Journal of Dentistry 31 (2003) 59–66
www.elsevier.com/locate/jdent
The effects of filling techniques and a low-viscosity
composite liner on bond strength to class II cavities
André Figueiredo Reisa, Marcelo Gianninia,*, Gláucia Maria
Bovi Ambrosanob, Daniel C.N. Chanc
a
Department of Restorative Dentistry/Operative Dentistry, Piracicaba School of Dentistry,
UNICAMP. Av Limeira, #901, Piracicaba, SP 13414-900, Brazil
b
Department of Social Dentistry/Statistics, Piracicaba School of Dentistry, UNICAMP. Av Limeira, #901,
Piracicaba, SP 13414-900, Brazil
c
Department of Oral Rehabilitation, School of Dentistry, Medical College of Georgia, Augusta,
GA 30912-1260, USA
KEYWORDS
Dentin bonding;
Microtensile testing;
Low-viscosity composite;
C-factor
Summary Objective. The aim of this study was to test the hypothesis that the effects
of filling technique, cavity configuration and use of a low-viscosity composite liner
influence resin bond strength to the dentin of class II cavities gingival floor; and
analyze the failure modes of fractured specimens.
Methods. Standardized class II cavities were prepared in the proximal surfaces of
freshly extracted third molars, which were randomly assigned to 10 experimental
groups. All prepared surfaces were acid-etched, bonded with Single Bond adhesive
system and restored with TPH composite, according to each technique: G1 and G2horizontal layering, G3 and G4-faciolingual layering, G5 and G6-oblique layering, G7 and
G8-bulk filling, G9 and G10-control (flat dentin surfaces). Groups were tested, with or
without a low-viscosity composite liner (Tetric Flow Chroma). After storage in water for
24 h, teeth were vertically serially sectioned to yield a series of 0.8 mm thick slabs. Each
slab was trimmed into an hourglass shape of approximately 0.8 mm2 area at the gingival
resin– dentin interface. Specimens were tested in tension at 0.5 mm/min until failure.
Fractured specimens were analyzed in an SEM to determine the failure modes.
Results. No significant difference was found between groups restored with and
without a low-viscosity composite liner (p . 0.05). Among filling techniques, the bulk
filling groups presented the lowest bond strength values ( p , 0.05), while incremental
filling groups did not differ from control (flat dentin surfaces). Failure modes varied
significantly among groups restored with and without the low-viscosity composite liner.
Significance. Bond strengths were not improved when a low-viscosity composite liner
was applied, but it remarkably influenced the failure modes. Incremental techniques
improved bond strength.
Q 2003 Elsevier Science Ltd. All rights reserved.
Introduction
*Corresponding author. Tel.: þ55-19-3412-5338; fax: þ55-193412-5218.
E-mail address: [email protected]
The conditions in which bond tests are normally
performed are far removed from real clinical
0300-5712/03/$ - see front matter Q 2003 Elsevier Science Ltd. All rights reserved.
doi: 1 0 . 1 0 1 6 / S 0 3 0 0 - 5 7 1 2 ( 0 2 ) 0 0 1 2 2 - 7
60
conditions. While the use of abrasive papers is
convenient in the laboratory, clinically, dentin is
never prepared with abrasive paper.1 Moreover, flat
dentin surfaces are not subjected to the same
polymerization contraction stresses faced on threedimensional cavities.2 The cavity configuration
factor, C-factor, is the ratio of the restorations
bonded to unbonded surfaces area. Only a few
studies have assessed bond strength to complex
cavity walls.3 – 5 However, tests realized under
clinically relevant conditions might better predict
restorations behavior in the oral environment.
Quality bonding to dentin depends on proper
infiltration of the adhesive system into demineralized dentin matrix in order to create a micromechanical interlocking between collagen and
adhesive resin, and form a hybrid layer.6 Several
studies have shown that adhesion is affected by
differences in dentin location and changes in the
different areas of a tooth.7 – 10 With the introduction
of the microtensile test by Sano et al.11 the
determination of specimens bond strengths using
small surface areas for bonding has been possible.
This test method has facilitated the study of
regional bond strengths, providing more information regarding adhesion within clinically relevant
cavity preparations.
Low-viscosity or flowable composites have been
advised for deep parts of class II cavities under
hybrid or packable composites to act as a stressabsorbing layer between the hybrid layer, the
adhesive resin and the shrinkage of the resin
composite, by partially relieving the polymerization
contraction stress.12,13 Their low moduli of elasticity and increased flow capacity might provide
more contraction stress relaxation, and could
reduce the frequency of marginal microleakage
and possible debonding.14,15
Restoration placement techniques are widely
recognized as a major factor in the modification of
shrinkage stresses.16 Thus, the purpose of this study
was to evaluate the effects of four different filling
techniques, placed with and without a low-viscosity
composite liner on cavities with different C-factor
values, on bond strength to the dentin of class II
cavities gingival floor; and analyze the failure
modes of fractured specimens.
Materials and methods
Cavity preparation
Freshly extracted sound third molars (stored in
0.05% thymol) were used in this study. The teeth
A. F. Reis et al.
were obtained by protocols (136/2001) that were
analyzed and approved by the Ethical Committee in
Research at the Piracicaba School of Dentistry/UNICAMP. After being cleaned and pumiced, teeth had
their roots apical third included in polystyrene resin
cylinders to facilitate handling. Standardized uniform box-shaped class II cavities were prepared with
a precision cavity preparation device on either the
mesial or distal surface, depending on which surface
would provide the most ideal preparation. Occlusal
enamel was abraded with 600-grit SiC paper to
obtain an occlusal height of 5 mm with the gingival
margin located 1 mm below the cemento– enamel
junction, and the proximal surface was slightly
abraded to remove any irregularities. The preparations were outlined with coarse diamond burs
#3145 (KG Sorensen, Barueri, SP, Brazil) operated in
a high-speed hand-piece using copious air – water
spray. The bucco-lingual width of the preparations
was 4 mm and the axial wall was prepared to a depth
of 2 mm, with the axial wall parallel to the long axis
of the tooth (Fig. 1(A)). Control groups (corresponding flat dentin surfaces) were prepared the same
way, but cavities were enlarged after preparation.
The precision cavity preparation device was
adjusted to remove all surrounding dental structures
and leave a flat dentin surface on the corresponding
gingival floor with the same texture for the bonding
procedures (Fig. 1(B)).
Restorative procedures
Cavities were acid-etched with 35% phosphoric acid
(3M Scotchbond Etchant) for 15 s, rinsed for 20 s
and gently air-dried to keep the dentinal surfaces
visibly moist. Single Bond adhesive system was
applied according to the manufacturer instructions
on all dentin surfaces, which were checked for a
shiny surface. The adhesive resin was thinned with a
directed low-pressure air stream and light-cured for
20 s using an XL 3000 light curing unit (3M ESPE, St
Paul, MN, USA). A polyester matrix band was fixed
around the tooth and it was restored according to
one of the four filling techniques: groups 1 and 2horizontal layering (Fig. 1(C)); groups 3 and 4faciolingual layering (Fig. 1(D)); groups 5 and 6oblique layering (Fig. 1(E)); groups 7 and 8-bulk
filling (Fig. 1(F)); groups 9 and 10-a 5 £ 2 £ 4 mm3
composite resin block was built in three increments
on the corresponding flat dentin surface (Fig. 1(G)).
Each increment was light cured for 40 s.
On groups 2, 4, 6, 8 and 10, a 0.5 mm layer of a
low-viscosity composite (Tetric Flow Chroma) was
placed and light cured for 40 s before the insertion
of Spectrum TPH hybrid composite resin. Materials,
The effects of filling techniques and a low-viscosity composite liner on bond strength
to class II cavities
61
Figure 1 Schematic representation of specimen preparation. (A) Class II cavity; (B) control groups cavity design (after
1A was prepared, surrounding walls were removed with the cavity preparation device); (C) horizontally filled group; (D)
faciolingually filled group; (E) obliquely filled group; (F) bulk filled group; (G) control group after restoration (note that
the gingival floor is the only bonded wall); (H) sectioned specimen; (I) three slabs were obtained from each tooth; (J)
0.8 mm thick slab; (K) specimen trimming; (L) microtensile testing.
manufacturers, composition and batch numbers are
listed in Table 1.
Microtensile testing
After 24 h of storage in water at 37 8C, the specimens were mesio-distally sectioned into 0.8 mm
thick slabs for micro-tensile testing using a diamond
impregnated saw (Isomet, Buehler Ltd, Lake Bluff,
IL, USA) (Fig. 1(H)). Three slabs were obtained from
each tooth (Figs. 1(I)). Each slab was trimmed to an
hourglass shape, yielding bonded surface areas of
approximately 0.8 mm2 (Fig. 1(J) and (K)). The
slabs were attached to the flat grips of a microtensile testing device with cyanoacrylate glue
(Zapit, DVA, Corona, CA, USA) and tested in tension
in a Universal Testing Machine (Instron CO., Canton,
MA, USA) with a cross-head speed of 0.5 mm/min
until failure (Fig. 1(L)). After testing, the specimens
were carefully removed from the fixtures with
a scalpel blade and the cross-sectional area at the
site of fracture measured to the nearest 0.01 mm
with a digital caliper (Starret 727 – 6/150, Starret,
SP, Brazil) to calculate the ultimate tensile bond
strength and express results in MPa. Differences in
microtensile bond strengths were evaluated for
statistical significance using a two-way analysis of
variance ANOVA, and Student – Newman – Keuls at
the 0.05 level of significance. All statistical analysis
was done using SAS for the personal computer (SAS
Institute, Cary, NC, USA).
SEM observations
After testing, the dentin sides of fractured specimens were mounted on an aluminum stub, goldsputter coated (MED 010, Balzers Union, Balzers,
Liechtenstein) and observed with a scanning electron microscope (LEO 435 VP, LEO Electron
Microscopy Ltd, Cambridge, United Kingdom) at
62
A. F. Reis et al.
Table 1 Materials used in resin –dentin bonding.
Materials
Composition
Batch number
Manufacturer
3M scotchbond etchant
Single Bond
35% phosphoric acid, colloidal silica
HEMA, Bis-GMA, PAA, ethanol and
water
Bis-GMA, urethane dimethacrylate,
triethylene
glycol dimethacrylate (35.0 wt%).
Barium glass, ytterbium trifluoride,
Ba-Al-fluorosilicate glass, highly dispersed
silicon dioxide and spheroid
mixed oxide (64.6 wt%)
Urethane modified Bis-GMA, silanated
Ba-Al-boro-silicate,
CQ, EDAB
1WE
9DE
3M ESPE, St Paul, MN, USA
3M ESPE, St Paul, MN, USA
CE0123
Vivadent-Ivoclar, Schaan,
Liechtenstein
63793/3
Dentsply De Trey, Konstanz,
Germany
Tetric Flow Chroma
Spectrum TPH
Abbreviations: HEMA: 2-hydroxyethyl methacrylate; PAA: polyalkenoic acid copolymer; Bis-GMA: bisphenol-glycidyl methacrylate;
CQ: dil-canforquinone.
200 £ or higher magnification for determination of
the mode of fracture. Failure mode was classified
into one of four types:
Type 1, adhesive failure between adhesive resin
and dentin, and partial cohesive failure in
adhesive resin;
Type 2, total cohesive failure in the adhesive
resin;
Type 3, partial cohesive failure in dentin;
Type 4, partial cohesive failure in the lowviscosity composite layer, or failure between
the low-viscosity composite and composite resin.
The frequency of fracture modes was analyzed
by Kruskal – Wallis test at the 0.05 level of
significance.
In order to observe composite adaptation,
hybrid layer formation and tubule orientation on
cavity bonded walls, additional specimens were
similarly prepared and filled as in test groups to
permit SEM examination. They were sectioned
perpendicular to the bonded surfaces with a
low speed wheel saw under water. Each interface
was finished with a 1000-grit SiC paper
under water and then polished with 6, 3, 1 and
0.25 mm diamond paste using a polish cloth.
Afterwards, specimens were demineralised with
37% phosphoric acid for 8 s. After each step,
specimens were rinsed, and debris were ultrasonically removed for 10 min. In order to observe
the fractured interface of a specimen that failed
cohesively in composite resin (group 8), the
fractured specimen was diamond polished as
mentioned above, and both sides of the slab
were fixed together on a stub.
For examination of the site available for bonding,
two additional specimens were similarly prepared
as in test groups, but were not restored. Cavity
walls were removed, and the gingival floor was
lightly wet abraded with 1000-grit SiC paper,
etched with 37% phosphoric acid for 15 s, washed
and allowed to air dry.
Each specimen was sputter-coated with gold
(MED 010) and observed under an SEM (LEO 435 VP).
Representative areas of the interfaces and of the
available site for bonding were photographed at
3000 £ magnification. The fractured interface was
photographed at 500 £ .
Results
Mean bond strength values obtained with combinations of cavity filling techniques and the low
viscosity composite resin are displayed in Table 2.
There was no statistically significant difference
within groups restored with or without a flowable
Table 2 Microtensile bond strength in MPa (SD) and number
of specimens of the groups evaluated.
Techniques
Composite
Control
Horizontal
Faciolingual
Oblique
Bulk
21.3
22.5
21.7
17.5
14.5
(5.5) 12
(9.1) 10
(6.6) 10
(4.3) 9
(5.2) 8
Composite þ flowable SNK
NS
NS
NS
NS
NS
24.4
19.1
19.4
18.3
15.2
(7.7) 11
(5.0) 9
(8.3) 12
(7.6) 12
(5.0) 9
A
A
A
AB
B
Means marked with different letters are statistically different
by Student –Newman–Keuls test ( p , 0.05). NS-no statistical
significant difference.
The effects of filling techniques and a low-viscosity composite liner on bond strength
to class II cavities
63
composite liner (p . 0.05). Student – Newman –
Keuls test revealed significant differences among
filling techniques ( p , 0.05). Bulk filling groups
presented the lowest bond strength means,
however, they were not significantly different
from the oblique layering technique. All incremental filling groups presented bond strength
means statistically similar to control groups,
which represent a flat dentin surface. Bond
strength values obtained on flat dentin surfaces
(C-factor below one) were only statistically
different from the bulk filling groups.
SEM observations
Illustrative SEM of the cavity gingival floor is shown
in Fig. 2. A characteristic middle to deep dentin is
noted, with high tubule density and low amount of
intertubular area for hybrid layer formation. Fig. 3
is a representative micrograph of the bonded
interface. Tubules are oriented obliquely to the
surface. Hybrid layer and resin tags can be observed
along the axial and gingival walls.
Failure mode observations showed considerable
variation among groups restored with and without
the low-viscosity composite liner (Table 3). A great
number of type 4 failures was observed on groups
restored with the low-viscosity composite resin. A
fracture between flow composite and a bulk-filled
restoration can be observed on Fig. 5. Type 1 failure
was the most predominant failure pattern on groups
restored without the low-viscosity composite resin.
Discussion
Direction of curing shrinkage, depth of cure and
polymerization contraction stresses are some of
Figure 2 Representative SEM micrograph of the site
available for bonding at the gingival floor.
Figure 3 Representative SEM micrograph of the bonded
interface at the gingival floor 1 mm below the cemento –
enamel junction (CR-composite resin, BA, bonding agent,
HL, hybrid layer, D, dentin).
the shortcomings of the curing pattern of lightcured resin composites, which may compromise the
achievement of a perfect seal at the cavity wall.3
The polymerization shrinkage of a resin composite
can create contraction forces that may disrupt the
bond to cavity walls. This competition between the
mechanical stress in polymerizing resin composites
and the bonds of adhesive resins to the walls of
restorations is one of the main causes of marginal
failure and subsequent microleakage observed with
composite restorations.17,18 Several researchers
have sought for techniques and materials to overcome composites undesirable curing effects.19 – 22
The introduction of the microtensile technique has
permitted the evaluation of regional bond strengths
in complex cavity restorations, allowing a better
comprehension of the bonding mechanism.23
In this study, test groups exhibited relatively low
bond strength means, ranging from 14.5 to 24.4 MPa.
These low values can be attributed to the availability
of solid dentin for hybrid layer formation at the site
of bonding.7,24 Fig. 2 is a representative photomicrograph of the site available for bonding at
the gingival floor of the cavity. Besides the great
tubule density and small area of solid dentin, tubules
are not oriented perpendicular to the bonding area
as in most bond strength tests (Fig. 3).
Flowable composites were created by retaining
the same small particle size of traditional hybrid
composites, but reducing the filler content and
consequently, reducing their viscosity.15,25 Lowviscosity composites are intended to act as stressabsorbers by partially relieving the polymerization
contraction stress.12,25,26 In the present investigation, results exhibited no significant differences
between bond strength values of groups restored
64
A. F. Reis et al.
Table 3 Failure modes: same letters indicate no significant difference by Kruskal–Wallis ( p . 0.05).
Groups
Type 2a
Type 2b
Type 3c
Type 4d
Grouping
Control
Horizontal
Faciolingual
Oblique
Bulk
Control þ flow
Horizontal þ flow
Faciolingual þ flow
Oblique þ flow
Bulk þ flow
10
10
6
8
6
5
6
2
6
3
1
0
1
0
2
0
0
1
0
0
1
0
3
1
0
1
1
1
0
0
nae
na
na
na
na
5
2
8
6
6
C
C
BC
C
BC
AB
BC
A
AB
AB
a
b
c
d
e
Type 1: adhesive failure between adhesive resin and dentin, and partial cohesive failure in adhesive resin.
Type 2: total cohesive failure in adhesive resin.
Type 3: partial cohesive failure in dentin.
Type 4: partial cohesive failure in the low-viscosity composite layer, or failure between the low-viscosity composite and
composite resin.
na: not available.
with and without a low-viscosity liner, showing that
low-viscosity composites had no effect on bond
strength. However, analysis of failure modes
revealed that the use of a low-viscosity composite
liner might improve the marginal seal of the dentin
tubules, because a great number of type 4 failures
was observed. The type 4 failure is characterized by
a partial or total failure within the flowable
composite, or between the flowable composite
and the composite resin (Figs. 4 and 5). Our findings
corroborate with the study performed by Montes
et al.15 which showed that a low-viscosity resin had
no effect on tensile bond strength, even though it
influenced failure modes.
Studies have found an improvement in marginal
sealing with the use of a flowable composite as the
first increment in class II cavities.20,27 A reduction in
the number of voids has also been demonstrated
when a low-viscosity composite was used as a liner
in class II cavities.28 The low-viscosity composite
liner may act as a stress-absorbing layer due to its
lower elastic modulus, and avoid disruption of the
bond promoted by the adhesive resin during
polymerization shrinkage; or, in the case it is
disrupted, there is a great chance that dentin
tubules will be kept sealed, as seen in the failure
mode analysis (Fig. 4). Flowable composites have
also been suggested to be used as filled adhesives,26
Figure 4 (A) SEM photomicrograph illustrating a Type 4 failure, showing that fracture partially occurred within the
flowable composite (FC), partially within the adhesive resin (AR) and between the adhesive resin and dentin (D).
Fracture occurred at either the bottom (left) or at the top (right) of the hybrid layer. Note the irregular surface
produced by the diamond bur during cavity preparation. Original magnification £ 220. (B) Close-up of the transition
zone of the fracture between the adhesive resin (AR) and the hybrid layer (HL). Original magnification £ 10,700.
The effects of filling techniques and a low-viscosity composite liner on bond strength
to class II cavities
Figure 5 Representative micrograph of the fractured
interface between composite resin and flow composite
(CR, composite resin, FC, flow composite, HL, hybrid
layer, D, dentin).
however, in a recent study, Frankenberger et al.29
stated that low-viscosity composites should not be
used to replace bonding agents.
Another approach to minimize the effects of
curing shrinkage is the insertion of the composite
resin in increments. The use of incremental
techniques has been extensively studied,30 – 33 however, there is not a common agreement among
authors.16 Our findings showed that the use of
reduced amounts of resin to be polymerized at each
increment helped achieving bond strength values
similar to flat dentin groups (control), which
presented a C-factor below one. Even though the
C-factor of each increment was higher than the one
found in flat surfaces, incremental filling assures
uniform and maximum polymerization of the
composite resin.34
Two factors may have contributed for the low
bond strength values at the bulk filling groups:
polymerization contraction stresses created during
light curing of a great volume of composite resin;35
or decreased effectiveness of polymerization at the
bottom of the cavity.34 Fig. 5 depicts a situation in
which fracture occurred between composite resin
and flowable composite. This is a strong evidence
that deficient polymerization may occur at the
deepest part of thick composite increments, leading to premature failure of bulk filled restorations.
In this situation, the use of a flow composite liner
contributed to keep the seal of dentin tubules at
the gingival floor.
In the current study, the class II configuration
factor was not of a high magnitude, about 1.7, but
sufficient to affect the bonded interfaces. It was
estimated that by filling the class II cavities
incrementally, the C-factor would have been
65
reduced to about 1.3. However, the assurance of
uniform and maximum polymerization in this type
of cavity, provided by incremental filling techniques, was probably more important than the Cfactor reduction. Among the incremental filling
techniques, oblique layering provided the smallest
bond strength values, and although it was statistically similar to control groups, it did not differ
statistically from the bulk filling groups. Although
incremental filling decreases shrinkage stresses due
to minimal contact with the cavity walls during
polymerization as well as to the reduced shrinkage
produced by a small volume material, this is valid
for each individual increment. Finite element
analysis and photoelastic models have shown that
the total shrinkage and its stress field are a result of
the combined effect of the contraction of all the
incremental layers of a restoration.16,32 The oblique
layering technique comprised the insertion of four
composite increments, whereas three increments
were inserted to fill the cavity in the horizontal and
faciolingual layering techniques. The bonded interface was probably more challenged in the oblique
than in the other incremental techniques.
Results of this study indicate that the use of a
low-viscosity composite liner had no effect on bond
strength of composite resin to dentin of a complex
cavity design. However, it significantly influenced
failure modes. The lower elastic modulus of the
flowable composite may help relieve the stresses
produced during polymerization shrinkage in a
cavity preparation. Bond strengths obtained on
flat dentin surfaces created with abrasive paper
may overestimate adhesive systems performance.
Additional research is required to determine optimal procedures for bonding to cavities prepared
and restored under clinical relevant conditions. The
bulk filling groups presented the lowest bond
strength values, however, bond strengths reached
values similar to those obtained on flat dentin
surfaces by incrementally filling the cavities.
Acknowledgements
The authors are indebted to Prof. E. W. Kitajima
(NAP-MEPA/ESALQ-USP) for allowing the use of SEM
equipment. The flowable composite used in this
study was kindly supplied by Vivadent-Ivoclar.
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