Keywords
resin composite - shear bond strength - universal adhesive - zirconia
Introduction
In restorative dentistry and prosthodontics, zirconia has been widely used, particularly
for inlays, onlays, crowns, bridges, and implant materials. Because of its biocompatibility,
strength, and esthetics in dentistry, zirconia has increased in popularity.[1] Despite their high-success rate, the most common cause of zirconia restoration failure
is chipping or fracture of the zirconia.[2] Resin composite repair methods reduce chair time and costs for patients.[3]
[4]
[5] Furthermore, intraoral repair has been suggested as a viable treatment alternative
option if the indication and treatment procedures are proper. In cases of chipping
or fracture of zirconia, the adhesion between resin composite and zirconia would influence
the prognosis of the intraoral repair. The zirconia surface modification, including
surface cleaning for micromechanical retention and surface modification for chemical
adhesion on the zirconia, should be performed to improve the bond strength between
resin composite and zirconia.[3]
Universal adhesives have become a new trend in adhesive and restorative dentistry.
It is further stated that universal adhesives can be used not only to bond to enamel
and dentin but as adhesive primers on materials such as zirconia, silica-based ceramics,
noble metals, base metals, and resin composites. The universal adhesives use phosphate
and/or carboxylate monomers as their primary adhesive functional monomer. Acidic functional
monomers are commonly utilized, including carboxylate monomers such as 4-methacryloyloxyethyl
trimellitate anhydride (4-META) and phosphate monomers such as 10-methacryloyloxydecyl
dihydrogen phosphate (10-MDP) or glycerol phosphate dimethacrylate (GPDM). These monomers
have many positive attributes, including the potential to bond chemically to zirconia,[6] metals,[7] and tooth structures via the formation of nonsoluble calcium salts.[8]
[9]
Thermocycling is a technique for simulating the restorations' artificial aging process.
This technique gives information on the bonding resin and zirconia adhesive failure
due to the dissimilar coefficients of thermal expansion between the adhesive and zirconia.
The aging process has been demonstrated using a variety of thermocycling regimens,
however, Gale et al have described that a cyclic procedure of 10,000 cycles per year
may be enough to indicate restorative adhesive failure.[10]
This laboratory study aimed to compare shear bond strength (SBS) of four universal
adhesives G-premio bond universal (GPU), Clearfil Tri-S bond universal (CTB), Optibond
Universal (OBU), Tetric N-bond universal (TNU) on the resin composite and zirconia
interface both before and after thermocycling with standard control Clearfil ceramic
primer plus (CCP) + single bond 2 (SB2) and negative control (no chemically surface
modification).
Materials and Methods
This was a randomized control group study using 120 fully sintered zirconia disc specimens
(VITA YZ HT, VITA Zahnfabrik, Germany) 6.0 mm in diameter and 4.0 mm in thickness.
At first, the specimens were embedded in polyvinyl chloride pipe with acrylic. The
specimen's surfaces were polished with 600 grit silicon carbide abrasive paper (3M
Wetordry Abrasive Sheet, 3M, MN, USA). The specimens were sandblasted with 50 µm aluminum
oxide (Rocatec, 3M ESPE, St. Paul, MN, USA) perpendicularly in the zirconia surface
(2.5 bars, 10 mm in distance, for 10 seconds) to create micromechanical retention.
All specimens were then ultrasonically cleaned (Ultrasonic cleaner VI, Yoshida dental
trade distribution Co., Tokyo, Japan) for 10 minutes in distilled water and then dried
with oil-free air for 10 seconds from a triple syringe. The specimens were randomly
divided into two groups (water storage for 24 hours and 10,000 cycles of thermocycling),
and each group was divided into six subgroups (n = 10) according to zirconia surface treatments: no Tx, CCP + SB2, GPU, CTB, OBU,
and TNU. The predictor variable was adhesives/primer, which was a nominal scale (GPU,
CTB, OBU, TNU, CCP, and SB2). The specimens treated by CCP + SB2, a standard adhesive
for zirconia/resin composite repair, as standard controls, while negative controls
were no chemically zirconia surface modification. [Table 1] showed the types, brand names, manufacturers' details, lot numbers, and chemical
composition of the adhesives/primers used in this study.
Table 1
Materials used in this study
|
Material
|
Composition
|
|
Clearfil ceramic primer plus (Kuraray Noritake Dental Inc., Okayama, Japan)
Lot: 410043
|
10-MDP, ethanol, 3-trimethoxysilylpropyl methacrylate
|
|
Clearfil Tri-S bond universal (Kuraray Noritake Dental Inc., Okayama, Japan)
Lot: 4K0025
|
10-MDP, Bis-GMA, HEMA, colloidal silica, ethanol, silane, sodium fluoride, camphoquinone,
ethanol, water
|
|
Optibond universal (Kerr Corporation, California, USA)
Lot: 6920782
|
GPDM, GDM, HEMA, dimethacrylate, acetone, ethanol
|
|
G-premio bond universal (GC Corporation, Tokyo, Japan)
Lot: 1611221
|
10-MDP, 4-MET, HEMA, dimethacrylate, ethanol, acetone
|
|
Tetric N-bond universal (Ivoclar vivadent, AG, FL-9494 Schaan, Liechtenstein)
Lot: X43844
|
10-MDP, 2-hydroxyethyl methacrylate, Bis-GMA, ethanol, 1, 10-decandiol dimethacrylate,
camphorquinone, 2-dimethylaminoethyl methacrylate
|
|
Single bond 2 (3M ESPE, St. Paul, Minnesota, USA)
Lot: N378816
|
Bis-GMA, HEMA, DMA, methacrylate functional copolymer, filler, photoinitiators, ethanol,
water
|
Abbreviations: 10-MDP, methacryloyloxydecyl dihydrogen phosphate; 4-MET, 4-methacryloxyethyl
trimellitic acid; Bis-GMA, bisphenol A-glycidyl methacrylate; DMA, dimethacrylate;
GDM, 1,3-glycerol dimethacrylate; GPDM, glycerol phosphate dimethacrylate; HEMA, 2-hydroxyethyl
methacrylate.
The samples were randomized into 12 subgroups (n = 10). All of them, except standard and negative control, were treated with one adhesive
according to their group using a microbrush, dried 10 seconds with oil-free air from
a triple syringe, and then light-cured for 20 seconds (Elipar FreeLight2 LED Curing
Light, 3M ESPE, MN, USA). The standard control samples were first conditioned with
CCP, dried 10 seconds with oil-free air from a triple syringe, subsequently treated
with SB2, dried again for 10 seconds in the same method, and finally light-cured for
20 seconds. The negative control samples were no chemically zirconia surface modification.
The Ultradent mold (Ultradent product, Inc., South Jordan, USA) 2.0 mm in diameter,
and 2.0 mm in thickness was located on the zirconia pretreated surfaces to help to
display the bonding region, filled with resin composite (Harmonize A4D shade, Kerr
Corporation, California, USA), and then light-cured 40 seconds.
The bonded samples were stored for 30 minutes at room temperature and then were kept
in distilled water at 37 °C in an incubator (Incubator, Humanlab instrument Co., Suwon,
Korea) for 24 hours; one half was then tested (24 hours) and the other half was thermocycled
(Proto-tech, Micoforce, Portland, OR, USA) for 10,000 cycles with different temperatures
of 5 °C and 55 °C, with a reside time of 30 seconds in each bath and 5 seconds of
a transfer time.
A universal testing equipment (AGS-X 500N, Shimadzu Corporation, Kyoto, Japan) was
used to measure the SBS with an external load tested in the direction parallel to
the zirconia/resin composite interface at 0.5 mm/min of crosshead speed. The SBS was
determined by dividing the force at which bond failure appeared at the zirconia/resin
composite interface.
Under a stereomicroscope (ML9300, Meiji Techno Co. Ltd., Saitama, Japan) with a magnification
of x40, the fracture pattern was identified to quantify the postloading failure mode
percentage at the fractured zirconia/resin composite interface. The fracture mechanism
was classified into three different types: adhesive failure at the zirconia/resin
composite interface, cohesive failure within the zirconia or resin composite substance,
and mixed failure was a result of a mix of the two.
During the study period, a collecting form was used to collect data and recorded into
a statistical database (SPSS, SPSS Inc., Chicago, IL, USA) for analysis. The statistical
analysis results, using one-way ANOVA and Tukey's test, were computed as appropriate.
p < 0.05 was used to determine statistical significance in all of the analyses.
Results
The SBS values of all test groups are presented in [Table 2]. After water storage for 24 hours, the SBS values of the specimens ranging from
high to low were as follows: GPU > CCP + SB2 = CTB = OBU = TNU > no Tx (p < 0.05). After thermocycling for 10,000 cycles, the SBS values were: CCP + SB2 = GPU = CTB = TNU > OBU > no
Tx (p < 0.05). Compared with water storage for 24 hours, the SBS values of the GPU, OBU
and no Tx groups were lower after thermocycling for 10,000 cycles, but the SBS values
of CCP + SB2, CTB, and TNU groups did not change significantly (p > 0.05).
Table 2
The mean SBS values of samples (X [SD], n = 10)
|
Groups
|
SBS (MPa)
|
|
24 hours
|
10,000 TC
|
|
1. No Tx
|
7.27 (1.72) Aa
|
3.29 (1.17) Ab
|
|
2. CCP + SB2
|
19.15 (1.87) Ba
|
18.48 (1.69) Ba
|
|
3. GPU
|
25.77 (2.12) Ca
|
17.93 (1.35) Bb
|
|
4. CTB
|
19.37 (1.74) Ba
|
18.88 (1.41) Ba
|
|
5. OBU
|
19.62 (1.22) Ba
|
12.75 (0.89) Cb
|
|
6. TNU
|
18.62 (1.21) Ba
|
18.38 (1.28) Ba
|
Abbreviations: MPa, megapascal; SBS, shear bond strength; SD, standard deviation;
TC, thermocycled.
Note: The superscripted letters indicate significant differences within the same column
(p < 0.05), the different lowercase letters indicate significant differences within
the same line (p < 0.05).
At the fractured zirconia/resin composite interface, the failure mode was detected
using a stereomicroscope ([Table 3]), which showed that the adhesive failure occurred in the no Tx group both of 24 hours
and 10,000 cycles. Groups of CCB + SB2, GPU, CTB, and TNU exhibited predominantly
mixed failures both before and after 10,000 cycles of thermocycling. OBU showed primarily
mixed failures of 24 hours water storage, but adhesive failures increased after 10,000
cycles of thermocycling.
Table 3
Percentage of failure modes
|
Groups
|
Adhesive
|
Mixed
|
Cohesive
|
|
1. No Tx
|
24 hours
|
100
|
0
|
0
|
|
10,000 TC
|
100
|
0
|
0
|
|
2. CCP + SB2
|
24 hours
|
30
|
70
|
0
|
|
10,000 TC
|
30
|
70
|
0
|
|
3. GPU
|
24 hours
10,000 TC
|
10
40
|
90
60
|
0
0
|
|
4. CTB
|
24 hours
10,000 TC
|
30
40
|
70
60
|
0
0
|
|
5. OBU
|
24 hours
10,000 TC
|
30
70
|
70
30
|
0
0
|
|
6. TNU
|
24 hours
10,000 TC
|
40
40
|
60
60
|
0
0
|
Abbreviation: TC, thermocycling.
Discussion
Zirconia is chemically inert. Surface modification with zirconia is necessary to achieve
micromechanical retention and/or chemical bonding.[3]
[11]
[12] To achieve a durable zirconia/resin composite connection, the zirconia cleaning
surface before the intraoral repair is important. The simplest method is air abrasion
using alumina oxide particles, which is the most effective method for producing micromechanical
retention of zirconia, as it improves the bond strength to zirconia.[13]
[14] It can also create a rough texture for zirconia surface, increase the zirconia surface
for mechanical and chemical retention, and improve wettability. The air-abrasion should
be conducted using 30 to 50 µm alumina oxide particles at 2.5 bars pressure in circular
motion at a distance of 10 mm perpendicular to the zirconia surface for 10 to 20 seconds.[15]
[16]
Phosphate and/or carboxylic monomers are included in commercially marketed zirconia
surface modification agents. The commonly-used adhesives/primers include phosphate
monomers, 10-MDP or GPDM, and carboxylic monomer, 4-META. These monomers are acidic
bifunctional monomers with two functions, with the hydrophilic portion being the phosphate/carboxylic
group and the hydrophobic portion being the vinyl group. Chemical bonds can be formed
between the phosphate/carboxylic group and oxide layer of zirconia. The vinyl group
can copolymerize with the resin monomer of the resin-based materials. Commercially
manufactured universal adhesives and metal/zirconia primers contained acidic functional
monomers that were efficient in improving the adhesion capacity of resin-based materials
to zirconia.[17]
[18]
In this research, we used one zirconia primer (CCP) and four universal adhesives (GPU,
CTB, OBU, TNU) containing phosphate and/or carboxylic monomers. The GPU has 10-MDP
and 4-MET containing monomers. CCP, CTB, and TNU have 10-MDP containing monomers.
OBU has GPDM containing monomers. After each of the primers and universal adhesives
had been applied to the zirconia surfaces, the SBS' significantly improved compared
with the no Tx groups. GPU showed the highest initial bond strength. In contrast to
other universal adhesives and CCP, GPU contains both phosphate monomer (10-MDP) and
carboxylic monomer (4-MET). These two acidic functional monomers could enhance the
initial bond strength of zirconia/resin composite. There was a need to differentiate
between the effects of other universal adhesives and GPU on SBS. However, the initial
bond strength for CCP + SB2, CTB, OBU, and TNU was not significant in between groups.
Both phosphate monomers, 10-MDP and GPDM, were successful in increasing the initial
bond strength of resin composite to zirconia. For this, the no Tx group served as
the negative control, as it showed the lowest initial bond strength. Similarly, Han
et al, Seabra et al, and Celik et al reported that the use of universal adhesives
proved successful in increasing resin composite and zirconia adherence.[4]
[19]
[20]
As part of the aging process, the SBS values of the GPU, OBU, and no Tx groups were
lower after thermocycling for 10,000 cycles, but the SBS values of CCP + SB2, CTB,
and TNU groups did not change significantly. Even after a long-term aging process,
the bonding strength of zirconia/resin composites must be maintained. Thermocycling
procedures were performed to compare the bond durability, according to aging. According
to Gale et al, a cyclic method of 10,000 cycles per year may be sufficient to detect
restorative failure.[10] This was because all universal adhesives and zirconia primer showed increased bonding
strengths through an initial chemical bond with 4-MET and GPDM monomer bond, subsequently
decreasing due to hydrolytic deterioration via 10,000 cycles of thermocycling procedures.
For this reason, the 10-MDP monomer features a lengthy and hydrophobic spacer chain
that improves bond strength and is stable even after 10,000 thermocycling cycles.[21]
[22] The 10-MDP has also been found to help improve and stabilize the bonding of resin
composites to zirconia. The use of an extra-long chain hydrophobic spacer when using
universal adhesives may improve the durability and resistance to deterioration of
the zirconia/resin composite interface and also increase the long-term durability
of resin composites.[23]
[24]
After the SBS test, the mode of failure distribution confirmed the bond strength data.
All of the samples for the no Tx group showed the lowest bond strength and exhibited
adhesive failures both after 24 hours and after 10,000 cycles. The samples for the
10-MDP containing monomers showed predominantly mixed failures both before and after
10,000 cycles of thermocycling. The samples for the GPDM containing monomers presented
primarily mixed failures after 24 hours, but adhesive failures increased after 10,000
cycles of thermocycling. Cohesive failures in the zirconia and resin composite were
not found, which mentions that the weakest bond in the bound specimen occurred at
the zirconia/resin composite interface.
Conclusion
Within the scope of this study's limitations, the universal adhesives containing 10-MDP
monomers performed best in terms of zirconia/resin composite interface' SBS both before
and after thermocycling. Moreover, the SBS of all universal adhesives using 10-MDP
monomers has not been substantially reduced by artificial aging. The universal adhesives
containing 10-MDP monomers have the potential to improve the clinical method of repairing
zirconia fractures, using resin composite, both in terms of initial bond and long-term
stability.
