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 Table of Contents  
Year : 2021  |  Volume : 13  |  Issue : 1  |  Page : 31-36

The effect of different curing units on the degree of polymerization of different composite resins

1 Department of Restorative Dentistry, Nuh Naci Yazgan University, Kayseri, Turkey
2 Department of Restorative Dentistry, Atatürk University, Erzurum, Turkey

Date of Submission29-May-2020
Date of Decision13-Oct-2020
Date of Acceptance19-Oct-2020
Date of Web Publication15-Feb-2021

Correspondence Address:
Ozcan Karatas
Department of Restorative Dentistry, Nuh Naci Yazgan University, Kayseri
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jorr.jorr_31_20

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Objective: The aim of this study was to investigate the degree of polymerization of composite resins cured with different light-curing units (LCUs).
Materials and Methods: Three bulk-fill composite (Beautifil Bulk-Fill Giomer, Filtek Bulk-Fill, and X-Tra Fill) and a methacrylate-based composite (Filtek Z250) were used in this study. Thirty disc-shaped specimens, 4 mm thick, were prepared from each composite resin. Specimens were divided into three groups and polymerized with light-emitting diode (LED), Plasma arch curing unit (PAC), and quartz-tungsten halogen LCU. The bottom and top surface microhardness of the specimens stored in distilled water for 24 h at 37°C after polymerization was measured with a universal test device. The hardness ratio (HR) of specimens was calculated by the bottom surface microhardness/top surface microhardness formula. Data were analyzed by two-way ANOVA and Tukey's least significant difference post hoc tests (α = 0.05).
Results: Statistically significant differences were detected between the mean HR values of the specimens polymerized with different LCUs (P < 0.05). The mean HR values of Beautifil Bulk-Fill Giomer specimens were found to be statistically significantly lower than other composites (P < 0.05). The mean HR values of the specimens polymerized with PAC LCU were found to be statistically significantly lower than other LCUs (P < 0.05).
Conclusion: The degree of polymerization of the composite resin restoration may be affected by the structural properties of the resin and the type of LCU. The clinician may use alternative techniques, especially in deep cavities.

Keywords: Bulk-fill composite resin, degree of polymerization, light-curing units

How to cite this article:
Karatas O, Yilmaz MN, Gul P, Sagsoz O, Yapar MI. The effect of different curing units on the degree of polymerization of different composite resins. J Oral Res Rev 2021;13:31-6

How to cite this URL:
Karatas O, Yilmaz MN, Gul P, Sagsoz O, Yapar MI. The effect of different curing units on the degree of polymerization of different composite resins. J Oral Res Rev [serial online] 2021 [cited 2021 Jun 21];13:31-6. Available from: https://www.jorr.org/text.asp?2021/13/1/31/309437

  Introduction Top

The amount of conversion of monomers into the polymer during the polymerization of composite resins is called the degree of conversion or polymerization. It is a general opinion that the degree of polymerization should be high during the application of composite resins.[1] As the degree of polymerization increases, the amount of residual monomer that does not participate in the resin decreases, and physical properties develop accordingly.[2] Inadequate polymerization may lead to adverse effects on the pulp due to nonpolymerized toxic monomers, defects in the restoration-tooth connection, and edge failure, postoperative sensitivity, coloration, abrasion, and secondary caries.[3],[4]

The ability of a material to resist a constant force application is called microhardness. If the microhardness of the material increases, its resistance to scratching and abrasion increases, preventing it from easily deforming against various forces.[5] Although various methods are used to measure surface hardness, which method should be used depends on the material to be tested. The Vickers hardness test may be used to measure the surface microhardness of composite resins. For this purpose, a pyramid-based diamond tip is used to create a collapse on the composite surface [Figure 1].[6] The figure obtained by dividing the bottom microhardness value of the composites by the top microhardness value is called the “hardness ratio (HR).” In the literature, it has been reported that the acceptable HR should be between 0.80 and 0.90 in order for the composite resin to polymerize sufficiently in a certain thickness.[7]
Figure 1: Vickers hardness measurement chart

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Different factors, such as the strength of the light source, the distance between the tip of the light device and the restoration, the thickness of the restoration, as well as the organic and inorganic nature of the composite material, may affect the degree of polymerization of light-cured composite resins.[8] Light-emitting diode (LED), plasma arch (PAC), and quartz-tungsten halogen (QTH) light-curing unit (LCU) are used for the polymerization of dental composite resins. QTH LCU has been used in the dental clinic for a long time. However, these systems have disadvantages such as long curing time, rapid deterioration, short working time, and high heat generation.[9] Solid-state LED systems have been proposed to overcome these disadvantages of halogens. LED systems have advantages such as long life of up to 10,000 h and shorter curing time. In addition, these systems are often preferred because they do not require extra coolants and they are light compared to halogens.[10] In recent years, PAC LCUs have also been used for curing composites. These systems contain two separate tungsten electrodes in an inert gas-filled pressure chamber. These electrodes create high electrical potential in the space between them. A spark occurs when high-voltage electricity is generated between the electrodes, and this spark ionizes the Xenon gas around the spark, forming the plasma. PAC units can produce more than 2000 mW/cm2 light intensity and provide polymerization of the composite in a very short time.[11],[12] In addition to its advantages, PAC systems also generate high heat, which can lead to an increase in intrapulpal heat in the restored tooth. Therefore, when there is not enough dentin and composite resin thickness to protect the pulp during curing, PAC LCUs should not be used for a long time.[13]

Conventional methacrylate-based composite resins should be placed in large and deep cavities in layers due to the limited polymerization degree and increased risk of polymerization shrinkage. The use of this technique, which is called the “incremental technique,” requires a lot of curing and causes time loss.[14] In order to overcome this negativity, manufacturers were sought to find ways to place composite resin with larger masses and reduce polymerization shrinkage. As a result, a new generation of composite resins called “bulk-fill” was developed.[15] Hydroxyl-free bisphenol A glycidyl methacrylate, aliphatic urethane dimethacrylate, partially aromatic urethane methacrylate, or high-branched methacrylate was added to the resin matrix structure of bulk-fill composites. Manufacturers stated that bulk-fill composites can be polymerized by placing 4–5 mm thick in the cavity.[16]

The degree of polymerization of the bulk-fill composites when curing by different LCUs is an issue to be investigated. The aim of this study is to investigate the effect of different LCUs on polymerization degrees of bulk-fill and methacrylate-based composite resins. Our null hypothesis is that there is no significant difference between the effects of different LCUs on the polymerization degree of different composite resins.

  Materials and Methods Top

In our study, the degree of polymerization of specimens prepared from three different bulk-fill composites (Beautifil Bulk-Fill Giomer, Filtek Bulk-Fill, and X-Tra Fill) and a methacrylate composite (Filtek Z250) [Table 1] was compared using the method determined by Bouschlicher et al.[17] Thirty specimens were obtained from each composite using 4 mm thick and 10 mm diameter cylindrical molds made of polytetrafluoroethylene. Composite resins were placed in the molds placed on the glass and covered with strip tape (Polydentia, Mezzovico, Switzerland) and glass slide to give the specimen a flat form. Specimens prepared from each composite were divided into three groups (n = 10) and polymerized with three different LCUs (Elipar S10 LED [1000 mW/cm2, 20 s, 3M ESPE, Seefeld, Germany], QTH Astralis7 [400 mw/cm2, 40 s, Ivoclar Vivadent, Australia], and Valo PAC [Xtra power mode, 3200 mW/cm2, 6 s, Ultradent, USA]). During the polymerization, curing units were kept in contact with the glass placed on the specimens. The curing units were positioned at the center of the specimens in accordance with the literature[18] and applied from a distance equal to the thickness of the glass (1 mm) placed on the specimen surface. The light intensity of the curing units was checked using a digital radiometer (Hilux Ultra Plus Curing Units, Benlioglu Dental, Ankara, Turkey), and calibration was repeated for each group. The specimens whose polymerization was completed were removed from their molds, and their bottom surfaces were marked to distinguish them.
Table 1: Details of the investigated restorative materials

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After polymerization, the surfaces of the specimens were polished for 30 s using a slow-speed handpiece with polishing discs (Sof-Lex, 3M ESPE, St. Paul, MN, USA) under water. Then, all specimens were kept in distilled water at 37°C for 24 h. The mean microhardness values of specimens were measured at randomly 3 points from the top and bottom surfaces of the specimens using the Vickers hardness device (Micromet 2001, Buehler, Illinois, USA) with a load of 200 g for 10 s waiting time. All hardness values (HVs) were calculated, where 1 HV = 1.854 P/d2, with P representing the indentation load and d the diagonal length.

The mean HR value of each specimen was determined with:

HRmean = HVbottom/HVtop formula. Specimens with a HR of 0.80 and above were considered to be sufficiently polymerized.

The data obtained from microhardness measurements were recorded and subjected to statistical analysis on SPSS 18 (SPSS Inc., Chicago, IL, USA) software. Kolmogorov–Smirnov test was used to determine the distribution of data. After determining that the data had a normal distribution, two-way ANOVA and Tukey's least significant difference post hoc tests were performed to examine the effect of restorative material and curing on the HR of the specimens. P < 0.05 value was considered statistically significant.

  Results Top

As a result of the ANOVA test, statistically significant differences were detected between the mean HR values of the specimens polymerized with different LCUs (P < 0.05) [Table 2]. When the composite resins were compared, the mean HR values of Beautifil Bulk-Fill Giomer specimens were found to be statistically significantly lower than other composites (P < 0.05). There were no statistically significant differences between Filtek Bulk-Fill, X-Tra Fill, and Filtek Z250 groups (P > 0.05). The highest mean HR values were determined in X-Tra Fill specimens.
Table 2: Analysis of variance results for hardness ratio measurements

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When the LCUs were compared, the mean HR values of the specimens polymerized with PAC LCU were found to be statistically significantly lower than other LCUs (P < 0.05). The highest mean HR values were determined in the specimens polymerized with LED LCU, while there were no statistically significant differences between LED and QTH LCU groups (P > 0.05). When the polymerization degree of the specimens was examined, the mean HR value was found above 0.80 only in the Filtek Bulk-Fill LED, X-Tra Fill QTH and LED, and Filtek Z250 LED groups. It was determined that the specimens in other groups did not have a sufficient degree of polymerization [Figure 2].
Figure 2: Mean hardness ratio values of different composite resins cured with different light-curing units

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  Discussion Top

In our study, the effects of different LCUs on polymerization degrees of different composite resins were investigated. The microhardness test method is effective to measure the mechanical strength and rigidity of the material.[19] Brinell, Rockwell, Knoop, and Vickers tests may be applied to measure surface microhardness. Rockwell and Brinell tests are not suitable for fragile materials but for elastic materials. Vickers test is used for measuring the hardness of the tooth structure as it is suitable for measuring the hardness of fragile materials.[20],[21] In addition, the tips of Vickers hardness tester are 1/3 shorter than the tips of other devices used in hardness measurement. This tip structure of the device allows the material to be less affected by the surface properties during hardness measurement and to make more precise measurements.[22] Therefore, in our study, hardness measurements were made with the Vickers hardness tester. The force applied to the specimen surface for microhardness measurement varies between 1 g and 1 kg. As the force exerted on the specimen increases, the tip of the measuring device advances to the deeper plates of the specimen.[23] For the ideal measurement, the notch tip should descent just below the specimen surface. Thus, more accurate results are obtained by not measuring the hardness of the light-affected surface of the material. In our study, 200 g of force was applied to the specimens with the universal test device. While preparing specimens in studies to examine the surface properties of different composite resin restorations in the literature, glass slide and strip bands were used to flatten the specimen's surface.[24],[25] Researchers stated that applying strip tape and glass slide to the composite resin surface increases the surface smoothness and that measurements such as hardness and roughness can be made more successfully.[26],[27] The use of a thin glass slide will improve the surface properties of the specimens without affecting the light intensity of the curing unit and thus the polymerization degree.[28] Hence, the surfaces of the specimens in our study were flattened with strip tape and a 1 mm thick glass slide.

It is an important issue at which thicknesses of composite restoration are applied for adequate polymerization. It is stated in the manufacturer's instructions that bulk-fill composites can be applied to a 4 mm thick cavity.[29] In our study, a 4 mm thickness polymerization degree of different bulk-fill composites was investigated and a methacrylate composite was used to compare. Although the methacrylate composite specimens are not expected to show a sufficient degree of polymerization at this thickness, the mean hardness rate of the Filtek QHT group specimens was found above 0.80. Contrary to the manufacturer's instructions, Beautifil Bulk-Fill Giomer specimens did not show sufficient polymerization degree when polymerized with different LCUs. The degree of polymerization of composite resins depends on its chemical structure, filler type and quantity, transmitting property, and properties of the light device.[30] The Beautifil Bulk-Fill Giomer was created by adding reacted glass ionomer filling to the resin matrix structure. This giomer-based material has been produced due to the clinical advantages of glass ionomers.[31] However, this structure of resin may have prevented the light from penetrating deep into the composite resin matrix.[32] In addition, the filling particle size of Beautifil Bulk-Fill Giomer is higher than the other composites used in the study. It is known that as the particle size increases, the degree of conversion of the composite resin decreases.[33],[34] Tsujimoto et al.[35] showed that Beautifil Bulk-Fill Giomer has lower polymerization shrinkage than other resin bulk-fill composites in their studies. Researchers found that this was due to lower curing depth. In our study, the fact that the Beautifil Bulk-Fill Giomer was not shown sufficient polymerization degree in 4 mm thickness may be caused by the glass ionomer content.

In our study, the mean hardness rates of Filtek and X-Tra Fill bulk-fill composites were found higher than the methacrylate-based composite by polymerizing with different LCUs. Jang et al.[36] found in their study that bulk-fill composites showed a higher degree of polymerization than 4 mm thick methacrylate-based composites. Similarly, Kuijs et al.[37] examined the effect of placing bulk-fill composites in layers and masses on the polymerization depth of these composites, and as a result, they reported that bulk-fill composites up to 4 mm thick can be placed in the cavity in the cavity. The bulk-fill composites' filler content, improved translucent structure, and photoactive groups placed in the methacrylate resin allow better control of polymerization kinetics, thereby demonstrating a better degree of polymerization than the methacrylate composites by the bulk technique.[38] In our study, the higher polymerization degree of Filtek and X-Tra Fill bulk-fill composites than methacrylate-based composites can be explained by these mechanisms. In addition, the filling size of methacrylate-based composite resin that we use in the study is higher than bulk-fill composites. As the filler content and size of the composite resin increases, the light transmittance of the material decreases, and the degree of polymerization decreases.[30]

The degree of polymerization of light-cured composite resins is influenced by the type of light device, light intensity, exposure time, and distance. In our study, standardization was achieved by contacting the light device with the glass on the specimen surface in specimen preparation. Yazici et al.[39] showed that the specimens polymerized with LED LCU had a higher polymerization degree than the other LCUs. In addition, the researchers were found that the specimens polymerized by PAC LCU showed the lowest degree of polymerization. Similarly, in our study, the highest mean hardness rate values were observed in the specimens polymerized by LED LCU, while the lowest mean hardness rate values were observed in PAC specimens. Although PAC LCU has high energy (3200 mW/cm2), its application time is short. Studies have reported that prolonged use of high-energy light devices leads to an irreversible increase in temperature in the pulp.[40] The short operating time of the PAC LCU may explain the lower mean hardness rate. Considering our results, our null hypothesis that there is no significant difference between the effects of different LCUs on the polymerization degree of different composite resins has been rejected.

  Conclusion Top

Within the limitations of this in vitro study, the degree of polymerization of the composite resin restoration may be affected by the structural properties of the resin and the type of LCU. The clinician should choose appropriate composite resin type and LCU during composite resin restorations, especially in deep cavities. Our data on the degree of polymerization should now be supported by further studies in which monomer release and temperature rise during polymerization are examined.

Ethical clearance

Since this study was an in-vitro material study, it was stated that there was no need for ethics committee approval by the clinical research ethics committee.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

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  [Table 1], [Table 2]


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