Long-term Bond Strength Between Layering Indirect Composite Material and Zirconia Coated with Silicabased Ceramics Ryosuke Fushikia / Futoshi Komineb / Junichi Hondac / Shingo Kamiod / Markus B. Blatze / Hideo Matsumuraf
Purpose: This study evaluated the long-term shear bond strength between an indirect composite material and a zirconia framework coated with silica-based ceramics, taking the effect of different primers into account. Materials and Methods: A total of 165 airborne-particle abraded zirconia disks were subjected to one of three pretreatments: no pretreatment (ZR-AB), airborne-particle abrasion of zirconia coated with feldspathic porcelain (ZR-PO-AB), and 9.5% hydrofluoric acid etching of zirconia coated with feldspathic porcelain (ZR-PO-HF). An indirect composite material (Estenia C&B) was then bonded to the zirconia disks after they were treated with one of the following primers: Clearfil Photo Bond (CPB), Clearfil Photo Bond with Clearfil Porcelain Bond Activator (CPB + Activator), Estenia Opaque Primer (EOP), Porcelain Liner M Liquid B (PLB), or no priming (CON, control group). Shear bond strength was tested after 100,000 thermocycles, and the data were analyzed using the Steel-Dwass U-test (α = 0.05). Results: For ZR-PO-AB and ZR-PO-HF specimens, bond strength was highest in the CPB+Activator group (25.8 MPa and 22.4 MPa, respectively). Bond strengths were significantly lower for ZR-AB specimens in the CON and PLB groups and for ZR-PO-AB specimens in the CON, CPB, and EOP groups. Conclusion: Combined application of a hydrophobic phosphate monomer (MDP) and silane coupling agent enhanced the long-term bond strength of indirect composite material to a zirconia coated with silica-based ceramics. Keywords: bond strength, coating, indirect composite, priming agent, zirconia. J Adhes Dent 2015; 17: 273–281. doi: 10.3290/j.jad.a34413
a
Assistant Professor, Department of Fixed Prosthodontics, Nihon University School of Dentistry, Tokyo, Japan. Idea, performed experiments, statistical analysis, wrote the manuscript.
b
Assistant Professor, Department of Fixed Prosthodontics, Nihon University School of Dentistry, Tokyo, Japan. Experimental design, interpreted the data, co-wrote manuscript.
c
Postgraduate Student, Division of Applied Oral Sciences, Nihon University Graduate School of Dentistry, Tokyo, Japan. Performed the specimen preparation and shear bond test.
d
Postgraduate Student, Division of Applied Oral Sciences, Nihon University Graduate School of Dentistry, Tokyo, Japan. Performed the stereomicroscopic and SEM observation.
e
Professor and Chair, Department of Preventive and Restorative Sciences, University of Pennsylvania School of Dental Medicine, Philadelphia, PA, USA. Co-supervised study, proofread the manuscript.
f
Professor and Chair, Department of Fixed Prosthodontics, Nihon University School of Dentistry, Tokyo, Japan. Supervised study, contributed to discussion, proofread the manuscript.
Correspondence: Futoshi Komine, Department of Fixed Prosthodontics, Nihon University School of Dentistry, 1-8-13, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8310, Japan. Tel: +81-3-3219-8145, Fax: +81-3-3219-8351. e-mail: [email protected]
Vol 17, No 3, 2015
Submitted for publication: 24.04.14; accepted for publication: 18.06.15
Z
irconium dioxide (zirconia) ceramics have high chemical and physical properties as well as high biocompatibility. 23,31 Zirconia ceramics are used as a framework material for single crowns and fixed dental prostheses (FDPs) even in the posterior region, where high occlusal loads are predominant. Clinical studies, including observation periods of over 5 years, found that zirconia-based restorations had a promising clinical performance, with a high survival rate (> 98%).12,29,34,35,37,40,42 Some researchers concluded that zirconia ceramics are a valid alternative to metal for frameworks in fixed restorations.35 However, the rate of chipping or fracture of the layering porcelain in posterior zirconia-based restorations (6% to 25%) was reported to be higher12,29,34,35,37,40,42 than that for metal-ceramic restorations (5% to 10% over 10 years;10 2.5% after 5 years32). Chipping/fracture of the layering porcelain can be caused by a thermal mismatch between layering porcelain and the underlying framework, mechanical deficiencies in 273
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the layering porcelain, inadequate framework support for the veneer, unfavorable framework:veneer thickness ratio, or inadequate bonding between the two materials.11,41 Several techniques to address these issues have been investigated, including overpressing the ceramic onto the zirconia framework,6 use of a supporting framework design for layering porcelain,24 the “CAD-on” technique using CAD/CAM systems (Ivoclar Vivadent; Schaan, Liechtenstein),5,13,17,38 monolithic structure with zirconia ceramics,33,44 and layering of an indirect composite material onto the zirconia framework.7,21 Composite materials have several shortcomings, such as insufficient wear resistance27 and plaque accumulation due to surface degradation.26 Nevertheless, composite materials for the veneer of zirconia-based restorations are an intriguing alternative and have functional advantages, especially in implant-supported restorations, because the indirect composite materials may absorb occlusal stresses. As compared to natural teeth, the implant has a more than 8-fold higher threshold value for tactile perception.16 Çiftçi and Canay9 evaluated the effects of various layering materials on stress distribution in implant-supported FDPs and noted that use of a composite material resulted in a 15% greater reduction in stress compared to feldspathic porcelain and gold alloy. In another study, the failure probability did not significantly differ between composite-layered restorations and metal-ceramic restorations supported with dental implants.39 The success of all-ceramic bilayered restorations may depend on the strength of the framework/veneer bond.1 Zirconia frameworks are typically veneered with a ceramic or composite material, which is built up in layers, to create definitive restorations with individualized and natural optical characteristics.3 While layered zirconia frameworks are esthetically advantageous, the framework/veneer interface, as one of the weakest aspects of zirconia-based restorations, contributes to chipping and cracking of the layering material.2 In vitro studies showed that a priming agent containing the functional monomer 10-methacryloyloxydecyl dihydrogen phosphate (MDP) yielded superior and durable bond strengths between an indirect composite and zirconia framework.7,21 The resin bond to silica-based ceramics is well documented. Application of a silane coupling agent to silica-based ceramics provides a strong chemical bond due to the formation of a siloxane network.4,8,22 Coating of the zirconia surface with a silica-based ceramic material, followed by silanization, could yield stable adhesive bonding between zirconia and resin cements.20 An earlier study assessed the effects of coating silica-based ceramics onto zirconia frameworks and priming agents on the shear bond strength between an indirect composite material and zirconia ceramic frameworks.14 The results confirmed that silica-based ceramic coating of zirconia frameworks enhanced the bond strength of an indirect composite material to zirconia, independent of surface treatment.14 The long-term stability of framework/veneer bonds is influenced by intraoral conditions, including water absorption,28 hydrolyzation,36 and thermal changes.15,18,25,30 To assess bond durability, long-term thermocycling in 274
water storage is a common artificial-aging method when testing the resin bond to ceramics.19,43 To date, however, only limited information is available on the durability of bonds between an indirect composite material and zirconia frameworks coated with silica-based ceramics. The purpose of the present study was to evaluate the long-term shear bond strength between an indirect composite material and a zirconia framework coated with silica-based ceramics and to investigate the effects of various ceramic priming agents. The following working hypotheses were tested: coating the zirconia with silicabased ceramics affects long-term bond strength, and long-term bond strength significantly differs with different priming agents.
MATERIALS AND METHODS Materials assessed in the present study and the experimental design are shown in Table 1 and Fig 1, respectively. A total of 165 zirconia disks (diameter, 11 mm; thickness, 2.5 mm) were fabricated with the Katana system (Kuraray Noritake Dental; Tokyo, Japan) as a bonding substrate. The surfaces of the zirconia disks were wet ground with 600-grit silicon carbide abrasive paper (Tri-M-ite Wetordry sheets, 3M ESPE; St Paul, MN, USA). The disks were then airborne-particle abraded (AB) with 50-μm Al2O3 particles (Hi-Aluminas; Shofu; Kyoto, Japan) at 0.2 MPa pressure for 20 s at a distance of 10 mm from the surface (Fig 1a). The area for feldspathic porcelain (Cerabien ZR SBA2, Kuraray Noritake Dental) coating was delimited by placing a piece of plastic tape (Star Traper-GP, Sakurai; Tokyo, Japan) with a circular hole (5.0 mm, diameter) on the surface of each zirconia disk (Fig 1b). The porcelain powder was mixed with the corresponding liquid (Meister Liquid, Kuraray Noritake Dental) supplied by the manufacturer, and a thin layer of porcelain slurry was layered on the coating area (Fig 1c). Then, the plastic tape was carefully removed from the specimen (Fig 1d), and the specimen was fired at 930°C for 1 min in a vacuum furnace (SingleMat Porcelain Furnace, Shofu) to create a porcelain coating on the surface of the zirconia specimen (Fig 1e). The zirconia specimens were randomly divided into 3 surface pretreatment groups (n = 55) as follows: no pretreatment (ZR-AB), airborne-particle abrasion of zirconia coated with feldspathic porcelain (ZR-PO-AB), and 9.5% hydrofluoric (HF) acid etching of zirconia coated with feldspathic porcelain (ZR-PO-HF) (Fig 1f). The surface of half the number of the porcelain-coated specimens was airborne-particle abraded with Al2O3 particles (Hi-Aluminas, Shofu) for 2 s at a pressure 0.2 MPa from a distance of 10 mm (ZR-PO-AB). The surfaces of the remaining coated specimens were acid etched with 9.5% HF acid gel (Porcelain Etch, Ultradent; South Jordan, UT, USA) for 1 min, rinsed in distilled water for 20 s, cleaned with methanol in an ultrasonic bath (Ultrasonic Cleaner SUC-110, Shofu), and dried with oil-free air spray (Air Duster AD400FL, Orientec; Misato, Japan) (ZR-PO-HF). The Journal of Adhesive Dentistry
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Table 1
Materials used
Material/Trade name
Abbreviation
Manufacturer
Zirconia ceramic material / Katana
ZR
Kuraray Noritake Dental; Tokyo, Japan
94.4% ZrO2, 5.4% Y2O3
Kuraray Noritake Dental
UTMA, methacrylate, photoinitiator, pigment, 92 filler (glass, macro-alumina), bis-GMA, methacrylate, photoinitiator, pigment, filler (quartz, composite, others)
Indirect composite material / Estenia C&B Dentin DA2
Lot no.
Components
Estenia C&B Opaque OA2 Feldspathic porcelain for zirconia / Cerabien ZR
PO
Kuraray Noritake Dental
4871
Shade base SBA2
Hydrofluoric acid / Porcelain Etch
HF
Ultradent; South Jordan, UT, USA
Priming agent / Clearfil Photo Bond
CPB
Kuraray Noritake Dental
00423B 00523A
Catalyst: MDP, HEMA, bis-GMA Universal: accelerators, ethanol
Clearfil Porcelain Bond Activator
Activator
Kuraray Noritake Dental
2273
3-TMSPMA
Estenia Opaque Primer
EOP
Kuraray Noritake Dental
00157A
MDP, monomer solvent
Porcelain Liner M Liquid B
PLB
Sun Medical; Moriyama, Japan
RS1
3-TMSPMA
9.5% hydrofluoric acid gel
MDP: 10-methacryloyloxydecyl dihydrogen phosphate; 3-TMSPMA: 3-trimethoxysilylpropyl methacrylate; HEMA: 2-hydroxyethyl methacrylate; bis-GMA: bisphenol A-glycidyl methacrylate.
Ø 5.0 mm
Zirconia
2.5 mm Ø 5.0 mm
Ø 11.0 mm
a
d
e
Ø 6.0 mm 2.0 mm
Ø 5.0 mm
g
Ø 5.0 mm
f
Ø 6.0 mm 2.0 mm 5° C
h
Surface roughness of all subgroups was measured using a profilometer (Surfcom 480A-12, Tokyo Seimitsu; Tokyo, Japan) with 2-μm-radius stylus tip. Cut-off length and transverse length were set at 0.8 mm and 4.0 mm, respectively. The specimens of all subgroups were scanned five times, and the average of Ra values was determined. Vol 17, No 3, 2015
c
Load
Fig 1 Experimental design. (a) Airborne-particle abrasion of zirconia disks; (b) delimitation of coating area with plastic tape; (c) layering of porcelain slurry; (d) removal of the plastic tape; (e) firing of the porcelain; (f) airborne-particle abrasion or 9.5% hydrofluoric (HF) acid etching of specimen surface; (g) applying a piece of double-coated tape and a stainless steel ring around the bonding area; (h) filling the indirect composite material into the ring and polymerization; (i) 100,000 thermocycles between 5°C and 55°C; (j) shear-bond testing.
b
55° C
i
j
For all subgroups, the specimens were divided into 5 groups containing 11 specimens each (n = 11). The surface of each specimen was then treated with one of the four priming agents, namely, Clearfil Photo Bond (CPB, Kuraray Noritake Dental), Clearfil Photo Bond with Clearfil Porcelain Bond Activator (CPB+Activator, Kuraray Noritake Dental), Estenia Opaque Primer (EOP, Kuraray Noritake 275
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Table 2 Shear bond strength (MPa) between Estenia C&B composite and Katana zirconia after 100,000 thermocycles Groups
ZR-AB
ZR-PO-AB
ZR-PO-HF
Median
Mean (SD)
Median
Mean (SD)
Median
Mean (SD)
0.0
0.1 (0.2)a,A
0.2
0.3 (0.2)d,A
13.5
14.1 (4.2)g,B
EOP
11.5
11.5
(2.6)b,C
0.3
(0.1)d,D
14.9
15.8 (4.1)g,E
PLB
12.2
12.3 (2.5)b,F
15.9
15.3 (3.7)e,F
16.2
15.8 (4.3)g,F
CPB
18.3
19.7
(4.0)c,G
0.2
(0.1)d,H
16.5
15.7 (3.6)g,G
CPB + Activator
21.9
20.5 (4.5)c,I
24.7
25.8 (3.4)f,J
22.1
22.4 (4.6)h,I,J
CON
0.3
0.2
ZR: Zirconia; AB: airborne-particle abrasion; PO: feldspathic porcelain; HF: hydrofluoric acid; CPB: Clearfil Photo Bond; Activator: Clearfil Photo Bond Activator; EOP: Estenia Opaque Primer; PLB: Porcelain Liner M Liquid B; CON: no priming (control). Identical superscript upper-case letters in the same row indicate that the values are not statistically different (Steel-Dwass test, p > 0.05). Identical superscript lower-case letters in the same column indicate that the values are not statistically different (Steel-Dwass test, p > 0.05).
Dental), and Porcelain Liner M Liquid B (PLB, Sun Medical; Moriyama, Japan), according to manufacturers’ instructions or not treated at all (CON, control). Except for the EOP and CON groups, each group was primed with EOP before bonding the opaque material to zirconia specimens. After surface treatment of the specimens, the bonding area was determined by applying a piece of doublecoated tape (Kincsem, Horse Care Products; Tokyo, Japan) containing a circular hole (diameter 5.0 mm). For all specimens in each group, a thin layer of opaque material (Estenia C&B opaque OA2, Kuraray Noritake Dental) as a high-flow bonding agent was applied and light cured for 90 s using a laboratory light-curing unit (α-Light II, J. Morita; Suita, Japan). An additional opaque material was placed on top of the primary opaque layer in the same manner. A stainless steel ring (inside diameter 6.0 mm; depth 2.0 mm) was then positioned to surround the opaque material (Fig 1g). A dentin shade of indirect composite material (Estenia C&B Dentin DA2, Kuraray Noritake Dental) was packed into the ring with a force of 5 N (Fig 1h). The specimen was light cured with the same polymerization unit for 5 min and then heat polymerized at 110°C for 15 min in a heat oven (KL-310, J. Morita). The specimens from each group were stored in distilled water at 37°C for 24 h, then placed in an automated thermocycling device (Thermal Shock Tester TTS-1 LM, Thomas Kagaku; Tokyo, Japan) for 100,000 cycles with water bath temperatures between 5°C and 55°C and a dwell time of 1 min (Fig 1i). Each specimen was subsequently positioned in a steel mold and placed in a shearbond testing jig (Tokyo Giken; Tokyo, Japan). Shear bond strength was tested in a universal testing device (Type 5567, Instron; Canton, MA, USA) at a crosshead speed of 0.5 mm/min (Fig 1j). Load at fracture was converted to shear bond strength (MPa) by dividing failure load (N) by bonding area (mm2). The results were primarily analyzed by the Levene test for equality of variance using statistical software (SPSS version 15.0; SPSS; Chicago, IL, USA). When the Levene test did not show homoscedasticity, the results were then analyzed with the Kruskal-Wallis test (SPSS version 15.0), followed by the Steel-Dwass test for multiple comparisons (Kyplot 5.0, KyensLab; Tokyo, Japan). The significance level was set at 0.05 in all analyses. 276
The fractured interfaces of tested specimens were examined with a stereomicroscope (32X) (StemiDV4, Carl Zeiss; Jena, Germany) to determine mode of failure after shear bond testing. Failure mode for the ZR-AB group was classified as A (adhesive failure at the zirconia/composite material interface) or B (combined adhesive and cohesive failure within the composite material). The failure mode for the ZR-PO-AB and ZR-PO-HF groups was classified as C (adhesive failure at the porcelain/composite material interface), D (adhesive failure at the porcelain/composite material interface, with cohesive failure inside the porcelain), or E (cohesive failure within the porcelain). Representative fractured specimens were osmium coated with a sputter coater (HPC-IS, Vacuum Device; Mito, Japan) and examined with a scanning electron microscope (SEM) (S-4300, Hitachi High-Technologies; Tokyo, Japan) operated at 15 kV. In addition, the specimens of all subgroups were embedded in acrylic resin (Unifast III, GC; Tokyo, Japan) and cut using a precision saw (Isomet Low Speed Saw, Buehler; Lake Bluff, IL, USA) equipped with a diamond wafering blade (15LC, Buehler). The interface between the indirect composite material and layering porcelain or zirconia of the representative specimens in all groups was observed with SEM.
RESULTS The shear bond strengths after thermocycling between the Estenia C&B composite and Katana zirconia material are presented in Table 2. The mean bond strength varied from 0.1 to 20.5 MPa in ZR-AB specimens, from 0.2 to 25.8 MPa in ZR-PO-AB specimens, and from 14.1 to 22.4 MPa in ZR-PO-HF specimens. Among ZR-AB specimens, the CPB (19.7 MPa) and CPB+Activator (20.5 MPa) groups had significantly higher bond strengths than the other groups. For ZR-PO-AB and ZR-PO-HF specimens, the CPB+Activator (25.8 MPa and 22.4 MPa, respectively) group had the highest bond strength of all groups. In the CPB+Activator group, the bond strength of ZR-PO-AB specimens was significantly higher than that of ZR-AB specimens. In addition, in the CON group, bond strength was significantly higher for the ZR-PO-HF specimens than for the other two subgroups. The Journal of Adhesive Dentistry
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Table 3 Mean (SD) surface roughness values Ra of the zirconia surface after the different surface pretreatments Specimens
Surface roughness (μm) (SD)
ZR-AB
0.49 (0.06)a
ZR-PO-AB
3.83 (0.54)b
ZR-PO-HF
2.85 (0.22)c
For abbreviations, see Table 2. Different superscript letters indicate significant difference (Steel-Dwass test, p > 0.05).
Table 4 Failure modes after 100,000 thermocycles and shear bond testing
a
Failure type Groups
Priming agents
A
ZR-AB
CON
11
ZR-PO-AB
ZR-PO-HF
B
C
D
E
CON
11
0
0
EOP
11
0
0
PLB
0
8
3
CPB
11
0
0
CPB + Activator
0
5
6
CON
2
8
1
EOP
1
8
2
PLB
0
9
2
CPB
2
8
1
CPB + Activator
0
6
5
0
EOP
9
2
PLB
10
1
CPB
7
4
CPB + Activator
7
4
See Table 2 for abbreviations. A: Adhesive failure at zirconia/composite material interface. B: Combined adhesive failure at zirconia/composite material interface and cohesive failure within composite material. C: Adhesive failure at porcelain/composite material interface. D: Adhesive failure at porcelain/composite material interface, with cohesive failure within the porcelain. E: Cohesive failure within the porcelain.
The surface roughness (Ra) of the zirconia surface after the different surface pretreatments are shown in Table 3. The surface roughness of ZR-PO-AB was significantly higher than that of the other two specimens. Table 4 shows the failure modes after shear bond testing. For ZR-AB specimens, adhesive failure at the zirconia/ composite material interface was observed in the CON group. Combined cohesive failure within the composite material and adhesive failure at the zirconia/composite material interface was detected in the other four groups (ie, EOP, PLB, CPB, and CPB+Activator). Cohesive failure within the porcelain was noted for ZR-PO-AB specimens Vol 17, No 3, 2015
b
c Fig 2 SEM images of (a) a zirconia surface after airborneparticle abrasion (ZR-AB); (b) a feldspathic porcelain-coated zirconia surface after airborne-particle abrasion (ZR-PO-AB); (c) a feldspathic porcelain-coated zirconia surface after HF acid etching (ZR-PO-HF) (original magnification 1000X).
in the CPB+Activator and PLB groups, and for ZR-PO-HF specimens in all groups. The ZR-PO-AB specimens in the CON, CPB, and EOP groups exhibited adhesive failure at the porcelain/composite interface. Figure 2 shows SEM images of the zirconia surface after the different surface pretreatments. As seen in Fig 2a and 2b, the surface of ZR-AB and ZR-PO-AB specimens was roughened and had sharp edges after airborne-par277
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Fig 3 SEM image of the debonded surface of a ZR-AB specimen from the Porcelain Liner M Liquid B (PLB) group after 100,000 thermocycles and shear bond testing (original magnification 1000X).
Fig 4 SEM image of the debonded surface of a ZR-AB specimen from the EOP group after 100,000 thermocycles and shear bond testing (original magnification 1000X).
Fig 5 SEM image of the debonded surface of a ZR-PO-AB specimen from the Clearfil Photo Bond (CPB) group after 100,000 thermocycles and shear bond testing (original magnification 1000X).
Fig 6 SEM image of the debonded surface of a ZR-PO-HF specimen from the CPB+Activator group after 100,000 thermocycles and shear bond testing (original magnification 1000X).
ticle abrasion with 50-μm Al2O3 particles. The surface of ZR-PO-AB specimens was rougher than that of ZR-AB specimens. In contrast, ZR-PO-HF specimens had major and minor irregularities and a honeycombed appearance (Fig 2c). Figures 3 and 4 show representative SEM images of the fractured interfaces of ZR-AB specimens after shear bond testing. Figure 3 shows adhesive failure. The composite material is completely separated from the zirconia surface, and the airborne-particle abraded surface can be seen. In contrast, the images of combined cohesive and adhesive failure show that the zirconia surface is partially covered with composite material (Fig 4). Figures 5 to 7 are SEM images of the debonded surface of ZR-PO-AB and ZR-PO-HF specimens after shear bond testing. Figure 5 depicts the debonded surface of adhesive failure at the porcelain/composite material interface. The surface
appearance is similar to that before priming in Fig 2b. An SEM image of adhesive failure at the porcelain/composite material interface with cohesive failure inside the porcelain (Fig 6) reveals part of the surface before priming. Figure 7 shows the surface of a specimen with cohesive failure within the porcelain and demonstrates the different appearance of the surface before priming and a thin layer of layering porcelain covering the debonded surface. The SEM images in Fig 8 show the interface between the indirect composite material and zirconia or layering material for the representative specimens. The bonding interface of indirect composite to the zirconia showed larger gaps and more defects (Fig 8a) compared with that of indirect composite to the layering porcelain (Figs 8b and 8c). For Figs 8b and 8c, the penetration of the composite material into layering porcelain in ZR-PO-HF was deeper than that of ZR-PO-AB.
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a Fig 7 SEM image of the debonded surface of a ZR-PO-AB specimen from the CPB+Activator group after 100,000 thermocycles and shear bond testing (original magnification 1000X).
DISCUSSION The present study evaluated long-term shear bond strengths between an indirect composite and zirconia framework coated with silica-based ceramics. The results of the present study indicate that the bond strength was influenced by coating with silica-based ceramics and application of priming agents. Thus, we confirm the working hypotheses that coating zirconia with silica-based ceramics influences long-term bond strength, and that long-term bond strength significantly differs with different priming agents. The results of the present study demonstrate that HF etching of the surfaces of zirconia frameworks coated with silica-based ceramics significantly enhanced bond strength durability between an indirect composite and the framework. Analysis of failure mode revealed cohesive failure within the porcelain in ZR-PO-HF specimens in all groups. In contrast, among ZR-PO-AB specimens, the failure mode of all specimens in the CON, CPB, and EOP groups was adhesive at the porcelain/composite material interface. One possible reason for this behavior might be that HF acid etching of the porcelain-coated zirconia surface enhanced micromechanical interlocking. The SEM image of ZR-PO-HF specimens, which showed that the penetration of the composite material into layering porcelain was deeper than that of ZR-PO-AB specimens, can confirm this finding. Previous studies found that the roughness caused by HF acid etching of silica-based ceramics improved the bond strength between composite resin and porcelain.8 Kato et al18 reported that airborneparticle abrasion of porcelain did not result in substantial mechanical retention, although the surface may be more retentive than polished or glazed surfaces. A silane coupling agent (3-TMSPMA) provided durable bond strengths of an indirect composite to a zirconia framework coated with silica-based ceramics, regardless of pretreatment with either airborne-particle abrasion or HF acid etching. This confirms findings of a previous Vol 17, No 3, 2015
b
c Fig 8 SEM images of (a) the interface of zirconia and indirect composite material (ZR-AB); (b) the interface of indirect composite material and layering porcelain for ZR-PO-AB; (c) the interface of indirect composite material and layering porcelain for ZR-PO-HF.
study, which noted that a siloxane network was successfully formed between an indirect composite material and zirconia coated with feldspathic porcelain.14 Several previous studies reported that siloxane network formation 279
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was more important than mechanical interlocking in the adhesion of composite to porcelain.4,22 The ZR-PO-AB and ZR-PO-HF specimens from the CPB+Activator group had the highest bond strength among the five groups. CPB+Activator contains a phosphate monomer (MDP) as well as a silane coupling agent (3-TMSPMA). Because formation of the siloxane network was facilitated by the acidic functional monomer, the durability of bonds between the indirect composite and zirconia ceramics was enhanced.4 This is supported by our observation that the incidence of cohesive failure within the porcelain was higher than in the other groups. For zirconia frameworks coated with silica-based ceramics, application of hydrophobic phosphate monomer (MDP) and a silane coupling agent should be considered to enhance bond strengths and their durability. The CPB and CPB+Activator groups had significantly higher bond strengths than the other groups of ZR-AB specimens, which indicates that application of MDP and a polymerization initiator results in durable bond strength between an indirect composite and zirconia framework. A chemical reaction might occur between the phosphate ester groups of the adhesive monomer MDP and the hydroxyl groups on the zirconia ceramic surface.19,43 The rate of combined adhesive failure at the zirconia/composite material interface and cohesive failure within the composite material was higher in the CPB and CPB+Activator groups than in the other groups. Previous studies of initial and longer-term bond strength reported similar results.7,21 Gale and Darvell15 estimated that 10,000 thermocycles can be correlated to one year of clinical service. A large number of thermal cycles were therefore applied in this study to further stress the bonding interfaces and to simulate multiple years of intraoral aging. After thermocycling, the mean bond strength in ZR-PO-AB specimens of the PLB and CPB+Activator groups and in ZR-PO-HF specimens of all groups was greater than 13 MPa, which suggests that coating the zirconia framework with feldspathic porcelain is an effective method of reaching a critical threshold for clinically successful bond strength between an indirect layering composite material and a zirconia framework.25 To eliminate chipping of layering porcelain in zirconiabased restoration, the “CAD-on” technique or monolithic zirconia restorations have currently received attention as an alternative to zirconia layered with feldspathic porcelain. In the CAD-on technique, a zirconia framework and layering ceramics are designed and milled together using CAD/CAM, and then the two parts are combined using a glass ceramic5,17,38 or a luting agent.13 Monolithic zirconia restorations have been introduced as an obvious way to circumvent the chipping of layering porcelain for replacing the bilayered structure (veneer/core) with a monolithic zirconia ceramic.33,44 Although these two approaches showed promising results in in vitro studies,5,17,38,44 long-term clinical studies are required to verify the clinical performance of the CAD-on technique or monolithic zirconia restorations. The primary objectives of in vitro studies are elimination of influential parameters and limitation of variables. 280
All in vitro studies have limitations, and randomized clinical trials are the ultimate tool to evaluate the benefits of a clinical procedure. Numerous factors, such as a framework design and 3-dimensional geometry of the restoration, may influence long-term clinical outcomes of zirconia-based composite-layered restorations and cannot be evaluated in an in vitro study. While the present study provides valuable information on possible future approaches to enhance composite/zirconia bonding interfaces, further laboratory and especially clinical studies are necessary before any of these protocols can be recommended for use in clinical practice.
CONCLUSIONS Within the limitations of this in vitro study, the following conclusions can be drawn: 1. Silica-based ceramic coating of zirconia specimens provides equal or superior long-term bond strength between indirect composite and zirconia compared with zirconia specimens without coating. 2. Combined application of a hydrophobic phosphate monomer (MDP) and a silane coupling agent enhances durable long-term bond strength between indirect composite material and zirconia specimens coated with silica-based ceramics.
ACKNOWLEDGMENTS This study was supported in part by JSPS KAKENHI, grant number 24592933, a grant from the Sato Fund, Nihon University School of Dentistry (2013 and 2014), and a grant from the Promotion and Mutual Aid Corporation for Private Schools of Japan (2014).
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Fushiki et al 10. Coornaert J, Adriaens P, De Boever J. Long-term clinical study of porcelain-fused-to-gold restorations. J Prosthet Dent 1984;51:338-342. 11. Douglas RD. Color stability of new-generation indirect resins for prosthodontic application. J Prosthet Dent 2000;83:166-170. 12. Edelhoff D, Florian B, Florian W, Johnen C. HIP zirconia fixed partial dentures–clinical results after 3 years of clinical service. Quintessence Int 2008;39:459-471. 13. Fabbri G, Sorrentino R, Brennan M, Cerutti A. A novel approach to implant screw-retained restorations: adhesive combination between zirconia frameworks and monolithic lithium disilicate. Int J Esthet Dent 2014;9:490-505. 14. Fushiki R, Komine F, Blatz MB, Koizuka M, Taguchi K, Matsumura H. Shear bond strength between an indirect composite layering material and feldspathic porcelain-coated zirconia ceramics. Clin Oral Investig 2012;16:1401-1411. 15. Gale MS, Darvell BW. Thermal cycling procedures for laboratory testing of dental restorations. J Dent 1999;27:89-99. 16. Hämmerle CH, Wagner D, Brägger U, Lussi A, Karayiannis A, Joss A, Lang NP. Threshold of tactile sensitivity perceived with dental endosseous implants and natural teeth. Clin Oral Implants Res 1995;6:83-90. 17. Kanat B, Comlekoglu EM, Dundar-Comlekoglu M, Hakan Sen B, Özcan M, Ali Gungor M. Effect of various veneering techniques on mechanical strength of computer-controlled zirconia framework designs. J Prosthodont 2014;23:445-455. 18. Kato H, Matsumura H, Atsuta M. Effect of etching and sandblasting on bond strength to sintered porcelain of unfilled resin. J Oral Rehabil 2000;27:103-110. 19. Kern M, Wegner SM. Bonding to zirconia ceramic: adhesion methods and their durability. Dent Mater 1998;14:64-71. 20. Kitayama S, Nikaido T, Maruoka R, Zhu L, Ikeda M, Watanabe A, Foxton RM, Miura H, Tagami J. Effect of an internal coating technique on tensile bond strengths of resin cements to zirconia ceramics. Dent Mater J 2009;28:446-453. 21. Kobayashi K, Komine F, Blatz MB, Saito A, Koizumi H, Matsumura H. Influence of priming agents on the short-term bond strength of an indirect composite veneering material to zirconium dioxide ceramic. Quintessence Int 2009;40:545-551. 22. Lacy AM, LaLuz J, Watanabe LG, Dellinges M. Effect of porcelain surface treatment on the bond to composite. J Prosthet Dent 1988;60:288-291. 23. Manicone PF, Rossi Iommetti P, Raffaelli L. An overview of zirconia ceramics: basic properties and clinical applications. J Dent 2007;35: 819-826. 24. Marchack BW, Futatsuki Y, Marchack CB, White SN. Customization of milled zirconia copings for all-ceramic crowns: a clinical report. J Prosthet Dent 2008;99:169-173. 25. Matsumura H, Yanagida H, Tanoue N, Atsuta M, Shimoe S. Shear bond strength of resin composite veneering material to gold alloy with varying metal surface preparations. J Prosthet Dent 2001;86:315-319. 26. Ohlmann B, Dreyhaupt J, Schmitter M, Gabbert O, Hassel A, Rammelsberg P. Clinical performance of posterior metal-free polymer crowns with and without fiber reinforcement: one-year results of a randomised clinical trial. J Dent 2006;34:757-762. 27. Ohlmann B, Uekermann J, Dreyhaupt J, Schmitter M, Mussotter K, Rammelsberg P. Clinical wear of posterior metal-free polymer crowns. Oneyear results from a randomized clinical trial. J Dent 2007;35:246-252. 28. Özcan M, Alkumru HN, Gemalmaz D. The effect of surface treatment on the shear bond strength of luting cement to a glass-infiltrated alumina ceramic. Int J Prosthodont 2001;14:335-339.
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29. Özer F, Mante FK, Chiche G, Saleh N, Takeichi T, Blatz MB. A retrospective survey on long-term survival of posterior zirconia and porcelain-fused-to-metal crowns in private practice. Quintessence Int 2014;45:31-38. 30. Palmer DS, Barco MT, Billy EJ. Temperature extremes produced orally by hot and cold liquids. J Prosthet Dent 1992;67:325-327. 31. Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials 1999;20:1-25. 32. Reuter JE, Brose MO. Failures in full crown retained dental bridges. Br Dent J 1984;157:61-63. 33. Rinke S, Fischer C. Range of indications for translucent zirconia modifications: clinical and technical aspects. Quintessence Int 2013;44: 557-566. 34. Sailer I, Fehér A, Filser F, Gauckler LJ, Lüthy H, Hämmerle CH. Five-year clinical results of zirconia frameworks for posterior fixed partial dentures. Int J Prosthodont 2007;20:383-388. 35. Sailer I, Gottner J, Känel S, Hämmerle CH. Randomized controlled clinical trial of zirconia-ceramic and metal-ceramic posterior fixed dental prostheses: a 3-year follow-up. Int J Prosthodont 2009;22:553-560. 36. Sailer I, Philipp A, Zembic A, Pjetursson BE, Hämmerle CH, Zwahlen M. A systematic review of the performance of ceramic and metal implant abutments supporting fixed implant reconstructions. Clin Oral Implants Res 2009;20 Suppl 4:4-31. 37. Sax C, Hämmerle CH, Sailer I. 10-year clinical outcomes of fixed dental prostheses with zirconia frameworks. Int J Comput Dent 2011;14: 183-202. 38. Schmitter M, Mueller D, Rues S. Chipping behaviour of all-ceramic crowns with zirconia framework and CAD/CAM manufactured veneer. J Dent 2012;40:154-162. 39. Takahashi Y, Hisama K, Sato H, Chai J, Shimizu H, Kido H, Ukon S. Probability of failure of highly filled indirect resin-veneered implantsupported restorations: an in vitro study. Int J Prosthodont 2002;15: 179-182. 40. Tartaglia GM, Sidoti E, Sforza C. Seven-year prospective clinical study on zirconia-based single crowns and fixed dental prostheses. Clin Oral Investig 2014;doi. 10.1007/s00784-014-1330-2. 41. Vanoorbeek S, Vandamme K, Lijnen I, Naert I. Computer-aided designed/computer-assisted manufactured composite resin versus ceramic single-tooth restorations: a 3-year clinical study. Int J Prosthodont 2010;23:223-230. 42. Vult von Steyern P, Carlson P, Nilner K. All-ceramic fixed partial dentures designed according to the DC-Zirkon technique. A 2-year clinical study. J Oral Rehabil 2005;32:180-187. 43. Wolfart M, Lehmann F, Wolfart S, Kern M. Durability of the resin bond strength to zirconia ceramic after using different surface conditioning methods. Dent Mater 2007;23:45-50. 44. Zhang Y, Lee JJW, Srikanth R, Lawn BR. Edge chipping and flexural resistance of monolithic ceramics. Dent Mater 2013;29:1201-1208.
Clinical relevance: Combined application of MDP and silane coupling agent is useful for bonding indirect composite material to a zirconia coated with silicabased ceramics.
281
Purpose: This study evaluated the long-term shear bond strength between an indirect composite material and a zirconia framework coated with silica-based ceramics, taking the effect of different primers into account. Materials and Methods: A total of 165 airborne-particle abraded zirconia disks were subjected to one of three pretreatments: no pretreatment (ZR-AB), airborne-particle abrasion of zirconia coated with feldspathic porcelain (ZR-PO-AB), and 9.5% hydrofluoric acid etching of zirconia coated with feldspathic porcelain (ZR-PO-HF). An indirect composite material (Estenia C&B) was then bonded to the zirconia disks after they were treated with one of the following primers: Clearfil Photo Bond (CPB), Clearfil Photo Bond with Clearfil Porcelain Bond Activator (CPB + Activator), Estenia Opaque Primer (EOP), Porcelain Liner M Liquid B (PLB), or no priming (CON, control group). Shear bond strength was tested after 100,000 thermocycles, and the data were analyzed using the Steel-Dwass U-test (α = 0.05). Results: For ZR-PO-AB and ZR-PO-HF specimens, bond strength was highest in the CPB+Activator group (25.8 MPa and 22.4 MPa, respectively). Bond strengths were significantly lower for ZR-AB specimens in the CON and PLB groups and for ZR-PO-AB specimens in the CON, CPB, and EOP groups. Conclusion: Combined application of a hydrophobic phosphate monomer (MDP) and silane coupling agent enhanced the long-term bond strength of indirect composite material to a zirconia coated with silica-based ceramics. Keywords: bond strength, coating, indirect composite, priming agent, zirconia. J Adhes Dent 2015; 17: 273–281. doi: 10.3290/j.jad.a34413
a
Assistant Professor, Department of Fixed Prosthodontics, Nihon University School of Dentistry, Tokyo, Japan. Idea, performed experiments, statistical analysis, wrote the manuscript.
b
Assistant Professor, Department of Fixed Prosthodontics, Nihon University School of Dentistry, Tokyo, Japan. Experimental design, interpreted the data, co-wrote manuscript.
c
Postgraduate Student, Division of Applied Oral Sciences, Nihon University Graduate School of Dentistry, Tokyo, Japan. Performed the specimen preparation and shear bond test.
d
Postgraduate Student, Division of Applied Oral Sciences, Nihon University Graduate School of Dentistry, Tokyo, Japan. Performed the stereomicroscopic and SEM observation.
e
Professor and Chair, Department of Preventive and Restorative Sciences, University of Pennsylvania School of Dental Medicine, Philadelphia, PA, USA. Co-supervised study, proofread the manuscript.
f
Professor and Chair, Department of Fixed Prosthodontics, Nihon University School of Dentistry, Tokyo, Japan. Supervised study, contributed to discussion, proofread the manuscript.
Correspondence: Futoshi Komine, Department of Fixed Prosthodontics, Nihon University School of Dentistry, 1-8-13, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8310, Japan. Tel: +81-3-3219-8145, Fax: +81-3-3219-8351. e-mail: [email protected]
Vol 17, No 3, 2015
Submitted for publication: 24.04.14; accepted for publication: 18.06.15
Z
irconium dioxide (zirconia) ceramics have high chemical and physical properties as well as high biocompatibility. 23,31 Zirconia ceramics are used as a framework material for single crowns and fixed dental prostheses (FDPs) even in the posterior region, where high occlusal loads are predominant. Clinical studies, including observation periods of over 5 years, found that zirconia-based restorations had a promising clinical performance, with a high survival rate (> 98%).12,29,34,35,37,40,42 Some researchers concluded that zirconia ceramics are a valid alternative to metal for frameworks in fixed restorations.35 However, the rate of chipping or fracture of the layering porcelain in posterior zirconia-based restorations (6% to 25%) was reported to be higher12,29,34,35,37,40,42 than that for metal-ceramic restorations (5% to 10% over 10 years;10 2.5% after 5 years32). Chipping/fracture of the layering porcelain can be caused by a thermal mismatch between layering porcelain and the underlying framework, mechanical deficiencies in 273
Fushiki et al
the layering porcelain, inadequate framework support for the veneer, unfavorable framework:veneer thickness ratio, or inadequate bonding between the two materials.11,41 Several techniques to address these issues have been investigated, including overpressing the ceramic onto the zirconia framework,6 use of a supporting framework design for layering porcelain,24 the “CAD-on” technique using CAD/CAM systems (Ivoclar Vivadent; Schaan, Liechtenstein),5,13,17,38 monolithic structure with zirconia ceramics,33,44 and layering of an indirect composite material onto the zirconia framework.7,21 Composite materials have several shortcomings, such as insufficient wear resistance27 and plaque accumulation due to surface degradation.26 Nevertheless, composite materials for the veneer of zirconia-based restorations are an intriguing alternative and have functional advantages, especially in implant-supported restorations, because the indirect composite materials may absorb occlusal stresses. As compared to natural teeth, the implant has a more than 8-fold higher threshold value for tactile perception.16 Çiftçi and Canay9 evaluated the effects of various layering materials on stress distribution in implant-supported FDPs and noted that use of a composite material resulted in a 15% greater reduction in stress compared to feldspathic porcelain and gold alloy. In another study, the failure probability did not significantly differ between composite-layered restorations and metal-ceramic restorations supported with dental implants.39 The success of all-ceramic bilayered restorations may depend on the strength of the framework/veneer bond.1 Zirconia frameworks are typically veneered with a ceramic or composite material, which is built up in layers, to create definitive restorations with individualized and natural optical characteristics.3 While layered zirconia frameworks are esthetically advantageous, the framework/veneer interface, as one of the weakest aspects of zirconia-based restorations, contributes to chipping and cracking of the layering material.2 In vitro studies showed that a priming agent containing the functional monomer 10-methacryloyloxydecyl dihydrogen phosphate (MDP) yielded superior and durable bond strengths between an indirect composite and zirconia framework.7,21 The resin bond to silica-based ceramics is well documented. Application of a silane coupling agent to silica-based ceramics provides a strong chemical bond due to the formation of a siloxane network.4,8,22 Coating of the zirconia surface with a silica-based ceramic material, followed by silanization, could yield stable adhesive bonding between zirconia and resin cements.20 An earlier study assessed the effects of coating silica-based ceramics onto zirconia frameworks and priming agents on the shear bond strength between an indirect composite material and zirconia ceramic frameworks.14 The results confirmed that silica-based ceramic coating of zirconia frameworks enhanced the bond strength of an indirect composite material to zirconia, independent of surface treatment.14 The long-term stability of framework/veneer bonds is influenced by intraoral conditions, including water absorption,28 hydrolyzation,36 and thermal changes.15,18,25,30 To assess bond durability, long-term thermocycling in 274
water storage is a common artificial-aging method when testing the resin bond to ceramics.19,43 To date, however, only limited information is available on the durability of bonds between an indirect composite material and zirconia frameworks coated with silica-based ceramics. The purpose of the present study was to evaluate the long-term shear bond strength between an indirect composite material and a zirconia framework coated with silica-based ceramics and to investigate the effects of various ceramic priming agents. The following working hypotheses were tested: coating the zirconia with silicabased ceramics affects long-term bond strength, and long-term bond strength significantly differs with different priming agents.
MATERIALS AND METHODS Materials assessed in the present study and the experimental design are shown in Table 1 and Fig 1, respectively. A total of 165 zirconia disks (diameter, 11 mm; thickness, 2.5 mm) were fabricated with the Katana system (Kuraray Noritake Dental; Tokyo, Japan) as a bonding substrate. The surfaces of the zirconia disks were wet ground with 600-grit silicon carbide abrasive paper (Tri-M-ite Wetordry sheets, 3M ESPE; St Paul, MN, USA). The disks were then airborne-particle abraded (AB) with 50-μm Al2O3 particles (Hi-Aluminas; Shofu; Kyoto, Japan) at 0.2 MPa pressure for 20 s at a distance of 10 mm from the surface (Fig 1a). The area for feldspathic porcelain (Cerabien ZR SBA2, Kuraray Noritake Dental) coating was delimited by placing a piece of plastic tape (Star Traper-GP, Sakurai; Tokyo, Japan) with a circular hole (5.0 mm, diameter) on the surface of each zirconia disk (Fig 1b). The porcelain powder was mixed with the corresponding liquid (Meister Liquid, Kuraray Noritake Dental) supplied by the manufacturer, and a thin layer of porcelain slurry was layered on the coating area (Fig 1c). Then, the plastic tape was carefully removed from the specimen (Fig 1d), and the specimen was fired at 930°C for 1 min in a vacuum furnace (SingleMat Porcelain Furnace, Shofu) to create a porcelain coating on the surface of the zirconia specimen (Fig 1e). The zirconia specimens were randomly divided into 3 surface pretreatment groups (n = 55) as follows: no pretreatment (ZR-AB), airborne-particle abrasion of zirconia coated with feldspathic porcelain (ZR-PO-AB), and 9.5% hydrofluoric (HF) acid etching of zirconia coated with feldspathic porcelain (ZR-PO-HF) (Fig 1f). The surface of half the number of the porcelain-coated specimens was airborne-particle abraded with Al2O3 particles (Hi-Aluminas, Shofu) for 2 s at a pressure 0.2 MPa from a distance of 10 mm (ZR-PO-AB). The surfaces of the remaining coated specimens were acid etched with 9.5% HF acid gel (Porcelain Etch, Ultradent; South Jordan, UT, USA) for 1 min, rinsed in distilled water for 20 s, cleaned with methanol in an ultrasonic bath (Ultrasonic Cleaner SUC-110, Shofu), and dried with oil-free air spray (Air Duster AD400FL, Orientec; Misato, Japan) (ZR-PO-HF). The Journal of Adhesive Dentistry
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Table 1
Materials used
Material/Trade name
Abbreviation
Manufacturer
Zirconia ceramic material / Katana
ZR
Kuraray Noritake Dental; Tokyo, Japan
94.4% ZrO2, 5.4% Y2O3
Kuraray Noritake Dental
UTMA, methacrylate, photoinitiator, pigment, 92 filler (glass, macro-alumina), bis-GMA, methacrylate, photoinitiator, pigment, filler (quartz, composite, others)
Indirect composite material / Estenia C&B Dentin DA2
Lot no.
Components
Estenia C&B Opaque OA2 Feldspathic porcelain for zirconia / Cerabien ZR
PO
Kuraray Noritake Dental
4871
Shade base SBA2
Hydrofluoric acid / Porcelain Etch
HF
Ultradent; South Jordan, UT, USA
Priming agent / Clearfil Photo Bond
CPB
Kuraray Noritake Dental
00423B 00523A
Catalyst: MDP, HEMA, bis-GMA Universal: accelerators, ethanol
Clearfil Porcelain Bond Activator
Activator
Kuraray Noritake Dental
2273
3-TMSPMA
Estenia Opaque Primer
EOP
Kuraray Noritake Dental
00157A
MDP, monomer solvent
Porcelain Liner M Liquid B
PLB
Sun Medical; Moriyama, Japan
RS1
3-TMSPMA
9.5% hydrofluoric acid gel
MDP: 10-methacryloyloxydecyl dihydrogen phosphate; 3-TMSPMA: 3-trimethoxysilylpropyl methacrylate; HEMA: 2-hydroxyethyl methacrylate; bis-GMA: bisphenol A-glycidyl methacrylate.
Ø 5.0 mm
Zirconia
2.5 mm Ø 5.0 mm
Ø 11.0 mm
a
d
e
Ø 6.0 mm 2.0 mm
Ø 5.0 mm
g
Ø 5.0 mm
f
Ø 6.0 mm 2.0 mm 5° C
h
Surface roughness of all subgroups was measured using a profilometer (Surfcom 480A-12, Tokyo Seimitsu; Tokyo, Japan) with 2-μm-radius stylus tip. Cut-off length and transverse length were set at 0.8 mm and 4.0 mm, respectively. The specimens of all subgroups were scanned five times, and the average of Ra values was determined. Vol 17, No 3, 2015
c
Load
Fig 1 Experimental design. (a) Airborne-particle abrasion of zirconia disks; (b) delimitation of coating area with plastic tape; (c) layering of porcelain slurry; (d) removal of the plastic tape; (e) firing of the porcelain; (f) airborne-particle abrasion or 9.5% hydrofluoric (HF) acid etching of specimen surface; (g) applying a piece of double-coated tape and a stainless steel ring around the bonding area; (h) filling the indirect composite material into the ring and polymerization; (i) 100,000 thermocycles between 5°C and 55°C; (j) shear-bond testing.
b
55° C
i
j
For all subgroups, the specimens were divided into 5 groups containing 11 specimens each (n = 11). The surface of each specimen was then treated with one of the four priming agents, namely, Clearfil Photo Bond (CPB, Kuraray Noritake Dental), Clearfil Photo Bond with Clearfil Porcelain Bond Activator (CPB+Activator, Kuraray Noritake Dental), Estenia Opaque Primer (EOP, Kuraray Noritake 275
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Table 2 Shear bond strength (MPa) between Estenia C&B composite and Katana zirconia after 100,000 thermocycles Groups
ZR-AB
ZR-PO-AB
ZR-PO-HF
Median
Mean (SD)
Median
Mean (SD)
Median
Mean (SD)
0.0
0.1 (0.2)a,A
0.2
0.3 (0.2)d,A
13.5
14.1 (4.2)g,B
EOP
11.5
11.5
(2.6)b,C
0.3
(0.1)d,D
14.9
15.8 (4.1)g,E
PLB
12.2
12.3 (2.5)b,F
15.9
15.3 (3.7)e,F
16.2
15.8 (4.3)g,F
CPB
18.3
19.7
(4.0)c,G
0.2
(0.1)d,H
16.5
15.7 (3.6)g,G
CPB + Activator
21.9
20.5 (4.5)c,I
24.7
25.8 (3.4)f,J
22.1
22.4 (4.6)h,I,J
CON
0.3
0.2
ZR: Zirconia; AB: airborne-particle abrasion; PO: feldspathic porcelain; HF: hydrofluoric acid; CPB: Clearfil Photo Bond; Activator: Clearfil Photo Bond Activator; EOP: Estenia Opaque Primer; PLB: Porcelain Liner M Liquid B; CON: no priming (control). Identical superscript upper-case letters in the same row indicate that the values are not statistically different (Steel-Dwass test, p > 0.05). Identical superscript lower-case letters in the same column indicate that the values are not statistically different (Steel-Dwass test, p > 0.05).
Dental), and Porcelain Liner M Liquid B (PLB, Sun Medical; Moriyama, Japan), according to manufacturers’ instructions or not treated at all (CON, control). Except for the EOP and CON groups, each group was primed with EOP before bonding the opaque material to zirconia specimens. After surface treatment of the specimens, the bonding area was determined by applying a piece of doublecoated tape (Kincsem, Horse Care Products; Tokyo, Japan) containing a circular hole (diameter 5.0 mm). For all specimens in each group, a thin layer of opaque material (Estenia C&B opaque OA2, Kuraray Noritake Dental) as a high-flow bonding agent was applied and light cured for 90 s using a laboratory light-curing unit (α-Light II, J. Morita; Suita, Japan). An additional opaque material was placed on top of the primary opaque layer in the same manner. A stainless steel ring (inside diameter 6.0 mm; depth 2.0 mm) was then positioned to surround the opaque material (Fig 1g). A dentin shade of indirect composite material (Estenia C&B Dentin DA2, Kuraray Noritake Dental) was packed into the ring with a force of 5 N (Fig 1h). The specimen was light cured with the same polymerization unit for 5 min and then heat polymerized at 110°C for 15 min in a heat oven (KL-310, J. Morita). The specimens from each group were stored in distilled water at 37°C for 24 h, then placed in an automated thermocycling device (Thermal Shock Tester TTS-1 LM, Thomas Kagaku; Tokyo, Japan) for 100,000 cycles with water bath temperatures between 5°C and 55°C and a dwell time of 1 min (Fig 1i). Each specimen was subsequently positioned in a steel mold and placed in a shearbond testing jig (Tokyo Giken; Tokyo, Japan). Shear bond strength was tested in a universal testing device (Type 5567, Instron; Canton, MA, USA) at a crosshead speed of 0.5 mm/min (Fig 1j). Load at fracture was converted to shear bond strength (MPa) by dividing failure load (N) by bonding area (mm2). The results were primarily analyzed by the Levene test for equality of variance using statistical software (SPSS version 15.0; SPSS; Chicago, IL, USA). When the Levene test did not show homoscedasticity, the results were then analyzed with the Kruskal-Wallis test (SPSS version 15.0), followed by the Steel-Dwass test for multiple comparisons (Kyplot 5.0, KyensLab; Tokyo, Japan). The significance level was set at 0.05 in all analyses. 276
The fractured interfaces of tested specimens were examined with a stereomicroscope (32X) (StemiDV4, Carl Zeiss; Jena, Germany) to determine mode of failure after shear bond testing. Failure mode for the ZR-AB group was classified as A (adhesive failure at the zirconia/composite material interface) or B (combined adhesive and cohesive failure within the composite material). The failure mode for the ZR-PO-AB and ZR-PO-HF groups was classified as C (adhesive failure at the porcelain/composite material interface), D (adhesive failure at the porcelain/composite material interface, with cohesive failure inside the porcelain), or E (cohesive failure within the porcelain). Representative fractured specimens were osmium coated with a sputter coater (HPC-IS, Vacuum Device; Mito, Japan) and examined with a scanning electron microscope (SEM) (S-4300, Hitachi High-Technologies; Tokyo, Japan) operated at 15 kV. In addition, the specimens of all subgroups were embedded in acrylic resin (Unifast III, GC; Tokyo, Japan) and cut using a precision saw (Isomet Low Speed Saw, Buehler; Lake Bluff, IL, USA) equipped with a diamond wafering blade (15LC, Buehler). The interface between the indirect composite material and layering porcelain or zirconia of the representative specimens in all groups was observed with SEM.
RESULTS The shear bond strengths after thermocycling between the Estenia C&B composite and Katana zirconia material are presented in Table 2. The mean bond strength varied from 0.1 to 20.5 MPa in ZR-AB specimens, from 0.2 to 25.8 MPa in ZR-PO-AB specimens, and from 14.1 to 22.4 MPa in ZR-PO-HF specimens. Among ZR-AB specimens, the CPB (19.7 MPa) and CPB+Activator (20.5 MPa) groups had significantly higher bond strengths than the other groups. For ZR-PO-AB and ZR-PO-HF specimens, the CPB+Activator (25.8 MPa and 22.4 MPa, respectively) group had the highest bond strength of all groups. In the CPB+Activator group, the bond strength of ZR-PO-AB specimens was significantly higher than that of ZR-AB specimens. In addition, in the CON group, bond strength was significantly higher for the ZR-PO-HF specimens than for the other two subgroups. The Journal of Adhesive Dentistry
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Table 3 Mean (SD) surface roughness values Ra of the zirconia surface after the different surface pretreatments Specimens
Surface roughness (μm) (SD)
ZR-AB
0.49 (0.06)a
ZR-PO-AB
3.83 (0.54)b
ZR-PO-HF
2.85 (0.22)c
For abbreviations, see Table 2. Different superscript letters indicate significant difference (Steel-Dwass test, p > 0.05).
Table 4 Failure modes after 100,000 thermocycles and shear bond testing
a
Failure type Groups
Priming agents
A
ZR-AB
CON
11
ZR-PO-AB
ZR-PO-HF
B
C
D
E
CON
11
0
0
EOP
11
0
0
PLB
0
8
3
CPB
11
0
0
CPB + Activator
0
5
6
CON
2
8
1
EOP
1
8
2
PLB
0
9
2
CPB
2
8
1
CPB + Activator
0
6
5
0
EOP
9
2
PLB
10
1
CPB
7
4
CPB + Activator
7
4
See Table 2 for abbreviations. A: Adhesive failure at zirconia/composite material interface. B: Combined adhesive failure at zirconia/composite material interface and cohesive failure within composite material. C: Adhesive failure at porcelain/composite material interface. D: Adhesive failure at porcelain/composite material interface, with cohesive failure within the porcelain. E: Cohesive failure within the porcelain.
The surface roughness (Ra) of the zirconia surface after the different surface pretreatments are shown in Table 3. The surface roughness of ZR-PO-AB was significantly higher than that of the other two specimens. Table 4 shows the failure modes after shear bond testing. For ZR-AB specimens, adhesive failure at the zirconia/ composite material interface was observed in the CON group. Combined cohesive failure within the composite material and adhesive failure at the zirconia/composite material interface was detected in the other four groups (ie, EOP, PLB, CPB, and CPB+Activator). Cohesive failure within the porcelain was noted for ZR-PO-AB specimens Vol 17, No 3, 2015
b
c Fig 2 SEM images of (a) a zirconia surface after airborneparticle abrasion (ZR-AB); (b) a feldspathic porcelain-coated zirconia surface after airborne-particle abrasion (ZR-PO-AB); (c) a feldspathic porcelain-coated zirconia surface after HF acid etching (ZR-PO-HF) (original magnification 1000X).
in the CPB+Activator and PLB groups, and for ZR-PO-HF specimens in all groups. The ZR-PO-AB specimens in the CON, CPB, and EOP groups exhibited adhesive failure at the porcelain/composite interface. Figure 2 shows SEM images of the zirconia surface after the different surface pretreatments. As seen in Fig 2a and 2b, the surface of ZR-AB and ZR-PO-AB specimens was roughened and had sharp edges after airborne-par277
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Fig 3 SEM image of the debonded surface of a ZR-AB specimen from the Porcelain Liner M Liquid B (PLB) group after 100,000 thermocycles and shear bond testing (original magnification 1000X).
Fig 4 SEM image of the debonded surface of a ZR-AB specimen from the EOP group after 100,000 thermocycles and shear bond testing (original magnification 1000X).
Fig 5 SEM image of the debonded surface of a ZR-PO-AB specimen from the Clearfil Photo Bond (CPB) group after 100,000 thermocycles and shear bond testing (original magnification 1000X).
Fig 6 SEM image of the debonded surface of a ZR-PO-HF specimen from the CPB+Activator group after 100,000 thermocycles and shear bond testing (original magnification 1000X).
ticle abrasion with 50-μm Al2O3 particles. The surface of ZR-PO-AB specimens was rougher than that of ZR-AB specimens. In contrast, ZR-PO-HF specimens had major and minor irregularities and a honeycombed appearance (Fig 2c). Figures 3 and 4 show representative SEM images of the fractured interfaces of ZR-AB specimens after shear bond testing. Figure 3 shows adhesive failure. The composite material is completely separated from the zirconia surface, and the airborne-particle abraded surface can be seen. In contrast, the images of combined cohesive and adhesive failure show that the zirconia surface is partially covered with composite material (Fig 4). Figures 5 to 7 are SEM images of the debonded surface of ZR-PO-AB and ZR-PO-HF specimens after shear bond testing. Figure 5 depicts the debonded surface of adhesive failure at the porcelain/composite material interface. The surface
appearance is similar to that before priming in Fig 2b. An SEM image of adhesive failure at the porcelain/composite material interface with cohesive failure inside the porcelain (Fig 6) reveals part of the surface before priming. Figure 7 shows the surface of a specimen with cohesive failure within the porcelain and demonstrates the different appearance of the surface before priming and a thin layer of layering porcelain covering the debonded surface. The SEM images in Fig 8 show the interface between the indirect composite material and zirconia or layering material for the representative specimens. The bonding interface of indirect composite to the zirconia showed larger gaps and more defects (Fig 8a) compared with that of indirect composite to the layering porcelain (Figs 8b and 8c). For Figs 8b and 8c, the penetration of the composite material into layering porcelain in ZR-PO-HF was deeper than that of ZR-PO-AB.
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a Fig 7 SEM image of the debonded surface of a ZR-PO-AB specimen from the CPB+Activator group after 100,000 thermocycles and shear bond testing (original magnification 1000X).
DISCUSSION The present study evaluated long-term shear bond strengths between an indirect composite and zirconia framework coated with silica-based ceramics. The results of the present study indicate that the bond strength was influenced by coating with silica-based ceramics and application of priming agents. Thus, we confirm the working hypotheses that coating zirconia with silica-based ceramics influences long-term bond strength, and that long-term bond strength significantly differs with different priming agents. The results of the present study demonstrate that HF etching of the surfaces of zirconia frameworks coated with silica-based ceramics significantly enhanced bond strength durability between an indirect composite and the framework. Analysis of failure mode revealed cohesive failure within the porcelain in ZR-PO-HF specimens in all groups. In contrast, among ZR-PO-AB specimens, the failure mode of all specimens in the CON, CPB, and EOP groups was adhesive at the porcelain/composite material interface. One possible reason for this behavior might be that HF acid etching of the porcelain-coated zirconia surface enhanced micromechanical interlocking. The SEM image of ZR-PO-HF specimens, which showed that the penetration of the composite material into layering porcelain was deeper than that of ZR-PO-AB specimens, can confirm this finding. Previous studies found that the roughness caused by HF acid etching of silica-based ceramics improved the bond strength between composite resin and porcelain.8 Kato et al18 reported that airborneparticle abrasion of porcelain did not result in substantial mechanical retention, although the surface may be more retentive than polished or glazed surfaces. A silane coupling agent (3-TMSPMA) provided durable bond strengths of an indirect composite to a zirconia framework coated with silica-based ceramics, regardless of pretreatment with either airborne-particle abrasion or HF acid etching. This confirms findings of a previous Vol 17, No 3, 2015
b
c Fig 8 SEM images of (a) the interface of zirconia and indirect composite material (ZR-AB); (b) the interface of indirect composite material and layering porcelain for ZR-PO-AB; (c) the interface of indirect composite material and layering porcelain for ZR-PO-HF.
study, which noted that a siloxane network was successfully formed between an indirect composite material and zirconia coated with feldspathic porcelain.14 Several previous studies reported that siloxane network formation 279
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was more important than mechanical interlocking in the adhesion of composite to porcelain.4,22 The ZR-PO-AB and ZR-PO-HF specimens from the CPB+Activator group had the highest bond strength among the five groups. CPB+Activator contains a phosphate monomer (MDP) as well as a silane coupling agent (3-TMSPMA). Because formation of the siloxane network was facilitated by the acidic functional monomer, the durability of bonds between the indirect composite and zirconia ceramics was enhanced.4 This is supported by our observation that the incidence of cohesive failure within the porcelain was higher than in the other groups. For zirconia frameworks coated with silica-based ceramics, application of hydrophobic phosphate monomer (MDP) and a silane coupling agent should be considered to enhance bond strengths and their durability. The CPB and CPB+Activator groups had significantly higher bond strengths than the other groups of ZR-AB specimens, which indicates that application of MDP and a polymerization initiator results in durable bond strength between an indirect composite and zirconia framework. A chemical reaction might occur between the phosphate ester groups of the adhesive monomer MDP and the hydroxyl groups on the zirconia ceramic surface.19,43 The rate of combined adhesive failure at the zirconia/composite material interface and cohesive failure within the composite material was higher in the CPB and CPB+Activator groups than in the other groups. Previous studies of initial and longer-term bond strength reported similar results.7,21 Gale and Darvell15 estimated that 10,000 thermocycles can be correlated to one year of clinical service. A large number of thermal cycles were therefore applied in this study to further stress the bonding interfaces and to simulate multiple years of intraoral aging. After thermocycling, the mean bond strength in ZR-PO-AB specimens of the PLB and CPB+Activator groups and in ZR-PO-HF specimens of all groups was greater than 13 MPa, which suggests that coating the zirconia framework with feldspathic porcelain is an effective method of reaching a critical threshold for clinically successful bond strength between an indirect layering composite material and a zirconia framework.25 To eliminate chipping of layering porcelain in zirconiabased restoration, the “CAD-on” technique or monolithic zirconia restorations have currently received attention as an alternative to zirconia layered with feldspathic porcelain. In the CAD-on technique, a zirconia framework and layering ceramics are designed and milled together using CAD/CAM, and then the two parts are combined using a glass ceramic5,17,38 or a luting agent.13 Monolithic zirconia restorations have been introduced as an obvious way to circumvent the chipping of layering porcelain for replacing the bilayered structure (veneer/core) with a monolithic zirconia ceramic.33,44 Although these two approaches showed promising results in in vitro studies,5,17,38,44 long-term clinical studies are required to verify the clinical performance of the CAD-on technique or monolithic zirconia restorations. The primary objectives of in vitro studies are elimination of influential parameters and limitation of variables. 280
All in vitro studies have limitations, and randomized clinical trials are the ultimate tool to evaluate the benefits of a clinical procedure. Numerous factors, such as a framework design and 3-dimensional geometry of the restoration, may influence long-term clinical outcomes of zirconia-based composite-layered restorations and cannot be evaluated in an in vitro study. While the present study provides valuable information on possible future approaches to enhance composite/zirconia bonding interfaces, further laboratory and especially clinical studies are necessary before any of these protocols can be recommended for use in clinical practice.
CONCLUSIONS Within the limitations of this in vitro study, the following conclusions can be drawn: 1. Silica-based ceramic coating of zirconia specimens provides equal or superior long-term bond strength between indirect composite and zirconia compared with zirconia specimens without coating. 2. Combined application of a hydrophobic phosphate monomer (MDP) and a silane coupling agent enhances durable long-term bond strength between indirect composite material and zirconia specimens coated with silica-based ceramics.
ACKNOWLEDGMENTS This study was supported in part by JSPS KAKENHI, grant number 24592933, a grant from the Sato Fund, Nihon University School of Dentistry (2013 and 2014), and a grant from the Promotion and Mutual Aid Corporation for Private Schools of Japan (2014).
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Clinical relevance: Combined application of MDP and silane coupling agent is useful for bonding indirect composite material to a zirconia coated with silicabased ceramics.
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