Efficacy of microtensile versus microshear bond testing for evaluation of bond strength of dental adhesive systems to enamel

Abstract

Objective

The aim of the study was to evaluate the efficacy of the microtensile bond test (μTBS) and the microshear bond test (μSBS) in ranking four dental adhesives according to bond strength to enamel and identify the modes of failure involved.

Materials and methods

Forty-four caries-free human molars were randomly assigned to one of two bond strength testing methods: 20 teeth were used for μTBS test and 24 teeth for μSBS test. Flat enamel surfaces were created by wet grinding. Four adhesive systems were applied to the ground enamel surfaces; a two-step self-etch (Clearfil SE Bond, SEB), two all-in-one self-etch (Adper Prompt L-Pop, APL; Hybrid Bond, HB) and a two-step etch-and-rinse (Adper Single Bond, ASB). Resin composite (Z100) was applied over the adhesive. The μTBS and μSBS were determined after 24 h of storage in water at 37 °C. The mode of failure was determined by light microscope and SEM. Data was analyzed with ANOVA, Tukey’s and Chi-square tests.

Results

μTBS test ranked the adhesives as follows: SEB = ASB = APL > HB, while μSBS test ranked the adhesives as follows: ASB > SEB = APL > HB. The highest percentage failure mode with μTBS testing was cohesive in enamel or at the DEJ: SEB (95%), APL (65%) and ASB (65%). As for HB, adhesive failure (95%) was the common finding. The predominant failure mode in case of the μSBS was adhesive (APL 50%, SEB 58.3%, ASB 75% and HB 91.7%).

Significance

Ranking appears to be test-dependant and μSBS test appears to be more accurate in differentiating among the stronger adhesives.

Introduction

The concept of bonding has revolutionized the dental world. Today, the science of adhesive dentistry is in a constant state of evolution. New or improved products, with claims to offer advantages over their predecessors, are continuously being introduced on the international market. Contemporary dental adhesives employ one of two approaches: the etch-and-rinse and the self-etch approach . Self-etch adhesives are further classified as ‘mild’ or ‘strong’ based on degree of acidity . Self-etch adhesives are available either as two-step or one-step (all-in-one adhesives). Some one-step self-etch adhesives require mixing, while others do not require mixing. This ever-growing diversity in adhesive materials has made in vitro mechanical testing of a paramount importance. Tensile and shear bond tests have long been the most common laboratory tests to evaluate adhesive strength of bonding systems to the tooth substrate . Unfortunately, studies have shown that tensile and shear testing is significantly influenced by the variability in specimen geometry, loading conditions and materials properties .

Sano et al. developed the microtensile bond test to overcome some of these limitations . Easier sample collection, ability to compare between a variety of substrates and areas in the same tooth, more uniform loading stress distribution over a smaller bonded area are but a few of the advantages of the microtensile bond test . This method requires that the bonded specimens be sectioned into rectangular or cylindrical sticks or bars, 0.5–1.5 mm thick, in the ‘non-trimming technique’ or even further trimmed with burs at the adhesive interface to produce a dumbbell or hourglass appearance in case of the ‘trimming technique’ .

Recently, the microshear bond test was introduced as an alternative to the microtensile bond test . The microshear bond test involves the application of a loading force by means of a blade from a universal testing machine to a resin composite cylinder bonded to a substrate disc . Advantages of the microshear bond test include less demanding specimen collection and easier control of the bond test area by means of microbore (tygon) tubes . Shimida et al. modified the microshear bond test by replacing the blade with a looped orthodontic wire . In a study by Foong et al. on bond strength to enamel, the wire microshear test was shown to be easier and more reliable compared to the blade microtensile test .

The data obtained by adhesion testing are useful mainly to compare and rank adhesives according to strength of the bond within a particular study. In other words, only relative study outcomes are relevant . One must also point out that as a consequence of variations present in tooth substrates, type of composite, sample size and geometry, and testing protocols, a direct comparison among different authors or tests is unfeasible . One recently published review of the bond testing literature showed a coefficient of variation of about 20–50% between similar studies .

The general belief is that the microtensile and microshear bond tests are the most suitable methods available to evaluate bond strength of dental adhesive. However, there is a lack of published evidence concerning the most accurate methodology when testing bond strength to enamel. Therefore, the aim of the study was to evaluate the accuracy of the microtensile and microshear bond tests in determining the relative outcomes (ranking) of four dental adhesives to enamel and determine the types of failures involved.

Materials and methods

Collection and grouping of experimental teeth

Forty four caries-free freshly extracted human molar teeth were obtained from Amsterdam, the Netherlands. All teeth were hand scaled to remove tissue remnants and debris, and stored, under refrigeration, for up to 3 months in a 0.5% Chloramine-T solution prior to use. The teeth were randomly assigned to one of two bond strength testing methods: 20 teeth were used for microtensile bond strength test and 24 teeth were used for the microshear bond strength test.

For bonding the resin composite to the ground flat enamel surfaces, four adhesive systems were used in this study. A mild two-step self-etch adhesive (Clearfil SE Bond, SEB), strong all-in-one self-etch adhesive (Adper Prompt L-Pop, APL) that required mixing, an all-in-one self-etch adhesive (Hybrid Bond, HB) that does not required mixing and a two-step etch-and-rinse system (Adper Single Bond, ASB). The manufacturers, general compositions and manufacturers’ instructions for use of the four adhesive systems used in the study are summarized in Table 1 .

Table 1
Bonding systems; manufacturers, general composition and manufacturers’ instructions.
Material/Manufacturer General composition Manufacturers’ instruction for use
Clearfil SE Bond (Kuraray Medical Inc., Tokyo, Japan) Primer : Water, MDP, HEMA, CQ, DET and hydrophilic DMA.
Bond : MDP, bis-GMA, HEMA, hydrophobic DMA, CQ, DET, silanated colloidal silica.
• Dispense primer in the well.
• Apply primer with a brush and leave for 20 s.
• Air-blow for 5–10 s.
• Dispense bond in the well.
• Apply bond with a brush and air-thin.
• Light cure for 10 s.
Adper prompt L-Pop (3M ESPE, St. Paul, MN, USA) Liquid 1: Methacrylate phosphoric esters, bis-GMA, CQ, stabilizers and PI.
Liquid 2: Water, HEMA, PCA copolymer, and stabilizers.
• Squeeze the material from the red reservoir into the yellow reservoir.
• Carefully fold back the red reservoir.
• Squeeze the liquid from the yellow reservoir into the green reservoir.
• Apply a spinning motion to the applicator for 5 s.
• Apply adhesive with a rubbing motion for 15 s.
• Gently air-dry.
• Light cure for 10 s.
Hybrid Bond (Sun Medical, Moriyama, Japan) Base : 4-META, multifunctional acrylate, HEMA, MMA, acetone, water, and PI.
Brush : p-toluene sulfinate, sodium-salt, and aromatic amine.
• Dispense one drop of Hybrid Base in the well.
• Stir the expressed liquid with a Hybrid Brush.
• Apply and keep it moist for 20 s.
• Air-dry for 5–10 s.
• Light cure for 3–5 s.
Adper Single Bond (3M ESPE, St. Paul, MN, USA) HEMA, ethanol, water, Bis-GMA, dimethacrylate, amines, methacrylic copolymer of polyacrylic and polyitaconic acids, and PI. • Etch tooth surface with Scotchbond Etchant for 15 s then rinse for 10 s.
• Blot excess water with cotton pellet.
• Apply 2–3 coats of bond with rubbing motion for 15 s.
• Air-thin for 5 s.
• Light cure for 10 s.
Abbreviations : Bis-GMA: bis-phenol A diglycidylmethacrylate; HEMA: 2-hydroxyethyl methacrylate; MDP: 10-methacryloyloxydecyl dihydrogen phosphate; DMA: dimethacrylate; DET: N,N-diethanol p-toluidine; CQ: camphorquinone; 4-META: 4-methacryloyloxyethyl trimellitate anhydride; MMA: methylmethacylate; PI: photoinitiators.

Experimental design

Teeth were sectioned mesio-distally after removal of their roots using a low speed cutting saw (Buehler Isomet 1000, Buehler Ltd., Lake Bluff, IL, USA). 0.5 mm deep, flat enamel surfaces were created by wet grinding of the mid-buccal and mid-palatal/lingual enamel on a polishing machine with 240 grit SiC paper (Buehler Ecomet V, Buehler Ltd., Lake Bluff, IL, USA). In order to standardize the smear layer, the ground enamel surfaces were finished for one minute by wet grinding with 600 grit SiC paper. The adhesives were then applied according to the manufacturers’ instructions ( Table 1 ).

For the microtensile bond strength test, after the adhesive resin was light cured using Elipar Highlight curing light (3 M ESPE, St. Paul, MN, USA, 800 mW/cm 2 ), the resin composite (Z100, 3 M ESPE, St. Paul, MN, USA) was applied to the enamel surface in five to six increments to a height of 5–6 mm, and each increment was polymerized separately for 40 s.

For the microshear test, prior to the adhesive resin polymerization, a tygon tube (R-3603, Norton Performance Plastic Co., Cleveland, OH, USA) with an internal diameter of 0.8 mm and a height of 0.5 mm was placed on the bonded area. After the adhesive resin was light cured according to manufacturers’ instructions, the Z 100 resin composite was placed into the tube and polymerized for 40 s. All bonded specimens were stored in distilled water at 37 °C for 24 h before testing.

Microtensile bond strength testing

After the storage period, each enamel-composite block assembly was sectioned in the x and y directions with a low speed cutting saw (Buehler Isomet 1000, Buehler Ltd., Lake Bluff, IL, USA) according to the technique described by Sano et al. to obtain microbars with a cross-sectional area of 0.75 ± 0.1 mm 2 . The specimens were checked under a stereo microscope (20X; Olympus, Tokyo, Japan). Defective specimens were excluded. After measurement of the cross-sectional area of each microbar with a digital caliper (Mitutoyo Co., Japan), they were fixed with a dental adhesive (Clearfil SE Bond, Kuraray Co., Japan) to a modified micro-tensile testing device (ACTA, Department of Dental Material Science, Amsterdam, The Netherlands) . The specimens were tested under tension using a universal testing machine (Model no. 6022; Instron, High Wycombe, Bucks, UK) at a cross-head speed of 1.0 mm/min .

Microshear bond strength testing

After the storage period, the tygon tube was removed to reveal composite cylinder with a cross-sectional area of 0.5 ± 0.02 mm 2 . The specimens were checked under a stereo-microscope (Olympus, Tokyo, Japan) at a magnification of 20× prior to microshear testing. Any specimens that presented a detectable interfacial defect were excluded. The specimens were attached to the testing device (Becor-Multi-T, Danvillee Engineering Co., San Ramon, CA, USA) using Clearfil SE Bond. The testing device was placed on the universal testing machine. A thin wire with a diameter of 0.2 mm was looped around the resin cylinder and placed as close as possible to the resin–enamel interface. A shear force was applied at cross-head speed of 1.0 mm/min .

Failure analysis

The mode of failure was determined light-microscopically at a magnification of 20× using a stereo-microscope. Four representative specimens of each group were selected and additionally examined by SEM (Phillips SEM XL 20, Eindhoven, The Netherlands) to confirm the stereomicroscope observations. Prior to the SEM observations, the specimens were air-dried and sputter-coated with gold.

Failure modes were categorized as: (a) interfacial failure between tooth and adhesive resin (adhesive); (b) cohesive failure of composite (cohesive composite); (c) cohesive failure of the enamel or at the DEJ (cohesive DEJ/enamel); (d) mixed adhesive–cohesive failure (mixed).

Statistical analysis

Numerical (quantitative) data were presented as mean and standard deviation values. Categorical (qualitative) data were presented as frequencies and percentages. One-way analysis of variance ( ANOVA ) was used to compare between means of the four adhesives. Tukey’s post hoc test was used for pair-wise comparison between the means when ANOVA test is significant. Chi-square ( χ 2 ) test was used to compare between failure modes of the four groups. The significance level was set at P 0.05. Statistical analysis was performed with SPSS 14.0 ® (SPSS, Inc., Chicago, IL, USA) for Windows.

Materials and methods

Collection and grouping of experimental teeth

Forty four caries-free freshly extracted human molar teeth were obtained from Amsterdam, the Netherlands. All teeth were hand scaled to remove tissue remnants and debris, and stored, under refrigeration, for up to 3 months in a 0.5% Chloramine-T solution prior to use. The teeth were randomly assigned to one of two bond strength testing methods: 20 teeth were used for microtensile bond strength test and 24 teeth were used for the microshear bond strength test.

For bonding the resin composite to the ground flat enamel surfaces, four adhesive systems were used in this study. A mild two-step self-etch adhesive (Clearfil SE Bond, SEB), strong all-in-one self-etch adhesive (Adper Prompt L-Pop, APL) that required mixing, an all-in-one self-etch adhesive (Hybrid Bond, HB) that does not required mixing and a two-step etch-and-rinse system (Adper Single Bond, ASB). The manufacturers, general compositions and manufacturers’ instructions for use of the four adhesive systems used in the study are summarized in Table 1 .

Table 1
Bonding systems; manufacturers, general composition and manufacturers’ instructions.
Material/Manufacturer General composition Manufacturers’ instruction for use
Clearfil SE Bond (Kuraray Medical Inc., Tokyo, Japan) Primer : Water, MDP, HEMA, CQ, DET and hydrophilic DMA.
Bond : MDP, bis-GMA, HEMA, hydrophobic DMA, CQ, DET, silanated colloidal silica.
• Dispense primer in the well.
• Apply primer with a brush and leave for 20 s.
• Air-blow for 5–10 s.
• Dispense bond in the well.
• Apply bond with a brush and air-thin.
• Light cure for 10 s.
Adper prompt L-Pop (3M ESPE, St. Paul, MN, USA) Liquid 1: Methacrylate phosphoric esters, bis-GMA, CQ, stabilizers and PI.
Liquid 2: Water, HEMA, PCA copolymer, and stabilizers.
• Squeeze the material from the red reservoir into the yellow reservoir.
• Carefully fold back the red reservoir.
• Squeeze the liquid from the yellow reservoir into the green reservoir.
• Apply a spinning motion to the applicator for 5 s.
• Apply adhesive with a rubbing motion for 15 s.
• Gently air-dry.
• Light cure for 10 s.
Hybrid Bond (Sun Medical, Moriyama, Japan) Base : 4-META, multifunctional acrylate, HEMA, MMA, acetone, water, and PI.
Brush : p-toluene sulfinate, sodium-salt, and aromatic amine.
• Dispense one drop of Hybrid Base in the well.
• Stir the expressed liquid with a Hybrid Brush.
• Apply and keep it moist for 20 s.
• Air-dry for 5–10 s.
• Light cure for 3–5 s.
Adper Single Bond (3M ESPE, St. Paul, MN, USA) HEMA, ethanol, water, Bis-GMA, dimethacrylate, amines, methacrylic copolymer of polyacrylic and polyitaconic acids, and PI. • Etch tooth surface with Scotchbond Etchant for 15 s then rinse for 10 s.
• Blot excess water with cotton pellet.
• Apply 2–3 coats of bond with rubbing motion for 15 s.
• Air-thin for 5 s.
• Light cure for 10 s.
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Nov 30, 2017 | Posted by in Dental Materials | Comments Off on Efficacy of microtensile versus microshear bond testing for evaluation of bond strength of dental adhesive systems to enamel
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