In vitro microleakage at the enamel and dentin margins of class II cavities of primary molars restored with a bulk‐fill and a conventional composite

Abstract Objectives This study assessed the enamel and dentin margin microleakage of class II cavities of primary molars restored with a bulk‐fill and a conventional composite. Materials and Methods In this in vitro, experimental study, standard class II cavities were created in the proximal surfaces of 60 extracted primary molars. The teeth were randomly divided into two groups, and restored with SonicFill bulk‐fill and Filtek Z250 conventional composite along with Single Bond 2 adhesive. The teeth were coated with two layers of nail varnish to 1 mm around the restoration margins, and the apices were sealed with wax. The teeth underwent 1500 thermal cycles and incubated at 37°C for 24 h. They were then immersed in 1 M silver nitrate in the dark, rinsed with water, immersed in developing solution for 12 h, and exposed to fluorescent light. Next, they were mesiodistally sectioned, and digitally photographed under a stereomicroscope at ×10 magnification. The dye penetration depth was measured by a blind observer, and analyzed by the Mann–Whitney U test (α = .05). Results No significant difference existed in microleakage between the two composite groups at the enamel (p = .76) or dentin (p = .16) margins. In both composite groups, microleakage at the dentin margins was significantly greater than that at the enamel margins (p = .000). Conclusion Considering the absence of a significant difference in microleakage, SonicFill bulk‐fill composite can be used as an alternative to Filtek Z250 conventional composite for restoration of primary molars to benefit from its advantages such as simpler and faster application.


| INTRODUCTION
By the advances in dental materials and clinical restoration techniques, direct composite resins are the most commonly used dental materials to meet the esthetic demands of patients in restoration of carious teeth, coronal fractures, dental erosions, and congenital defects (Kwon et al., 2012). Despite the optimal physical properties, polymerization shrinkage, and stress are the main drawbacks of conventional composite resins. Management of stress caused by polymerization shrinkage of dental composite resins is imperative to achieve optimal marginal integrity and guarantee the durability of restorations. Polymerization stress causes small cracks in the composite mass, and results in debonding of adhesive from the cavity walls, and subsequent gap formation and marginal microleakage, which results in postoperative tooth hypersensitivity. Marginal discoloration, low fracture resistance, caries recurrence, and tooth deformation are among other complications caused by polymerization stress (Radhika et al., 2010;Van der Vyver, 2010).
Microleakage refers to passage of bacteria, liquids, molecules, and ions through the cavity wall-restoration interface, which is not clinically detectable (Vicente et al., 2009). Microleakage is an important factor that adversely affects the durability of restorations, and can cause tooth hypersensitivity, recurrent caries, and pulpal damage (Gong et al., 2019). To seal restoration margins, a uniform interface between the cavity walls and restoration is imperative (Gogna et al., 2011).
New chemical formulations of resins have been introduced to minimize microleakage. Also, it is important to facilitate and accelerate the restorative procedure by using restorative materials with fewer procedural steps. Bulk-fill composite resins were introduced to the market to simply the restorative procedures and overcome the limitations of incremental application of conventional composite resins by the advances in their monomer, initiator, and filler technology. Compared with conventional resin composites, bulk-fill composite resins have lower filler content and larger filler particles, as well as improved translucency .
Moreover, different chemical structures of monomers in bulk-fill composites decrease their polymerization stress (Abbasi et al., 2018).
However, bulk-fill composite resins have a curing time comparable to that of conventional composites and are cured by the same curing units (Bucuta & Ilie, 2014). Bulk-fill composite resins can be applied in 4-mm thick increments and can preserve their optimal mechanical properties and degree of conversion in all thicknesses, which may be due to their decreased polymerization stress and high reactivity to light (Czasch & Ilie, 2013). Bulk-fill composites have higher molecularweight monomers and novel initiator systems compared with conventional composites. Bulk-fill composites are suitable for use in patients with poor cooperation since they accelerate the restorative procedure. Thus, bulk-fill composites are particularly appealing for use in pediatric restorative procedures (Moorthy et al., 2012).
Bulk-fill composites are available in low viscosity (flow) and high viscosity (restorative) types, and can be injected into the cavity and easily applied. The newest generation of bulk-fill composites can also be used for posterior restorations in depths over 4 mm due to improved mechanical properties. In new-generation bulk-fill composites, the advanced monomer technology decreases the polymerization shrinkage and has advantages such as lower cuspal flexure in standard class II cavities, and optimal adaptation to the cavity walls (Dahl, 2016). Adverse complications such as postoperative tooth hypersensitivity, microleakage and debonding also have a lower frequency in use of new-generation bulk-fill composites (El-Damanhoury & Platt, 2014). The bulk application technique has fewer clinical steps and accelerates the procedure as such (Didem et al., 2014). However, some studies reported that use of newgeneration bulk-fill composites had no significant effect on cervical microleakage (Gallo et al., 2000;Moorthy et al., 2012).
The main advantages of bulk-fill composite resins are their increased depth of cure probably due to their high translucency, and low polymerization stress . SonicFill is a bulk-fill composite which is polymerized in 4-mm-thick increments, and possesses the properties of both flowable and packable composite resins at the same time. SonicFill has its own specific handpiece, that decreases the viscosity of composite by sonic energy activation, and aids in fast application of composite and its optimal adaptation to cavity walls. When the sonic energy is discontinued, the composite turns into viscous state, which is more suitable for shaping and carving (Leprince et al., 2014).
Since the introduction of bulk-fill composite resins, their properties such as bond strength, cuspal flexure, degree of conversion, depth of cure, gap formation, mechanical properties, microleakage, and polymerization shrinkage have been the topic of many studies. However, the majority of such investigations have been conducted on permanent teeth. Thus, this study aimed to assess the microleakage at the enamel and dentin margins of standard class II cavities restored with a bulk-fill and a conventional composite resin.

| MATERIALS AND METHODS
This in vitro, experimental study was conducted on 60 sound primary molars extracted within the past 3 months due to their physiologic exfoliation time or serial extraction for orthodontic treatments. The The collected teeth were stored in saline. Before the onset of the experiment, the teeth were immersed in 0.5% chloramine T solution at 4°C for 1 week. Standard class II cavities were created in the mesial and distal surfaces such that they had 2 mm buccolingual width, and 1.5 mm isthmus width, and their cervical margin was 1 mm below the cementoenamel junction (CEJ). The teeth were randomly MOSHARRAFIAN ET AL. | 513 divided into two groups (n = 30). The cavities were restored with SonicFill bulk-fill composite (Kerr) in group 1, and Filtek Z250 conventional composite (3 M ESPE) in group 2. Single Bond 2 adhesive was used in both groups ( Table 1). The conventional composite was applied incrementally by manual instruments while the bulk-fill composite was applied as bulk.
In group 1, the cavities were rinsed and dried, enamel was etched for 20 s, and dentin was etched for 15 s, followed by 10 s of rinsing and drying. Two layers of Single Bond 2 were applied, air-thinned for 5 s at 10 mm distance, and cured for 20 s. Bulk-fill composite was applied as mass, cured for 40 s using a LED curing unit (Woodpecker) with a light intensity of 800-1000 mW/cm 2 , and the surface of specimens was polished.
In group 2, the same steps were performed for adhesive application as explained for group 1. Next, conventional composite resin was applied incrementally in two steps, and after curing for 20 s, the surface of specimens was polished.
The surface of the teeth was then coated with two layers of nail varnish except for a 1 mm margin around the restorations. Tooth apices were also sealed with wax. The teeth then underwent 1500 thermal cycles between 5°C and 55°C with a dwell time of 30 s and a transfer time of 15 s. Next, the teeth were immersed in water at 37°C for 24 h. They were then immersed in 1 M silver nitrate solution in the dark for 6 h, and after rinsing with water, they were immersed in developing solution for 12 h, and exposed to fluorescent light. After drying, the teeth were mesiodistally sectioned by a high-speed diamond cutter (Mecatome R210A) under water coolant. The sections were inspected under a stereomicroscope (EZ4D; Leica, Olympus) at ×10 magnification and digital photographs were taken from the specimens to measure the dye penetration depth. The dye penetration depth was measured and recorded by an examiner blinded to the group allocation of the teeth (Figure 1). The microleakage data were analyzed by SPSS version 25. The measures of central dispersion were reported for microleakage at the enamel and dentin margins in the two composite groups. Since the Kolmogorov-Smirnov test showed non-normal distribution of data (p < .05), comparisons were made using the nonparametric Mann-Whitney U test. Level of significance was set at 0.05. Table 2 presents the measures of central dispersion for dye penetration depth (indicative of microleakage) in the two composite groups. According to the Mann-Whitney U test, microleakage at the enamel margins of bulk-fill composite group was slightly greater than that in the conventional composite group but this difference was not significant (p = .76). Conversely, microleakage at the dentin margins of the conventional composite group was slightly, but not significantly, higher than that in bulk-fill composite group (p = .16).

| RESULTS
However, in both Filtek Z250 conventional composite and SonicFill bulk-fill composite groups, microleakage at the dentin margins was significantly greater than that at the enamel margins (p = .0001 in both).

| DISCUSSION
This study assessed the enamel and dentin microleakage of class II cavities of primary molars restored with a bulk-fill and a conventional composite. The results showed the occurrence of microleakage at the enamel and dentin margins in both groups. However, no significant difference existed in microleakage between the two composite groups at the enamel or dentin margins. In other words, both T A B L E 1 Characteristics of composite resins and adhesive used in this study.

Material Composition Manufacturer
Filtek Z250   Rengo et al. (Rengo et al., 2015). compared bulk-fill and conventional composite resins using 50% silver nitrate and micro-CT and digital microscopy, and demonstrated their comparable performance with T A B L E 2 Measures of central dispersion for the dye penetration depth (indicative of microleakage) at the enamel and dentin margins in the bulk-fill and conventional composite groups. SonicFill is among the best bulk-fill composite resins available in the market. Also, increased translucency of bulk-fill composite resins improves their curing depth and leads to better light penetration into deeper layers, resulting in greater depth of cure .
Furthermore, SonicFill is activated by sonic energy, and can be applied in one layer with 5 mm thickness without increased translucency, which guarantees optimal esthetics (Jackson, 2012).
Its modifiers decrease its viscosity with sonic energy by 87%, and increase its flow for better marginal adaptation with the cavity walls, and then it can be shaped following discontinuation of sonic energy (Jackon, 2011). Use of Single Bond 2 adhesive in the present study is another possible reason for lack of a significant difference in microleakage between the two composite groups (Campos et al., 2014).
The majority of available studies on bulk-fill composites have been conducted on permanent teeth. Güngör et al (Güngör et al., 2014). measured the microleakage in class II cavities in permanent and primary teeth restored with a conventional composite and found no significant difference in microleakage at the occlusal margins. However, microleakage was significantly greater in the gingival margin of primary teeth than permanent teeth, which may be due to different enamel structure and thickness in primary teeth. In general, primary enamel has lower calcium and phosphorous content than permanent enamel, is thinner, and has higher density of rods (De Menezes Oliveira et al., 2010). Primary dentin has higher number of dentinal tubules with larger diameters, compared with permanent dentin, and thus, the available substrate for bonding is smaller in primary teeth (Sardella et al., 2005). All these factors can explain greater microleakage in primary teeth.
In the present study, microleakage at the dentin margins was significantly greater than that at the enamel margins in both groups.