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adhesive interface
International Journal of Adhesion & Adhesives 35 (2012) 114–119
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International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Etch and rinse versus self-etching adhesives systems: Tridimensional micromechanical 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
´, Brazil Institute Federal of Parana-IFPR, Londrina, Parana ~ Paulo, Brazil Department of Dental Materials and Prosthodontics, Sao Paulo State University, Faculty of Dentistry of Arac- atuba-UNESP, Arac- atuba, Sao c ´-UTFPR, Brazil ´gica Federal do Parana Department of Mechanical Engineering, Universidade Tecnolo b
a r t i c l e i n f o
a b s t r a c t
Article history: Accepted 13 November 2011 Available online 27 December 2011
The 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 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.
Keywords: Finite element stress analysis Mechanical properties of adhesives Dentin bonding agents Hybrid layer Etch-and-rinse adhesives Self-etch adhesives
1. Introduction The clinical efficacy of a dentin adhesive system depends mainly on the molecular entanglement promoted by the etched dentinal substrate, allowing the infiltration of hydrophilic resinous monomers among the collagen fibers (mainly in the intertubular dentin), thereby creating a biopolymer to characterize the hybrid 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: the etch-and-rinse and the self-etching technique [1–3]. Etch-and-rinse, or the total etch technique, is employed through the total etching of the dental substrate with phosphoric acid followed by the application of a primer and bond resin. The characteristics of the HL created by these systems indicate the formation of a HL ranging from 4.2 to 7.4 mm thickness with a regular and uniformly distributed pattern of resin tags (tags) in its surface. The tags are relatively lengthy (25 mm) and funnelshaped with several lateral branches, as shown by scanning
n
Corresponding author. Tel.: þ 55 18 36363290. E-mail addresses: [email protected] (M. M. Junior), [email protected] (E.P. Rocha), [email protected] (R.B. Anchieta), [email protected] (C.M. Archangelo), [email protected] (M.A. Luersen). 0143-7496/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2011.11.012
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 and highly sensitive, because the over-dried dentin causes both demineralized collagen fibers to collapse and low monomer diffusion among the fibers, hampering the formation of a functionally suitable HL. In addition, the excessive presence of humidity may result in incomplete monomer polymerization and water adsorption in the HL. These effects can decrease the mechanical quality of the HL formed, causing its early degradation [6,7]. Self-etching systems were introduced to control the sensitivity to humidity of the etch-and-rinse technique as well as to simplify the clinical procedures of adhesive application, reducing clinical time [8]. Two-step self-etching adhesive systems (the first bottle containing primer and acid and the second bottle containing hydrophobic bond resin) are less acidic and less injurious to the dental substrate than etch-and-rinse adhesives [9,10]. Possibly, self-etching systems alter the ‘‘smear layer’’ that covers the dentin after tooth burr preparation, creating a thin HL of 0.5–1.2 mm thickness [5–11]. For this system, the created tags are short (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 resin tag formation. In spite of forming a thin HL, this system exhibits
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a chemical bond to the dentin substrate. This occurs through ionic links among the monomer’s hydrophilic phosphate group, water, and calcium salts of the hydroxyapatite, as observed with Clearfil SE Bond, a 10-methacryloxydecyl dihydrogen phosphate (10-MDP) monomer-based adhesive system, whose bond strength values are similar to those of gold standard etch-and-rinse adhesives [13–17]. Despite the technological advances in research on bonding techniques, most studies involving microtensile bond strength tests, shear bond strength tests, or other tests commonly show situations where failures occur, such as mixed, adhesive, or cohesive failures [18–21], but do not show the exact points where fractures may initiate. Thus, it is not possible to accurately determine, which structure of the dentin/adhesive (d/a) interface is likely to fail [22–24]. Finite element analyses (FEA) allow researchers to observe the mechanical behavior of the dental structure in detail, analyze stress concentration and its distribution quantitatively and qualitatively [25–27], as well as determine which system favors the reduction of stresses at the d/a interface. In recent studies involving FEA in the analysis of the d/a interface, there is the prevalence of two-dimensional studies investigating the influence of the elastic modulus in the propagation of adhesive failures through the HL, considering tag adhesion in the dentinal tubule walls as imperfect. In contrast, only one study evaluating the influence of flaws in the d/a interface is tridimensional [28–30]. Other two-dimensional studies have analyzed the variation of the HL thickness and tag length in the dentinal tubules, disregarding the influence of adhesive lateral branches on internal stress distribution [31]. There is still no published data showing the behavior of etchand-rinse and self-etching adhesive systems by modeling the complex features of the d/a interface. Therefore, the purpose of this study was to evaluate and compare the micromechanical behavior of etch-and-rinse adhesive systems, with or without the presence of lateral branches, and self-etching systems, with or without the presence of resin tags. The null hypothesis, that no differences in stress concentration between the two types of adhesive systems, was tested.
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2. Materials and methods In this study, all dentin structures were built to create a geometric model based on the main characteristics of the dentin substrate, which is composed of minerals (50%) in the form of carbonate-rich, calcium-deficient apatite, organic matter (30%) made up of type I collagen fibers, and fluid (20%) [32]. Three major structures are found in the coronal dentin: intertubular dentin, dentin tubules, and peritubular dentin [32]. The dentin structural composition includes the elements of orientation and convergence degree of dentin tubules, with a high density of tubules near the pulp. The dentin tubules are surrounded by a highly mineralized peritubular dentin, and the intertubular dentin matrices are among these structures (tubules and peritubular dentin), consisting of type I collagen fibrils reinforced with apatite [20,33]. The contributions of the intertubular dentin, peritubular dentin, and dentin tubules to adhesion vary significantly with location of the crown dentin where the restoration will be done [20,33]. The demineralized intertubular dentin is the main substrate to create the HL, and opened tubules might be useful for resin tag formation inside the tubules [11,12]. In line with these characteristics, a dentin specimen restored with composite resin (41 41 82 mm) (Fig. 1) was synthesized using the solid modeling software Solidworks 2009 (SolidWorks Corporation, Concord, MA, USA) [29,31,34]. The dimensions and mechanical properties of each structure were obtained from specific studies and are listed in Table 1 [25,29,31,35,37]. Based on this model (M), four geometric models were considered: (a) Mc, the representation of a deep 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, partially demineralized dentin, peritubular dentin, intertubular dentin, and intratubular content, as expected for the dentin/adhesive interface created 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 the tags created between the top and base of the HL in addition to some in the beginning of resin tags, as expected for a dentin/adhesive interface created from using a three-step etch-and-rinse adhesive system (Scotchbond Multipurpose, 3M ESPE, St. Paul, USA) (Fig. 1D);
Fig. 1. (A) Geometric model of a dentin specimen restored with resin composite (41 41 82 mm). The base of all models was fixed in x, y, and z axes; (B) Magnification of dentin adhesive interface (Mc), with emphasis on HL and adhesive layer (a); (C) Adhesive interface of conventional adhesive without lateral branches; (D) Adhesive interface of conventional adhesive with lateral branches; (E) Adhesive interface of self-etching adhesive, without resin tags; (F) Adhesive interface of self-etching adhesive with resin tags. (HL) hybrid layer; a—adhesive layer; rt—resin tags; lb—lateral branches.
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Table 1 Mechanical properties (E, v) and dimensions of materials. Material
Base (width) Length Composite resin Tubule orifice Resin tags
Elastic Poisson’s modulus (GPa) ratios
Dimension (lm)
References
30
0.3
5
0.28
5 20 13
0.3 0.3 0.3
41 41 82 41 2.5 19 17 1.2 36 3
0.3 0.3 0.28
41 6 1
[29] [25,29] [25,29]
Pulp
28.6 5 4 3 2 1 0.002
0.3
[31]
Smear plug
1
0.28
27 25 2.5 1.0
Lateral branches Intertubular dentin Inter-tubular dentin close to HL Peritubular dentin Adhesive layer Hybrid layer
[25,29] [35,37] [25,36] [35] [25,29] [25,29] 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.
[25]
(c) Ma, similar to Mc, but with a 6-mm-thick adhesive layer, 1-mmthick HL, and a smear plug obliterating the tubule orifice, with no tags, as expected for a dentin/adhesive interface created from using a mild two-step self-etch adhesive (Clearfil SE Bond, Kuraray Medical Inc, Tokio, Japan) (Fig. 1E); (d) Mat, similar to Ma, but without smear plugs obliterating the dentin tubules and with presence of 17-mm-length tags inside the dentin tubules, as expected for a dentin/adhesive interface created after using a mild two-step self-etch adhesive (Clearfil SE Bond, Kuraray Medical Inc, Tokio, Japan) (Fig. 1F). The simulation of the infiltrated adhesive among the collagen fibers (HL, resin tags, and lateral branches) was considered in a total and close relationship with adjacent and subjacent structures with a 5 GPa elastic modulus [29]. With respect to the degree of stiffness in structures, the elastic modulus of the peritubular dentin was 28.6 GPa, and that of the intertubular dentin was 20 GPa. However, close to the HL interface, the dentin elastic modulus was lower due to acid etching, rated at 13 GPa [29]. Following the same study [29], a 4-mm-thick layer of intertubular dentin was created under the HL for Mc and Mr. This layer was divided into 4 sublayers of 1-mm-thickness each. For the first layer, the elastic modulus (E) adopted was 4 GPa, with 3 GPa for 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 established, with an E of 4 GPa, simulating a self-etching adhesive system, which presents lower HL thickness when compared to those of etch-and-rinse adhesive systems [4,5,11]. Ma simulated the adhesive layer infiltrated in the ‘‘smear layer’’, maintaining a 1-mm smear plug thickness [12]. Mat was similar to Ma, exhibiting the presence of thinner tags of 17-mm in length [12]. The modeling process took into account regular geometric figures such as cone, sphere, cylinder, and rectangle rounded corners. Quadratic tetrahedral elements were used for finite element mesh generation, which was driven by the convergence of analysis (6%) [34]. This technique allows for a balanced concentration of elements in the main areas, such as the intertubular dentin, to avoid the occurrence of excessive stress in small regions. Models showed up to 35,786 elements and 93,188 nodes. The bases of all models were fixed in the x, y, and z axes. For loading definition, an hourglass-shaped macro-specimen of dentin, restored with composite resin, with 1.1 mm2 area of adhesive interface was built to generate tension stress equal to 18 MPa at the adhesive interface after 20 N tensile loading at the
Fig. 3. Maximum principal stress (MPa) for peritubular dentin, hybrid layer, and adhesive layer in all models considered (Mc, Mr, Ma, and Mat).
top of the composite resin [38]. Taking into account the ratios of the surface area, force applied, and mean tensile stress obtained between the macro- and micro-models of the present study, a vertical and distributed tensile loading of 0.03 N was selected to be applied at the top of the composite resin (Fig. 2). For numerical analysis, the finite element software ANSYS Workbench 10.0 (Swanson Analysis System, Canonsburg, PA, USA) was used.
3. Results The maximum principal stress (smax) was used because it acts as a good index for the identification of failures that might initiate from small defects [30]. Even considering the precautions taken during modeling and mesh generation based on the convergence of analysis, maximum stress values (the peak of smax) were not employed in all models to avoid misinterpretation. The peritubular dentin, intertubular dentin, adhesive layer, and HL were analyzed individually (Fig. 3). 3.1. Peritubular and intertubular dentin In all models, the smax concentrations were located at the upper area of the peritubular dentin close to the HL (Fig. 4). Model Mr had the highest stress concentration for the peritubular dentin, followed by Mc, Ma, and Mat (Fig. 3). For the intertubular
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Fig. 6. Stress concentration in upper area of HL in contact with adhesive layer in model Mr (black arrow). 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.)
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 layer close to the HL for all models, except for Mr, where it took place at the bottom, more specifically in the lateral branches (Fig. 5). Among the different models, the highest smax value occurred in model Mr (Fig. 3). 3.3. Hybrid layer The HL structure presented the lowest stresses among all the structures analyzed. The peak of smax was located at the upper area of 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 not allow for the individual evaluation of the d/a interface structures
[39,40]. However, bond strength values reported by the literature for deep dentin (49–120 MPa) are similar to values obtained in this study [41]. However, more important than correlating the values of the present study with the structural failure risk is to understand the stress distribution in d/a interface structures and interpreting the mechanical behavior of each structure individually with the variables used in this study. For the etch-and-rinse adhesive models (Mc and Mr), the stress concentration in Mr was 43% higher than in Mc for the peritubular dentin. Perhaps this increase of stress concentration was influenced by the presence of adhesive lateral branches in Mr, which was considered perfectly adhered to adjacent structures in the present study, characterizing good hybridization. This observation can be explained by an increase in the micromechanical retentive area, as shown for etch-and-rinse adhesive systems at different dentin depths, through mathematical calculations [42,43]. These data diverge from previous studies that consider the lateral branches of the dentinal tubules only as a type of dentinal permeability, with low relevance to the HL retentive capacity, due to its low bond value [44–46]. For the self-etching models (Ma and Mat), the smax was similar indicating that the presence of resin tags did not differentially influence stress concentration in the peritubular dentin. Although Ma presented higher stress values than Mat, Lohbauer et al. [46] showed in an in vitro study that resin tag formation in a selfetching system does not contribute to dentin adhesion once it possesses low bond strength with the dentin. When the smax of the peritubular dentin is compared between the etch-and-rinse and self-etching models (Mc and Mat), stress concentration for the etch-and-rinse was 52% higher than for the self-etching system. This difference could be explained by the smaller thickness of the HL produced by the self-etching systems. According to some FEA studies, the increase in HL thickness also increases stress concentration in the peritubular dentin [29,31]. Stress dissipation in the peritubular dentin occurred in the apical direction, which reinforces the influence of the structure’s modulus of elasticity, since stress is dissipated from structures with higher elastic modulus to structures with lower elastic modulus [26]. The stress distribution behavior at the d/a interface for the intertubular dentin was similar to the stress distribution in the peritubular dentin. It is important to note that one of the most interesting regions for analysis was the intertubular dentin subjacent to the HL, as it is an area of probable failure close to the dentinal tubules [47]. HL was not influenced by the presence of adhesive lateral branches for etch-and-rinse models, nor was it influenced by the
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presence of resin tags in the self-etching models (Fig. 6). Comparing the smax and ultimate tensile strength (UTS) presented in the literature for the HL, the smax values found in the etch-and-rinse models (Mc—27.4 MPa; Mr—25.8 MPa) were close to values for structural failure reported in the literature, between 20 and 30 MPa [41]. The peak of smax for Mr was located near the lateral branches, indicating once again the influence of the lateral branches in the increase of stress concentration. Studies have identified the top of HL near the adhesive layer as the region for possible cohesive and adhesive failures [38,48] with UTS and bond strength values between 19 and 38 MPa for etch-and-rinse adhesives [6,10,16,19] and between 18 and 27 MPa for self-etching systems [10,16,19]. The present study predicts the base of the adhesive layer as the region of probable structural failure since it shows higher smax value than those reported by the literature (Fig. 3). However, there is no agreement on failure terminology with respect to its localization. Many studies have shown through scanning or transmission electron microscopy images that the adhesive layer region in contact with the dental substrate is the failure area, similar to the present study [6,10,16,19]. In the present study, etch-and-rinse models produced higher stresses in comparison with self-etching models and based on these results the null hypothesis was rejected. However, results on the bond strength performance of etch-and-rinse adhesive systems or self-etching systems are still controversial [2,13–17,48]. The stress distribution in the present study, using tridimensional models analyzed with FEA, allowed the visualization of the probable area where fractures may initiate: the adhesive layer. This FEA study showed interesting findings on adhesion in the dentin substrate by comparing two distinct adhesive systems and variables (lateral branches and resin tag formation), but some limitations must be underlined. All of the d/a interface structures were considered isotropic and linearly elastic to reduce computational demands. However, some structures, such as dentin, show anisotropic or orthotropic behavior depending on the tubule orientation. Furthermore, due to the difficulty of assessing laboratory tests in such small micro-specimens and loading conditions to develop an experimental tensile test, the validation of 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 technologies might be helpful to validate future results.
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Etch and rinse versus self-etching adhesives systems: Tridimensional micromechanical 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
´, Brazil Institute Federal of Parana-IFPR, Londrina, Parana ~ Paulo, Brazil Department of Dental Materials and Prosthodontics, Sao Paulo State University, Faculty of Dentistry of Arac- atuba-UNESP, Arac- atuba, Sao c ´-UTFPR, Brazil ´gica Federal do Parana Department of Mechanical Engineering, Universidade Tecnolo b
a r t i c l e i n f o
a b s t r a c t
Article history: Accepted 13 November 2011 Available online 27 December 2011
The 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 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.
Keywords: Finite element stress analysis Mechanical properties of adhesives Dentin bonding agents Hybrid layer Etch-and-rinse adhesives Self-etch adhesives
1. Introduction The clinical efficacy of a dentin adhesive system depends mainly on the molecular entanglement promoted by the etched dentinal substrate, allowing the infiltration of hydrophilic resinous monomers among the collagen fibers (mainly in the intertubular dentin), thereby creating a biopolymer to characterize the hybrid 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: the etch-and-rinse and the self-etching technique [1–3]. Etch-and-rinse, or the total etch technique, is employed through the total etching of the dental substrate with phosphoric acid followed by the application of a primer and bond resin. The characteristics of the HL created by these systems indicate the formation of a HL ranging from 4.2 to 7.4 mm thickness with a regular and uniformly distributed pattern of resin tags (tags) in its surface. The tags are relatively lengthy (25 mm) and funnelshaped with several lateral branches, as shown by scanning
n
Corresponding author. Tel.: þ 55 18 36363290. E-mail addresses: [email protected] (M. M. Junior), [email protected] (E.P. Rocha), [email protected] (R.B. Anchieta), [email protected] (C.M. Archangelo), [email protected] (M.A. Luersen). 0143-7496/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2011.11.012
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 and highly sensitive, because the over-dried dentin causes both demineralized collagen fibers to collapse and low monomer diffusion among the fibers, hampering the formation of a functionally suitable HL. In addition, the excessive presence of humidity may result in incomplete monomer polymerization and water adsorption in the HL. These effects can decrease the mechanical quality of the HL formed, causing its early degradation [6,7]. Self-etching systems were introduced to control the sensitivity to humidity of the etch-and-rinse technique as well as to simplify the clinical procedures of adhesive application, reducing clinical time [8]. Two-step self-etching adhesive systems (the first bottle containing primer and acid and the second bottle containing hydrophobic bond resin) are less acidic and less injurious to the dental substrate than etch-and-rinse adhesives [9,10]. Possibly, self-etching systems alter the ‘‘smear layer’’ that covers the dentin after tooth burr preparation, creating a thin HL of 0.5–1.2 mm thickness [5–11]. For this system, the created tags are short (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 resin tag formation. In spite of forming a thin HL, this system exhibits
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a chemical bond to the dentin substrate. This occurs through ionic links among the monomer’s hydrophilic phosphate group, water, and calcium salts of the hydroxyapatite, as observed with Clearfil SE Bond, a 10-methacryloxydecyl dihydrogen phosphate (10-MDP) monomer-based adhesive system, whose bond strength values are similar to those of gold standard etch-and-rinse adhesives [13–17]. Despite the technological advances in research on bonding techniques, most studies involving microtensile bond strength tests, shear bond strength tests, or other tests commonly show situations where failures occur, such as mixed, adhesive, or cohesive failures [18–21], but do not show the exact points where fractures may initiate. Thus, it is not possible to accurately determine, which structure of the dentin/adhesive (d/a) interface is likely to fail [22–24]. Finite element analyses (FEA) allow researchers to observe the mechanical behavior of the dental structure in detail, analyze stress concentration and its distribution quantitatively and qualitatively [25–27], as well as determine which system favors the reduction of stresses at the d/a interface. In recent studies involving FEA in the analysis of the d/a interface, there is the prevalence of two-dimensional studies investigating the influence of the elastic modulus in the propagation of adhesive failures through the HL, considering tag adhesion in the dentinal tubule walls as imperfect. In contrast, only one study evaluating the influence of flaws in the d/a interface is tridimensional [28–30]. Other two-dimensional studies have analyzed the variation of the HL thickness and tag length in the dentinal tubules, disregarding the influence of adhesive lateral branches on internal stress distribution [31]. There is still no published data showing the behavior of etchand-rinse and self-etching adhesive systems by modeling the complex features of the d/a interface. Therefore, the purpose of this study was to evaluate and compare the micromechanical behavior of etch-and-rinse adhesive systems, with or without the presence of lateral branches, and self-etching systems, with or without the presence of resin tags. The null hypothesis, that no differences in stress concentration between the two types of adhesive systems, was tested.
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2. Materials and methods In this study, all dentin structures were built to create a geometric model based on the main characteristics of the dentin substrate, which is composed of minerals (50%) in the form of carbonate-rich, calcium-deficient apatite, organic matter (30%) made up of type I collagen fibers, and fluid (20%) [32]. Three major structures are found in the coronal dentin: intertubular dentin, dentin tubules, and peritubular dentin [32]. The dentin structural composition includes the elements of orientation and convergence degree of dentin tubules, with a high density of tubules near the pulp. The dentin tubules are surrounded by a highly mineralized peritubular dentin, and the intertubular dentin matrices are among these structures (tubules and peritubular dentin), consisting of type I collagen fibrils reinforced with apatite [20,33]. The contributions of the intertubular dentin, peritubular dentin, and dentin tubules to adhesion vary significantly with location of the crown dentin where the restoration will be done [20,33]. The demineralized intertubular dentin is the main substrate to create the HL, and opened tubules might be useful for resin tag formation inside the tubules [11,12]. In line with these characteristics, a dentin specimen restored with composite resin (41 41 82 mm) (Fig. 1) was synthesized using the solid modeling software Solidworks 2009 (SolidWorks Corporation, Concord, MA, USA) [29,31,34]. The dimensions and mechanical properties of each structure were obtained from specific studies and are listed in Table 1 [25,29,31,35,37]. Based on this model (M), four geometric models were considered: (a) Mc, the representation of a deep 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, partially demineralized dentin, peritubular dentin, intertubular dentin, and intratubular content, as expected for the dentin/adhesive interface created 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 the tags created between the top and base of the HL in addition to some in the beginning of resin tags, as expected for a dentin/adhesive interface created from using a three-step etch-and-rinse adhesive system (Scotchbond Multipurpose, 3M ESPE, St. Paul, USA) (Fig. 1D);
Fig. 1. (A) Geometric model of a dentin specimen restored with resin composite (41 41 82 mm). The base of all models was fixed in x, y, and z axes; (B) Magnification of dentin adhesive interface (Mc), with emphasis on HL and adhesive layer (a); (C) Adhesive interface of conventional adhesive without lateral branches; (D) Adhesive interface of conventional adhesive with lateral branches; (E) Adhesive interface of self-etching adhesive, without resin tags; (F) Adhesive interface of self-etching adhesive with resin tags. (HL) hybrid layer; a—adhesive layer; rt—resin tags; lb—lateral branches.
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Table 1 Mechanical properties (E, v) and dimensions of materials. Material
Base (width) Length Composite resin Tubule orifice Resin tags
Elastic Poisson’s modulus (GPa) ratios
Dimension (lm)
References
30
0.3
5
0.28
5 20 13
0.3 0.3 0.3
41 41 82 41 2.5 19 17 1.2 36 3
0.3 0.3 0.28
41 6 1
[29] [25,29] [25,29]
Pulp
28.6 5 4 3 2 1 0.002
0.3
[31]
Smear plug
1
0.28
27 25 2.5 1.0
Lateral branches Intertubular dentin Inter-tubular dentin close to HL Peritubular dentin Adhesive layer Hybrid layer
[25,29] [35,37] [25,36] [35] [25,29] [25,29] 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.
[25]
(c) Ma, similar to Mc, but with a 6-mm-thick adhesive layer, 1-mmthick HL, and a smear plug obliterating the tubule orifice, with no tags, as expected for a dentin/adhesive interface created from using a mild two-step self-etch adhesive (Clearfil SE Bond, Kuraray Medical Inc, Tokio, Japan) (Fig. 1E); (d) Mat, similar to Ma, but without smear plugs obliterating the dentin tubules and with presence of 17-mm-length tags inside the dentin tubules, as expected for a dentin/adhesive interface created after using a mild two-step self-etch adhesive (Clearfil SE Bond, Kuraray Medical Inc, Tokio, Japan) (Fig. 1F). The simulation of the infiltrated adhesive among the collagen fibers (HL, resin tags, and lateral branches) was considered in a total and close relationship with adjacent and subjacent structures with a 5 GPa elastic modulus [29]. With respect to the degree of stiffness in structures, the elastic modulus of the peritubular dentin was 28.6 GPa, and that of the intertubular dentin was 20 GPa. However, close to the HL interface, the dentin elastic modulus was lower due to acid etching, rated at 13 GPa [29]. Following the same study [29], a 4-mm-thick layer of intertubular dentin was created under the HL for Mc and Mr. This layer was divided into 4 sublayers of 1-mm-thickness each. For the first layer, the elastic modulus (E) adopted was 4 GPa, with 3 GPa for 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 established, with an E of 4 GPa, simulating a self-etching adhesive system, which presents lower HL thickness when compared to those of etch-and-rinse adhesive systems [4,5,11]. Ma simulated the adhesive layer infiltrated in the ‘‘smear layer’’, maintaining a 1-mm smear plug thickness [12]. Mat was similar to Ma, exhibiting the presence of thinner tags of 17-mm in length [12]. The modeling process took into account regular geometric figures such as cone, sphere, cylinder, and rectangle rounded corners. Quadratic tetrahedral elements were used for finite element mesh generation, which was driven by the convergence of analysis (6%) [34]. This technique allows for a balanced concentration of elements in the main areas, such as the intertubular dentin, to avoid the occurrence of excessive stress in small regions. Models showed up to 35,786 elements and 93,188 nodes. The bases of all models were fixed in the x, y, and z axes. For loading definition, an hourglass-shaped macro-specimen of dentin, restored with composite resin, with 1.1 mm2 area of adhesive interface was built to generate tension stress equal to 18 MPa at the adhesive interface after 20 N tensile loading at the
Fig. 3. Maximum principal stress (MPa) for peritubular dentin, hybrid layer, and adhesive layer in all models considered (Mc, Mr, Ma, and Mat).
top of the composite resin [38]. Taking into account the ratios of the surface area, force applied, and mean tensile stress obtained between the macro- and micro-models of the present study, a vertical and distributed tensile loading of 0.03 N was selected to be applied at the top of the composite resin (Fig. 2). For numerical analysis, the finite element software ANSYS Workbench 10.0 (Swanson Analysis System, Canonsburg, PA, USA) was used.
3. Results The maximum principal stress (smax) was used because it acts as a good index for the identification of failures that might initiate from small defects [30]. Even considering the precautions taken during modeling and mesh generation based on the convergence of analysis, maximum stress values (the peak of smax) were not employed in all models to avoid misinterpretation. The peritubular dentin, intertubular dentin, adhesive layer, and HL were analyzed individually (Fig. 3). 3.1. Peritubular and intertubular dentin In all models, the smax concentrations were located at the upper area of the peritubular dentin close to the HL (Fig. 4). Model Mr had the highest stress concentration for the peritubular dentin, followed by Mc, Ma, and Mat (Fig. 3). For the intertubular
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Fig. 6. Stress concentration in upper area of HL in contact with adhesive layer in model Mr (black arrow). 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.)
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 layer close to the HL for all models, except for Mr, where it took place at the bottom, more specifically in the lateral branches (Fig. 5). Among the different models, the highest smax value occurred in model Mr (Fig. 3). 3.3. Hybrid layer The HL structure presented the lowest stresses among all the structures analyzed. The peak of smax was located at the upper area of 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 not allow for the individual evaluation of the d/a interface structures
[39,40]. However, bond strength values reported by the literature for deep dentin (49–120 MPa) are similar to values obtained in this study [41]. However, more important than correlating the values of the present study with the structural failure risk is to understand the stress distribution in d/a interface structures and interpreting the mechanical behavior of each structure individually with the variables used in this study. For the etch-and-rinse adhesive models (Mc and Mr), the stress concentration in Mr was 43% higher than in Mc for the peritubular dentin. Perhaps this increase of stress concentration was influenced by the presence of adhesive lateral branches in Mr, which was considered perfectly adhered to adjacent structures in the present study, characterizing good hybridization. This observation can be explained by an increase in the micromechanical retentive area, as shown for etch-and-rinse adhesive systems at different dentin depths, through mathematical calculations [42,43]. These data diverge from previous studies that consider the lateral branches of the dentinal tubules only as a type of dentinal permeability, with low relevance to the HL retentive capacity, due to its low bond value [44–46]. For the self-etching models (Ma and Mat), the smax was similar indicating that the presence of resin tags did not differentially influence stress concentration in the peritubular dentin. Although Ma presented higher stress values than Mat, Lohbauer et al. [46] showed in an in vitro study that resin tag formation in a selfetching system does not contribute to dentin adhesion once it possesses low bond strength with the dentin. When the smax of the peritubular dentin is compared between the etch-and-rinse and self-etching models (Mc and Mat), stress concentration for the etch-and-rinse was 52% higher than for the self-etching system. This difference could be explained by the smaller thickness of the HL produced by the self-etching systems. According to some FEA studies, the increase in HL thickness also increases stress concentration in the peritubular dentin [29,31]. Stress dissipation in the peritubular dentin occurred in the apical direction, which reinforces the influence of the structure’s modulus of elasticity, since stress is dissipated from structures with higher elastic modulus to structures with lower elastic modulus [26]. The stress distribution behavior at the d/a interface for the intertubular dentin was similar to the stress distribution in the peritubular dentin. It is important to note that one of the most interesting regions for analysis was the intertubular dentin subjacent to the HL, as it is an area of probable failure close to the dentinal tubules [47]. HL was not influenced by the presence of adhesive lateral branches for etch-and-rinse models, nor was it influenced by the
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presence of resin tags in the self-etching models (Fig. 6). Comparing the smax and ultimate tensile strength (UTS) presented in the literature for the HL, the smax values found in the etch-and-rinse models (Mc—27.4 MPa; Mr—25.8 MPa) were close to values for structural failure reported in the literature, between 20 and 30 MPa [41]. The peak of smax for Mr was located near the lateral branches, indicating once again the influence of the lateral branches in the increase of stress concentration. Studies have identified the top of HL near the adhesive layer as the region for possible cohesive and adhesive failures [38,48] with UTS and bond strength values between 19 and 38 MPa for etch-and-rinse adhesives [6,10,16,19] and between 18 and 27 MPa for self-etching systems [10,16,19]. The present study predicts the base of the adhesive layer as the region of probable structural failure since it shows higher smax value than those reported by the literature (Fig. 3). However, there is no agreement on failure terminology with respect to its localization. Many studies have shown through scanning or transmission electron microscopy images that the adhesive layer region in contact with the dental substrate is the failure area, similar to the present study [6,10,16,19]. In the present study, etch-and-rinse models produced higher stresses in comparison with self-etching models and based on these results the null hypothesis was rejected. However, results on the bond strength performance of etch-and-rinse adhesive systems or self-etching systems are still controversial [2,13–17,48]. The stress distribution in the present study, using tridimensional models analyzed with FEA, allowed the visualization of the probable area where fractures may initiate: the adhesive layer. This FEA study showed interesting findings on adhesion in the dentin substrate by comparing two distinct adhesive systems and variables (lateral branches and resin tag formation), but some limitations must be underlined. All of the d/a interface structures were considered isotropic and linearly elastic to reduce computational demands. However, some structures, such as dentin, show anisotropic or orthotropic behavior depending on the tubule orientation. Furthermore, due to the difficulty of assessing laboratory tests in such small micro-specimens and loading conditions to develop an experimental tensile test, the validation of 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 technologies might be helpful to validate future results.
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