Graphical abstract
Highlights
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MDP containing universal adhesive alone provides effective chemical bonding to Y-TZP.
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Silica coating + silanization provided higher hydrolytic stability than applying MDP.
Abstract
Objectives
To evaluate the bonding of resin-cement to yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) via silica coating followed by silanization, and three one-bottle universal adhesives, with or without prior conditioning using a zirconia primer.
Methods
Y-TZP specimens ( n = 160) were conditioned by tribochemical silica coating and silanization (CS), or alumina sandblasting with one of the following MDP containing adhesives or primers: Z-Prime Plus™ (zirconia primer, ZP), Single Bond Universal™ (SU), Clearfil Universal Bond™ (CU) or All-Bond Universal™ (AU). Additionally, some specimens (ZPSU, ZPCU and ZPAU) received Z-Prime Plus™ followed by one of the three adhesives. After 24 h water storage and “aging” (20,000 thermocycles plus additional 40-day water storage), shear bond strength (SBS) was measured. Fourier-transform infrared spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) were employed for characterization of the chemical bonds between the primer/adhesives and the zirconia. Thermodynamic calculations were used to examine the hydrolytic stability between the MDP-zirconia chemical bonds and the SiO 2 -silane chemical bonds.
Results
The CS and ZPCU groups showed higher SBS than the other six groups. There were no significant pairwise differences amongst ZP, SU and ZPSU, or amongst ZP, AU and ZPAU. Aging led to significantly decreased SBS for all groups except CS and ZPCU. There was no statistically significant interaction between surface treatment and aging. XPS determined the chemical bonds between MDP and zirconia. FTIR showed similar shifts in characteristic phosphate peaks for all the primer and/or adhesive groups. Result of thermodynamic calculation showed that equilibrium constant of SiO 2 -silane system is much larger than the one of MDP-tetragonal phase zirconia system.
Significance
The application of one-bottle universal adhesives after alumina sandblasting is an alternative to tribochemical silica coating with silanization for bonding to zirconia, while bonding between resin and Y-TZP is more susceptible to hydrolysis when zirconia primer or one-bottle universal adhesive is used.
1
Introduction
Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramic has superior mechanical properties compared to other all-ceramic materials. Because of the intrinsic brittleness of Y-TZP and its tendency to undergo low temperature degradation, long-term survival of Y-TZP restorations still relies on adequate resin bonding . After cementation, Y-TZP restoration-resin and cement-dentin combine tightly to form a “sandwich-like” structure consisting of two bonding interfaces. Both interfaces are important and methods have been developed to strengthen bonding at each of those interfaces. Bonding at the ceramic/resin interface is based on micro-mechanical interlocking and chemical bonding via primer conditioning . Surface roughening alone was found to provide insufficient bond strength, although bond durability was improved . Chemical bonding is more effective in increasing bond strength. Accordingly, surface roughening followed by acidic phosphate monomer conditioning (usually zirconia primers), and tribochemical silica coating followed by silanization, are the two most popular methods for Y-TZP bonding .
Although these aforementioned surface treatments work well on the Y-TZP/resin interface, they cannot be simultaneously used for improving resin/dentin bonding. Because surface treatments focusing on either dentin or Y-TZP usually employ many different products with different aims, it becomes necessary to adopt different operating procedures for each bonding substrate. This usually involves two, three or more steps, causing confusion to the clinician. Thus, there is a need to simplify such procedures to satisfy the clinical demand for faster, more consistent and less technique-sensitive methodologies.
Recently, one-bottle universal adhesives have been developed to bond with almost all indirect restoration materials, including resin composites, zirconia-based and alumina-based ceramics, silica-based glass ceramics, alloys, enamel and dentin. Manufacturers claim that components such as methacryloxydecyl dihydrogen phosphate (MDP) enable bonding to these surfaces without the use of primers. Amaral and Seabra found that application of one-bottle universal adhesives alone provided higher bonding strength to zirconia than application of zirconia primers alone. Nevertheless, literature on the influence of one-bottle universal adhesives on resin bonding to zirconia is rare . In particular, there is no information on whether these adhesives provide stronger or more stable bonds when zirconia primers are used in advance. In addition, since tribochemical silica coating followed by silanization is still the most commonly used practice for pretreating Y-TZP bonding surface, it is more valuable to compare the bonding performance of one-bottle universal adhesives to zirconia with this gold standard, which will make the clinicians evaluate the new generation of adhesives more accurately. Thus, the objective of the present study was to evaluate the bonding of resin-cement to Y-TZP via silica coating followed by silanization, and three one-bottle universal adhesives, with or without prior conditioning using a zirconia primer. The null hypothesis tested was that pre-conditioning with a zirconia primer has no effect on improvement of bond strength and durability when one-bottle universal adhesives are used for bonding to zirconia.
2
Materials and methods
2.1
Bonding specimens
One hundred and sixty ceramic plates (10 × 10 × 2 mm 3 ) were cut from a machinable Y-TZP block using a low-speed saw (Isomet 100, Buehler Ltd., Lake Bluff, IL, USA). Each plate was completely sintered according to the manufacturer’s instructions. These Y-TZP plates were randomly assigned to eight groups) according to the conditioning method employed.
2.1.1
Group CS
Tribochemical silica coating was performed with 30 μm Cojet™ particles (3M ESPE, St. Paul, MN, USA) at a distance of 10 mm from the bonding surface for 15 s. Sandblasting was achieved using an intraoral sandblaster (Micro Etcher, Danville Materials, San Ramon, CA, USA) and an air pressure of 0.3 MPa. This was followed by the application of a silane coupling agent (Porcelain Primer, Bisco Inc.) to the sandblasted Y-TZP surface.
2.1.2
Group ZP
The bonding surface was sandblasted with 50 μm alumina particles from a distance of 10 mm for 20 s, at 0.3 MPa, using a sandblasting device (JNBP-2, Jianian Futong Medical Equipment Co., Ltd., Tianjin, China). A coat of Z-Prime Plus™ (Bisco Inc., Schaumburg, IL, USA) was applied to the sandblasted surface. After 15–20 s of volatilization, the primed Y-TZP surface was dried with low-pressure, oil-free air for 15 s.
2.1.3
Group SU
The Y-TZP surface was sandblasted with alumina as previously described. A coat of Single Bond Universal™ (3 M ESPE) was applied to the sandblasted surface and allowed to react for 15–20 s. The ceramic plate was air-dried to remove excess solvent. The adhesive was light-cured for 10 s.
2.1.4
Group CU
Procedures described in Group SU was repeated, with the adhesive replaced by Clearfil Universal Bond™ (Kuraray Noritake Dental Inc., Tokyo, Japan).
2.1.5
Group AU
Procedures described in Group SU was repeated, with the adhesive replaced by All-Bond Universal™ (Bisco, Inc.).
2.1.6
Group ZPSU
The alumina-sandblasted Y-TZP plate surface was conditioned with Z-Prime Plus™ followed by the application of Single Bond Universal™ in the manner previously described.
2.1.7
Group ZPCU
The alumina-sandblasted Y-TZP plate surface was conditioned with Z-Prime Plus™ followed by the application of Clearfil Universal Bond™ in the manner previously described.
2.1.8
Group ZPAU
The alumina-sandblasted Y-TZP plate surface was conditioned with Z-Prime Plus™ followed by the application of All-Bond Universal™ in the manner previously described.
One hundred and sixty nylon tubes, each 6 mm in inner diameter and 3 mm in height, were filled with a resin composite (Valux Plus, 3M ESPE). Each composite-filled tube was light-cured for 40 s using an LED light-curing unit (Elipar FreeLight 2, 3M ESPE). The polymerized resin-composite cylinders were removed from the nylon tubes. A layer of resin composite cement was applied to the surface of the pretreated Y-TZP plate, and a resin-composite cylinder was placed on the resin cement under a constant load. Excess cement was removed and then the remaining cement was light-cured for 40 s. Details of the materials used in the present study are given in Table 1 .
| Material/trade name | Main composition a | Manufacturer |
|---|---|---|
| Y-TZP/Everest ZS-Ronde | ZrO 2 + HfO 2 (94.4 wt%), Y 2 O 3 (5.2 wt%), Al 2 O 3 (0.2–0.5 wt%) | KAVO, Kaltenbach & Voigt GmbH & Co. KG, Bismarckring, Germany |
| Tribochemical silica coating sands (CoJet) | Silicatized alumina sand | 3M ESPE, St. Paul, MN, USA |
| Single bond universal | Vitrebond copolymer, MDP, silane | 3M ESPE |
| Clearfil universal bond | bis-GMA, MDP, HEMA, hydrophilic aliphatic dimethacrylate, colloidal silica, silane, dl-camphorquinone, ethanol | Kuraray Noritake Dental Co., Tokyo, Japan |
| Porcelain primer | Ethanol (30–70%), acetone (30–70%), silane (γ-MPS) (1–10%) | Bisco Inc., Schaumburg, IL, USA |
| Zirconia primer (Z-Prime Plus) | Ethanol (<90%), biphenyl dimethacrylate (<10%); HEMA (<20%); MDP | Bisco Inc. |
| All-bond universal | Ethanol (>20%), bis-GMA (>20%), MDP | Bisco Inc. |
| Light-polymerized resin cement (Variolink N) | Filler: barium glass, ytterbium trifluoride, Ba-Al-fluorosilicate glass, spheroid mixed oxide (73.4 wt%) Base resin: bis-GMA, urethane dimethacrylate, triethylene glycol dimethacrylate (26.3 wt%) |
Ivoclar-Vivadent AG, Schaan, Liechtenstein |
| Light-polymerized resin composite (Valux Plus) | Filler: Barium aluminum fluoride glass + highly dispersive silica (80–90%) Base resin: bis-GMA (5–10%) |
3M ESPE |
a Details of major compositions were obtained from the material safety data sheets provided by manufacturers.
2.2
Shear bond strength (SBS) testing
One half of the bonded specimens were stored in distilled water at room temperature for 24 h. The other half was subjected to aging by first thermocycling for 20,000 cycles (TC-501F, Suzhou Weier Labware Co., Ltd., China) between two water baths, one at 5 °C and the other at 55 °C, with a dwell time of 30 s. After thermocycling, the specimens were further soaked in distilled water for 40 days at room temperature. All the bonded specimens were subjected to shear bond strength testing using a universal testing machine (Instron Model 3365, ElectroPuls, MA, USA) at a crosshead speed of 1.0 mm/min. Each bonded specimen was embedded in self-curing acrylic resin base before testing.
Shear bond strength (in MPa) was calculated by dividing the load at fracture (in Newtons), with the bonding interface area (28.26 mm 2 ).
2.3
Statistical analyses
Bond strength data from the eight groups were statistically analyzed using Two-way analysis of variance and an LSD multiple comparison test to examine the effects of the two categorical independents, surface treatment and aging, and the interaction of these two factors on shear bond strength. Parametric statistical methods were employed after validating the normality and equal variance of the data sets. Post-hoc analysis was performed using the Tukey test, to compare bond strength means between individual groups, with or without aging. Statistically analyses were performed using SPSS 17.0 software (SPSS Inc., Chicago, IL, USA). For all tests, statistical significance was set as 0.05.
2.4
Surface characterization of sandblasted Y-TZP surface
The Y-TZP surfaces that had been tribochemically silica coated with Cojet™ sands, or sandblasted with alumina were sputter-coated with gold. Cojet™ sand particles were similarly sputter-coated. The specimens were examined with a field emission-scanning electron microscope (FE-SEM; LEO 1530VP, Oberkochen, Germany) at 20 kV. Energy dispersive X-ray microanalysis (EDS; INCAx-sight, Oxford Instruments, United Kingdom) was used to characterize the elemental distribution of Si and Al on the sandblasted Y-TZP surfaces.
2.5
Chemical-bond characterization of primers and adhesives on Y-TZP
In order to find the chemical bond between Y-TZP and MDP, the main functional adhesive composition in the current testing primers or adhesives, Y-TZP plates treated with 10 wt% MDP (supplied by Watson International Ltd., China) ethanol solution were detected by X-ray Photoelectron Spectroscopy (XPS, PHI Quanter, USA).
Y-TZP powders were generated by grinding a machinable Y-TZP block. The powders were mixed with the primers and/or adhesives in order, according to the different group assignments. Untreated Y-TZP powders, the primers and adhesives and the treated Y-TZP powders were characterized by Fourier-transform infrared spectroscopy (FTIR; NEXUS870, Nicolet, USA) in transmission mode.
2.6
Thermodynamic calculations to examine the hydrolytic stability between the MDP-tetragonal phase zirconia chemical bonds and the SiO 2 -silane chemical bonds
In our previous study, a MDP- tetragonal phase zirconia system has been modeled . According to these data, the hydrolytic stability of the MDP-tetragonal phase zirconia chemical bonds was analyzed by thermodynamic calculations. Calculations were performed using the “Our-own N-layered Integrated molecular Orbital and Molecular mechanics” (ONIOM) method. All calculations relating to the calculation of thermodynamic functions were undertaken using the Gaussian 09 software. All data were calculated at standard conditions (1 atm, 298 K).
As similar as the method for evaluation hydrolytic stability of the MDP-tetragonal phase zirconia chemical bonds, a SiO 2 -γ-Methacryloxypropyltrimethoxysilane (γ-MPS) system was modeled, and hydrolysis of complex of γ-MPS and SiO 2 cluster was evaluated by thermodynamic calculations.
2
Materials and methods
2.1
Bonding specimens
One hundred and sixty ceramic plates (10 × 10 × 2 mm 3 ) were cut from a machinable Y-TZP block using a low-speed saw (Isomet 100, Buehler Ltd., Lake Bluff, IL, USA). Each plate was completely sintered according to the manufacturer’s instructions. These Y-TZP plates were randomly assigned to eight groups) according to the conditioning method employed.
2.1.1
Group CS
Tribochemical silica coating was performed with 30 μm Cojet™ particles (3M ESPE, St. Paul, MN, USA) at a distance of 10 mm from the bonding surface for 15 s. Sandblasting was achieved using an intraoral sandblaster (Micro Etcher, Danville Materials, San Ramon, CA, USA) and an air pressure of 0.3 MPa. This was followed by the application of a silane coupling agent (Porcelain Primer, Bisco Inc.) to the sandblasted Y-TZP surface.
2.1.2
Group ZP
The bonding surface was sandblasted with 50 μm alumina particles from a distance of 10 mm for 20 s, at 0.3 MPa, using a sandblasting device (JNBP-2, Jianian Futong Medical Equipment Co., Ltd., Tianjin, China). A coat of Z-Prime Plus™ (Bisco Inc., Schaumburg, IL, USA) was applied to the sandblasted surface. After 15–20 s of volatilization, the primed Y-TZP surface was dried with low-pressure, oil-free air for 15 s.
2.1.3
Group SU
The Y-TZP surface was sandblasted with alumina as previously described. A coat of Single Bond Universal™ (3 M ESPE) was applied to the sandblasted surface and allowed to react for 15–20 s. The ceramic plate was air-dried to remove excess solvent. The adhesive was light-cured for 10 s.
2.1.4
Group CU
Procedures described in Group SU was repeated, with the adhesive replaced by Clearfil Universal Bond™ (Kuraray Noritake Dental Inc., Tokyo, Japan).
2.1.5
Group AU
Procedures described in Group SU was repeated, with the adhesive replaced by All-Bond Universal™ (Bisco, Inc.).
2.1.6
Group ZPSU
The alumina-sandblasted Y-TZP plate surface was conditioned with Z-Prime Plus™ followed by the application of Single Bond Universal™ in the manner previously described.
2.1.7
Group ZPCU
The alumina-sandblasted Y-TZP plate surface was conditioned with Z-Prime Plus™ followed by the application of Clearfil Universal Bond™ in the manner previously described.
2.1.8
Group ZPAU
The alumina-sandblasted Y-TZP plate surface was conditioned with Z-Prime Plus™ followed by the application of All-Bond Universal™ in the manner previously described.
One hundred and sixty nylon tubes, each 6 mm in inner diameter and 3 mm in height, were filled with a resin composite (Valux Plus, 3M ESPE). Each composite-filled tube was light-cured for 40 s using an LED light-curing unit (Elipar FreeLight 2, 3M ESPE). The polymerized resin-composite cylinders were removed from the nylon tubes. A layer of resin composite cement was applied to the surface of the pretreated Y-TZP plate, and a resin-composite cylinder was placed on the resin cement under a constant load. Excess cement was removed and then the remaining cement was light-cured for 40 s. Details of the materials used in the present study are given in Table 1 .
