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In 1962, Bowen of the American Dental Association developed a new type of composite material. Bisphenol-A glycerylmethacrylate (bis-GMA), which is called Bowen’s resin, and an organic silane coupling agent were his great achievements (Anusavice, 2003). These new composites solved problems such as excessive thermal expansion and contraction, the poor wear resistance of acrylic resins, and a bond between the filler particles and the resin matrix.

However, commonly used resin-based composites (RBC) still have major drawbacks such as high polymerization shrinkage (Davidson & Feilzer, 1997; Ozer et al., 2014). Polymerization stress is caused by volumetric contraction, which causes the bond integrity to result in secondary caries and restoration loss (Zorzin et al., 2015; Taneja et al., 2016). Many clinical methods have been proposed for reducing polymerization shrinkage. The layering technique that sequentially fills resin composite into the tooth cavity and performs light curing reduces polymerization shrinkage effectively by reducing the C-factor (ratio of bonded to unbonded surface) (Opdam et al., 1998; Park et al., 2008). Still, this incremental layering method of filling the cavity with resin composite has caused problems such as incorporation of voids, contamination between composite layers, bonding failure between layers (Abbas et al., 2003), increased deformation of the restored tooth (Versluis et al., 1996), and long working time. Another limitation in the use of resin composite is an insufficient depth of cure (Sakaguchi et al., 1992). Although the light curing time varies in different RBCs and light curing units (LCUs), typical Quartz-tungsten-halogen (QTH) and light-emitting diode (LED) curing lights require 20 to 40 seconds to cure resin composites (Christensen et al., 2000), and the depth of cure of resin composites is 2 mm (Christensen et al., 2000; Moore et al., 2008).

To overcome these limitations, a new type of RBCs was developed (Zorzin et al., 2005; Ilie et al., 2013b). This newly developed type of RBCs has a comparable flexural strength to nano-and microhybrid RBCs and a higher flexural strength than flowable RBCs (Ilie et al., 2013a). These bulk-fill RBCs have less shrinkage stress and a lower shrinkage rate (Ilie & Hickel, 2011), reduced cuspal deflection (Moorthy et al., 2012), and similar marginal adaptation (Campos et al., 2014) compared with traditional RBCs. These bulk-fill RBCs show improvement in light transmittance due to a reduction in the amount of filler and increased filler size, which enhances the depth of cure (Ilie et al., 2013a). Because curing an RBC with a 4mm bulk for 20 seconds can maintain the degree of cure and micro-mechanical properties (Czasch & Ilie, 2012), this bulk-fill RBC allows the use of up to 4 mm increments of material (Ilie et al., 2013a; Ilie et al., 2013b; Finan et al., 2013; Garoushi et al., 2013; Li et al., 2015). Therefore, the layering process can be skipped with this bulk-fill RBC while maintaining a high shear strength (Ilie et al., 2014) and saving precious chair time (Garoushi et al., 2013).

Furthermore, flowable bulk-fill RBCs such as SureFil SDR (Dentsply, Milford, DE, USA) have been introduced to have low, slow elastic modulus development and low-stress polymerization without compromising the cure depth (Burgess et al., 2010). SureFil SDR with the low-stress behavior of flowable bulk-fill RBCs is suitable for both post-cementation and core build-up (Giovannetti et al., 2012). Traditional RBCs have been used in diverse dentistry fields such as posterior occlusal restoration, sealing pits and fissures, porcelain veneer cementation, and other dental restoration cementation. Considering the availability for both post-cementation and core build-up, bulk-fill RBCs can be used in cementation like traditional RBCs.

However, there is limited knowledge about the bonding properties of bulk-fill RBCs. Therefore, the aim of this study was to evaluate and compare the shear bond strength (SBS) of orthodontic brackets bonded with bulk-fill RBCs. The tested null hypotheses were that: (1) there would be no significant difference in the shear bond strength (SBS) among the four materials (bulk-fill RBCs or conventional adhesive cement) and (2) there would be no significant difference in the shear bond strengths (SBS) among light-curing units (LCU).

The first null hypothesis that adhesive cement (bulk-fill RBCs and conventional adhesive cements) has no significant influence on shear bond strength (SBS) has to be rejected. All three bulk-fill RBCs had a lower SBS value than conventional adhesive cement Transbond XT. When the brackets were attached using bulk-fill RBCs, SonicFill had a significantly higher SBS than the other two bulk-fill RBCs. However, the SBS of all bulk-fill RBCs was the higher than the SBS value recommended for adhesive cements (5.9 - 7.8MPa), indicating that bulk-fill RBCs can be used to attach orthodontic brackets. In a previous study, when bulk-fill RBCs and conventional RBCs were bonded to dentin, the SBS of conventional RBCs was higher than that of bulk-fill RBCs, and the SBS of SonicFill (12.19 ± 5.48 MPa) was slightly higher than that of Tetric EvoCeram Bulk Fill (11.16 ± 2.76 MPa)(Colak et al., 2016), similar to the findings of the present study.

Bulk-fill RBCs have been introduced into the market as a result of the demand for simple and convenient restoration procedures (Ilie et al., 2014; Caixeta et al., 2015). Bulk-fill RBCs are classified according to viscosity as low- and high-viscosity bulk-fill RBCs. Flowable, low-viscosity bulk-fill RBCs (SureFil SDR, Dentsply; Venus BulkFill, Heraeus Kulzer; x-trabase, VOCO; Filtek BulkFill, 3MESPE) entered the market first. Low-viscosity bulk-fill RBCs have weak physical properties; thus it is necessary to cap them with traditional RBCs. Later, high-viscosity bulk-fill RBCs were released (SonicFill, Kerr; Tetric EvoCeram Bulk Fill, Ivoclar Vivadent; X-traFil, VOCO) that do not require a capping layer and can, therefore, be used as a single step bulk filling material (Braga et al., 2005).

RBCs have viscoelastic behavior and convert during polymerization from a viscous plastic structure to a rigid elastic structure (Caixeta et al., 2015). Polymerization shrinkage occurs in RBCs while bonding composites to dental tissue, causing a problem with the stability of the restoration (Caixeta et al., 2015). In composites, development of polymerization shrinkage stress (PSS) is influenced by a multiplicity of factors such as the type of filler and content, filler/matrix interaction, type of monomer, and the degree and rate of polymerization (Braga et al., 2005; Caixeta et al., 2015). Increased inorganic filler decreases the concentration of double bonds, which reduce volumetric contraction (Gonçalves et al., 2011).The reduced polymerization rate due to high inorganic filler content was related to a high modulus of elasticity (Gonçalves et al., 2011). In conventional RBCs, higher filler content resulted in a higher modulus of elasticity (Ilie & Hickel, 2009; Gonçalves et al., 2010; Ilie et al., 2013). Similarly, in bulk-fill RBCs, higher filler content resulted in a higher modulus of elasticity (Ilie et al., 2013a). On the other hand, materials with reduced elastic modulus due to lower filler content will result in greater deformability under masticatory stresses when placed in a load-bearing area, which will cause disastrous failure (Ilie et al., 2013a).

The volumetric percent of the filler weight of Filtek BulkFill (64.5%) used in the present study was remarkably lower than that of the other RBCs. It is assumed that this lower volumetric percent of the filler weight significantly influenced the lower SBS of the Filtek BulkFill. Tetric N Ceram Bulk Fill, a high-viscosity bulk-fill RBC, had a significantly lower SBS than SonicFill, also a high-viscosity bulk-fill RBC. The volumetric percent of the filler weight of Tetric N-Ceram Bulk Fill (78%) is high, but polymer filler (16%) is included within the filler content. This polymer filler may have influenced the lower SBS of the Tetric N-Ceram Bulk Fill. SonicFill has higher filler content (83.5%) than the other two bulk-fill RBCs and also had higher SBS values, which is accordance with a previous study (Colak et al., 2016).

The effect of inorganic filler content on PSS is still under debate. That is, in a previous study, an increase in filler content disrupted polymeric chain propagation, which resulted in increased polymerization shrinkage stress (Braga et al., 2005). In another previous study, an increase of the filler content of RBCs was related to less PSS, which can be explained by the reduced volumetric shrinkage observed in heavily filled composites (Gonçalves et al., 2010).

An increased degree of conversion (DC) in RBCs results in increased polymerization shrinkage and elastic modulus. The bis-GMA (2,2-bis[4-(2-hydroxy-3-methacrylyloxypropoxy) phenyl] propane) concentration influences the increase of elastic modulus (Asmussen et al., 1998).That is, a monomer matrix such as bis-GMA, triethylene glycol dimethacrylate (TEGDMA), or urethane dimethacrylate (UDMA) influences the conversion of resin composites (Zorzin et al., 2015).

The high viscosity of bis-GMA reduces mobility, which reduces DC (Zorzin et al., 2015).To reduce the high viscosity of bis-GMA, TEGDMA and UDMA were added to the resin matrix. Low molecular weight monomers such as TEGDMA reduce the viscosity of resin composites and enhance the %DC but increase PSS and polymerization volume shrinkage (Zorzin et al., 2015). In order to reduce viscosity without increasing the TEGDMA content, high molecular weight monomers such as UDMA and ethoxylated dimethacrylate have been used (Zorzin et al., 2015).

The bonding ability to tooth substrate could also be influenced by the composition of the agents. In RBCs, the main factors that influence the bonding to tooth tissues are the DC, wettability, and shrinkage stress (Moraes et al., 2008).With a higher DC, the mechanical properties are excellent, which improves bonding to the tooth tissues (Moraes et al., 2008). Filtek BulkFill and Tetric EvoCeram Bulk Fill include bisphenol-A polyethylene glycol diether dimethacrylate (bis-EMA) and UDMA as a resin matrix; on the other hand, SonicFill includes TEGDMA and ethoxylated bisphenol-A-dimethacrylate, which have a low molecular weight, as a resin matrix. Bis-EMA and UDMA have a higher molecular weight and a stiffer structure than TEGDMA. The high viscosity of bis-EMA and UDMA reduces wettability with the substrate surface and disturbs adhesion, which might influence the low SBS of Filtek BulkFill and Tetric EvoCeram Bulk Fill (Moraes et al., 2008).

In traditional RBCs, the filler content is increased in order to enhance the mechanical properties, and the filler size is decreased in order to improve aesthetics. On the other hand, bulk-fill RBCs have a smaller filler amount and a larger filler size (Colak et al., 2016).A reduction in filler content was related to a reduction of hardness (Ilie et al., 2009).Commonly, hardness and the indentation modulus of high-viscosity bulk-fill RBCs with a high filler content were similar to conventional RBCs, and the hardness and indentation modulus of low-viscosity bulk-fill RBCs with a low filler content were lower than conventional RBCs (Bucutaet et al., 2014; Kim et al., 2015).Other than EvoCeram Bulk that contains 17% prepolymer filler, the increased filler content of bulk-fill RBCs increased the flexural strength (Ilie et al., 2013a). The flexural strength of bulk-fill RBCs is similar to nano-and hybrid RBCs and higher than flowable RBCs (Ilie et al., 2013a).

The filler content and transparency of RBCs are correlated. For all traditional RBCs and bulk-fill RBCs, lower transparency was correlated with higher filler content (Zorzin et al., 2015; Ilie et al., 2013a; Bucuta & Ilie, 2014).Other than SonicFill, which had a high filler content, the transparency of bulk-fill RBCs was the higher than that of traditional RBCs (Bucuta & Ilie, 2014). The transparency was affected by the difference of the refractive index between filler particles and resin matrix (Primus et al., 2002; Shortall et al., 2007).When the refractive index between the RBC components was similar, the transparency of the dental materials increased (Azzopardi et al., 2009).The good depth of cure with SonicFill may be due to a matching refractive index between the resin and filler, which enhances light transmission (Colak et al., 2016). In this study, the higher bond strength of SonicFill in comparison to the other bulk-fill RBCs could be attributed to the properties of the SonicFill (Colak et al., 2016).

In addition, the transparency of dental materials increases with a larger filler size (Ilie et al., 2013a).The filler dimension of bulk-fill RBCs increased (filler size>20mm as observed in several materials such as x-trafil and x-trabase, VOCO, Cuxhaven, Germany; SureFil SDRflow, Dentsply Caulk, Milford, DE, USA; SonicFill, Kerr, Orange, CA, USA), which reduced the entire filler surface and filler-matrix interface as if filler content was the same. That is, light scattering at the filler-matrix interface reduced, which made light focus on the matrix and increased the depth of cure (Ilie et al., 2013a). In this study, SonicFill had a higher SBS value than the other bulk-fill RBCs. It was considered that this was attributed to DC enhancement due to an increase in the transparency according to the SonicFill filler size increase.

The second null hypothesis that the type of LCU has no significant influence on SBS was accepted. For all light-curing units, SBS values did not show a significant difference. Light curing with DPSS showed the comparable SBS values to QTH and LED. Thus, light curing with DPSS is possible in clinical practice (Park et al., 2013).

In ARI evaluation, Transbond XT (68.9%) and SonicFill (75.6%) displayed the highest SBS value and had a high distribution of ARI score 5 (all the adhesive cement remained on the bracket). In other words, more frequent failure occurred on the tooth-adhesive cement interface. Filtek Bulk Fill and Tetric N-Ceram displayed lower SBS values and had more frequent cohesive fractures within the adhesive cement.

The purpose of this study was to evaluate the utility of bulk-fill RBCs for bonding orthodontic brackets. In the present study, orthodontic brackets were bonded to the enamel surface using three different light curing units. For all LCUs, Transbond XT showed a higher SBS than bulk-fill resin composites. For all LCUs, the three bulk-fill resin composites showed clinically acceptable SBS values. There were significant differences between the bulk-fill RBC groups, as SonicFill had a significantly higher SBS than the other two bulk-fill resin composites. However, the LCUs showed no significant impact on the SBS. ARI scores were significantly different among adhesive cement groups, whereas ARI scores were not significantly different among LCUs. In all groups, some or all the adhesive cement remained on the bracket (ARI score 4 and 5).