INTRODUCTION

Nano-filled resin-modified glass ionomer (nano-filled RMGI) is a newly introduced version of resin-modified glass ionomer cement (RMGIC). It is a new technical development that combines the benefits of a resin-modified light-cure glass ionomer with bonded nano-filler technology. Its chemistry is based on the methacrylate modified polyalkenoic acid, which was first employed in Vitrebond liner/base material (Ketac Nano-100, technical product profile, 3M ESPE). The use of nano primer is recommended by the manufacturer to bond the nano-filled RMGI to hard tooth surfaces. Nano primer is a one-component, acidic primer that does not need a preconditioning step. However, it is not clear how the nano primer can react with the smear layer covered dentin; the use of the nano-filled RMGI without the prior application of the nano primer should not be performed.¹

One of the indications of nano-filled RMGI is as a base material under restorative resin composite restorations, the so-called “sandwich technique.” This technique first requires the application of the nano primer to bond the nano-filled RMGI and then application of either etch-and-rinse or self-etch adhesives to bond the overlying resin composite. These double treatments of the tooth structure will increase not only the application procedure's steps but also, consequently, the restorative procedure's time. The trend today is to reduce application steps, so one-step self-etch adhesives were introduced.² Self-etch adhesives are user-friendly and lessen clinical application time. The elimination of the rinsing step makes such adhesives less technique sensitive and reduces the risk of making errors during application.^3-5 Although a study by Wang and others⁶ showed that the use of the etch-and-rinse single-bottle adhesive did not improve the bond strength of RMGI to dentin, it was reported that the use of different self-etch adhesives improved the bond strength of RMGI to dentin.^7,8 This approach will facilitate the application of both RMGI and composite using a one-step bonding procedure, thus reducing application time. Today, different self-etch adhesives are available with different chemical formulations. Some adhesives contain polyalkenoic acid copolymers in their chemical compositions, while others do not. The question is, can different self-etch adhesives bond the nano-filled RMGI to dentin with the same efficiency?

The purpose of this study was to investigate the effect of different self-etch adhesives with different chemical formulations on the shear bond strength of a nano-filled RMGI to dentin. The null hypothesis tested was that the use of the self-etch adhesives had no effect on the shear bond strength of nano-filled RMGI to dentin.

MATERIALS AND METHODS

In addition to the nano primer, four different self-etch adhesives were used; three were one-step self-etch adhesives and the fourth was a two-step self-etch adhesive. These adhesives were chosen according to their chemical formulations. Two different restorative materials were used: a nano-filled RMGI and a nano-hybrid resin composite. Compositions and manufacturers of adhesives and restorative materials used in this investigation are presented in Table 1.

Table 1:  Materials, Compositions and Manufacturers

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One hundred bovine anterior teeth were collected and used in the present study. The teeth were thoroughly cleaned and stored in 0.1% thymol solution for not more than 1 month. Subsequent to root removal, the pulp tissue was removed and the pulp chamber was blocked with sterile cotton. The labial enamel was wet ground on a laboratory trimmer to expose approximately 10 mm of flat dentin surface. The teeth were fixed from their labial side on the base of a rectangular mold with double-sided adhesive tape. Auto-cured acrylic resin was poured, and the mold was immersed in tap water to reduce the temperature rise during polymerization of the acrylic resin. After resin polymerization, the dentin surfaces were wet ground over 600 grit SiC paper for 60 seconds to create a standardized smear layer. The specimens were then ultrasonically cleaned in distilled water for 5 minutes prior to the bonding procedure to remove any remaining silicon carbide dust particles.

The teeth were divided into two main groups (50 each) according to the restorative material used, either the nano-filled RMGI or the resin composite. Each main group was then divided into five subgroups (10 each) according to the self-etch adhesive used. The nano primer and the self-etch adhesives were applied according to manufacturer instructions. Experimental grouping and bonding procedures are summarized in Table 2.

Table 2:  Experimental Groups and Application Procedures in This Study

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Before light curing each adhesive, a polyethylene tube (3 mm internal diameter and 2 mm in height) was positioned over the dentin surface. The adhesives were light cured using a halogen light curing unit (Hilux, Benlioglu; Ankara, Turkey) with an output of 600 mW/cm². The light intensity was checked periodically using a radiometer (Demetron-100, Demetron Research Corp, Danbury, CT, USA). The tube was filled with either the nano-filled RMGI or the resin composite, a polyester strip was applied and pressed gently, and the material was light cured according to manufacturer instructions using the halogen light curing unit. The teeth and their attached material cylinders were stored in distilled water at 37°C for 24 hours.

After 24 hours, each specimen was mounted to the universal testing machine (LR5K series, Lloyd Instrument, Fareham, UK). A metallic chisel-bladed instrument was positioned as accurately as possible to the restoration/dentin interface. The test was run at a crosshead speed of 0.5 mm/min until failure. The load recorded in Newtons was divided over the calculated surface area, and the shear bond strength was retrieved in MPa. Group 6 was not tested because all specimens of this group were spontaneously dissociated from the dentin surface during the 24-hour storage period, and their shear bond strength was expressed as 0.0 ± 0.0 MPa for statistical analysis.

Specimens from each group were evaluated under a stereomicroscope (S2-PT Olympus, Tokyo, Japan) at 40× magnification. The dentin side was evaluated, and the fracture pattern was analyzed as adhesive, mixed, or cohesive either in the restorative materials or in dentin. Two specimens were randomly chosen from each subgroup for scanning electron microscope (SEM) evaluation. Dentin blocks of approximately 6×6×2 mm, completely including the bonded area, were obtained, ultrasonically cleaned for 5 minutes, and dehydrated in ascending concentrations of ethanol, 50%, 70%, and 90% for 20 minutes each and in 100% ethanol for 1 hour. The specimens were left to dry on absorbent paper in a closed container overnight.

The specimens were fixed on a specimen holder using double-faced stickers and sputter coated (BAL-TEC, SCD 005, , Cologne, Germany). Dentin sides of the fracture specimens of the nano-filled RMGI and resin composite were evaluated at 25× and 1500× using SEM (Philips XL series, XL30, Eindhoven, The Netherlands) operated at 20 Kv. All data for shear bond strength in MPa were expressed as mean ± standard deviation (SD). Statistical analysis was carried out using SAS program (SAS, 1988, STAT/User's Guide, Release 6.03 ed., Cary NC, USA). Two-way analysis of variance (ANOVA) was run to test the effects of the restorative materials, adhesives, and their interaction on shear bond strength. One-way ANOVA (SAS) followed by the Duncan multiple range test were used to test the effects of adhesives within each restorative material. Student t-test (procedure TTEST of SAS) was run to compare the effect of restorative materials on shear bond strength within each adhesive.

RESULTS

The shear bond strength data for each restorative material and self-etch adhesive are presented in Table 3. Two-way ANOVA (Table 4) revealed that both materials and adhesives have a significant effect of the shear bond strength to dentin. The interactions between the two independent variables (materials and adhesives) also have a significant effect on shear bond strength.

Table 3:  Mean^a ± SD of the Shear Bond Strength of Nano-filled RMGI and Resin Composite Using Different Self-etch Adhesives

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Table 4:  Two-way Analysis of Variance for the Effect of Adhesives, Restorative Materials, and Their Interaction on the Shear Bond Strength

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One-way ANOVA followed by the Duncan multiple range test (Table 3) showed that three of the self-etch adhesives used (Adper Easy Bond, One Coat, and G Bond) significantly improve the shear bond strength of nano-filled RMGI compared with the nano primer. No statistically significant difference was shown in the shear bond strength between the Futurabond M and the nano-filled RMGI. Nano primer was not able to bond composite to dentin (Group 6), with all specimens failing before testing, and the shear bond strength was expressed as 0.0 ± 0.0 MPa, showing a statistically significant difference with all self-etch adhesives used. No statistically significant difference was recorded between the self-etch adhesives when composite was bonded to dentin, except between Adper Easy Bond and G Bond, where Adper Easy Bond shows a significantly higher shear bond strength.

Student t-test showed that nano primer records statistically significant higher shear bond strength when used with nano-filled RMGI than when used with composite. All self-etch adhesives show statistically significant higher shear bond strength with composite than with nano-filled RMGI except the One Coat adhesive, which shows no statistically significant difference between nano-filled RMGI and resin composite.

The Failure Mode Analysis

Failure mode analysis (Table 5) showed that the mixed type of failure is the predominant failure type in all evaluated groups except for Group 6, in which all specimens failed adhesively between the nano primer and resin composite. SEM microphotographs of the different failure modes are presented in Figures 1 through 3.

Table 5:  Failure Mode Analysis

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DISCUSSION

Direct posterior restorations are commonly used in daily dental practice because of their good esthetics and low cost.⁹ The success of composite restorations, especially in deep interproximal cavities, depends on several clinical factors,^10–16 among them the proper isolation as well as the optimum solvent evaporation and adequate light curing of the adhesive resin. These factors render the composite restoration a technique sensitive to most general dental practitioners.¹⁷ The “sandwich technique” was developed to overcome such difficulties in the bonding of resin composite to dentin.⁹ It was reported that the clinical durability of the glass ionomer (GI) is, to some extent, more predictable than the resin composite, through its chemical bonding to dentin.³ This technique is based on the application of either conventional GI or RMGI under composite restoration to overcome the technique sensitivity of composite restoration.¹⁸

With the increased interest in using nano materials in dentistry, a nano-filled RMGI is now available for dental practitioners.¹⁹ The use of the nano primer is a mandatory step to bond nano-filled RMGI to enamel and dentin.¹ Double bonding procedures to tooth substrates should be performed in the nano-filled RMGI/composite “sandwich technique.” These double bonding procedures are required to bond both the nano-filled RMGI and resin composite separately to tooth substrates. This will increase the clinical bonding steps and, consequently, the application time of the “sandwich technique” using the nano-filled RMGI/composite combinations.

Contemporary adhesives can be classified into three main categories: etch-and-rinse, self-etch, and glass ionomers.⁴ Self-etch adhesives were introduced as alternatives to the etch-and-rinse types. According to their clinical application steps, self-etch adhesives are either two-step or one-step.³ With the elimination of the rinsing step in the self-etch adhesives, the clinical application time was reduced.⁴ The chemical compositions of adhesives should aim to fulfill the requirements of the adhesive systems meeting our clinical needs. To some extent, all contemporary adhesives contain similar components.²⁰ However, additional elements may be present in some adhesives and not included in the chemical composition of others. In this study, the choice of adhesives was based on the presence or lack of ingredients such as methacrylized polyalkenoate or the presence of acidic monomer that contains carboxylic groups. Besnault and others⁷ report the improvement in the shear bond strength of the RMGI to dentin using different self-etch adhesives. In their study, they used the RMGI, which did not include the “Vitremere copolymer” in its composition. Accordingly, the present study aimed to evaluate if nano-filled RMGI, which does contain “Vitrebond copolymer,” can be bonded to dentin using the four different self-etch adhesives containing or not containing the polyalkenoic acid copolymer in their chemical compositions. Further, this study was aimed to evaluate the self-etch adhesive or adhesives that can bond both with the nano-filled RMGI and composite in a single bonding step to reduce clinical application steps and, consequently, application time in the “sandwich technique.”

In the present study, self-etch adhesives have a significant effect on the shear bond strength of the nano-filled RMGI to dentin, so the null hypothesis must be rejected. Three of the self-etch adhesives used in this study improve the shear bond strength of the nano-filled RMGI to dentin; however, the Futurabond M does not improve bond strength and shows similar bond strength with the nano primer. Two of the self-etch adhesives—Adper Easy Bond and One Coat—that improve the bond strength of the nano-filled RMGI contain methacrylized polyalkenoate in their chemical compositions and the other one (G Bond) contains acidic monomer 4-MET containing two carboxylic groups. Futurabond M contains neither a methacrylized polyalkenoate nor an acidic monomer such as 4-MET in its chemical composition. Therefore, the presence or absence of the polyalkenoate copolymer or the carboxylic group affects the bond strength of the nano-filled RMGI to dentin. This could be the reason that Futurabond M did not improve the bond strength of the nano-filled RMGI over the nano primer.

One of the advantages of the glass ionomer cements is their self-adhesive capacity to tooth tissue.^21,22 Unfortunately, this property was not supported in a previous study that evaluated bonding effectiveness of the nano-filled RMGI.¹ For this reason, the manufacture recommends the use of the nano primer to bond the nano-filled RMGI to tooth structure. Nano primer is an acidic primer with a pH of 3 (Ketac Nano-100, technical product profile, 3M ESPE). This high pH value does not allow the nano primer to remove or dissolve completely the smear layer (Figures 1A and 3A). In the SEM photomicrograph of the fractured surface of the nano-filled RMGI bonded with nano primer, the presence of the smear plugs occluding the dentinal tubules was shown. This observation was in agreement with Coutinho and others¹ and Korkmaz and others¹⁹ who reported no evidence of demineralization or hybrid layer formation and the absence of cement extensions inside the dentinal tubules.

The high incidence of the mixed failure type or even the presence of some specimens that failed cohesively with the nano-filled RMGI (Table 5, Figure 1) could be attributed to the presence of voids (Figure 3B), which might be due to air entrapment within the material during mixing.

In the present study, nano primer failed to bond resin composite to dentin, and all specimens failed prematurely during their storage period. This might be attributed to the absence of the intermediate adhesive layer following the application of the nano primer.¹⁹ One-step self-etch adhesives did not meet the clinical predictions in terms of their bond durability, as high incidence of failure during their clinical service was reported.³ One-step self-adhesives could bond resin composite to dentin, statistically, with the same results as the etch-and-rinse adhesives.²³ The different self-etch adhesives used in this study bonded the resin composite to dentin with the same efficiency. However, G bond showed the lowest shear bond strength results among all adhesives used. It was reported that when the application of the G bond was doubled, the bond strength was improved, which could lead to an explanation that the short time recommended by the manufacturer could not be enough for the chemical bonding mechanism to take place.²⁴

The shear bond strength of the resin composite to dentin was statistically higher than the bond strength of the nano-filled RMGI, except for the One Coat self-etch adhesive. Although, bond strength of RMGI using One Coat was still lower than the One Coat/resin composite bond, the difference was not statistically significant. One Coat is a two-step self-etch adhesive: the One Coat self-etch primer is applied first, followed by the application of the One Coat Bond 2. The bond (One Coat Bond 2) of the One Coat self-etch adhesive does not contain solvent in its chemical composition (One Coat, technical product profile, Coltène). This hydrophobic, nonsolvated layer might be the cause in bonding the RMGI and composite with the same efficiency. This is an explanation that needs further investigation.

From the results of this study, it would appear that when completing an interproximal “open sandwich” procedure, the bonding step for both the RMGI and composite can be accomplished simultaneously with a single, self-etching bonding agent. In this technique, the use of nano primer may not be necessary, which may make the application of the nano primer prior to the nano-filled RMGI dependent on the clinical condition.

CONCLUSIONS

Under the limitations of this study, it can be concluded that:

1. The self-etch adhesives used in this study appear to adequately bond nano-filled RMGI (Ketac Nano-100) to bovine dentin.

2. Absence of a polyalkenoic acid copolymer or an acidic monomer containing a carboxylic group in the adhesive negatively affects the shear bond strength of the nano-filled RMGI.

3. Nano primer appears to be unable to bond Grandio composite to bovine dentin.