Short- and Long-term Evaluation of Dentin-Resin Interfaces
Formed by Etch-and-Rinse Adhesives on Plasma-treated
Dentin
Ronaldo Hirata
a/ Camila Sampaio
b/ Lucas S. Machado
c/ Paulo G. Coelho
d/ Van P. Thompson
e/
Simone Duarte
f/ Ana Paula Almeida Ayres
g/ Marcelo Giannini
hPurpose: To investigate the influence of atmospheric pressure plasma (APP) treatment on the microtensile dentin
bond strength of two etch-and-rinse adhesive systems, after one week and one year of water storage, and addition-ally to observe the micromorphology of resin/dentin interfaces under scanning electronic microscopy (SEM).
Materials and Methods: The occlusal enamel was removed from third human molars to expose a flat dentin
sur-face. The teeth were then randomly divided into six groups (n = 7), according to two adhesives (Optibond FL and XP-Bond) and three APP treatments (untreated dentin [control], APP application before or after acid etching). After performing the composite resin buildup on bonded dentin, the teeth were sectioned perpendicularly to the bonded interface to obtain beam-shaped specimens (cross-sectional area of ~0.9 mm2). The specimens were tested in
ten-sion until failure after one week and one year of water storage (1.0 mm/min rate). Bond strength data were ana-lyzed by three-way ANOVA and Tukey’s post-hoc test (α = 0.05%). Bonded beam specimens from each tooth were also prepared for interfacial SEM investigation.
Results: At one week, APP treatment applied after acid etching increased the dentin bond strength for XP Bond,
while no effect was observed for Optibond FL. After one year, the bond strength of XP Bond decreased in groups where APP was applied after etching. The evaluation time did not influence the bond strength for Optibond FL.
Conclusion: One-year evaluation did not show any sign of degradation of interfacial structures in any group.
Appli-cation of APP to etched dentin combined with a two-step etch-and-rinse adhesive significantly increased bond strength at one week, but the effect was not stable after one year and was adhesive dependent.
Keywords: plasma, microtensile, dentin adhesion.
J Adhes Dent 2016; 18: 215–222. Submitted for publication: 27.08.15; accepted for publication: 09.03.16 doi: 10.3290/j.jad.a36134
a Assistant Professor, Department of Biomaterials and Biomimetics, New York University, College of Dentistry, New York, USA. Experimental design, per-formed the experiments, wrote the manuscript.
b PhD Student, Department of Restorative Dentistry, State University of Campi-nas, Piracicaba Dental School, Piracicaba, SP, Brazil. Performed the fracture mode analysis.
c PhD Student, Department of Restorative Dentistry, São Paulo State University, Araçatuba Dental School, Araçatuba, SP, Brazil. Performed the microtensile text. d Associate Professor, Department of Biomaterials and Biomimetics, New York
University, College of Dentistry, New York, USA. Idea, experimental design, contributed to the discussion.
e Associate Professor, Department of Biomaterials, Biomimetics and Biophoton-ics, Kings College London Institute, London, UK. Idea, contributed to the dis-cussion and analysis.
A
dhesion to tooth structures has been a well-established procedure in restorative dentistry since the first experi-ment using phosphoric acid on dental enamel prior to application of acrylic resins.7 The bonding to enamel ispro-vided by interlocking of monomers applied on micro-retentions created by partial dissolution of enamel rods by
phosphoric acid. On dentin, the bonding mechanism relies on the penetration of adhesive monomers into demineralized dentin, creating an interface called the hybrid layer.54 The
hybridization process is characterized as a three-dimen-sional collagen-resin biopolymer that provides a continuous and stable link between restoration and dentin.2
f Associate Professor, Department of Basic Sciences, New York University, Col-lege of Dentistry, New York, USA. APP treatment, reviewed manuscript. g PhD Student, Department of Restorative Dentistry, State University of
Campi-nas, Piracicaba Dental School, Piracicaba, SP, Brazil. Performed the SEM analysis.
h Associate Professor, Department of Restorative Dentistry, State University of Campinas, Piracicaba Dental School, Piracicaba, SP, Brazil. Hypothesis, ex-perimental design, statistical analysis, mentored project, reviewed manu-script.
Correspondence: Ronaldo Hirata, 433 First Avenue, 10010, Dept. Biomaterials
and Biomimetics, New York University, New York, NY, USA. Tel: +1-212-998-9214; e-mail: rh1694@nyu.edu
For etch-and-rinse adhesives, previous etching of the enamel and dentin is mandatory. In dentin, phosphoric acid etching promotes hydroxyapatite removal from intra- and intertubular dentin up to a depth of 12 μm.47 The main
con-cerns related to bonding durability are still the structural stability of adhesive polymers and the enzymatic degrada-tion by metalloproteinase enzymes of poorly infiltrated col-lagen fibrils that are exposed after water rinsing of acid from the etching procedure.9,14,35,51,58,59 Approaches to
inhibiting metalloproteinase activity and reducing the colla-gen degradation include the development of new mono-mers,28,53 the use of mild self-etching adhesives,56
applica-tion of chlorhexidine after etching,50 the use of EDTA as
etchant,42 and the application of natural or synthetic
cross-linkers after etching.4
In order to completely fill the interfibrilar spaces created by acid etching and improve the bonding durability, low mo-lecular-weight monomers such as 2-hydroxyethyl methacry-late (HEMA) are used in adhesive compositions.34 In
addi-tion, chemical, electrical or biological modification of dentin has been suggested.4,38 The rationale for plasma treating
dentin before applying an adhesive system would be to im-prove adhesive infiltration into demineralized dentin by al-tering the dentin surface energy and dentin matrix wettabil-ity. It is also speculated that enhanced dentin interactions with adhesive monomers may occur after plasma surface modification.60
When a gas is exposed to an electric or electromagnetic field, it is ionized, that is, it becomes a plasma of elec-trons, ions, and neutrons.3,24 Studies have reported
modi-fications of surfaces by application of low temperature at-mospheric-pressure plasma (APP).46,49 In dentistry, the use
of APP is promising for many reasons, including the ability to disinfect contaminated tooth structures,16,33 enhance
the effectiveness of bonding procedures,45 and treat
or-ganic structures such as the dental tissues, increasing their surface wettability.49 Plasma can also induce
polymer-ization; the polymers synthesized by plasma exposure have demonstrated high cross linking and a higher degree of con-version.11,17,21 Additionally, APP treatment does not
signifi-cantly heat the tooth surface: the temperature generated on the surface by the plasma jet is approximately 32°C to 40ºC.12,26,41
APP dentin treatment has been recently examined, with results indicating improved resin adhesive penetration.10,60
Previous studies found a significant increase in bond strength in peripheral dentin surrounding the tooth’s center, but no improvement in the tooth’s central dentin area was observed.40 High immediate bond strength of etch-and-rinse
adhesives has been reported,15 while the analysis of
self-etching adhesives in APP-treated dentin demonstrated that the APP effect was material dependent.22
The aim of this study was to determine the short- (one week) and long-term (one year) effect of APP treatment, be-fore or after dentin etching, on the bond strength and micro-morphology of plasma-treated dentin/resin interfaces of two etch-and-rinse adhesive systems. The hypothesis tested was that the application of plasma on the dentin surface affects the bond strength of etch-and-rinse adhesives and changes the morphology of the dentin/resin interfaces.
MATERIALS AND METHODS
APP Device
The APP generator used in this study (KinPen 09, Leibnitz Institute for Plasma Research and Technology, INP; Greifs-wald, Germany)39 is a hand-held unit (170 mm long, 20 mm
diameter, 170 g) connected to a high-frequency power sup-ply (frequency 1.1 MHz, 2–6 kV peak-to-peak, 8 W system power) for the generation of a plasma jet at atmospheric pressure. The hand-held unit has a pin-type electrode (1 mm diameter) surrounded by a 1.6-mm quartz capillary. An operating gas consisting of argon at a flow rate of 5 slm was used. The plasma plume emerging at the exit nozzle was about 1.5 mm in diameter and extends into the sur-rounding air for a distance of up to 15 mm, causing no tem-perature change.
Microtensile Bond Strength Test
Forty-two freshly extracted noncarious third molars were ob-tained and used according to the protocol approved by the New York University College of Medicine Institutional Re-view Board. The teeth were cleaned and kept in distilled water at 5°C until use. The occlusal enamel of each tooth was removed perpendicular to the long axis of the tooth
Table 1 Composition and lot number of the etch-and-rinse adhesives tested in this study
Adhesives Optibond FL XP Bond Universal
Composition Primer: 2-hydroxyethyl methacrylate ethanol,
2-(2-(methacryloyloxy)ethoxycarbonyl)benzoic acid, glycerol phosphate dimethacrylate
Adhesive: 2-hydroxyethyl methacrylate, 3-trimethoxysilylpropyl methacrylate, 2-hydroxy-1,3-propanediyl bismethacrylate, alkaline fluorosilicate (Na), 48% filler (fumed SiO2, barium aluminoborosilicate, Na2SiF6)
Carboxylic acid modified dimethacrylate (TCB resin), phosphoric acid modified acrylate resin (PENTA), urethane dimethacrylate (UDMA), triethyleneglycol dimethacrylate (TEG-DMA), 2-hydroxyethylmethacrylate (HEMA), butylated benzenediol (stabilizer), ethyl-4-dimethylaminobenzoate, camphorquinone, functionalized amorphous silica, t-butanol Batch number Primer: 4648047 Adhesive: 4633638 1201000039
with a diamond saw (Buehler; Lake Bluff, IL, USA) to expose a flat dentin surface. Teeth were sectioned at the middle of the anatomic crown, and the dentin surface was dried to verify if any remnants of enamel were still present on the occlusal surface. Afterwards, the cut surfaces were pol-ished with 600-grit SiC papers (Buehler) to remove the rem-nants of enamel and prepare the dentin for bonding proced-ures. The teeth were randomly divided into six groups according to two adhesive systems and the use of APP treatment. Two etch-and-rinse adhesive systems were se-lected for this study (Table 1): Optibond FL (Kerr; Orange, CA, USA) and XP Bond (Dentsply De Trey; Konstanz, Ger-many). The control groups comprised adhesive application according to the manufacturers’ instructions on untreated dentin, while in the experimental groups, the dentin was treated with a continuous APP wave plume for 30 s, before or after acid etching for 15 s (Ultra-etch, Ultradent; South Jordan, UT, USA). The samples were moved horizontally dur-ing plasma application to sweep the entire surface. The plasma tip-to-sample distance was maintained using a de-vice developed to fix the position of the plasma torch during treatment. The experiment was conducted by a calibrated operator trained to perform the experiment in a standard-ized manner in order to minimize bias. Afterwards, the ad-hesive systems were applied and light cured, and a com-posite resin block (Amelogen, Ultradent) was built up 6 mm high on the bonding dentin surfaces. The teeth were stored in distilled water at 37°C for 24 h.27
Teeth were serially sectioned perpendicular to the com-posite/dentin interface with a low-speed diamond saw (Buehler) under water cooling to obtain beam specimens (parallelepiped samples) with a cross-sectional area of ap-proximately 0.9 mm2.40 Twelve beams were obtained from
each tooth. Four specimens were tested immediately after sectioning and four other beams were stored in distilled water for one year before testing. The remaining four beams were used to analyze the micromorphology of resin/dentin interfaces. The cross-sectional areas of all specimens were measured individually with a digital caliper (Mitutoyo Sul Americana; São Paulo, SP, Brazil) before testing. After-wards, each specimen was fixed with cyanoacrylate-based glue (Krazy Glue Gel, Products Advanced Formula, Elmer; Columbus, OH, USA) to a microtensile device that is at-tached to a universal testing machine (EZ test, Shimadzu; Tokyo, Japan). The specimens were tested at a crosshead speed of 1.0 mm/min until failure.
The cross-sectional area of each specimen was divided by the peak tensile load at failure to calculate the bond strength in MPa. A single, mean tensile strength value was calculated for each group by averaging 4 specimens for each tooth (n = 7). After testing for normality of the data distribution and equality of the variances, the bond strength data were subjected to a three-way repeated measures ANOVA (adhesive system, plasma treatment, and storage time) and a post-hoc Tukey’s test (α = 0.05).
Fractured surfaces of tested beams were examined using an optical microscope (Olympus SZX12, Shibuya-ku; Tokyo, Japan) at 9X. Failure patterns were classified as: (1)
adhesive, (2) cohesive within dentin, (3) cohesive within composite resin, and (4) mixed when simultaneously involv-ing adhesive failure and cohesive failure within dentin, the adhesive layer, and/or the resin composite.
Scanning Electron Microscopy (SEM)
Four bonded beams obtained after sectioning were used for micromorphological interfacial analysis. Two of them were analyzed after 1 week of water storage, while the other two beams were stored for 1 year in distilled water before analyz-ing. After storage, beams were embedded in epoxy resin (Buehler; Lake Bluff, IL, USA) and polished with Al2O3 (800-,
1000-, and 1200-grit), followed by diamond pastes (6, 3, and 1 μm). The beams were rinsed with distilled water, and pol-ishing debris was ultrasonically removed during a 5-min ultra-sonic cleaning in distilled water after each polishing step. After polishing, specimens were etched with 50% phosphoric acid for 15 s, washed with distilled water, treated with 0.1% with NaOCl for 10 min and washed again. Afterwards, they were immersed in hexamethydisilazane for 10 min, dried overnight at 37°C, mounted on aluminum stubs, sputter coated with gold (SCD 050, Bal-Tec; Balzers, Liechtenstein), and examined using an SEM (JSM 5600, JEOL; Peabody, MA, USA). Representative areas of the adhesive/dentin inter-faces were photographed at 2000X magnification.
RESULTS
Dentin bond strength means of adhesive systems for the two evaluation periods and according to plasma regimens are presented in Table 2. Three-way ANOVA revealed that there were statistically significant differences for the factors “adhesive system” (p = 0.0006), “plasma treatment” (p = 0.0340), and “storage time” (p = 0.0012). In addition, the interaction “adhesive system” x “plasma treatment” was also statistically significant (p = 0.0166).
The APP dentin treatment after acid etching resulted in significantly increased bond strength for XP Bond at one week (p < 0.05); however, this decreased after one year
Table 2 Bond strength means (standard deviation) as a function of adhesive, type of treatment, and time
Adhesive Treatment Time
1 week 1 year
XP Bond
Control (no plasma) *45.1 (4.6)Aa *42.2 (3.4)Aa Plasma pre etching 45.9 (6.0)Aa *41.0 (6.4)Aa Plasma post etching 60.5 (6.7)Ab 45.0 (9.0)Ba
Optibond FL
Control (no plasma) 57.4 (7.8)Aa 56.4 (6.7)Aa Plasma pre-etching 52.8 (5.8)Aa 52.5 (6.8)Aa Plasma post-etching 55.9 (10.1)Aa 52.4 (8.4)Aa Means followed by different superscript letters (upper case in rows and lower case in columns) were statistically different (p ≤ 0.05). * Differs from Optibond FL for the same treatment and time (p ≤ 0.05).
*
chief failure mode for the XP Bond control group. APP treat-ment prior to acid etching did not change the failure pat-tern, while APP treatment after acid etching substantially increased the adhesive failures at the 1-week evaluation. After 1-year water storage, failure patterns similar to those observed at the 1-week evaluation were observed in the control groups (no plasma treatment), as well as in the APP pre-etching treatment groups. For groups of APP treatment after acid etching, there was a significant increase in cohe-sive failures within composite after one year of storage, and more adhesive failures were observed for both adhesives.
DISCUSSION
The research hypothesis, which tested whether APP applica-tion on dentin would influence bond strengths and interfacial dentin/resin micromorphology regardless of the etch-and-rinse adhesive system used, was rejected. The results showed that APP treatment after acid etching increased the (p < 0.05), and did not differ from other experimental
groups (control and plasma pre-etching) (p > 0.05). For Optibond FL, the APP treatment and evaluation time did not influence the dentin bond strength (p > 0.05). When applied according to the manufacturer’s instructions, Opti-bond FL adhesive showed higher Opti-bond strength than that obtained for XP Bond (p < 0.05).
The adhesive systems formed a hybrid layer with resin tags inside the dentinal tubules, independent of APP appli-cation (Figs 1 and 2). The hybrid layers formed by adhesives after acid etching were approximately 2.5 to 3 μm thick. The hybrid layers created by the experimental (APP dentin treatment) and control techniques showed similar thick-nesses. SEM images of the resin/dentin interfaces of spec-imens stored for one year did not show signs of resin tag degradation.
The distribution of failure modes for the experimental and control groups is presented in Table 3. Cohesive failure within composite resin was the main failure mode observed in the Optibond control group, and adhesive failure was the
AL HL HL D
*
15kV X2,000 10μm XP Control a AL HL HL D*
*
15kV X2,000 10μm XP Plasma b AL HL HL D*
*
15kV X2,000 10μm XP Plasma c AL HL HL D*
*
15kV X2,000 10μm XP Plasma dFig 1 Representative SEM micrograph of the resin/dentin interface formed by the XP Bond, 2000X magnification. a. Untreated dentin, con-trol group: storage in water for one year. b. Plasma before etching, storage in water for one year. c. Plasma after etching, storage in water for one week. d. Plasma after etching: storage in water for one year. AL: adhesive layer; HL: hybrid layer; D: dentin. * Resin tag.
Fig 2 Representative SEM micrographs of the resin/dentin interface formed by the Optibond FL, 2000X. a. Untreated dentin, control group: storage in water for one week. b. Untreated dentin, control group: storage in water for one year. c. Plasma before etching, storage in water for one year. d. Plasma after etching, storage in water for one year. AL: adhesive layer; HL: hybrid layer; D: dentin. * Resin tag.
AL HL HL D
*
15kV X2,000 10μm OptControl a AL HL HL D*
15kV X2,000 10μm OptControl*
b AL HL HL D*
15kV X2,000 10μm Opt Plasma*
c*
AL HL HL D*
15kV X2,000 10μm Opt Plasma d*
Table 3 Percentage of specimens distributed according to the fracture mode for each experimental condition
Adhesive Fracture mode Control Plasma pre-etching Plasma post-etching
1 week 1 year 1 week 1 year 1 week 1 year
XP Bond Adhesive 45.2 18.4 47.6 11.8 84.5 32.6 Mixed 9.5 2.6 16.7 11.8 6.7 13.0 Cohesive in dentin 7.2 10.5 4.8 17.6 4.4 8.7 Cohesive in resin 38.1 68.5 30.9 58.8 4.4 45.7 Optibond FL Adhesive 5.1 0.0 4.9 0.0 72.1 21.0 Mixed 20.5 12.5 9.8 7.3 4.6 15.8 Cohesive in dentin 7.7 22.5 7.3 2.5 7.0 8.0 Cohesive in resin 66.7 65 78.0 90.2 16.3 55.2
dentin bond strength for XP Bond adhesive at one week, but had no influence on bond strength mediated by Optibond FL. APP application before acid etching produced the same re-sults as the respective control groups. Interfacial micromor-phology analysis was similar for all groups, independent of evaluation time. The increase in bond strength for XP Bond was approximately 25% when compared to the groups in which APP was applied before dentin etching and the control group. However, this higher bond strength was not stable after one year of water storage, and no significant differ-ences were observed among XP Bond groups at one year.
Direct exposure to water decreased the bond strength of only one group, which may be related to the bond degrada-tion commonly reported for simplified bonding systems.1,14
Regarding the organic content of dentin, the host-derived proteolytic enzymes trapped within mineralized dentin ma-trix can be activated by the etchant agents or acidic solu-tions, which are responsible for the degradation of collagen fibrils at the resin/dentin interface.36,37 Further studies
should be performed to investigate the effects of APP on matrix metalloproteinase and cysteine cathepsin activity.
Regarding polymer degradation, because simplified (two-step) etch-and-rinse adhesives contain a higher concentra-tion of hydrophilic monomers than do three-step adhesives, the dentin sealing promoted by these simplified adhesives was found to be highly permeable after polymerization. Re-ports have demonstrated that simplified etch-and-rinse ad-hesives form hybrid layers that are more sensitive to aging and clinically less durable.6,52,55 In this study, direct
expo-sure of the resin/dentin interface to water was expected to have a greater effect on samples bonded with the simplified self-etching adhesive (XP Bond).9,48 As opposed to
simpli-fied adhesives, the three-step etch-and-rinse adhesives have been considered the “gold standard” material due to the presence of a hydrophobic bonding resin layer as third step during the adhesive system application.39
Small APP devices are feasible in clinical situations, with safe operating parameters and enough power to increase surface energy.29 Despite the potential to improve
interac-tions between materials and biological systems,13,49 APP
treatment effects on dentin are still unclear.15,40 The
sur-face energy increase22 may enhance monomeric
penetra-tion,60 potentially promoting better interaction among
adhe-sive monomers and the dentin collagen matrix.10 In this
study, plasma treatment before or after acid etching did not improve the dentin bond strength of Optibond FL, and one-year water storage did not affect the dentin bond strength of any Optibond FL groups. Besides the hydrophobic layer, which has a high filler load (48%), the presence of glycerol phosphate dimethacrylate in the primer solution may chem-ically interact with hydroxyapatite, increasing the bond strength.30,44
Furthermore, dentin could benefit from APP treatment due to its known antimicrobial effects, which has several advan-tages over traditional antimicrobial applications, including chlorhexidine. Among these advantages, APP can be used for site-specific treatment, as it provides an almost instanta-neous antimicrobial response, and there is no known
devel-opment of resistance against AAP.25 In fact, APP treatment
mimics aspects of naturally occurring host inflammatory re-sponses to bacteria – including reactive oxygen species that are inflammatory mediators secreted by neutrophils and mac-rophages – which indicates its safety.19,43
Previous investigations also have reported that the use of APP prior to an etch-and-rinse adhesive application in-creased the dentin bond strength, supporting the findings of this study.15 It is possible that during APP application,
electrical energy forms in the dentin surface and facilitates the infiltration of adhesive into demineralized dentin. Such a mechanism has been discussed in studies that utilized EletroBond combined with an adhesive.5,31,38,57 SEM
evalu-ation of the interface did not detect improved monomer in-filtration into dentin when samples treated with APP were compared with untreated control groups.
Previous studies have evaluated the duration of APP ap-plication; however, further investigations should be per-formed to determine the optimal APP application time, con-sidering that an overexposure to plasma may result in collagen denaturation.15,40,60 Although most APP is
gener-ated by argon plasma and does not change the temperature of treated surfaces, other parameters, eg, power, outlet-to-sample distance, and argon flow rate, also need to be de-fined.23 The plasma torch contains etching species which
may possess different energy levels, thus affecting the sur-faces differently depending on the equipment used. Dong et al15 described higher dentin bond strengths applying 30 s
of APP at a lower power level than that used in this study.15
A significant difference was observed between adhesive systems when they were applied on untreated dentin, re-gardless of the evaluation time. Previous studies observed that the three-step etch-and-rinse adhesive showed higher bond strength than did a simplified system.8,18 Using the
primer of the three-step adhesive system does not seem to interfere with the effects of plasma, while for XP Bond a better interaction with plasma-treated (etched) dentin may occur due to increased surface energy and hydrophilic-ity.20,29 However, both of these APP effects can also
facili-tate water uptake into the dentin/adhesive interface, ac-celerating hydrolytic degradation, which was made evident in bond strength testing of XP Bond after one year, but was not observed in the SEM analysis.
The use of APP before acid etching was evaluated in this study in order to address its potential effects in cases where the tooth structure was disinfected after cavity prep-aration.32 The results showed that APP application on intact
dentin and before etching did not influence the bond strength of either adhesive tested. APP could influence acid etching, producing deeper acid penetration or enhancing the interaction of phosphoric acid with dentin. However, the interfacial SEM images did not detect any micromorphologi-cal differences in the structures formed during the hybridiza-tion process. Future studies using transmission electron microscopy could show the adhesive monomer interactions with the intertubular dentin.
More adhesive failures were observed when APP was ap-plied after acid etching for both adhesives systems. The
prevalence of this fracture mode indicates that APP treat-ment modified the interaction between etched dentin and the adhesive components. In contrast, APP applied before dentin acid etching produced the same results as in the control groups. It is plausible, however, that acid etching may neutralize the effects of APP on the dentin surface, as this procedure removes smear layer and demineralizes the underlying dentin. Because this in vitro study used ex-tracted teeth, further clinical studies using vital teeth are warranted to validate the potential mechanisms of the im-provement of resin infiltration that is associated with the use of the APP bonding technique.
CONCLUSION
The influence of APP application on dentin adhesion de-pends on the adhesive system used and whether APP is applied before or after acid etching the dentin. It is specu-lated that resin infiltration may be improved by the modifica-tion of the dentin surface, which can result in higher bond strength. The three-step etch-and-rinse adhesive was not affected by AAP. Conversely, the dentin bond strength of the two-step etch-and-rinse adhesive increased when APP application was performed after acid etching, although this effect was not stable after one year of water storage.
ACKNOWLEDGMENTS
This study was supported by Capes (#1777-2014), FAPESP (#2013/15952-7) and CNPq (#307217-2014-0), Brazil.
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Clinical relevance: APP applied to etched dentin can
improve the short-term bond strength and quality of resin monomer infiltration into dentin, depending on type of adhesive systems. However, the long-term ef-fect of APP treatment is still questionable, and further studies must examine the influence of APP on durability of composite restorations.
