The use of tooth-colored restorative materials has increased in recent decades. Improvements in the various physical properties of composite resins have contributed to this rise in popularity. However, composite resins continue to suffer from polymerization shrinkage, which can cause stress at the interface of the material and tooth structure. If shrinkage stress exceeds bond strength, a marginal gap forms and leakage occurs at the interface. ¹ , ² Microleakage can contribute to marginal discoloration, recurrent caries, and pulpal irritation. ³

When the gingival margin of a preparation is in dentin, the risk of the microleakage is increased. Many materials and methods have been proposed to improve marginal adaptation and reduce microleakage at the gingival margin. One such method is the bonded-base restorative technique. In this technique, an intermediate layer such as a glass ionomer or a low modulus resin-based material is placed between the restorative material and the dentin floor as the initial increment of the restoration. ⁴ Because this intermediate layer absorbs stress, it can decrease the effects of polymerization shrinkage. When glass ionomer is used as an intermediate material and is left exposed at the margin, this method is called the open-sandwich technique. ⁵ , ⁶

The use of resin-modified glass ionomers (RMGIs) in the sandwich technique provides chemical bonding to dentin, micromechanical bonding to composite resin, protective effects for the pulp, and potential cariostatic activity. Some studies have reported the effect of RMGI on the reduction of polymerization shrinkage, microleakage, and secondary caries. ⁷ , ⁸ , ⁹ , ¹⁰

Flowable composite resins can reduce microleakage because of their low viscosity, ease of adaptation with tooth structure, and low modulus of elasticity. ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁵ However, flowable composite (Grandio Flow: 6.85 Mpa) has higher elastic modulus than RMGI (Fuji II LC: 5.33 Mpa), so it is less effective in reducing the effects of restorative material shrinkage. ¹⁶ Nevertheless, one study showed no difference in microleakage of Class II restorations in flowable composite resins and RMGI when used as intermediate materials. ¹¹

Many in vitro studies ¹² , ¹³ , ¹⁴ have shown reduction of microleakage when using flowable composite resins as the base, whereas other studies ¹⁷ , ¹⁸ , ¹⁹ have shown no improvement in marginal adaptation.

Considering the varying results obtained from different methods for Class II composite restorations, especially the introduction of new products such as bulk-fill composite resins and various materials as bases, the purpose of this study was to compare the microleakage of new base materials used in bonded-base restorations along with bulk-fill composite resin. The null hypothesis was that there is no significant difference between techniques and base materials used for placing Class II composite restorations.

The present in vitro study was approved by the Research Ethics Committee of Isfahan University of Medical Sciences, Isfahan, Iran (with the ethics code of 395188), 60 extracted human third molars (free from cracks, fracture, caries, abrasion, previous restoration, and structural defects) were used. The teeth were stored in 0.2% thymol, cleaned using curettes, and randomly divided into five groups, 12 samples in each group. Sample size was calculated according to a previous study by Poggio et al. ²⁰ using PASS II software considering alpha = 0.05, beta = 0.2, and effect size of 0.42.

Class II box preparations were made in the mesial and distal surfaces of each tooth using a #245 bur (C21FG, Jota AG, Rüthi, Switzerland). Preparations had a buccolingual width of 4 mm, axial depth of 2 mm, and occlusogingival length of 8 mm, with gingival floors 1 mm below the CEJ. Pre-contoured clear matrices (Polyester Matrix, TDV, Santa Catarina, Brazil) were used to confine materials during placement. The thickness of the composite layers was marked on the strip to ensure uniform thickness of the composite layers. The materials used in the study, their manufacturers, and compositions are listed in Table 1.{Table 1}

Group 1 RMGI: The gingival floor of the preparation was conditioned for 20 s with polyacrylic acid 10% (Dentin Conditioner, GC, Tokyo, Japan) and then a 1-mm thick layer of RMGI (Fuji II LC) was placed on the gingival floor, measured by graduated periodontal probe, and light-activated for 20 s. After that, enamel margins were etched with 37% phosphoric acid (Ultra-Etch, Ultradent, South Jordan, UT, USA) for 20 s and then were rinsed and dried for 20 s. Self-etch primer was rubbed on the cavity walls for 20 s using a microbrush. Mild air pressure was applied for 10 s to remove solvent. The adhesive (Clearfil SE Bond) was applied to the entire preparation's walls. After removing the excess using a microbrush, the adhesive was light-activated for 20 s. The preparation was restored using two layers of bulk-fill composite (x-tra fil). The first and second increments were placed horizontally and had 3 and 4 mm thickness, respectively, measured by graduated periodontal probe, and then each layer was cured for 10 s. All light-curing was done using a halogen light-curing unit (Optilux 501, Kerr Demetron, Orange, CA, USA) with an intensity of 1000 mW/cm ².

Group 2: First, the preparations were etched, primed, and bonded as described for Group 1 and then a 1-mm thickness of resin-reinforced glass ionomer (Ionoseal) was applied to the gingival floor and was light-activated for 20 s. The restorations were completed by placing x-tra fil bulk-fill composite. The first and second increments were 3 and 4 mm thick, respectively, and each layer was cured for 10 s.

Group 3: All etching, priming, and bonding steps were accomplished as in the previous groups. For restoration, a 1-mm layer of a composite resin base (x-tra base) was inserted into the gingival floor and was cured for 20 s. The restoration was completed using x-tra fil composite in the same manner as described before.

Group 4: All etching, priming, and bonding steps were accomplished as in the previous groups. A 1-mm layer of flowable composite resin (Grandio Flow) was placed on the gingival floor and light-cured for 20 s. The restoration was completed using two increments of x-tra fil composite, as described before.

Group 5 (Control) (x-tra fil): All etching, priming, and bonding steps were accomplished as in the previous groups. The preparation was restored using x-tra fil composite resin in two layers as in the previous groups.

The restored specimens were finished and polished using a series of abrasive discs (Sof-Lex, 3M Oral Care, St. Paul, MN, USA) and were stored for 24 h at 37°C with 100% relative humidity in an incubator (BehdadIncub, Behdad, Tehran, Iran). Then, they were subjected to thermocycling procedures consisting of 500 cycles ²¹ (Mp Based, KARA 1000, Tehran, Iran) between 5°C and 55°C with a dwell time of 30 s in each bath and a transfer time of 10 s.

A dye penetration method was used to evaluate the microleakage. All the root apices were sealed with wax and to prevent dye penetration; two layers of nail varnish were applied to all surfaces, up to 1 mm from the restoration margins. The teeth were immersed in 0.5% basic fuchsine solution for 24 h. Specimens were then mounted in acrylic resin to be placed in the cutting device and the middle increment of the restoration was marked for sectioning. The specimens were sectioned half in the mesiodistal plan using two-sided diamond disks and gingival floor was observed under a stereomicroscope with a magnification of ×32 (MGC-10N9116734). The highest dye penetration rate in each half was selected to score the microleakage. Dye penetration was scored according to a four-point scale (ISO/TS 11405: 2003), including:

0 = No dye penetration

Two different operators performed the scoring and measured the highest dye penetration. The data were statistically analyzed using SPSS software version 20. The differences were considered statistically significant for P < 0.05. Kruskal–Wallis test was used to detect significant differences and Mann–Whitney test for comparisons between the independent groups.

Microleakage mean and median scores of the five groups are presented in Table 2. The greatest dye penetration occurred in the Ionoseal group and in the control group and was not significantly different. Dye penetration in each of the other three groups was significantly less than the control and Ionoseal groups (P < 0.02). Except for the Ionoseal group, all other groups had significantly less microleakage than the control group (P < 0.02), but no significant dye penetration was observed among groups (P > 0.05) Table 3.{Table 2}{Table 3}

Microleakage is an inherent defect of dental restorations. If restoration's margins are not completely sealed, fluids, bacteria, and debris can enter the cavity preparation. Leaky margins result in the development of caries, pulpal irritation with tooth sensitivity, and staining on the margins. Microleakage refers to microscopic openings between the margins of the resin filling and tooth structure. ²⁰ , ²² Several methods have been designed and used for this purpose. In this study, as in many previous studies, dye penetration was used to evaluate microleakage. This is a simple technique that does not require complex procedures or advanced equipment. ²³ , ²⁴

The statistical analysis revealed significant differences in microleakage between some treatment groups. The control group, in which x-tra fil composite resin was used without a base, had the highest microleakage scores. Among the four types of bases used in the experimental groups, Fuji II LC had the least microleakage and Ionoseal had the greatest. All except the Ionoseal group were significantly different from the control.

X-tra fil is a bulk-fill composite with low polymerization shrinkage (1.7%). According to its manufacturer, the physical properties of this material are due to the combination of multihybrid filler technology with the initiator system required for photopolymerization. Although the material is claimed to have low shrinkage stress, when no base was used, microleakage was relatively high.

It would appear that modification of the resin-based composite (RBC) restoration protocol of some conventional RBCs or bulk-fill resin restoratives may significantly improve bond integrity. ²⁵

The reason for the higher microleakage of Ionoseal, despite the presence of glass ionomer chemistry, could be a higher resin content. Its elastic modulus is greater than that of RMGI and it can therefore absorb less stress from the polymerization of composite resin in bonded base restorations. ⁸

Compared with previous studies, few studies were found to be similar in terms of materials and methods, but their results were consistent with those of the present study. For example, Zajkani et al. compared microleakage in two composite resin restoration techniques (open-sandwich and bonding technique) and reported that the least microleakage was observed in the open-sandwich restorations, including RMGI. RMGIs have a lower elastic modulus than highly filled composite resins. ²⁶ This results in less stress during setting, which can reduce the likelihood of microleakage. ²⁷

In addition, the reduction of composite volume in the sandwich technique can reduce polymerization shrinkage and microleakage of composite restorations. ²⁸

Kim assessed the microleakage of Class II composite resin restorations in a nonbased group (control) and three groups with flowable, compomer, and RMGI bases. They reported that the use of RMGI as a base was effective in reducing microleakage. In contrast, compomer and flowable composite bases did not reduce microleakage. ⁸

There was a significant difference between Ionoseal group and flowable and x-tra base groups, which may be due to the high percentage of filler in the flowable and x-tra base. The higher filler content could result in less polymerization shrinkage of these materials and therefore less leakage. ²⁹

Other in vitro studies have shown a decrease in microleakage for flowable composite resins as bases. ¹³ , ¹⁵ However, flowable composite (Grandio Flow: 6.85 Mpa) has higher elastic modulus than RMGI (Fuji II LC: 5.33 Mpa), so it is less effective in reducing the effects of restorative material shrinkage. ¹⁶ Thickness of the flowable layer might have some effect. For example, Hernandes et al. compared microleakage in Class II composite restorations by varying restorative techniques and thickness of two flowable composite resins. No significant differences were observed between materials. However, the lower thicknesses of the flowable composite resulted in less leakage. ³⁰

In another study, Toledano et al. examined the microleakage of Class V composite resin restorations filled with RMGI and compomer. They reported that microleakage in the three groups was not significantly different. ³¹ In addition, the results of the study by Sadeghi et al. suggested that the use of flowable liners, regardless of the type of material (composite resin or compomer), effectively reduced the microleakage in the gingival margin of Class II composite resin restorations. ¹⁵ However, according to the results of Moorthy et al., flowable composites used as bases did not result in less cervical microleakage in Class II restorations than an incrementally placed highly filled composite. ³² Furthermore, Politi et al. claimed that the conventional resin base composite materials had significantly lower microleakage scores compared with their bulk-fill resin counterpart materials. ²⁵

According to the results of this study, despite the introduction of bulk-fill composite resins with low polymerization shrinkage, the use of a base under the Class II composite restorations is recommended. The base can be either RMGI or flowable composite resin. Furthermore, microleakage of these materials should be investigated under more challenging conditions, such as artificial aging, use of hydrolytic enzymes, or exposure to an oral environment.

According to the results of this study, despite the introduction of bulk-fill composite resins with low polymerization shrinkage, the use of a base under the Class II composite restorations is recommended. The base can be either RMGI or flowable composite resin.

Limitations

In this study, mechanical loading was not performed, which is one of the limitations, since different results could have been obtained if the restorations had received occlusal loads. Another limitation of this study was the evaluation of specimens only under a stereomicroscope.

Acknowledgment

This report is based on a thesis that was submitted to the School of Dentistry, Isfahan University of Medical Sciences, in partial fulfillment of the requirement for the DDS degree (#395188). The authors are grateful for the financial support received by Isfahan University of Medical Sciences, Isfahan, Iran, who also approved the report.

Financial support and sponsorship

Dentistry School, Isfahan University of Medical Sciences, supported the study.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.