra fill (with conventional curing) showed the highest shrinkage stress. There were no statistical differences between other groups (P > 0.05) [Figure 2]a,b.
Figure 2 a) shrinkage stress at 40 s. b) Comparison of the shrinkage stress pattern as a function of time for three bulk-fill resin composites cured with two photo-activation methods

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Degree of conversion

The highest DC was related to the Filtek group with soft-start curing (66.2%), but there was no significant difference with conventional curing. Xtra fil soft-start group showed the lowest DC (40%). [Table 2].


  Discussion Top


In this study polymerization shrinkage, shrinkage stresses, and DC of three bulk-fill resin composites were evaluated after two photo-activation protocols. The results of this study showed that the photo-activation method had no effect on the polymerization shrinkage reduction except for Tetric-N Ceram. Different methods to assess polymerization shrinkage include water and mercury dilatometer, cuspal deflection, strain gauge transducer, Bioman method, LVDT, and optical methods.[12],[13] In this study, we evaluated the polymerization shrinkage by making the customized LVDT machine, which records the instantaneous dimensional changes with laser. Bulk-fill resin composites are inhomogeneous and differ in terms of filler type, size and content, viscosity, and chemical components. Previous studies reported a typical polymerization shrinkage of 2–5% for conventional and 1.5–3% for bulk-fill resin composites, which are consistent with the values obtained in our study.[3],[12],[13],[14] Polymerization shrinkage of resin composites has been reported to conversely correlate with the filler content.[9] Although having higher filler content than Filtek composite, Tetric N-Ceram showed significant higher polymerization shrinkage than other groups. According to the results of this study, soft-start curing was very effective in reducing this contraction.

Decreasing the amount of low molecular weight monomers such as TEGDMA polymerization shrinkage of flowable bulk-fill resin composites would reduce. Higher molecular weight monomers such as UDMA and BisEMA are used to maintain the low viscosity without increased amount of TEGDMA. It should be noted that UDMA and BisGMA have a synergistic effect on the shrinkage rate.[15] The high molecular weight of UDMA (UDMA; 470.0) reaches a low polymerization shrinkage.[16] This might explain the significant lower polymerization shrinkage of Filtek than that of Tetric N-Ceram since it may contain a higher concentration of UDMA monomer. Filtek flowable bulk-fill composite contains different monomers including BisGMA, UDMA, BisEMA, and procrylat. These monomers have high molecular weight and can reduce the polymerization shrinkage. In addition, procrylat would cause more flowability and less contraction stress.

In this study, Filtek composite showed more shrinkage than Xtra fil after conventional curing possibly due to less BisGMA and fillers in Filtek flowable composite (42.4% vol) when compared with Xtra fil (70% vol). In other words, with increasing the filler content, polymerization shrinkage decreases. This was shown in this study as the highly filled X-tra fil composite (70%vol) had significant inferior polymerization shrinkage than Filtek flowable and Tetric-N Ceram composites after conventional curing.

Based on the results of this study, soft-start curing reduced the polymerization shrinkage of all bulk-fill composites when compared to conventional method. But only in Tetric N-Ceram composite photo-activation method had a significant effect on the polymerization shrinkage. According to some studies, low radiation rate increases the marginal integrity but some physical properties such as tensile strength and hardness may be compromised.[17],[18] To solve this problem, the soft-start method is recommended so that in the first 10 s we will have low intensity of radiation, then increase the light intensity.[9] Previous studies have shown that the maximum rate of polymerization shrinkage occurs during the phase transition from a viscous into a viscoelastic state and then during elastic phase.[19] The lower exposure rate leads to a long pre-gel phase, more flowability of the material, and decreases the stress without reduction of the DC.[14] In addition, enhancing the light intensity leads to increase in depth of curing and to improve the physical properties of composite resin.[15] In general, polymerization shrinkage decreases by gradual increasing of light intensity with soft-start photo-activation method.[20].

During composite polymerization, the monomer chains and silanes start a cross-linking procedure and create stress at the tooth-restoration interfaces. This stress is affected by the elastic modulus and volumetric polymerization shrinkage. As previously stated, control of the curing speed leads to more flowability of the composite during polymerization and reduces the amount of polymerization stress.[13] Similar to our results, Oliveira et al.[21] stated that there was no significant difference between shrinkage stress of resin composites using conventional and soft-start curing. Also based on the results of this study, Filtek group showed least amount of stress that was not consistent with the results of DC, which could be due to the filler content and the filler size of the composite and also because the smaller filler particles scatter light more.

FTIR is the most common method for measuring the DC in the most studies.[22] It has been shown that the DC is affected by the light source and the type of composite. The light source variables are the type of light, wavelength, exposure time, radiation distance, and photo-activation method.[23] All above variables were standardized in this study and only light intensity (conventional and soft-start photo-activation methods) was changed. Our study showed that although the DC was product dependent and there were significant differences between different resin composites, each bulk-fill composite exhibited similar amounts of DC using both photo-activation methods. In another study, Atria and Sampaio evaluated the DC and found that there were no significant differences between different resin composites after conventional and soft-start curing methods.[24]

Bulk-fill resin composites have a higher translucency when compared to conventional composites.[25] Considering that the light transmission rate is highly dependent on the opacity of material, DC is the result of reduced opacity in the bulk-fill composites. More translucencies can be obtained by reducing the filler content.[26] According to the literatures, resin composites with more filler content would show less DC.[27] In this study, the DC decreased with increasing the filler content. The highest DC was related to Filtek flowable bulk-fill resin composite with 42.5% volume filler, then Tetric N-Ceram with 61% volume, and the lowest amount was related to Xtra fil with 70.1% volume filler.On the other hand, the type of monomer used in the composite structure is also effective on DC.[16] All investigated composites in this study contain UDMA and BISGMA. Since BisGMA molecule has high viscosity and less freedom results in reduced DC and inferior shrinkage stress. UDMA is a high molecular weight monomer with low viscosity and high concentration of double bonds. Copolymerization of this monomer with BisGMA leads to create a highly cross-linked polymer and increased DC. Therefore the DC reduces with increased fillers and BisGMA content and also has a direct relationship with UDMA content.[28],[29]

However previous studies have shown that the use of bulk-fill composites in 4 mm thickness is accompanied by sufficient curing but all of these products don’t have low shrinkage. Therefore clinicians should be careful when choosing these materials. Clinical significance of this study was that the soft-start photo-activation method had no effect on decreasing the polymerization shrinkage when compared to conventional method. However photo-activation method affected the shrinkage stress pattern but the ultimate shrinkage stress for both photo-activation techniques was almost equal. DC was product dependent and should be evaluated for each product separately. This study showed that photo-activation method had no effect on decreasing the polymerization shrinkage except for Tetric N-Ceram; also, Filtek flowable resin composite showed least amount of shrinkage stress and highest DC. Therefore our first null hypothesis was rejected. One of the limitations of this study was the lack of evaluation of the effect of cavity preparation and C-factor on the shrinkage behavior. More studies should be conducted to evaluate the effect of these parameters on the polymerization shrinkage and contraction stress of bulk-fill resin composites.


  Conclusions Top


  1. Tetric N-Ceram (conventional mode) showed the highest polymerization shrinkage rate and photo-activation method had no effect on decreasing the polymerization shrinkage except for Tetric N-Ceram.
  2. Polymerization shrinkage stress in Filtek flowable composites with both curing methods was less than other groups and curing method had no effect on polymerization stress.
  3. The DC was product dependent and there were significant differences between different resin composites.


Acknowledgment

This article is based on a research project No. 941275 and the permission of the Ethics Committee No. Mums REC.1395.48 IR.

Financial support and sponsorship

Nil.

Conflicts of interest

The authors of this manuscript certify that they have no proprietary, financial, or other personal interest of any nature or kind in any product, service, and/or company that is presented in this article.



 
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    Tables

  [Table 1], [Table 2]



 

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