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MDJ Storage effect on the bond strength of orthodontic metal… Vol.:5 No.:1 2008
53
Storage effect on the bond strength of orthodontic
metal brackets bonded by three types of adhesive
generations
Dr. Suhad M. A. Sadiq. B.D.S., M.Sc.*
Abstract
The effect of food simulants on the bond strength of orthodontic metal brackets
bonded to enamel with light cured composite was studied. One hundred twenty
extracted human premolars were selected and randomly divided into three equal
groups each with 40 teeth, representing the adhesive bonding generation (5th, 6th and
7th). Each group was subdivided in to two subgroups which represented the storage
media, which are distilled water (DW) and 75% aqueous ethanol (Food simulating
solution-FSS). Then the storage media group was subdivided into two subgroups with
10 teeth each, representing two storage periods (1 day and 30 days).
At the end of the storage period in the immersion media the brackets were
debonded by an Instron universal testing machine to measure the shear bond strength.
It was found immersion in the food simulants for 30 days significantly reduces the
bond strength of light cured composite brackets.
Key words: Storage media, ethanol, Adhesive bonding generations.
Introduction
Adhesive techniques have been
developed to such an extent that they
are now involved in a large number of
the clinical procedures. Ease of bracket
placement coupled with a reasonable
clinical success rate and a reduction in
chair side time have removed the need
for banding all the teeth. The resin
dentin bonds created by current
hydrophilic adhesive systems can
severely degrade over time. (1,2)
Lee et al, (3) reported the effects of
oral fluid simulants and food simulants
upon the bond strength of dentine
bonding composites. They found that
exposure to 75% ethanol significantly
decreased the bond strength after 30
days. It has been shown that Bis-GMA
based composites are susceptible to
chemical softening by certain solvents.
(4) The diffusion of moisture through
the resin may also lead to the initiation
and propagation of micro cracks at the
interface and in the resin. (5)
Storage time and medium may be
deleterious to the mechanical property
and durability of resin bond. The
durability of resin dentin bonds
depends upon the stability of their
components over time. Morphological
in vitro studies indicated that both resin
and collagen matrices may degrade
upon storage. (6,7)
Failure of resin bonds may initiate
in one specific component of the
interface. The identification of which
component is more likely to be
responsible for the over all reduction of
the bond strength is impossible to be
Lecturer, P.O.P. department, College of Dentistry, Al-Mustansiria University
MDJ
MDJ Storage effect on the bond strength of orthodontic metal… Vol.:5 No.:1 2008
54
evaluated. The evaluation can only be
able to determine which is least stable
during various storage conditions.
Thus, the objective of this study was to
investigate the effect of prolong aging
in distilled water and food simulating
solution (75% ethanol solution) on
shear bond strength of three types of
current dentinal adhesive generations.
Materials and Methods
One hundred twenty sound
extracted human premolar teeth were
collected, which have been extracted
from 15-20 years old Iraqi patients
seeking orthodontic treatment. All
teeth were examined for any visible
fracture or crack by using light curing
unit, any tooth that had a visible
fracture or crack was discarded. The
teeth were cleaned under running water
then stored in distilled water
containing a crystal of thymol to
prevent dehydration and bacterial
growth with closed container at room
temperature (22 Cº ± 3). (8)
One hundred twenty new stainless
steel brackets standard edge wise (0
torque, 0 angulations) were used. The
stainless steel brackets with coarse
mesh base (Dentaurum, Germany) with
surface area of 12.30 mm2.
The roots of the teeth were serrated
by diamond disk, made a retentive
wedge shaped to increase the retention
of the teeth inside the self-cured acrylic
blocks, then each tooth was fixed on a
glass slide in a vertical position using
soft sticky wax at the root apex, so that
the middle third of the buccal surface
was oriented to be parallel to the
analyzing rod of the surveyor, so that
the force could be applied at right
angle to the enamel-bracket interface.
(9) Another tooth was fixed on the
glass slide about 1 cm away from the
first tooth and was oriented in the same
manner. Then two more teeth were
placed and fixed on the glass slide in
the same way of the second tooth in
order to have four premolars fixed on
the glass slide 1 cm apart having the
middle third of the buccal surface of
each tooth parallel to the analyzing rod
of the surveyor and the occlusal
surface of each tooth oriented to same
height by using a stone disc bur. (10)
Then the two L-shaped metal plates
were painted with a thin layer of
separating medium (Vaseline) and
placed opposite to each other in such
away to form a box around the
vertically positioned teeth with the
crowns protruding. Then the powder
and liquid of the cold cured acrylic
were mixed and poured around the
teeth to the level of the cemento-
enamel junction of each tooth. After
setting of the cold cured acrylic resin,
the two L- shaped metal plates were
removed, the sticky wax used for
fixation of teeth in the proper
orientation removed too and the
resulting holes filled with cold cure
acrylic, slight adjustment of the acrylic
blocks was done using the portable
engine. After mounting, the teeth were
stored in normal saline solution to
prevent dehydration until bonding. (11)
Thirty blocks were made to hold 120
teeth.
120 teeth were divided in to three
equal main groups according to the
type of the adhesive generations:
Group I: 40 teeth used single bond, 5th
generation bonding material
(Adper Single Bond, 3M ESPE,
Scotch bond, USA).
Group II: 40 teeth used self etching,
6th generation bonding material
(Ivoclar, Vivadent, Germany).
Group III: 40 teeth used Go! 7th
generation bonding material
(SDI, Australia).
Each main group was divided
equally into two groups, containing
twenty teeth each, according to the
type of storage media (Distilled water
and 75% aqueous ethanol), and then
MDJ Storage effect on the bond strength of orthodontic metal… Vol.:5 No.:1 2008
55
each storage media group was
subdivided into two subgroups with ten
teeth each, representing the time of
storage period, which are 1 day and 30
days.
The buccal surface of each tooth
was polished with a rubber cup using
low speed hand piece and non-
fluoridated pumice. The teeth were
washed and dried with oil-free air.
Each subgroup has bonded with
different type of bonding:
First group: Forty teeth were treated
with 5th generation, SB (Adper
single-bond, 3M ESPE Adper,
Scotch bond dental product, USA)
two step total-etch adhesive system.
Adhesive bond was applied
according to manufacturing
instruction, first of all apply etching
gel for 15 sec. (35% phosphoric
acid), then thoroughly rinsed off with
water for 15 sec. gently dried with air
stream for 2 sec. SB was applied to
the etched area with disposable brush
tip then gently dried with air stream
for 2-5sec. Light cured for 10 sec. by
LED light cure (SDI, Australia).
Second group: Forty teeth were
treated with 6th generation (Self-
etching adhesive, Ivoclar, Vivadent).
Adhesive bond was applied
according to manufacturer
instruction, first of all applied the
self-etch primer to the bonded area
for 15 sec. and brush into the surface
for another 15 sec. then disperses
primer with a strong stream of air.
SE bond was applied and light-cured
immediately for 10 sec.
Third group: Forty teeth were treated
with Go! 7th generation bonding
material (SDI, Australia). Adhesive
bond was applied according to
manufacturer's instruction. Applied
7th generation to entire dried bonded
area, leave undisturbed for 10 sec.,
dry thoroughly under maximum air
pressure for 5 sec. then light cured
for 10 sec.
Light cure Microfilled composite
material (Hellio Molar, Ivoclar,
Vivadent) was used, dispensing the
composite directly into the base of the
bracket (to decrease the air
entrapment). The bracket positioned on
the tooth, on the center of the buccal
aspect of the tooth.
A standard pressure was added for
each tooth after bracket placement, the
pressure instrument is of 200 gm and
adapted into the dental surveyor and
the sample place in position on the
metal base, which kept vertically on
the surveyor base. The excess material
was removed using a dental probe from
around the base of the bracket. Then
with LED light curing machine, we
cured the composite from three
directions, first from the lingual side
and then from the mesio-buccal side
and lastly from disto-buccal side for
each time, the curing time was 20 sec.
After storing all the specimens in
normal saline at 37Cº for 24 hours,
they were immersed into 250ml of
distal water or 75% ethanol and
maintained in the incubator at 37Cº for
the decided period. (12)
Each container was closed by Para
film to control evaporation; the
substrates were changed weekly during
the month period. (13) At the end of
each conditioning period (1 day and 30
days), the specimens were washed
under running water and ready for the
shear bond strength test.
Shear bond strength was measured
by using Instron machine with a cross-
head speed 0.5 mm/min. (14) The
sample was seated in mounting base of
the testing machine; the chisel of the
Instron machine is vertical to the tooth
and applies the force to the sample of
the bracket base enamel interface. The
conversion of Newton to Mega Pascal
(Mpa) was made by divided force by
the bracket base area. (15)
The statistical method that had been
used in this study to analyze and to
MDJ Storage effect on the bond strength of orthodontic metal… Vol.:5 No.:1 2008
56
assess results includes the descriptive
statistical and inferential statistics.
Results
Shear bond strength of the present
study of two main groups (DW and
75% ethanol) are shown in table I. The
mean shear bond strength, standard
deviation values, minimum and
maximum values for each storage
intervals group.
One-way ANOVA test was
performed for all main groups to
compare among the means of the shear
bond strength values of the different
generation groups within each storage
period, which is presented in tables II.
Using student t-test was done to
compare statistically between the
means of SBS values of three
generation groups (5th, 6th, and 7th)
within different storage time intervals.
Student t-test was done for timing
effects to compare statistically between
the means of SBS values for each
generation groups (5th, 6th and 7th)
between the two aging intervals (1 day
and 30 days) of DW and FSS groups as
presented in tables IV.
Student t-test was done to compare
statistically between the SBS values of
the two main groups (DW and FSS) at
different aging time intervals (1 day
and 30 days) of three generation
subgroups ( 5th, 6th and 7th), which was
presented in table V.
Discussion
* Bond strength of three adhesive
generations:
The present study demonstrates that
tooth structure and Shear Bond
Strength can be significantly affected
by types of current bonding adhesive
generations and prolonged aging in
distilled water or Food Simulating
Solution like (75% Ethanol solution).
According to the food and drug
Administration Guidelines of the
United States (1976) this solution is a
recommended food simulator and may
be considering clinically relevant.
The results of this study show their
was statistically a high significant
difference among the two-step (5th
generation) and two self-etch (6th and
7th generation) adhesive systems.
Spencer and Wang,2002; (16) concluded
that the combination of primer and the
adhesive resin in one-bottle 5th
generation will lead to a higher
viscosity of this component which will
decrease the penetration and
hybridization effectiveness. Also, the
Bis-GMA/HEMA mixtures with 5th
generation adhesive when combined
with water at concentration 50-65%.
Macrophase separation in the Bis-
GMA/HEMA water mixtures was
detected based on SEM analysis; there
was substantial porosity at the adhesive
interface with tooth.
Miyazaki M. et al, 2002 (17)
reported that the layer of
demineralization seemed to consist of
two different phases (6.8-7.3 µm)
thickness. The Bis-GMA could not
penetrate into the deepest area and the
depth of this Bis-GMA unsaturated
layer was estimated as 1.0-1.2 µm
.These nano space in turn cause
interfacial defects and inhibit the
infiltration of hydrophobic resin
monomer.
Van Landuyt K. et al, 2005, (18)
concluded that 5th generation adhesive
contains hydrophilic monomers such as
HEMA. This monomer is soluble in
water, acetone and alcohol. It is also an
organic material that has an affinity to
hydrophobic monomers. Hence, it can
be a useful medium for the hydrophilic
and hydrophobic components. On the
other hand, HEMA creates a hydrogel
within the hybrid layer and adhesive
resin (when primer is mixed with
adhesive resin in one bottle) in some
cases. The hydrogel may provide a
MDJ Storage effect on the bond strength of orthodontic metal… Vol.:5 No.:1 2008
57
channel for water permeation that has
the potential to affect the durability of
bonds, especially when poly-HEMA of
low molecular weight is created.
Also, the results of this study show
that there was difference in SBS value
between the two self-etch adhesive
systems (6th and 7th generation) but of
non significant difference. This results
is in agreement with Abate et al, 2000,
Inoue et al ,2001, (19, 20) reported that
one-step-self-etch adhesive tend to
have lower bond strength than two-
step self-etch adhesive even of no
significant difference value.
Also this is in agreement with
Gallo et al, 2001. (21) He concluded that
the bond strength of all-in-one
adhesive is reported not to exceed that
of two-steps self- etch system.
This may be due to the difference
between the monomer composition (4-
META) of the (GO) one-step self-etch
adhesive system and the (SE) two-step
self-etch adhesive systems. This
monomer was not present in the
composition of other adhesive systems
or the difference could be due to the
different type and concentration of
solvent used in (GO) bond acetone
which has the higher vapor pressure
(180 mmHg) and lower boiling
temperature(56.5Cº) when compared to
ethanol (43.9mmHg ;78.3Cº) and water
(17.5mmHg ; 100Cº). (22)
* Effect of food simulants:
The results in one day storage
period statistically showed no
significant differences in shear bond
strength values between the groups
(Table V). These small effects may be
explained that one day storage is not
enough for the different chemical
solutions to exert a notable change in
the bond strength values. This confirms
the findings of Asmussen, 1984, (4)
who found that one day soaking in
ethanol does not induce softening of
polymers.
The presence of fillers in a polymer
network can strongly influence solvent
uptake and softening effect, as it
reduces the overall volume of the
absorbing material. In addition,
alteration in the organic matrix
components may result in changes in
the magnitude of composites`
solubility parameter. (23)
The result of this study shows that
there was statistically a high significant
difference in SBS value of three types
of adhesive systems used after one
month storage in 75% ethanol solution
in comparison with one day storage
period (Table IV).
The significant effect of ethanol,
which simulates alcoholic drinks and
beverages, on bond strength may be
explained as follows, cross linked
dimethacrylate resins are virtually
insoluble, yet they are capable of
swelling in good solvent, so it is
possible to predict the efficiency of a
given solvent for a given polymer by
matching their solubility parameters.
The solubility parameter describes the
ease with which a molecule will
penetrate and dissolve within another
substance. (24) The diffusion of ethanol
into the composite may cause micro-
cracking in structure that may
subsequently weaken the bonding.
Ethanol has solubility characteristics
similar to that of Bis-GMA and this
may further promote the infusion of
ethanol into the composite leading to
further damage. (3)
The finding of this study comes in
accordance with Yap et al, 2001, (25)
who found that all composites are
softened by 25-75% ethanol-water
solution, nevertheless different
concentrations were used. It also
agrees with Akova et al, 2007, (12) who
showed that food simulants
significantly decrease the bond
strength between bracket and tooth
surface, with ethanol, which simulates
MDJ Storage effect on the bond strength of orthodontic metal… Vol.:5 No.:1 2008
58
alcoholic drinks and beverages, being
the most effective chemical.
* Effect of storage period:
The difference between the two
storage periods (1 day and 30 days)
was highly significant for 75% ethanol
while non significant difference for
distilled water storage media (Table
IV).
This can be explained in terms of
diffusivity and solubility parameter.
The diffusivity of the ethanol solution
in dental composite specimens is
greater than that of water in dental
composites. (26, 27) The solubility
parameter describes the ease with
which a molecule will penetrate and
dissolve within another substance, and
the solubility parameter of ethanol is
closer to that for the dental composite
3x104 J1/2/m3/2, while the solubility
parameter of water is about 4.8x104
J1/2/m3/2 this value has been shown to
fall beyond the solubility parameter
ranges for Bis-GMA based composite
and thus has little influence on matrix
softening, while ethanol has greater
permeability. (24)
The results further confirmed
earlier results by Lee et al, (3, 28, 29) they
suggested that the materials were not
very susceptible to chemical
breakdown by artificial saliva which
contains 90-95% water for up to 30
days of immersion, but a significant
decrease in shear bond strength was
noticed after exposure to 75% ethanol.
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Table I: Descriptive statistics for all main groups.
Times
Subgroup
Mean
SD
Min.
Max.
1day
DW
5th
6th
7th
25.4
28.67
28.0
0.821
1.661
0.55
23.9
24.4
27.1
26.8
30.5
28.8
1 Day
FSS
5th
6th
7th
24.85
28.78
27.96
0.61
0.522
0.576
23.9
27.9
27.1
26.1
29.8
28.9
30 day
DW
5th
6th
7th
25.79
28.33
27.72
2.969
0.953
0.607
23.2
26.9
27.1
32.9
30.1
29.1
30 day
FSS
5th
6th
7th
15.93
20.31
19.52
0.283
0.538
0.601
15.4
19.6
18.8
16.3
21.2
20.6
MDJ Storage effect on the bond strength of orthodontic metal… Vol.:5 No.:1 2008
60
Tables II: One-way ANOVA test.
Times
S.O.V.
S.S.
d.f.
M.S.
F-test
P-value
1 day
DW
Between
Within
Total
59.67
33.62
93.29
2
27
29
29.84
1.25
23.96
0.000
(H.S.)
1 day
FSS
Between
Within
Total
85.965
8.785
94.750
2
27
29
42.982
0.325
132.1
0.000
(H.S.)
30 days
DW
Between
Within
Total
35.16
90.81
125.97
2
27
29
17.58
3.36
5.23
0.012
(N.S.)
30 days
FSS
Between
Within
Total
108.989
6.586
115.575
2
27
29
54.494
0.244
223.41
0.000
(H.S.)
Tables III: Student t-test between generation groups within different storage time
intervals.
Storage time
Groups
Means
t-test
P-value
Sig.
1 day
DW
5th
6th
25.4
28.67
5.58
0.000
H.S.
5th
7th
25.4
28.0
8.32
0.000
H.S.
6th
7th
28.67
28.0
1.12
0.254
N.S.
1 day
FSS
5th
6th
24.85
28.78
15.48
0.000
H.S.
5th
7th
24.85
27.96
11.73
0.000
H.S.
6th
7th
28.78
27.96
3.34
0.004
S.
30 days
DW
5th
6th
25.79
28.33
2.58
0.028
S.
5th
7th
25.79
27.72
2.01
0.075
N.S.
6th
7th
28.33
27.72
1.71
0.108
N.S.
30 days
FSS
5th
6th
15.93
20.31
22.77
0.000
H.S.
5th
7th
15.93
19.52
17.08
0.000
H.S.
6th
7th
20.31
19.52
3.09
0.007
S.
MDJ Storage effect on the bond strength of orthodontic metal… Vol.:5 No.:1 2008
61
Tables IV: Student t-test for timing effects.
Groups
Subgroups
time
Mean
t-test
P-value
Sig.
DW
5th
1 day
30 days
25.4
25.79
0.4
0.697
N.S.
6th
1 day
30 days
28.67
28.33
0.56
0.583
N.S.
7th
1 day
30 days
28.0
27.72
1.08
0.295
N.S.
FSS
5th
1 day
30 days
24.85
15.93
41.97
0.000
H.S.
6th
1 day
30 days
28.78
20.31
35.7
0.000
H.S.
7th
1 day
30 days
27.96
19.52
32.05
0.000
H.S.
Table V: Student t-test for timing effects between the two main groups of three
generation subgroups at two time intervals.
Time
Subgroups
Storage
media
Mean
t-test
P-value
Sig.
1 day
5th
DW
FSS
25.4
24.85
1.7
0.108
N.S.
6th
DW
FSS
28.67
28.78
0.2
0.846
N.S.
7th
DW
FSS
28.0
27.96
0.16
0.876
N.S.
30 days
5th
DW
FSS
25.79
15.93
10.46
0.000
H.S.
6th
DW
FSS
28.33
20.31
23.16
0.000
H.S.
7th
DW
FSS
27.72
19.52
30.34
0.000
H.S.