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Four-year water degradation of a total-etch and two self-etching adhesives bonded to dentin
journal of dentistry 36 (2008) 611–617
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/jden
Four-year water degradation of a total-etch and two self-etching adhesives bonded to dentin Ali I. Abdalla a,*, Albert J. Feilzer b a b
Department of Restorative Dentistry, Faculty of Dentistry, University of Tanta, Tanta, Egypt Department of Dental Materials Science (ACTA), Universiteit van Amsterdam and Vrije Universiteit, Amsterdam, The Netherlands
article info
abstract
Article history:
Objectives: To evaluate effect of direct and indirect water storage on the microtensile dentin
Received 18 November 2007
bond strength of one total-etch and two self-etching adhesives.
Received in revised form
Methods: The adhesive materials were: one total-etch adhesive; ‘Admira Bond’ and two self-
18 April 2008
etch adhesives; ‘Clearfil SE Bond’ and ‘Hybrid Bond’. Freshly extracted human third molar
Accepted 21 April 2008
teeth were used. In each tooth, a Class I cavity (4 mm 4 mm) was prepared in the occlusal surface with the pulpal floor extending approximately 1 mm into dentin. The teeth were divided into three groups (n = 18). Each group was restored with the resin composite ‘Clearfil
Keywords:
APX’ using one of the tested adhesives. For each experimental group 3 test procedures (n = 6)
Self-etch adhesives
were carried out: Procedure A: the teeth were stored in water for 24 h (control), then
Water storage
sectioned longitudinally, buccolingually and mesiodistally to get rectangular slabs of 1.0–
Bond strength
1.2 mm thickness on which a microtensile test was carried out. Procedure B: the teeth were
Dentin
also sectioned; however, the slabs were stored in water at 37 8C for 4 years before microtensile testing (direct water storage). Procedure C: the teeth were kept in water at 37 8C 4 years before sectioning and microtensile testing (Indirect water storage). During microtensile testing the slabs were placed in a universal testing machine and load was applied at cross-head speed of 0.5 mm/min. Results: For the 24 h control, there was no significant difference in bond strength between the three tested adhesives. After 4 years of indirect water storage, the bond strength decreased but the reduction was not significantly different from those of 24 h. After 4 years of direct water storage, the bond strengths of all tested adhesives were significantly reduced compared to their 24 h results. Conclusion: All the tested adhesives showed no reduction in bond strength after indirect water exposure for 4 years. After 4-year direct water exposure, the bond produced by all tested adhesives was unable to resist deterioration. # 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
The durability of the adhesive bond between resin and tooth structure is of significant importance for longevity of adhesive restorations. Clinically, marginal deterioration of resin composite remains problematic and forms the major factor
that dramatically shorten the lifetime of composite–tooth bond. The immediate bonding effectiveness of most current adhesive systems is quite favorable1 regardless of the adhesive used. However, when these adhesives are tested in a clinical trial, the bonding effectiveness of some materials appears
* Corresponding author. Tel.: +20 40 3336654; fax: +20 40 331800. E-mail address: [email protected] (A.I. Abdalla). 0300-5712/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2008.04.011
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journal of dentistry 36 (2008) 611–617
dramatically low, whereas the bonds of other materials are more stable.2,3 Bond strength tests are the most frequently used tests to screen adhesives. The rationale behind this testing method is that the stronger the adhesion between tooth and adhesives, the better it will resist stress imposed by resin polymerization and oral function. Different bond strength tests have been developed. Currently, the shear and microtensile bond strength test methods are used the most. In addition, different artificial aging techniques were used to reveal valuable clinical information. The most commonly used artificial aging technique is water storage. In this technique, the bonded specimens are stored in fluid at 37 8C for a specific period. This period may vary from a few months4 up to 4–5 years5–7 or even longer. Most of these studies report significant decreases in bond strengths, even after relatively short storage periods.8–15 The decrease in bonding effectiveness after water storage was supposed to be caused by degradation of interface components by hydrolysis (mainly resin and/or collagen). Also, water can infiltrate and decrease the mechanical properties of the polymer matrix, by swelling and reducing the frictional forces between the polymer chains, a process known as ‘plasticization’.16,17 Most in vitro bond strength studies use flat surfaces to test the bonding effectiveness of dental adhesives. Clinically, however, adhesives are applied in cavities, which result in higher polymerization contraction stress. This stress puts the resin–tooth interfaces under severe tension during the critical setting of the adhesive, particularly when restoring cavities with a high C-factor.18 Such prestressed interfaces are more susceptible to degradation19 by gaps and micro-voids that facilitate fluid exchange along the interface. In this study, the degradation of resin–dentin bonds formed in Class I cavities was studied by exposure to water for 4 years at 37 8C. In addition, the restored teeth were either stored in water intact or after sectioning. The first case represents a clinical situation in which the occlusal seal produced by bonding to the enamel margin may protect the bond of the adhesive to cavity dentin against degradation. The second case represents a situation in which degradation of bond occurs in cavity with margin entirely in dentin. The present study was designed to evaluate the influence of direct and indirect water storage on the microtensile bond strength of one total-etch adhesive and two self-etching adhesives to dentin.
2.
Materials and methods
The materials used in this study (Table 1) include a total-etch adhesive; Admira Bond; and two self-etch Adhesives; Clearfil SE Bond and Hybrid Bond. Clearfil AP-X was used as restorative resin composite.
2.1.
Test methods
Fifty-four extracted human sound lower molar teeth were collected and stored in 0.5% chloramine solution in water. The teeth were used within 1 month after extraction. The root of each tooth was embedded in a cylindrical plastic tube up to 1 mm from cemento-enamel junction with cold curing acrylic resin. A standard box-type Class I cavity (4 mm 4 mm) was prepared in the occlusal surface of all teeth using a #56 carbide fissure bur at high speed with water coolant and finished with a straight fissure bur at low speed handpiece. The pulpal floor of the cavity was created approximately 1 mm into dentin. The enamel margins were beveled (458, 1 mm) using fine diamond points. The teeth were divided into 3 groups of 18 teeth. Each group was restored with resin composite using one of the adhesives. The adhesive materials were applied following the manufacturers’ instructions, as follows.
2.1.1.
Admira Bond
The entire cavity preparation was etched for 15 s with 36% phosphoric acid (Vococid, Voco, Cuxhaven, Germany), rinsed with water spray. Excess water was removed with air blast for 3 s leaving the dentin moist. Admira Bond (Voco) was applied with a disposable brush, thinned with mild air for 2–3 s and light cured for 20 s using a visible light curing device (Heliolux DLX, Ivoclar Vivadent, Schaan, Liechtenstein). The output of the light curing unit was regularly checked periodically with radiometer (Demetron Research Corp., Danbury, CT, USA) to ensure that the light was always about 500 mW/mm2.
2.1.2.
Clearfil SE Bond
Clearfil SE primer (Kuraray Medical Inc., Tokyo, Japan) was applied to the cavity for 20 s using a disposable brush and air thinned. Clearfil SE Bond (Kuraray) was then applied an, thinned with gentle stream of air and light cured for 20 s.
Table 1 – Composition of the adhesive systems used in the study Adhesive system
Component
Composition
Manufacturer
Admira Bond
Acid Bond
36% phosphoric acid Acetone, bonding ormocer, dimethacrylate, functionizing methacrylates, initiators, stabilizer
Voco, Cuxhaven, Germany
Clearfil SE Bond
Primer
HEMA, hydrophilic dimethacrylate, 10-MDP toluidine, camphorquinone, water 10-MDP, BIS-GMA, HEMA, hydrophilic dimethacrylate, microfiller
Kuraray, Tokyo, Japan
Sodium p-toluenesulfinate, Sodium N-phenylglycine (NPG-Na) Methylmethacrylate (MMA), 4-methacryloxyethyltrimetillic acid anhydride tri(2-hydroxyethyl)-isocyanurat-triacrylate (THIT), HEMA, acetone, water
Sun-Medical, Shiga, Japan
Adhesive
Hybrid Bond
Brush Adhesive
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journal of dentistry 36 (2008) 611–617
2.1.3.
Hybrid Bond
Hybrid Bond (Sun Medical Inc., Shiga, Japan) was dispensed into the mixing well. A Hybrid Bond brush was dipped into the solution, stirred shortly and then applied to the cavity for 20 s. The adhesive was thinned with gentle blast of air for 5 s and light cured for 20 s. The cavities were restored with resin composite using an incremental condensation technique. Each increment was (thickness <2 mm) light cured for 40 s. All restorations were finished with a set of carbide finishing burs (Komet, Gebr, Brasseler, Germany) and polished with polishing discs (SofLex Pop On, 3M Espe, AG, Seefeld, Germany). Three test procedures were carried out for each adhesive including sex teeth for each procedure: Procedure A: After preparation and resin composite placement, the teeth were stored in water at 37 8C for 1 day, and then microtensile bond strength measurements were carried out (24 h indirect). Procedure B: The teeth were stored for 4 years at 37 8C in water that contained 0.5% chloramine to prevent bacterial growth. Then, microtensile bond strength measurements were carried out (4-year indirect water storage). Procedure C: The restored teeth were sectioned, and the slabs were stored in water containing 0.5 chloramine at 37 8C for 4 years, where after microtensile bond strength measurements were carried out (4-year direct water storage).
2.2.
from each tooth and there cross-sectional areas were measured with a digital calliper (Mitutoyo Corp., Japan) before testing. For each test procedures 20 beams were prepared. For microtensile testing, the beams were glued to a testing device as previously described by El Zohairy et al.20 using a light curing adhesive (Clearfil SE Bond, Kuraray Co., Japan) and placed in a universal testing machine (Instron, Corp., High Wycombe, UK). Tensile load was applied at cross-head speed of 0.5 mm/min until failure occurred.
2.3.
Statistical analysis
The results were analyzed using a two-way ANOVA, with the adhesive system and testing procedure as the main factors. When the F-factor was significant, the Student–Newman– Keuls multiple comparison test was used.
2.4.
Fracture surfaces observation
After microtensile testing, the fractured surface of the specimen was inspected by stereomicroscope (Olympus, Tokyo, Japan) to evaluate the mode of failure. In addition, for some specimens of each test group impressions of the fractured surfaces were made using a light-body polyvinylsiloxane impression material (Extrude, Kerr Gmbh, Karlsruhe, Germany). The impressions were casted in epoxy resin (Epoxy Die), then mounted on aluminum stubs, sputter-coated with gold and observed by using SEM (Philips XL30, Eindhoven, the Netherlands) operating at 15 kV.
Specimen preparation for microtensile bond strength
The restored teeth were sectioned longitudinally, perpendicular to adhesive interface, buccolingually and mesiodistally with low speed water cooled diamond saw (Isomet, Buehler, Ltd. Lake Bluff, IL, USA), then the mounted tooth are rotated 908 and sectioned at its cervical portion to separate the micro-specimens. This serial sectioning leads to the formation of numerous rectangular ‘‘beams’’ or ‘‘sticks’’ with approximately 1–1.2 mm2 of cross-sectional area. Only beams from the central portion of the restoration were selected as peripheral beams may not have had the same dentin thickness. Four-six beams were obtained
3.
Results
3.1.
Microtensile bond strength test
The mean bond strengths are shown in Table 2. After 24 h water storage, there were no significant differences (P > 0.05) for the different adhesives tested. After 4 years of indirect water storage, the bond strength of each adhesive was decreased but this reduction was not significant (P > 0.05). Also there was no significant difference between the different
Table 2 – Bond strength of the tested materials (MPa W S.D.) Adhesive system
Number of slabs
24 h (control)
Four-year indirect water storage
20 20 20
39 5.2 41 3.7 37 3.4
36 4.1 39 5.1 32 2.9
Admira Bond Clearfil SE Bond Hybrid Bond
Four-year direct water storage 22 4.7 21 2.9 12 2.5
Table 3a – Fracture patterns of bonded specimens after 24 h Adhesive system
Mixed
Admira Bond (n = 20) Clearfil SE Bond (n = 20) Hybrid Bond (n = 20)
7 5 9 21 (35%)
n: number of slabs tested.
Cohesive/dentin
Cohesive/resin composite
Adhesive
2 3 1
3 2 2
8 10 8
6 (10%)
7 (11.6%)
26 (43.3%)
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journal of dentistry 36 (2008) 611–617
Table 3b – Fracture patterns of bonded specimens after 4-year indirect water storage Adhesive system
Mixed
Admira Bond (n = 20) Clearfil SE Bond (n = 20) Hybrid Bond (n = 20)
4 3 5 12 (20%)
Cohesive/dentin
Cohesive/resin composite
Adhesive
1 1 0
2 1 1
13 15 14
2 (3.3%)
4 (6.6%)
42 (70%)
n: number of slabs tested.
Table 3c – Fracture patterns of bonded specimens after 4-year direct water storage Adhesive system Admira Bond (n = 20) Clearfil SE Bond (n = 20) Hybrid Bond (n = 20)
Mixed
Cohesive/dentin
Cohesive/resin composite
Adhesive
5 2 0
0 0 0
1 1 0
14 17 20
7 (11.6%)
0 (0%)
2 (3.4%)
51 (85%)
n: number of slabs tested.
adhesive. After 4-year direct water storage, the bond strength of all adhesives were significantly (P < 0.05) reduced compared to their 24 h results and to their 4-year indirect water storage.
3.2.
Fracture surface observation
The fractured pattern of bonded specimens is shown in Tables 3a–3c and in Figs. 1–3. At 24 h water storage, 21.6% of the samples failed either cohesive in dentin or in composite, 43.4% failed purely adhesively or 35% showed mixed type of failure. After indirect water storage for 4 years 70% of the samples failed adhesively at the interface between adhesive and resin composite or between adhesive and dentin while 10% failed cohesively and 20% showed mixed failure pattern. After 4 years of direct water storage, 85% of samples showed adhesive failure at the interface, while 3.4% showed cohesive failure and 11.6% showed mixed failure.
4.
Discussion
In the present study the effect of by water storage on the bond strength of two self-etching adhesives and one total-etch adhesive to dentin was evaluated. Under clinical situation, cycling masticatory function has been reported to fatigue the integrity of resin enamel bond, thereby permitting micro- or nanoleakage of the peripheral enamel seal.21 This in turn could lead to degradation of both resin and exposed collagen fibrils by indirect exposure to water, saliva and enzymes attack.22 In addition restorations with margins that extend into the cementum are more susceptible to degradation by direct water contact.3 In the present study, both these situations were represented by direct and indirect exposure of the bonded interface to water in order to visualize the possible behavior of the tested materials under similar clinical condition. With the total-etch system, Admira Bond, bond strength of 37 4.9 MPa was found at 24 h. The primary bonding mechanism of Admira Bond was thought to be diffusionbased and depends on hybridization or infiltration of resin
Fig. 1 – (a) SEM photograph of the fractured surface of Admira Bond specimen after 4-year indirect water storage showed a dense hybrid layer that consisted of resin enveloped collagen fibrils and resin matrix. (b) SEM photograph of the fractured surface of Admira Bond specimen after 4 years of direct water storage showed exposed collagen fibrils with loss of resin contents.
journal of dentistry 36 (2008) 611–617
Fig. 2 – (a) Fractured surface of Clearfil SE Bond after 4-year indirect water storage. Failure occurred at the top of Hybrid layer. Resin seemed to infiltrate well into the interfibrillar spaces as well as into the dentinal tubules. (b) Fractured surface of Clearfil SE Bond after direct water storage for 4 years. Resin was lost and dentinal tubules were empty.
within the exposed collagen mesh as well as into dentinal tubules. After polymerization, the hybrid layer provides micromechanical retention. Accordingly, exposed collagen fibrils seemed to be well enveloped with resinous component. The resin will protect the fibrils from hydrolysis. After 4 years of indirect water storage, the bond strength was not significantly affected. Again, the protective role of surrounding resin–enamel bond against degradation and the optimal dentin hybridization of Admira Bond could explain such findings.23 SEM observation of the fractured surface of specimen showed good resin impregnation (Fig. 1a). After direct water exposure for 4 years, the bond strength of Admira Bond was significantly reduced compared with those after 24 h control or after 4-year indirect water storage. Several investigations24–27 have the attributed degradation of resinbond strength for total-etch system to the disintegration of collagen fibrils and the loss of associated resin in the area of exposed collagen fibrils in the deminerlized zone of dentin. This area was created by the discrepancy between the depth of acid etching and resin infiltration. If infiltration depth is less
615
Fig. 3 – (a) Fractured surface of specimen of hybrid bond after 24 h water storage. Failure occurred at the top of the hybrid layer with a dense resin impregnation. (b) Fractured surface of Hybrid Bond specimen after direct water storage for 4 years. Resin seemed to be extracted from the hybrid layer with increase in the size of the interfibrillar spaces.
than the deminerlized depth, a zone of hydroxyapatite depleted collagen fibrils is left exposed and unsupported. These naked collagen fibrils will undergo more strain than the overlying well resin infiltrated hybrid layer27 since the modulus of elasticity of the deminerlized dentin collagen matrix is far lower than that of the hybrid layer.28 Thus, the deminerlized dentin at the bottom of hybrid layer would became a weak link in the bonding interface over time. SEM observation of the fractured surface showed exposed collagen fibrils with loss of resin contents (Fig. 1b). The bond strength of Clearfil SE Bond showed no deterioration by indirect water storage. Clearfil SE Bond is a two-step mild self-etching adhesive. The primer of Clearfil SE Bond contains 10-MDP as functional monomer dissolved in water to result in a pH around 2.29 On dentin, Clearfil SE Bond does not remove the smear layer. It impregnates the smear plugs, fixing it at the tubules. The bonding mechanism of Clearfil SE Bond was suggested to result from the simultaneous demineralization and infiltration of enamel and dentin to form a continuum in the substrate incorporating the smear plug in the resin tag.30 This will led to a shallow but uniform resin infiltrated dentin layer. Besides to a simplification of the bonding technique, the elimination of both rinsing and drying
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journal of dentistry 36 (2008) 611–617
steps reduces the possibility of over-wetting or over-drying as they have a negative effect on adhesion. Also, the presence of the highly hydrophilic 10-MDP monomer is believed to improve the wetting to moist tooth surface. In addition, 10MDP has two hydroxyl groups that may chelate to calcium ions of dentin.31 Moreover, the residual hydroxyapatite around the exposed collagen fibrils remains available for additional chemical interaction with the functional monomers.31 This bonding mechanism seems to be able to tolerate indirect water storage for 4 years. Fig. 2(a) shows fractured surface of Clearfil SE Bond after 4-year indirect water storage. Failure occurred at the top of Hybrid layer. Resin seemed to infiltrate well into the interfibrillar spaces as well as into the dentinal tubules. However, after direct water storage, lower bond strength values have been reported. In this case, the bonded dentin surface could not completely prevent fluid movement. Slow water penetration under pressure might accelerate degradation of resin dentin bond. Fig. 2(b) shows the fractured surface of Clearfil SE Bond after direct water storage for 4 years. Resin was lost and dentinal tubules were empty. These results are in agreement with the values obtained by Armstrong and others,12 who observed a median mTBS value of 21.6 MPa after 15 months of direct water exposure, which was about half of the baseline mTBS. Hybrid Bond is one-step self-etching adhesive. In contrast to Clearfil SE Bond, Hybrid Bond contains 4-META as an active monomer component. In an aqueous environment this monomer is converted to the dicarboxylic acid 4-MET, the etching component of Hybrid Bond with a pH around 1.32 This high acidity resulted in rather deep demineralization effect. Collagen was exposed and nearly all hydroxyapatite is dissolved, consequently, their underlying mechanism of bonding is primarily diffusion-based like that of the totaletch approach. Also the amphiphilic monomer wets the exposed collagen network and bonds via hydrogen bridges.17 At the same time, the collagen coated surface is rendered hydrophobic via the methacrylate group and is thus prepared to bond to the hydrophobic monomers of the resin composite. Such bonding mechanism seems to protect the resin–dentin interface during indirect water storage for 4 years. However, after 4-year direct storage, the bond strength was significantly decreased. A possible reason for such findings could be the absence of coupling hydrophobic bonding agent, which made such material to behave as permeable membranes after polymerization. In the absence of more hydrophobic coating in the simplified adhesive system, rapid water sorption can occur via the hydrophilic and permeable adhesive layer.8 In addition, Yoshida et al.31 found a lower bonding potential of 4MET to residual hydroxyapatite around exposed collagen fibrils. This means low chemical bonding efficiency to tooth structure. Observation of the fractured surfaces after microtensile test seemed to confirm this speculation. Fig. 3a shows a fractured surface of specimen after 24 h water storage. Failure occurred at the top of the hybrid layer with a dense resin impregnation. After direct 4-year water storage most specimens failed at the bottom of the hybrid layer (Fig. 3b), suggesting degeneration of collagen and resin parts over time. The mean bond strength of the tested adhesives in our study was lower than that reported in a previous one with a similar storage condition.7 This could be attributed to the
difference in testing method employed in these two studies. In the previous one, the materials were applied to flat dentin surface. In our study, however, materials were applied in Class I cavity with high C-factor. This may induce additional polymerization stress in bonding interface which could render the bonding more vulnerable to water degradation. Another factor which could contribute to the deterioration of resin–dentin bond after water storage is the fact that the physical properties of the hydrophilic dental adhesives, such as those present in current adhesives, were reduced by 30–40% after 3–6 month water storage.33 This resulted from the plasticizing effect of water on the mechanical properties of resin. Water sorption swells the polymer chain causing decrease in their mechanical properties with passive hydrolysis and leaching effect. This passive hydrolysis and leaching effect is the most important mode of degradation of resin– dentin bond.34
5.
Conclusion
This study showed that water sorption could significantly affect the durability of the resin–dentin bond for certain adhesives. Both the residual water within the polymerized adhesive and water uptake from the media could deteriorate the adhesive bond. In restorations with margins in enamel acid etching of enamel could protect the resin–dentin bond from such effect. Restorations with margin extending into cementum or dentin, unless a bonding resin with favorable properties that can resist water degradation, is used, the bond will deteriorate with time.
references
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available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/jden
Four-year water degradation of a total-etch and two self-etching adhesives bonded to dentin Ali I. Abdalla a,*, Albert J. Feilzer b a b
Department of Restorative Dentistry, Faculty of Dentistry, University of Tanta, Tanta, Egypt Department of Dental Materials Science (ACTA), Universiteit van Amsterdam and Vrije Universiteit, Amsterdam, The Netherlands
article info
abstract
Article history:
Objectives: To evaluate effect of direct and indirect water storage on the microtensile dentin
Received 18 November 2007
bond strength of one total-etch and two self-etching adhesives.
Received in revised form
Methods: The adhesive materials were: one total-etch adhesive; ‘Admira Bond’ and two self-
18 April 2008
etch adhesives; ‘Clearfil SE Bond’ and ‘Hybrid Bond’. Freshly extracted human third molar
Accepted 21 April 2008
teeth were used. In each tooth, a Class I cavity (4 mm 4 mm) was prepared in the occlusal surface with the pulpal floor extending approximately 1 mm into dentin. The teeth were divided into three groups (n = 18). Each group was restored with the resin composite ‘Clearfil
Keywords:
APX’ using one of the tested adhesives. For each experimental group 3 test procedures (n = 6)
Self-etch adhesives
were carried out: Procedure A: the teeth were stored in water for 24 h (control), then
Water storage
sectioned longitudinally, buccolingually and mesiodistally to get rectangular slabs of 1.0–
Bond strength
1.2 mm thickness on which a microtensile test was carried out. Procedure B: the teeth were
Dentin
also sectioned; however, the slabs were stored in water at 37 8C for 4 years before microtensile testing (direct water storage). Procedure C: the teeth were kept in water at 37 8C 4 years before sectioning and microtensile testing (Indirect water storage). During microtensile testing the slabs were placed in a universal testing machine and load was applied at cross-head speed of 0.5 mm/min. Results: For the 24 h control, there was no significant difference in bond strength between the three tested adhesives. After 4 years of indirect water storage, the bond strength decreased but the reduction was not significantly different from those of 24 h. After 4 years of direct water storage, the bond strengths of all tested adhesives were significantly reduced compared to their 24 h results. Conclusion: All the tested adhesives showed no reduction in bond strength after indirect water exposure for 4 years. After 4-year direct water exposure, the bond produced by all tested adhesives was unable to resist deterioration. # 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
The durability of the adhesive bond between resin and tooth structure is of significant importance for longevity of adhesive restorations. Clinically, marginal deterioration of resin composite remains problematic and forms the major factor
that dramatically shorten the lifetime of composite–tooth bond. The immediate bonding effectiveness of most current adhesive systems is quite favorable1 regardless of the adhesive used. However, when these adhesives are tested in a clinical trial, the bonding effectiveness of some materials appears
* Corresponding author. Tel.: +20 40 3336654; fax: +20 40 331800. E-mail address: [email protected] (A.I. Abdalla). 0300-5712/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2008.04.011
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dramatically low, whereas the bonds of other materials are more stable.2,3 Bond strength tests are the most frequently used tests to screen adhesives. The rationale behind this testing method is that the stronger the adhesion between tooth and adhesives, the better it will resist stress imposed by resin polymerization and oral function. Different bond strength tests have been developed. Currently, the shear and microtensile bond strength test methods are used the most. In addition, different artificial aging techniques were used to reveal valuable clinical information. The most commonly used artificial aging technique is water storage. In this technique, the bonded specimens are stored in fluid at 37 8C for a specific period. This period may vary from a few months4 up to 4–5 years5–7 or even longer. Most of these studies report significant decreases in bond strengths, even after relatively short storage periods.8–15 The decrease in bonding effectiveness after water storage was supposed to be caused by degradation of interface components by hydrolysis (mainly resin and/or collagen). Also, water can infiltrate and decrease the mechanical properties of the polymer matrix, by swelling and reducing the frictional forces between the polymer chains, a process known as ‘plasticization’.16,17 Most in vitro bond strength studies use flat surfaces to test the bonding effectiveness of dental adhesives. Clinically, however, adhesives are applied in cavities, which result in higher polymerization contraction stress. This stress puts the resin–tooth interfaces under severe tension during the critical setting of the adhesive, particularly when restoring cavities with a high C-factor.18 Such prestressed interfaces are more susceptible to degradation19 by gaps and micro-voids that facilitate fluid exchange along the interface. In this study, the degradation of resin–dentin bonds formed in Class I cavities was studied by exposure to water for 4 years at 37 8C. In addition, the restored teeth were either stored in water intact or after sectioning. The first case represents a clinical situation in which the occlusal seal produced by bonding to the enamel margin may protect the bond of the adhesive to cavity dentin against degradation. The second case represents a situation in which degradation of bond occurs in cavity with margin entirely in dentin. The present study was designed to evaluate the influence of direct and indirect water storage on the microtensile bond strength of one total-etch adhesive and two self-etching adhesives to dentin.
2.
Materials and methods
The materials used in this study (Table 1) include a total-etch adhesive; Admira Bond; and two self-etch Adhesives; Clearfil SE Bond and Hybrid Bond. Clearfil AP-X was used as restorative resin composite.
2.1.
Test methods
Fifty-four extracted human sound lower molar teeth were collected and stored in 0.5% chloramine solution in water. The teeth were used within 1 month after extraction. The root of each tooth was embedded in a cylindrical plastic tube up to 1 mm from cemento-enamel junction with cold curing acrylic resin. A standard box-type Class I cavity (4 mm 4 mm) was prepared in the occlusal surface of all teeth using a #56 carbide fissure bur at high speed with water coolant and finished with a straight fissure bur at low speed handpiece. The pulpal floor of the cavity was created approximately 1 mm into dentin. The enamel margins were beveled (458, 1 mm) using fine diamond points. The teeth were divided into 3 groups of 18 teeth. Each group was restored with resin composite using one of the adhesives. The adhesive materials were applied following the manufacturers’ instructions, as follows.
2.1.1.
Admira Bond
The entire cavity preparation was etched for 15 s with 36% phosphoric acid (Vococid, Voco, Cuxhaven, Germany), rinsed with water spray. Excess water was removed with air blast for 3 s leaving the dentin moist. Admira Bond (Voco) was applied with a disposable brush, thinned with mild air for 2–3 s and light cured for 20 s using a visible light curing device (Heliolux DLX, Ivoclar Vivadent, Schaan, Liechtenstein). The output of the light curing unit was regularly checked periodically with radiometer (Demetron Research Corp., Danbury, CT, USA) to ensure that the light was always about 500 mW/mm2.
2.1.2.
Clearfil SE Bond
Clearfil SE primer (Kuraray Medical Inc., Tokyo, Japan) was applied to the cavity for 20 s using a disposable brush and air thinned. Clearfil SE Bond (Kuraray) was then applied an, thinned with gentle stream of air and light cured for 20 s.
Table 1 – Composition of the adhesive systems used in the study Adhesive system
Component
Composition
Manufacturer
Admira Bond
Acid Bond
36% phosphoric acid Acetone, bonding ormocer, dimethacrylate, functionizing methacrylates, initiators, stabilizer
Voco, Cuxhaven, Germany
Clearfil SE Bond
Primer
HEMA, hydrophilic dimethacrylate, 10-MDP toluidine, camphorquinone, water 10-MDP, BIS-GMA, HEMA, hydrophilic dimethacrylate, microfiller
Kuraray, Tokyo, Japan
Sodium p-toluenesulfinate, Sodium N-phenylglycine (NPG-Na) Methylmethacrylate (MMA), 4-methacryloxyethyltrimetillic acid anhydride tri(2-hydroxyethyl)-isocyanurat-triacrylate (THIT), HEMA, acetone, water
Sun-Medical, Shiga, Japan
Adhesive
Hybrid Bond
Brush Adhesive
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2.1.3.
Hybrid Bond
Hybrid Bond (Sun Medical Inc., Shiga, Japan) was dispensed into the mixing well. A Hybrid Bond brush was dipped into the solution, stirred shortly and then applied to the cavity for 20 s. The adhesive was thinned with gentle blast of air for 5 s and light cured for 20 s. The cavities were restored with resin composite using an incremental condensation technique. Each increment was (thickness <2 mm) light cured for 40 s. All restorations were finished with a set of carbide finishing burs (Komet, Gebr, Brasseler, Germany) and polished with polishing discs (SofLex Pop On, 3M Espe, AG, Seefeld, Germany). Three test procedures were carried out for each adhesive including sex teeth for each procedure: Procedure A: After preparation and resin composite placement, the teeth were stored in water at 37 8C for 1 day, and then microtensile bond strength measurements were carried out (24 h indirect). Procedure B: The teeth were stored for 4 years at 37 8C in water that contained 0.5% chloramine to prevent bacterial growth. Then, microtensile bond strength measurements were carried out (4-year indirect water storage). Procedure C: The restored teeth were sectioned, and the slabs were stored in water containing 0.5 chloramine at 37 8C for 4 years, where after microtensile bond strength measurements were carried out (4-year direct water storage).
2.2.
from each tooth and there cross-sectional areas were measured with a digital calliper (Mitutoyo Corp., Japan) before testing. For each test procedures 20 beams were prepared. For microtensile testing, the beams were glued to a testing device as previously described by El Zohairy et al.20 using a light curing adhesive (Clearfil SE Bond, Kuraray Co., Japan) and placed in a universal testing machine (Instron, Corp., High Wycombe, UK). Tensile load was applied at cross-head speed of 0.5 mm/min until failure occurred.
2.3.
Statistical analysis
The results were analyzed using a two-way ANOVA, with the adhesive system and testing procedure as the main factors. When the F-factor was significant, the Student–Newman– Keuls multiple comparison test was used.
2.4.
Fracture surfaces observation
After microtensile testing, the fractured surface of the specimen was inspected by stereomicroscope (Olympus, Tokyo, Japan) to evaluate the mode of failure. In addition, for some specimens of each test group impressions of the fractured surfaces were made using a light-body polyvinylsiloxane impression material (Extrude, Kerr Gmbh, Karlsruhe, Germany). The impressions were casted in epoxy resin (Epoxy Die), then mounted on aluminum stubs, sputter-coated with gold and observed by using SEM (Philips XL30, Eindhoven, the Netherlands) operating at 15 kV.
Specimen preparation for microtensile bond strength
The restored teeth were sectioned longitudinally, perpendicular to adhesive interface, buccolingually and mesiodistally with low speed water cooled diamond saw (Isomet, Buehler, Ltd. Lake Bluff, IL, USA), then the mounted tooth are rotated 908 and sectioned at its cervical portion to separate the micro-specimens. This serial sectioning leads to the formation of numerous rectangular ‘‘beams’’ or ‘‘sticks’’ with approximately 1–1.2 mm2 of cross-sectional area. Only beams from the central portion of the restoration were selected as peripheral beams may not have had the same dentin thickness. Four-six beams were obtained
3.
Results
3.1.
Microtensile bond strength test
The mean bond strengths are shown in Table 2. After 24 h water storage, there were no significant differences (P > 0.05) for the different adhesives tested. After 4 years of indirect water storage, the bond strength of each adhesive was decreased but this reduction was not significant (P > 0.05). Also there was no significant difference between the different
Table 2 – Bond strength of the tested materials (MPa W S.D.) Adhesive system
Number of slabs
24 h (control)
Four-year indirect water storage
20 20 20
39 5.2 41 3.7 37 3.4
36 4.1 39 5.1 32 2.9
Admira Bond Clearfil SE Bond Hybrid Bond
Four-year direct water storage 22 4.7 21 2.9 12 2.5
Table 3a – Fracture patterns of bonded specimens after 24 h Adhesive system
Mixed
Admira Bond (n = 20) Clearfil SE Bond (n = 20) Hybrid Bond (n = 20)
7 5 9 21 (35%)
n: number of slabs tested.
Cohesive/dentin
Cohesive/resin composite
Adhesive
2 3 1
3 2 2
8 10 8
6 (10%)
7 (11.6%)
26 (43.3%)
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Table 3b – Fracture patterns of bonded specimens after 4-year indirect water storage Adhesive system
Mixed
Admira Bond (n = 20) Clearfil SE Bond (n = 20) Hybrid Bond (n = 20)
4 3 5 12 (20%)
Cohesive/dentin
Cohesive/resin composite
Adhesive
1 1 0
2 1 1
13 15 14
2 (3.3%)
4 (6.6%)
42 (70%)
n: number of slabs tested.
Table 3c – Fracture patterns of bonded specimens after 4-year direct water storage Adhesive system Admira Bond (n = 20) Clearfil SE Bond (n = 20) Hybrid Bond (n = 20)
Mixed
Cohesive/dentin
Cohesive/resin composite
Adhesive
5 2 0
0 0 0
1 1 0
14 17 20
7 (11.6%)
0 (0%)
2 (3.4%)
51 (85%)
n: number of slabs tested.
adhesive. After 4-year direct water storage, the bond strength of all adhesives were significantly (P < 0.05) reduced compared to their 24 h results and to their 4-year indirect water storage.
3.2.
Fracture surface observation
The fractured pattern of bonded specimens is shown in Tables 3a–3c and in Figs. 1–3. At 24 h water storage, 21.6% of the samples failed either cohesive in dentin or in composite, 43.4% failed purely adhesively or 35% showed mixed type of failure. After indirect water storage for 4 years 70% of the samples failed adhesively at the interface between adhesive and resin composite or between adhesive and dentin while 10% failed cohesively and 20% showed mixed failure pattern. After 4 years of direct water storage, 85% of samples showed adhesive failure at the interface, while 3.4% showed cohesive failure and 11.6% showed mixed failure.
4.
Discussion
In the present study the effect of by water storage on the bond strength of two self-etching adhesives and one total-etch adhesive to dentin was evaluated. Under clinical situation, cycling masticatory function has been reported to fatigue the integrity of resin enamel bond, thereby permitting micro- or nanoleakage of the peripheral enamel seal.21 This in turn could lead to degradation of both resin and exposed collagen fibrils by indirect exposure to water, saliva and enzymes attack.22 In addition restorations with margins that extend into the cementum are more susceptible to degradation by direct water contact.3 In the present study, both these situations were represented by direct and indirect exposure of the bonded interface to water in order to visualize the possible behavior of the tested materials under similar clinical condition. With the total-etch system, Admira Bond, bond strength of 37 4.9 MPa was found at 24 h. The primary bonding mechanism of Admira Bond was thought to be diffusionbased and depends on hybridization or infiltration of resin
Fig. 1 – (a) SEM photograph of the fractured surface of Admira Bond specimen after 4-year indirect water storage showed a dense hybrid layer that consisted of resin enveloped collagen fibrils and resin matrix. (b) SEM photograph of the fractured surface of Admira Bond specimen after 4 years of direct water storage showed exposed collagen fibrils with loss of resin contents.
journal of dentistry 36 (2008) 611–617
Fig. 2 – (a) Fractured surface of Clearfil SE Bond after 4-year indirect water storage. Failure occurred at the top of Hybrid layer. Resin seemed to infiltrate well into the interfibrillar spaces as well as into the dentinal tubules. (b) Fractured surface of Clearfil SE Bond after direct water storage for 4 years. Resin was lost and dentinal tubules were empty.
within the exposed collagen mesh as well as into dentinal tubules. After polymerization, the hybrid layer provides micromechanical retention. Accordingly, exposed collagen fibrils seemed to be well enveloped with resinous component. The resin will protect the fibrils from hydrolysis. After 4 years of indirect water storage, the bond strength was not significantly affected. Again, the protective role of surrounding resin–enamel bond against degradation and the optimal dentin hybridization of Admira Bond could explain such findings.23 SEM observation of the fractured surface of specimen showed good resin impregnation (Fig. 1a). After direct water exposure for 4 years, the bond strength of Admira Bond was significantly reduced compared with those after 24 h control or after 4-year indirect water storage. Several investigations24–27 have the attributed degradation of resinbond strength for total-etch system to the disintegration of collagen fibrils and the loss of associated resin in the area of exposed collagen fibrils in the deminerlized zone of dentin. This area was created by the discrepancy between the depth of acid etching and resin infiltration. If infiltration depth is less
615
Fig. 3 – (a) Fractured surface of specimen of hybrid bond after 24 h water storage. Failure occurred at the top of the hybrid layer with a dense resin impregnation. (b) Fractured surface of Hybrid Bond specimen after direct water storage for 4 years. Resin seemed to be extracted from the hybrid layer with increase in the size of the interfibrillar spaces.
than the deminerlized depth, a zone of hydroxyapatite depleted collagen fibrils is left exposed and unsupported. These naked collagen fibrils will undergo more strain than the overlying well resin infiltrated hybrid layer27 since the modulus of elasticity of the deminerlized dentin collagen matrix is far lower than that of the hybrid layer.28 Thus, the deminerlized dentin at the bottom of hybrid layer would became a weak link in the bonding interface over time. SEM observation of the fractured surface showed exposed collagen fibrils with loss of resin contents (Fig. 1b). The bond strength of Clearfil SE Bond showed no deterioration by indirect water storage. Clearfil SE Bond is a two-step mild self-etching adhesive. The primer of Clearfil SE Bond contains 10-MDP as functional monomer dissolved in water to result in a pH around 2.29 On dentin, Clearfil SE Bond does not remove the smear layer. It impregnates the smear plugs, fixing it at the tubules. The bonding mechanism of Clearfil SE Bond was suggested to result from the simultaneous demineralization and infiltration of enamel and dentin to form a continuum in the substrate incorporating the smear plug in the resin tag.30 This will led to a shallow but uniform resin infiltrated dentin layer. Besides to a simplification of the bonding technique, the elimination of both rinsing and drying
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steps reduces the possibility of over-wetting or over-drying as they have a negative effect on adhesion. Also, the presence of the highly hydrophilic 10-MDP monomer is believed to improve the wetting to moist tooth surface. In addition, 10MDP has two hydroxyl groups that may chelate to calcium ions of dentin.31 Moreover, the residual hydroxyapatite around the exposed collagen fibrils remains available for additional chemical interaction with the functional monomers.31 This bonding mechanism seems to be able to tolerate indirect water storage for 4 years. Fig. 2(a) shows fractured surface of Clearfil SE Bond after 4-year indirect water storage. Failure occurred at the top of Hybrid layer. Resin seemed to infiltrate well into the interfibrillar spaces as well as into the dentinal tubules. However, after direct water storage, lower bond strength values have been reported. In this case, the bonded dentin surface could not completely prevent fluid movement. Slow water penetration under pressure might accelerate degradation of resin dentin bond. Fig. 2(b) shows the fractured surface of Clearfil SE Bond after direct water storage for 4 years. Resin was lost and dentinal tubules were empty. These results are in agreement with the values obtained by Armstrong and others,12 who observed a median mTBS value of 21.6 MPa after 15 months of direct water exposure, which was about half of the baseline mTBS. Hybrid Bond is one-step self-etching adhesive. In contrast to Clearfil SE Bond, Hybrid Bond contains 4-META as an active monomer component. In an aqueous environment this monomer is converted to the dicarboxylic acid 4-MET, the etching component of Hybrid Bond with a pH around 1.32 This high acidity resulted in rather deep demineralization effect. Collagen was exposed and nearly all hydroxyapatite is dissolved, consequently, their underlying mechanism of bonding is primarily diffusion-based like that of the totaletch approach. Also the amphiphilic monomer wets the exposed collagen network and bonds via hydrogen bridges.17 At the same time, the collagen coated surface is rendered hydrophobic via the methacrylate group and is thus prepared to bond to the hydrophobic monomers of the resin composite. Such bonding mechanism seems to protect the resin–dentin interface during indirect water storage for 4 years. However, after 4-year direct storage, the bond strength was significantly decreased. A possible reason for such findings could be the absence of coupling hydrophobic bonding agent, which made such material to behave as permeable membranes after polymerization. In the absence of more hydrophobic coating in the simplified adhesive system, rapid water sorption can occur via the hydrophilic and permeable adhesive layer.8 In addition, Yoshida et al.31 found a lower bonding potential of 4MET to residual hydroxyapatite around exposed collagen fibrils. This means low chemical bonding efficiency to tooth structure. Observation of the fractured surfaces after microtensile test seemed to confirm this speculation. Fig. 3a shows a fractured surface of specimen after 24 h water storage. Failure occurred at the top of the hybrid layer with a dense resin impregnation. After direct 4-year water storage most specimens failed at the bottom of the hybrid layer (Fig. 3b), suggesting degeneration of collagen and resin parts over time. The mean bond strength of the tested adhesives in our study was lower than that reported in a previous one with a similar storage condition.7 This could be attributed to the
difference in testing method employed in these two studies. In the previous one, the materials were applied to flat dentin surface. In our study, however, materials were applied in Class I cavity with high C-factor. This may induce additional polymerization stress in bonding interface which could render the bonding more vulnerable to water degradation. Another factor which could contribute to the deterioration of resin–dentin bond after water storage is the fact that the physical properties of the hydrophilic dental adhesives, such as those present in current adhesives, were reduced by 30–40% after 3–6 month water storage.33 This resulted from the plasticizing effect of water on the mechanical properties of resin. Water sorption swells the polymer chain causing decrease in their mechanical properties with passive hydrolysis and leaching effect. This passive hydrolysis and leaching effect is the most important mode of degradation of resin– dentin bond.34
5.
Conclusion
This study showed that water sorption could significantly affect the durability of the resin–dentin bond for certain adhesives. Both the residual water within the polymerized adhesive and water uptake from the media could deteriorate the adhesive bond. In restorations with margins in enamel acid etching of enamel could protect the resin–dentin bond from such effect. Restorations with margin extending into cementum or dentin, unless a bonding resin with favorable properties that can resist water degradation, is used, the bond will deteriorate with time.
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