Keywords zirconia - brackets - cementation - surface treatment
Introduction
The aesthetic demands of patients are increasing, and all-ceramic fixed partial dentures
(FPDs) meet their needs. Research has led to the development of zirconium oxide, or
zirconia (ZrO2 ), a material that presents many advantages such as enhanced aesthetic compared with
traditional metal-ceramic restorations, good chemical properties, dimensional stability,
and high mechanical strength. Moreover, it presents a Young's modulus of 210 GPa,
which is similar to that of stainless steel alloy.[1 ]
[2 ] Its high physical properties come from a phenomenon called “transformation toughening.”[1 ]
[2 ] According to Sailer et al,[3 ] if an all-ceramic FPD has to be placed in the posterior region, the use of zirconia
is recommended. However, when zirconia is used as a framework, these restorations
present a higher rate of veneering ceramic chipping compared with metal-ceramic one.
The reasons for these chippings are numerous, such as differences in coefficient of
thermal expansion between framework and porcelain, firing shrinkage of porcelain,
porosities, poor wetting of veneering, flaws on veneering, inadequate framework design
to support veneer porcelain, overloading, and fatigue.[4 ] One alternative to avoid these chipping is to use nonveneering or monolithic zirconia
restorations.[4 ]
[5 ]
Orthodontists are regularly faced with patients who present monolithic zirconia restorations.
But despite all its qualities,[6 ]
[7 ] bonding on zirconia represents a real challenge,[8 ]
[9 ] and as a result the bond failure rate is higher than that with enamel.[10 ] Practitioners seek to obtain a bond strong enough to reduce bracket detachment from
zirconia surfaces. Zirconia has no glass phase,[11 ]
[12 ] so surface preparation using hydrofluoric acid cannot be used to improve bond strength.[4 ]
[13 ] Besides, using this acid in the oral cavity can be dangerous for dental and soft
tissues.[8 ]
Laser treatments are constantly evolving, and are used in general practice, for certain
treatments in periodontics, as well as in dental surgery, surgery, and other fields.[14 ] If certain parameters are observed, laser treatment can be used to roughen the surface
of a zirconia restoration. For this reason, it has been recommended in some studies.[9 ]
[15 ] Methods such as airborne particle abrasion,[7 ]
[9 ]
[11 ]
[13 ]
[16 ]
[17 ]
[18 ]
[19 ] laser treatment,[7 ]
[9 ]
[10 ]
[11 ]
[15 ]
[16 ]
[17 ] or even silanization[17 ]
[18 ] have been investigated in previous studies.
Some studies have shown that ceramic brackets are recommended on surfaces like zirconia
ahead of metal brackets,[17 ] yet a recent study has claimed the opposite, finding that metal brackets seemed
to adhere more strongly to zirconia surfaces because of their better base surface
design and their method of retention.[19 ]
The aim and objective of this study are to evaluate the influence of orthodontic bracket
material (metallic or ceramic) and zirconia surface treatment (airborne particle abrasion
or laser-etching) on the shear bond strength of these brackets to surface treated
monolithic zirconia blocks. We also investigated the amount of residual cement on
the blocks after failure by means of the adhesive remnant index (ARI) using an electron
microscope and an optical microscope. We then observed the occurrence of adhesive
or cohesive failures to determine whether the bonds created between the interfaces
were stronger than the bonds within the materials themselves (or vice versa), as well
as the occurrence of restoration material fractures.
Materials and Methods
Samples Preparation
Forty blocks of polychromatic, super translucent monolithic zirconia (Ceramill Zolid
FX Multilayer; Amann Girrbach, Koblach, Austria), shade B2-B3, 10 mm in diameter and
10 mm in length, were prepared, and randomly divided into two groups (n = 20): metallic (Victory Series Low Profile Bracket, Univ L Anterior, 0.022, 3M)
and ceramic (Clarity Advanced Ceramic Brackets, Lower Anterior, Roth Rx, 0.022, 3M)
([Table 1 ]), and subsequently divided in subgroups: SMB (airborne particle abrasion/metallic
brackets), SCB (airborne particle abrasion/ceramic brackets), LMB (laser/metallic
brackets), and LCB (laser/ceramic brackets).
Table 1
Group names depending on zirconia preparation and type of bracket cemented to the
surface of the block
Name of the group
Surface treatment
Bracket type
SMB
Airborne particle abrasion
Metal
SCB
Airborne particle abrasion
Ceramic
LMB
Laser
Metal
LCB
Laser
Ceramic
Surface Treatment
For half of the samples (n = 20), the surfaces were prepared by airborne particle abrasion using 25 μm aluminum
oxide (Basic Master sandblasting unit; Renfert, Hilzingen, Germany) for 20 seconds
at 2.5 bar and a distance of 10 mm.[7 ]
For the other half (n = 20), the surfaces were covered with graphite powder (HB pencil) to increase its
energy absorption and then subjected to erbium-doped yttrium aluminum garnet (Er:YAG)
laser radiation (Fidelis Plus III; Fotona, Ljubljana, Slovenia). The laser was set
at a wavelength of 2940 nm, pulse duration of 50μs (SSP), power of 2 W, pulse repetition
rate of 10 Hz, and an energy density of 200 mJ. An R14 handpiece was used and equipped
with a sapphire tip of a diameter of 0.8 mm. The air/water spray ratio was set at
4/4. The sapphire tip was held perpendicular to the surface of the block at an approximate
distance of 0.5 mm. The surface of the zirconia block (78.54 mm2 ) was then subjected to radiation at a speed of around 2 mm/s for 10 seconds using
horizontal scanning.[15 ] All samples were rinsed using the air/water spray for 30 seconds.[12 ]
Bonding
The bonding steps were performed in accordance with the manufacturer's instructions.
A layer of silane (Clearfil Ceramic Primer Plus, Kuraray Noritake Dental Inc., Osaka,
Japan) was applied to the adherend surface of the zirconia block for 20 seconds using
an application brush, after which the entire adherend was suitably dried using a moderate,
oil-free air spray.
The primer (BrackFix Primer, Voco) was measured into a mixing palette and applied
to the surface of the conditioned zirconia surface in a thin, uniform film using a
microbrush. Since the primer is photopolymerizable, intense exposure to ambient light
was avoided and the intensity of the surgical lighting was reduced during application.
A sufficient quantity of bonding agent (BrackFix; Voco GmbH, Cuxhaven, Germany) was
applied to the base surface of each bracket. As soon as the bonding agent had been
applied, the bracket was lightly placed on the surface of the zirconia block, its
position adjusted, and it was then firmly pressed down. Excess adhesive around the
bracket base was delicately removed with a probe without moving the bracket. Lastly,
we performed photopolymerization using a curing light (Elipar DeepCure-S, 3M, St Paul,
MN, United States) with an intensity of 1470mW/cm2 (–10%/+20%): light oriented either on the interproximal surfaces of the metallic
brackets for 10s or perpendicularly to the ceramic brackets for 20 s.
The blocks were then placed in a cold-curing resin (Selacryl Cold powder pink + Selacryl
Cold liquid, Selexion) except for the surface where the bracket had been cemented.
Shear Strength Test
The shear strength test was conducted using a universal testing machine (Autograph
AGS-X; Shimadzu, Osaka, Japan). Shear stress was applied in a downwards direction
parallel to the surface of the zirconia block at a speed of 0.5 mm/m ([Fig. 1 ]).
Fig. 1 Universal testing machine (Autograph AGS-X, Shimadzu, 1000 N unit) and direction
of stress applied to the brackets.
The load applied was recorded in N and shear strength in MPa. An optical microscope
(magnification x20) (Leica; Wetzlar, Germany) was used to determine the ARI score
after failure.
ARI Score
The ARI score is represented by a scale with four levels, from 0 to 3:
0. No adhesive left on the surface
1. Less than half of the adhesive left on the surface
2. More than half of the adhesive left on the surface
3. All of the adhesive left on the surface after failure.
Type of Failure/Fracture
Using the optical (Leica) and electron (iT 300; Jeol, Akishima, Tokyo, Japan) microscopes,
we were able to check for adhesive, cohesive, and mixed failures as well as fractures
of the zirconia restoration material. The steps in the protocol were all performed
by the same operator.
Statistical Analysis
Statistical analysis was conducted using IBM SPSS Statistics, version 23 (Statistical
Package for Social Sciences; SPSS Inc., Chicago, IL, United States).
Results
The values obtained on bracket debonding during shear testing are described in [Table 2 ] ([Fig. 2 ]).
Table 2
Shear strength values (MPa) for the samples of the different groups
Name of the group
SMB
SCB
LMB
LCB
No. 1
24.53
19.29
24.43
14.32
No. 2
19.62
22.83
27.86
16.69
No. 3
19.27
15.13
21.34
18.73
No. 4
19.27
19.46
20.32
12.53
No. 5
17.14
17.29
24.31
25.43
No. 6
22.48
14.29
17.15
17.63
No. 7
23.34
27.89
24.51
14.31
No. 8
30.03
19.45
18.15
22.19
No. 9
19.00
23.21
23.25
15.70
No. 10
23.12
21.83
14.78
18.02
Fig. 2 Graph comparing the shear strength (MPa) of the samples depending on surface treatment
and type of bracket.
Shear Strength
Descriptive statistics, including the mean, standard deviation, median, and minimum
and maximum values for shear strength (MPa), were calculated. Because this test was
not significant (p > 0.05), we accepted the null hypothesis that the variances were equal. The variances
were therefore deemed to be similar.
Two-way analysis of variance (ANOVA) was conducted to determine whether there existed
a statistically significant difference between the groups. The differences were significant
between the metal (SMB, LMB) and ceramic (SCB, LCB) bracket groups with regard to
shear bond strength, with respectively 23.29 ± 5.34 MPa, 21.59 ± 4.03 MPa, 20.06 ±
4.05 MPa, and 17.55 3.88 MPa. The SMB group (airborne particle abrasion and metal
bracket) showed the highest shear bond strength values, whereas the lowest was that
of the LCB group (Er:YAG laser and ceramic bracket).
The ANOVA was significant (p < 0.05) between the different types of bracket. Conversely, the groups treated with
airborne particle abrasion or laser-etching did not display any statistically significant
differences in terms of their shear strength.
ARI Score
The frequency distribution and percentage of ARI scores of the four groups are shown
in [Table 3 ]. A sample of blocks and brackets analyzed and used to evaluate the ARI score is
shown in [Figs. 3 ]
[4 ].
Table 3
Frequency distribution and percentage of adhesive remnant index (ARI) scores of the
four groups (n = 10 for each group)
Name of the group
0
1
2
3
SMB
0
0
6 (60%)
4 (40%)
SCB
0
0
3 (30%)
7 (70%)
LMB
0
0
8 (80%)
2 (20%)
LCB
0
1 (10%)
3 (30%)
6 (60%)
Fig. 3 Bracket base of each group (SMB, SCB, LMB, and LCB) and corresponding surface after
debonding on electron microscopy (x20). (A ) Sample from SMB group. (B ) Sample from SCB group. (C ) Sample from LMB group. (D ) Sample from LCB group.
Fig. 4 Bracket base of each group (SMB, SCB, LMB, and LCB) and corresponding surface after
debonding on optical microscopy (x20). (A ) Sample from SMB group. (B ) Sample from SCB group. (C ) sample from LMB group. (D ) Sample from LCB group.
Statistical analysis in the form of the Kruskal–Wallis test was conducted on the variable
“ARI score” and was not significant (p > 0.05; data not shown). Thus, there was no statistically significant difference
between the groups.
The LMB group displayed the lowest mean ARI score, followed by the SMB, LCB, and SCB
groups. The groups with ceramic brackets had higher mean ARI scores than those with
metal brackets. However, after analysis, there was no statistically significant difference
between these groups.
Types of Failures and Fractures
Examination of the samples after bracket debonding revealed mixed fractures and adhesive
fractures between the bracket and the resin. No cohesive fracture within the zirconia
or the brackets was observed.
Discussion
The objective of this study is to evaluate the shear bond strength of metal and ceramic
orthodontic brackets bonded to monolithic zirconia blocks, with their surface treated
with two different treatments (Er:YAG laser treatment, and airborne particle abrasion
using 25μm aluminum oxide). The aims are to evaluate the influence of brackets type
and influence of zirconia surface treatment on the shear bond strength of orthodontic
brackets to zirconia surfaces.
It is difficult to achieve a long-term bond to ceramic surfaces. The findings of previous
studies confirm that applying hydrofluoric acid to zirconia does not result in effective
retention.[4 ]
[13 ] Other method of surface treatment had to be applied. To date, several studies have
demonstrated the benefit of airborne particle abrasion using aluminum oxide (Al2 O3 ),[7 ]
[11 ]
[13 ]
[16 ]
[17 ]
[20 ] since abrasion with particles of aluminum oxide does indeed roughen the surface
of restorations. However, we cannot compare the results of this study with those of
previously studies, since the grain size chosen are different.
Laser is employed in multiple fields of dentistry,[14 ] including periodontics, dental surgery, and minor surgery. Several studies have
used laser treatment to prepare a surface for the bonding of a bracket. For zirconia
restorations, it has been demonstrated that Er:YAG laser treatment is recommended
ahead of others such as Nd:YAG and CO2 , which create micro-cracks.[9 ]
[10 ] If the parameters discussed in the material and method section are adhered to, Er:YAG
can be used to roughen the surface of zirconia without altering its structure.
We did not observe any statistically significant difference in this study between
the samples based on the surface treatment they were subjected to, although the highest
mean shear strength was achieved by the airborne particle abrasion group.
Some studies have shown that ceramic brackets are recommended for surfaces like zirconia
ahead of metal brackets,[17 ] yet a recent study has claimed the opposite, finding that metal brackets seemed
to adhere more strongly to zirconia surfaces because of their better base surface
design and their method of retention.[19 ]
In this study, the samples that were given metal brackets displayed greater shear
strength, and a statistically significant difference was indeed found between the
metal bracket group and ceramic bracket group. Our finding matches that of the study
by Mehmeti et al[19 ] but contradicts the article by García-Sanz et al[17 ] This may be due to the design of the metal bracket's base surface, that is, to an
uneven surface that creates better mechanical retention compared with ceramic brackets.
It would seem that the brackets' mechanical bond to zirconia is stronger than their
chemical bond.
Hobson et al[21 ] defined the lowest acceptable shear strength for routine clinical use as being no
less than 5.9 to 7.5 MPa. The values obtained in this study exceed this objective.
Ceramill Zolid monolithic zirconia has a bending strength of 700 ± 150 MPa and a Young's
modulus ≥ 200 GPa. When debonding the bracket, the risk of causing fractures is therefore
small. However, care must be taken with the adhesion between the prosthetic restoration
and the tooth, since the strength of this bond depends on several factors, including
the type of cementation used. Thus, the right balance must be found to avoid debonding
the crown attached to the bracket. The studies have been vague on this issue and have
not given any exact maximum strength limit for zirconia crowns.[13 ]
When debonding a bracket, it is important not to alter the structure of the enamel
while ensuring that the residual adhesive on the surface of the tooth is minimal.
This applies to ceramic materials as well. Our objective during debonding is to minimize
cohesive damage to the zirconia and leave as little bonding agent as possible on the
surface of the restoration. That is why a low ARI score is desirable, as well as avoiding
any cohesive failure within the zirconia.[22 ]
The LMB group displayed the lowest mean ARI score, followed by the SMB, LCB, and SCB
groups. The groups with ceramic brackets had higher mean ARI scores than those with
metal brackets. However, after analysis, there was no statistically significant difference
between these groups.
There are two categories of failure when two indirectly bonded materials come apart:
A cohesive failure occurs within the bonding agent, the bracket, or the zirconia and
indicates that the bond in the interface is stronger than that within the material.
An adhesive failure occurs at the bracket/cement or zirconia/cement interface and
it indicates that the bond is weaker at the interface between the cement and the material
(the zirconia or bracket).
In this study, we found no pure adhesive failures or cohesive failures within the
brackets or zirconia blocks, only mixed and cohesive failures within the resin. None
of these failures damaged the zirconia or the brackets.
Conclusion
Within the limits of this study, we can conclude that metal brackets have a greater
bond strength than ceramic brackets when cemented to zirconia. No statistically significant
difference in shear strength was uncovered between the surface treatments.
As regards ARI score, the sample groups did not appear to have any statistically significant
differences between them.
Authors’ Contributions
Sibel Cetik: Data analysis, manuscript preparation/editing/review.
Thaï Hoang Ha: Literature search, English translation, manuscript review.
Léa Sitri: Experimental studies
Hadrien Duterme: Data acquisition and analysis.
Viet Pham: Date analysis and literature search
Ramin Atash: Concept, design, protocol.
All authors approved the final version of the article.
