Four-year water degradation of a total-etch and twoself-etching adhesives bonded to dentin

Ali I. Abdalla a,*, Albert J. Feilzer b

aDepartment of Restorative Dentistry, Faculty of Dentistry, University of Tanta, Tanta, EgyptbDepartment of Dental Materials Science (ACTA), Universiteit van Amsterdam and Vrije Universiteit, Amsterdam, The Netherlands

j o u r n a l o f d e n t i s t r y 3 6 ( 2 0 0 8 ) 6 1 1 – 6 1 7

a r t i c l e i n f o

Article history:

Received 18 November 2007

Received in revised form

18 April 2008

Accepted 21 April 2008

Keywords:

Self-etch adhesives

Water storage

Bond strength

Dentin

a b s t r a c t

Objectives: To evaluate effect of direct and indirect water storage on the microtensile dentin

bond strength of one total-etch and two self-etching adhesives.

Methods: The adhesive materials were: one total-etch adhesive; ‘Admira Bond’ and two self-

etch adhesives; ‘Clearfil SE Bond’ and ‘Hybrid Bond’. Freshly extracted human third molar

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

APX’ using one of the tested adhesives. For each experimental group 3 test procedures (n = 6)

were carried out: Procedure A: the teeth were stored in water for 24 h (control), then

sectioned longitudinally, buccolingually and mesiodistally to get rectangular slabs of 1.0–

1.2 mm thickness on which a microtensile test was carried out. Procedure B: the teeth were

also sectioned; however, the slabs were stored in water at 37 8C for 4 years before micro-

tensile 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 micro-

tensile 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.

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journal homepage: www. int l .e lsev ierhea l th .com/ journa ls / jden

1. Introduction

The durability of the adhesive bond between resin and tooth

structure is of significant importance for longevity of adhe-

sive restorations. Clinically, marginal deterioration of resin

composite remains problematic and forms the major factor

* Corresponding author. Tel.: +20 40 3336654; fax: +20 40 331800.E-mail address: ali_abdalla79@yahoo.com (A.I. Abdalla).

0300-5712/$ – see front matter # 2008 Elsevier Ltd. All rights reserveddoi:10.1016/j.jdent.2008.04.011

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

.

j o u r n a l o f d e n t i s t r y 3 6 ( 2 0 0 8 ) 6 1 1 – 6 1 7612

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 informa-

tion. 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 thefrictional forces between thepolymer

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.

Table 1 – Composition of the adhesive systems used in the st

Adhesive system Component Com

Admira Bond Acid 36% phosphoric acid

Bond Acetone, bonding ormocer, dim

methacrylates, initiators, stabi

Clearfil SE Bond Primer HEMA, hydrophilic dimethacry

camphorquinone, water

Adhesive 10-MDP, BIS-GMA, HEMA, hydr

microfiller

Hybrid Bond Brush Sodium p-toluenesulfinate, Sod

Adhesive Methylmethacrylate (MMA), 4-

acid anhydride tri(2-hydroxyet

HEMA, acetone, water

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 man-

ufacturers’ instructions, as follows.

2.1.1. Admira BondThe 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, Liechten-

stein). 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 BondClearfil 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.

udy

position Manufacturer

Voco, Cuxhaven, Germany

ethacrylate, functionizing

lizer

late, 10-MDP toluidine, Kuraray, Tokyo, Japan

ophilic dimethacrylate,

ium N-phenylglycine (NPG-Na) Sun-Medical, Shiga, Japan

methacryloxyethyltrimetillic

hyl)-isocyanurat-triacrylate (THIT),

j o u r n a l o f d e n t i s t r y 3 6 ( 2 0 0 8 ) 6 1 1 – 6 1 7 613

2.1.3. Hybrid BondHybrid 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 (Sof-

Lex 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 place-

ment, 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. Specimen preparation for microtensile bond strength

Therestored teethweresectioned 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-speci-

mens. 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

Table 2 – Bond strength of the tested materials (MPa W S.D.)

Adhesive system Number of slabs 24 h (control) Four-y

Admira Bond 20 39 � 5.2

Clearfil SE Bond 20 41 � 3.7

Hybrid Bond 20 37 � 3.4

Table 3a – Fracture patterns of bonded specimens after 24 h

Adhesive system Mixed Cohesive/de

Admira Bond (n = 20) 7 2

Clearfil SE Bond (n = 20) 5 3

Hybrid Bond (n = 20) 9 1

21 (35%) 6 (10%)

n: number of slabs tested.

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 speci-

men 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, Ger-

many). 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.

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

ear indirect water storage Four-year direct water storage

36 � 4.1 22 � 4.7

39 � 5.1 21 � 2.9

32 � 2.9 12 � 2.5

ntin Cohesive/resin composite Adhesive

3 8

2 10

2 8

7 (11.6%) 26 (43.3%)

Table 3b – Fracture patterns of bonded specimens after 4-year indirect water storage

Adhesive system Mixed Cohesive/dentin Cohesive/resin composite Adhesive

Admira Bond (n = 20) 4 1 2 13

Clearfil SE Bond (n = 20) 3 1 1 15

Hybrid Bond (n = 20) 5 0 1 14

12 (20%) 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 Mixed Cohesive/dentin Cohesive/resin composite Adhesive

Admira Bond (n = 20) 5 0 1 14

Clearfil SE Bond (n = 20) 2 0 1 17

Hybrid Bond (n = 20) 0 0 0 20

7 (11.6%) 0 (0%) 2 (3.4%) 51 (85%)

n: number of slabs tested.

j o u r n a l o f d e n t i s t r y 3 6 ( 2 0 0 8 ) 6 1 1 – 6 1 7614

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.

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.

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 diffusion-

based and depends on hybridization or infiltration of resin

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.

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.

j o u r n a l o f d e n t i s t r y 3 6 ( 2 0 0 8 ) 6 1 1 – 6 1 7 615

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 surround-

ing 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 speci-

men 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 resin-

bond 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

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 simulta-

neous 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

j o u r n a l o f d e n t i s t r y 3 6 ( 2 0 0 8 ) 6 1 1 – 6 1 7616

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, 10-

MDP 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 total-

etch 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 4-

MET 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 hydro-

lysis 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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