Bond strength of a composite resin to glass ionomer cements using different adhesive systems

Introduction: Glass ionomer cements are often used as a base or cavity lining prior to restorative material. Objective: To evaluate the bond strength of a composite resin to different glass ionomer cements, when using a two-step conventional and self-etching adhesive systems. Material and method: Three glass ionomer cements (Ketac Molar Easymix, Vitremer and Vitrebond), the composite resin Filtek Z350 XT and the adhesive systems Adper Single Bond 2, Clearfil SE Bond and Adper Easy One were used. As negative control, resin was bonded to cement without using an adhesive system. Holes (4 mm diameter, 2 mm deep) prepared in acrilic bloks were filled with the glass ionomer cements (n=12/group). On the surface, an area of 1mm in diameter was delimited, the adhesive system was applied, and a specimen of composite resin with 1 mm height was made. After 24 hours storage (37 °C and 100% humidity), the microshear test was performed. Data were analyzed using two-way ANOVA and Tukey test for comparison between groups (α=0.05). Result: The adhesive systems significantly improved the bond strenght of composite resin to glass ionomer cements (p≤0.001). There was no significant difference in bond strength when self-etching adhesive systems were compared with the simplified etch-and-rinse adhesive, except for Vitrebond where Clearfil SE Bond determined higher bond strength when compared to Adper Single Bond 2 (p=0.003). Conclusion: Self-etching adhesive systems are a good option for establishing the bond between the composite resin and the glass ionomer cement. Descriptors: Glass ionomer cements; dentin-bonding agents; composite resins; shear strength. Rev Odontol UNESP. 2017 July-Aug; 46(4): 214-219 Bond strength of a composite resin... 215


INTRODUCTION
The increasing demand for aesthetic restorations and the evolution of composite resins (CR) have encouraged the use of these materials in posterior teeth.However, when they are associated with adhesive systems in deep dentin without a protection of the pulpodentin complex, they are an important factor contributing to pulpal irritation 1,2 .
Glass ionomer cement (GIC) lining prior to restoration seems to increase clinical success, because this procedure associates the low coefficient of thermal expansion, biocompatibility 3 and anticariogenic activity 4,5 of GIC with the aesthetic and fracture resistance properties of composite resins.However, the lack of chemical bonding between composite resin and conventional GICs may interfere in the final properties of the restoration, and consequently, its longevity 6 .To improve the GIC/composite resin bond, the use of resin-modified glass ionomer cements may be indicated 3,7 .
Another method to optimize the bond GIC/composite resin is to perform phosphoric acid etching on the GIC surface, which increases the retentive microporosities and consequently improves the micromechanical retention 8 .Nonetheless, moisture contamination during setting of the GICs may cause dissolution of the calcium polyacrylate chains altering their physical properties 9 , therefore it is advisable to wait for the initial setting to occur before carrying out the acid etching and washing 6,10 .
The use of self-etching adhesive systems may solve this problem, since they do not require the washing step 6 .In addition, one or more carboxylic or phosphate groups may be incorporated into these self-etching adhesive systems, and studies have shown that they exhibit enamel and dentin bond strength similar to that of total acid etching (etch and rinse) adhesive systems 11,12 .Although these adhesive systems have shown promising results, to date few studies have evaluated the composite resin bond to glass ionomer cements when using these materials.
Thus, the objective of this study was to evaluate the bond strength of a composite resin and both conventional and resin modified glass ionomer cements when using a simplified etch-and-rinse and self-etching adhesive systems.The null hypothesis was that the type of adhesive system did not interfere with the bond strength of a composite resin to different glass ionomer cements.

Materials
The materials used in this study are shown in Table 1.

Experimental Design
The Groups were distributed according to the variation factors, totalizing 12 levels of variation (Table 2):

Preparation of the Specimens
For specimen preparation, blocks (20 mm high, 18 mm in diameter) containing three holes (4 mm in diameter, 2 mm deep) in the upper surface, were made of acrylic resin (OrtoClas, Artículos Odontológicos Clássico Ltda.Ind. Bras, São Paulo, SP, Brazil).
The holes were filled with the GICs which were manipulated in the powder/liquid ratio (by weight) recommended by the manufacturers and inserted by means of a Centrix syringe (DFL, São Paulo, SP, Brazil), in a single increment.After application of the material, a polyester matrix strip, followed by a glass plate and a 500 g weight were placed on its surface for 30 seconds to ensure a flat, smooth surface.Resin-modified glass ionomer cements Vitremer and Vitrebond, were photoactivated for the time recommended by the manufacturer by using a photocuring unit (UltraLed, Dabi Atlante Inds., Medical Odontology Ltda, Ribeirão Preto, SP, Brazil) previously evaluated for light intensity (450 ± 10 mW / cm 2 ).For the conventional glass ionomer cement (Ketac Molar Easymix), the time required for the initial setting (5 min) was waited.The matrix/GIC set was stored at 37 °C with 100% humidity for 24 h.
In the second stage of specimen preparation, the GIC surfaces were sanded with 320 grit abrasive paper to simulate wear with a diamond drill.The bond area was delimited by means of resistant double-sided adhesive tape (Tectape, Manaus, AM, Brazil) with a central perforation 1.0 mm in diameter, made by means of an adapted rubber sheet perforator (model Ainsworth, Wilcos do Brasil Indústria e Comércio Ltda., Petrópolis, RJ, Brazil).The dentin adhesive systems: Adper Single Bond 2, Clearfil SE Bond and Adper Easy One were applied on the GICs in accordance with the respective manufacturers' recommendations and then were photoactivated.
A plastic microtube (1.0 mm internal diameter and 2.0 mm high) (Tygon, Norton Performance Plastic Co, Cleveland, OH, USA) was fitted with the internal area coinciding with the area bounded by the adhesive tape and was used as a matrix for preparing the specimens.The composite resin Filtek Z350 XT was inserted into the microtube in a single increment, with the aid of a clinical probe (Duflex, Rio de Janeiro, RJ, Brazil) and photoactivated for 20 seconds as recommended by the manufacturer.As a control group, composite resin was applied on the glass ionomer cements without any previous surface treatment.
The specimens were stored at 37 °C with 100% humidity for 48 hours.After this period, the plastic microtube was removed with the help of a n o .15scalpel blade (Embramed, São Paulo, SP, Brazil) and the specimens were observed under a stereoscopic magnifying glass at approximately 50x magnification to certify the absence of defects at the bond interface.
Traction movements were made at a speed of 0.5 mm/min.The tests started by means of a specific computerized program (Tesc-Test Script, EMIC Equipamentos de Ensaio Ltda, São José dos Pinhais, PR, Brazil) and proceeded until fracture.The maximum stress values in MegaPascal supported by the union composite resin/GIC were recorded.

Fracture Pattern Analysis
After the microshear test, the specimens were stored in closed recipients at room temperature.The failure type was identified by examination under a stereomicroscope (Mod SZX7, Olympus, São Paulo, SP, Brazil), at approximately 50x magnification.The failures were classified as: adhesive (failure at composite resin/GIC interface), cohesive (failure within GIC or composite resin), or mixed (combination of adhesive and cohesive failure).

Statistical Analysis
To evaluate the influence of using the different adhesive systems on the bond strength of composite resin to the different GICs, the normality and the homoscedasticity of the data were initially observed.Based on these assumptions, a parametric test (two-way ANOVA) was used, and the Tukey post-test was applied to identify the differences between the groups.The level of significance adopted for decision making was 5% (α = 0.05).
The fracture patterns were descriptively analyzed.

RESULT
The means and standard deviations of bond strength are presented in Table 3.The use of adhesive systems significantly improved the bond strength of the composite resin to GICs (p≤0.001), and bond strength was influenced by the type of glass ionomer cement and adhesive system (p<0.001).
When bonding was produced on a conventional glass ionomer cement (KM), the CSE adhesive system showed bond strength values similar to SB (p=0.072) and higher than EO (p=0.005).For bonding produced on VM, adhesive systems did not influence the bond strength results (p≥0.126), while for VB, the CSE adhesive system showed bond strength values higher than SB (p=0.003) and similar to EO (p=0.287).The EO and SB adhesive systems did not show statistically significant difference for the different GICs (p>0.05).
The distribution of failure types observed, considering the GICs and adhesive systems used, is presented as a percentage of occurrences in Figure 1.Adhesive failures were identified in 100% of cases when no surface treatment of GIC was performed before placement of the composite resin.No cohesive fracture of the composite resin was observed.For conventional GIC, Ketac Molar Easymix, irrespective of the adhesive system used, 50% of the failures observed were cohesive within GIC, 27.5% were adhesive and 22.5% mixed.In the resin-modified GICs, the failures were distributed as follows: Vitremer (47.9% adhesive, 27.1% cohesive within the material and 25% mixed); and Vitrebond (44.7% adhesive, 23.7% cohesive within the material and 31.6%mixed failure).Adhesive systems had a significant influence on the failure type (Figure 1).

DISCUSSION
Our findings showed that bond strength of the composite resin to different glass ionomer cements was influenced by adhesive system used.Few studies have evaluated the bond strength of composite resin to glass ionomer cements when different types of dentin adhesive systems were used.Sá et al. 13 compared the bond strength of composite resin to different conventional GICs, by using three adhesive systems, and either performing etching with 37% phosphoric acid, or not, before their application.They  concluded that acid etching was dispensable for both base/lining and restoration GICs.In our study, the bond strength between composite resin and GICs when using a simplified etch-and-rinse and self-etching adhesive systems improved significantly when compared with application of the composite resin directly onto the GIC.Similar results were observed by Kundabala et al. 14 .According to Hinoura et al. 15 adhesive systems improved the wettability of the GIC, and consequently, improved the bond to the composite resin.
Pamir et al. 16 comparing a simplified etch-and-rinse and a single-step self-etching adhesive did not observe significant difference in bond strength of a composite resin to the conventional and resin-modified GICs .In the present study, we found no significant difference between the single-step self-etching (EO) and simplified etch-and-rinse (SB) adhesive systems.But there was difference between CSE (two-step self-etching adhesive) and SB only when the glass ionomer VB was used as base.Although the statistically significant difference was not always present, self-etching adhesive systems determined higher mean bond strength values between composite resin and resin-modified GICs than the conventional adhesive system.Kundabala et al. 14 and Zhang et al. 17 also observed higher values of bond strength when using self-etching adhesive systems.This may have been due to a lower viscosity of self-etching adhesive systems, which determines a lower angle of contact with the surface of the GICs, resulting in better wettability, and consequently, better bonding between composite resin and GICs 18 .
Venkateshbabu et al. 19 , studying the bonding capacity of composite resin to GIC when using strong self-etching adhesives (pH=1.0),intermediate strength (pH=1.4) and weak (pH=2.2) adhesives, concluded that weak self-etching adhesive systems significantly improved bond strength when compared with other adhesive systems.When strong acids are used higher cation neutralization and formation of fragile structure salts occur, which adversely affect the bond strength.
In this study, as in the study of Zhang et al. 17 , the only significant difference in composite resin/GIC bond strength between two-step (pH 2.0) and one-step (pH 2.3) self-etching adhesive systems was observed when they were applied on conventional GIC.According to these authors 17 , the cohesive type of failure of conventional GIC was relatively common and when this type of failure occurs, the real strength of the interfacial bond between the GIC and composite resin must not being evaluated, since the cohesive strength of GIC's acts as a limiting factor in bond strength tests.
When the composite resin was applied directly onto the GIC, we obtained low bond strength values and 100% of the failures that occurred were adhesive.When the bond of the composite resin to the conventional GIC was established using adhesive systems 50% of the fractures observed were cohesive of the GIC.According to Gupta, Mahajan 10 , the bond strength between composite resin and conventional GIC was reduced by the low cohesive strength of the GIC and the absence of chemical bonding, due to the different setting reactions occurring in these materials.Sneed, Looper 20 , considered that the bond strength between composite resin, adhesive system and GIC was greater than the cohesive force of the GIC.Moreover, when the etch-and-rinse adhesive system was used on the conventional GIC, phosphoric acid etching could dissolve surface charged particles 21,22 creating a zone of fragility that resulted in cohesive failure of the material, possibly resulting in lower bond strengths 17,22 .
According to Kerby, Knobloch 23 , resin-modified glass ionomer cements exhibit significantly higher cohesive strength when compared with conventional GICs.In the present study, resin-modified glass ionomer cement (Vitremer) showed the highest bond strength to composite resin.These results suggested that because this material exhibited a higher cohesive strength, the evaluation reflected higher real bond strength values at the composite resin/adhesive/GIC interface.According to Kundabala et al. 14 due to the presence of resinous monomers in their composition, resin-modified GICs showed a chemical bond to the composite resin, which favoured the increase in bond strength.
In this study, when the composite resin bond to the resinmodified GICs was established with the use of the conventional adhesive system, about 80% of the fractures were of the adhesive or mixed type.Whereas, when the self-etching adhesive systems were used, cohesive of GIC and mixed failures prevailed.
Although only CSE adhesive system showed significant difference in bond strength when compared with SB, there was considerable difference in behaviour with regard to the type of failure that occurred during the microshear test when the simplified etch-and-rinse and self-etching adhesive systems were considered.Therefore, further studies are needed to evaluate the longevity of composite resin/GIC bonding when using conventional and self-etching adhesive systems.

CONCLUSION
The authors verified that the composite resin/GIC bond strength improved significantly with the use of adhesive systems.Although there was no significant difference in bond strength when self-etching adhesive systems were compared with the simplified etch-and-rinse adhesive, Clear Fil SE bond determined the highest bond strength values.Therefore, self-etching adhesive systems are a good option for establishing the bond between the composite resin and the glass ionomer cement.