Effect of fiber-reinforced composite at the interface on bonding of resin core system to dentin.

Abstract

The aim of this study was to evaluate the effect of fiber-reinforced composite (FRC) at the interface on bonding of resin core systems to bovine dentin using different adhesive systems. To this end, the labial surfaces of 60 bovine incisors were ground to obtain flat dentin surfaces and then divided into two groups according to the adhesive system used: total-etching (Solobond Plus) versus self-etching (Clearfil SE Bond). Resin core systems were bonded to tooth structure either without or with a FRC layer (everStick Net, StickTech). For groups with FRC layer, a silicon forming aid was used to adapt the latter on the dentin surfaces. After resin core was polymerized with Optilux 501 for 40 seconds, the specimens were tested in a universal testing machine. ANOVA revealed that presence of FRC at the interface had a significantly positive effect on bond strength (p < 0.001). However, differences between groups were not significant for either adhesive system (p = 0.076) or with the use of silicon forming aid (p = 0.348).

Full text

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Dental Materials Journal 2008; 27(5): 736－743
Original Paper

Effect of fiber-reinforced composite at the interface on bonding of resin
core system to dentin

Isil CEKIC-NAGAS1, Gulfem ERGUN1, Arzu TEZVERGIL2, Pekka K. VALLITTU2 and Lippo V. J. LASSILA2
1Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey
2Department of Prosthetic Dentistry and Biomaterials Science, Institute of Dentistry, University of Turku, Turku, Finland
Corresponding author, Gulfem ERGUN; E-mail: ergungulfem@yahoo.com, isilcekic@gmail.com

The aim of this study was to evaluate the effect of fiber-reinforced composite (FRC) at the interface on bonding of resin core
systems to bovine dentin using different adhesive systems. To this end, the labial surfaces of 60 bovine incisors were
ground to obtain flat dentin surfaces and then divided into two groups according to the adhesive system used: total-etching
(Solobond Plus) versus self-etching (Clearfil SE Bond). Resin core systems were bonded to tooth structure either without or
with a FRC layer (everStick Net, StickTech). For groups with FRC layer, a silicon forming aid was used to adapt the latter
on the dentin surfaces. After resin core was polymerized with Optilux 501 for 40 seconds, the specimens were tested in a
universal testing machine. ANOVA revealed that presence of FRC at the interface had a significantly positive effect on
bond strength (p<0.001). However, differences between groups were not significant for either adhesive system (p=0.076) or
with the use of silicon forming aid (p=0.348).
Key words: Bond strength, FRC, Silicon device

Received Jan 17, 2008: Accepted Apr 30, 2008
INTRODUCTION
The outcome of endodontic treatment for all patients
should be high levels of comfort, function, and
longevity. On this note, preservation of the
remaining tooth structure is instrumental to
augmenting the longevity of restored teeth1). On the
techniques available to preserve sound tooth
structure, core foundation systems are frequently
used to repair endodontically treated teeth after
excessive loss of the coronal portion and to stabilize
the weakened parts of tooth structure2). Further, to
combine the advantages of chemical- and light-
polymerized materials so as to improve the dentin
bond strengths of direct core foundation systems,
dual-polymerized core foundation
developed3).
　　Key to the success of tooth restoration treatments
are an appropriate adhesive system and core
foundation system4). Current adhesive systems
interact with dentin using two different strategies:
remove the smear layer (total-etch technique) or
maintain it as the substrate for bonding (self-etch
technique)5). The introduction of self-etching
adhesives has since eliminated the use of separate
acid-etching step and significantly reduced post-
operative sensitivity associated with the removal of
smear layer6). For two-step self-etch adhesives, they
require separate application steps of a mild self-etch
primer and a hydrophobic resin. These mild self-etch
systems (pH<2) are able to partially remove the
smear layer and penetrate the dentin surface,
resins were
creating a less pronounced resin tag formation and
hybrid layers that are thinner than those of total-
etch systems7).
　　Adhesion to dentin substrate provides retention
for the core foundation resin as well as reinforcement
characteristics that are beneficial in the treatment of
compromised teeth8). In view of the significant
impact of interfacial adhesion on the mechanical
properties and long-term durability of foundation
restorations, it is therefore needful to establish
appropriate methods to assess changes in the
strength and stability of the interface9). Indeed, bond
failures at dentin-resin composite interface have been
shown to occur due to cohesive fracture within the
resin composite-bonding agent complex in the
presence of concentrated stress at the interface10).
Therefore, to increase bond strength, there must be
means and measures to reduce or eliminate stress
concentration at the interface10,11).
　　One available method is to use fiber-reinforced
composites (FRCs) at the interface10,12). It should be
mentioned that orientation of the fibers is an
important factor that affects the strength of FRCs13).
Unidirectional fibers provide reinforcement only in
one direction and confer anisotropic properties to the
composite resins. When fibers are oriented in two or
three directions, they provide orthotropic or even
isotropic mechanical properties to the FRC14). The
employment of fibers with different orientations can
change the dynamics of the adhesive interface and
play a role in influencing interfacial bond failures11).
Indeed, previous studies had demonstrated the

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Dent Mater J 2008; 27(5): 736－743737
ability of glass fibers to withstand tensile stress and
stop crack propagation in composite materials15,16).
However, only a few studies have examined the effect
of FRC layer at the
interface10,11,17,18). Moreover, none of the published
research has investigated the effect of silicon forming
aids, which were developed for forming and
positioning fibers to their right positions in clinical
applications.
　　Based on these considerations, the purpose of
this study was to investigate the effect of bidirec-
tional FRC at the interface on bond strength of resin
core system to bovine dentin by using different
adhesive systems. In addition, the possible
contribution of silicon forming aid to interfacial
tooth-resin composite
bonding was examined.
MATERIALS AND METHODS
Specimen preparation
Sixty freshly extracted bovine incisors were used as
test substrates. The labial surface of each tooth was
wet-ground with silicon carbide abrasive papers up
to No. 320 (Federation of European Producers of
Abrasives (FEPA), Paris, France) under water cooling
with a grinding machine (Struers RotoPol 11, Struers
A/S, Rodovre, Denmark) to create a flat dentin
surface. Following which, the root of each tooth was
removed with a diamond wafering blade (Ernst Leitz
GMBH, Wetzlar 1600, Germany). Specimens were
Trade nameType
Batch
number
Composition Manufacturer
Bonding
procedure
Solobond plus Two-stage
universal
bonding agent
Etching
agent
(Vococid)
59072236％ phosphoric acid
Voco GmbH,
Cuxhaven,
Germany
Etch for 15 s,
rinse, dry
gently
Primer591582Water, acetone, maleic acid,
acid-functionalized
methacrylates, fluorides
Apply primer
for30 s
Adhesive591583Acetone, dimethacrylate,
hydroxymetahcrylate
Apply
adhesive for
15 s, dry and
light
polymerize for
10 s
Clearfil SE
Bond
Self-etch
bottle system
two Primer00648AWater, HEMAa, MDPb,
camphorquinone,
dimethacrylate
hydrophilic
Kuraray Co.
Ltd, Osaka,
Japan
Apply primer
for 20 s,
gently dry
Adhesive00920AMDPb, Bis-GMAc, HEMAa, N,
N-diethanol-p-toluidine,
hydrophobic dimethacrylate,
silanated colloidal silica
Apply
adhesive for
15 s, dry for
5 s and light
polymerize
for10 s
Clearfil DC
Core Automix
Dual-cured core
build-up material
00027ABis-GMAc, MDPb,
dimethacrylate, filler,
photo/chemical initiotor
Kuraray Co.
Ltd, Osaka,
Japan
Stick-NetGlass fiber weave2050712
-w-0054
Porous PMMAd impregnated
bidirectional E-glass fibers
Stick Tech,
Turku, Finland
Stick resinLight curing resin 5504765Bis-GMAc- TEGDMAe
Stick Tech,
Turku, Finland
aHEMA, hydroxyethyl methacrylate;
A-glycidyl dimethacrylate; dPMMA, polymethyl methacrylate; eTEGDMA, triethylenglycoldimethacrylate.
bMDP, 10-methacryloyloxydecyl dihydrogen phosphate;
cBis-GMA, bisphenol
Table 1 Materials used in this study

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Dent Mater J 2008; 27(5): 736－743738
maintained in a fully hydrated state throughout the
entire preparation procedure.
　　A Mylar strip (DuPont Corp., Wilmington, USA)
with a 3-mm-diameter hole was centered on the
dentin surface to standardize the exposed area, and
then burnished on the dentin surface to ensure tight
adhesion to the specimen. Finally, the specimens
were mounted in the countersunk hole in plate A of
the shearing apparatus (Fig. 1) using a die stone
(Fuji Rock, GC, Tokyo, Japan).
Bonding procedures
The prepared teeth were divided into two groups:
total-etching versus self-etching. Each group was
further divided into two sub-groups: with or without
FRC (control group). FRC groups were in turn
subdivided into two more groups: presence or absence
of silicon forming aid to FRC layer to the dentin
surface.
　　Following complete set of the die stone, the
dentin surfaces were treated with the materials
listed in Table 1 according to manufacturers’
instructions. In FRC groups, after primer application
on dentin, the adhesive resin of FRC (Stick Resin,
StickTech, Turku, Finland) was applied together
with one layer of bidirectional, light-polymerizable,
polymer-impregnated E-glass fibers (Stick Net,
StickTech) of 3 mm diameter. For FRC groups to be
examined for the effect of silicon forming aid, the
fibers were adapted to the tooth surface with a
transparent silicon device (Refix D, StickTech).
Then, for all FRC groups with or without silicon
forming aid, they were light-polymerized for 10
seconds with a quartz-tungsten-halogen (QTH) unit
(Optilux 501, Kerr, Danbury, CA, USA) under
continuous mode. To improve the adhesion of resin
core to the polymer-impregnated fibers, the fiber
layers were treated with an adhesive resin (Stick
Resin, StickTech) for 24 hours prior to their
application. Finally, the resin core system (Clearfil
DC Core, Kuraray Co. Ltd.) was condensed into the
countersunk hole in plate B (Fig. 1) and polymerized
for 40 seconds with a QTH unit (Optilux 501, Kerr).
Shear bond test
After 24-hour storage in 100％ humidity at 37ºC, the
test specimens were mounted into the single plane
shear test assembly, aligning the abrasion marks to
shear direction. The screws securing plate A to plate
B were removed just before loading, and then
shearing force was applied at a crosshead speed of 1
mm/min using a universal testing machine (Lloyd
LRX, Lloyd Instruments Ltd., Fareham Hants, UK)
until failure occurred. Shear bond strength was
calculated by dividing the maximum load at failure
(N) with the bonding area (mm2) and recorded in
megapascals (MPa).
　　Failure modes were examined visually with an
optical microscope at ×40 magnification (Stereo-
microscope; Wild M3B, Heerbrugg, Switzerland).
Two specimens representative of the fracture mode
from each group were prepared for scanning electron
microscope (SEM) analysis. The specimens were
sputter-coated (Bal-Tec SCD 050 Sputter Coater,
Bal-Tec AG, Liechtenstein) with gold and observed
with a SEM (JSM-5500, JEOL Ltd., Tokyo, Japan).
Furthermore, one specimen from each adhesive
system was stored in 2 M hydrochloric acid (HCl) for
48 hours to demineralize the tooth structure and
reveal the resin tag formation19). After being
extensively rinsed, specimens were freeze-dried20),
gold sputter-coated, and then observed with a SEM
(JSM-5500).
Statistical evaluation
Shear bond strength (SBS) data were analyzed sta-
tistically by three-factor analysis of variance
(ANOVA) and Tukey’s post hoc test, with the level of
significance set at p<0.05. All statistical analyses
were performed using a statistical software package
(SPSS Inc., Chicago, IL, USA).
RESULTS
Figure 2 summarizes the mean SBS values and
standard deviations of the test groups. Three-way
ANOVA (Table 2) indicated a significant effect of the
Fig. 1 Single plane shear test assembly.

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Dent Mater J 2008; 27(5): 736－743739
presence of FRC at the interface on SBS (p<0.001).
However, the effects of adhesive system (p=0.076)
and the silicon forming aid (p=0.348) were not
statistically significant. The highest mean SBS value
was seen in Group TFP (19.3±2.4 MPa) and the
lowest in Group SC (11.1±1.7 MPa).
　　Table 3 shows the distribution of failure modes
of the test groups. In the control groups (Group TC
and SC), it was solely adhesive failure between
dentin and the resin core. In groups that had an
FRC layer adapted with silicon forming aid, failures
were mostly cohesive within the FRC layer (Group
TFP: 80％; Group SFP: 70％). However, in groups
without silicon forming aid, failures were predomi-
nantly adhesive between dentin and resin core
(Group TFW: 30％; Group SFW: 30％) and cohesive
within the FRC layer (Group TFW: 60％; Group
SFW: 50％).
　　Figure 3 shows the stereomicroscopic images of
cohesive failure within the FRC layer. SEM
micrographs of adhesive failure at the dentin-resin
core interface, cohesive failure within the FRC layer,
and mixed failure are shown in Fig. 4. SEM photo-
micrographs after removal of tooth structure are
shown in Fig. 5.
Source variationdf
5
1
1
1
1
1
1
0
0
54
60
59
Sum of squares
628.493a
13065.250
18.766
401.259
5.119
1.548
1.380
Mean square
125.699
13065.250
18.766
401.259
5.119
1.548
1.380
FP
Corrected model
Intercept
Adhesive system
FRC
Silicon device
Adhesive system X FRC
Adhesive system X Silicon device
FRC X Silicon device
Adhesive system X FRC X Silicon device
Error
Total
Correceted total
21.961
2282.664
3.279
70.105
.000
.000
.076
.000
.348
.605
.625
.894
.271
.241
.000
.000
309.079
15745.818
937.572
5.724
a. R Squared = .670 (Adjusted R Squared = .640)
Table 2 Analysis of variance of shear bond strength results
Fig. 2 Shear bond strength (MPa) values of resin core
system to bovine dentin.
Group nameGroup code
Adhesive between
resin-core and
dentin
10 (100％)
Cohesive in FRCMixed
Total-etch
controlTC00
FRC with silicon deviceTFP 1 ( 10％)8 (80％)1 (10％)
FRC without silicon deviceTFW 3 ( 30％)6 (60％)1 (10％)
Self-etch
controlSC 10 (100％)00
FRC with silicon deviceSFP 1 ( 10％)7 (70％)2 (20％)
FRC without silicon deviceSFW 3 ( 30％)5 (50％)2 (20％)
Table 3 Failure mode analysis of the test groups (n=10)

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Dent Mater J 2008; 27(5): 736－743740
Fig. 3 Fractured surfaces of dentin and resin core material showing cohesive failure in FRC.
Fig. 5 SEM photomicrographs of the resin tag formation after dissolving tooth structure with 2 M HCl.
Resin replica of the dentin surface with: (A) Solobond Plus adhesive system, where a regular
distribution could be observed with numerous, longer resin tags; and (B) Clearfil SE Bond, where a
non-uniform distribution of shorter and thinner resin tags extending longitudinally to the dentin
wall could be observed. Original magnification ×1000, bar=10 μm.
Fig. 4 SEM micrographs of fracture surfaces. A: Mixed failure in Group SFP; B: Cohesive failure in FRC in Group TFP;
C: Adhesive failure between resin core and dentin. Original magnification ×25, bar=1 mm.

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Dent Mater J 2008; 27(5): 736－743741
DISCUSSION
Amid the diverse spectrum of bond strength testing
methods, shear bond testing has become very
popular9,11,21). This is chiefly because shear stress is
believed to be the major stress which accounts for the
bond failures of restorative materials in vivo22).
Several testing configurations have been used ―
including loops, points, and knife edges ― to apply
shear force. However, a major problem with these
methods lies in the difficulty of placing the shear
loading device in close alignment with the bond
interface23). As for the single plane shear test
assembly used in this study, the shear loading device
was positioned in line with the bond interface zone
and applied the stress through this zone in a specific
plane22). This design supported the restorative
material and that there was no point contact during
load application24).
　　The mean shear bond strength values found in
this study were lower than those in previous
studies8,25). Giannini et al. used the wire loop
technique in a study and reported a mean bond
strength of 37.06±4.88 MPa for Clearfil DC Core
with Clearfil SE Bond25). However, in the present
study, the mean bond strength for this combination
was found to be 11.1±1.7 MPa. On the other hand,
another study ― which employed a knife-edge shear
test method ― showed a more similar shear bond
strength value for Clearfil DC Core with Clearfil
PhotoBond (14.6±2.4 MPa)8). Put together, these
aforementioned variations could be attributed to one
or a combination of causes arising from bonding
methodology, storage environment, and testing
techniques.
　　The silicon forming aid investigated in this study
is currently used by dentists for better positioning of
the FRC layer, as suggested by the manufacturer. In
view of this recommendation, it was hypothesized in
this study that the application of FRC to the dentin-
resin core interface with the use of silicon forming
aid would increase the bond strength. Based on the
results obtained, this hypothesis was partially
accepted in both FRC test groups. It must be
clarified that whereas the FRC significantly increased
the bond strength, the silicon forming aid did not
lead to a significant increase in bond strength.
　　The addition of FRC improved the shear bond
strength as the layer thus created acted as an
impediment to crack formation and debonding. As
for the bonding ability of FRC to the characteristics
of adhesive resins to the tooth surface and FRC26). In
the current study, the bidirectional FRC used
probably mimicked the biomechanics of tooth
structure and hence increased the bonding surface
area of dentin. Furthermore, the continuous fibers
could transfer stress to a wider surface area, thereby
diminishing stress ― and hence the possibility of
debonding ― at the interface17).
　　Previous studies have reported that HEMA-
containing resins or methyl methacrylate monomer
in combination with some dimethacrylate monomer
systems can promote the diffusion of the monomer
and to some extent dissolve the linear polymer phase
of the FRC on the bonding surface27,28). On this note,
the positive effect of FRC on bond strength could be
related with the adhesive systems used in this study.
Clearfil SE and Solobond Plus contained HEMA and
some dimethacrylate monomers respectively. The
preimpregnation of FRC using a light-polymerizable
resin system formed a semi-interpenetrating polymer
network (semi-IPN) after being polymerized. The
FRC with a semi-IPN matrix then adhered with the
resin composite by means of interdiffusion bonding11).
Consequently, the results of the present study were
in good agreement with studies that showed good
adhesion between the adhesive resin and FRC26-28).
　　To date, few studies have investigated the effect
of thermocycling on the stability of bonding layers
with FRC. These studies have yielded encouraging
results with slight increase in bond strength
values11,28). Nevertheless, although FRC seemingly
has a positive effect on bond durability in clinical
practice, its long-term stability needs further
investigation.
　　In the present study, the bond failures of all the
control groups (TC and SC) occurred adhesively
between the resin core and dentin (Fig. 4C). In the
FRC groups with silicon forming aid, failures were
predominantly cohesive within FRC (TFP: 80％; SFP:
70％) (Fig. 4B). It was observed that the number of
cohesive failures within FRC tended to increase as
higher shear bond strength values were yielded.
Results of this study were in accordance with
previous studies which showed changes in fracture
pattern owing to the addition of FRC to the
interface14,17,18). Fennis et al. investigated the effect of
FRC on the fracture resistance of cusp-replacing
composite restorations, whereby it was concluded
that glass FRCs had a beneficial effect on the failure
mode and thereby on re-restorability in the event of
a fracture18). In FRC groups with silicon forming aid
(TFP, SFP), the adhesive failure percentage was 10％
whereas it was 30％ in FRC groups without silicon
forming aid (TFW, SFW). The increase in adhesive
failure percentage could be related to the inadequate
adaptation of FRC to the tooth surface.
　　Previous studies have investigated the effective-
ness of self-etching adhesive systems and their
adhesion to both dentin and enamel, but controver-
sial results were yielded regarding the bonding
performance of these systems6,13,14,21,29,30). Clearfil SE
Primer, used in this study, is considered as a mild
acidic agent with a pH of 2.0. Mild self-etch

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Dent Mater J 2008; 27(5): 736－743742
adhesives produce hybrid layers thinner than total-
etch systems7). In this study, the SEM images of
specimens prepared from the adhesive systems
showed difference in surface texture between the two
systems. Solobond Plus adhesive system yielded a
uniform formation of numerous resin tags that were
longer in length. With Clearfil SE Bond, a non-
uniform formation of shorter resin tags was obtained
(Fig. 5). These findings were consistent with those of
previous studies, which reported that resin tags
made only a minor contribution to bond strength7,31,32).
As a result, the shear bond strengths obtained with
Clearfil SE Bond were statistically similar (p=0.076)
to those obtained with Solobond Plus in this study.
　　The comparable bonding performance of Clearfil
SE Bond could be attributed to the synergy of an
unsaturated methacrylate phosphate ester (10-MDP)
as the acidic monomer in combination with
hydroxyethyl methacrylate
believed to improve wetting of the tooth surface and
chelate to calcium ions of dentin33). Additionally,
adhesive strength might be governed by mechanical
interlocking with hydroxyl apatite crystals within the
hybrid layer, thereby resulting in a more rigid and
compact interface34). In a review by Peumans et al.,
two-step self-etch adhesives have been shown to
approach the gold standard of three-step total-etch
adhesive systems in terms of clinical bonding
effectiveness35). Therefore, these simplified adhesives
wield numerous clinical advantages: faster and easier
to use, and reduced
Nonetheless, long-term clinical studies are still
required to further understand the bonding interface
produced by self-etching systems.
　　Although bovine and human teeth have different
features especially in terms of morphology, several
studies have demonstrated similarities between these
two types of substrates. In bonding tests, similar
results were reported between bovine and human
dentin as well as in the number and distribution of
dentinal tubules36,37). In this study, bovine dentin
was chosen as the substrate because of the
convenient size of the teeth. Moreover, the use of
bovine teeth facilitates the ease of obtaining uniform
surfaces for bonding, which are suitable for primary
screening tests38). It has been demonstrated that
bovine teeth are a good substitute for human teeth,
although slightly lower values are frequently
obtained39). It is therefore noteworthy that all these
non-standardized parameters in bond strength
studies make it difficult to compare results of
different studies.
(HEMA), which is
technique sensitivity.
CONCLUSIONS
Within the limitations of this in vitro study, the
following conclusions were drawn:
(1) Addition of an FRC layer did significantly
improve the shear bond strength of resin core
to bovine dentin. However, use of silicon
forming aid did not result in significant
improvement of bond strength.
(2) Solobond Plus (three-step total-etching system)
and Clearfil SE Bond (two-step self-etching
system) provided equally
strengths to bovine dentin.
effective bond
ACKNOWLEDGEMENTS
This study was financed by a fund from the Scientific
Research Projects Unit of Gazi University and in
part by a contribution
Nanopolymers Research Group, Center of Excellence
of the Academy of Finland.
from the Bio and
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25)
26)
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glass fiber-reinforced
28)
29)
30)
31)
32)
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34)
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adhesives: A
36)
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