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SCIENCE
Shear Bond Strength of Orthodontic
Brackets Bonded to Different Preparations
of a Porcelain Surface
Tasanee Tengrungsuna, Saksamut Prombutrab, Chatri Kaewsuriyathamrongc,
Wanna Suchatod, Poompada Jaochakarasirie
a
b
c
d
e
Assistant Professor, Department of Hospital Dentistry, Faculty of Dentistry, Mahidol University,
Bangkok,Thailand.
Dental Government Officer, Dental Division, Naval Medical Center, Royal Thai Navy, Bangkok, Thailand.
Orthodontist in Private Practice, 33 Dental Clinic, Bangkok, Thailand.
Associate Professor, Department of Orthodontics, Faculty of Dentistry, Mahidol University, Bangkok,
Thailand.
Assistant Professor, Department of Hospital Dentistry, Faculty of Dentistry, Mahidol University,
Bangkok, Thailand.
Purpose: To evaluate the effect of various porcelain surface preparations on the shear bond strength of orthodontic brackets bonded to feldspathic metal ceramic porcelains and to observe the porcelain surfaces after each
preparation.
Materials and Methods: Sixty-four porcelain-fused-to-nonprecious-metal specimens were divided into four
groups as follows. Group 1: glazed porcelain without surface preparation as a control group; group 2: porcelain
surface treated with intraoral sandblasting; group 3: porcelain surface treated with Nd:YAG laser etching; group
4: porcelain surface etched with 9.5% hydrofluoric acid. One representative specimen from each group was observed under SEM after surface preparation. Silane coupling agent and System 1+ adhesive resin were applied to
the surface-treated porcelain specimens. All specimens were stored in distilled water at 37°C for 24 h. Shear
bond strength was tested with an Instron testing machine.
Results: The mean shear bond strength achieved on surface-treated porcelain specimens was significantly greater
than in the control group (p < 0.05). No significant difference was observed among the mean shear bond
strengths of three surface-treated groups. SEM demonstrated that all three surface preparations created micromechanical retention.
Conclusion: This study indicates that preparing porcelain surfaces by the methods described here increases the
shear bond strength of orthodontic brackets bonded to porcelain surfaces.
Key words: porcelain surface preparation, shear bond strength, orthodontic bracket, Nd:YAG laser, sandblasting,
hydrofluoric acid etching.
J Oral Laser Applications 2004; 4: 47-53..
Submitted for publication: 28.08.03; accepted for publication: 16.12.03.
considerable number of orthodontic patients are
adults1-3 who have crown and bridge restorations
fabricated from porcelain; therefore, orthodontists
must develop a method that permits excellent bond
strength of orthodontic attachment to porcelain. Appli-
A
Vol 4, No 1, 2004
cation of a silane coupling agent is one of the methods
used to enhance the bond strength to either glazed or
roughened dental porcelains,4 and is now commonly
used in clinical orthodontic practice.3,5 However, the
bond strength achieved is still low,6 and long-term suc-
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SCIENCE
cess of this material is doubtful.7 Moreover, an in vitro
study about the effect of long-term water storage on
silane-treated resin/porcelain bonds revealed that the
bond strength is substantially lower after 6 months.8
The effect of surface preparation of porcelain on
bond strength has been examined by many researchers. 1,5,9-13 Smith et al 1 evaluated shear bond
strengths of orthodontic composite adhesives to treated porcelain surfaces compared with those to acidetched enamel. They found that roughening the
porcelain surface with silane treatment provided clinically acceptable bond strengths (8.1 to 8.6 MPa).
Phoenix and Shen14 stated that surface treatments may
enhance the wettability of porcelain surfaces and/or
create microporosity that facilitates mechanical interlocking with low-viscosity resin.
Currently, air-abrasion technology has been examined as a potential application in dentistry. 15-17 Researchers are now studying the use of air abrasive
systems for mechanical etching, or modifying the surface of enamel, dentin, and restorative materials such
as gold and porcelain.13,16-19 Zachrisson13 investigated
the effect of variously treated porcelain surfaces on the
tensile strength of orthodontic brackets bonded to
feldspathic metal ceramic porcelain. The finding of this
study demonstrated that sandblasted porcelain surfaces also treated with silane significantly increased the
bond strength.
In recent years, advances in laser technology have
greatly decreased its undesirable effects and made successful application of lasers in dentistry possible. In the
past decade, there has been an innovation in utilizing
the Nd:YAG laser to prepare enamel surfaces for direct bonding of orthodontic attachments.20-22 In addition, it has been used for roughening the porcelain
surface before luting with a hybrid resin cement. 23
The purpose of this study is to evaluate shear bond
strengths between feldspathic porcelain and meshbacked orthodontic brackets after three different
methods of preparing porcelain surfaces: intraoral
sandblaster with 50-µm aluminum oxide particles,
pulsed Nd:YAG laser etching, and 1 min of hydrofluoric acid etching. The surface morphology after preparation is examined with SEM.
MATERIALS AND METHODS
Specimen Preparation
Sixty-four porcelain-fused-to-metal specimens were fabricated with nonprecious metal. The square-shaped
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substructure cast with cross-shaped retentive projections for attachment of the acrylic resin block to
mounting rings were fabricated by one dental technician. Prior to testing, they were stereomicroscopically
inspected to rule out samples with surface defects or
cracks. They were then stored in a clean, dry environment.
Surface Preparation of Porcelain
All specimens were rinsed with distilled water and ultrasonically treated in 70% ethanol for 5 min to eliminate potential organic deposits. Then they were
polished and cleaned with pumice slurry, and washed
and dried with oil-free compressed air. All specimens
were randomly divided into four groups.
Group 1: Glazed porcelain with no surface preparation (control group)
Group 2: Intraoral sandblasting. Each specimen in
this group was air abraded with aluminum oxide particles, grain size 50 µm (KCP 2000, American Dental
Technologies, Troy, MI, USA) for 10 s. An area of approximately 20 mm2 of the porcelain surface at the
center of each specimen was subjected to the intraoral
sandblaster until the selected porcelain surface appeared uniformly frosted. The pressure used was
689.5 KPa and the distance from the nozzle to porcelain surface was 10 mm.
Group 3: Nd:YAG laser. Each specimen in this group
was treated with pulsed Nd:YAG laser (Twinlight Dental Laser Unit, Fotona, Ljubljana, Slovenia). The parameters were 150 mJ/pulse, 20 pps, with surface
energy density of 375 J/cm2 for 60 s as described by
Hess.23 Prior to Nd:YAG laser irradiation, a thin coat
of the laser initiator material (black drawing ink,
Camel, Bombay, India) was applied to the porcelain
surface with a nylon brush over an area of approximately 20 mm2 and let stand for 1 min. Each specimen
was exposed to the laser until the laser initiator material had completely disappeared (within 60 s) with a
constant rate of 1.5 mm/s.
Group 4: Hydrofluoric acid treatment. Each specimen in this group was treated with 9.5% buffered hydrof luoric acid (Ultradent porcelain etch, South
Jordan, Utah, USA) for 1 min according to the manufacturer’s instruction. The porcelain surface at the center of each specimen was coated with hydrofluoric acid
using an Inspiral brush tip (Ultradent, South Jordan,
UT, USA) over an area of approximately 20 mm2, then
rinsed with clean water for 30 s and dried with oil-free
compressed air.
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SEM Examination
One specimen was randomly selected from each group
to be examined in terms of surface texture and surface
characteristics. Each specimen was ultrasonically cleaned in distilled water, dried in a desiccator for 12 h,
sputter coated with gold (sputter coater, SPI-module,
West Chester, PA, USA), and the surface topography
was examined with a scanning electron microscope
(JEOL, Model JSM-5410LV, Tokyo, Japan) at an accelerating voltage of 20 KV.
Silanization of the Specimens
All specimens in each group were cleaned with distilled
water in an ultrasonic cleaner (Tru-Sweep, model-275
D, Crest Ultrasonics, NJ, USA) for 5 min and dried
with oil-free compressed air for 10 s. Each specimen
was treated with 37% phosphoric acid for 90 s, then
Ormco porcelain primer (Ormco, Glendora, CA, USA)
was applied. The specimens were left in situ for 60 s
and the porcelain primer was reapplied for another 60
s. The specimen was then cleaned with water for 10 s
and dried with oil-free compressed air for 10 s.
Direct Bonding Procedure
A thin film of System 1+ bonding resin (Ormco) was
applied to the base of the stainless steel standard
mesh-backed orthodontic bracket (Minidiamond, Ormco). Each bracket was placed at the center of the
prepared porcelain specimen. To standardize the placement of each bracket, the samples were arranged on
the surface of a surveyor table and the squared end of
a chisel-shaped surveying rod was lowered with a seating pressure of 101.43 grams for direct bonding in a
perpendicular direction for 60 s to make the thin film
of adhesive resin. Excess resin was removed with an
explorer. The specimens were let stand for 30 min before storage in distilled water at 37°C for 24 h in an incubator (Memmert model 400, Schwabach, Germany).
Shear Bond Strength Testing
until bond failure occurred. The shear force values at
bond failure were recorded in kilonewtons (KN) and
converted to megapascals (MPa) .
Statistical Analysis
The SPSS program for Windows version 7.5 was used
to calculate means and standard deviations of 24-h
shear bond strengths for all four tested groups. The
homogeneity of variances was tested with the Levene
statistic. The data were then analyzed using one-way
analysis of variance (ANOVA) for overall significance.
The Scheffe multiple comparison test was used to identify which groups were significantly different at the
95% level of confidence.
RESULTS
The effects of the three different porcelain surface
preparations on the shear bond strength between
mesh-backed orthodontic brackets and feldspathic
porcelain using System 1+ as an adhesive are demonstrated in Fig 1. The highest mean shear bond strength
was observed in group 4, and the bond strength values
decreased from group 2 to group 3 to group 1. The
sites of bond failure were observed under a stereomicroscope at 10X magnification, showing that bond failure was mainly cohesive within adhesive material
(Table 1).
Comparison of Shear Bond Strengths
The Levene test showed no statistically significant difference among variances of 24-h shear bond strength
of all four tested groups (p > 0.05). One-way ANOVA
showed a significant difference between the four
groups (p < 0.05).
The Scheffe multiple comparison test, as shown in
Table 2, revealed that there were statistically significant
differences in the mean shear bond strength between
the glazed porcelain (control) and the surface preparations. However, there was no statistically significant difference between the intraoral sandblasting, Nd:YAG
laser, and hydrofluoric acid etching groups.
Each specimen was mounted on a universal testing machine (Model 5566, Instron, Buckinghamshire, England). The shear load was applied with a monobevel
chisel blade parallel to the porcelain/adhesive-resin interface. A crosshead speed of 0.5 mm/min was used
Vol 4, No 1, 2004
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Table 1 Frequency distribution of bond failure site
Surface Preparation
No surface preparation
Intraoral sandblasting
Nd:YAG laser etching
HF acid etching
Site of bond failure
Within porcelain
At adhesive/
porcelain interface*
Within adhesive**
At bracket/
adhesive interface***
At bracket
–
–
–
–
4 (26.7%)
5 (33.3%)
–
8 (53.3%)
9 (60.0%)
5 (33.3%)
4 (26.7%)
3 (20.0%)
6 (40.0%)
5 (33.3%)
11 (73.3%)
–
–
–
–
* 0% to 25% of the adhesive left on the porcelain.
** 25% to 75% of the adhesive left on the porcelain.
***75% to 100% of the adhesive left on the porcelain.
Fig 1 Bar graph shows the mean and standard deviation of 24-h shear bond strengths.
Table 2 The comparison of 24-h shear bond strengths: results of the Scheffe multiple comparison test
Surface preparation
No surface treatment
Intraoral sandblasting
Nd:YAG laser etching
Hydrofluoric acid etching
No surface treatment
–
0.00**
0.003**
0.00**
Intraoral sandblasting
–
–
0.768
0.650
Nd:YAG laser etching
–
–
–
0.150
** α=0.01
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Fig 2 The scanning electron micrograph of glazed porcelain with
no surface preparation.
Fig 3 The scanning electron micrograph of a porcelain surface
sandblasted with 50-µm aluminum oxide particles.
Fig 4 The scanning electron micrograph of Nd:YAG laser- etched
porcelain.
Fig 5 The scanning electron micrograph of a porcelain surface
etched for 1 min with hydrofluoric acid.
SEM Examination
dissolution of the continuous glass matrix. The irregularly shaped crystalline particles of porcelain appeared
at the porcelain surface (Fig 5).
In group 1 (control), the SE micrograph of the glazed
porcelain specimen with no surface preparation was
extremely smooth, displaying no grooves or fissures
(Fig 2). In group 2, the SE micrograph of the porcelain
specimen after sandblasting was intricate and complex,
with an angular appearance (Fig 3). In group 3, the SE
micrograph of the Nd:YAG laser-prepared specimen indicated that the laser energy altered the smooth surface of feldspathic porcelain, creating homogeneous
craters and pores with microcracks all over the surface
(Fig 4). The SE micrograph of the group 4 specimen,
etched for 1 min with hydrofluoric acid, showed that
there was a complex, homogeneous pattern of numerous small channels all over the surface, resulting from
Vol 4, No 1, 2004
DISCUSSION
The results of this in vitro study indicated that surface
preparation significantly increased the shear bond
strength compared with the control group. This finding
is in agreement with the previous studies by Smith et
al 1 and Kao et al, 11 who found that roughening the
porcelain surface together with the application of
porcelain primers significantly increased bond strength.
According to Cochran et al,5 shear bond strengths between 7 and 10 MPa or higher, with no porcelain frac-
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ture on debonding, would be clinically acceptable. The
use of a silane coupling agent alone as the control
group in this study still achieved the minimum bond required. However, Thurmond et al24 proved that the
use of the chemical agent without any physical alteration of the porcelain surface would not yield high
bond strengths in long-term water storage.
Sandblasting followed by silanization produced the
high shear bond strength of 12.41 MPa. This resulted
from the additive effect of both mechanical and chemical adhesion. Sandblasting is considered to create mechanical retention.25,26 Wolf et al27and Zachrisson et
al13 also showed a similar result; silane application to
the sandblasted porcelain surface significantly increased
the bond strength.
In the Nd:YAG laser-etched plus silane group, a significantly greater mean shear bond strength (11.71
MPa) was found than in the control group (9.08 MPa).
The high bond strengths resulted from the additive effect of mechanical bonding from laser treatment and
chemical bonding from the silane coupling agent. The
use of Nd:YAG etching of porcelain required a laser-initiating material. The application of a laser-initiating material made it possible to create the specific laseretched areas of porcelain surface. The uncoated surface of the porcelain remained unchanged when the
beam was passed beyond the edge of the laser-initiating
material. It has been shown that Nd:YAG laser works
on a dark/light basis: the darker the material, the better the absorption.22 Thus, the use of laser-initiating
material in this study enhanced the laser absorption
into the porcelain surface.
The 9.5% hydrofluoric acid-etched group with silane
yielded the highest mean shear bond strength (13.25
MPa), which was a result of the combination of mechanical and chemical bonding mechanisms. It was suggested that hydrofluoric acid etching would alter the
feldspathic porcelain surface by creating microretention
with undercuts. The results of this study were in agreement with Thurmond et al 24 that 24-h shear bond
strengths of the silanized, hydrofluoric acid-etched
group were greater than the silanized, sandblasted
group. However, their results were not statistically significantly different.
The differences in shear bond strength can be explained by the differences in microscopic surface texture among all tested groups. The glazed porcelain
without surface preparation had no mechanical adhesion because the glazed surface did not permit resin
tag formation. Therefore, the specimen in this group
yielded the lowest shear bond strength. Sandblasted
porcelain, which exhibited the roughest surface, slightly
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improved the shear bond strength compared to the
controls. This may be due to the fact that despite the
roughness from air abrasion with aluminum oxide particles, the method could not create microscopic undercuts sufficient for greatly improving bond strength. This
finding was in agreement with Wood et al’s study,28
which stated that sandblasting creates surface irregularities without undercuts.
Hydrofluoric acid etching of feldspathic porcelain
enabled an adequate resinous bond to be created to
these materials.27,29 It was proved to be an effective
procedure for enhancing and retaining bond integrity.30 This result was in agreement with the findings
of other studies.31-33 They found that acid etching of
porcelain yielded bond strengths that did not change as
a function of water storage, unlike the use of silane
alone.24 These results suggested that surface topography was crucial in enhancing the bond strength. Surface roughening method and storage conditions
interact strongly, and etched samples retained their
strength after water storage.30 However, the intraoral
use of hydrofluoric acid poses certain dangers, and
must be seen critically.34,35
The findings of this in vitro study have produced a
database for the direct bonding technique of orthodontic attachments to porcelain surfaces. Further clinical
research should be conducted to assess the bond integrity in an oral environment and to determine
whether each type of surface preparation would be clinically successful in direct bonding.
CONCLUSION
Different approaches to bonding orthodontic brackets
to a porcelain surface have been suggested. Both mechanical and chemical adhesion are considered to improve the bond strength. This study clearly indicates
that different methods of porcelain surface preparation
can produce micromechanical retention and increase
the shear bond strength of orthodontic brackets to
porcelain surfaces.
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Contact address: Tasanee Tengrungsun, Department of Hospital Dentistry, Faculty of Dentistry, Mahidol University, Yothi
Road, Bangkok 10400, Thailand. Tel: +662-6448644-6 Ext.
1521 or 107, Fax: +662-2466910. e-mail address: [email protected]
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