Bonding of self-adhesive resin cements to enamel using different surface treatments: bond strength and etching pattern evaluations.

Abstract

This study evaluated the shear bond strengths and etching patterns of seven self-adhesive resin cements to human enamel specimens which were subjected to one of the following surface treatments: (1) Polishing with #600 polishing paper; (2) Phosphoric acid; (3) G-Bond one-step adhesive; or (4) Phosphoric acid and G-Bond. After surface treatment, the human incisor specimens were bonded to a resin composite using a self-adhesive resin cement [Maxcem (MA), RelyX Unicem (UN), Breeze (BR), BisCem (BI), seT (SE), Clearfil SA Luting (CL)] or a conventional resin cement [ResiCem (RE)]. Representative morphology formed with self-adhesive resin cements showed areas of etched enamel intermingled with areas of featureless enamel. In conclusion, etching efficacy influenced the bonding effectiveness of self-adhesive resin cements to unground enamel, and that a combined use of phosphoric acid and G-Bond for pretreatment of human enamel surfaces improved the bond strength of self-adhesive resin cements.

Full text

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Dental Materials Journal 2010; 29(4): 425–432
Bonding of self-adhesive resin cements to enamel using different surface
treatments: bond strength and etching pattern evaluations
Jie LIN1, Akikazu SHINYA1,2, Harunori GOMI1 and Akiyoshi SHINYA1
1Department of Crown and Bridge, School of Life Dentistry at Tokyo, The Nippon Dental University, 1-9-20 Fujimi, Chiyoda-ku, Tokyo 102-8159,
Japan
2Department of Prosthetic Dentistry and Biomaterials Science, Institute of Dentistry, University of Turku, Lemminkaisenkatu 2, 20520
Turku, Finland
Corresponding author, Akiyoshi SHINYA; E-mail: shinya-a@tky.ndu.ac.jp

This study evaluated the shear bond strengths and etching patterns of seven self-adhesive resin cements to human enamel specimens
which were subjected to one of the following surface treatments: (1) Polishing with #600 polishing paper; (2) Phosphoric acid; (3) G-
Bond one-step adhesive; or (4) Phosphoric acid and G-Bond. After surface treatment, the human incisor specimens were bonded to a
resin composite using a self-adhesive resin cement [Maxcem (MA), RelyX Unicem (UN), Breeze (BR), BisCem (BI), seT (SE), Clearfil
SA Luting (CL)] or a conventional resin cement [ResiCem (RE)]. Representative morphology formed with self-adhesive resin cements
showed areas of etched enamel intermingled with areas of featureless enamel. In conclusion, etching efficacy influenced the bonding
effectiveness of self-adhesive resin cements to unground enamel, and that a combined use of phosphoric acid and G-Bond for pre-
treatment of human enamel surfaces improved the bond strength of self-adhesive resin cements.
Keywords: Self-adhesive resin cement, Bond strength, Enamel

INTRODUCTION
In current dental practice, minimal intervention is a
widely advocated concept which promotes minimally
invasive procedures —treatment approaches that
preserve as much sound tooth structure as possible1). In
accordance with the minimal intervention principle, the
use of enamel adhesive techniques has greatly
increased in dentistry in recent years, with many
innovative applications being found in prosthodontics
such as veneers and resin-bonded fixed partial
dentures2,3). An apparent advantage of applying enamel
adhesive techniques to these prosthodontic restorations
is the preservation of dental hard tissues.
Inspired by the industrial use of 85% phosphoric
acid to facilitate the adhesion of paints and resins to
metallic surfaces, Buonocore envisioned the use of acids
to etch enamel for sealing pits and fissures in 19554).
Adhesion to enamel is achieved through acid etching of
this highly mineralized substrate, which substantially
enlarges its surface area for bonding. Further research
into the underlying mechanism of the bond suggested
that tag-like resin extensions were formed and
micromechanically interlocked
microporosities created by etching5,6). Conventional
resin cements are based upon the use of an etch-and-
rinse or self-etch adhesive followed by a low-viscosity
resin composite. However, this multi-step application
technique is complex and rather technique-sensitive7).
Now that conventional resin cements have
established a reputation for acceptable bonding
effectiveness8), recent efforts focused on how to simplify
the multi-step bonding process and reduce its
sensitivity to errors during clinical handling. Recently,
with the enamel
so-called universal, all-purpose or multipurpose, self-
adhesive resin cements are commercially available now,
and they purportedly bond to a multitude of substrates
such as enamel, dentin, amalgam, metal, and
porcelain9-12). In addition, self-adhesive cements that
require only single-step application have been proposed
for luting zirconium-based restorations13,14). For these
systems, their resin matrix consists of multifunctional
acid methacrylates that purportedly react with the
substrate and contribute to the adhesion mechanism5).
However, with regard to adhesion between self-
adhesive resin cements and enamel, no conclusive
results have been obtained for the bond strength,
failure mode, and etching pattern.
For successful long-term retention of restorations15)
and for good marginal adaptation16), it is imperative
that a luting material be reliably bonded to both the
restorative material and tooth structures. RelyX
Unicem, which features a simplified application
procedure, has been proposed as an alternative to the
currently used systems for luting conventional ceramics
as well as metal-based and high-strength ceramic
restorations17,18). In light of the importance of reliable
bonding, the following null hypotheses were examined
in this study: (1) The use of phosphoric acid and G-
Bond for luting does not improve bonding effectiveness;
(2) There are no differences in shear bond strength
between self-adhesive resin cements and conventional
resin cements.
MATERIALS AND METHODS
Specimen preparation
Non-carious human incisors were stored in 0.5%

Received Dec 25, 2009: Accepted Mar 29, 2010
doi:10.4012/dmj.2009-140 JOI JST.JSTAGE/dmj/2009-140

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Dent Mater J 2010; 29(4): 425–432 426
chloramine in water at 4°C for a maximum of 6 months
until use. The teeth were embedded in chemically cured
acrylic resin (d=25 mm, h=30 mm). They were ground
flat with 600-grit polishing paper used on a polisher
(EcoMet 3, Buehler, IL, USA) to obtain a flat enamel
surface of about 6 mm diameter.
Specimens were randomly assigned to four groups
according to the type of surface treatment applied:
Type I (#600): Enamel surfaces were polished with
600-grit polishing paper used on a polisher.
Type II (Phosphoric acid): 35% phosphoric acid
(Panavia Etching Agent V, Kuraray Medical,
Tokyo, Japan) was applied for 60 seconds, then
rinsed with an air-water spray from a dental three-
way syringe and air-dried.
Type III (G-Bond): One-step adhesive (G-Bond, GC,
Tokyo, Japan) was applied for 10 seconds and
gently air-blown. Excess agent was removed with
strong air, and light curing was applied for 10
seconds.
Type IV (PG): Both phosphoric acid and G-Bond
were applied.
Product/Code/Lot No./
Manufacturer
Main composition Application
Resin cements
Maxcem/MA/2772209/
Kerr (CA, USA)
Bis-GMA, UDMA, TEGDMA,
GPDM, barium glass filler,
fluoroaluminosilicate glass
filler, fumed silica
Mix cement through a dual-barrel syringe.
Apply, then light-cure for 20 s from each side.
RelyX Unicem/UN/304133/
3M ESPE (MN, USA)
dimethacrylate, acetate ,
methacrylated phosphoric
ester, glass powder, silica,
calcium hydroxide
Insert capsule into activator. Press down handle
completely. Insert activated capsule into mixing
device. Mix 15 s. Apply, then light-cure for 30 s
from each side.
Breeze/BR/162835/
Pentron Clinical Technologies
(CT, USA)
Bis-GMA, UDMA, TEGDMA,
HEMA, 4-MET, silane treated
barium glass, silica
(amorphous), Ca-Al-F-silicate
Mix cement through a dual-barrel syringe.
Apply, then light-cure for 20 s from each side.
BisCem/BI/700009638/
Bisco (IL, USA)
TEGDMA, HEMA, phosphate,
dental glass
Mix cement through a dual-barrel syringe.
Apply, then light-cure for 20 s from each side.
seT/SE/S0711272/
SDI (Victoria, Australia)
UDMA, phosphate,
fluoroaluminosilicate glass,
silica,
Insert capsule into activator. Press down handle
completely. Insert activated capsule into mixing
device. Mix 10 s. Apply, then light-cure for 20 s
from each side.
Clearfil SA Luting/CL/0005AA/
Kuraray Medical (Tokyo, Japan)
Bis-GMA, TEGDMA, MDP,
barium glass, silica, sodium
fluoride
Mix cement through a dual-barrel syringe.
Apply, then light-cure for 10 s from each side.
ResiCem/RE/0107/
Shofu (Kyoto, Japan)
UDMA, TEGDMA,
fluoroaluminosilicate glass, 4-
AET, HEMA
Mix cement through a dual-barrel syringe.
Apply, then light-cure for 20 s from each side.
Primer from the same manufacturer was not
applied.
One-step self-etch adhesive
G-Bond/ /0610051/
GC (Tokyo, Japan)
4-MET, MMA, water, acetone Apply adhesive to the enamel surface for 10 s.
Gently air-blow and strongly air-dry by a three-
way syringe for 10 s. Light-cure for 10 s.
Resin composite
Gradia/ /0306241/
GC (Tokyo, Japan)
UDMA, dimethacrylate co-
monomers, silica,
prepolymerized filler
Bis-GMA: bisphenol-A-diglycidyl methacrylate; GPDM: glycerol dimethacrylate dihydrogen phosphate;
HEMA: 2-hydroxyethyl methacrylate; MDP: 10-methacryloyloxydecyl dihydrogen phosphate; MMA: methyl methacrylate;
TEGDMA: triethyleneglycol dimethacrylate; UDMA: urethane dimethacrylate; 4-AET: 4-acryloxyethyltrimellitic acid;
4-MET: 4-methacryloxyethyl trimellitic acid.
Table 1 List of materials used in this study

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Dent Mater J 2010; 29(4): 425–432427
Specimens from each group were further divided
into seven subgroups according to the number of resin
cements investigated in this study (total: 168, n=6 per
subgroup). Table 1 lists the materials used in this
study, including their chemical compositions and
application procedures, while Fig. 1 shows the bonding
procedure. The resin cements used in this study were
six self-adhesive resin cements [Maxcem (MA; Kerr,
CA, USA), RelyX Unicem (UN; 3M ESPE, MN, USA),
Breeze (BR; Pentron Clinical Technologies, CT, USA),
BisCem (BI; Bisco, IL, USA), seT (SE, SDI, Victoria,
Australia), Clearfil SA Luting (CL; Kuraray Medical,
Tokyo, Japan)] and one conventional resin cement
[ResiCem (RE; Shofu, Kyoto, Japan)].
After surface treatment, each bonding area was
delineated by a masking tape with a 5-mm-diameter
hole. All self-adhesive resin cements were applied using
the bonding procedures according to the manufacturers’
instructions. The conventional resin cement, ResiCem,
was not used in conjunction with the primer according
to the manufacturer’s instruction; instead, it was
applied like a self-adhesive resin cement. After
applying the resin cement, a tube with an internal
diameter of 8 mm and a height of approximately 2 mm
was placed on the uncured-resin bonding surface. The
tube was filled with a resin composite (Gradia, GC,
Tokyo, Japan) and cured with a light curing unit (G-
Light, GC, Tokyo, Japan; 1200 mW/cm2 light intensity).
The specimens were then immersed in distilled water
at a temperature of 37°C for 24 hours.
Shear bond strength test
Shear bond strength was determined according to ISO/
TS 11405:200319) using a universal testing machine
(Servo Pulser EHF-FDI, Shimadzu, Kyoto, Japan) at a
crosshead speed of 0.5 mm/min. Shear bond strength
was expressed in MPa and was derived from dividing
the imposed force (N) at the time of fracture by the
bonding area (approx. 20 mm2).
Fracture surfaces of the samples were examined
using an optical light microscope (MZ7.5, Leica
Microsystems, Germany) at 32× magnification. Failure
modes which were thus observed were classified as
follows: (A) Adhesive failure at resin-enamel interface;
(B) Mixed failure, where adhesive failure occurred with
a thin layer of luting material remaining on the enamel
surface; (C) Cohesive failure in luting material; (D)
Partial cohesive failure in enamel or resin composite.
Statistical analysis
Statistical analysis was performed using two-way and
one-way analysis of variance (ANOVA) models and
Tukey’s HSD test for post hoc pairwise comparisons
(p<0.05). Mean shear bond strengths of seven different
resin cement groups with four different surface
treatments were compared using a two-factor ANOVA
model. One-way ANOVA model was used to evaluate
the differences among the resin cements within each of
the four surface treatment groups. Then, for each resin
cement, a similar series of one-way ANOVA models
was used to evaluate the differences among the four
surface treatment methods.
Scanning electron microscopy (SEM)
Twelve additional teeth were selected for SEM
examination to assess the effect of surface treatments
on enamel surface morphology and the etching patterns
of resin cements and G-Bond.
1. Effect of surface treatments on enamel surface
morphology
After sputter-coating with a gold-palladium alloy
conductive layer (Ion Sputter E-1030, Hitachi, Tokyo,
Japan), enamel surfaces which had been subjected to
four different surface treatments were examined using
a scanning electron microscope (SEM; S-4000, Hitachi,
Tokyo, Japan) with an acceleration voltage of 5 kV.
2. Etching patterns of resin cements and G-Bond
Enamel specimens polished with 600-grit polishing
paper were applied with one of the seven resin cements
or G-Bond for 60 seconds. The surfaces were then
removed of all agents, soluble organic and inorganic
products using alternating washes of distilled water
and acetone, air-dried, and examined by SEM.
RESULTS
Shear bond strength
ANOVA revealed that the factors of resin cement and
surface treatment (p<0.01), and their interaction
(p<0.01), had a significant effect on shear bond
strength.
Fig. 1 Bonding procedure used in this study. (a) Enamel
surface was pretreated with four different
treatment methods. (b) A piece of masking tape
was attached to delineate an area for bonding. (c)
Resin cement was applied to the bonding area on
enamel surface. (d) A tube was formed and used
to hold the resin composite. (e) Resin composite
was condensed into the tube and cured with a
light curing unit. (f) After storage, shear bond
strength of each specimen was determined using
a universal testing machine.

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Dent Mater J 2010; 29(4): 425–432 428
Figure 2 shows the mean shear bond strength
values for all the seven resin cement groups and the
four surface treatments. The mean values were 6.4±2.8
MPa for #600, 13.9±1.9 MPa for phosphoric acid,
11.3±1.7 MPa for G-Bond, and 15.9±2.2 MPa for PG.
For shear bond strength of enamel to MA, UN, BR,
BI, SE, CL, and RE, the results were in ascending
order of #600 polishing<one-step self-etch adhesive G-
Bond<phosphoric acid etching<PG.
The bond strength of RE with #600 polishing was
relatively low and the mean value was 1.4±1.0 MPa.
Conversely, the bond strength of the MDP-containing
self-adhesive cement CL with #600 polishing was
relatively high with a mean value of 9.8±1.9 MPa.
Similarly, the use of CL with PG treatment resulted in
the highest bond strength value (17.6±1.4 MPa).
The failure modes are shown in Table 2. For #600
polishing, phosphoric acid etching, and G-Bond, most of
the failures were observed to be adhesive failure at the
resin-enamel interface for all the resin cements. In
contrast, more occurrences of mixed failure and
cohesive failure were found for PG treatment.
SEM
Figure 3 shows the SEM photographs of the four
surface treatments. With #600 polishing, a smear layer
covering the underlying substrate was observed on the
enamel surface. With phosphoric acid etching, typical
keyhole-shaped enamel prisms or rods were observed.
With G-Bond application, a thick adhesive layer was
observed on the enamel surface. On the other hand, the
thick adhesive layer was not observed with PG
treatment but seemed to have infiltrated into the
enamel surface.
As for the etching patterns of the resin cements
and G-Bond, their representative SEM photographs are
shown in Fig. 4.
DISCUSSION
Smear layer is defined as “any debris, calcific in nature,
produced by reduction or instrumentation of dentin,
enamel or cementum”20,21). However, this iatrogenically
produced layer of debris has an adverse influence on
any adhesive bond formed between a cut tooth and the
restorative material21-23). Two strategies are used to
overcome the low attachment strengths of the smear
layer: removal of the smear layer prior to bonding
(etch-and-rinse approach, such as phosphoric acid
etching), or use of bonding agents that can penetrate
beyond the smear layer and incorporate the latter into
the bonding layer (self-etch approach, such as self-
adhesive resin cements and G-Bond)24). With self-
adhesive resin cements, the rationale is to superficially
demineralize the enamel and simultaneously infiltrate
the etched, exposed enamel with resin to create a resin-
reinforced hybrid layer8).
Pashley et al. have classified self-etching systems
into three categories according to their etching efficacy:
#600
6/0/0/0
6/0/0/0
6/0/0/0
6/0/0/0
6/0/0/0
6/0/0/0
6/0/0/0
Phosphoric acid
6/0/0/0
6/0/0/0
4/0/2/0
5/1/0/0
6/0/0/0
5/0/0/1
3/1/1/1
G-Bond
6/0/0/0
6/0/0/0
6/0/0/0
6/0/0/0
6/0/0/0
6/0/0/0
6/0/0/0
PG
MA
UN
BR
BI
SE
CL
RE
2/1/3/0
1/2/3/0
2/0/4/0
2/3/1/0
1/2/1/2
0/1/4/1
0/1/5/0
Failure modes: A/B/C/D
A: Adhesive failure at resin-enamel interface; B: Mixed failure, where adhesive failure occurred with a thin layer of luting
material remaining on the enamel surface; C: Cohesive failure in luting material; D: Partial cohesive failure in enamel or
resin composite.
Table 2 Failure mode distribution recorded in the experimental groups
Fig. 2 Box plots of shear bond strength results. The box
represents the spread of data between the first
and third quartiles. The central horizontal line in
the box represents the median value. The range of
data is represented by whiskers extending to the
minimum and maximum values.

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Dent Mater J 2010; 29(4): 425–432429
mild, moderate, or aggressive25). Clearfil Mega Bond
(Kuraray Medical, Osaka, Japan), the least aggressive
system (mild), had a pH value of 2.0, while the Non-
Rinse Conditioner (Dentsply
Germany) that produced moderate etching had a pH
value of 1.2. Prompt L-Pop (3M ESPE, Seefeld,
Germany), the most aggressive system at etching
unground enamel, had a pH value of 1.0 and that
approached the effect achieved using 32% phosphoric
acid etching. Further, SEM examination results of
demineralization correlated well with the pH values.
Regarding their effects on bond strength, mild self-
etching systems tend to provide excellent dentin bond
strengths but poorer enamel bonds, whereas more
aggressive self-etch systems provide the reverse25).
In the present study, a similar difference in
aggressiveness was also observed. SEM examination of
enamel surface morphology showed that self-adhesive
resin cements produced a very mild etching effect on
unground enamel, with the bulk of the surface
remaining unetched. G-Bond produced a moderate
DeTrey, Konstanz,
‘coral-like’ etching pattern. On the other hand, 37%
phosphoric acid etching, being the most acidic of the
three self-etching adhesives, produced a highly porous
surface on aprismatic enamel.
SEM photographs of the etching patterns of self-
adhesive resin cements showed that the smear layer
was partially dissolved (Figs. 4(a)−(f)). Although the
etching patterns of the resin cements presented some
micromorphological differences, these differences were
not strong enough to affect their failure modes —which
was predominantly adhesive failure at the resin-enamel
interface. Further, shear bond strength test revealed
that with #600 polishing, self-adhesive resin cements
failed to exhibit acceptable bond strengths. Minute
amounts of water left in the smear layer and intrinsic
water in enamel most probably led to limited ionization
of the demineralizing monomers, which was one more
indication that resin cements or adhesives are able to
absorb water from their bonding surfaces through
diffusion26).
As for self-etching adhesive systems, their
Fig. 3 Sample SEM photographs of different surface treatments: (a) #600; (b) phosphoric acid; (c) G-Bond; (d) PG. In (a),
the smear layer completely covered the underlying substrate. In (b), typical keyhole-shaped enamel prisms or rods
were seen. In (c), a thick adhesive layer was observed. In (d), the thick adhesive layer was not observed but
seemed to have infiltrated into the enamel surface.

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Dent Mater J 2010; 29(4): 425–432 430
Fig. 4 SEM photographs of the etching patterns of resin cements and G-Bond, where: (a) MA; (b) UN; (c) BR; (d) BI; (e)
SE; (f) CL; (g) RE; (h) G-Bond. Arrowheads indicate etched enamel (10,000×magnification).
In (a, b, d, e, f), areas of etched enamel intermingled with areas of featureless enamel. However, (f) was better
defined than (a, b, d, e).
In (c, h), similar morphology was seen as BR and G-Bond contained the same adhesive monomer 4-MET.
However, (c) was mostly featureless and less defined than (h).
RE contained a small amount of adhesive monomer (4-AET), but (g) showed no evidence of acid attack. With G-
Bond, a moderate ‘coral-like’ etching pattern was produced in (h).

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Dent Mater J 2010; 29(4): 425–432 431
composition comprises water because it is needed to
enable ionization of
demineralization of hard dental tissues. On the other
hand, the presence of water can be a bane because
residual water and hydrophilic solvent within the
interfacial structure weakens bond integrity, providing
channels for nanoleakage and may even affect the
polymerization of infiltrated monomers27,28). Against
this background, the tooth surface must be dried by air
after applying G-Bond. However, due to excessive
simplification of the clinical application procedure in
the present study, residual water and adhesive solvent
was not removed and the resultant interfacial structure
becane more hydrophilic29,30), thereby rendering it to be
more prone to hydrolytic degradation31).
RE contained a small amount of adhesive monomer
(4-AET). However, when it was applied with a
simplified application procedure on enamel surface
pretreated with #600 polishing, poor bond strength
(1.4±1.0 MPa) was obtained. SEM photograph of the
etching pattern of RE showed that the smear layer was
not dissolved (Fig. 4g). Nonetheless, when RE was
applied on enamel surfaces pretreated with phosphoric
acid and G-Bond, shear bond strength test revealed
that bond strengths comparable to those of self-
adhesive resin cements were achieved.
The main objectives of acid conditioning are
removing the smear layer and rendering the enamel
surface more receptive for bonding32,33). Similarly in this
study, enamel was treated with phosphoric acid with a
view to achieving the objectives of smear layer removal
and enamel demineralization. Consequently, the acid-
treated enamel surface had a high surface energy34,35)
so that resin monomer flowed into, intimately adhered
to, and polymerized within the pores to form retentive
resin tags. In contrast, when enamel was pretreated
with #600 polishing, the smear layer was left intact
which then interfered with adhesive layer information.
As a result, comparison of the shear bond strengths of
#600 and phosphoric acid revealed that the thick and
compact smear layer apparently prevented the viscous
cement from reaching the deeper unaffected enamel.
When phosphoric acid was used in combination
with G-Bond, SEM photograph showed that after
phosphoric acid removed mineral deposits from the
enamel surface, the one-step adhesive, G-Bond,
infiltrated the exposed enamel surface (Fig. 3d).
Further, shear bond strength test revealed that a
combined use of phosphoric acid and G-Bond resulted
in significantly greater bond strength to enamel than
use of G-Bond alone. In contrast, use of G-Bond alone
resulted in only a moderate etching pattern (Fig. 3c);
the adhesive did not infiltrate into the smear layer, and
hence a thick adhesive layer was created. In light of
these findings, the first null hypothesis was rejected.
On pretreatment with G-Bond, no significant
differences in shear bond strength were observed
among the seven resin cements. This finding showed
that the differing compositions and solvents of self-
adhesive and conventional resin cements did not affect
acidic monomers and
bond strength. Therefore, the second null hypothesis
was partially proved.
CONCLUSION
A combined use of phosphoric acid and G-Bond for pre-
treatment of human enamel surfaces improved the
bonding effectiveness of self-adhesive resin cements.
This was chiefly because etching efficacy was an
important contributing factor
effectiveness of self-adhesive resin cements to unground
enamel. Representative morphology formed with self-
adhesive resin cements showed areas of etched enamel
intermingled with areas of featureless enamel, and
shear bond strength test further revealed that self-
adhesive resin cements failed to achieve acceptable
bonding effectiveness. Therefore, pre-treatment with
phosphoric acid is recommended when using self-
adhesive resin cements to bond to enamel.
to the bonding
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