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Effect of adhesive liner on the shear bond strength of indirect
restoration to enamel and dentin
Yasushi Shimada, DDS, PhD,a Vanthana Sattabanasuk, DDS,a Ayano Ishikawa,
DDS,a Chieko Imamiya, DDS,a Michael F. Burrow, MDS, PhD,b and Junji Tagami,
DDS, PhDaa

Cariology and Operative Dentistry, Department of Restorative Sciences,
Graduate School, Tokyo Medical and Dental University, Tokyo, Japan, andbSchool of Dental Science, Faculty of Medicine, Dentistry and Health Sciences,
University of Melbourne, Australia
Purpose: This study investigated whether the bonding of a resin inlay to enamel and dentin could be
improved by applying both an adhesive system and a low-viscosity microfilled resin to the underlying tooth
substrate.
Materials and Methods: A self-etching primer system (Clearfil SE Bond) and a low-viscosity microfilled
resin composite (Protect Liner F) was applied to one half of enamel or dentin slices, with the remaining
slices served as non-lined specimens, with only Clearfil SE Bond applied. Composite inlays (Estenia) were
fabricated indirectly and cemented with a dual-cured resin cement (Panavia Fluoro Cement). After 24-hourstorage in 37˚C water, the bonded inlays were subjected to a micro-shear bond test, whereby a shear force
was applied to the inlays. The data were compared with observations of the directly placed controls(Clearfil SE bond and Clearfil AP-X); statistical analysis was carried out using ANOVA and Fisher's PLSD
test. Morphological observations using confocal laser scanning microscopy were also performed.
Results: The enamel surfaces did not show any a statistical difference between the shear bond strength
values observed in the lined and non-lined groups (p>0.05). However, the bonding capacity of Estenia
inlay with the liner to dentin was slightly higher than that of the non-lined restoration. The bonding of
indirect restorations to enamel was almost the same as that of the directly placed resin composite (p>0.05).
Conclusion: The resin liner might improve the bonding of resin cement to dentin, whereas no effect wasobserved on enamel. The results of the present study suggest that, from the perspective of shear stress, it
seems advantageous if an adhesive layer on the cavosurface enamel were thin rather than thick.
(Int Chin J Dent 2003; 3: 36-44.)
Clinical Significance: Using a low-viscosity resin in indirect restorations showed an improvement inbonding of resin cement to dentin, whereas no significant difference was seen in the case involving ofenamel.
Key Words: low-viscosity microfilled resin, micro-shear bond strength, self-etching primer.

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Introduction
Indirect adhesive procedures are being used more widely for esthetic restorations. Metal and metal-free
inlays, veneers, crowns, resin-bonded fixed prostheses, and even posts are now routinely bonded to tooth
structures via the use of resin cement.1-9 On the other hand, evidence of insufficient marginal sealing ofbonded restorations has also been shown in microleakage studies.9,10 A clinical study using bonded inlays
reported the incidence of post-cementation sensitivity as high as 30%.11 The application of dentin bonding
systems to the prepared cavity wall has been advocated to prevent postoperative sensitivity in cases
involving indirect restoration of vital teeth.4
Recently, application of a dentin bonding agent and low-viscosity microfilled resin on a prepared dentin
surface has been advocated for indirect restorations.12,13 It has been reported that the application of
low-viscosity microfilled resin could reduce the influence of polymerization shrinkage of resin composite
and resin cements.14,15 Although many studies have focused on the bonding of resin cement to enamel,
dentin, or indirect restorative materials, little is known about the bonding of indirect restorative materials to
the tooth structure via the use of resin cement.1-9 This lack of information is probably due to the difficulties
of fabricating samples and designing a simple method of testing two adhesive interfaces (i.e., one interface
between the tooth and the resin cement, and the other between the indirect restoration and the resin
cement).
Recently, a micro-shear bond test was developed that enabled measurement of bonding of adhesivematerials to a variety of tooth substrates; the method allows for ease of sample preparation and gives
precise results with relatively small standard deviations.8,16 The aim of this study was to determine whether
the bonding of a resin inlay to enamel or dentin could be improved by applying an adhesive system andlow-viscosity microfilled resin. Bond strengths were assessed by means of a micro-shear bond test, and thefailure mode was observed using confocal laser scanning microscopy (CLSM). CLSM has been widelyused in biology for noninvasive and nondestructive imaging in vivo of many organ tissues and has foundnumerous applications in dental research.17 Specimens examined by CLSM do not require any special
preparation and are not subjected to the distortions caused by dehydration that results from procedures suchas scanning electron microscopy.

Materials and Methods
Tooth Preparation
Bonding was performed on buccal surfaces of enamel and dentin surfaces of extracted human molars thatwere stored at 4°C in saline. Sixty teeth were used in this study. Thirty enamel and dentin slices, eachapproximately 1.0 mm thick, were cut parallel to the buccal surface from the mid-coronal region using aslowly rotating diamond blade (Struers Minitom, Struers, Copenhagen, Denmark) and water lavage and wet
600-grit SiC paper was used to create flat surfaces with a uniform smear layer thickness. The enamel ordentin surfaces were treated with Clearfil SE Bond Primer (Kuraray Medical, Inc., Tokyo, Japan, Lot #00295A) for 20 s, dried, Clearfil SE Bond (Kuraray Medical, Inc., Lot #00364A) applied, air-thinned, and37

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Int Chin J Dent 2003; 3: 36-44.

light-cured for 10 s. Prior to irradiation of the bonding resin, an iris that was cut from micro bore tygon
tubing (R-3603, Norton Performance Plastic Co., Cleveland, OH, USA) with an internal diameter and aheight of approximately 0.8 mm and 0.5 mm respectively (Fig. 1), and mounted on the enamel or dentin
slice to delineate the bonding area.
Preparation of Composite Resin Inlay
A cylinder from the micro bore tygon tubing (R-3603) with an internal diameter of 0.8 mm and a heightof 0.5 mm was cut and used as a mold for the composite resin inlay (Fig. 1). A resin composite for indirect
restorations (Estenia, Shade DA2, Kuraray Medical Inc., Lot #00209B) was placed into the tubing on a flat
surface covered with a paper mixing pad, a clear plastic matrix strip was placed over the resin, gently
pressed flat and irradiated for 60 s using a visible light curing unit for laboratory (α-Light II, J. Morita Co.,Tokyo, Japan), prior to heat curing at 110˚C for 15 minutes in air (KL 100, Kuraray Medical, Inc.). Verysmall cylinders of resin inlay, approximately 0.8 mm in diameter and 0.5 mm high, were removed from thetygon tubing and silanized with the application of a mixture of acidic primer (Clearfil SE Bond Primer) anda silane agent (Clearfil porcelain bond activator, Kuraray Medical, Inc., Lot #00128A) for 20 s.

Load
Tygon tubing
0.8 mmEstenia
Estenia inlay
Wire

Set

Load

Tooth slice
Wire
Tooth slice
Lining or Non-lining,
Panavia Fluoro Cement

Resin cylinder

Fig. 1. Bonding procedure for Estenia inlay. Fig. 2. Schematic diagram of the micro-shear
bond test apparatus.
Bonding of Composite Resin Inlay
Thirty enamel slices and thirty dentin slices with tygon tubing were divided and assigned to threetreatment sequences (10 enamel or dentin specimens each), and then each specimen received one of thefollowing treatments:
Group 1 (Estenia, non-lining): Dual-cured resin cement (Panavia Fluoro Cement, Kuraray Medical, Inc.,

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Lot A paste: #00158A, B paste: #00090A) was mixed and injected into the tubing on the tooth surface. The
small cylinder of resin inlay was then inserted into the tubing, bonded and light cured for 30 s.
Group 2 (Estenia, lining): A low-viscosity microfilled resin (Protect liner F, Kuraray Medical, Inc., Lot#0052) was applied to the tooth surface inside the iris of the tubing and was light cured for 20 s, prior to theinjection of mixed Panavia Fuoro Cement into the tubing. The small cylinder of resin inlay was theninserted into the tubing, bonded and light cured for 30 s.
Group 3 (resin composite): A hybrid restorative resin composite, shade A3 (Clearfil AP-X, Shade A3,
Kuraray Medical, Inc., Lot #00765A) was placed into the cylinder, and a clear plastic matrix strip wasplaced over the resin; the specimen was then gently pressed flat and irradiated for 30 s.
In this manner, very small cylinders of resin inlay or resin composite were bonded to the surface. The
specimens were stored at room temperature (23˚C) for 1 hour prior to removal of the tygon tubing. All
specimens were placed in 37˚C water for 24 hours.
Micro-Shear Bond Test
The micro-shear bond test apparatus is shown in Fig. 2. The tooth slice, with the resin cylinder, wasattached to the testing device (Bencor-Multi-T, Danville Engineering Co, San Ramon, CA, USA) with acyanoacrylate adhesive (Zapit, Dental Ventures of America, Corona, CA, USA), which in turn was placedin a Universal testing machine (EZ-test-500N, Shimadzu, Kyoto, Japan) for shear bond testing. A thin wire
(diameter: 0.20 mm) was looped around the resin cylinder making contact with half of the cylinder base,and the wire was held flush against the resin/tooth interface. A shear force was applied to each specimen ata crosshead speed of 1.0 mm/minute until failure occurred.
Ten specimens from each group were tested.

The data were statistically analyzed using two-way

analysis of variance (ANOVA). After two-way ANOVA, one-way ANOVA and multiple comparisonswere carried out using Fisher’s PLSD test. The bonding methods (i.e., Estenia with no lining, Estenia withlining, or resin composite) and tooth substrates (enamel or dentin) served as the two factors. Statisticalsignificance was defined as p<0.05.
All of the debonded tooth surfaces were examined with an optical microscope at 30x magnification and
CLSM (1LM21H/W, Lasertec Co., Yokohama, Japan) to determine the mode of failure. Failure modes
were categorized into one of six types: A: 100% cohesive failure of the adherend materials, resin cement,
resin inlay, resin liner, or resin composite; B: 100% adhesive failure between tooth substrate or hybrid-likelayer and adhesive resin; C: 100% cohesive failure in the tooth substrate; AB: Mixed failure with cohesive
failure of the adherend materials (A) and adhesive failure (B); BC: Mixed failure with adhesive failure (B)and cohesive failure in the tooth substrate (C); ABC: Mixed failure with cohesive failure of the adherend
materials (A), adhesive failure (B), and cohesive failure in the tooth substrate (C). The modes of failure
were analyzed using the Kruskal-Wallis test, with significance defined as p<0.05.

Results
The mean shear bond strength values (MPa), standard deviations, and the modes of failure are shown in39

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Int Chin J Dent 2003; 3: 36-44.

Table 1. The ANOVA indicated that there was a statistically significant variance among the mean bondstrength values (two-way ANOVA, F=5.007, p=0.0101). Statistical significance was observed among theshear bond strengths of the indirect restorative materials that were bonded to dentin (one-way ANOVA,
F=12.636, p=0.0001), whereas no significance was found among the specimens bonded to enamel(one-way ANOVA, F=1.701, p=0.2006). When the indirect and direct restorations were compared, thebonding of the indirect restorative materials to dentin was found to be significantly lower than that of thedirect restorations (Fisher’s PLSD test, p<0.05). The bonding of Estenia with lining to dentin gave anintermediate bond strength that was also significantly different from that of the direct-restoration specimens(Fisher’s PLSD test, p<0.05).
Table 1. Results of the micro-shear bond test.
Group

Shear bond strength (MPa)Failure mode

Mean

SD

A

B

C

AB

BC

ABC

Estenia, non-lining

39.6

7.79

0

8

0

0

1

1

Estenia, lining

36.7

5.12

1

6

0

1

2

0

AP-X

41.9

5.39

0

10

0

0

0

0

Estenia, non-lining

29.1

8.26

0

6

1

2

1

0

Estenia, lining

33.7

5.26

1

1

0

7

1

0

AP-X

43.0

6.71

4

2

0

4

0

0

EnamelDentinSample size=10 for each group. SD: Standard deviation.

CLSM observation also revealed that the mode of failure after the shear test was significantly different
among the groups (Kruskall-Wallis, p=0.0013). For the three groups on enamel surfaces, a substantial
percentage of the failure modes was adhesive failure along the enamel/resin cement or adhesive linerinterface (Type B failure, Fig. 3); no significant difference was observed among the enamel surfaces(Kruskall-Wallis, p=0.7049). However, in the case of dentin, various failure patterns were observed(Kruskal-Wallis, p=0.0063). When the dentin surface was coated with low-viscosity resin prior to thebonding of the Estenia inlay, failures were predominantly mixed (i.e., cohesive within dentin and adhesive
along the dentin/resin cement or adhesive liner interface; Type AB failure, Fig. 4). Without the liner in the
Estenia inlay, adhesive failure (Type B failure) was seen in most cases. Direct composite resin underwent
Type B or Type AB failure.

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Fig. 3. CLSM image of a debonded enamel surface after the shear bond test. Estenia inlay withno-lining. Adhesive failure below the bonding resin and enamel can be seen (left).
Fig. 4. CLSM image of a debonded dentin surface after the shear bond test. Estenia inlay withresin lining. Mixed failure with adhesive failure (A) and dentin failure (D) can be seen.

Discussion
The hardness and bonding of dual-cured resin cement have both been reported to be low underconditions of self-cure alone without light irradiation.2,6,8 Moreover, the intensity of the light penetrating
through the indirect restorative materials has been shown to be reduced by thickness and shade.18,19
Considering these factors, it becomes clear that a curing unit must be able to effectively transmit lightthrough an indirect restoration in order to obtain sufficient hardness and bonding of the resin cement
immediately after cementation.5,19 In this study, the light from the curing unit was able to transmit throughthe Estenia inlay, with a thickness of 0.5 mm, to the resin cement. Tashiro et al.19 reported that a 1-mm
thick Estenia inlay did not adversely affect the bonding of a photo-irradiated dual-cured resin cement.
Since the bond strength of resin cement through the Estenia inlay obtained for the enamel was similar tothat of the direct resin composite filling, this finding appears to be correct.
Composite restorative materials undergo significant volumetric shrinkage when polymerized.20
Although the use of indirect composite inlays significantly reduces polymerization shrinkage during
placement, contraction of the luting resin composite may also remain problematic.9,10,21 The usage ofelastic materials as a liner during the early stage of setting has been reported to relieve this contraction
stress.22 Jayasooriya et al.9 observed gap formation under an MOD inlay, and reported that the percentageof the length of the gap formation with the resin liner was significantly lower at the internal
dentin-restoration interface compared with that of non-lined specimens. It is likely that the resin liner
decreases the microleakege of resin cement bonded to the dentin surface.9,15,19 In the present study, whenProtect liner F was applied to the dentin surface, the bonding of the Estenia inlay was significantly
improved, i.e., 33.7 MPa. This finding was in accordance with previous studies.9,15,19 An explanation forthese findings remains to be determined, but it is possible that the additional application of a low-viscosity
resin composite might have promoted the polymerization of the adhesive, due to the double-curing of theadhesive and air inhibition at the adhesive surface.3

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Several clinical and laboratory investigations have revealed that the luting space is the weakest part ofindirect restorations.11,21 Secondary caries following occlusal wear and marginal leakage has been reported,which always occurs at this location.23

Consequently, the film thickness of the resin cement at thecavosurface enamel margin needs to be thin in order to prevent secondary caries.9 In this study, the enamelbonding of the Estenia inlay did not improve with the application of Protect Liner F (p>0.05). Although nostatistical significance was noted between the bond strength values obtained from enamel surfaces, theapplication of micro-filled resin slightly decreased the bonding of indirect restorative materials (non-lined,
39.6 MPa; lined, 36.7 MPa).

In addition, the mode of failure was also similar, and no statisticalsignificance was seen among the groups (p>0.05). Previous studies have shown that the film thickness of
low-viscosity resin composite and resin cement was over 200 µm, which was obviously thicker than thatobtained with the application of bonding resin alone.9 In this study, the film thickness of low-viscosity
resin composite or resin cement might also be thick because of the smallness of specimen dimension. It
would therefore be reasonable to speculate that lining with the micro-filled resin composite is not necessary
in the case of enamel surfaces; the application of the adhesive only might be more beneficial. However, theusage of a thinner liner might also be an alternative in cases involving enamel margins.
Comparing the bonding of indirect composite restorative materials with that of direct resin composite
materials, no significant difference was seen in the specimens involving enamel bonding (p>0.05), whereas
the bond was significantly reduced when the resin inlay was bonded directly to the dentin surfaces (p<0.05).
Consequently, the findings indicated that in terms of bonding, direct composite restorations would be thepreferred treatment over indirect composite restorations. Furthermore, direct composite restorations require
minimal intervention and also minimal cavity preparation.24

However, a major limitation of directcomposites should be noted in this context, namely, they are associated with the inability to control bothpolymerization shrinkage and the depth of cure.25 Therefore, for larger restorations, indirect methods aresuperior alternatives to direct resin composite fillings. With regard to tooth-colored inlays and onlays,
factors affecting overall restoration longevity may be related to luting and finishing procedures, and may be
dependent on the width and performance of the resin composite lute.4,11,21 Furukawa et al.5 reported thatreinforcement by the unification of the tooth substrate and the indirect restoration was achieved if thebonding of resin cement to the tooth substrate was adequate.

Even though the application of the

low-viscosity resin improved the bonding of the resin cement to the dentin surface, the capacity for bonding
was still lower than that of the direct composite (p<0.05; Estenia, resin lining, 33.7 MPa; AP-X, 43.0 MPa).
Development of new luting materials or methods that can effectively bond both to tooth and to the indirect
restorative materials will most likely be necessary in the future.
The cervical enamel/composite interface and the dentin/composite interface have repeatedly beenreported to be more vulnerable to microleakage than the incisal enamel/composite interface in vitro.10 Evenat the enamel margin, the enamel structure of the cervical border frequently exhibits a disturbed prismarrangement lacking a normal keyhole appearance; the bonding capacity of this margin has been reported to
be low.16 Again, further investigation will be necessary to develop bonding resins and cements that42

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produce high, uniform bond strength and/or gap-free margins in cases involving indirect restorations.

Conclusion
In spite of the limitations of this laboratory study, it was concluded that resin lining with a low-viscosity
composite was necessary when indirect resin composite restorative materials were bonded to a dentinal
surface. In cases involving an enamel surface, the influence of the low-viscosity composite on the bondingof indirect restorative materials was unclear. As the performance of the materials was not considered in aclinical setting, further investigation using clinical trials are necessary.

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Reprint request to:
Dr. Yasushi Shimada
Cariology and Operative Dentistry, Department of Restorative Sciences
Graduate School, Tokyo Medical and Dental University
1-5-45, Yushima, Bunkyo-ku, Tokyo, 113-8549, Japan
FAX: +81-3-5803-0195 E-mail: shimada.ope@tmd.ac.jp
Received on April 12, 2003. Revised on April 30, 2003. Accepted on May 7, 2003.
Copyright ©2003 by the Editorial Council of the International Chinese Journal of Dentistry.

44