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International Journal of Adhesion & Adhesives 35 (2012) 114–119
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International Journal of Adhesion & Adhesives
0143-74
doi:10.1
n Corr
E-m
eduardo
rodolfoa
carlosar
luersen
journal homepage: www.elsevier.com/locate/ijadhadh
Etch and rinse versus self-etching adhesives systems: Tridimensionalmicromechanical analysis of dentin/adhesive interface
Manoel M. Junior a, Eduardo P. Rocha b,n, Rodolfo B. Anchieta b, Carlos Marcelo Archangelo a,Marco Antonio Luersen c
a Institute Federal of Parana-IFPR, Londrina, Parana, Brazilb Department of Dental Materials and Prosthodontics, Sao Paulo State University, Faculty of Dentistry of Arac-atuba-UNESP, Arac-atuba, S ~ao Paulo, Brazilc Department of Mechanical Engineering, Universidade Tecnologica Federal do Parana-UTFPR, Brazil
a r t i c l e i n f o
Article history:
Accepted 13 November 2011The purpose of this study was to evaluate stress distribution in the hybrid layer produced by two
adhesive systems using three-dimensional finite element analysis (FEA). Four FEA models (M) were
Available online 27 December 2011Keywords:
Finite element stress analysis
Mechanical properties of adhesives
Dentin bonding agents
Hybrid layer
Etch-and-rinse adhesives
Self-etch adhesives
96/$ - see front matter & 2012 Elsevier Ltd. A
016/j.ijadhadh.2011.11.012
esponding author. Tel.: þ 55 18 36363290.
ail addresses: [email protected] (M. M. Ju
[email protected] (E.P. Rocha),
[email protected] (R.B. Anchieta),
[email protected] (C.M. Archangelo),
@utfpr.edu.br (M.A. Luersen).
a b s t r a c t
developed: Mc, a representation of a dentin specimen (41�41�82 mm) restored with composite resin,
exhibiting the adhesive layer, hybrid layer (HL), resin tags, peritubular dentin, and intertubular dentin
to simulate the etch-and-rinse adhesive system; Mr, similar to Mc, with lateral branches of the
adhesive; Ma, similar to Mc, however without resin tags and obliterated tubule orifice, to simulate the
environment for the self-etching adhesive system; Mat, similar to Ma, with tags. A numerical
simulation was performed to obtain the maximum principal stress (smax). The highest smax in the
HL was observed for the etch-and-rinse adhesive system. The lateral branches increased the smax in the
HL. The resin tags had a little influence on stress distribution with the self-etching system.
& 2012 Elsevier Ltd. All rights reserved.
1. Introduction
The clinical efficacy of a dentin adhesive system dependsmainly on the molecular entanglement promoted by the etcheddentinal substrate, allowing the infiltration of hydrophilic resi-nous monomers among the collagen fibers (mainly in the inter-tubular dentin), thereby creating a biopolymer to characterize thehybrid layer (HL). Resin tags may form inside the dentin tubules,and contribute less to the total bond strength in the dentin [1–3].To produce a dentin adhesive system, there are two protocols: theetch-and-rinse and the self-etching technique [1–3].
Etch-and-rinse, or the total etch technique, is employedthrough the total etching of the dental substrate with phosphoricacid followed by the application of a primer and bond resin. Thecharacteristics of the HL created by these systems indicate theformation of a HL ranging from 4.2 to 7.4 mm thickness with aregular and uniformly distributed pattern of resin tags (tags) in itssurface. The tags are relatively lengthy (25 mm) and funnel-shaped with several lateral branches, as shown by scanning
ll rights reserved.
nior),
electron microscopy (SEM) and transmission electron microscopy(TEM) studies for a three-step etch-and-rinse adhesive [4,5].
The etch-and-rinse technique is considered to be critical andhighly sensitive, because the over-dried dentin causes bothdemineralized collagen fibers to collapse and low monomerdiffusion among the fibers, hampering the formation of a func-tionally suitable HL. In addition, the excessive presence ofhumidity may result in incomplete monomer polymerizationand water adsorption in the HL. These effects can decrease themechanical quality of the HL formed, causing its early degrada-tion [6,7].
Self-etching systems were introduced to control the sensitivityto humidity of the etch-and-rinse technique as well as to simplifythe clinical procedures of adhesive application, reducing clinicaltime [8]. Two-step self-etching adhesive systems (the first bottlecontaining primer and acid and the second bottle containinghydrophobic bond resin) are less acidic and less injurious tothe dental substrate than etch-and-rinse adhesives [9,10]. Possi-bly, self-etching systems alter the ‘‘smear layer’’ that coversthe dentin after tooth burr preparation, creating a thin HL of0.5–1.2 mm thickness [5–11]. For this system, the created tags areshort (16 mm) and narrow [5–12]. However, due to low acidity,the presence of a ‘‘smear layer’’ that obliterates the tubule orifices(also called ‘‘smear plugs’’) is common after adhesive procedures[11], limiting hybridization of the peritubular dentin and resintag formation. In spite of forming a thin HL, this system exhibits
M. Martin Junior et al. / International Journal of Adhesion & Adhesives 35 (2012) 114–119 115
a chemical bond to the dentin substrate. This occurs throughionic links among the monomer’s hydrophilic phosphate group,water, and calcium salts of the hydroxyapatite, as observed withClearfil SE Bond, a 10-methacryloxydecyl dihydrogen phosphate(10-MDP) monomer-based adhesive system, whose bond strengthvalues are similar to those of gold standard etch-and-rinseadhesives [13–17].
Despite the technological advances in research on bondingtechniques, most studies involving microtensile bond strengthtests, shear bond strength tests, or other tests commonly showsituations where failures occur, such as mixed, adhesive, orcohesive failures [18–21], but do not show the exact points wherefractures may initiate. Thus, it is not possible to accuratelydetermine, which structure of the dentin/adhesive (d/a) interfaceis likely to fail [22–24]. Finite element analyses (FEA) allowresearchers to observe the mechanical behavior of the dentalstructure in detail, analyze stress concentration and its distribu-tion quantitatively and qualitatively [25–27], as well as deter-mine which system favors the reduction of stresses at the d/ainterface.
In recent studies involving FEA in the analysis of the d/ainterface, there is the prevalence of two-dimensional studiesinvestigating the influence of the elastic modulus in the propaga-tion of adhesive failures through the HL, considering tag adhesionin the dentinal tubule walls as imperfect. In contrast, only onestudy evaluating the influence of flaws in the d/a interface istridimensional [28–30]. Other two-dimensional studies haveanalyzed the variation of the HL thickness and tag length in thedentinal tubules, disregarding the influence of adhesive lateralbranches on internal stress distribution [31].
There is still no published data showing the behavior of etch-and-rinse and self-etching adhesive systems by modeling thecomplex features of the d/a interface. Therefore, the purpose ofthis study was to evaluate and compare the micromechanicalbehavior of etch-and-rinse adhesive systems, with or without thepresence of lateral branches, and self-etching systems, with orwithout the presence of resin tags. The null hypothesis, that nodifferences in stress concentration between the two types ofadhesive systems, was tested.
Fig. 1. (A) Geometric model of a dentin specimen restored with resin composite (41�4
dentin adhesive interface (Mc), with emphasis on HL and adhesive layer (a); (C) Adh
interface of conventional adhesive with lateral branches; (E) Adhesive interface of self-e
with resin tags. (HL) hybrid layer; a—adhesive layer; rt—resin tags; lb—lateral branch
2. Materials and methods
In this study, all dentin structures were built to create ageometric model based on the main characteristics of the dentinsubstrate, which is composed of minerals (50%) in the form ofcarbonate-rich, calcium-deficient apatite, organic matter (30%) madeup of type I collagen fibers, and fluid (20%) [32]. Three majorstructures are found in the coronal dentin: intertubular dentin,dentin tubules, and peritubular dentin [32]. The dentin structuralcomposition includes the elements of orientation and convergencedegree of dentin tubules, with a high density of tubules near thepulp. The dentin tubules are surrounded by a highly mineralizedperitubular dentin, and the intertubular dentin matrices are amongthese structures (tubules and peritubular dentin), consisting of typeI collagen fibrils reinforced with apatite [20,33]. The contributions ofthe intertubular dentin, peritubular dentin, and dentin tubules toadhesion vary significantly with location of the crown dentin wherethe restoration will be done [20,33]. The demineralized intertubulardentin is the main substrate to create the HL, and opened tubulesmight be useful for resin tag formation inside the tubules [11,12]. Inline with these characteristics, a dentin specimen restored withcomposite resin (41�41�82 mm) (Fig. 1) was synthesized usingthe solid modeling software Solidworks 2009 (SolidWorks Corpora-tion, Concord, MA, USA) [29,31,34]. The dimensions and mechanicalproperties of each structure were obtained from specific studies andare listed in Table 1 [25,29,31,35,37]. Based on this model (M), fourgeometric models were considered: (a) Mc, the representation of adeep dentin specimen restored with composite resin. The dentin/adhesive interface was composed of an 8-mm-thick adhesive layer,4-mm-thick HL, 19-mm-length resin tags, 16 dentin tubules, partiallydemineralized dentin, peritubular dentin, intertubular dentin, andintratubular content, as expected for the dentin/adhesive interfacecreated from the use of a three-step etch-and-rinse adhesive system(Scotchbond Multipurpose, 3M ESPE, St. Paul, USA) (Fig. 1C) [31]; (b)Mr, similar to Mc, with representations of lateral branches of thetags created between the top and base of the HL in addition to somein the beginning of resin tags, as expected for a dentin/adhesiveinterface created from using a three-step etch-and-rinse adhesivesystem (Scotchbond Multipurpose, 3M ESPE, St. Paul, USA) (Fig. 1D);
1�82 mm). The base of all models was fixed in x, y, and z axes; (B) Magnification of
esive interface of conventional adhesive without lateral branches; (D) Adhesive
tching adhesive, without resin tags; (F) Adhesive interface of self-etching adhesive
es.
Table 1Mechanical properties (E, v) and dimensions of materials.
Material Elasticmodulus (GPa)
Poisson’sratios
Dimension(lm)
References
Base (width) 41�41
Length 82
Composite resin 30 0.3 41 [25,29]
Tubule orifice 2.5 [35,37]
Resin tags 5 0.28 19 [25,36]
17
Lateral branches 5 0.3 1.2 [35]
Intertubular dentin 20 0.3 36 [25,29]
Inter-tubular dentin
close to HL
13 0.3 3 [25,29]
Peritubular dentin 28.6 0.3 41 [29]
Adhesive layer 5 0.3 6 [25,29]
Hybrid layer 4 0.28 1 [25,29]
3
2
1
Pulp 0.002 0.3 27 [31]
25
Smear plug 1 0.28 2.5�1.0 [25]
Fig. 2. (A) Macro-specimen (hourglass-shaped) with 1.1 mm2 sectional area
(dentin/adhesive interface) and a micro-fragment of dentin/adhesive interface;
(B) Micro-specimen with dimensions proportional to those of macro-specimen.
Loading tension (0.03 N) on top of composite resin.
Fig. 3. Maximum principal stress (MPa) for peritubular dentin, hybrid layer, and
adhesive layer in all models considered (Mc, Mr, Ma, and Mat).
M. Martin Junior et al. / International Journal of Adhesion & Adhesives 35 (2012) 114–119116
(c) Ma, similar to Mc, but with a 6-mm-thick adhesive layer, 1-mm-thick HL, and a smear plug obliterating the tubule orifice, with notags, as expected for a dentin/adhesive interface created from usinga mild two-step self-etch adhesive (Clearfil SE Bond, KurarayMedical Inc, Tokio, Japan) (Fig. 1E); (d) Mat, similar to Ma, butwithout smear plugs obliterating the dentin tubules and withpresence of 17-mm-length tags inside the dentin tubules, asexpected for a dentin/adhesive interface created after using a mildtwo-step self-etch adhesive (Clearfil SE Bond, Kuraray Medical Inc,Tokio, Japan) (Fig. 1F).
The simulation of the infiltrated adhesive among the collagenfibers (HL, resin tags, and lateral branches) was considered in atotal and close relationship with adjacent and subjacent struc-tures with a 5 GPa elastic modulus [29].
With respect to the degree of stiffness in structures, the elasticmodulus of the peritubular dentin was 28.6 GPa, and that of theintertubular dentin was 20 GPa. However, close to the HL interface,the dentin elastic modulus was lower due to acid etching, rated at13 GPa [29]. Following the same study [29], a 4-mm-thick layer ofintertubular dentin was created under the HL for Mc and Mr. Thislayer was divided into 4 sublayers of 1-mm-thickness each. For thefirst layer, the elastic modulus (E) adopted was 4 GPa, with 3 GPafor the second, 2 GPa for the third, and 1 GPa for the fourth layer[29]. For Ma and Mat models, only a 1-mm-thick HL was estab-lished, with an E of 4 GPa, simulating a self-etching adhesivesystem, which presents lower HL thickness when compared tothose of etch-and-rinse adhesive systems [4,5,11]. Ma simulatedthe adhesive layer infiltrated in the ‘‘smear layer’’, maintaining a1-mm smear plug thickness [12]. Mat was similar to Ma, exhibitingthe presence of thinner tags of 17-mm in length [12].
The modeling process took into account regular geometricfigures such as cone, sphere, cylinder, and rectangle roundedcorners. Quadratic tetrahedral elements were used for finiteelement mesh generation, which was driven by the convergenceof analysis (6%) [34]. This technique allows for a balancedconcentration of elements in the main areas, such as the inter-tubular dentin, to avoid the occurrence of excessive stress insmall regions. Models showed up to 35,786 elements and 93,188nodes. The bases of all models were fixed in the x, y, and z axes.
For loading definition, an hourglass-shaped macro-specimenof dentin, restored with composite resin, with 1.1 mm2 area ofadhesive interface was built to generate tension stress equal to18 MPa at the adhesive interface after 20 N tensile loading at the
top of the composite resin [38]. Taking into account the ratios ofthe surface area, force applied, and mean tensile stress obtainedbetween the macro- and micro-models of the present study, avertical and distributed tensile loading of 0.03 N was selected tobe applied at the top of the composite resin (Fig. 2).
For numerical analysis, the finite element software ANSYSWorkbench 10.0 (Swanson Analysis System, Canonsburg, PA, USA)was used.
3. Results
The maximum principal stress (smax) was used because it actsas a good index for the identification of failures that might initiatefrom small defects [30]. Even considering the precautions takenduring modeling and mesh generation based on the convergenceof analysis, maximum stress values (the peak of smax) were notemployed in all models to avoid misinterpretation. The peritub-ular dentin, intertubular dentin, adhesive layer, and HL wereanalyzed individually (Fig. 3).
3.1. Peritubular and intertubular dentin
In all models, the smax concentrations were located at theupper area of the peritubular dentin close to the HL (Fig. 4). ModelMr had the highest stress concentration for the peritubulardentin, followed by Mc, Ma, and Mat (Fig. 3). For the intertubular
Fig. 4. Stress concentration localized in upper area of peritubular dentin in
contact with HL (black arrow) for Mc.
Fig. 5. Stress concentration at base of adhesive layer above HL (red arrow). Black
arrow shows stress in lateral branches in model Mr. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version
of this article.)
Fig. 6. Stress concentration in upper area of HL in contact with adhesive layer in
model Mr (black arrow).
M. Martin Junior et al. / International Journal of Adhesion & Adhesives 35 (2012) 114–119 117
dentin, the smax was located in contact with the HL. The smax
occurred in the Mc (59.5 MPa) model, followed by Mr (55.2 MPa),Ma (23 MPa), and Mat (21 MPa).
3.2. Adhesive layer
The highest smax occurred at the bottom of the adhesive layerclose to the HL for all models, except for Mr, where it took place at thebottom, more specifically in the lateral branches (Fig. 5). Among thedifferent models, the highest smax value occurred in model Mr (Fig. 3).
3.3. Hybrid layer
The HL structure presented the lowest stresses among all thestructures analyzed. The peak of smax was located at the upper areaof the HL, in contact with the adhesive layer for all models (Fig. 6),exhibiting values ranging from 19.8 MPa to 27.4 MPa (Fig. 3).
4. Discussion
Tensile, shear bond strength, and micro-tensile analyses do notallow for the individual evaluation of the d/a interface structures
[39,40]. However, bond strength values reported by the literaturefor deep dentin (49–120 MPa) are similar to values obtained inthis study [41]. However, more important than correlating thevalues of the present study with the structural failure risk is tounderstand the stress distribution in d/a interface structures andinterpreting the mechanical behavior of each structure individu-ally with the variables used in this study.
For the etch-and-rinse adhesive models (Mc and Mr), thestress concentration in Mr was 43% higher than in Mc for theperitubular dentin. Perhaps this increase of stress concentrationwas influenced by the presence of adhesive lateral branches inMr, which was considered perfectly adhered to adjacent struc-tures in the present study, characterizing good hybridization.This observation can be explained by an increase in the micro-mechanical retentive area, as shown for etch-and-rinse adhesivesystems at different dentin depths, through mathematical calcu-lations [42,43]. These data diverge from previous studies thatconsider the lateral branches of the dentinal tubules only as atype of dentinal permeability, with low relevance to the HLretentive capacity, due to its low bond value [44–46].
For the self-etching models (Ma and Mat), the smax was similarindicating that the presence of resin tags did not differentiallyinfluence stress concentration in the peritubular dentin. AlthoughMa presented higher stress values than Mat, Lohbauer et al. [46]showed in an in vitro study that resin tag formation in a self-etching system does not contribute to dentin adhesion once itpossesses low bond strength with the dentin.
When the smax of the peritubular dentin is compared betweenthe etch-and-rinse and self-etching models (Mc and Mat), stressconcentration for the etch-and-rinse was 52% higher than for theself-etching system. This difference could be explained by thesmaller thickness of the HL produced by the self-etching systems.According to some FEA studies, the increase in HL thickness alsoincreases stress concentration in the peritubular dentin [29,31].
Stress dissipation in the peritubular dentin occurred in theapical direction, which reinforces the influence of the structure’smodulus of elasticity, since stress is dissipated from structureswith higher elastic modulus to structures with lower elasticmodulus [26]. The stress distribution behavior at the d/a interfacefor the intertubular dentin was similar to the stress distribution inthe peritubular dentin. It is important to note that one of the mostinteresting regions for analysis was the intertubular dentinsubjacent to the HL, as it is an area of probable failure close tothe dentinal tubules [47].
HL was not influenced by the presence of adhesive lateralbranches for etch-and-rinse models, nor was it influenced by the
M. Martin Junior et al. / International Journal of Adhesion & Adhesives 35 (2012) 114–119118
presence of resin tags in the self-etching models (Fig. 6). Compar-ing the smax and ultimate tensile strength (UTS) presented in theliterature for the HL, the smax values found in the etch-and-rinsemodels (Mc—27.4 MPa; Mr—25.8 MPa) were close to values forstructural failure reported in the literature, between 20 and30 MPa [41]. The peak of smax for Mr was located near the lateralbranches, indicating once again the influence of the lateralbranches in the increase of stress concentration. Studies haveidentified the top of HL near the adhesive layer as the region forpossible cohesive and adhesive failures [38,48] with UTS and bondstrength values between 19 and 38 MPa for etch-and-rinse adhe-sives [6,10,16,19] and between 18 and 27 MPa for self-etchingsystems [10,16,19]. The present study predicts the base of theadhesive layer as the region of probable structural failure since itshows higher smax value than those reported by the literature(Fig. 3). However, there is no agreement on failure terminologywith respect to its localization. Many studies have shown throughscanning or transmission electron microscopy images that theadhesive layer region in contact with the dental substrate is thefailure area, similar to the present study [6,10,16,19].
In the present study, etch-and-rinse models produced higherstresses in comparison with self-etching models and basedon these results the null hypothesis was rejected. However,results on the bond strength performance of etch-and-rinseadhesive systems or self-etching systems are still controversial[2,13–17,48]. The stress distribution in the present study, usingtridimensional models analyzed with FEA, allowed the visualiza-tion of the probable area where fractures may initiate: theadhesive layer.
This FEA study showed interesting findings on adhesion in thedentin substrate by comparing two distinct adhesive systems andvariables (lateral branches and resin tag formation), but somelimitations must be underlined. All of the d/a interface structureswere considered isotropic and linearly elastic to reduce computa-tional demands. However, some structures, such as dentin, showanisotropic or orthotropic behavior depending on the tubuleorientation. Furthermore, due to the difficulty of assessinglaboratory tests in such small micro-specimens and loadingconditions to develop an experimental tensile test, the validationof these models becomes very difficult if not nearly impossible.Efforts should be considered when evaluating the present results,based on the characteristics of the present study. New technolo-gies might be helpful to validate future results.
5. Conclusion
Within the limitations of this study, it was concluded that thepresence of lateral branches increased stress concentration inetch-and-rinse adhesive models, mainly in the peritubular dentinand adhesive layer. The presence of resin tags in self-etchingmodels promoted a small alteration in smax values. In addition,self-etching models showed less stress concentration than etch-and-rinse models, suggesting better micromechanical behavior.
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