Effect of Ethylene diamine tetra acetic acid and sodium hypochlorite solution conditioning on microtensile bond strength of one-step self-etch adhesives.

BACKGROUND: Attempts to improve bond strength of self-etch adhesives can enhance the durability of composite restorations. AIMS: The aim of the present study was to evaluate the effect of collagen and smear layer removal with sodium hypochlorite solution (NaOCl) and EDTA on micro-tensile bond strength (μTBS) of self-etch adhesives to dentin. SETTINGS AND
DESIGN: It was an in-vitro study.
MATERIALS AND METHODS: Seventy-two teeth were divided into eight groups and their crowns were ground perpendicular to their long axis to expose dentin. The teeth were polished with silicon-carbide papers. The groups were treated as follows: No conditioning, 0.5-M EDTA conditioning, 2.5% NaOCl conditioning, NaOCl + EDTA conditioning. The surfaces were rinsed and blot-dried. Clearfil S3 and I-Bond were applied according to manufacturers' instructions and restored with Z100 composite. After 500 cycles of thermo-cycling between 5°C and 55°C, the samples were sectioned and tested for μTBS. STATISTICAL ANALYSIS: Data were analyzed by two-way ANOVA and Tukey-HSD test.
RESULTS: The highest μTBS was recorded with Clearfil S3 + NaOCl + EDTA, and the lowest was recorded with I-Bond without conditioning. μTBS in EDTA-and EDTA + NaOCl-treated groups was significantly higher than the control and NaOCl-conditioned groups.
CONCLUSIONS: Application of EDTA or EDTA + NaOCl before one-step self-etch adhesives increased μTBS.

INTRODUCTION

Inadequate adhesion of composite resin restorations to dentin results in reduced retention, micro-leakage and finally recurrent caries.[1] Progressive loss of bond strength of etch-and-rinse adhesives has been demonstrated in some studies.[23] One of the factors that accounts for this degradation is incomplete infiltration of resin monomers into denuded collagen networks after acid etching with strong acids such as phosphoric acid, which produces a zone of collagen without any support of either minerals or resins in the base of the hybrid layer.[2] This discrepancy is less likely to occur in self-etch systems as they demineralize and infiltrate to the same extent,[4] a phenomenon which prevents collagen network collapse and creation of a resin-poor zone under the hybrid layer.[23]

Some studies have reported higher bond strength values for self-etch adhesives after removal of the smear layer and remnants of organic materials from prepared dentin,[25] although some other studies do not confirm this.[67]

When dentin is cut, it is recommended to remove the smear layer in order to attain an acceptable bond to the underlying dentin.[8] If surface contaminants are removed from dentin, a substrate rich in minerals will remain, which can establish a suitable surface for bonding procedures.[3]

EDTA is a mild chelator at normal pH,[9] with various effects on dentin, depending on application time and concentration.[10] EDTA is known to remove the smear layer and slightly demineralize dentin surface.[11] Some previous studies have reported that higher bond strength would be obtained by removing the smear layer with EDTA without additional etching of dentin surface.[21213]

Another factor which might affect the bond quality is the formation of an amorphous and partially impermeable gel on the surface of denuded collagen network, which is created as a result of pH variations, heat produced during preparation, and collagen denaturation, and is called “collagen smear layer”.[14] In etch- and-rinse systems, collagen smear layer can prevent complete infiltration of resin tags into the demineralized dentin.[15] Since this layer is not removed with the use of self-etch adhesives, but is incorporated into the hybridized complex, it probably causes a reduction in the amount of resins in the adhesive layer and consequently affects bond strength. Short-term application of weak sodium hypochlorite solution (NaOCl) is proposed for removal of this organic gel.[16]

The aim of the present study was to evaluate the effect of EDTA and NaOCl treatment on μTBS of one-step self-etch adhesives to dentin.

MATERIALS AND METHODS

This in-vitro study was performed on seventy-two sound human premolars, which had been extracted for orthodontic reasons. The teeth were randomly divided into 8 groups (n = 9), were cleaned of debris and calculus and kept in distilled water at room temperature. The occlusal surfaces of the teeth were ground on a trimmer (DENTAURUM, Ptorzheim, Germany) perpendicular to the long axis of the teeth to expose the superficial dentin with a surface area of about 5 mm2 without pulp exposure. The samples were polished with 400- and 600-grit silicon carbide papers under running water to create a flat surface and a homogenous smear layer. The teeth were distributed into the following groups: In the first group, two layers of the one-step self-etch adhesive, I-Bond (Heraeus Kulzer, Hanau, Germany) were applied according to manufacturer's instructions and light-cured by a halogen light-curing unit with 700 mW/cm² irradiance (Degulux II; Degussa AG, Gschttsbereich Dental, D-63457 hanau-Wolfgang, Germany) for 20 s. The bonded surfaces were restored with Z100 composite resin (3M ESPE, St Paul, MN, USA) using the incremental technique to a thickness of 5 mm and each layer was cured separately for 40 s.

In the second group, the procedure was similar to the previous group, except that the teeth were treated with 2.5% NaOCl (Golrang Co, Tehran, Iran) before application of the adhesive. NaOCl was rubbed on the prepared surface of the teeth by a brush for 15 s, and was not disturbed for 15 s, then rinsed with water for 15 s and blot-dried.

In the third group, the steps were the same as those in the second group, but 0.5-M EDTA disodium salt (Sigma-Aldrich Corp., St Louis, MO, USA) with a pH of 7.2 was applied for 30 s instead of NaOCl, followed by rinsing with water.

In the fourth group, NaOCl and EDTA were applied sequentially, and the application method was the same as that in the previous groups.

The samples in the fifth group were treated with the one-step self-etch adhesive, Clearfil S3 Bond (Kuraray Co, Okayama, Japan) according to manufacturer's instructions. In groups 6-8 the procedures were the same as those in groups 2-4, but the adhesive used was Clearfil S3 Bond. All the bonding procedures were carried out by a single operator.

After storage in distilled water (37°C/24 h), and thermo cycling for 500 cycles at 5-55°C with a dwell time of 30 s, the samples were sectioned parallel to the long axis of the teeth creating 1 mm thick slabs. Each slab was cut in beams with a bonded area of 1 ± 0.1 mm2. The cross-sectional area of each specimen was measured with a digital caliper (Absolute Digimatic, Mitutoyo, Tokyo, TYO, Japan). The samples were tested for μTBS in a microtensile tester (Bisco, Schaumburg, USA) at a crosshead speed of 0.5 mm/min. Data were analyzed using two-way ANOVA and post hoc Tukey HSD tests. Significance was set at 0.05.

RESULTS

As shown in Table 1, the highest μTBS values were recorded in the group treated with NaOCl + EDTA + Clearfil S3 Bond, and the lowest values were recorded in the I-Bond group. Two-way ANOVA indicated that the main effect of the adhesive system and surface conditioning were significant (P = 0.001, P = 0.0001, respectively). The interactions of these two factors were also significant (P = 0.001), which implies that depending on the type of the self-etch system used, conditioning the dentin surface prior to application of the adhesive system can result in different amounts of micro tensile bond strength.

Table 1

Mean and SD of μTBS of groups of the study

Pairwise comparisons of μTBS values by Tukey HSD test revealed that the effect of EDTA and NaOCl + EDTA conditioning is significant in comparison with the control group (P = 0.005, P = 0.005), while the effect of NaOCl conditioning was not statistically significant (P = 0.85) [Table 2].

Table 2

Comparison of the effect of conditioners on micro-tensile bond strength of self-etch adhesives

DISCUSSION

Adhesion to tooth structure depends on various factors, including the type of the adhesive system, tooth structure, load cycling, and surface contaminations.[1718] We used EDTA to remove the smear layer[11] and NaOCl to remove remaining organic material[16] before one-step self-etch adhesives in this study.

The results showed that EDTA at a concentration of 0.5 M and a pH of 7.2 increased dentin bond strength of one-step self-etch adhesives (P = 0.005).

Previous studies on the effect of EDTA on dentin bond strength of adhesive systems have reported different results. Jacques et al. concluded that application of 0.5 M EDTA for 30 s increases bond strength of Clearfil SE Bond.[2] Torii et al. reported that EDTA conditioning is effective in improving dentin bonding of all-in-one adhesives.[19] The results of the present study concur with these findings. In a study in which surface conditioning was carried out with 0.5 M EDTA for 2 min, no significant changes were observed in the dentin bond strength of self-etch systems.[6] Severe reduction in μTBS was reported after tooth conditioning with 0.5 M EDTA for 5 days.[7]

EDTA contains carboxylic acid groups and can chelate calcium.[20] It has been used to dissolve the mineral phase of dentin without altering dentin proteins and causing major alterations of the fibrillar structure of dentin collagen network.[711] These unaltered collagen fibrils preserve most of their intra-fibrillar mineral, so that resin infiltration is probably facilitated.[9]

The effects of EDTA depend on its pH, application time and concentration,[1013] which can be a reason for different results in different studies. For example, treating dentin with 1.5% or 5% concentrations of EDTA is not effective in dissolving the smear layer, although its 15% concentration dissolves the smear layer and exposes a small amount of collagen fibrils.[11] In addition, according to a study carried out by Bogra et al. when the conditioning time is prolonged, there is an appreciable increase in the widening of the tubule orifices and depth of demineralization within the tubules.[21]

Removing the smear layer by EDTA conditioning and permitting the direct contact of functional monomers in one-step self-etching adhesives with dentin may provide a more intimate chemical interaction on a molecular level and help increase bond strength.[6]

Aqueous solution of NaOCl is a deproteinizing agent that forms superoxide radicals and induces oxidation that fragments peptide chains of proteins.[22] NaOCl also results in the formation of amino-acid-derived chloramines and hypochlorous acid that have been shown to increase the proteolytic susceptibility of these collagen proteins.[23]

Deproteinizing dentin surface with NaOCl results in an increase in surface free energy and its wettability by hydrophilic resins.[24]

The results of the present study are indicative of higher μTBS in I-Bond group and lower μTBS in Clearfil S3 Bond group after treatment with NaOCl, although the changes were not statistically significant (P > 0.05).

I-Bond contains glutaraldehyde in its chemical composition. Aldehyde groups react with amine, amino, and amido groups in the collagen molecules to create N-hydroxyl alkyl compound, which reacts with the hydroxyl group in Hydroxy ethyl methacrylate (HEMA) and bonds to HEMA by releasing a water molecule. The other end of HEMA is capable of co-polymerizing and creating the bond.[25] Since the amount of collagen-derived organic materials decreases after treatment with NaOCl, we expected a reduction in the bond strength in the I-Bond group; whereas the bond strength showed a non-significant increase, which can be attributed to other factors, such as the reduction in collagen smear and also preventing the creation of a layer with a low amount of resin and high amount of organic materials above the hybrid layer, which plays a role in reduction of the bond strength of composite resin to dentin.

Clearfil S3 Bond contains monomers with phosphate groups, whereas the monomers in I-Bond contain carboxylic acid groups. Since phosphate and phosphonate groups are more sensitive to oxidizing agents, such as NaOCl, than carboxylic acid groups,[16] it can be expected that NaOCl has a more deleterious effect on μTBS of Clearfil S3 Bond. Furthermore, the solvent in I-Bond is acetone, while in Clearfil S3 Bond, the solvent is water-ethanol. It is reported that acetone-based adhesive systems establish a better bond after treatment with NaOCl than ethanol-water systems.[5]

Numerous studies have evaluated the effects of NaOCl on adhesion process and different results have been achieved.[5726] Increase in bond strength of self-etch systems is reported after application of NaOCl.[26] Some studies, however, have indicated lower bond strength by using NaOCl.[7] This decrease in bond strength can be attributed to generation of oxygen after disintegration of NaOCl into NaCl and O2. The released oxygen in this chemical reaction is a strong factor in preventing the polymerization of adhesive agents. In other words, there might be some reactive residual free radicals in NaOCl-treated dentin, which might compete with the propagation of vinyl-free radicals generated during light activation of the adhesive system, resulting in premature chain termination and incomplete polymerization.[27]

NaOCl may exert different effects on bond strength, depending on the chemical structure of the adhesive system and the type of the initiator in the adhesive system used.

One of the advantages of the present study was evaluation of groups with the sequential application of NaOCl and EDTA. Statistical analysis revealed higher μTBS of self-etch adhesives after sequential application of NaOCl and EDTA (P < 0.05). According to the results of the present study, the P value in both EDTA and NaOCl + EDTA groups was similar, while the effect of NaOCl on μTBS was not significant; therefore, it seems that the increase in μTBS is related to EDTA and its effects on adhesion process.

Within the limitations of the present study, it was concluded that:

Smear layer removal with 0.5% EDTA before application of one-step self-etch adhesives produced higher μTBS, which was closely related to the adhesive system used.

Collagen removal from dentin surface with NaOCl did not result in significant changes in μTBS of one-step self-etch adhesives to dentin.

Sequential application of NaOCl and EDTA increased μTBS of one-step self-etch adhesives to dentin.

Although both EDTA and NaOCl + EDTA resulted in higher bond strength of self-etch adhesives to dentin, the use of EDTA before the application of the adhesive system is recommended in order to decrease the number of steps in application procedure and reduce chair time during clinical procedures.