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Table of Contents   
ORIGINAL ARTICLE  
Year : 2021  |  Volume : 24  |  Issue : 1  |  Page : 77-82
Comparative evaluation of the degree of conversion of four different composites polymerized using ultrafast photopolymerization technique: An in vitro study


1 Department of Conservative Dentistry and Endodontics, Tagore Dental College and Hospital, Chennai, India
2 Department of Conservative Dentistry and Endodontics, Adhiparasakthi Dental College and Hospital, Melmaruvathur, Tamil Nadu, India

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Date of Submission24-Dec-2020
Date of Decision24-Jan-2021
Date of Acceptance09-Feb-2021
Date of Web Publication05-Jul-2021
 

   Abstract 


Context: Lower degree of conversion (DC%) of monomer to polymer in a resin composite restoration could be a health hazard for the patient as well as it could affect the longevity of the restoration.
Aims: This study is aimed to compare and evaluate the DC% of four different composites polymerized using ultrafast photopolymerization.
Settings and Design: In-vitro study.
Materials and Methods: A total of 40 disc-shaped composite samples were used in the study. Twenty samples were prepared for each group using 2 mm height and 6 mm diameter Tygon tube as a matrix. All of the composites were cured using the Woodpecker i Led light-curing unit with an intensity of 2300–2500 mW/cm2 (TURBO mode). Samples in Group 1 were cured for 1 s and samples in Group 2 were cured for 3 s. Each group had 4 subgroups of five samples of the 4 resin composites tested. After photo-activation, the specimens were stored under dark dry conditions at room temperature for 24 h before testing. The DC% was measured using Fourier-transform infrared spectroscopy.
Statistical Analysis Used: The DC% were analyzed using ANOVA, and Tukey HSD post hoc test using IBM SPSS 21 software.
Results: Among the experimental groups, Group 2 showed a higher DC% which ranges from 93.7% to 95.4% than Group 1 which ranges from 58.5% to 65.5%. There was a statistically significant difference in the DC% among the materials tested (P < 0.05).
Conclusions: Within the limitations of the study, it was concluded that composites cured for 3 s showed a higher DC% which ranges from 93.7% to 95.4% than those cured for 1 s. The DC% also varied among the four different composites tested.

Keywords: Degree of conversion; Fourier-transform infrared spectroscopy; microhybrid composite; nanohybrid composite; polymerization

How to cite this article:
Balagopal S, Geethapriya N, Anisha S, Hemasathya BA, Vandana J, Dhatshayani C. Comparative evaluation of the degree of conversion of four different composites polymerized using ultrafast photopolymerization technique: An in vitro study. J Conserv Dent 2021;24:77-82

How to cite this URL:
Balagopal S, Geethapriya N, Anisha S, Hemasathya BA, Vandana J, Dhatshayani C. Comparative evaluation of the degree of conversion of four different composites polymerized using ultrafast photopolymerization technique: An in vitro study. J Conserv Dent [serial online] 2021 [cited 2021 Aug 4];24:77-82. Available from: https://www.jcd.org.in/text.asp?2021/24/1/77/320684



   Introduction Top


The demand for tooth-colored restorations has helped with the evolution of esthetic composite resin restoratives and lead to its continuous improvements and innovations.[1] The unpolymerized resin restoratives are light-activated after placement to undergo polymerization for hardening.[2] During the polymerization process, resin monomer units bond with one another to make long and heavy polymers. The strength of those restoration depends on the degree of conversion (DC%) of the monomers to polymers. DC% is an vital parameter that determines the final physical, mechanical and biological properties of photo-activated composite resins.[3],[4] Several factors influence the DC% like light source used, power density,[5] increment thickness,[6] wavelength, light-curing duration,[7] light-tip size, distribution of filler particle size, type of fillers, source intensity, and shade of the composite resins.[8]

The DC% may be a critical factor that greatly influences several properties associated with the composite restoration longevity such as solubility, color stability, mechanical properties, and even biocompatibility.[9] The minimum DC% for a clinically satisfactory restoration has not yet been established precisely. Nevertheless, an indirect correlation of in vivo abrasive wear depth with DC has been established for DC values within the range of 55%–65%.[10] This suggests that, a minimum of for occlusal restorative layers, DC values <55% is also contraindicated.[11] While higher DC% ends up in an improved clinical performance of composite resin materials,[12] resins with a lower DC% have more free monomer and so a larger amount of leachable monomer.[13]

Furthermore, the poorly polymerized resin composite will be a possible risk. Sulekha Siddharth Gosavi, Siddharth Yuvraj Gosavi, Rama Krishna Alla, have documented that the monomers (Methyl methacrylate widely used monomer in dentistry) may cause a good range of adverse health effects such as irritation to the skin, eyes, and mucous membranes, allergic dermatitis, stomatitis, asthma, neuropathy, disturbances of the central nervous system, liver toxicity, and fertility disturbances.[14]

The light cure unit and the irradiation time play an important role in obtaining satisfactory DC%. A curing time of 20 s is usually recommended for curing a composite layer of 2 mm thick restorations and this is often still in common clinical practice.[15] Currently, many ultrafast curing lights are available within the market, which claims to polymerize the composites in an exceedingly shorter time by increasing the intensity of light. The influence of those light-curing units on the DC% of composites is not well documented. Hence, this study is aimed to compare and evaluate the DC% of four different composites using ultrafast photopolymerization.


   Materials and Methods Top


In this study, two microhybrid composites-Charisma smart (Kulzer GmbH, Hanau Germany, K010514) and Spectrum (DENTSPLY DETREY GmbH 78467 Konstanz, GERMANY, 1810000332) and two nanohybrid composites-Tetric N Ceram (Ivoclar Vivadent AG 9494 Schaan/Liechtenstein, X43749) and Solare Sculpt (GC Dental Products Corp, Japan, 1807306) were tested for their DC% with 1 s and 3 s polymerization using Woodpecker i Led Light curing unit (WOODPECKER-GUILIN WOODPECKER medical instrument Co., Ltd. Information Industrial Park, National High-Tech Zone, Guilin, Guangxi, 541004 P. R. China).

Experimental groups

The test specimen was divided into two groups according to the curing time used.

Group 1 specimen were subjected to 1-s curing and the test samples in Group 2 were cured for 3 s. Both Group 1 and Group 2 had 4 subgroups. Subgroup 1a and 2a consisted of samples prepared with resin composite “Charisma Smart;” Subgroup 1b and 2b had samples prepared with “Spectrum;” Subgroup 1c and 2c were samples prepared with “Tetric N Ceram;” and Subgroup 1d and 2d samples were prepared with “Solare Sculpt.”

Methodology

Sample preparation

A total of 40 disc-shaped samples, 5 numbers for each subgroup were prepared. A mylar strip was placed on a glass plate and 2 mm height Tygon tube of inner diameter 6 mm was positioned over it. Employing a Teflon coated instrument, the respective composite resin in line with the subgroups were placed. A second mylar strip was placed over the composite resin and another glass plate was slightly pressed over it to even the external surface of the samples. After removing the glass slab, the light-guide tip of the light-curing unit was placed over the composite resin at a distance of 2 mm from the top surface of the material and photo-activated for 1 second within the samples in Group 1 and for 3 s within the samples in Group 2. After photo-activation, the specimens were stored under dark dry conditions at room temperature.

Degree of conversion

After 24 h, the composite resin samples were submitted for Fourier transform infrared spectroscopy (FTIR) analysis. To measure DC%, the composite resin was pulverized into a fine powder. The pulverized composite resin was maintained during a dark room until the instant of the FTIR analysis. Five milligrams of the ground powder were thoroughly mixed with 100 mg of potassium bromide powder. This mixture was placed into a pelleting device and so pressed with a load of 10 tons for 1 min to get a pellet and it had been placed into a holder attachment into the spectrometer. The FTIR spectra of both uncured and cured samples were analyzed by obtaining the quantity of carbon double-bonds that are converted into single bonds. This provides the DC% of the composite resin.[16]

DC% was determined by the percentage of unreactive carbon-carbon double bonds (% C = C) was determined from the ratio of the absorbance intensities of aliphatic C = C (peak at 1638 cm−1) against an internal standard before and after the curing of the specimen: aromatic C-C (peak at 1608 cm−1). The DC% was determined by subtracting the % C = C from 100%, according to the equation given by Ribeiro et al.[16] and Abed et al.,[17]



Statistical analysis

The values were entered into a Microsoft Excel sheet for calculation. The obtained data were subjected to ANOVA and Tukey HSD post hoc test using IBM SPSS 21 software (SPSS version 21.0; IBM Corporation, Armonk, NY, USA).


   Results Top


Degree of conversion

[Table 1] shows the DC% mean values obtained from different curing time and different dental composites.
Table 1: Mean degree of conversion (degree of conversion %) values and standard deviation of the study group 1 (1 s curing)

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On comparing the subgroups in Group 1, subgroup 1c - Tetric N Ceram, a nanohybrid composite showed the maximum DC% (65.5%) followed by subgroup 1d - Solare, a nanohybrid composite (65.3%), subgroup 1b - spectrum, the microhybrid composite (62.3%) and subgroup 1a - Charisma smart, the microhybrid composite showed the minimum DC% (58.5%). Group 1 exhibited a statistical difference in the DC% (P < 0.05). The subgroups within group 1 showed statistically significant difference in the DC% (P < 0.05).

On comparing the subgroups in Group 2, subgroup 2b - Spectrum, the microhybrid composite showed the maximum DC% (95.4%) followed by subgroup 2a - charisma smart, the microhybrid composite (95.3%), subgroup 2d - Solare sculpt, a nanohybrid composite (93.7%) and subgroup 2c - Tetric N Ceram, a nanohybrid composite showed the least DC% (93.5%). The subgroups within group 2 does not exhibit any statistically significant difference in the DC% (P > 0.05).

ANOVA showed the DC% was influenced by different curing time (P < 0.05), according to the results presented, composites cured for 3 s presented higher values of DC% when compared to 1 s curing time.

Using the irradiation time of 1 s and 3 s recommended by the manufacturers the DC% of micro-hybrid and nanohybrid resin were statistically different. Therefore, the results suggested that the differences in the curing time had a significant impact on DC% (P < 0.05). The DC% values of the specimen's photo-activated for 1 s showed lower mean values when compared to the specimen's photo-activated for 3 s. There was also a statistically significant difference in the DC% among the experimental sub groups within the two groups tested (P < 0.05).

  • [Graph 1]: Comparison of group 1 (1-second curing) DC%
  • [Graph 2]: Comparison of group 2 (3 s curing) DC%.




   Discussion Top


The current study was conducted to evaluate the DC% of four different composites polymerized using two ultrafast photopolymerization durations employing a particular light curing unit using the recommended settings for every duration. A lower DC% of monomers to polymer in an exceedingly resin composite restoration could also be a hazard for the patient yet because it could affect the longevity of the restoration, because an incomplete conversion may cause unreacted monomers, which could dissolve in an exceedingly wet environment.[18]

The number of carbon double-bonds that are converted into single bonds provides DC% of the composite resin. Chung and Greener[19] obtained a DC% of photocured composites ranging from 43.5% to 73.8%. Silikas et al.[11] suggested that, a minimum of for occlusally restorative layers, DC values <55% is also contraindicated. When DC% levels are inadequate, the mechanical properties such as tensile strength, flexural modulus, and temperature resistance will be compromised and color stability may decline. Inadequate DC% might cause increased cytotoxity,[20] increased wear, increased marginal breakdown,[10] and increased microleakage.[9]

Significant toxic effects of monomers were reported on isolated human gingival fibroblasts. The cytotoxicity of the monomer's levels was within the following order: HEMA < TEGDMA < UDMA < BisGMA. Monomers released from resin composites were also found to be toxic to human gingival fibroblasts and immortalized human keratinocytes.[21] Substances such as TEGDMA and HEMA cause gene mutations in vitro[22] and unpolymerized resin-based materials contain various amounts of residual monomers and polymerization additives that will leach from restorations. Resin monomers from dental restorative materials are released into saliva and diffuse into the tooth pulp, gingiva, mucosa and salivary glands, which is able to potentially contribute to tumorigenesis.[23] Its been stated by Gupta et al.[24] that unbound monomers and/or additives are eluted by solvents or polymer degradation within the first few hours after initial polymerization. The quantity of such toxic leachable substances is reduced when there's high DC%.

FTIR was chosen to analyse DC% during this study because it is an efficient and regularly used technique. It provides a quantitative measure of the number of carbon double bonds that are converted to single bonds which reflect the DC% and effect of photopolymerization. The FTIR spectroscopy is relies on the very fact that molecules absorb electromagnetic radiation within the infrared region. This method could be a reliable method that detects the C = C stretching before and after the curing of the material, thus determining the number of unreacted monomers.[4],[16],[17]

Woodpecker i Led Light curing unit was utilized in this study. According to Roy et al., LED-curing unit has performed better when compared with QTH unit.[25] They are 2 modes within the light curing unit, TURBO Mode (2300–2500 mW/cm2) and NORMAL Mode (1000 mW/cm2–1200 mW/cm2). The recommended time for curing by the manufacturers is 1 second and 3 s in TURBO mode and 5, 10, 15, 20 s in NORMAL mode. We chose this light curing unit due to the high intensity of about 2300–2500 mW/cm2, which cures the 2 mm layer of composite in a very short period. Its known that physical properties of light-cured composites may change according to the distance from the irradiated surface and to attenuate the probabilities of its interference during this study, the space between the light guide tip and therefore, the samples was standardized at a distance of 2 mm as suggested by Medikasari et al.[26]

Within the present study, two microhybrid composites and two nanohybrid composites were used. Charisma smart and Spectrum are microhybrid resin composites and Tetric N Ceram and Solare sculpt are nanohybrid composites. Nanohybrid resin composites encompass large particles (0.4–5 μ) with added nanometer sized particles. In microhybrid resin composites, the fine particles of a lower average particle size (0.04–1 μm) are blended with microfine silica.[27]

We have chosen these materials to grasp whether the filler particle size has any influence on the DC%. Among Group 1 subgroup 1c - Tetric N Ceram, a nanohybrid composite exhibited maximum DC% (65.5%) followed by subgroup 1d - Solare, a nanohybrid composite (65.3%), subgroup 1b - Spectrum, the microhybrid composite (62.3%) and subgroup 1a - Charisma Smart, the microhybrid composite exhibited minimum DC% (58.5%).

Among Group 2, subgroup 2b - Spectrum, the microhybrid composite exhibited a maximum DC% (95.4%) followed by subgroup 2a - Charisma smart, the microhybrid composite (95.3%), subgroup 2d - Solare sculpt, a nanohybrid composite (93.7%) and subgroup 2c - Tetric N Ceram, a nanohybrid composite exhibited minimum DC% (93.5%). All the subgroups in Group 2 had a higher DC%, which exceeded the DC% values suggested by Chung and Greener[19] and Ferracane et al.[10] DC% values were nearly as good as indirect lab processing composites that are cured under higher intensity and longer durations. These results correlate with the previous study by da Silva et al.[28]

Within the studies by da Silva et al.,[28] the DC% of the nanohybrid composites was observed to be below microhybrid composites. The possible explanation for this performance was that the nonagglomerated silica nanoparticles which are commonly utilized in nanohybrid composites may cause a light scattering effect which can even attenuate the DC%. On other hand, the microhybrid composites aid in light transmittance which successively ends up in high DC% in composites than nanohybrid composites.

According to Knezević et al.,[29] the possible reason could also be because of the difference within the composition of the organic matrix and filler particle size, volume, and type. It is been observed that these factors interfere with the depth of cure and different spread pattern of the incident light that influence the DC%. The higher DC% of nanohybrid composites than microhybrid among group 1 during this study could also be because of the decreased curing time and its probable influence on the light scattering effect of nonagglomerated silica nanoparticles.

Mouhat et al.,[30] suggested that the use of light curing units with higher irradiance values than 1200 mW/cm2 may harm the pulp tissue. Park et al.,[31] advised that clinicians should limit the exposure time to 20 s when the irradiance from LED units ranges from 1200 to 1600 mW/cm2, while exposure period should not be longer than 10 s when the light curing unit irradiance ranges from 2000 to 3000 mW/cm2. However in vitro simulation does not reproduce the complexities of an in vivo scenario, which incorporates the presence of pulp tissue and also the dynamic blood flow mechanisms to control pulp temperature.

Within this study, higher DC% of composites can prevent the microleakage that leads to recurrent caries and its very less time-consuming procedure, as good as indirect laboratory procedures. The effect of higher light intensity and shorter curing time to the pulp is not known with the light curing unit used in our study, so further studies must be done.


   Conclusions Top


Within the limitations of this study, it was concluded that all the four types of composites cured for 3 s with Woodpecker i Led light-curing unit showed a high DC% which ranges from 93.7% to 95.4%. All the four composites cured for 1 s had much lower DC% but were still within clinically acceptable values. The results obtained during this study indicated that higher power curing reduced the time of cure to 1–3 s when conventional units required an exposure time of a minimum of 20 s.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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Correspondence Address:
Dr. Sundaresan Balagopal
Department of Conservative Dentistry and Endodontics, Tagore Dental College and Hospital, Chennai, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jcd.jcd_648_20

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