Abstract
Objectives
This study aimed to evaluate the adhesive properties of a MDP-containing resin cement to a colored zirconia ceramic, using an experimental zirconia–silica coating technique with different priming conditions.
Methods
18 zirconia ceramic discs (Cercon base colored) were divided into two groups: the control group and the experimental zirconia–silica coating group. Specimens in each group were further divided into 3 subgroups ( n = 3) according to the priming conditions: no primer, a MDP-containing primer (ED Primer II) or a silane coupling primer (RelyX™ Ceramic Primer). Then resin-composite discs (Filtek™ Z250) were bonded to the treated surface using a MDP-containing resin cement (Panavia F 2.0). The bi-layered specimens were cut into microbars and 20 microbars were randomly selected from each specimen, half of which were stored in 37 °C water bath for 24 h, and the other half were stored for 30 days. After water storage, the samples were exposed to a micro tensile bond strength test (MTBS). The results were analyzed by ANOVA, while the fracture surfaces were examined by SEM.
Results
After 24 h water storage, zirconia–silica coating followed by silanization showed a significantly ( P < 0.001) higher MTBS value 45.0 (10.9) MPa. Water storage affected ( P < 0.05) MTBS in the control group (24.1–30.3 MPa to 2.8–3.1 MPa), but only partially in the zirconia–silica coating group (20.0–45.1 MPa to 17.4–25.9 MPa). SEM analysis revealed a failure mode change after water storage.
Significance
The combination of zirconia–silica coating with silane coupling can improve the bonding of resin cement to this colored zirconia.
1
Introduction
The popularity of zirconia based core materials has increased in recent years because of its favorable biocompatibility, esthetic and mechanical properties. However, a long-term durable bonding to zirconia ceramics is reported to be difficult , because of its surface stability. When zirconia is aimed to be used in adhesive dentistry applications this might be a determining factor in obtaining clinical success.
As zirconia is a monolithic ceramic not containing a glass phase, conventional hydrofluoric acid etching does not lead to changes of the surface roughness of zirconia, nor does it enhance the bond strength . Without surface pretreatment, a single silane coupling agent may also not improve the bond strength to zirconia , which lacks the required silica phase for the chemical reaction to form a siloxane network. Although the real function is still not clear, MDP (10-methacryloyloxydecyl dihydrogen phosphate) is assumed to have a chemical reaction to zirconia surface, and can improve the initial bond strength of resin cement to zirconia ceramics. However, this bond strength will always be compromised by artificial aging, indicating the high bond strength based on MDP is not stable in water .
The pretreatment of airborne-particle abrasion is suggested to roughen the zirconia surface, since it could increase the micro mechanical interlocks and surface bonding area . However, it can also produce surface defects, which will result in a reduced strength of the zirconia restorations . Moreover, it could also affect the long-term performance of zirconia ceramics , due to the surface flaws and the tetragonal to monoclinic phase transformation . Therefore, alternative surface modification methods are required for improving the resin bond strength to zirconia ceramics. Several alternatives to roughen zirconia surface are reported in literature, such as SIE (selective infiltration etching) technique , plasma spraying , coating with nano-structured alumina (Al 2 O 3 ) , coating with zirconia ceramic powder and pore former , different ways of coating with silica-based ceramic , and coating with silica-like seed layers .
Among these techniques, coating with a silica-based layer seems to be easy and more reliable, since the bonding of resin cement to zirconia ceramics may be further improved via silane coupling agents . However, according to previous reports, there are still several weaknesses of the existing silica coating methods. For example, the coating layer could be too thick, hence influencing the clinical fitness ; the zirconia surface may not be fully covered by the coating layer or the coating layer may not firmly attached to the zirconia surface ; most of them were still based on air-abrasion which may cause micro defects due to a high blasting pressure.
The purpose of this study was to examine the effect of an experimental zirconia–silica coating technique on durability of the bond strength between resin cement and a colored zirconia ceramic, under different priming conditions. The micro tensile bond strength test (MTBS) and Scanning Electron Microscopy (SEM) were used as a method to evaluate the bond quality.
2
Materials and methods
2.1
Zirconia specimens
Eighteen zirconia ceramic discs were cut from Cercon base colored blanks (Cercon base 38 colored, Shade: ivory; Degudent, Germany) Table 1 , with a diamond-coated cutting disc (Diamond Wafering Blade, No. 11-4254; Buehler, USA) applied in a sawing machine (Isomet 1000; Buehler, USA). The blank zirconia discs were then placed in a sintering oven (Cercon Heat, Degudent, Germany) and sintered for 6:40 h at the maximum temperature of 1350 °C, according to manufacturer’s instructions. The fully sintered zirconia discs (3.0 mm in thickness and 19.5 mm in diameter) were cleaned in an ethanol ultrasonic bath (Bransonic 3510; Branson Ultrasonics Corp., USA) for 10 min. The 18 discs were divided into two groups (i) the control group with no pretreatment and (ii) the zirconia–silica coating group.
| Material | Basic composition | LOT | Manufacturer |
|---|---|---|---|
| Cercon base colored | Zirconia 93 wt% | 20023697 | DeguDent |
| Yttrium oxide 5 wt% | GmbH | ||
| Hafnium oxide <2 wt% | Germany | ||
| Aluminum oxide and silicon oxide <1 wt%. | |||
| Filtek supreme XT | Bis-GMA a , TEGDM b , Bis-EMA c | 6CB | 3M ESPE |
| Flowable restorative | 75 nm silica nanofiller | USA | |
| 5–10 nm zirconia nanofiller | |||
| 0.6–1.4 μm zirconia/silica nanoclusters | |||
| 65% by weight (55% by volume). | |||
| Filtek Z250 | Bis-GMA, UDMA d , Bis-EMA | N193019 | 3M ESPE |
| Universal restorative | 0.01–3.5 μm silica/zirconia particles | USA | |
| 82% by weight (60% by volume) | |||
| ED Primer II | 2-Hydroxyethyl methacrylate, 10-methacryloyloxydecyl dihydrogen phosphate | 00283B | Kuraray Medical Inc. |
| Liquid A | N-Methacryloyl-5-aminosalicylic acid | Japan | |
| Water | |||
| Accelerators | |||
| ED Primer II | N-Methacryloyl-5-aminosalicylic acid | 00158A | Kuraray Medical Inc. |
| Liquid B | Water | Japan | |
| Catalysts | |||
| Accelerators | |||
| Panavia F 2.0 | 10-Methacryloyloxydecyl dihydrogen phosphate | 00437A | Kuraray Medical Inc. |
| Paste A | Hydrophobic aromatic dimethacrylate | Japan | |
| Hydrophobic aliphatic dimethacrylate | |||
| Hydrophilic aliphatic dimethacrylate | |||
| Silanated silica filler | |||
| Silanated colloidal silica | |||
| dl -Camphorquinone | |||
| Catalysts | |||
| Initiators | |||
| Others | |||
| Panavia F 2.0 | Sodium fluoride | 00223B | Kuraray Medical Inc. |
| Paste B | Hydrophobic aromatic dimethacrylate | Japan | |
| Hydrophobic aliphatic dimethacrylate | |||
| Hydrophilic aliphatic dimethacrylate | |||
| Silanated barium glass filler | |||
| Catalysts | |||
| Accelerators | |||
| Pigments | |||
| Others | |||
| RelyX Ceramic Primer | Ethyl alcohol 70–80 wt% | N198785 | 3M ESPE |
| Water 20–30 wt% | USA | ||
| Methacryloxypropyltrimethoxysilane <2 wt% |
a Bis-GMA, bisphenyl glycidylmethacrylate.
b TEGDMA, triethylene glycol dimethacrylate.
2.2
The experimental coating technique
The experimental zirconia–silica coating technique applied in this study was based on sintering the filler particles (at 1200 °C for 10 min) of a flowable composite resin which was coated on the zirconia surfaces. A thin layer of Filtek supreme XT Flowable Restorative (A3 Shade; 3M ESPE, USA) was spread on one surface of the zirconia disc by a micro brush and remained uncured. Filtek supreme XT Flowable is a bis-GMA based resin composite containing silica and zirconia nanofillers and nanoclusters (65% by weight, 55% by volume). During firing of the nine discs, the resin matrix will burn, leaving the filler particles on the zirconia surface to be sintered. For the sintering procedure, a computer programmed dental porcelain furnace (STRATOS; Elephant Dental BV, The Netherlands) was used, at a heating rate of 15 °C/min and holding on for 10 min at the highest temperature of 1200 °C. After that the specimens were cooled down to room temperature in 100 min. All the coated specimens were ultrasonically cleaned in ethanol for 10 min.
2.3
Composite resin discs
Eighteen resin composite discs (3.0 mm in thickness and 19.5 mm in diameter) were made of Filtek Z250 Universal Restorative (A3 Shade, 3M ESPE, USA) with a plastic mold. The composite discs were cured using Elipar FreeLight 2 (3M-ESPE, St. Paul, MN, USA), according to the manufacturer’s instructions. The discs were stored in water at 37 °C for at least 24 h before bonding.
2.4
Priming and luting
The specimens in each group were further divided into three subgroups ( n = 3), according to the priming conditions: (A) no primer, (B) a MDP-containing primer (ED Primer II; Kuraray Medical Inc., Japan) and (C) a silane coupling agent containing primer (RelyX Ceramic Primer; 3M-ESPE, USA). A MDP-containing resin cement (Panavia F 2.0; Kuraray Medical Inc., Japan) was mixed and applied on the surface of the composite resin disc, which was then seated on top of the zirconia disc at a constant load of 50 N for 60 s. Excess cement was wiped off and the specimen was light cured at four different locations for 60 s each.
2.5
Micro tensile bonding strength (MTBS) test
All the cemented bi-layered specimens were stored in water at 37 °C for 24 h and sectioned into microbars of 1.0 mm × 1.0 mm by using the sawing machine and a 0.3 mm thick diamond-coated cutting disc. The microbars were examined under a stereomicroscope (SZ; Olympus, Tokyo, Japan) and only intact microbars were selected. 20 microbars from each specimen were selected for the micro tensile bonding strength (MTBS) test, ten of them were tested after 24 h in 37 °C water, and the other ten microbars were tested after 30 days storage in 37 °C water. Each microbar was bonded to a stainless steel attachment unit using a light-polymerized adhesive resin (Clearfil SE bond; Kuraray Medical Inc., Japan). The zirconia–resin MTBS was measured by applying tensile load to the bonded interface using a universal testing machine (Instron 6022; Instron Corp., High Wycombe, England) at a crosshead speed of 0.5 mm/min. Microbars which debonded spontaneously before test were included as 0 MPa in the statistical analysis.
2.6
Analysis of failure mode
The fractured zirconia surfaces after MTBS test were examined under a Scanning Electron Microscopy (SEM) (XL 20; Philips, The Netherlands), after gold coating. Failure mode was classified either as the four types below:
- Type 1
adhesive mode, complete zirconia surface was visible;
- Type 2
a mixed mode in zirconia surface and cement, both (partial) zirconia surface and a (partial) cement cover were visible;
- Type 3
a cohesive fracture within the cement layer, almost all of the fracture surface was covered with cement;
- Type 4
a mixed mode in zirconia surface, cement and resin composite, both cement and resin composite were detected on the zirconia surface.
2.7
Statistics
MTBS values were analyzed by three-way analysis of variance (ANOVA) to examine the effects of zirconia–silica coating pretreatment, priming conditions and water storage periods. Post hoc multiple comparisons were conducted using Dunnett T3 tests at α = 0.05 level, for the variances were not equal from the homogeneity test.
2
Materials and methods
2.1
Zirconia specimens
Eighteen zirconia ceramic discs were cut from Cercon base colored blanks (Cercon base 38 colored, Shade: ivory; Degudent, Germany) Table 1 , with a diamond-coated cutting disc (Diamond Wafering Blade, No. 11-4254; Buehler, USA) applied in a sawing machine (Isomet 1000; Buehler, USA). The blank zirconia discs were then placed in a sintering oven (Cercon Heat, Degudent, Germany) and sintered for 6:40 h at the maximum temperature of 1350 °C, according to manufacturer’s instructions. The fully sintered zirconia discs (3.0 mm in thickness and 19.5 mm in diameter) were cleaned in an ethanol ultrasonic bath (Bransonic 3510; Branson Ultrasonics Corp., USA) for 10 min. The 18 discs were divided into two groups (i) the control group with no pretreatment and (ii) the zirconia–silica coating group.
