Investigation of polymerization of methacrylate-based composite resins those belong to different categories

INTRODUCTION

Since the introduction of Bis-GMA by Bowen in 1960s, countless methacrylate-based composite resins have been introduced in dentistry for the restoration of damaged teeth. To serve as a reliable and durable restorative material, composite resins are needed to satisfy two aspects: functional and aesthetic. To be compatible with host teeth, composite resins are needed to have high strength, modulus, and surface hardness, low water sorption, solubility, and thermal expansion (Ferracane, 2011; Drummond, 2008). Also, due to the increased interest in the aesthetics of teeth, color matching through diverse color options and color stability are important (Tsubone et al., 2012, Lee et al., 2010). According to reviewing of many studies, one of the main reasons attributable to the restoration fracture and secondary caries is connected to the polymerization shrinkage that occurred during and after light curing (Kemp-Scholte & Davidson, 1988; Lai & Johnson, 1993; Davidson & Feilzer, 1997). Since polymerization shrinkage is the result of the reduction of molecular spacing in the resin matrix due to the change of governing force between molecules, it is inevitable in the monomer-based composite resins. To overcome this problem, increase of filler content has been suggested to reduce monomers content those are responsible for the shrinkage. Inclusion of nanofillers enhances translucency by increasing the optical path length of incident light through the reduction of light scattering with nanofillers those are shorter than the wavelength of the incident light, but such increase of filler content may reversely decrease mechanical properties and hinder easy handling due to reduced resin matrix (Ferracane, 1995; Kim et al., 2007). Introduction of a new monomer system using a ring-opening polymerization mechanism may challenge its use over the free radicalmediated conventional composite resins in addition to its first impact of low shrinkage modality (Weinmann et al., 2005; Ilie & Hickel, 2006). The oxirane and siloxane molecules allow the silorane-based composite resins to have low shrinkage, water sorption, and solubility features.

Composite resins available in dentistry can be differently classified basically by the filler system such as filler size and filler loading (Ferracane, 2011). Many categories can be made according to the ways of classification, but based on the products produced by the same manufacturer, only several categories are possible. The purpose of the present study was to evaluate the polymerization shrinkage and flexural properties of the composite resins which can be categorized into four ways according to function and filler size: low shrinkage, nanohybrid, microhybrid, and flowable resins.

MATERIALS AND METHODS

Specimens and light-curing units

For the study, four different composite resins (all A3 shade) from three different manufacturers were used. Products were chosen based on their characteristics such as low shrinkage, nanohybrid, microhybrid, and flowable by the manufacturers’ claim. Details of the tested products were summarized in Table 1.

For the light curing, a QTH [Optilux 501, Kerr, Danbury, CT, USA] and a LED [L.E.Demetron, Kerr, Danbury, CT, USA] light-curing units were used as light sources. The output light intensity of QTH and LED was approximately 900 mW/cm² as measured using the built-in radiometer.

Table 1.

Materials tested in the present study

RESULTS

The filler weight determined by ash method is shown in Table 1. The nominal wt% (provided by the manufacturers) of the tested products ranged from 60 to 92%, on the other hand, wt% determined by ash method ranged from 56.2 to 85.8%. Depending on product, the difference of both values ranged from 0.5 to 36.8%.

Table 2 shows the polymerization shrinkage of the specimens for different light-curing units. Regardless of product and manufacturer, low shrinkage and flowable resins showed the lowest (7.7-12.3 μm) and highest (26.2-35.4 μm) values, respectively. Nanohybrid and microhybrid products showed less difference in their values, and two microhybrid products (AB and Z2) showed slightly lower polymerization shrinkage than that of their corresponding nanohybrid products (AE and ZX). Shrinkage difference by the different lightcuring units was statistically insignificant (p>0.05).

Table 2.

Polymerization shrinkage (μm) of the tested specimens

Table 3 shows the flexural properties (FS and FM) of the specimens before and after immersion test. Before immersion, FS and FM ranged 99.2-188.6 MPa and 5.24-20.87 GPa, respectively, depending on product and light-curing unit. After immersion, these values changed to 82.9-176.7 MPa and 3.89-21.80 GPa. Most products showed decreased FS and FM values after immersion (approximately 0.2-44.7% for FS and 0.5-32.7% for FM depending on product and light-curing unit). FS change was statistically significant (p<0.05), whereas FM change was statistically insignificant (p>0.05).

Table 3.

Flexural properties of the tested specimens before and after immersion at 37˚C distilled water for 2 weeks

DISCUSSION

The tested products in the present study can be differently classified based on the claimed characteristics by the manufacturers: low shrinkage, nanohybrid, microhybrid, and flowable resins. The basis of these classifications is the restoration function, filler type, and flowability of the tested products.

Low shrinkage nature in important in the dental restorative materials to minimize unwanted clinical problems such as marginal leakage, restoration fracture, postoperative sensitivity, and recurrence of secondary caries (Kemp-Scholte & Davidson, 1988; Lai & Johnson, 1993; Davidson & Feilzer, 1997). To overcome shrinkage problems, so far, there were two main approaches: increasing filler content and adopting new monomer system. AP and CP were claimed to contain much higher filler content than the other products. To increase the filler content, recently nanofillers are routinely contained in the dental composite resins. In the case of FL, it showed much lower filler content than AP and CP, but, contains completely different monomer system. Unlike most free radical-mediated methacrylatebased composite resins, FL adopts silorane-based ringopening polymerization system. Silorane is a hybrid compound of siloxane and oxirane molecules, oxirane molecules in silorane achieve low polymerization shrinkage through a cationic ring-opening mechanism (Tilbrook et al., 2000; Eick et al., 2002). In the case of AP and FL, they certainly have less shrinkage values than the other products (7.7-10.1 μm vs 13.6-35.4 μm) within the same manufacturer, but, CP has statistically similar shrinkage values compared to the other products (CE and CX) within the same manufacturer. High shrinkage in flowable resins (AF, CF, and ZF) is mainly due to low filler content to increase the content of diluents such as TEGDMA even though CF contains exceptionally high content of fillers as a flowable resin. The filler content of nanohybrid products was lower than that of microhybrid products even though nanofillers were claimed to be included in their resin matrix. However, the resultant shrinkage difference between them is low (11.3-24.7 μm vs 12.8-21.0 μm). Furthermore, the shrinkage pattern was not consistent in these two classes. According to the linear fit, there was an inverse linear correlation between the filler content and polymerization shrinkage (R=0.5-0.72 depending on filler content and light-curing unit), so containing high content of fillers may produce less shrinkage if it is methacrylate-based composite resin.

The flexural properties (FS and FM) are the measure of material resistance against transverse stress. FS values of products of different categories had no consistent pattern among products of different manufacturers. In the case of Bisco and Kuraray products, microhybrid and nanohybrid products (AB and CX; AE and CE) showed the highest and the lowest FS, respectively. On the other hand, in the case of 3M, except low shrinkage product (FL), there was no significant difference in FS among products. A linear correlation with filler content was not over 0.56 according to the linear fit, indicates other unknown factors those affect the resultant values. One probable factor that affects the result is the inclusion of internal defects during manufacturing or specimen preparation process (Zeng et al., 1996; Della Bona et al., 2003). Such formed defects become the origin of fracture when there is external stress. The FS values evaluated are similar to those of many other methacrylate-based composite resins (Ilie & Hickel, 2009). Modulus is the measure of material’s stiffness. In the load-displacement curve of the bending test, modulus is the slope of the curve in the elastic range, so high modulus implies a great stiffness (Ferracane, 2001). FM of the tested products has a linear correlation (R=0.87 and 0.88 for DE and OP, respectively) with the filler content (wt% determined by ash method). The FM values obtained are lower than that of dentin (17-25 GPa), so those products can compatible with the underlying dentin (Kinney et al., 2003; Xu et al., 1998). After immersion in distilled water, most products tested showed decrease of FS and FM values even though such decrease had no consistent dependence on product category.

CONCLUSION

Within the limitations of the present study, the following conclusions could be reached:

• 1. Regardless of product and manufacturer, low shrinkage and flowable resins showed the lowest (7.7-12.3 μm) and highest (26.2-35.4 μm) polymerization shrinkage, respectively. The effect by light-curing units is insignificant (p>0.05).
• 2. In most cases tested, composite resins produced by the same manufacturer but belong to different categories have inconsistently different flexural properties. Also, composite resins those belong to the same category but products of different manufacturers showed inconsistently different flexural properties as well.
• 3. Accordingly, the user needs more attention to the detailed flexural properties of each product than nominal classification (category) of the resin products for the reliable and durable restoration.

Acknowledgments

이 논문은 부산대학교 자유과제 학술연구비(2년)에 의해 연구되었음

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