Effect of reducing agents on bond strength to NaOCl-treated dentin

Abstract

Objective

The purpose of this study was to evaluate the effect of three antioxidant/reducing agents with different application times on microtensile bond strengths to sodium hypochlorite-treated dentin.

Methods

The occlusal surfaces of 24 extracted human third molars were horizontally cut to expose sound dentin. The teeth were divided into eight groups. The dentin surfaces of the teeth were treated as follows: group 1, no treatment; group 2, treated with 6% sodium hypochlorite (NaOCl) for 30 s; groups 3–8, applications of 10% sodium ascorbate solution, 100 μM rosmarinic acid solution or Accel for 5 or 10 s after the same treatment as in group 2. All treated dentin surfaces were bonded with a 2-step self-etching adhesive system (Clearfil Protect Bond) and restored with a resin composite (Clearfil AP-X). After storage in water for 24 h, the bonded specimens were subjected to the microtensile bond test at a crosshead speed of 1.0 mm/min. Data were analyzed by a one-way ANOVA and Tukey test ( p < 0.05).

Results

The NaOCl-treated group had significantly lower bond strength than the control group ( p < 0.05). The application of sodium ascorbate solution for 5 or 10 s did not significantly increase the compromised bonding to NaOCl-treated dentin ( p > 0.05). On the other hand, Accel and rosmarinic acid solution had significant reversal effects with the same application times ( p < 0.05).

Significance

The reversal effect on compromised bonding to NaOCl-treated dentin depended upon the type of antioxidant within the short application time. Applying Accel or rosmarinic acid for 5 or 10 s improved bond strengths to NaOCl-treated dentin.

Introduction

Sodium hypochlorite (NaOCl) is widely used as a chemical irrigant for endodontic therapy due to its antibacterial and organic tissue dissolution properties . Many researchers have demonstrated that NaOCl reduces the bond strength between resin composites and dentin . This is thought to be due to remnants and by-products of NaOCl exhibiting a negative effect on the polymerization of dental adhesive systems . On the other hand, these compromised bond strengths to NaOCl-treated dentin could be restored by more than 60 s application of 10% sodium ascorbate (antioxidant) solution before the adhesive procedure , because it can interact with the by-products of NaOCl , resulting in neutralization and reversal of the oxidizing effect of the NaOCl-treated dentin surface .

Recently, a product Accel (Sun Medical Co. Ltd., Kyoto, Japan), that contains p -toluenesulfinic acid sodium salt , has been introduced as a pretreatment agent for adhesive root canal sealers to reduce the oxidative effect of NaOCl irrigation. The p -toluenesulfinic acid sodium salt has also been used as accelerator for the polymerization of resin composite for a long time . It has been reported that an application of Accel for 30 s could improve the bond strength to NaOCl-treated normal dentin and caries-affected dentin . A shorter time application would increase convenience and comfort to dentists and patients.

On the other hand, some antioxidants extracted from plants are known to be matrix metalloproteinases (MMPs) inhibitors . MMPs are a group of 23 human enzymes capable of degrading all extracellular matrix components. Human dentin contains collagenase (MMP-8), gelatinases (MMP-2, -9), and enamelysin (MMP-20) , which could cause degeneration of exposed dentin collagen fibrils within the hybrid layer, leading to the loss of hybrid layer integrity and the reduction of resin–dentin bond stability . Recent studies have demonstrated that the application of a MMPs-inhibitor such as chlorhexidine could prevent auto-degradation of hybrid layer and improve bonding durability of resin–dentin interface . Rosemary extract (rosmarinic acid) has a high antioxidant ability and MMPs-inhibitor ability as well . It is possible that rosmarinic acid could improve compromised bond strengths to NaOCl-treated dentin in a shorter application time and might also contribute to the long-term stability of the resin–dentin interface due to its antioxidant and MMPs-inhibitor ability. However, there is little published research on the effect of rosmarinic acid on bond strengths to NaOCl-treated dentin.

Therefore, the purpose of this study was to evaluate the effect of sodium ascorbate, rosmarinic acid solutions and Accel (Sun Medical Co. Ltd., Kyoto, Japan) with a short application time on the microtensile bond strength of a 2-step self-etching adhesive system (Clearfil Protect Bond, Kuraray Medical Inc., Tokyo, Japan) to NaOCl-treated dentin. The null hypotheses tested were that neither the application time nor the type of antioxidant/reducing agents has any affect on reversing compromised bonding to NaOCl-treated dentin.

Materials and methods

Preparation of materials

Twenty-four extracted human non-carious third molars, stored frozen, were used in this study. The occlusal enamel was ground perpendicular to the long axis of the tooth to expose flat surfaces of sound dentin under water lubrication. The occlusal dentin surfaces were then polished using 600-grit silicon carbide paper under running water. The teeth were divided into eight groups of three teeth each. The dental materials included in the study are listed in Table 1 .

Table 1
Materials used.
Material Manufacturer Batch number Composition
Accel Sun Medical Co. Ltd., Kyoto, Japan MM2F p -toluenesulfinic acid sodium salt, ethanol, water
Clearfil Protect Bond Kuraray Medical Inc., Tokyo, Japan 00056A Primer: 10-MDP, MDPB, HEMA, hydrophilic aliphatic dimethacrylate, N,N-diethanol- p -toluidine, CQ, water
Bond: Bis-GMA, HEMA, hydrophobic dimethacrylate, 10-MDP, MDPB, toluidine, silanated silica, CQ, sodium fluoride
Clearfil AP-X Kuraray Medical Inc., Tokyo, Japan 1016AB Bis-GMA, TEGDMA, silanated barium glass filler, silanated silica filler, silanated colloidal silica, CQ, initiators, accelerators, pigments
Abbreviations : 10-MDP: 10-methacryloyloxydecyl dihydrogen phosphate; MDPB: 12-methacryloyloxydodecylpyridinium bromide; HEMA: 2-hydroxyethyl methacrylate; Bis-GMA: 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]propane; TEGDMA: triethyleneglycol dimethacrylate; CQ: camphorquinone.

Group 1 : dentin surfaces were irrigated with distilled water for 30 s (control group).

Group 2 : dentin surfaces were irrigated with 6% NaOCl (Jiaen 6%, Yoshida Co., Tokyo, Japan) for 30 s, then rinsed with water for 10 s.

Groups 3 and 4 : after treatment as in group 2, freshly made 10% sodium ascorbate (Wako Pure Chemical Industries, Ltd., Osaka, Japan) was applied to the dentin surfaces for 5 or 10 s.

Groups 5 and 6 : after treatment as in group 2, Accel (Sun Medical Co. Ltd., Kyoto, Japan) was applied to the dentin surface for 5 or 10 s.

Groups 7 and 8 : after treatment as in group 2, 100 μM rosmarinic acid (Cayman Chemical Co., Ann Arbor, MI, USA) in 5% ethanol was applied to the dentin surface for 5 or 10 s.

After pretreatment, all of the specimen surfaces were dried with air, and then a 2-step self-etching adhesive system (Clearfil Protect Bond, Kuraray Medical Inc., Tokyo, Japan) was applied according to the manufacturer’s instructions. Resin composites build-ups (Clearfil AP-X, Kuraray Medical Inc., Tokyo, Japan) were constructed in three 1.5 mm increments. Each increment was light-activated for 20 s with a light curing unit (Optilux 501, Kerr Corp., Orange, CA, USA).

Microtensile bond strengths (μTBS)

After water storage at 37 °C for 24 h, the bonded teeth were vertically sectioned into four or five 0.7 mm thick slabs using a low-speed diamond saw (IsoMet Low Speed Saw, Buehler Ltd., Lake Bluff, IL, USA) under water lubrication. Each slab was hand-trimmed by using a cylindrical shaped fine diamond bur (Intensive SA, Swiss Dental, Zurich, Switzerland) with a high-speed handpiece under water spray, to a hourglass shape form with the narrowest portion at resin/dentin bonded interface to produce a bonded area of approximately 0.98 mm 2 . The final thickness and width of the bonded interface was measured using a digital micrometer (Digimatic Solar, Mitutoya Corp., Tokyo, Japan). Four or five specimens were prepared from each tooth. The specimens were attached to a universal testing machine (EZ Test, Shimadzu Crop., Kyoto, Japan) with a cyanoacrylate adhesive (Zapit, Dental Ventures of America Inc., Corona, CA, USA) and subjected to the μTBS test at a cross-head speed of 1 mm/min ( Fig. 1 ).

Fig. 1
Schematic illustration of sample preparation for microtensile bond strength testing.

The μTBS data were analyzed for statistically significant differences by a one-way analysis of variance and Tukey test as post hoc multiple comparisons.

Failure mode analysis

After the μTBS test, the dentin sides of fractured specimens in each group were observed using a stereomicroscope (Nikon SMZ1000, Nikon Corp., Kanagawa, Japan) at 120× magnification for failure mode determination. Failure modes were classified according to one of four types:

Type 1 : mixed failure (mixed with adhesive failure between resin and dentin, and cohesive failure in bonding agent and/or dentin).

Type 2 : adhesive failure (80–100% of the failure occurred between resin and dentin).

Type 3 : cohesive failure in dentin (80–100% of the failure occurred in the underlying dentin).

Type 4 : cohesive failure in resin (80–100% of the failure occurred in the adhesive resin and/or overlying composite).

Failure modes were analyzed for statistically significant differences by the nonparametric Pearson Chi-Square test. All statistical analyzes were performed at a confidence level of 95% using SPSS software version 16 (SPSS Inc., Chicago, IL, USA).

Materials and methods

Preparation of materials

Twenty-four extracted human non-carious third molars, stored frozen, were used in this study. The occlusal enamel was ground perpendicular to the long axis of the tooth to expose flat surfaces of sound dentin under water lubrication. The occlusal dentin surfaces were then polished using 600-grit silicon carbide paper under running water. The teeth were divided into eight groups of three teeth each. The dental materials included in the study are listed in Table 1 .

Table 1
Materials used.
Material Manufacturer Batch number Composition
Accel Sun Medical Co. Ltd., Kyoto, Japan MM2F p -toluenesulfinic acid sodium salt, ethanol, water
Clearfil Protect Bond Kuraray Medical Inc., Tokyo, Japan 00056A Primer: 10-MDP, MDPB, HEMA, hydrophilic aliphatic dimethacrylate, N,N-diethanol- p -toluidine, CQ, water
Bond: Bis-GMA, HEMA, hydrophobic dimethacrylate, 10-MDP, MDPB, toluidine, silanated silica, CQ, sodium fluoride
Clearfil AP-X Kuraray Medical Inc., Tokyo, Japan 1016AB Bis-GMA, TEGDMA, silanated barium glass filler, silanated silica filler, silanated colloidal silica, CQ, initiators, accelerators, pigments
Abbreviations : 10-MDP: 10-methacryloyloxydecyl dihydrogen phosphate; MDPB: 12-methacryloyloxydodecylpyridinium bromide; HEMA: 2-hydroxyethyl methacrylate; Bis-GMA: 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]propane; TEGDMA: triethyleneglycol dimethacrylate; CQ: camphorquinone.
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Effect of reducing agents on bond strength to NaOCl-treated dentin
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