A comparative evaluation of shear bond strength of composite resin to pulp chamber dentin treated with sodium thiosulfate and proanthocyanidin: An in vitro study.

AIM: Based on the importance of coronal sealing right after endodontic treatment, this in vitro study aimed to evaluate the difference of shear bond strength to pulp chamber dentin treated with sodium thiosulfate and proanthocyanidin (PA).
MATERIALS AND METHODS: Fifteen extracted mandibular molar teeth were decoronated at the level of cementoenamel junction horizontally. The individual teeth were then sectioned mesiodistally to expose the pulp chamber dentin using a diamond disc. The specimens were randomly divided into three groups (n = 10). Group A: control group, treated with 5.25% sodium hypochlorite (NaOCl) for 30 min followed by a final rinse with 17% ethylenediaminetetraacetic acid (EDTA) solution for 3 min; Group B: after pretreatment with NaOCl and EDTA, it is further treated with 5% sodium thiosulfate (Na2S2O3) for 10 min; and Group C: after pretreatment with NaOCl and EDTA, it is further treated with 6.5% PA for 10 min. After drying the specimens, a bonding agent namely One Coat Bond SL (SwissTEC, Coltene) was applied to the pulp chamber dentin followed by a composite restoration (SwissTEC, Coltene). Each specimen was then tested under a universal testing machine at the dentin/resin interface to determine the shear bond strength.
RESULTS: Both the groups in which 5% Na2S2O3 and 6.5% PA were used for 10 min showed bond strength that was found to be statistically higher than the positive control (P < 0.05).
CONCLUSION: The use of Na2S2O3 and PA can significantly increase the bond strength of composite resin to NaOCl/EDTA-treated dentin, allowing adhesive restorations to be immediately applied after endodontic treatment.

INTRODUCTION

Successful endodontic treatment relies on effective cleaning and shaping of root canal as well as creation of an apical seal. However, regardless of the materials and obturation techniques employed, root canal fillings exposed to saliva may become contaminated[123] causing coronal leakage, which is a negative contributor to the prognosis of endodontic treatment.[45] Thus, even though apical leakage is still being considered as an important factor of endodontic failure, in the last few years, more focus has been given to procedures performed to achieve an effective coronal sealing soon after the completion of root canal filling.

The immediate sealing of endodontically treated teeth using restorative materials is very important in preventing early coronal leakage[678] and is associated with healthy apical tissues.[459] Composite resin-based materials have been suggested for the restoration of nonvital teeth[10] because of their benefits such as bonding to dentin by hybrid layer formation and reducing marginal leakage.[610] Moreover, teeth restored with resin composite have been shown to exhibit better fracture resistance than those restored with amalgam[101112] as the physico-mechanical properties of resin composites are closer to those of dentin.[13]

Nevertheless, the structure of dentin surface may change due to the chemical substances used during cleaning and shaping[14] and this in turn may affect the interaction of dentin with adhesive restorative materials.[1516]

Sodium hypochlorite (NaOCl) has antimicrobial activity and the ability to dissolve organic matter and is therefore widely used as a chemical irrigant for endodontic therapy.[17] The remnants and oxidation by-products of NaOCl can be one of the reasons due to which there is a negative effect on the polymerization of dental adhesive systems.[181920] This adverse effect of NaOCl on the resin–dentin bond strength has been investigated and previously confirmed.[1516181920] The application of an antioxidant solution before the adhesive procedure could restore the compromised bond strength to NaOCl-treated dentin by neutralizing and reversing the oxidizing effect of NaOCl treatment on the dentin surface.[20212223]

Grape seed extract (GSE) is a naturally occurring antioxidant. It contains oligomeric proanthocyanidin (PA) complexes (OPCs). PAs have free radicle scavenging activity, antibacterial, antiviral, anti-cariogenic, anti-inflammatory, anti-allergic properties, and vasodilatory action.[24]

Type I collagen in dentin forms a rigid, strong, space-filling biomaterial which is essential for many structural roles such as scaffolding for mineralization and viscoelasticity. PAs cause hydroxylation of proline which is an essential step of collagen biosynthesis, thereby stabilizing and increasing the cross-linking of Type I collagen fibrils. This intermolecular cross-linking is a prerequisite for viscoelasticity, stability, and tensile strength of collagen fibrils. This strengthening of collagen fibrils leads to increased mechanical properties and decreased enzymatic degradation and is a significant step in adhesive dentistry which can be achieved by cross-linking agents such as PAs.[25]

Another antioxidant/reducing agent which may reverse the NaOCl-compromised dentin bonding is sodium thiosulfate (Na2S2O3). Na2S2O3, being a potent antioxidant, has some potential to neutralize the oxidizing agents through redox reaction of the NaOCl-treated surface. This in turn facilitates complete polymerization of resin-bonding material.[1821] It can react with oxidants to form a stable product by neutralizing unpaired electrons.[26] Thus, Na2S2O3 can be used to recover the lost bond strength of NaOCl-treated dentin.

The purpose of this study was to determine the microtensile shear bond strength of pulp chamber dentin to composite resin material after pretreatment of exposed dentin with Na2S2O3 and PA with the null hypothesis that there is no significant difference existing between the tested groups.

Endodontic treatment and final restoration cannot be regarded as separate modalities because both are elements of a single concept called endodontic restorative continuum. Adjustment of the adhesive system to the irrigating solution may lead to more predictable treatment outcomes, but in this study, depending on the concentration and time application, the use of Na2S2O3 and PAs can recover the bond strength to dentin. The null hypothesis was that pretreatment of dentin with neither PA nor Na2S2O3 would be effective in reversing the bond strength to dentin compromised by NaOCl.

MATERIALS AND METHODS

Specimen preparation

Fifteen extracted mandibular molar teeth were sectioned in a mesiodistal orientation using a circular diamond disc to give a total of thirty sectioned samples. The samples were then randomly divided into three groups of ten samples each. Initially, all the sectioned samples were immersed in a solution of 3% NaOCl solution [Figure 1a] for 30 min, with the solution being renewed every 10 min. The samples were then irrigated and cleaned with saline. Any remaining pulpal tissue was removed with a spoon excavator. After treatment with NaOCl, all the samples were then immersed in a solution of 17% ethylenediaminetetraacetic acid (EDTA) for 3 min [Figure 1b]. The samples were then irrigated and cleaned with saline. After this pretreatment of all the samples, they were randomly divided into three groups. Group A was a positive control group comprising of ten samples which were only treated with NaOCl and EDTA. Group B (n = 10) consisted of samples which were treated with NaOCl and EDTA, and then immersed in a solution of 5% Na2S2O3 for 10 min [Figure 1d]. The third group, Group C (n = 10), consisted of samples which were treated with NaOCl and EDTA, and then immersed in a solution of 6.5% PA for 10 min [Figure 2c]. All the time periods were assessed with a timer.

Figure 1

(a) Pretreatment of samples with sodium hypochlorite. (b) Pretreatment with ethylenediaminetetraacetic acid. (c) Tooth samples treated with proanthocyanidin. (d) Tooth samples treated with sodium hypochlorite

Figure 2

Stereomicroscopic view of mode of failures. (a) Cohesive failure. (b) Adhesive failure. (c) Mixed failure

Bonding procedure

The sectioned samples were then blot dried using cotton pellets. Each sample was then etched with 37% phosphoric acid for 20 s. All the samples were coated with One Coat SL (Coltene, Switzerland) bonding agent using a micro applicator tip (Dentsply) and were cured for 20 s. After the application of the bonding agent, each sample received a packable composite resin (SwissTEC, Coltene) built upon the treated and exposed dentinal surface of the tooth. The composite resin material was placed incrementally, where each increment was of 1 mm thickness. Two such increments were placed with successive curing of each increment with a light-curing unit (Woodpecker (Artspa industrial company limited, Foshan, Guangdong)) for 20 s. After the bonding procedure, the samples were stored in distilled water until testing.

Microtensile shear bond testing

For testing, the samples were embedded in a cylindrical acrylic mold and were then tested for microtensile shear bond strength of the composite resin to the tooth surface which was pretreated with NaOCl, EDTA, Na2S2O3, and PA. Each sample was individually tested and was subjected to a tensile load in a universal testing machine at a crosshead speed of 1 mm/min until failure.

Analysis of mode of failure

The fractured, sectioned samples were then assessed under a stereomicroscope for determining the resultant mode of failure of the composite resin after testing for microtensile bond strength in the following manner:

Cohesive: Fracture of the composite resin material only [Figure 2a]

Adhesive: Fracture of the composite resin completely from the tooth–resin interface [Figure 2b]

Mixed: Fracture of the composite resin material itself and partial or complete de-attachment from the tooth–resin interface [Figure 2c].

RESULTS

The microtensile bond strength means and standard deviations are shown in Table 1. Statistical analysis revealed that Group A (control group) with NaOCl + EDTA showed a significantly lower bond strength to pulp chamber dentin (P < 0.05). Conversely, the experimental groups immersed in 5% Na2S2O3 (Group B) showed higher values than the group immersed in 6.5% PA (Group C), but the difference was not statistically significant.

Table 1

Microtensile bond strengths (MPa) and standard deviations

Stereomicroscopic analysis of the mode of failure revealed that cohesive failures were maximum with Group B followed by Group C. Adhesive mode of failure was least in Group B followed by Group C, showing statistically significant results [Table 2].

Table 2

Failure modes (%) according to the experimental groups

DISCUSSION

The methodology[15] was adjusted to evaluate the bond strength achieved by adhesive procedures performed on the buccal and lingual walls of the intracoronary dentin, which is the area fully affected by chemical substances during endodontic treatment.

This study is one of the few evaluating the resin bond strength to dentin pulp chamber after the use of typical endodontic substances, and it was performed to assess the applicability of Na2S2O3 and PAs to recover the dentin bond strength lost with the use of these products. The null hypothesis, which held that using Na2S2O3 and PAs would not be effective to recover the compromised bond strength after the use of NaOCl, was not validated.

NaOCl treatment reduces the mechanical properties of dentin, such as flexural strength, elastic modulus, and microhardness,[27] producing a “new” dentin surface that might be caused by damage to the organic matrix, mainly collagen (Type I) and leaving a porous and mineralized surface.[14] The use of EDTA also contributes to the erosion of this dentin.[14] In addition, NaOCl is an oxidizing substance which causes strong inhibition of the interfacial polymerization of adhesive materials.[28] Thus, there might be some reactive residual-free radicals in NaOCl-treated dentin that might compete with the propagating vinyl-free radicals generated during light activation of the adhesive system, resulting in premature chain termination and incomplete polymerization.[2021]

On the other hand, an antioxidant/reducing agent (e.g., PA and Na2S2O3) may reverse the NaOCl-comprised dentin bonding. Na2S2O3 is a potent inorganic antioxidant and is used for treatment in cases of cyanide poisoning. PAs are also called condensed tannins, are basically oligomers and polymers of monomeric flavonoids. These secondary plant metabolites also possess substantial antioxidant activity.[2930] Na2S2O3 and PA have potential to neutralize the oxidizing agents through redox reaction of the treated substrate, thus facilitating complete polymerization of resin-bonding materials.[1821] Furthermore, they can react with oxidants to neutralize unpaired electrons by relatively easy release of protons and form a stable product.[26] Na2S2O3 has been used in many microbiology studies to neutralize NaOCl,[3132] including clinical studies, showing biocompatibility. An initial cytotoxic assessment of Na2S2O3 in contact with fibroblastic cells has been performed by means of the methyl-thiazolyl-diphenyl tetrazolium bromide assay, the results of which show that concentrations of 0.5% and 5% are compatible with cell viability, suggesting a great potential for routine clinical use.

On the other hand, PAs are natural extracts and therefore lack toxicity. They are known to stabilize and increase the cross-linkage of Type-1 collagen fibrils. They have been reported to produce exogenous cross-links.[33] The interaction between PAs and collagen fibrils involves the formation of complexes that are primarily stabilized by hydrogen bonding between the carbonyl and hydroxyl functional groups of amide linkages and phenols, respectively.[34] They are theoretically known to counter the oxygen inhibition layer and thus ensure complete polymerization. GSE, which has been used in the current study, mainly comprises of OPCs. These OPCs are made up of dimers and trimers of catechins and epicatechins.[353637] Esterification of these two occurs in the presence of gallic acid. This process is mainly responsible for increasing their free radical scavenging ability. (Gallic acid is found only in GSE.)[24] Xie et al. reported that at a controlled pH of 7.4, GSE positively affected the demineralization of artificial root caries.[38]

Microtensile bond strength[39] allows bond testing of small areas, promoting a better stress distribution throughout the specimen, and induces failure of materials that are closer to their true ultimate strengths and are mostly adhesive failures.[40]

On comparison of shear bond strength of composite resin to pulp chamber dentin, it was seen that the mean shear bond strength of samples treated with 5% Na2S2O3 for 10 min was maximum as compared to 6.5% PA (P < 0.05, significant). Cho and Dickens reported that acetone in bonding agents results in poor wetting of dentin surface and incomplete penetration of hybrid layer. Furthermore, the residual acetone may lead to pores that can act as flaw-initiating sites during debonding.[41] Acetone used during the extraction of PAs can be a confounding factor, resulting in lower shear bond strength.

Postuniversal testing, the samples obtained were examined under a stereomicroscope for mode failures. On assessment, minimum adhesive failures of composite resin to pulp chamber dentin were seen in samples of Group B. These findings complied with the results in Table 1. Among all the three groups, maximum cohesive failures were again seen with Group B, but the findings were not statistically significant. An equal number of samples presented with mixed failures in Group B and Group C. Hence, the null hypothesis was rejected as there is significant difference existing between the tested and control groups.

CONCLUSION

Endodontic treatment and final restoration cannot be regarded as separate modalities because both are elements of a single concept called endodontic restorative continuum. Adjustment of the adhesive system to the irrigating solution may lead to more predictable treatment outcomes,[42] but in this study, depending on the concentration and time application, the use of Na2S2O3 and PAs can recover the bond strength to dentin.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.