Effect of Sonic Application Mode on the Resin-Dentin Bond Strength and Dentin Permeability of Self-etching Systems Alexandra Mena-Serranoa / Thays Regina Ferreira da Costab / Rafael Tiago Patzlaffc / Alessandro Dourado Loguerciod / Alessandra Reise
Purpose: To compare manual and sonic adhesive application modes in terms of the permeability and microtensile bond strength of a self-etching adhesive applied in the one-step or two-step protocol. Materials and Methods: Self-etching All Bond SE (Bisco) was applied as a one- or a two-step adhesive under manual or sonic vibration modes on flat occlusal dentin surfaces of 64 human molars. Half of the teeth were used to measure the hydraulic conductance of dentin at 200 cm H2O hydrostatic pressure for 5 min immediately after the adhesive application. In the other half, composite buildups (Opallis) were constructed incrementally to create resin-dentin sticks with a cross-sectional area of 0.8 mm2 to be tested in tension (0.5 mm/min) immediately after restoration placement. Data were analyzed using a two-way ANOVA and Tukey’s test (α = 0.05). Results: The fluid conductance of dentin was significantly reduced by the sonic vibration mode for both adhesives, but no effect on the bond strength values was observed for either adhesive. Conclusion: The sonic application mode at an oscillating frequency of 170 Hz can reduce the fluid conductance of the one- and two-step All Bond SE adhesive when applied on dentin. Keywords: tensile strength, dentin bonding agents, dentin, dentin permeability. J Adhes Dent 2014; 16: 435–440. doi: 10.3290/j.jad.a32810
T
he prevention of adhesive interface degradation is still a challenge for dentistry. The literature points out that resin hydrolysis is one of the causes of resin degradation. 4 This phenomenon initiates with water
a
Professor, Department of Restorative Dentistry, School of Dentistry, Universidad de las Américas, Quito, Pichincha, Ecuador. Performed study in partial fulfillment of her PhD degree, performed the experiments, co-wrote manuscript.
b
PhD Student, Department of Restorative Dentistry, School of Dentistry, State University of Ponta Grossa, Ponta Grossa, Paraná, Brazil. Performed microtensile bond strength tests.
c
Mechanical Engineer, Research and Development Department, Odeme Biotechnology, Joaçaba, Santa Catarina, Brazil. Developed of the fluid conductance equipment, followed up the sonic device’s performance.
d
Adjunct Professor, Department of Restorative Dentistry, School of Dentistry, State University of Ponta Grossa, Ponta Grossa, Paraná, Brazil. Consulted on the statistical evaluation, co-wrote manuscript.
e
Adjunct Professor, Department of Restorative Dentistry, School of Dentistry, State University of Ponta Grossa, Ponta Grossa, Paraná, Brazil. Idea, hypothesis, experimental design, statistical analysis, contributed substantially to manuscript writing, proofread manuscript.
Correspondence: Alessandra Reis, Universidade Estadual de Ponta Grossa, Pós-Graduação em Odontologia, Rua Carlos Cavalcanti, 4748, Bloco M, Sala 64A Uvaranas, Ponta Grossa, Paraná, Brazil 84030-900. Tel/Fax: +55-423-220-3741. e-mail: [email protected]
Vol 16, No 5, 2014
Submitted for publication: 02.10.13; accepted for publication: 10.07.14
sorption within the hybrid layer29 and the consequent release of monomers/oligomers from the hydrid layer to the surrondings.4,29 This appears to be more pronounced in single-step adhesives.35 Due to their intrinsic hydrophilic characteristics,36 these materials behave as semi-permeable membranes,17 allowing fluid flow movement across the adhesive interface. In regard to self-etching adhesives, which demineralize hard dental tissues, the presence of water is also an obstacle to achieving optimal polymer cross linking.18 Additionally, the entrapment of water and solvents within the adhesive39 favors water sorption and degradation over time.29 With the purpose of improving the quality of the adhesive interfaces produced with self-etching adhesives, some studies have suggested the application of multiple adhesive coats16,26 or the application of an extra hydrophobic layer.28,31 Even though these studies found satisfactory results, these alternatives add steps to the bonding protocol, which is contrary to the clinician’s preference for simplification when choosing one-step selfetching adhesives. To improve solvent evaporation and resin monomer infiltration into the dental substrate,10,11 several authors have demonstrated under in vitro10,11,22 and in vivo21 conditions that active and vigorous adhesive scrubbing 435
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Table 1
Adhesive system, batch number, composition and application mode according to the manufacturer
Adhesive
Composition (batch number)
Application mode*
All Bond SE
Part I: Ethanol, sodium benzene sulfinate (0700007204) Part II: Hydroxyethyl methacrylate, bis(glyceryl 1,3 dimethyacrylate) phosphate, bisphenyl dimethacrylate (0700007204) All Bond SE liner: bisphenol-A glycidyl methacrylate, urethane dimetyhacrylate, hydroxyethyl methacrylate, glass filler (0700001908)
One-step All Bond SE Dispense an equal number of drops of All Bond SE Parts I and II (1:1 ratio) into a mixing well. Apply two coats on the dry preparation and agitate each coat for 10 s. Air dry for 5 s. Light cure for 10 s at 500 mW/cm2. Two-step All Bond SE After steps 1 to 4, apply the All Bond SE liner in a layer less than 1 mm thick on the surface. Light cure for 10 s at 500 mW/cm2.
* For the sonic mode, each coat was applied with the microbrush attached to the sonic device Smart (FGM).
Fig 1 Prototype of the Smart Sonic Device (FGM Prod Odontológicos; Joinville, SC, Brazil).
no significant difference will be found on the fluid conductance of dentin between manual and sonic vibration application modes using one- and two-step self-etching adhesives and 2) no significant difference on the microtensile bond strengths will be found between manual and sonic vibration application modes using one- and two-step self-etching adhesives.
MATERIALS AND METHODS is superior to gentle adhesive application. Other studies went further and proposed the use of electrical3,38 and ultrasonic devices.1,2,12 The latter application mode could not improve the bond strength results of some of the self-etching adhesive systems evaluated. Ultrasonics is a branch of acoustics concerned with sound vibrations in frequency ranges above the audible level. The frequency of the oscillating instruments in dental practice is sonic when the frequency ranges from 1000 to 6000 Hz or ultrasonic for frequencies ranging from 20,000 to 40,000 Hz. Therefore, there is still room for investigation of ultrasonic technology for adhesive application. For instance, a recent study demonstrated that the use of a sonic device with an oscillating frequency of 170 Hz improved the immediate microtensile bond strength and delayed the degradation of one-step self-etching adhesives.23 The bristle motion of the applicator under sonic vibration imparts energy to the fluids that surround the dental structure (such as the bonding resin). In turn, these agitated fluids are able to achieve areas beyond those touched by the bristles. The high-speed vibration of the microbrush creates pressure waves and shear forces in the adhesive. It also generates microscopic bubbles that are forcefully propelled against surfaces to which the adhesive solution is applied. All combined, these fluid dynamics are able to increase monomer diffusion inward while aiding in the evaporation of solvents outward.23 Given these positive findings, the present study aimed to compare the manual and sonic vibration application modes in terms of dentin permeability and the bond strength of a self-etching adhesive applied in the one-step or two-step protocol. Two null hypothesis were tested: 1) 436
Sixty-four extracted, caries-free human third molars were used in this study. The teeth were collected after obtaining the patients’ informed consent under protocol 11592/10, approved by the Ethics Committee of the State University of Ponta Grossa. The self-etching adhesive All Bond SE (Bisco; Schaumburg, IL, USA) was used as a one-step (AB1) or as a twostep (AB2) self-etching system and applied according to the description in Table 1. The only exception was the method used to spread the adhesive onto the dentin surface. The self-etching adhesive system was applied for 20 s with a regular-size microbrush (Microbrush tube series, Microbrush International; Grafton, WI, USA) using either manual (agitation) or sonic vibration modes. For the latter, the microbrush was first attached to the prototype of the sonic device Smart (Fig 1), to be released on the dental market by FGM Prod Odontológicos (Joinville, SC, Brazil). The prototype produced an oscillating vibration of 10,200 rpm or 170 Hz as measured by the BlackmanHarris sound method.13 The Smart device (FGM) has five different oscillating frequencies (144.5, 150, 170, 223.5, and 167.5 Hz). This study employed the middle frequency of the device. In the AB2 group, the resin liner was applied manually with a regular-size microbrush not attached to the sonic device (Table 1). Measurement of Hydraulic Conductance of Dentin The roots of 32 teeth were sectioned perpendicular to their longitudinal axis 2 mm bellow the CEJ by means of an Isomet saw (Buehler; Lake Bluff, IL, USA) under water cooling. The pulp tissue was carefully removed with small The Journal of Adhesive Dentistry
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forceps and care was taken to avoid touching the pulp chamber walls and, consequently, crushing the pre-dentin toward the dentinal tubules, which could alter the final permeability of dentin.9 Then, the occlusal enamel was removed with a second cut parallel to the section of the dentin. The thickness of the specimens was reduced by polishing with wet 400-grit silicon carbide paper and measured with a caliper until a final thickness of approximately 0.5 mm was achieved. Any specimen presenting pulp horn exposure was excluded from the sample. This study used an in vitro fluid transport model (Odeme Biotechnology; Joaçaba, SC, Brazil) to measure the fluid conductance induced by hydrostatic pressure. This equipment used a glass capillary tube 125 mm long and 0.75 mm in internal diameter, and measured the bubble displacement with a digital caliper mounted close to the glass capillary. The hydrostatic pressure was calibrated at 200 cm H2O5 and measured for 5 min. Each measurement was repeated twice to confirm the values; in case of discrepancies, the procedure was repeated. To access the maximum baseline water conductance of each tooth, the dentin surfaces of 32 specimens were etched with 37% phosphoric acid (Dentsply DeTrey; Konstanz, Germany) for 15 s. With the baseline fluid flow values, the teeth were classified as low, medium, and highly permeable, and then distributed equally into four groups (n = 8 for each group) resulting from the combination of the main factors adhesive system and application mode. The occlusal dentin surface of each specimen was polished with a wet 600-grit SiC paper for 60 s to form a standardized smear layer. Then the fluid displacement was measured again to ensure that the etched superficial dentin surface was eliminated. The adhesive system All Bond SE was applied according to each experimental condition and light cured for 10 s using a quartz-halogen light unit set at 500 mW/cm2 (VIP, Bisco). After adhesive application, the fluid displacement across the bonded dentin was measured again, as previously described. The linear displacement was automatically converted to fluid flow (μL • min-1) according to the following formula: Q = (98.17 × ΔL)/125 × t where 98.17 is the internal total volume of the glass capillary (μL), ΔL is the displacement of the void during the test (mm), 125 mm is the glass capillary length, and t is measuring time during the test (5 min). For each specimen, the fluid flow (μL • min-1) across the adhesively bonded dentin was expressed as a percentage of the maximum permeability derived from the acid-etched dentin (designated as 100%). This allowed each specimen to serve as its own control, since the same surface area was used in the same two measurements.6 Microtensile Bond Strength Test (μTBS) Thirty-two teeth were used for the μTBS test. A flat, superficial dentin surface was exposed on each tooth after wet grinding the occlusal enamel on 180-grit SiC paper. The enamel-free, exposed dentin surfaces were further polished on wet 600-grit silicon-carbide paper for 60 s Vol 16, No 5, 2014
to standardize the smear layer. The teeth were randomly divided into four groups (n = 8 teeth) resulting from the combination of the main factors adhesive systems and application mode. For the restorative procedure, the adhesive system All Bond SE was applied according to each experimental condition and light cured for 10 s using a quartz-halogen light unit set at 500 mW/cm2 (VIP, Bisco). Crowns were built up with a composite resin (Opallis, FGM) in 3 increments of 1 mm each, which were individually light activated for 40 s using the same light-curing unit. After 24 h of storage in distilled water at 37°C, the specimens were sectioned in both “x” and “y” directions across the bonded interface with a diamond blade in an Isomet 1000 saw (Isomet 1000, Buehler) at 400 rpm to obtain sticks with a crosssectional area of approximately 0.8 mm2. The bonded sticks were attached to a Geraldeli’s device (Odeme Biotechnology) with cyanoacrylate resin (Super Bonder Gel, Loctite; São Paulo, SP, Brazil) and subjected to a tensile force in a universal testing machine (Kratos Dinamometros; São Paulo, SP, Brazil) at a crosshead speed of 0.5 mm/min. The failure modes were evaluated at 40X (Eclipse E200, Nikon; Melvill, NY, USA) and classified as cohesive (failure exclusively within the dentin [CD] or resin [CR]) or adhesive/mixed (failure at the resin/dentin interface and failure at resin/dentin interface that included cohesive failure of the neighboring substrates [A/M]). Statistical Analysis The mean μTBS of all sticks (with adhesive/mixed failure) from the same tooth was averaged for statistical purposes. As few specimens from all groups showed cohesive or premature failures, they were not included in the tooth mean. Before submitting the data to statistical analysis, the Kolmogorov-Smirnov test was performed to assess whether the data followed a normal distribution, and the Barlett’s test was used to determine whether the assumption of equal variances was valid. After confirming the normality of data distribution and the equality of variances, the μTBS values (MPa) and the hydraulic conductance (%) were submitted to a two-way ANOVA (adhesive vs application mode). The level of significance was pre-set at 5%. Tukey’s test was used for pair comparisons (α = 0.05).
RESULTS Hydraulic Conductance of Dentin Fluid conductance is expressed as percentages of the maximum permeability that occurred in the baseline acid-etched dentin. None of the bonding treatments was able to completely interrupt the transudation of fluid across the adhesively bonded interface (Table 2). Two-way ANOVA revealed that the cross-product interaction adhesive vs application mode was not statistically significant (p > 0.05). Only the main factor application mode was significant (p = 0.017). The use of sonic vibration application significantly reduced the fluid conductance of dentin (p = 0.017) compared to manual application. 437
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Table 2 Overall mean fluid conductance and the respective standard deviations (%) obtained in each experimental condition Application technique
Adhesive
Main factor technique
One-step All Bond SE
Two-step All Bond SE
Manual
9.2 ± 4.9
6.1 ± 3.2
7.4 ± 4.2A
Sonic
3.0 ± 0.9
4.8 ± 3.6
4.0 ± 2.8B
Means identified with different superscript letters are statistically different (p < 0.05).
Table 3 Number and percentage (%) of specimens distributed according to the fracture mode and premature failures for each experimental condition Adhesive One-step All Bond SE
Two-step All Bond SE
Fracture mode
Manual
Sonic
A/M
87 (82.6)
112 (81.8)
CD
0 (0)
1 (0.7)
CR
0 (0)
1 (0.7)
PF
34 (17.4)
23 (17.2)
A/M
104 (89.7)
96 (81.4)
CD
0 (0)
0 (0.0)
CR
0 (0)
0 (0.0)
PF
12 (10.3)
22 (18.6)
A/M: adhesive/mixed; CD: cohesive in dentin; CR: cohesive in resin; PF: premature failures.
Table 4 Overall mean μTBS values and the respective standard deviations (MPa) obtained in each experimental condition Application technique
Adhesive One-step All Bond SE
Two-step All Bond SE
Manual
31.5 ± 8.3a
27.9 ± 7.8a
Sonic
28.3 ± 6.4a
31.3 ± 4.6a
Means identified with identical superscript letters are statistically similar (p > 0.05).
Microtensile Bond Strength The majority of the specimens showed adhesive/mixed failures, irrespective of the experimental group. Cohesive failures were observed in only a few specimens (Table 3). Two-way ANOVA revealed that the cross-product interaction as well as the main factors were not statistically 438
significant (p > 0.05). The application mode and the application of a hydrophobic coating did not affect the μTBS of the adhesive All Bond SE (Table 4, p > 0.05).
DISCUSSION The results of the present study showed that sonic application was not able to increase the microtensile bond strength values of the one- and two-step self-etching adhesives, which led us to accept the first null hypothesis. On the other hand, the permeability of the dentin was reduced when the adhesive All Bond SE, in one or two steps, was applied with sonic vibration. This led us to reject the second null hypothesis. In accordance with previous studies,17,27 the current findings showed that the application and light curing of a self-etching adhesive used in a one-step and two-step mode to dentin does not prevent fluid passage across the bonding interface. On the other hand, the sonic application mode significantly decreased the fluid conductance of the adhesive system independent of the bonding protocol tested. Sonic vibration propagates pressure waves due to the stimulation of the adhesive molecules. This movement promotes fluid agitation and increases monomer diffusion inward. Additionally, this movement enables solvent molecules trapped between high molecular-weight monomers to be brought to the adhesive surface, thus facilitating their evaporation.8,24 The benefits of this application mode were observed in a very recent study which reported lower nanoleakage at the immediate and 6-month periods when one-step adhesives were applied with this same sonic device.23 These findings were not surprising, since earlier studies reported that the active and vigorous manual application of adhesives could reduce the nanoleakage at the immediate period11 and after water storage for 6 months10 and 3 years.22 From a clinical standpoint, the use of a sonic device can be considered superior to active manual application, as it reduces the finger pressure variations of different operators and can guarantee homogeneous vibration of the adhesive. Contrary to our expectations, sonic application did not yield improved bond strength values. In an earlier study,23 the authors reported that the same sonic device employed here increased the bond strength of two out of three adhesive systems. This suggests that the benefit of this application protocol may be adhesive dependent. Several factors may be responsible for low resin-dentin bond strength of adhesive systems, such as improper demineralization of the substrates and monomer infiltration,22 high solvent retention,30 low conversion degree,15 and adhesive strength,14 etc. The literature reports on the effect of ultrasonic application mode on the bond strength of self-etching adhesives; however, no satisfactory results were found.1,12 It is important to point out that the devices used in those studies worked within ultrasonic frequencies, 25 to 30 kHz12 and 1 MHz,1 while the device proposed in the present study functions in the sonic range (170 Hz). The Journal of Adhesive Dentistry
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Sonic application can allow better adhesive demineralization and infiltration and can aid in solvent evaporation as the adhesive solution is agitated, but it cannot improve the strength of the adhesive layer. This explains why the sonic device did not improve the bond strength of the Futurabond NR adhesive (Voco) in an earlier study.23 As opposed to Futurabond NR, All Bond SE (Bisco) has a higher ultimate tensile strength compared to other competitive products.15 Additionally, All Bond SE employed as one-step or two-step self-etching reached high immediate29,33 and short-term40 resin-dentin bond strengths.29 More recently, high retention rates were observed after 24 months when All Bond SE was applied according to onestep and two-step self-etching protocols.20 Perhaps this material’s properties are already maximized with manual application and therefore no further increase could be detected with sonic application, at least when measured at the immediate period. As is true of application mode, the application of an extra hydrophobic layer improved neither the bond strength nor dentin sealing. Although there are many studies reporting that the hydrophobic resin coating improves the immediate resin-dentin bonds,9,28 reduces degradation over time,28 and reduces the fluid conductance,34 the fact that this approach was not beneficial for some adhesives under in vitro19,28 and in vivo studies31 again suggests that the protocol is adhesive dependent. The findings of a recent publication33 strengthen this hypothesis. Regarding the resin-dentin bond strength, one-step All Bond SE was the only all-in-one adhesive that was statistically similar to two-step All Bond SE29,33 and the gold standard two-step self-etching Clearfil SE Bond.29 This product is a two-component self-etching system where water and acidic monomers are supplied in separate containers. It is well known that one-component self-etching systems are chemically unstable and are more prone to degrade into the bottles, which might jeopardize their bonding efficacy.25 Whether or not this could have influenced the bond strength results of the present study must be determined in further investigations. Although sonic application did not improve the bond strength of the adhesives tested, it did not reduce the materials’ performance in the present and earlier studies.23 Furthermore, it reduced the flow conductance of the bonded interface and this may have a positive impact on the longevity of the dentin bonds as already reported in studies where the vigorous application mode was employed.22,32 It has already been demonstrated that only “aged” bond strength values are correlated with clinical performance of adhesives.37 A limitation of the present study is that it did not test the specimens after aging; therefore, this should be the focus of future studies. Additionally, it is known that under clinical conditions, there is an outward fluid flow across exposed dentin in response to the low but positive pulpal tissue pressure,7 which is completely absent in extracted teeth and was not simulated in the present study. Whether or not this may affect the study results has yet to be determined. Vol 16, No 5, 2014
Future studies should focus on a wide range of adhesive systems and the longevity of the resin-dentin bonds to provide comprehensive knowledge about the potential benefits of a sonic device for adhesive application.
CONCLUSIONS The sonic application of one- and two-step All Bond SE at an oscillating frequency of 170 Hz can significantly reduce the fluid conductance of dentin, although this did not yield significant improvement in the resin-dentin bond strengths of the adhesive systems.
ACKNOWLEDGMENTS This study was performed by Alexandra Mena Serrano in partial fulfillment of her PhD degree at the State University of Ponta Grossa. This study was partially supported by the National Council for Scientific and Technological Development (CNPq) under grants 301937/ 2009-5 and 301891/2010-9. The authors are grateful for the collaboration of Eugenio Jose Garcia on the microtensile bond experiment. Conflict of interest: R. T. Patzlaff, A. Reis, and A. D. Loguercio are involved in the development of the sonic device employed in this study.
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Clinical relevance: Sonic application mode at an oscillating frequency of 170 Hz can be helpful to reduce the permeability of a self-etching adhesive system.
The Journal of Adhesive Dentistry
Purpose: To compare manual and sonic adhesive application modes in terms of the permeability and microtensile bond strength of a self-etching adhesive applied in the one-step or two-step protocol. Materials and Methods: Self-etching All Bond SE (Bisco) was applied as a one- or a two-step adhesive under manual or sonic vibration modes on flat occlusal dentin surfaces of 64 human molars. Half of the teeth were used to measure the hydraulic conductance of dentin at 200 cm H2O hydrostatic pressure for 5 min immediately after the adhesive application. In the other half, composite buildups (Opallis) were constructed incrementally to create resin-dentin sticks with a cross-sectional area of 0.8 mm2 to be tested in tension (0.5 mm/min) immediately after restoration placement. Data were analyzed using a two-way ANOVA and Tukey’s test (α = 0.05). Results: The fluid conductance of dentin was significantly reduced by the sonic vibration mode for both adhesives, but no effect on the bond strength values was observed for either adhesive. Conclusion: The sonic application mode at an oscillating frequency of 170 Hz can reduce the fluid conductance of the one- and two-step All Bond SE adhesive when applied on dentin. Keywords: tensile strength, dentin bonding agents, dentin, dentin permeability. J Adhes Dent 2014; 16: 435–440. doi: 10.3290/j.jad.a32810
T
he prevention of adhesive interface degradation is still a challenge for dentistry. The literature points out that resin hydrolysis is one of the causes of resin degradation. 4 This phenomenon initiates with water
a
Professor, Department of Restorative Dentistry, School of Dentistry, Universidad de las Américas, Quito, Pichincha, Ecuador. Performed study in partial fulfillment of her PhD degree, performed the experiments, co-wrote manuscript.
b
PhD Student, Department of Restorative Dentistry, School of Dentistry, State University of Ponta Grossa, Ponta Grossa, Paraná, Brazil. Performed microtensile bond strength tests.
c
Mechanical Engineer, Research and Development Department, Odeme Biotechnology, Joaçaba, Santa Catarina, Brazil. Developed of the fluid conductance equipment, followed up the sonic device’s performance.
d
Adjunct Professor, Department of Restorative Dentistry, School of Dentistry, State University of Ponta Grossa, Ponta Grossa, Paraná, Brazil. Consulted on the statistical evaluation, co-wrote manuscript.
e
Adjunct Professor, Department of Restorative Dentistry, School of Dentistry, State University of Ponta Grossa, Ponta Grossa, Paraná, Brazil. Idea, hypothesis, experimental design, statistical analysis, contributed substantially to manuscript writing, proofread manuscript.
Correspondence: Alessandra Reis, Universidade Estadual de Ponta Grossa, Pós-Graduação em Odontologia, Rua Carlos Cavalcanti, 4748, Bloco M, Sala 64A Uvaranas, Ponta Grossa, Paraná, Brazil 84030-900. Tel/Fax: +55-423-220-3741. e-mail: [email protected]
Vol 16, No 5, 2014
Submitted for publication: 02.10.13; accepted for publication: 10.07.14
sorption within the hybrid layer29 and the consequent release of monomers/oligomers from the hydrid layer to the surrondings.4,29 This appears to be more pronounced in single-step adhesives.35 Due to their intrinsic hydrophilic characteristics,36 these materials behave as semi-permeable membranes,17 allowing fluid flow movement across the adhesive interface. In regard to self-etching adhesives, which demineralize hard dental tissues, the presence of water is also an obstacle to achieving optimal polymer cross linking.18 Additionally, the entrapment of water and solvents within the adhesive39 favors water sorption and degradation over time.29 With the purpose of improving the quality of the adhesive interfaces produced with self-etching adhesives, some studies have suggested the application of multiple adhesive coats16,26 or the application of an extra hydrophobic layer.28,31 Even though these studies found satisfactory results, these alternatives add steps to the bonding protocol, which is contrary to the clinician’s preference for simplification when choosing one-step selfetching adhesives. To improve solvent evaporation and resin monomer infiltration into the dental substrate,10,11 several authors have demonstrated under in vitro10,11,22 and in vivo21 conditions that active and vigorous adhesive scrubbing 435
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Table 1
Adhesive system, batch number, composition and application mode according to the manufacturer
Adhesive
Composition (batch number)
Application mode*
All Bond SE
Part I: Ethanol, sodium benzene sulfinate (0700007204) Part II: Hydroxyethyl methacrylate, bis(glyceryl 1,3 dimethyacrylate) phosphate, bisphenyl dimethacrylate (0700007204) All Bond SE liner: bisphenol-A glycidyl methacrylate, urethane dimetyhacrylate, hydroxyethyl methacrylate, glass filler (0700001908)
One-step All Bond SE Dispense an equal number of drops of All Bond SE Parts I and II (1:1 ratio) into a mixing well. Apply two coats on the dry preparation and agitate each coat for 10 s. Air dry for 5 s. Light cure for 10 s at 500 mW/cm2. Two-step All Bond SE After steps 1 to 4, apply the All Bond SE liner in a layer less than 1 mm thick on the surface. Light cure for 10 s at 500 mW/cm2.
* For the sonic mode, each coat was applied with the microbrush attached to the sonic device Smart (FGM).
Fig 1 Prototype of the Smart Sonic Device (FGM Prod Odontológicos; Joinville, SC, Brazil).
no significant difference will be found on the fluid conductance of dentin between manual and sonic vibration application modes using one- and two-step self-etching adhesives and 2) no significant difference on the microtensile bond strengths will be found between manual and sonic vibration application modes using one- and two-step self-etching adhesives.
MATERIALS AND METHODS is superior to gentle adhesive application. Other studies went further and proposed the use of electrical3,38 and ultrasonic devices.1,2,12 The latter application mode could not improve the bond strength results of some of the self-etching adhesive systems evaluated. Ultrasonics is a branch of acoustics concerned with sound vibrations in frequency ranges above the audible level. The frequency of the oscillating instruments in dental practice is sonic when the frequency ranges from 1000 to 6000 Hz or ultrasonic for frequencies ranging from 20,000 to 40,000 Hz. Therefore, there is still room for investigation of ultrasonic technology for adhesive application. For instance, a recent study demonstrated that the use of a sonic device with an oscillating frequency of 170 Hz improved the immediate microtensile bond strength and delayed the degradation of one-step self-etching adhesives.23 The bristle motion of the applicator under sonic vibration imparts energy to the fluids that surround the dental structure (such as the bonding resin). In turn, these agitated fluids are able to achieve areas beyond those touched by the bristles. The high-speed vibration of the microbrush creates pressure waves and shear forces in the adhesive. It also generates microscopic bubbles that are forcefully propelled against surfaces to which the adhesive solution is applied. All combined, these fluid dynamics are able to increase monomer diffusion inward while aiding in the evaporation of solvents outward.23 Given these positive findings, the present study aimed to compare the manual and sonic vibration application modes in terms of dentin permeability and the bond strength of a self-etching adhesive applied in the one-step or two-step protocol. Two null hypothesis were tested: 1) 436
Sixty-four extracted, caries-free human third molars were used in this study. The teeth were collected after obtaining the patients’ informed consent under protocol 11592/10, approved by the Ethics Committee of the State University of Ponta Grossa. The self-etching adhesive All Bond SE (Bisco; Schaumburg, IL, USA) was used as a one-step (AB1) or as a twostep (AB2) self-etching system and applied according to the description in Table 1. The only exception was the method used to spread the adhesive onto the dentin surface. The self-etching adhesive system was applied for 20 s with a regular-size microbrush (Microbrush tube series, Microbrush International; Grafton, WI, USA) using either manual (agitation) or sonic vibration modes. For the latter, the microbrush was first attached to the prototype of the sonic device Smart (Fig 1), to be released on the dental market by FGM Prod Odontológicos (Joinville, SC, Brazil). The prototype produced an oscillating vibration of 10,200 rpm or 170 Hz as measured by the BlackmanHarris sound method.13 The Smart device (FGM) has five different oscillating frequencies (144.5, 150, 170, 223.5, and 167.5 Hz). This study employed the middle frequency of the device. In the AB2 group, the resin liner was applied manually with a regular-size microbrush not attached to the sonic device (Table 1). Measurement of Hydraulic Conductance of Dentin The roots of 32 teeth were sectioned perpendicular to their longitudinal axis 2 mm bellow the CEJ by means of an Isomet saw (Buehler; Lake Bluff, IL, USA) under water cooling. The pulp tissue was carefully removed with small The Journal of Adhesive Dentistry
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forceps and care was taken to avoid touching the pulp chamber walls and, consequently, crushing the pre-dentin toward the dentinal tubules, which could alter the final permeability of dentin.9 Then, the occlusal enamel was removed with a second cut parallel to the section of the dentin. The thickness of the specimens was reduced by polishing with wet 400-grit silicon carbide paper and measured with a caliper until a final thickness of approximately 0.5 mm was achieved. Any specimen presenting pulp horn exposure was excluded from the sample. This study used an in vitro fluid transport model (Odeme Biotechnology; Joaçaba, SC, Brazil) to measure the fluid conductance induced by hydrostatic pressure. This equipment used a glass capillary tube 125 mm long and 0.75 mm in internal diameter, and measured the bubble displacement with a digital caliper mounted close to the glass capillary. The hydrostatic pressure was calibrated at 200 cm H2O5 and measured for 5 min. Each measurement was repeated twice to confirm the values; in case of discrepancies, the procedure was repeated. To access the maximum baseline water conductance of each tooth, the dentin surfaces of 32 specimens were etched with 37% phosphoric acid (Dentsply DeTrey; Konstanz, Germany) for 15 s. With the baseline fluid flow values, the teeth were classified as low, medium, and highly permeable, and then distributed equally into four groups (n = 8 for each group) resulting from the combination of the main factors adhesive system and application mode. The occlusal dentin surface of each specimen was polished with a wet 600-grit SiC paper for 60 s to form a standardized smear layer. Then the fluid displacement was measured again to ensure that the etched superficial dentin surface was eliminated. The adhesive system All Bond SE was applied according to each experimental condition and light cured for 10 s using a quartz-halogen light unit set at 500 mW/cm2 (VIP, Bisco). After adhesive application, the fluid displacement across the bonded dentin was measured again, as previously described. The linear displacement was automatically converted to fluid flow (μL • min-1) according to the following formula: Q = (98.17 × ΔL)/125 × t where 98.17 is the internal total volume of the glass capillary (μL), ΔL is the displacement of the void during the test (mm), 125 mm is the glass capillary length, and t is measuring time during the test (5 min). For each specimen, the fluid flow (μL • min-1) across the adhesively bonded dentin was expressed as a percentage of the maximum permeability derived from the acid-etched dentin (designated as 100%). This allowed each specimen to serve as its own control, since the same surface area was used in the same two measurements.6 Microtensile Bond Strength Test (μTBS) Thirty-two teeth were used for the μTBS test. A flat, superficial dentin surface was exposed on each tooth after wet grinding the occlusal enamel on 180-grit SiC paper. The enamel-free, exposed dentin surfaces were further polished on wet 600-grit silicon-carbide paper for 60 s Vol 16, No 5, 2014
to standardize the smear layer. The teeth were randomly divided into four groups (n = 8 teeth) resulting from the combination of the main factors adhesive systems and application mode. For the restorative procedure, the adhesive system All Bond SE was applied according to each experimental condition and light cured for 10 s using a quartz-halogen light unit set at 500 mW/cm2 (VIP, Bisco). Crowns were built up with a composite resin (Opallis, FGM) in 3 increments of 1 mm each, which were individually light activated for 40 s using the same light-curing unit. After 24 h of storage in distilled water at 37°C, the specimens were sectioned in both “x” and “y” directions across the bonded interface with a diamond blade in an Isomet 1000 saw (Isomet 1000, Buehler) at 400 rpm to obtain sticks with a crosssectional area of approximately 0.8 mm2. The bonded sticks were attached to a Geraldeli’s device (Odeme Biotechnology) with cyanoacrylate resin (Super Bonder Gel, Loctite; São Paulo, SP, Brazil) and subjected to a tensile force in a universal testing machine (Kratos Dinamometros; São Paulo, SP, Brazil) at a crosshead speed of 0.5 mm/min. The failure modes were evaluated at 40X (Eclipse E200, Nikon; Melvill, NY, USA) and classified as cohesive (failure exclusively within the dentin [CD] or resin [CR]) or adhesive/mixed (failure at the resin/dentin interface and failure at resin/dentin interface that included cohesive failure of the neighboring substrates [A/M]). Statistical Analysis The mean μTBS of all sticks (with adhesive/mixed failure) from the same tooth was averaged for statistical purposes. As few specimens from all groups showed cohesive or premature failures, they were not included in the tooth mean. Before submitting the data to statistical analysis, the Kolmogorov-Smirnov test was performed to assess whether the data followed a normal distribution, and the Barlett’s test was used to determine whether the assumption of equal variances was valid. After confirming the normality of data distribution and the equality of variances, the μTBS values (MPa) and the hydraulic conductance (%) were submitted to a two-way ANOVA (adhesive vs application mode). The level of significance was pre-set at 5%. Tukey’s test was used for pair comparisons (α = 0.05).
RESULTS Hydraulic Conductance of Dentin Fluid conductance is expressed as percentages of the maximum permeability that occurred in the baseline acid-etched dentin. None of the bonding treatments was able to completely interrupt the transudation of fluid across the adhesively bonded interface (Table 2). Two-way ANOVA revealed that the cross-product interaction adhesive vs application mode was not statistically significant (p > 0.05). Only the main factor application mode was significant (p = 0.017). The use of sonic vibration application significantly reduced the fluid conductance of dentin (p = 0.017) compared to manual application. 437
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Table 2 Overall mean fluid conductance and the respective standard deviations (%) obtained in each experimental condition Application technique
Adhesive
Main factor technique
One-step All Bond SE
Two-step All Bond SE
Manual
9.2 ± 4.9
6.1 ± 3.2
7.4 ± 4.2A
Sonic
3.0 ± 0.9
4.8 ± 3.6
4.0 ± 2.8B
Means identified with different superscript letters are statistically different (p < 0.05).
Table 3 Number and percentage (%) of specimens distributed according to the fracture mode and premature failures for each experimental condition Adhesive One-step All Bond SE
Two-step All Bond SE
Fracture mode
Manual
Sonic
A/M
87 (82.6)
112 (81.8)
CD
0 (0)
1 (0.7)
CR
0 (0)
1 (0.7)
PF
34 (17.4)
23 (17.2)
A/M
104 (89.7)
96 (81.4)
CD
0 (0)
0 (0.0)
CR
0 (0)
0 (0.0)
PF
12 (10.3)
22 (18.6)
A/M: adhesive/mixed; CD: cohesive in dentin; CR: cohesive in resin; PF: premature failures.
Table 4 Overall mean μTBS values and the respective standard deviations (MPa) obtained in each experimental condition Application technique
Adhesive One-step All Bond SE
Two-step All Bond SE
Manual
31.5 ± 8.3a
27.9 ± 7.8a
Sonic
28.3 ± 6.4a
31.3 ± 4.6a
Means identified with identical superscript letters are statistically similar (p > 0.05).
Microtensile Bond Strength The majority of the specimens showed adhesive/mixed failures, irrespective of the experimental group. Cohesive failures were observed in only a few specimens (Table 3). Two-way ANOVA revealed that the cross-product interaction as well as the main factors were not statistically 438
significant (p > 0.05). The application mode and the application of a hydrophobic coating did not affect the μTBS of the adhesive All Bond SE (Table 4, p > 0.05).
DISCUSSION The results of the present study showed that sonic application was not able to increase the microtensile bond strength values of the one- and two-step self-etching adhesives, which led us to accept the first null hypothesis. On the other hand, the permeability of the dentin was reduced when the adhesive All Bond SE, in one or two steps, was applied with sonic vibration. This led us to reject the second null hypothesis. In accordance with previous studies,17,27 the current findings showed that the application and light curing of a self-etching adhesive used in a one-step and two-step mode to dentin does not prevent fluid passage across the bonding interface. On the other hand, the sonic application mode significantly decreased the fluid conductance of the adhesive system independent of the bonding protocol tested. Sonic vibration propagates pressure waves due to the stimulation of the adhesive molecules. This movement promotes fluid agitation and increases monomer diffusion inward. Additionally, this movement enables solvent molecules trapped between high molecular-weight monomers to be brought to the adhesive surface, thus facilitating their evaporation.8,24 The benefits of this application mode were observed in a very recent study which reported lower nanoleakage at the immediate and 6-month periods when one-step adhesives were applied with this same sonic device.23 These findings were not surprising, since earlier studies reported that the active and vigorous manual application of adhesives could reduce the nanoleakage at the immediate period11 and after water storage for 6 months10 and 3 years.22 From a clinical standpoint, the use of a sonic device can be considered superior to active manual application, as it reduces the finger pressure variations of different operators and can guarantee homogeneous vibration of the adhesive. Contrary to our expectations, sonic application did not yield improved bond strength values. In an earlier study,23 the authors reported that the same sonic device employed here increased the bond strength of two out of three adhesive systems. This suggests that the benefit of this application protocol may be adhesive dependent. Several factors may be responsible for low resin-dentin bond strength of adhesive systems, such as improper demineralization of the substrates and monomer infiltration,22 high solvent retention,30 low conversion degree,15 and adhesive strength,14 etc. The literature reports on the effect of ultrasonic application mode on the bond strength of self-etching adhesives; however, no satisfactory results were found.1,12 It is important to point out that the devices used in those studies worked within ultrasonic frequencies, 25 to 30 kHz12 and 1 MHz,1 while the device proposed in the present study functions in the sonic range (170 Hz). The Journal of Adhesive Dentistry
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Sonic application can allow better adhesive demineralization and infiltration and can aid in solvent evaporation as the adhesive solution is agitated, but it cannot improve the strength of the adhesive layer. This explains why the sonic device did not improve the bond strength of the Futurabond NR adhesive (Voco) in an earlier study.23 As opposed to Futurabond NR, All Bond SE (Bisco) has a higher ultimate tensile strength compared to other competitive products.15 Additionally, All Bond SE employed as one-step or two-step self-etching reached high immediate29,33 and short-term40 resin-dentin bond strengths.29 More recently, high retention rates were observed after 24 months when All Bond SE was applied according to onestep and two-step self-etching protocols.20 Perhaps this material’s properties are already maximized with manual application and therefore no further increase could be detected with sonic application, at least when measured at the immediate period. As is true of application mode, the application of an extra hydrophobic layer improved neither the bond strength nor dentin sealing. Although there are many studies reporting that the hydrophobic resin coating improves the immediate resin-dentin bonds,9,28 reduces degradation over time,28 and reduces the fluid conductance,34 the fact that this approach was not beneficial for some adhesives under in vitro19,28 and in vivo studies31 again suggests that the protocol is adhesive dependent. The findings of a recent publication33 strengthen this hypothesis. Regarding the resin-dentin bond strength, one-step All Bond SE was the only all-in-one adhesive that was statistically similar to two-step All Bond SE29,33 and the gold standard two-step self-etching Clearfil SE Bond.29 This product is a two-component self-etching system where water and acidic monomers are supplied in separate containers. It is well known that one-component self-etching systems are chemically unstable and are more prone to degrade into the bottles, which might jeopardize their bonding efficacy.25 Whether or not this could have influenced the bond strength results of the present study must be determined in further investigations. Although sonic application did not improve the bond strength of the adhesives tested, it did not reduce the materials’ performance in the present and earlier studies.23 Furthermore, it reduced the flow conductance of the bonded interface and this may have a positive impact on the longevity of the dentin bonds as already reported in studies where the vigorous application mode was employed.22,32 It has already been demonstrated that only “aged” bond strength values are correlated with clinical performance of adhesives.37 A limitation of the present study is that it did not test the specimens after aging; therefore, this should be the focus of future studies. Additionally, it is known that under clinical conditions, there is an outward fluid flow across exposed dentin in response to the low but positive pulpal tissue pressure,7 which is completely absent in extracted teeth and was not simulated in the present study. Whether or not this may affect the study results has yet to be determined. Vol 16, No 5, 2014
Future studies should focus on a wide range of adhesive systems and the longevity of the resin-dentin bonds to provide comprehensive knowledge about the potential benefits of a sonic device for adhesive application.
CONCLUSIONS The sonic application of one- and two-step All Bond SE at an oscillating frequency of 170 Hz can significantly reduce the fluid conductance of dentin, although this did not yield significant improvement in the resin-dentin bond strengths of the adhesive systems.
ACKNOWLEDGMENTS This study was performed by Alexandra Mena Serrano in partial fulfillment of her PhD degree at the State University of Ponta Grossa. This study was partially supported by the National Council for Scientific and Technological Development (CNPq) under grants 301937/ 2009-5 and 301891/2010-9. The authors are grateful for the collaboration of Eugenio Jose Garcia on the microtensile bond experiment. Conflict of interest: R. T. Patzlaff, A. Reis, and A. D. Loguercio are involved in the development of the sonic device employed in this study.
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Clinical relevance: Sonic application mode at an oscillating frequency of 170 Hz can be helpful to reduce the permeability of a self-etching adhesive system.
The Journal of Adhesive Dentistry
