Immediate bonding properties of universal adhesives to dentine
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Miguel AngelMuñozabIssisLuqueaVivianeHassaAlessandraReisaAlessandro DouradoLoguercioaNara Hellen CampanhaBombardaa
Open Access funded by Brazilian Government
Under an Elsevier user license
To evaluate the dentine microtensile bond strength (μTBS), nanoleakage (NL), degree of conversion (DC) within the hybrid layer for etch-and-rinse and self-etch strategies of universal simplified adhesive systems.
forty caries free extracted third molars were divided into 8 groups for μTBS (n = 5), according to the adhesive and etching strategy: Clearfil SE Bond [CSE] and Adper Single Bond 2 [SB], as controls; Peak Universal Adhesive System, self-etch [PkSe] and etch-and-rinse [PkEr]; Scotchbond Universal Adhesive, self-etch [ScSe] and etch-and-rinse [ScEr]; All Bond Universal, self-etch [AlSe] and etch-and-rinse [AlEr]. After restorations were constructed, specimens were stored in water (37 °C/24 h) and then resin–dentine sticks were prepared (0.8 mm2). The sticks were tested under tension at 0.5 mm/min. Some sticks from each tooth group were used for DC determination by micro-Raman spectroscopy or nanoleakage evaluation (NL). The pH for each solution was evaluated using a pH metre. Data were analyzed with one-way ANOVA and Tukey’s test (α = 0.05).
For μTBS, only PkSe and PkEr were similar to the respective control groups (p > 0.05). AlSe showed the lowest μTBS mean (p < 0.05). For NL, ScEr, ScSe, AlSe, and AlEr showed the lowest NL similar to control groups (p < 0.05). For DC, only ScSe showed lower DC than the other materials (p < 0.05).
Performance of universal adhesives was shown to be material-dependent. The results indicate that this new category of universal adhesives used on dentine as either etch-and-rinse or self-etch strategies were inferior as regards at least one of the properties evaluated (μTBS, NL and DC) in comparison with the control adhesives (CSE for self-etch and SB for etch-and-rinse).
Microtensile bond strengthNanoleakageDegree of conversionEtch-and-rinseSelf-etchUniversal simplified adhesive systems
The bonding mechanism of adhesive systems basically involves the replacement of minerals removed from the hard dental tissue by resin monomers, in such a way that a polymer becomes micro-mechanically interlocked to the dental substrate.1 However, the adhesive systems available on the market can be classified into two categories: etch-and-rinse (Er) and those applied using self-etch strategies (Se), in versions of three (only Er), two or one application step.2, 3
When using the Er strategy, the first step involves the application of a phosphoric acid gel to both dental substrates, which allows removal of the smear layer, exposure of the collagen fibrils in dentine, and increase in surface area and surface energy in the enamel substrate. The primer is then applied (second step) followed by the bond (third step) resin separately or in a single solution.2, 3, 4 Irrespective of the number of steps, the main disadvantage of the Er system, mainly two-step versions, is that there is risk of collagen fibre collapse during the process of demineralized dentine drying, which leads to a decrease in bond strength.5, 6 The collagen collapse is prevented by keeping demineralized dentine moist, which is a difficult task to perform clinically. In fact, adequate moisture depends on both the solvent used in the material7 and on the clinician’s interpretation of the manufacturer’s directions.
The incomplete impregnation of collagen fibers8 and the need to protect them against the degrading mechanisms present in the oral cavity environment,9, 10 led to the development of the second category, an adhesive using the self-etch strategy.
In the Se strategy (one-step or two-step), there is no need to apply a preliminary phosphoric acid gel on dental substrates as dentine demineralization and priming occur simultaneously.3, 11 The dissolved hydroxyapatite crystals and residual smear layer are incorporated in the hybridized complex.3, 12 Except for very acidic Se systems,13, 14 the whole extension of the demineralized dentine depth is impregnated by resin monomers, which may be the reason why Se systems are not associated with the technique sensitivity characteristic of bonding to moist etched dentine.7, 15, 16 This advantage makes Se materials suitable for areas where adequate control of moisture is rather difficult, such as in posterior restorations.
A clear disadvantage of the Se protocol is the reduction in enamel bonding effectiveness.17, 18 The increase in surface area in intact and ground enamel obtained with Se adhesives is lower than that achieved with phosphoric acid, and it depends on the pH of the Se adhesive.18 The performance of Se adhesives has improved when these systems were applied to phosphoric acid-treated enamel.12, 19, 20 However, this procedure has been shown to be unsuitable for use on the dentine substrate,21, 22, 23 because accidental dentine etching may occur during the enamel-etching process, particularly when a low-viscosity etchant is used. The effect of intentionally etching dentine with phosphoric acid prior to the application of self-etch adhesives has been studied.21, 23, 24, 25, 26 The results are controversial and material-dependent.
Considering the differences in professional judgement regarding the selection of the adhesive strategy and number of steps, some manufacturers have released more versatile adhesive systems that include etch-and-rinse (two step) and self-etch (one or two step) options. These new materials are called “Universal”, “Multi-purpose” or “Multi-mode” adhesives.23, 27 There is little information in the literature about the performance of this new class of universal adhesives.23, 27 Thus, this study compared the immediate microtensile bond strengths (μTBS), nanoleakage (NL), in situ degree of conversion (DC) of three universal adhesives applied to dentine according to the etch-and-rinse and the self-etch strategies. The two-step etch-and-rinse, Adper Single Bond 2 (SB, 3M ESPE, St. Paul, MN, USA), and two-step self-etch, Clearfil SE Bond (CSE, Kuraray, Okayama, Japan) were also evaluated as control groups. The following null hypotheses were tested in this study: (1) universal adhesives applied to dentine according to the Er and the Se strategies when compared to their respective control groups do not affect the immediate resin–dentine bond strength; (2) universal adhesives applied to dentine according to the Er and the Se strategies when compared to their respective control groups do not affect the immediate silver nitrate deposition and (3) universal adhesives applied to dentine according to the Er and the Se strategies when compared to their respective control groups do not affect the degree of conversion of the adhesives.
2. Materials and methods
2.1. Tooth selection and preparation
Forty extracted, caries-free human third molars were used. The teeth were collected after obtaining the respective patients’ informed consent under a protocol approved by the local Ethics Committee Review Board. The teeth were disinfected in 0.5% chloramine, stored in distilled water and used within six months after extraction. A flat dentine surface was exposed after wet grinding the occlusal enamel on a #180 grit SiC paper. The exposed dentine surfaces were further polished on wet #600-grit silicon-carbide paper for 60 s to standardize the smear layer.
2.2. Experimental design
The teeth were randomly assigned into eight groups (n = 5) according to the different bonding strategies of the selected adhesive system. As control materials, the 2-step etch-and-rinse (Er), Adper Single Bond 2 (SB, 3M ESPE, St. Paul, MN, USA); and the 2-step self-etch (Se), Clearfil SE Bond (CSE, Kuraray, Okayama, Japan) were used. The following three universal adhesive systems were tested: Peak Universal Adhesive System (Peak LC Bond and Peak SE Primer, Ultradent Products Inc., South Jordan, UT, USA), applied as a 2-step Er (PkEr) and 2-step Se (PkSe); Scotchbond Universal Adhesive (3M ESPE, St. Paul, MN, USA), applied as a 2-step Er (ScEr) and 1-step Se (ScSe); and All Bond Universal (Bisco Inc., Shaumburg, IL, USA) applied as a 2-step Er (AlEr) and 1-step Se (AlSe).
2.3. Restorative procedure and specimen preparation
The adhesive systems were applied strictly in accordance with the respective manufacturer’s instructions, described in Table 1. After the bonding procedures, all teeth received a microhybrid composite restoration (Opallis, FGM Produtos Odontológicos, Joinville, SC, Brazil) in two increments of 2 mm. Each increment was light polymerized for 40 s using a LED light curing unit set at 1200 mW/cm2 (Radii-cal, SDI Limited, Bayswater, Victoria, Australia).
Table 1. Adhesive system (batch number), composition and application mode* of the adhesive systems used (*) according to the manufacturer’s instructions.
|Adhesive system (batch number)||Composition||Self-etch strategy||Etch-and-rinse strategy|
|Adper Single Bond 2 (BPBR)||1. Etchant: 35% phosphoric acid (Scotchbond Etchant)
2. Adhesive: bis-GMA, HEMA, dimethacrylates, ethanol, water, photoinitiator, methacrylate functional copolymer of polyacrylic and poly(itaconic) acids,10% by weight of 5 nm-diameter spherical silica particles
|N.A||1. Apply etchant for 15 s
2. Rinse for 10 s
3. Blot excess water
4. Apply 2–3 consecutive coats of adhesive for 15 s with gentle agitation
5. Gently air dry for 5 s
6. Light polymerize for 10 s at 1200 mW/cm2
|Clearfil SE Bond (Primer: 00954 A – Bond: 01416A)||1. Primer: water, MDP, HEMA, camphorquinone, hydrophilic dimethacrylate
2. Bonding: MDP, bis-GMA, HEMA, camphorquinone, hydrophobic dimethacrylate, N,N-diethanol p-toluidine bond, colloidal silica
|1. Apply primer to tooth surface and leave in place for 20 s
2. Dry with air stream to evaporate the volatile ingredients
3. Apply bond to the tooth surface and then create a uniform film using a gentle air stream
4.Light polymerize for 10 s at 1200 mW/cm2
|Peak Universal Adhesive System (Peak SE Primer: 0N062 – Peak LC Bond: Y062)||1. Etchant: 35% phosphoric acid (Ultraetch)
2. Peak SE Primer: ethyl alcohol, methacrylic acid, 2-hydroxyethyl methacrylate (Peak SE Primer)
3. Peak LC Bond resin: ethyl alcohol, 2-hydroxyethyl methacrylate (Peak LC Bond)
|1. Initial use of Peak SE requires activation of the two components separated in the syringe
2. Application of the Peak SE with microbrush for 20 s using continuous scrubbing on dentine. Do not scrub enamel
3. Thin/dry for 3 s using air/water syringe or high volume suction directly over preparation
4. Apply a puddle coat of Peak LC Bond and gently agitate for 10 s
5. Thin/dry 10 s using ¼ to ½ air pressure
6. Light polymerize for 10 s at 1200 mW/cm2
|1. Apply etchant for 20 s
2. Rinse for 5 s
3. Air dry 2 s
4. Apply a puddle coat of Peak LC Bond with gently agitate for 10 s
5. Dry 10 s using ¼ to ½ air pressure
6. Light polymerize for 10 s at 1200 mW/cm2
|Scotchbond Universal Adhesive (D-82229)||1. Etchant: 34% phosphoric acid, water, synthetic amorphous silica, polyethylene glycol, aluminium oxide. (Scotchbond Universal Etchant)
2. Adhesive: MDP phosphate monomer, dimethacrylate resins, HEMA, methacrylate-modified polyalkenoic acid copolymer, filler, ethanol, water, initiators, silane
|1. Apply the adhesive to the entire preparation with a microbrush and rub it in for 20 s. If necessary, rewet the disposable applicator during treatment
2. Direct a gentle stream of air over the liquid for about 5 s until it no longer moves and the solvent has evaporated completely
3. Light polymerize for 10 s
|1. Apply etchant for 15 s
2. Rinse for 10 s
3. Air dry 2 s
4. Apply adhesive as for the self-etch mode
|All-Bond Universal (1200006111)||1. Etchant Uni-Etch: 32% phosphoric acid, benzalkonium Chloride
2. Adhesive: MDP, bis-GMA, HEMA, ethanol, water, initiators
|1. Apply two separate coats of adhesive, scrubbing the preparation with a microbrush for 10–15 s per coat. Do not light polymerize between coats
2. Evaporate excess solvent by thoroughly air-drying with an air syringe for at least 10 s, there should be no visible movement of the material. The surface should have a uniform glossy appearance
3. Light polymerize for 10 s at 1200 mW/cm2
|1. Apply etchant for 15 s
2. Rinse thoroughly
3. Remove excess water with absorbent pellet or high volume suction for 1–2 s
4. Apply adhesive as for the self-etch mode
After the restored teeth had been stored in distilled water at 37 °C for 24 h, the specimens were sectioned longitudinally in the mesio-distal and buccal-lingual directions across the bonded interface, using a slow-speed diamond saw (Isomet, Buehler Ltd., Lake Bluff, IL, USA) to obtain 15–20 resin–dentine sticks with a cross sectional area of approximately 0.8 mm2 measured with a digital calliper (Digimatic Calliper, Mitutoyo, Tokyo, Japan).
All the specimens from each tooth were used for the microtensile bond strength evaluation (μTBS), except for six that were randomly selected. These six resin–dentine bonded specimens were divided for measurement of the in situ degree of conversion (DC) and nanoleakage (NL).
2.4. Microtensile bond strength test (μTBS)
Resin–dentine bonded sticks were attached to a Geraldeli’s jig28 with cyanoacrylate adhesive and tested under tension (Kratos Dinamometros, Cotia, SP, Brazil) at 0.5 mm/min until failure. The μTBS values were calculated by dividing the load at failure by the cross-sectional bonding area.
The failure mode of the specimens was classified as cohesive ([C] failure exclusive within dentine or resin composite), adhesive ([A] failure at resin/dentine interface), or mixed ([M] failure at resin/dentine interface, which included cohesive failure of the neighbouring substrates). The classification was performed under a stereomicroscope at 100× magnification (Olympus SZ40, Tokyo, Japan). Specimens with premature failures (PF) were included in the tooth mean.
2.5. Degree of conversion in situ (DC)
Three resin–dentine bonded sticks from each tooth were wet polished with #1500, 2000 and 2500-grit SiC paper. Then they were ultrasonically cleaned for 20 min and stored in water at 37 °C for 24 h, before taking the DC readings. The micro-Raman spectroscopy analysis was performed using the Senterra equipment (Bruker Optik GmbH, Ettlingen, Baden-Württemberg, Germany). The micro-Raman spectrometer was first calibrated by resetting to zero, and then for coefficient values using a silicon specimen. Specimens were analyzed using the following micro-Raman parameters: 20 mW Neon laser with 532 nm wavelength, spatial resolution of ≈3 μm, spectral resolution ≈5 cm−1, accumulation time of 30 s with 6 co-additions, and 110× magnification (Olympus UK, London, UK) to a ≈1 μm beam diameter. Spectra were taken at the dentine–adhesive interface at three different sites within intertubular dentine for each resin–dentine bonded stick. The average value of the measurements taken from the same tooth was used for statistical purposes. Spectra of unpolymerized adhesives were taken as reference. Post-processing of spectra was performed using the dedicated Opus Spectroscopy Software version 6.5. The ratio of double-bond content of monomer to polymer in the adhesive was calculated according to the following formula:
where “R” is the ratio of aliphatic and aromatic peak areas at 1639 cm−1 and 1609 cm−1 in polymerized and unpolymerized adhesives. The in situ DC of all resin–dentine bonded sticks from the same tooth was averaged for statistical purposes.
2.6. Nanoleakage (NL) evaluation
Three resin-bonded sticks, from each tooth, were used for NL evaluation. Ammoniacal silver nitrate was prepared according to the protocol previously described by Tay et al.29 The sticks were placed in the ammoniacal silver nitrate solution in darkness for 24 h, rinsed thoroughly in distilled water, and immersed in photo developing solution for 8 h under a fluorescent light to reduce silver ions into metallic silver grains within voids along the bonded interface. Specimens were polished with a wet #600, 1000, 1200, 1500, 2000 and 2500-grit SiC paper and 1 and 0.25 μm diamond paste (Buehler Ltd., Lake Bluff, IL, USA) using a polishing cloth. They were ultrasonically cleaned, air dried, mounted on stubs, and coated with carbon-gold (MED 010, Balzers Union, Balzers, Liechtenstein). Resin–dentine interfaces were analyzed in a field-emission scanning electron microscope operated in the backscattered mode (LEO 435 VP, LEO Electron Microscopy Ltd., Cambridge, UK).
Three images were captured of each resin–dentine bonded stick. The relative percentage of NL within the adhesive and hybrid layers in each specimen was measured in all images using the UTHSCSA ImageTool 3.0 software (Department of Dental Diagnostic Science at The University of Texas Health Science Centre, San Antonio, Texas) by a blinded researcher. The values originating from the same specimen were averaged for statistical purposes. The mean NL of all sticks from the same tooth was averaged for statistical purposes.
2.7. pH measurement
The pH of the adhesive systems was measured with a pH metre E520 (Metrohm, Herisau, Switzerland). Three blends were prepared by mixing each adhesive solution and deionized water in a proportion of 1:9 (by weight). This blend was kept stored in darkness for 12 h to allow uptake of H+. For each blend, three readings were performed at room temperature, in darkness and a mean was obtained.
2.8. Statistical analysis
Data from μTBS, NL, DC and pH were analyzed by one-way ANOVA and Tukey’s post hoc test at α = 0.05.
3.1. Microtensile bond strength test
The overall values of all tested properties are shown in Table 2. Significant differences were detected in the mean μTBS among the adhesives tested (p < 0.05). Only the universal adhesive PK, used in both the Er and Se approach, showed mean μTBS statistically similar to those of the controls SB and CSE respectively (p > 0.05). The other universal materials resulted in lower mean μTBS than those of the Er and Se controls (p < 0.05). The lowest mean μTBS was observed for AISe (p < 0.05).
Table 2. Microtensile bond strength (μTBS), nanoleakage (NL) and degree of conversion (DC) values (means ± standard deviations) of the different experimental groups.*
|Adhesive system||Application mode||Test|
|μTBS (MPa)||NL (%)||DC (%)|
|Adper Single Bond 2||Etch-and-rinse||49.3 ± 4.6A||12.4 ± 2.5b||85.4 ± 4.7a|
|Clearfil SE Bond||Self-etch||43.0 ± 4.5A,B||7.6 ± 2.a||87.7 ± 2.8a|
|Peak Universal Adhesive System||Etch-and-rinse||43.6 ± 4.6A,B||23.4 ± 5.9c||89.2 ± 6.3a|
|Self-etch||39.9 ± 4.5B,C||34.4 ± 11c||78.9 ± 9.5a|
|Scotchbond Universal Adhesive||Etch-and-rinse||35.1 ± 6.6B,C||5.1 ± 2.5a||88.3 ± 5.6a|
|Self-etch||32.4 ± 4.5C||5.5 ± 3.2a||69.1 ± 9.8b|
|All Bond Universal||Etch-and-rinse||39.3 ± 3.7B||9.3 ± 2.9b||77.9 ± 0.1a|
|Self-etch||13.4 ± 1.9D||6.2 ± 4.1a||77.8 ± 0.1a|
*Similar capital (μTBS), lower (NL) and superscript letter (DC) are not statistically significant (p < 0.05)
The majority of the specimens (83.1%) showed adhesive/mixed failures. Cohesive failures were observed in 8.3% of the specimens. A small number of premature failures (8.6%) were observed in the present study (Table 3).
Table 3. Number and percentage of specimens (%) according to the fracture mode and premature failures of all experimental group.
|Adhesive system||Application mode||Fracture pattern|
|Adper Single Bond 2||Etch-and-rinse||50 (56.8)||18 (20.5)||8 (9.1)||12 (13.6)|
|Clearfil SE Bond||Self-etch||66 (76.7)||4 (4.7)||8 (9.3)||8 (9.3)|
|Peak Universal Adhesive System||Etch-and-rinse||60 (68.2)||6 (6.8)||12 (13.6)||10 (11.4)|
|Self-etch||56 (66.7)||8 (9.5)||12 (14.3)||8 (9.5)|
|Scotchbond Universal Adhesive||Etch-and-rinse||64 (82.1)||0 (0)||12 (15.4)||2 (2.5)|
|Self-etch||58 (76.3)||4 (5.3)||12 (15.8)||2 (2.6)|
|All Bond Universal||Etch-and-rinse||46 (60.5)||10 (13.2)||16 (21.1)||4 (5.2)|
|Self-etch||52 (66.7)||0 (0)||20 (25.6)||6 (7.7)|
3.2. Nanoleakage evaluation
Significant differences were observed among the adhesives tested (p = 0.0001; Table 2 and Fig. 1). PKEr and PKSe adhesives showed the highest percentage of NL (p < 0.05). AlEr and AlSe adhesives were statistically similar to their respective control groups (SB and CSE; p > 0.05). Scotchbond Universal (ScEr and ScSe) showed the lowest nanoleakage (p < 0.05); in the Er approach, the NL was lower than that of SB; and in the Se approach NL was similar to that of CSE.
Fig. 1. Representative backscattered SEM images of the resin–dentine adhesive interfaces of each experimental group (Control etch-and-rinse group = Adper Single Bond 2 and control self-etch group = Clearfil SE Bond). The amount of nanoleakage was lower and practically occurred within the hybrid layer for Scotchbond Universal (C, G); All Bond Universal (D, H) and controls (white arrows) (A, E). For Peak Universal Adhesive System, the amount of nanoleakage was higher than the other materials; with most silver nitrate uptake occurring throughout the entire thickness of the HL (white arrows) and in the AL, forming the so-called “water trees (white hands) (B, F).
3.3. Degree of conversion
All materials showed similar DC (p > 0.05, Table 2), except for ScSe, which showed a statistically lower DC compared with the other materials (p = 0.01).
3.4. pH measurement
The pH of the adhesives is shown in Table 4. SB showed the highest pH (4.1) and the Peak LC Bond showed the lowest (1.2).
Table 4. The pH values (means ± standard deviations) of each adhesive system.
|Adper Single Bond 2||4.1 ± 0.02|
|Clearfil SE Bond||Primer||2.1 ± 0.01|
|Bond||2.6 ± 0.08|
|Peak Universal Adhesive System||Peak SE Primer||1.2 ± 0.01|
|Peak LC Bond resin||2.0 ± 0.04|
|Scotchbond Universal Adhesive||3.0 ± 0.05|
|All Bond Universal||2.4 ± 0.05|
Although all the new universal adhesive systems tested share the same versatility of being used in the Er and Se approaches, there are differences in their compositions and number of steps, which seems to be the key reason for the different performance of these materials when compared with the controls.
The 2-step Se CSE and, the 1-step Se ScSe and AlSe have a pH within a similar range (around 2). Se adhesives within this pH range partially demineralize dentine, leaving a substantial amount of hydroxyapatite crystals around the collagen fibrils.30 As these three materials have 10-methacryloyloxydecyl dihydrogen phosphate monomer (MDP) in their composition, they bond chemically to dentine.31, 32, 33 Yoshida et al.34 showed that an effective chemical interaction occurs between MDP and hydroxyapatite forming a stable nano-layer that could form a stronger phase at the adhesive interface, which increases the mechanical strength of the adhesive interface. In addition, stable MDP-Ca salt deposition along with nano-layering may explain the high bond stability,34, 35 which has previously been proven both in laboratory and clinical research.33, 36, 37
Based on these similarities one would expect that these two universal adhesives (ScSe and AlSe) would be equivalent to the control 2-step Se CSE; however, this was not observed in the present investigation. The slight difference in terms of μTBS between CSE and ScSe may be due to the presence of polyalkenoic acid copolymer in the composition of ScSe and this leads to partially reject the 1st. null hypothesis. This copolymer may have competed with the MDP by binding to the calcium of the hydroxyapatite.34 In addition to impairing the bond of MDP to dentine, the polyalkenoic acid copolymer could have prevented monomer approximation during polymerization, due to its high molecular weight. Consequently a lower DC was observed for this material in comparison with the others leads to partially reject the 3rd. null hypothesis. Furthermore, ScSe and AlSe are 1-step self-etch adhesives and this probably makes the MDP concentration lower than that of CSE, which has this monomer incorporated into both the primer and bond components.34
Among all Se adhesives, AlSe showed the poorest performance in terms of μTBS, which might be attributed to its application mode. AlSe was the only material that was not applied actively on dentine, as the materials were applied in accordance with the respective manufacturer’s directions. Previous literature findings clearly pointed out that active application improves the bonding performance of Se adhesive systems to dentine.38, 39 Future studies need to be conducted to test this hypothesis.
Among adhesives that are used with the Se strategy, PKSe provided μTBS values similar to those of the control CSE. The placement of an additional layer has been shown to increase the performance of 1-step Se materials both in vitro37, 40, 41, 42 and clinically.43, 44 This is also a disadvantage of ScSe and AlSe. In general, 1-step SE adhesives present inferior performance to 2-step Se adhesives.45, 46 Usually, 1-step Se adhesives result in thinner adhesive layers, which are likely to be prone to polymerization inhibition by oxygen.47With regard to the Er approach, the universal adhesives showed improved bond strength in comparison with the Se strategy, in spite of ScPK being statistically similar to the ScSe. It is known that the smear layer constitutes a true physical barrier and makes it extremely difficult for the bonding and hybrid layer formation to be fully integrated with the dentine.48 After preliminary etching with phosphoric acid, the smear layer is removed and superficial dentine is demineralized. This increases impregnation by the adhesive, allowing the creation of a well impregnated hybrid layer, without modifying the potential for interfacial nanoleakage, as has been shown by several authors when a 1-step self-etch adhesive was applied on phosphoric acid etched dentine.23, 25, 26, 27
NL represents the location of defects at the resin–dentine interface that might serve as the pathway for degradation of resin/dentine bonds over time.49 Silver nitrate is capable of occupying the nanometer-sized spaces around naked collagen fibrils, where resin failed to infiltrate, or where residual water has not been displaced by adhesive or even in areas of incomplete monomer conversion.49
Among all the adhesives, PKEr and PKSe showed the highest NL at the adhesive interfaces, and this leads to partially reject the 2nd. null hypothesis. In the case of PkSe, Peak SE primer has a low pH value (pH = 1.2), which is within the range of the most aggressive Se adhesives.30, 45 It has previously been reported that unpolymerized, acidic and aggressive monomers from Se adhesives are able to continue to demineralize the dentine even after polymerization.13, 14 This is probably the result of hydrolysis of the ester bond from the acid monomer that results in a strong phosphoric acid14 and might explain the intense silver nitrate deposition within the hybrid layer.
Although one could expect reduced NL within the adhesive layer due to the fact that PKSe is a 2-step Se, the second adhesive layer seems to be as hydrophilic as the primer, since no hydrophobic monomer was listed in the composition of the Peak LC Bond (Table 1). Thus, although increased impregnation of resin monomers might have occurred during the application of an additional adhesive coat, the nature of this impregnation is hydrophilic, which resulted in extensive NL in the hybrid layer. In fact, Peak LC bond (pH = 2.0) may cause an additional etching and an increased conditioning pattern of the dentine substrate. This is likely to result in increased demineralization and production of a hydroxyapatite-denuded, collagen-rich, network,45, 50, 51 increasing the risk of NL 10, 52, 53, 54 for PkEr.
Taking all the properties together, none of the materials showed similar behaviour to the controls (a gold standard two-step self-etch Clearfil SE Bond and a gold standard two-step etch-and-rinse Adper Single Bond 2) leads to reject all null hypothesis tested. Future studies need to be conducted to evaluate the long-term performance of this new category of adhesives.
Within the limitations of the present study, the results indicate that when the universal adhesives were tested using the self-etch or etch-and-rinse strategy on dentine, they were inferior to the respective controls (Clearfill SE Bond, a two-step etch-and-rinse or Adper Single Bond 2, a two-step etch-and-rinse) with respect to at least one of the properties tested (microtensile bond strength, nanoleakage and in situ degree of conversion.
This study was partially supported by the National Council for Scientific and Technological Development (CNPq) (grants # 301937/2009-5 and # 301891/2010-9).The authors would like to thank to the manufacturers for the donation of adhesive systems employed in this study.
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