Resin–dentin bonds to EDTA-treated vs. acid-etched dentin using ethanol wet-bonding. Part II: Effects of mechanical cycling load on microtensile bond strengths

Abstract

Objective

To compare microtensile bond strengths (MTBS) subsequent to load cycling of resin bonded acid-etched or EDTA-treated dentin using a modified ethanol wet-bonding technique.

Methods

Flat dentin surfaces were obtained from extracted human molars and conditioned using 37% H 3 PO 4 (PA) (15 s) or 0.1 M EDTA (60 s). Five experimental adhesives and one commercial bonding agent were applied to the dentin and light-cured. Solvated experimental resins (50% ethanol/50% comonomers) were used as primers and their respective neat resins were used as the adhesives. The resin-bonded teeth were stored in distilled water (24 h) or submitted to 5000 loading cycles of 90 N. The bonded teeth were then sectioned in beams for MTBS. Modes of failure were examined by scanning electron microscopy.

Results

The most hydrophobic resin 1 gave the lowest bond strength values to both acid and EDTA-treated dentin. The hydrophobic resin 2 applied to EDTA-treated dentin showed lower bond strengths after cycling load but this did not occur when it was bonded to PA-etched dentin. Resins 3 and 4, which contained hydrophilic monomers, gave higher bond strengths to both EDTA-treated or acid-etched dentin and showed no significant difference after load cycling. The most hydrophilic resin 5 showed no significant difference in bond strengths after cycling loading when bonded to EDTA or phosphoric acid treated dentin but exhibited low bond strengths.

Significance

The presence of different functional monomers influences the MTBS of the adhesive systems when submitted to cyclic loads. Adhesives containing hydrophilic comonomers are not affected by cycling load challenge especially when applied on EDTA-treated dentin followed by ethanol wet bonding.

Introduction

Resin bonding agents with high hydrophilicity are characterized by substantial water sorption and plasticization . Adhesive bond strengths to water-saturated, acid-etched dentin matrices are directly related to the hydrophilicity of the resin bonding agents . Dentin can be bonded with hydrophobic resins to decrease water sorption and increase the longevity of the resin–dentin interfaces using ethanol wet-bonding . Indeed, it is possible to coax hydrophobic monomers into the hydrophilic dentin collagen matrix by using absolute ethanol without any sign of phase change and/or micropermeability at the resin–dentin interface . This technique leads to better infiltration of hydrophilic and/or hydrophobic dimethacrylates into ethanol–dentin matrices also when applied on 37% phosphoric acid-demineralized or EDTA-treated dentin for only 1 min .

Moreover, the use of EDTA-conditioning, combined with ethanol wet-bonding seems allow better penetration of adhesive monomers into a thinner layer of demineralized dentin and to offer high and more durable microtensile bond strength (μTBS) .

However, restored teeth are constantly subjected to cyclic stresses during physiologic chewing and swallowing. This occlusal stress may cause mechanical degradation and accelerate chemical degradation within resin–dentin interfaces .

No information is available on the effect of mechanical load cycling on the resin–dentin bonds to EDTA-treated vs. acid-etched dentin using ethanol wet-bonding. Thus, the purpose of this study was to compare resin–dentin bond strengths before and after mechanical load cycling of resin bonding agents applied to acid-etched or EDTA-treated dentin created with a simplified (1 min) ethanol wet-bonding technique. The test null hypotheses were that there is no difference in bond strength following cyclic loading of EDTA vs. phosphoric acid pre-treatment using hydrophobic vs. hydrophilic resins.

Materials and methods

Specimen preparation

The materials and methods sessions of this paper are similar to those of Sauro and collaborators as it reports the second part of the experiments performed on the resin–dentin bonds to EDTA-treated vs. acid-etched dentin.

In brief, human molars extracted for surgical reasons were stored at 4 °C in 0.5% chloramine T for up to 1 month before use. The specimens were sectioned below the dentin–enamel junction using a water-cooled diamond saw (330-CA RS-70300, Struers, Copenhagen, Denmark). The occlusal surfaces were ground flat (LaboPol-4, Struers, Copenhagem, Denmark) using 500 grit SiC abrasive paper under constant water irrigation to provide standardized smear layer-covered dentin surfaces. Two principal groups were created according to the dentin conditioning treatments (i.e. EDTA or H 3 PO 4 ). The teeth were subsequently divided into seven subgroups according to the resin adhesives used in this study. A further sub-division of each subgroup was performed according to whether or not the teeth were cyclically loaded. All bonded teeth were stored in water for 24 h prior to cyclic loading or water storage for the same time.

Experimental resins

The five experimental comonomer blends used in this study as dentin bonding agents (DBAs) were formulated based on known concentrations of all ingredients, including 50 wt.% ethanol-solvated resin mixtures used as primers ( Table 1 ) . All experimental neat resins contained 0.25 wt.% camphoquinone and 1.0 wt.% ethyl-dimethyl-4-aminobenzoate. Resins 1 and 2 were hydrophobic resins similar to those used in pit-and-fissure sealants. Resin 3 represented the formulation of typical two-step, etch-and-rinse adhesives, while resins 4 and 5 had a hydrophilicity similar to one-step, self-etching adhesives, containing carboxylic- or phosphate-substituted methacrylates, respectively .

Table 1
Composition of neat and solvated resins used in this study.
Neat resin (pH ∼ 7) Solvated primer (pH ∼ 7)
Resin 1 70 wt.% E-BisADM 34.4 wt.% E-BisADM
28.75% TEGDMA 14.35% TEGDMA
50 wt.% ethanol
Neat resin (pH ∼ 7) Solvated primer (pH ∼ 7)
Resin 2 70 wt.% BisGMA 34.4 wt.% BisGMA
28.75% TEGDMA 14.35% TEGDMA
50 wt.% ethanol
Neat resin (pH ∼ 7) Solvated primer (pH ∼ 7)
Resin 3 70 wt.% BisGMA 34.4 wt.% BisGMA
28.75% HEMA 14.35 wt.% HEMA
50 wt.% ethanol
Neat resin (pH ∼ 4) Solvated primer (pH ∼ 4)
Resin 4 40 wt.% BisGMA 20 wt.% BisGMA
30 wt.% TCDM 14.4 wt.% TCDM
28.75% HEMA 14.35% HEMA
50 wt.% ethanol
Neat resin (pH ∼ 3) Solvated primer (pH ∼ 3)
Resin 5 40 wt.% BisGMA 20 wt.% BisGMA
30 wt.% BisMP 14.4 wt.% BisMP
28.75% HEMA 14.35% HEMA
50 wt.% ethanol
Neat adhesive (pH 8.2) Solvated adhesive (pH ∼ 8.2)
a Scotchbond Multi-Purpose (Adhesive-bottle) (3M ESPE, St. Paul, MN, USA) BisGMA BisGMA
HEMA HEMA
Polyalkenoic acid polymer Polyalkenoic acid polymer
Tertiary amines Tertiary amines
Photo-initiator Photo-initiator
50 wt.% ethanol
Adhesive (pH 8.2) Primer (pH 3.3)
a Scotchbond Multi-Purpose (Primer and Adhesive) (3M ESPE, St. Paul, MN, USA) BisGMA HEMA
Polyalkenoic acid polymer Polyalkenoic acid polymer
HEMA Water
Tertiary amines
Photo-initiator
Abbreviations . E-BisADM = ethoxylated Bisphenol A dimethacrylate; BisGMA = 2,2-bis[4-(2-hydroxy-3-methacryloylpropoxy)]-phenyl propane; TEGDMA = triethyleneglycol dimethacrylate; HEMA = 2-hydroxyethylmethacrylate; TCDM = di(hydroxyethyl-methacrylate) ester of 5-(2,5-dioxotetrahydrofurfuryl)-methyl-3-cyclohexane-1,2′-dicarboxylic acid; BisMP = Bis[2-(methacryloyloxy)ethyl]phosphate.

a Percentage of the constituents of the primer and adhesive of the commercial Adper Scotchbond Multi-Purpose are not given by the 3M ESPE, St. Paul, MN, USA.

Bonding procedures

Dentin surfaces were acid-etched for 15 s with 37% phosphoric acid (PA) or treated with 0.5 M EDTA (pH 7.8) for 60 s and copiously rinsed with deionized water. The dentin surface was covered with absolute ethyl alcohol for 1 min and kept visibly moist with ethanol prior to the application of the resin blends .

Two consecutive coats of the five experimental primers (50% ethanol/50% resin) were applied to the conditioned dentin. Gentle air-drying was performed for 3 s to evaporate the excess solvent from the primed dentin. A layer of each respective neat comonomer adhesive was spread thin with a microbrush and light-cured for 15 s (Translux EC halogen light-curing unit, Kulzer GmBh, Bereich Dental, Werheim, Germany). A light output intensity of 600 mW/cm 2 intensity was employed for the experiments (Demetron Radiometer Model 100, Demetron Research, Danbury, CT, USA). A commercial adhesive, Scotchbond Multi-Purpose (SBMP) (3M ESPE, St. Paul, MN, USA) was also applied with the ethanol wet-bonding either as per manufactures’ instructions (i.e. application of the primer and adhesive layers) or the adhesive was diluted in 50% ethanol and applied as a primer in two consecutive coats, followed by one layer of the neat Scotchbond MP adhesive. Five mm high composite build-ups were constructed for each specimen with a light-cured flowable resin composite, Tetric EvoFlow ® (Ivoclar, Vivadent, Schaan, Liechtenstein – batch number: L26398) in 1-mm-thick increments. The resin-bonded specimens were stored in de-ionized water for 24 h at 37 °C.

Mechanical cycling load

The resin-bonded teeth of each subgroup that were created for the mechanical cycling load test were mounted in plastic rings with dental stone for placement in the load cycling machine using load control (5000 cycles, 12 Hz, 90 N). This compressive load was applied to the flat resin composite build-ups using a 5-mm diameter spherical stainless steel plunger, attached to a cyclic loading machine (model S-MMT-250NB; Shimadzu, Tokyo, Japan) while immersed in deionized water .

Microtensile bond strength (μTBS) test

The resin–dentin specimens were sectioned with a diamond wafering blade (Accutom-50, Struers, Copenhagem, Denmark) using a hard tissue saw (330-CA RS-70300, Struers, Copenhagem, Denmark) in both x and y directions across the adhesive interface to obtain beams with cross-sectional areas of 1 mm 2 .

Each beam was attached to a modified Bencor Multi-testing apparatus (Danville Engineering Co., Danville, CA, USA) with cyanoacrylate adhesive (Zapit, Dental Ventures of America Inc., Corona, CA, USA) and stressed to failure in tension using a microtensile bond strength testing machine (Instron 4411, Instron Corporation, Canton, MA, USA) at a crosshead speed of 0.5 mm/min. Bond strength data were calculated in MPa. Premature failures were included in the statistical analysis as zero values . Two-way ANOVA including interactions and Student–Newman–Keuls multiple comparisons were used for the statistical analysis. Adhesive systems, dentin surface treatment and mechanical stress were considered as independent variables and μTBS as the dependent variable. Statistical significance level was set in advance at α = 0.05.

SEM observations of the failed bonds

Modes of failure were classified as adhesive (A), cohesive (C) or mixed (M) when the failed bonds were examined at 30× by stereoscopic light microscopy and by SEM. Ten representative fractured specimens from each group were critical-point dried and then mounted on aluminum stubs with carbon cement. They were sputter-coated with gold (SCD 004 Sputter Coater; Bal-Tec, Vaduz, Liechtenstein) and viewed using a scanning electron microscope (SEM) (S-3500; Hitachi, Wokingham, UK) with an accelerating voltage of 15 kV and a working distance of 25 mm at increasing magnifications from 60× to 5000×.

Materials and methods

Specimen preparation

The materials and methods sessions of this paper are similar to those of Sauro and collaborators as it reports the second part of the experiments performed on the resin–dentin bonds to EDTA-treated vs. acid-etched dentin.

In brief, human molars extracted for surgical reasons were stored at 4 °C in 0.5% chloramine T for up to 1 month before use. The specimens were sectioned below the dentin–enamel junction using a water-cooled diamond saw (330-CA RS-70300, Struers, Copenhagen, Denmark). The occlusal surfaces were ground flat (LaboPol-4, Struers, Copenhagem, Denmark) using 500 grit SiC abrasive paper under constant water irrigation to provide standardized smear layer-covered dentin surfaces. Two principal groups were created according to the dentin conditioning treatments (i.e. EDTA or H 3 PO 4 ). The teeth were subsequently divided into seven subgroups according to the resin adhesives used in this study. A further sub-division of each subgroup was performed according to whether or not the teeth were cyclically loaded. All bonded teeth were stored in water for 24 h prior to cyclic loading or water storage for the same time.

Experimental resins

The five experimental comonomer blends used in this study as dentin bonding agents (DBAs) were formulated based on known concentrations of all ingredients, including 50 wt.% ethanol-solvated resin mixtures used as primers ( Table 1 ) . All experimental neat resins contained 0.25 wt.% camphoquinone and 1.0 wt.% ethyl-dimethyl-4-aminobenzoate. Resins 1 and 2 were hydrophobic resins similar to those used in pit-and-fissure sealants. Resin 3 represented the formulation of typical two-step, etch-and-rinse adhesives, while resins 4 and 5 had a hydrophilicity similar to one-step, self-etching adhesives, containing carboxylic- or phosphate-substituted methacrylates, respectively .

Table 1
Composition of neat and solvated resins used in this study.
Neat resin (pH ∼ 7) Solvated primer (pH ∼ 7)
Resin 1 70 wt.% E-BisADM 34.4 wt.% E-BisADM
28.75% TEGDMA 14.35% TEGDMA
50 wt.% ethanol
Neat resin (pH ∼ 7) Solvated primer (pH ∼ 7)
Resin 2 70 wt.% BisGMA 34.4 wt.% BisGMA
28.75% TEGDMA 14.35% TEGDMA
50 wt.% ethanol
Neat resin (pH ∼ 7) Solvated primer (pH ∼ 7)
Resin 3 70 wt.% BisGMA 34.4 wt.% BisGMA
28.75% HEMA 14.35 wt.% HEMA
50 wt.% ethanol
Neat resin (pH ∼ 4) Solvated primer (pH ∼ 4)
Resin 4 40 wt.% BisGMA 20 wt.% BisGMA
30 wt.% TCDM 14.4 wt.% TCDM
28.75% HEMA 14.35% HEMA
50 wt.% ethanol
Neat resin (pH ∼ 3) Solvated primer (pH ∼ 3)
Resin 5 40 wt.% BisGMA 20 wt.% BisGMA
30 wt.% BisMP 14.4 wt.% BisMP
28.75% HEMA 14.35% HEMA
50 wt.% ethanol
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Resin–dentin bonds to EDTA-treated vs. acid-etched dentin using ethanol wet-bonding. Part II: Effects of mechanical cycling load on microtensile bond strengths
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