Effect of Solvent Evaporation Strategies On

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Effect of solvent evaporation strategies on

regional bond strength of one-step self-etch

adhesives to root canal dentine

S. Thitthaweerat1,2,3, M. Nakajima1, R. M. Foxton4 & J. Tagami1,3

1Cariology and Operative Dentistry, Department of Oral Health Sciences, Tokyo Medical and Dental University, Tokyo, Japan;2Department of Operative Dentistry and Endodontics, Faculty of Dentistry, Mahidol University, Bangkok, Thailand; 3Global

Center Excellence (GCOE) Program, International Research Center for Molecular Science in Tooth and Bone Diseases, Tokyo

Medical and Dental University, Tokyo, Japan; and 4Division of Conservative Dentistry, Kings College London Dental Institute

at Guys, Kings and St Thomas Hospitals, Kings College London, London, UK

Abstract

Thitthaweerat S, Nakajima M, Foxton RM, Tagami

J. Effect of solvent evaporation strategies on regional bond

strength of one-step self-etch adhesives to root canal dentine.

International Endodontic Journal, 46, 10231031, 2013.

Aim To evaluate the efficacy of different solvent

evaporation strategies on bonding of one-step self-etch

adhesives to root canal dentine.

Methodology Two dual-cure resin core systems

(Clearfil DC Bond/Clearfil DC Core Automix and Clear-

fil Tri-S Bond Plus/Clearfil DC Core Plus; Kuraray

Noritake Dental, Tokyo, Japan) were equally applied

in 24 post spaces from extracted human mandibular

premolars. After the adhesive application, specimens

were randomly assigned into four water/solvent evap-

oration strategies as follows (I) insertion of absorbent

paper point for 10 s: P, (II) 10 s air-blowing: A, (III)

as (II) followed by insertion of absorbent paper point:

AP, (IV) as (III) followed by 10 s additional air-

blowing: APA. Then, the adhesives were light cured,

and resin core materials were placed into the post

space, followed by light curing for 60 s. After water

storage for 24 h, 0.6 9 0.6 mm-thick beams were

prepared to measure the regional lTBS. The mode of

failure was also observed. The lTBS values were

statistically analysed using three-way ANOVA andDuncan HSD test (a = 0.05).

Results In the coronal region, there were no signifi-

cant differences in lTBS between each evaporation

strategy (P > 0.05), except P group. However, in the

apical region, APA and AP groups significantly increased

in lTBS compared with A and P groups (P < 0.05).

Only in the APA group of Clearfil Tri-S Bond Plus/

Clearfil DC Core Plus, was there no significant differ-

ence in lTBS between the coronal and apical regions

(P > 0.05).

Conclusion The use of paper points with additional

air-blowing for removing excessive adhesive and

evaporating residual water/solvent would be effective

in producing higher bond strength for the tested one-

step self-etch adhesives and fewer blister formations

in deeper regions of the post space.

Keywords: bonding, micro-tensile bond strength,

one-step self-etch adhesive, root canal dentine, solvent

evaporation.

Received 20 November 2012; accepted 18 February 2013

Introduction

Generally, one-step self-etch adhesives are a mixture

of hydrophilic and hydrophobic resin monomers,

dissolved in water and organic solvents, such as

ethanol, acetone etc. The organic solvents improve the

miscibility of hydrophilic and hydrophobic compo-

nents and the diffusion of monomer into the hydrated

Correspondence: Masatoshi Nakajima, Cariology and Opera-

tive Dentistry, Department of Oral Health Sciences, Tokyo

Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku,

Tokyo 113-8549, Japan (Tel.: +81(0)3 5803 5483; fax:

+81(0)3 5803 0195; e-mail: [email protected]).

2013 International Endodontic Journal. Published by John Wiley & Sons Ltd International Endodontic Journal, 46, 10231031, 2013

doi:10.1111/iej.12093

1023


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demineralized matrix. Meanwhile, they play an impor-

tant role in removal of water from the adhesive sur-

face due to azeotropic dehydration (Van Landuyt et al.

2007, Loguercio et al. 2009). Air-drying after adhe-

sive application can facilitate the evaporation of

water/solvent from the applied dentine substrate

(Spreafico et al. 2006) and thin the adhesive layer,leading to diminishing amounts of water/solvent in

the adhesive layer (Zheng et al. 2001). However, com-

plete water/solvent evaporation is difficult to achieve

in the clinical situation because the air-blowing proce-

dure is restricted in the oral environment (Ikeda et al.

2005). Many studies have demonstrated that residual

water and solvent can dilute the monomer and inhibit

the polymerization reaction, resulting in a reduction of

bond strength (Galan et al. 1991, De Munck et al.

2005). Moreover, blister formation and higher perme-

ability of the adhesive layer would occur, leading to a

reduction in the quality of adhesive interface (Hashim-

otoet al. 2005, Ferrari et al. 2008).

To improve the bonding performance of one-step

self-etch adhesives, many studies have investigated

solvent evaporation strategies using flat coronal den-

tine surfaces, such as using warm air stream (Galan

et al. 1991, Klein-Junior et al. 2008, Garcia et al.

2009, Reis et al. 2010), extending the air-drying time

(Furuse et al. 2008, Giannini et al. 2008, Ikeda et al.

2008, Garcia et al. 2009) and increasing the air-

pressure (El-Askary & Van Noort 2011). However, it

has been indicated that these evaporation strategies

were not enough to completely remove residual

water/solvent from the adhesive layer (De Muncket al. 2005, Hashimoto et al. 2006).

Unlike coronal flat dentine surfaces, the root canal

dentine cavity appears to be a challenging bonding

substrate due to various factors, such as attenuation

of the light energy in the deeper region (Foxton et al.

2003, Aksornmuang et al. 2006, 2009, Mao et al.

2011) and limited accessibility to the root canal post

for bonding application and/or solvent evaporation

(Schwartz 2006, Breschi et al. 2009), which could

adversely affect the adhesive performance to root

canal dentine. Moreover, due to the narrower orifice

of the post cavity and/or further distance from the

air-blowing source to deeper region, efficient water/

solvent evaporation would deteriorate in the deeper

region. Spreading of the adhesive by air-blowing

would contribute to better solvent evaporation of the

adhesives and more polymerization (Galan et al.

1991). El-Askary & Van Noort (2011) have men-

tioned that increasing the distance from the dentine

substrate during air-drying significantly decreases

solvent evaporation from one-step self-etch adhesives,

reducing their bond strength. Therefore, the water/

solvent evaporation strategy prior to light curing is

very important for bonding to root canal dentine.

A previous study (Souza et al. 2007) reported that

using a paper point was useful for removing excessadhesive in root canal, leading to an improvement in

the bond strength to bovine root dentine. However,

there was a little information on which is the most

effective water/solvent evaporation strategy for bond-

ing a one-step self-etch adhesive to human root canal

dentine.

Therefore, the aim of this study was to determine

the effectiveness of different solvent evaporation strat-

egies in improving adhesion to human root canal

dentine. The null hypothesis was that applying differ-

ent solvent evaporation strategies had no effect on

the bond strength of one-step self-etch adhesives to

human root canal dentine.

Materials and methods

Preparation of bonded specimens

Twenty-four single-rooted human mandibular premo-

lar teeth extracted from adolescents for orthodontic

reasons, with similar root length and free of cracks,

caries and fractures, were collected following ethnical

approval by the Ethics Committee of Tokyo Medical

and Dental University under protocol No. 725 and

stored in refrigerated distilled water at 4 C beforebeing used. The crown was sectioned 2 mm above the

cementoenamel junction using a low-speed diamond

saw (Isomet; Buehler, Lake Bluff, IL, USA) with water

lubrication. The root canals were mechanically

enlarged using endodontic K-files with distilled water

irrigation until a size 25 file reached the canal

terminus. Then, post spaces were prepared using

Gates-Glidden drills (Matsutani Seisakusho Co. Ltd.,

Takanezawa, Japan) and FiberKor drills (Pentron

Corp., Wallingford, CT, USA) in a low-speed handpiece

under copious water-cooling to a depth of 8 mm from

the sectioned root surface and a diameter of 1.5 mm

at the apical end. Prior to the bonding procedure, the

external surfaces of the roots were built up with a

resin composite (Clearfil Majesty and Clearfil SE bond;

Kuraray Noritake Dental, Tokyo, Japan) to make grips

for testing and to prevent the external curing light,

which could pass through thin portions of the dentine

wall to the adhesive resin during photo-irradiation.

Solvent evaporation on bonding to root canal dentine Thitthaweerat et al.

2013 International Endodontic Journal. Published by John Wiley & Sons LtdInternational Endodontic Journal, 46, 10231031, 20131024


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Two dual-cure resin core systems (Clearfil DC

Bond/Clearfil DC Core Automix, Clearfil Tri-S Bond

Plus/Clearfil DC Core Plus; Kuraray Noritake Dental,

Tokyo, Japan) were used. Each resin core system con-

sists of a one-step self-etching adhesive and a resin

core material, which the manufacturer suggests

should be used together. The chemical compositionsand bonding procedure of the materials are presented

in Table 1. After applying the adhesives to the walls

of the post spaces using a micro-brush disposable

applicator (Pentron Clinical Technologies), the post

spaces were randomly divided into four groups

according to the solvent evaporation technique, each

consisting of three teeth: Group 1(P): insertion of a

size 60 absorbent paper point (Dentsply Tulsa Dental,

Tulsa, OK, USA) for 10 s into the root canal; Group 2

(A): air-blowing for 10 s; Group 3(AP): Group 2

followed by insertion of an absorbent paper point for

10 s; Group 4 (APA): Group 3 followed by additive

air-blowing for 10 s. Air-blowing the adhesives was

performed using a dental triple syringe from 1 cm

above post orifice, whose air-pressure was approxi-

mately 3.5 kg cm2.

Then, all the adhesives were light cured in accor-

dance with the manufacturers instructions using a

halogen light source (Optilux 501; Demetron Kerr,

Danbury, CA, USA), whose mean light intensity was

550 mW cm2. For Clearfil DC Bond, the manufac-

turers instructions recommend 20 s light curing time

to adhesive, while Clearfil Tri-s Bond Plus is recom-

mended 10 s light curing time. Afterward, all the post

spaces were filled with dual-cure resin core material

using a guide tip ( 1.1 mm.) and light cured for 60 s.

Bond strength testing

After 24 h water storage at 37 C, each bonded speci-

men was sectioned perpendicular to the bonded inter-face using a low-speed diamond saw (Isomet) under

water-cooling to create 8, 0.6-mm-thick slabs. Four

coronal slabs were considered to represent the coronal

portion of the post space corresponding to the coronal

third of root canal, while those of four apical slabs

were considered to represent the apical region

corresponding to the middle third of root canal. Then,

each slab was cut transversely at the central part

of post space in mesio-distal direction to produce

0.6 9 0.6 mm-thick beam (Aksornmuang et al.

2009). One of the two interfaces of each beam was

randomly selected for lTBS testing. The ends of the

beam and the remaining interface were carefully

attached onto a testing device in universal testing

machine (EZ Test; Shimadzu, Kyoto, Japan) using cya-

noacrylate glue (Zapit; Dental Ventures of America,

Anaheim, CA, USA) and subjected to a tensile force at

a crosshead speed of 1 mm min1. After the speci-

mens were fractured, the cross-sectional area of each

beam was measured with digital calipers (Mitutoyo

CD15; Mitutoyo Co., Kawasaki, Japan) to 0.01 mm

accuracy, which was approximately 0.37 mm2. The

value was recorded in kilogram9force (kgf) and trans-

formed to lTBS value in MPa.

Table 1 The chemical compositions and bonding procedure of the materials used in this study

Materials Chemical compositions Bonding procedure

Clearfil DC Core Automix

(Kuraray Noritake Dental;

Tokyo, Japan)

Adhesive(Clearfil DC Bond)

A: 10-MDP, HEMA, Bis-GMA, colloidal silica, CQ, BPO

B: ethanol, water

Resin core material (Clearfil DC Core Automix)

Bis-GMA, TEGDMA, Hydrophobic aromatic dimethacrylate,

silanated barium glass filler, silanated silica, CQ, BPO, accelerators

Apply a mixture of liquid

A&B and leave for 20 s,

strong air blow for 10 s,

Light cured for 20 s

Clearfil DC Core Plus

(Kuraray Noritake Dental;

Tokyo, Japan)

Adhesive(Clearfil Tri-S Bond Plus)

10-MDP, HEMA, Bis-GMA, hydrophilic aliphatic dimethacrylate,

hydrophobic aliphatic methacrylate, colloidal silica, CQ, NaF,

accelerators, initiators, ethanol, water

Resin core material (Clearfil DC Core Plus)

Bis-GMA, TEGDMA, hydrophobic/hydrophilic aliphatic

dimethacrylate, hydrophobic aromatic dimethacrylate, silanated

barium glass filler, silanated colloidal silica, colloidal silica,

aluminum oxide filler, CQ, accelerator, initiator

Apply and leave for 10 s,

mild air blowing for more

than 5 s, light-curing for 10 s

10-MDP, 10-Methacryloyloxydecyl dihydrogen phosphate; HEMA, 2-hydroxyethyl methacrylate; Bis-GMA, 2,2-bis[4-(2-hydroxy-3-

methacryloyloxypropoxy)phenyl]propane; CQ, Camphorquinone; BPO, Benzoyl peroxide; TEGDMA, Triethylene glycol dimethacry-

late.

Thitthaweerat et al. Solvent evaporation on bonding to root canal dentine

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Fracture analysis

After testing, both the resin and dentine sides of the

fractured beams were mounted on brass tablets and

gold sputter-coated. Fracture surfaces were observed

using a scanning electron microscope (JSM-5310;

JEOL, Tokyo, Japan) at magnifications of 150 and500. The failure mode was classified as one of the fol-

lowing: adhesive failure, cohesive failure in resin,

cohesive failure in dentine and mixed adhesive. To

observe the presence of blister formation in each root

canal region within the adhesive layer, adhesive fail-

ure areas in the fractured beams were observed using

SEM at the magnification of 500.

Statistical analysis

Three-way ANOVA was performed to evaluate interac-

tions among the three variables (material tested, sol-

vent evaporation strategy, root region), and the

Duncan post hoc test was used for multiple compari-

sons (a = 0.05). The failure mode data were analysed

using the chi-squared test. All statistical analyses

were performed using SPSS software version 17 (SPSS

Inc., Chicago, IL, USA).

Results

The means and standard deviations of the regional

microtensile bond strengths of the materials and evap-

oration strategies are illustrated in Table 2. Three-

way ANOVA revealed that material tested, solvent evap-oration strategy and root region significantly affected

the lTBS to root canal dentine (P = 0.002,

P < 0.0001 and P < 0.0001, respectively). Significant

interactions between evaporation technique and root

region, material tested and root region were observed

(P = 0.01, P = 0.024, respectively).

For both regions of post space, the P group was

significantly lower lTBS than the other evaporation

strategies (P < 0.05). In the coronal region, the lTBS

of both materials were not significantly different

between the A, APand APA groups (P > 0.05); how-ever, in the apical region, Clearfil DC Core Plus

showed significant differences in the lTBS between

them (P < 0.05), in which the APA group exhibited

the highest bond strength. For Clearfil DC Core Auto-

mix, the AP and APA groups exhibited significantly

higher lTBS compared to the A group (P < 0.05).

Only for the APA group of Clearfil DC Core Plus, was

there no significant difference in lTBS between the

coronal and apical regions (P > 0.05), whereas for

the other experimental groups, the coronal region

were significantly higher lTBS than that of the apical

region (P < 0.05).

There was no pretesting failure during specimen

preparation. Figure 1 presents the failure mode per-

centage of the debonded specimens. In this study,

there was no failure within the dentine substrate.

The predominant failure modes occurred at the

adhesive interface between dentine and adhesive

resin. Chi-square testing revealed significant differ-

ences in failure mode between solvent evaporation

techniques in the apical region of the post space

(P < 0.05). The high incidence of cohesive failure in

resin was found in the A and P groups (41.7% and

33.3% for Clearfil DC Core Automix, respectively;

33.3% and 41.7% for Clearfil DC Core Plus, respec-tively). On the other hand, a notably lower number

of cohesive failures in resin were found in the APA

group (16.7% for Clearfil DC Core Automix, 8.3% for

Clearfil DC Core Plus).

Table 2 The lTBS to root canal dentin in each resin core system, evaporation technique and root region. There were signifi-

cant differences in lTBS between coronal and apical region, except in APAgroup of Clearfil DC Core Plus (P > 0.05).

Materials

Root regions

(n = 12 )

Solvent evaporation technique

P A AP APA

Clearfil DC Core Automix Coronal 28.01 3.971A 38.39 4.282A 38.40 3.422A 38.73 3.502A

P < 0.05 P < 0.05 P < 0.05 P < 0.05

Apical 20.78 2.871a 26.79 3.612a 30.86 3.333a 31.56 3.843a

Clearfil DC Core Plus Coronal 29.15 3.081A 38.43 4.232A 38.72 3.482A 39.00 3.842A

P < 0.05 P < 0.05 P < 0.05 NS

Apical 24.21 2.941b 28.52 4.172a 32.05 3.973a 36.53 3.434b

The same superscript number indicates no significant differences in lTBS for each row, while the same superscript capital and

small letter indicates no significant differences within each column at coronal and apical region ( P > 0.05).

P, paper point; A, air blow; AP, air blow+ paper point; APA, air blow+ paper point+ air blow.




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The SEM micrographs of the adhesive and/or mixed

failure in the apical region of each material are presented

in Fig. 2, which show different amounts and sizes of

blisters within the adhesive layer in each experimental

group. Numerous blisters and large blisters were

observed in Clearfil DC Core Automix. On the other

hand, for Clearfil DC Core Plus, few blisters formed and

small blisters were found. Regarding the solvent evapo-

ration strategy, the APA group produced less blister

formation than the other experimental groups.

Discussion

Clinically, the placement of a post is critical for the

restoration of root filled teeth (Schwartz & Robbins

2004). However, post placement was not performed

in this study because the focus was on evaluating the

bonding performance of one-step self-etch adhesives

to root canal dentine. Aksornmuang et al. (2011)

recently reported that post placement decreased the

bond strength due to the increase of C-factor and

increased the number of pretesting failure specimen

comparing with no post placement.

Push-out tests and microtensile tests have been used

for the assessment of bonding to root canal dentine.

Push-out tests are useful to evaluate the retention of

posts luted in root canals (Van Meerbeek et al. 2010),

but the measured value includes a friction effect as well

as the bonding effect (Goracci et al.2005, Faria-e-Silva

et al. 2008). On the other hand, microtensile tests can

accurately measure the bond strength between resin

and dentine or post and resin (Bouillaguet et al. 2003,

Aksornmuanget al. 2011). However, sometimes there

are pretesting failures of specimens during specimen

preparation, which makes it difficult to investigate the

bonding performance (Goracci et al. 2004). In this

study, there were no pretesting failures. Therefore, in

this study, bonding efficacy between resin and dentine

could be evaluated.

For both materials used in this study, the apical

region exhibited lower lTBS than the coronal region.These results can be attributed to attenuation of the

light energy in deeper regions of the post space due to

increasing distance from the light source (Le Bell et al.

2003, Wu et al. 2009, Beriat et al. 2012). Previous

studies have demonstrated that the reduced light

energy in deeper regions reduced the bond strength of

dual-cure one-step self-etch adhesives in the apical

region because of incomplete polymerization of the

adhesive (Foxton et al. 2003, Aksornmuang et al.

2009), and it was suggested that extending the light

irradiation time and/or using a higher light-output

power unit can increase the bond strength to the apical

region by increasing the degree of polymerization of the

adhesive. Insufficient light energy would be one of the

factors responsible for a reduction in the bond strength

values of the one-step self-etch adhesives in the deeper

region. Furthermore, the occurrence of tubular sclero-

sis in the root canal may be the other factor affecting

the lower bond strength in the apical region of a post

Figure 1 The percentages of failure modes for each root region in different evaporation techniques. There were significant

differences in failure mode among different evaporation techniques at the apical regions in both materials.




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space. Tubular sclerosis is the physiological deposition

of increasing amounts of peritubular dentine, which

increases from the apex towards the coronal direction

with age and is more prominent in mesial and distal

root aspects (Vasiliadis et al. 1983, Paqueet al. 2006).

Some studies have demonstrated that the obliteration

of dentinal tubules and the hypermineralized surface

layer is more resistant to demineralization by acidic

monomer than normal dentine (Tay & Pashley 2004,

Lopeset al.2011), preventing optimal resin infiltration

(a)

(b)

(c)

(d)

Figure 2 Representative SEM micrographs of the adhesive and/or mixed failure among solvent evaporation strategies at the

apical region of post space in Clearfil DC Core Automix (left) and Clearfil DC Core Plus (right); a) The paper point application

only [P]; b) The air-blowing only [A]; c) The use of paper point without additional air-blowing [AP]; d) The use of paper point

with additional air-blowing technique [APA]. Numerous blisters formation could be found when the adhesive was air blown

solely and only the paper point was used, while the use of paper point with additional air-blowing produced few blister forma-

tion within adhesive layer. The smaller size of blister formation was found in Clearfil DC Core Plus comparing to Clearfil DC

Core Automix.




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into the dentinal tubules and subsequently lowering

the apical bond strength.

In the present study, the solvent evaporation strate-

gies except use of the absorbent paper point only

significantly affected the bond strength in the apical

region, but did not exert an affect at the coronal

region. Therefore, the null hypothesis that solventevaporation strategies did not affect the regional bond

strength to root canal dentine was partially rejected.

For water/solvent containing adhesives, water and solvents

should be completely eliminated from the adhesive

layer prior to light curing, because any residual

water/solvent may have an adverse effect on polymeri-

zation of the adhesive layer (Jacobsen & Soderholm

1995, Carvalho et al. 2003, Reis et al. 2003). Never-

theless, in the root canal, water/solvent evaporation

would be limited because of its configuration. Even

when using flat dentine surfaces, increasing the air-

blowing distance and reducing air-pressure caused a

reduction in the bond strength of one-step self-etch

adhesive (El-Askary & Van Noort 2011). Reducing the

air-pressure at the deeper region in the root canal

would cause further insufficient water/solvent evapo-

ration, resulting in a reduction in bond strength in the

apical region. These speculations are supported by the

failure modes in this study, in which there were a

large number of cohesive failures in resin, and numer-

ous blister formations were observed within the adhe-

sive layer (Fig. 2). Excess water/solvent may dilute the

concentration of co-monomer in the hybrid layer

(Galan et al. 1991, Hashimoto et al. 2005) and con-

tribute to the growth of non-uniformity in the adhe-sive layer, resulting in a reduction in bond strength

(Cho & Dickens 2004). Residual water/solvent within

the adhesive layer would be another responsible factor

for the reduction in bond strength values of the one-

step self-etch adhesives in the deeper regions.

In addition, the narrow and deep configuration of

the root canal would also cause the pooling of excess

adhesive in the apical region. The thick adhesive layer

pooled at the apical region in the root canal would

contain higher amounts of water/solvent, which

would increase the difficulty of solvent/water evapora-

tion (Zhenget al. 2001, De Munck et al. 2007). In this

study, additional usage of the paper point after air-

blowing significantly increased the bond strength in

the apical region compared with only air-blowing and

using the paper point solely. Souza et al. (2007) also

demonstrated that the use of a paper point after air-

blowing the adhesive could improve the bond strength

to bovine root canal dentine. These results would indi-

cate that only air-blowing the adhesive was not

enough to evaporate water/solvent from the adhesive

layer at the apical region. The paper point application

could minimize pooling of the adhesive layer at the api-

cal region, facilitating further residual solvent/water

removal. However, using only a paper point without

air-blowing produced the lowest bond strength at bothcoronal and apical region of post space. This result

indicates the importance of air-blowing to evaporate

solvent from adhesive layer. Previous studies have

demonstrated that increasing the air-blowing pressure

could improve solvent evaporation from adhesive layer

when the adhesive surface was further away from air-

blowing source (Shinkai et al. 2006, El-Askary & Van

Noort 2011). Therefore, in the apical region, higher

air-blowing pressure would be effective in evaporating

solvent evaporation from the adhesive layer, removing

the pooled adhesive. However, using a higher pressure

might adversely affect bonding at the coronal region,

because it might cause incomplete envelopment of the

collagen fibrils due to excessive removal of the adhesive

resin (Shinkai et al. 2006, Spreafico et al. 2006).

Therefore, the paper point application might be useful

for bonding to root canal dentine.

When additional air-blowing was applied after using

the paper point, cohesive failure in resin decreased and

fewer blisters at the adhesive surface were found

compared with the other experimental group (Figs 1

and 2). These results would indicate that for both mate-

rials, additional air-blowing was effective in evaporat-

ing solvent from the adhesive layer. However, Clearfil

Tri-S Bond Plus with Clearfil DC Core Plus significantlyincreasedlTBS to the apical region, whereas for Clear-

fil DC Bond with Clearfil DC Core Automix, the additive

air-blowing could not improve lTBS to the apical

region. These results might be due to different chemi-

cal-polymerization systems for Clearfil Tri-S Bond Plus

and Clearfil DC Bond. With both materials, lTBS were

lower in the apical region than the coronal region

although Clearfil Tri-S Bond Plus exhibited no signifi-

cant difference in lTBS between the coronal and apical

regions. These results would indicate that the apical

region was not exposed to sufficient light energy for

light-polymerization of the adhesive, and its lTBS was

dependent upon a chemical-polymerization reaction.

Clearfil DC Bond is a two-bottle self-etch dual-cure

adhesive system containing a chemical initiator and

accelerator in different bottles, which slowly chemical-

polymerizes after mixing. On the other hand, Clearfil

Tri-S Bond Plus is developed as one-bottle self-etch

light-cure adhesive with a Touch-cure system, which




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can rapidly chemical-polymerize in combination with a

chemical accelerator in the adhesive and a specific

chemical initiator in Clearfil DC Core Plus of resin core

material after contact with the adhesive and resin core

material. This different chemical-polymerization behav-

iour might affect lTBS to the apical region. Addition-

ally, Clearfil Tri-S Bond Plus contains multifunctionalhydrophilic monomers as well as 2-hydroxyethyl meth-

acrylate (HEMA), while Clearfil DC Bond contains

HEMA only as hydrophilic monomer. This might con-

tribute to less blister formation in Clearfil Tri-S Bond

Plus than Clearfil DC Bond in each experimental group.

Blister formation would affect the quality of the

adhesive interface to dentine. In this study, it is diffi-

cult to compare quantitatively the blister formation

between each experimental group in the failure analy-

sis because the existence or non-existence of the blis-

ter formation could be confirmed only in the adhesive

failure specimens, but not in the cohesive failure spec-

imens. However, blister formation area would be the

weakest point in the specimen because of less poly-

merization due to residual water content. Therefore,

most of the specimen with the blister formation would

fail in the regions with this area. Further research

should be established to quantitate the blister forma-

tion in adhesive interface between one-step self-etch

adhesive and root canal dentine.

Conclusion

Within the limitations of this study, it can be

concluded that the use of paper points with additionalair-blowing for removing excessive adhesive and

evaporating residual water/solvent would be effective

in producing higher bond strength for one-step self-

etch adhesives (Clearfil Tri-S Bond Plus and Clearfil

DC Bond) and fewer blister formations in deeper

regions of the post space.

Acknowledgements

The authors gratefully acknowledge the financial support

received from the Japanese Ministry of Education, Global

Center of Excellence (GCOE) Program, International

Research Center for Molecular Science in Tooth and

Bone Diseases, Tokyo Medical and Dental University.

References

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Effect of prolonged photo-irradiation time of three self-etch

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