Evaluation of a Commercial Primer for Bonding of Zirconia to Two Different Resin Composite Cements Chen Chena / Haifeng Xieb / Xin Songc / Michael Francis Burrowd / Gang Chene / Feimin Zhangf Purpose: To evaluate the effects of a commercial zirconia primer (Choice or RelyX Unicem) on shear bond strength (SBS) of two different resin composite cements – Choice (a conventional bis-GMA-based resin cement) and RelyX Unicem (self-adhesive resin cement) – to zirconia. Materials and Methods: Zirconia blocks were manufactured and randomly divided into 5 main groups (n = 20) that received surface treatments and cements as follows: no surface treatment, Choice and RelyX Unicem (groups C and U, resp.); tribochemical silica coating followed by silanization, Choice (group SSC); application of a zirconia primer, Choice and RelyX Unicem (groups ZC and ZU, resp.). Light-curing composite resin cylinders were prepared and bonded on the prepared zirconia blocks using the two different resin cements. Half of the specimens in each group were stored in water for 24 h, and half were aged by 50 days of water storage followed by thermocycling (12,000 cycles between 5°C and 55°C). Thereafter, all of them were submitted to the SBS test. Fourier transmission infrared (FT-IR) spectrum analysis and gas mass spectrometry (MS) analysis were adopted for characterization of the zirconia primer. Results: Statistical analysis of the SBS test showed that group C presented the lowest SBS values and group SSC the highest (p < 0.01). Artificial aging exerted no influence on the SBS of groups U, SSC, ZC, or ZU. FT-IR analysis suggested that benzene rings and carboxylic groups exist in the zirconia primer. MS analysis detected that 2-hydroxyethyl methacrylate, triethylamine, ethyl-4-dimethylaminobenzoate, ethanol, and water are contained in the primer. Conclusion: The zirconia primer and self-adhesive resin cement increased the SBS of zirconia. Keywords: zirconia, ceramic, bond, adhesion, chemical, primer, surface treatment. J Adhes Dent 2014; 16: 169-176. doi: 10.3290/j.jad.a31809
Y
ttria-stabilized tetragonal zirconia polycrystals (Y-TZP) offers a wide variety of clinical applications due to their higher strength; however, its difficulty in adhering to composite resin cements via conventional cementing methods is still one of the major limitations to its use.2,4,14,21 Many surface treatments to improve the bonding of Y-TZP have been developed, including creating a microretentive
a
Lecturer, Institute of Stomatology, Nanjing Medical University, Nanjing, China. Performed the experiments.
b
Lecturer, Institute of Stomatology, Nanjing Medical University, Nanjing, China. Experimental Design, wrote manuscript.
c
PhD Student, Institute of Stomatology, Nanjing Medical University, Nanjing, China. Performed the experiments.
d
Professor, School of Dental Science, University of Hong Kong, Hong Kong, China. Proofread manuscript.
e
Ph D Student, Institute of Stomatology, Nanjing Medical University, Nanjing, China. Performed statistical evaluation.
f
Professor, Institute of Stomatology, Nanjing Medical University, Nanjing, China. Idea.
Correspondence: Professor Feimin Zhang, Institute of Stomatology, Nanjing Medical University, No. 136, Han Zhong Road, Nanjing, JiangSu Province, China 210029. Tel: +86-25-8503-1831, Fax: +86-25-8651-6414. e-mail: [email protected]
Vol 16, No 2, 2014
Submitted for publication: 23.05.12; accepted for publication: 08.08.13
surface texture through sandblasting, hot acid etching, selective infiltration etching, laser etching, sintering microporcelain pearls, nano-structured alumina coating, and zirconia coating.1,3,5,16,32 Just as with silica-based ceramics, strong resin bonding of Y-TZP relies on chemical adhesion as well as micromechanical interlocking.3,16 As is known, silane is capable of forming chemical bonds with organic and inorganic surfaces. Its bonding to resin occurs by an addition polymerization reaction between methacrylate groups in the matrix resin and alkene bonds in the silane molecule during light or chemical curing, whereas bonding to ceramics occurs via a condensation reaction between Si-OH groups on the ceramic surface and Si-OH groups in hydrolyzed silane molecules, yielding siloxane bonds and water molecules as by-products.3,24 However, because fewer or no hydroxy groups exist in Y-TZP, a coating containing SixOy must be formed on the bonding surface of Y-TZP before silanization.3,5,16 The literature reports several methods for SixOy coating, such as the conventional pyrolytical method developed in 1984, and later the tribochemical method.30 In recent years, some new, tentative coating techniques have been introduced to increase the silica content on Y-TZP surfaces. According to the study by Derand et al,8 the bond strength of Y-TZP 169
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Table 1
Summary of experimental groups
Group
Surface pre-treatment
Primer
Resin cement
C
Sandblasting*
None
Choice
U
Sandblasting
None
RelyX Unicem
SSC
Tribochemical silica coating using Cojet system*
Silane coupling agent
Choice
ZC
Sandblasting
Z-Prime Plus
Choice
ZU
Sandblasting
Z-Prime Plus
RelyX Unicem
*Vertically sandblasted with 110-μm alumina particles for 20 s at 3 bar from a distance of 10 mm using a sandblasting device (Lndp-III, Jianian Futong Medical Equipment; Tianjin, China). **Vertically sandblasted with 30-μm Cojet particles for 15 s at 3 bar from a distance of 10 mm (3M ESPE; St Paul, MN, USA).
almost tripled through deposition of hexamethyldisiloxane ((CH3)3SiOSi(CH3)3) via plasma spraying techniques. Another innovative approach is sol-gel technique to form silica coating.16,22,33,34 However, there is no doubt that either extra silica coating or complex surface-roughening methods for Y-TZP would be time consuming and technique sensitive. Dentists prefer more convenient methods in practice, such as using resins containing phosphate monomers, or phosphoric acid acrylate primers.27 In pursuit of the goals of optimizing the bond as well as simplifying application procedures, new adhesive resin cements or primers for Y-TZP conditioning have been developed. Recently, a commercial single-bottle zirconia primer (Z-Prime Plus, Bisco; Schaumburg, IL, USA) has been introduced which, according to the manufacturer, contains biphenyl dimethacrylate and hydroxyethyl methacrylate and can be used as a surface treatment for zirconia and alumina metal oxide ceramics as well as other types of metals/alloys to increase resin bond strength without Table 2
applying a silica coating and silane.23 Unfortunately, the manufacturer disclosed little of its product information, especially its active ingredients. The aim of this study was thus to evaluate the enhancement of the primer Z-Prime Plus on the shear bond strength (SBS) of Y-TZP to two different resin composite cements (one conventional, based on bis-GMA; one self-adhesive), and discuss the chemical role of this zirconia primer in improving the bonding of Y-TZP. The hypothesis is that this zirconia primer would improve the SBS of Y-TZP to both resin composite cements.
MATERIALS AND METHODS One hundred Y-TZP (Everest ZS-Ronde, KaVo; Biberach, Germany) blocks (12 mm x 8 mm x 2 mm) were produced and randomly assigned to 5 groups (n = 20), according to surface treatment, primer, and resin cement used: C: sandblasting, no primer, Choice resin cement (Bisco; Schaumburg, IL, USA); U: sandblasting, no primer, RelyX Unicem cement (3M ESPE; Seefeld, Germany; SSC: tribochemical silica coating (Cojet, 3M ESPE), silane coupling agent, Choice; ZC: sandblasting, Z-Prime Plus (Bisco), Choice; ZU: sandblasting, Z-Prime Plus, RelyX Unicem (Table 1). One hundred resin cylinders were prepared as follows: nylon tubes with an inner diameter of 5 mm and a height of 2 mm were filled with light-curing composite resin (Valux Plus, 3M ESPE) using a spatula and were photopolymerized for 20 s using an LED lamp (Elipar Freelight 2; 3M ESPE). Then the polymerized resin cylinders were removed from the nylon tubes for the subsequent tests. A layer of resin cement was applied to the prepared Y-TZP block, and a composite resin cylinder was pressed on the resin cement using a continual compressive force. The resin cement was light cured for 2 to 3 min to reach an initial setting. The excess resin cement in this state was then easily removed with a probe; final light curing was performed for 20 s. The details of the materials are presented in Table 2. Each bonded specimen was embedded in a self-curing acrylic resin base.
Description of main materials used*
Material/Trade name
Main compositions
Manufacturer
Light-curing composite resin cement/ Choice
Filler: Glass frit (40-70 wt%), amorphous silica (10-40 wt%) Base resin: bisphenol A-glycidyl methacrylate (Bis-GMA) (5-30 wt%)
Bisco; Schaumburg, IL, USA
Dual-polymerized, self-adhesive resin cement/RelyX Unicem
Liquid: Methacrylate monomers containing phosphoric acid groups (MP), dimethacrylate Powder: Glass powder, silanated silica, calcium hydroxide
3M ESPE; Seefeld, Germany
Adhesive primers, specific to oxidebased materials/Z-Prime Plus
Biphenyl dimethacrylate (BPDM), 2-hydroxyethyl methacrylate (HEMA), ethanol
Bisco
Silane coupling agent/Porcelain Primer
Ethanol (30-70%), acetone (30-70%); silane (1-10%)
Bisco
Light-curing composite resin/ Valux Plus
Filler: zirconia/silica (66 vol%) Base resin: bis-GMA, triethylene glycol dimethacrylate
3M ESPE
*From the material safety data sheet or technical data sheet.
170
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a
SEM images (magnification 600X) of the sandblasted zirconia surface treated with zirconia primer (left) or not (right).
Half of the bonded specimens were stored in distilled water at room temperature for 24 h and the other half was aged by storing in water (25°C) for 50 days, followed by thermocycling (12,000 cycles between 5°C and 55°C). Then, all the specimens were submitted to the SBS test (universal testing machine, Instron 3365, ElectroPuls; Norwood, MA, USA) with a reglet tap at a crosshead speed of 1.0 mm/min. One-way ANOVA and the Least Significant Difference (LSD) test for multiple comparisons were used for statistical analysis, performed with SPSS 17.0 (Chicago, IL, USA). The failure mode was observed using a microscope (WYSK-100X, Shu Yao Equipment; Shanghai, China) at 10X magnification. The modes of failure were classified as adhesive (fracture occurred at the resin/ resin cement and Y-TZP interface), cohesive (fracture occurred within the resin/resin cement), or mixed (both adhesive and cohesive failure occurred). Typical specimens were examined with a scanning electron microscope (SEM) (S-4800, Hitachi; Tokyo, Japan). Two additional Y-TZP blocks were produced and alumina sandblasted. One of them was conditioned with Z-Prime Plus and another was not. Both of them were examined at 600X magnification using SEM. Fourier transmission infrared (FT-IR) spectrum analysis (Nexus 870FT; Los Angeles, CA, USA) in transmission mode and gas mass spectrometry (MS) analysis (6890 Series GC system, Agilent Technologies; Santa Clara, CA, USA) were employed for characterization of Z-Prime Plus.
RESULTS SEM images of the sandblasted Y-TZP surfaces before and after treatment with the zirconia primer are shown in Fig 1. The FT-IR spectrum of Z-Prime Plus is depicted in Fig 2. As shown, the peaks located at 1637.2, 1510.2, and 1455.1 cm-1 are associated with benzene rings, the peak located at 1609.6 cm-1 is associated with stretching of the C=C linkage, the peak located Vol 16, No 2, 2014
100 90 80 70
% Reflectance
Fig 1
b
60 50 40 30 20 10 0 4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm–1)
Fig 2
FT-IR spectrum of Z-Prime Plus.
at 3419.9 cm-1 is associated with stretching of the -OH group, and the peak located at 1717.1 cm-1 is associated with stretching of the C=O group. The groups C=C, -OH, and C=O suggest the possible existence of carboxylic groups. the MS spectra of Z-Prime Plus are presented in Fig 3. A search of the NIST08.L computer database showed that 2-propenoic acid, 2-methyl-, 2-hydroxyethyl ester, triethylamine, ethyl-4-dimethylaminobenzoate, ethanol, and water are found in Z-Prime Plus. After aging, all specimens of groups U, SSC, ZC, and ZU remained bonded, while debonding occurred in half the specimens of group C. Mean (± SD) SBS values before and after aging are illustrated in Fig 4. One-way ANOVA revealed that SBS values from 24-h water storage (F = 31.3, p = 0.0001) and aging (50-day water storage, thermocycling; F = 55.1, p = 0.0001) differed significantly among the experimental groups considering the surface treatment factor. According to the results of LSD tests, group C presented the lowest SBS values (p < 0.01), group SSC presented the highest (p < 0.01), 171
Chen et al
600000 550000 500000 450000 400000 350000 300000 250000 200000 150000 100000 50000
a
2.00
4.00
6.00
8.00
10.00
12.00 14.00 16.00 18.00 20.00 22.00 24.00 148.0
m/z Æ
Benzoid acid, 4-(dimethylamino)-, ethyl ester
9000 8000 7000 193.1 6000
164.0
5000 4000 3000 2000
77.0
104.0
42.0
1000 15.1
29.1
51.0
91.0
65.0
120.0 132.0
178.0
0
b
10
30
50
70
90
110
69.1
41.1
130
150
170
190
2-Propenoic acid, 2-methyl-, 2-hydroxyethyl ester
9000 8000 7000 6000 87.0
5000 4000 3000 2000 1000
c
0 5 m/z Æ
31.1 26.1 15.1 15
99.0
46.0 53.0 59.0 25
35
45
55
112.0
74.0 81.9 65
75
85
95
105
115
86.1
Triethylamine 9000 8000 7000 6000 5000 4000 3000
30.1
58.0
2000 1000
d
0 m/z Æ 5
172
101.0
42.0 15.0 15
70.0 25
35
45
55
65
75
85
95
105
Fig 3 a) MS spectra of Z-Prime Plus; b) spectrum matched with standard map of benzoic acid, 4-(dimethylamino)-, ethyl ester; c) spectrum matched with standard map of 2-propenoic acid, 2-methyl-, 2-hydroxyethyl ester; d) spectrum matched with standard map of triethylamine.
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16
MPa
13.36 (2.03)c
before aging after aging
11.99 (2.49)c
14 9.56 (2.55)b
12
8.77 (2.34)b 9.27 (0.92)b 8.17 (1.71)b
7.79 (2.08)b
10 7.70 (0.02)b
8
6 3.10 (1.26)a
4 0.85 (1.44)d
2
a 0 group
C
U
SSC
ZC
ZU
Fig 4 Bar graph of SBS values as means (standard deviation) before and after aging (n=10). Different letters over bars indicated statistically significant differences. C: Choice; U: RelyX Unicem; SSC: tribochemical silica coating followed by silanization, Choice; ZC and ZU: application of a zirconia primer, Choice and RelyX Unicem, resp.
Table 3 Failure modes Group
Artificial aging
C
before after
U
SSC
ZC
ZU
Adhesive failure
Cohesive failure
Mixed failure
10
0
0
10
0
0
before
4
1
5
after
0
2
8
before
6
0
4
after
0
3
7
before
3
0
7
after
0
2
8
before
2
2
6
after
0
0
10
and the other three groups, in which Z-Prime Plus and/ or RelyX Unicem were used, presented intermediate SBS values (p < 0.01) both before and after aging. One-way ANOVA also showed that the combination of 50-day water storage and 12,000 thermal cycles exerted no statistical influence on the SBS of groups U, SSC, ZC, and ZU (p < 0.05), while post-aging SBS values were lower in group C than before aging. The failure modes are shown in Table 3. Typical SEM images of the fracture interface are shown in Fig 5. Vol 16, No 2, 2014
b
c Fig 5 SEM images of fracture interfaces. a) adhesive failure, (14X magnification); b) mixed failure (15X magnification); c) cohesive failure (15X magnification).
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DISCUSSION Original roughness produced by milling during manufacture is reportedly not sufficient to promote adhesion between Y-TZP and resin. The most commonly used method of increasing their bonding is to create a rougher surface texture through air abrasion with alumina particles.3,5,32 According to a study by Kern,15 a combination of either lower-pressure or higher-pressure air abrasion and priming improved long-term resin bonding to Y-TZP significantly, while omitting air abrasion resulted in debonding during artificial aging, independent of primer use. Although some authors reported that alumina sandblasting would negatively influence the mechanical strength, the positive effects of alumina sandblasting are widely recognized, include increasing the surface roughness, surface energy, and wettability, increasing the surface area for bonding, and even enhancing the mechanical strength caused by phase transformation toughening.3,23,33 Therefore, standardization of the Y-TZP surfaces was achieved by using the alumina air-abrasion technique instead of polishing in current study. In the SBS test, half of the specimens of group C, in which only the conventional bis-GMA-based resin cement, Choice, was applied, debonding occurred after aging. Failure mode analysis also showed exclusively adhesive failure in this group. The SEM image of sandblasted Y-TZP also showed that because of the high rigidity of Y-TZP, it was not possible to obtain a well-roughened surface through alumina sandblasting. Thus, a lack of sufficient micromechanical interlocking and chemical adhesion may be the main reason for the weaker bond strength and poor bond durability. It is generally believed that some adhesive monomers present in luting agents or zirconia/metal primers provide a possible chemical bonding mechanism with metal oxides such as alumina and zirconia.22-24,37 The chemical adhesion results through the reaction between acidic functional groups in the adhesive monomers and oxide groups on the Y-TZP surface, which is similar to the reaction between silane and silica-based ceramics.11,23,36 These kinds of adhesive monomers typically include the carboxylic acid group (-COOH) or its anhydride, and the phosphoric [-O-P(=O)(OH)2] or phosphonic acid group [-P(=O)(OH)2].11,36 Representative examples are 10-methacryloyloxydecyldihydrogenphosphate (10-MDP) and 4-methacryloyloxyethyl trimellitic acid (4-MET).11,25,28,37 According to current SBS test results, no matter which resin cement was used, application of the zirconia primer always provided higher SBS values compared to the control group. Referring to the product’s safety data sheet, the main functional components of Z-Prime Plus are biphenyl dimethacrylate (BPDM) and 2-hydroxyethyl methacrylate (HEMA). BPDM features a double aromatic ring structure.10,29 HEMA is a low-molecularweight monomer, which is used in many adhesives for its positive influence on the bond strength.9,38 One study reported that Z-Prime Plus combined both phosphate and carboxylic monomers, with the former bonding chemically with oxide groups on the Y-TZP surface, and the latter participating in the development of the chemical bonds.23 174
However, the FT-IR analysis of the current study could not detect phosphate groups in Z-Prime Plus. As shown in Fig 2, carboxylic groups might exist in Z-Prime Plus, but whether the carboxylic groups contributed mainly to the chemical bonds between Z-Prime Plus and Y-TZP needs to be addressed by further tests. The MS analysis in the present study identified several components of Z-Prime Plus. The main functions of these components are as follows: Ethanol and water are the solvents. Ethyl-4-dimethylaminobenzoate is often used in curing systems in combination with other photoinitiators, or used as an intermediate in organic synthesis. HEMA is often used in dentin adhesive systems and has been described above. Triethylamine is often used for organic synthesis of materials, or as an organic solvent and wetting agent. Therefore, there is still no evidence to suggest that Z-Prime Plus could condition zirconia surfaces via the chemical reaction between phosphate monomer and zirconia. Choosing the right resin cement has been considered to be another important factor in achieving adhesion to zirconia or alumina-based ceramics in practice, since they are difficult to condition.20,35 In the current study, one self-adhesive resin cement and one conventional bis-GMA-based resin cement were evaluated. According to the chemical formula, two functional phosphoric acid groups are contained in the functional monomer of the self-adhesive resin cement RelyX Unicem. The manufacturer claims the special molecular formula provides high reactivity and excellent mechanical properties. Theoretically, the main chain structure of a polymer decides its molecular flexibility. Higher molecular flexibility means greater molecular motion, which brings the molecules in the adhesive system closer together and increases their adsorbability.26 The main chain structure of methacrylate monomers in the RelyX Unicem resin cement consists in single C-C bonds. Because internal rotation occurs in each bond, this kind of polymer theoretically possesses higher flexibility and better adhesion. Choice is a conventional bis-GMA-based material. In the molecular formula of bis-GMA, two benzene rings exist in the main chain structure, which induces higher rigidity because of the weaker molecular motion of the heteroaromatic ring compared with the single C-C bond. The monomer of RelyX Unicem contains at least two phosphoric acid groups and a minimum of two double-bonded carbon units (C=C) per molecule; thus, it might seem logical that this self-adhesive resin cement might provide better adhesion to Y-TZP. Conforming to this, the current SBS test showed increased SBS values when this self-adhesive resin cement was used. According to the results of the failure mode analysis, cohesive and mixed failures occurred in all bond specimens of groups U, SSC, ZC, ZU, and exclusively adhesive failure occurred in almost all bonded specimens of group C, which suggested that the resin/ceramic bond interface was similar to or even stronger than the cohesive strength of the resin cement itself when Z-Prime Plus or the self-adhesive resin cement were used. The manufacturer has also claimed that the Z-Prime Plus associated with self-adhesive resin cement can provide synergistic bondimproving effects for the adhesion of Y-TZP, alumina, and metal. However, the combination of Z-Prime Plus with the The Journal of Adhesive Dentistry
Chen et al
resin cement RelyX Unicem in the current study failed to increase initial SBS values and bond durability compared with using RelyX Unicem by itself. It has been proven that strong resin bond strength to alumina- and zirconia-based ceramics was achieved with the use of tribochemical silica coating. Its principal mechanism is that the silica coating provides an outermost atomic silica surface layer to bond with the silane agent.12,13 According to the SBS results of the present study, although the groups using Z-Prime Plus or RelyX Unicem showed advantages of simplified procedures and improved SBS, a combination of tribochemical silica coating and silanization yielded both the highest initial SBS values and the highest bond strengths after aging, which suggests that further improving bond strength and bond durability needs to be realized by chemically conditioning the zirconia with adhesive monomers. Water absorption and hydrolization are conditions that influence the long-term stability of the resin bond. Aging by water storage and thermocycling is the most commonly used method to evaluate the durability of resin bonds.3,16,20 The present results showed that aging decreased the SBS values for group C, while it did not significantly affect the SBS values for groups U, SSC, ZC, or ZU, which suggests that the chemical bonding possibly conditioned Y-TZP and improved its bond durability. Similarly, other authors have reported unchanged bond strength between zirconia and resin after artificial aging: de Castro et al7 found that 60-day water storage did not significantly reduce the bond strength to Y-TZP, and Matinlinna and Lassila19 showed that 6000 thermal cycles significantly increased the SBS values of zirconia conditioned with silane monomers. When analyzing the results of studies such as these, it is important to bear the limitations of the SBS test in mind. Although it is a popular method and has been employed widely because of its convenience, comparableness, and high efficiency,3,17,18,22,28 the SBS test has been criticized for uneven stress distribution, difficulty in duplicating in vivo conditions, the influence of a variety of loading edge/point shapes for applying the shear force, etc.6,31
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CONCLUSIONS 21.
The null hypothesis was only partially accepted. Within the limitations of this study, it may be reasonably concluded that Z-Prime Plus improves the resin bond strength of Y-TZP when conventional bis-GMA-based resin cement is used. There was no evidence that Z-Prime Plus could condition zirconia surface via a chemical reaction between phosphate monomer and zirconia.
ACKNOWLEDGMENTS This study was supported by the National High Technology Research and Development Program of China (863 Program, 2012AA030309), Research Grant JH10-27 from the Education Ministry, Jiangsu, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (2011-137).
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Amaral R, Özcan M, Bottino MA, Valandro LF. Microtensile bond strength of a resin cement to glass-infiltrated zirconia-reinforced ceramic: The effect of surface conditioning. Dent Mater 2006;22:283-290. Aboushelib MN. Evaluation of zirconia/resin bond strength and interface quality using a new technique. J Adhes Dent 2011;13:255-260. Blatz MB, Sadan A, Kern M. Resin-ceramic bonding: a review of the literature. J Prosthet Dent 2003;89:268-274. Cura C, Özcan M, Isik G, Saracoglu A. Comparison of alternative adhesive cementation concepts for zirconia: glaze layer vs zirconia primer. J Adhes Dent 2012;14:75-82. Chen C, Kleverlaan CJ, Feilzer AJ. Effect of an experimental zirconiasilica coating technique on micro tensile bond strength of zirconia in different priming conditions. Dent Mater 2012;28:e127-134. DeHoff PH, Anusavice KJ, Wang Z. Three-dimensional finite element analysis of the shear bond test. Dent Mater 1995;11:126-131. de Castro HL, Corazza PH, Paes-Júnior Tde A, Della Bona A. Influence of Y-TZP ceramic treatment and different resin cements on bond strength to dentin. Dent Mater 2012;28:1191-1197. Derand T, Molin M, Kvam K. Bond strength of composite luting cement to zirconia ceramic surfaces. Dent Mater 2005;21:1158-1162. Ertuùrul F, Türkün M, Türkün LS, Toman M, Cal E. Bond strength of different dentin bonding systems to fluorotic enamel. J Adhes Dent 2009;11:299-303. Garcia EJ, Reis A, Arana-Correa BE, Sepúlveda-Navarro WF, Higashi C, Gomes JC, Loguercio AD. Reducing the incompatibility between two-step adhesives and resin composite luting cements. J Adhes Dent 2010;12: 373-379. Ikemura K, Endo T. A review of our development of dental adhesives Effects of radical polymerization initiators and adhesive monomers on adhesion. Dent Mater J 2010;29:109-121. Janda R, Roulet JF, Wulf M, Tiller HJ. A new adhesive technology for allceramics. Dent Mater 2003;19:567-573. Kern M, Thompson VP. Sandblasting and silica coating of a glass-infiltrated aluminum oxide ceramic: volume loss, morphology, and changes in the surface composition. J Prosthet Dent 1994;71:453-461. Koizumi H, Nakayama D, Komine F, Blatz MB, Matsumura H. Bonding of resin-based luting cements to zirconia with and without the use of ceramic priming agents. J Adhes Dent 2012;14:385-392. Kern M, Barloi A, Yang B. Surface conditioning influences zirconia ceramic bonding. J Dent Res 2009;88:817-822. Kern M. Resin bonding to oxide ceramics for dental restorations. J Adhes Sci Technol 2009;23: 1097-1111. Kulunk S, Kulunk T, Ural C, Kurt M, Baba S. Effect of air abrasion particles on the bond strength of adhesive resin cement to zirconia core. Acta Odontol Scand 2011;69:88-94. Lung CY, Botelho MG, Heinonen M, Matinlinna JP. Resin zirconia bonding promotion with some novel coupling agents. Dent Mater 2012;28: 863-872. Matinlinna JP, Lassila LV. Enhanced resin-composite bonding to zirconia framework after pretreatment with selected silane monomers. Dent Mater 2011;27:273-380. Mirmohammadi H, Aboushelib MN, Salameh Z, Feilzer AJ, Kleverlaan CJ. Innovations in bonding to zirconia based ceramics: Part III. Phosphate monomer resin cements. Dent Mater 2010;26:786-792. Miragaya L, Maia LC, Sabrosa CE, de Goes MF, da Silva EM. Evaluation of self-adhesive resin cement bond strength to yttria-stabilized zirconia ceramic (Y-TZP) using four surface treatments. J Adhes Dent 2011;13:473-480. Mair L, Padipatvuthikul P. Variables related to materials and preparing for bond strength testing irrespective of the test protocol. Dent Mater 2010;26:e17-23. Magne P, Paranhos MP, Burnett LH Jr. New zirconia primer improves bond strength of resin-based cements. Dent Mater 2010;26:345-352. Nagai T, Kawamoto Y, Kakehashi Y, Matsumura H. Adhesive bonding of a lithium disilicate ceramic material with resin-based luting agents. J Oral Rehabil 2005;32:598-605. Nishiyama N, Tay FR, Fujita K, Pashley DH, Ikemura K, Hiraishi N, King NM. Hydrolysis of functional monomers in a single-bottle self-etching primercorrelation of 13C NMR and TEM findings. J Dent Res 2006;85:422-426. Rabitz H, Zhu W. Optimal control of molecular motion: design, implementation, and inversion. Acc Chem Res 2000;33:572-578. Sasse M, Eschbach S, Kern M. Randomized clinical trial on single retainer all-ceramic resin-bonded fixed dental prostheses: Influence of the bonding system after up to 55 months. J Dent 2012;40:783-786.
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Chen et al 28. Thompson JY, Stoner BR, Piascik JR, Smith R. Adhesion/cementation to zirconia and other non-silicate ceramics: where are we now? Dent Mater 2011;27:71-82. 29. Tay FR, Gwinnett JA, Wei SH. Relation between water content in acetone/alcohol-based primer and interfacial ultrastructure. J Dent 1998;26:147-156. 30. Vallittu PK, Kurunmäki H. Bond strength of fibre-reinforced composite to the metal surface. J Oral Rehabil 2003;30:887-892. 31. Van Noort R, Cardew GE, Howard IC, Noroozi S. The effect of local interfacial geometry on the measurement of the tensile bond strength to dentin. J Dent Res 1991;70:889-893. 32. 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. 33. Xie H, Wang X, Wang Y, Zhang F, Chen C, Xia Y. Effects of Sol-gel Processed Silica Coating on Bond Strength of Resin Cements to GlassInfiltrated Alumina Ceramic. J Adhes Dent 2009;11:49-55. 34. Xie H, Zhu Y, Chen C, Zhang F, Xia Y. Effect of silica coating on fracture strength of alumina ceramic cemented to dentin. J Adhes Dent 2011;13:467-472.
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35. Yang B, Barloi A, Kern M. Influence of air-abrasion on zirconia ceramic bonding using an adhesive composite resin. Dent Mater 2010;26:44-50. 36. Yoshida K, Yamashita M, Atsuta M. Zirconate coupling agent for bonding resin luting cement to pure zirconium. Am J Dent 2004;17:249-252. 37. Yoshida K, Tsuo Y, Atsuta M. Bonding of dual-cured resin cement to zirconia ceramic using phosphate acid ester monomer and zirconate coupler. J Biomed Mater Res B Appl Biomater 2006;77:28-33. 38. Zou Y, Jessop JL, Armstrong SR. In vitro enzymatic biodegradation of adhesive resin in the hybrid layer. J Biomed Mater Res A 2010;94: 187-192.
Clinical relevance: Primers or resin cements containing acidic monomers are able to improve bonding of Y-TZP through simple application procedures.
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Y
ttria-stabilized tetragonal zirconia polycrystals (Y-TZP) offers a wide variety of clinical applications due to their higher strength; however, its difficulty in adhering to composite resin cements via conventional cementing methods is still one of the major limitations to its use.2,4,14,21 Many surface treatments to improve the bonding of Y-TZP have been developed, including creating a microretentive
a
Lecturer, Institute of Stomatology, Nanjing Medical University, Nanjing, China. Performed the experiments.
b
Lecturer, Institute of Stomatology, Nanjing Medical University, Nanjing, China. Experimental Design, wrote manuscript.
c
PhD Student, Institute of Stomatology, Nanjing Medical University, Nanjing, China. Performed the experiments.
d
Professor, School of Dental Science, University of Hong Kong, Hong Kong, China. Proofread manuscript.
e
Ph D Student, Institute of Stomatology, Nanjing Medical University, Nanjing, China. Performed statistical evaluation.
f
Professor, Institute of Stomatology, Nanjing Medical University, Nanjing, China. Idea.
Correspondence: Professor Feimin Zhang, Institute of Stomatology, Nanjing Medical University, No. 136, Han Zhong Road, Nanjing, JiangSu Province, China 210029. Tel: +86-25-8503-1831, Fax: +86-25-8651-6414. e-mail: [email protected]
Vol 16, No 2, 2014
Submitted for publication: 23.05.12; accepted for publication: 08.08.13
surface texture through sandblasting, hot acid etching, selective infiltration etching, laser etching, sintering microporcelain pearls, nano-structured alumina coating, and zirconia coating.1,3,5,16,32 Just as with silica-based ceramics, strong resin bonding of Y-TZP relies on chemical adhesion as well as micromechanical interlocking.3,16 As is known, silane is capable of forming chemical bonds with organic and inorganic surfaces. Its bonding to resin occurs by an addition polymerization reaction between methacrylate groups in the matrix resin and alkene bonds in the silane molecule during light or chemical curing, whereas bonding to ceramics occurs via a condensation reaction between Si-OH groups on the ceramic surface and Si-OH groups in hydrolyzed silane molecules, yielding siloxane bonds and water molecules as by-products.3,24 However, because fewer or no hydroxy groups exist in Y-TZP, a coating containing SixOy must be formed on the bonding surface of Y-TZP before silanization.3,5,16 The literature reports several methods for SixOy coating, such as the conventional pyrolytical method developed in 1984, and later the tribochemical method.30 In recent years, some new, tentative coating techniques have been introduced to increase the silica content on Y-TZP surfaces. According to the study by Derand et al,8 the bond strength of Y-TZP 169
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Table 1
Summary of experimental groups
Group
Surface pre-treatment
Primer
Resin cement
C
Sandblasting*
None
Choice
U
Sandblasting
None
RelyX Unicem
SSC
Tribochemical silica coating using Cojet system*
Silane coupling agent
Choice
ZC
Sandblasting
Z-Prime Plus
Choice
ZU
Sandblasting
Z-Prime Plus
RelyX Unicem
*Vertically sandblasted with 110-μm alumina particles for 20 s at 3 bar from a distance of 10 mm using a sandblasting device (Lndp-III, Jianian Futong Medical Equipment; Tianjin, China). **Vertically sandblasted with 30-μm Cojet particles for 15 s at 3 bar from a distance of 10 mm (3M ESPE; St Paul, MN, USA).
almost tripled through deposition of hexamethyldisiloxane ((CH3)3SiOSi(CH3)3) via plasma spraying techniques. Another innovative approach is sol-gel technique to form silica coating.16,22,33,34 However, there is no doubt that either extra silica coating or complex surface-roughening methods for Y-TZP would be time consuming and technique sensitive. Dentists prefer more convenient methods in practice, such as using resins containing phosphate monomers, or phosphoric acid acrylate primers.27 In pursuit of the goals of optimizing the bond as well as simplifying application procedures, new adhesive resin cements or primers for Y-TZP conditioning have been developed. Recently, a commercial single-bottle zirconia primer (Z-Prime Plus, Bisco; Schaumburg, IL, USA) has been introduced which, according to the manufacturer, contains biphenyl dimethacrylate and hydroxyethyl methacrylate and can be used as a surface treatment for zirconia and alumina metal oxide ceramics as well as other types of metals/alloys to increase resin bond strength without Table 2
applying a silica coating and silane.23 Unfortunately, the manufacturer disclosed little of its product information, especially its active ingredients. The aim of this study was thus to evaluate the enhancement of the primer Z-Prime Plus on the shear bond strength (SBS) of Y-TZP to two different resin composite cements (one conventional, based on bis-GMA; one self-adhesive), and discuss the chemical role of this zirconia primer in improving the bonding of Y-TZP. The hypothesis is that this zirconia primer would improve the SBS of Y-TZP to both resin composite cements.
MATERIALS AND METHODS One hundred Y-TZP (Everest ZS-Ronde, KaVo; Biberach, Germany) blocks (12 mm x 8 mm x 2 mm) were produced and randomly assigned to 5 groups (n = 20), according to surface treatment, primer, and resin cement used: C: sandblasting, no primer, Choice resin cement (Bisco; Schaumburg, IL, USA); U: sandblasting, no primer, RelyX Unicem cement (3M ESPE; Seefeld, Germany; SSC: tribochemical silica coating (Cojet, 3M ESPE), silane coupling agent, Choice; ZC: sandblasting, Z-Prime Plus (Bisco), Choice; ZU: sandblasting, Z-Prime Plus, RelyX Unicem (Table 1). One hundred resin cylinders were prepared as follows: nylon tubes with an inner diameter of 5 mm and a height of 2 mm were filled with light-curing composite resin (Valux Plus, 3M ESPE) using a spatula and were photopolymerized for 20 s using an LED lamp (Elipar Freelight 2; 3M ESPE). Then the polymerized resin cylinders were removed from the nylon tubes for the subsequent tests. A layer of resin cement was applied to the prepared Y-TZP block, and a composite resin cylinder was pressed on the resin cement using a continual compressive force. The resin cement was light cured for 2 to 3 min to reach an initial setting. The excess resin cement in this state was then easily removed with a probe; final light curing was performed for 20 s. The details of the materials are presented in Table 2. Each bonded specimen was embedded in a self-curing acrylic resin base.
Description of main materials used*
Material/Trade name
Main compositions
Manufacturer
Light-curing composite resin cement/ Choice
Filler: Glass frit (40-70 wt%), amorphous silica (10-40 wt%) Base resin: bisphenol A-glycidyl methacrylate (Bis-GMA) (5-30 wt%)
Bisco; Schaumburg, IL, USA
Dual-polymerized, self-adhesive resin cement/RelyX Unicem
Liquid: Methacrylate monomers containing phosphoric acid groups (MP), dimethacrylate Powder: Glass powder, silanated silica, calcium hydroxide
3M ESPE; Seefeld, Germany
Adhesive primers, specific to oxidebased materials/Z-Prime Plus
Biphenyl dimethacrylate (BPDM), 2-hydroxyethyl methacrylate (HEMA), ethanol
Bisco
Silane coupling agent/Porcelain Primer
Ethanol (30-70%), acetone (30-70%); silane (1-10%)
Bisco
Light-curing composite resin/ Valux Plus
Filler: zirconia/silica (66 vol%) Base resin: bis-GMA, triethylene glycol dimethacrylate
3M ESPE
*From the material safety data sheet or technical data sheet.
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a
SEM images (magnification 600X) of the sandblasted zirconia surface treated with zirconia primer (left) or not (right).
Half of the bonded specimens were stored in distilled water at room temperature for 24 h and the other half was aged by storing in water (25°C) for 50 days, followed by thermocycling (12,000 cycles between 5°C and 55°C). Then, all the specimens were submitted to the SBS test (universal testing machine, Instron 3365, ElectroPuls; Norwood, MA, USA) with a reglet tap at a crosshead speed of 1.0 mm/min. One-way ANOVA and the Least Significant Difference (LSD) test for multiple comparisons were used for statistical analysis, performed with SPSS 17.0 (Chicago, IL, USA). The failure mode was observed using a microscope (WYSK-100X, Shu Yao Equipment; Shanghai, China) at 10X magnification. The modes of failure were classified as adhesive (fracture occurred at the resin/ resin cement and Y-TZP interface), cohesive (fracture occurred within the resin/resin cement), or mixed (both adhesive and cohesive failure occurred). Typical specimens were examined with a scanning electron microscope (SEM) (S-4800, Hitachi; Tokyo, Japan). Two additional Y-TZP blocks were produced and alumina sandblasted. One of them was conditioned with Z-Prime Plus and another was not. Both of them were examined at 600X magnification using SEM. Fourier transmission infrared (FT-IR) spectrum analysis (Nexus 870FT; Los Angeles, CA, USA) in transmission mode and gas mass spectrometry (MS) analysis (6890 Series GC system, Agilent Technologies; Santa Clara, CA, USA) were employed for characterization of Z-Prime Plus.
RESULTS SEM images of the sandblasted Y-TZP surfaces before and after treatment with the zirconia primer are shown in Fig 1. The FT-IR spectrum of Z-Prime Plus is depicted in Fig 2. As shown, the peaks located at 1637.2, 1510.2, and 1455.1 cm-1 are associated with benzene rings, the peak located at 1609.6 cm-1 is associated with stretching of the C=C linkage, the peak located Vol 16, No 2, 2014
100 90 80 70
% Reflectance
Fig 1
b
60 50 40 30 20 10 0 4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm–1)
Fig 2
FT-IR spectrum of Z-Prime Plus.
at 3419.9 cm-1 is associated with stretching of the -OH group, and the peak located at 1717.1 cm-1 is associated with stretching of the C=O group. The groups C=C, -OH, and C=O suggest the possible existence of carboxylic groups. the MS spectra of Z-Prime Plus are presented in Fig 3. A search of the NIST08.L computer database showed that 2-propenoic acid, 2-methyl-, 2-hydroxyethyl ester, triethylamine, ethyl-4-dimethylaminobenzoate, ethanol, and water are found in Z-Prime Plus. After aging, all specimens of groups U, SSC, ZC, and ZU remained bonded, while debonding occurred in half the specimens of group C. Mean (± SD) SBS values before and after aging are illustrated in Fig 4. One-way ANOVA revealed that SBS values from 24-h water storage (F = 31.3, p = 0.0001) and aging (50-day water storage, thermocycling; F = 55.1, p = 0.0001) differed significantly among the experimental groups considering the surface treatment factor. According to the results of LSD tests, group C presented the lowest SBS values (p < 0.01), group SSC presented the highest (p < 0.01), 171
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600000 550000 500000 450000 400000 350000 300000 250000 200000 150000 100000 50000
a
2.00
4.00
6.00
8.00
10.00
12.00 14.00 16.00 18.00 20.00 22.00 24.00 148.0
m/z Æ
Benzoid acid, 4-(dimethylamino)-, ethyl ester
9000 8000 7000 193.1 6000
164.0
5000 4000 3000 2000
77.0
104.0
42.0
1000 15.1
29.1
51.0
91.0
65.0
120.0 132.0
178.0
0
b
10
30
50
70
90
110
69.1
41.1
130
150
170
190
2-Propenoic acid, 2-methyl-, 2-hydroxyethyl ester
9000 8000 7000 6000 87.0
5000 4000 3000 2000 1000
c
0 5 m/z Æ
31.1 26.1 15.1 15
99.0
46.0 53.0 59.0 25
35
45
55
112.0
74.0 81.9 65
75
85
95
105
115
86.1
Triethylamine 9000 8000 7000 6000 5000 4000 3000
30.1
58.0
2000 1000
d
0 m/z Æ 5
172
101.0
42.0 15.0 15
70.0 25
35
45
55
65
75
85
95
105
Fig 3 a) MS spectra of Z-Prime Plus; b) spectrum matched with standard map of benzoic acid, 4-(dimethylamino)-, ethyl ester; c) spectrum matched with standard map of 2-propenoic acid, 2-methyl-, 2-hydroxyethyl ester; d) spectrum matched with standard map of triethylamine.
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16
MPa
13.36 (2.03)c
before aging after aging
11.99 (2.49)c
14 9.56 (2.55)b
12
8.77 (2.34)b 9.27 (0.92)b 8.17 (1.71)b
7.79 (2.08)b
10 7.70 (0.02)b
8
6 3.10 (1.26)a
4 0.85 (1.44)d
2
a 0 group
C
U
SSC
ZC
ZU
Fig 4 Bar graph of SBS values as means (standard deviation) before and after aging (n=10). Different letters over bars indicated statistically significant differences. C: Choice; U: RelyX Unicem; SSC: tribochemical silica coating followed by silanization, Choice; ZC and ZU: application of a zirconia primer, Choice and RelyX Unicem, resp.
Table 3 Failure modes Group
Artificial aging
C
before after
U
SSC
ZC
ZU
Adhesive failure
Cohesive failure
Mixed failure
10
0
0
10
0
0
before
4
1
5
after
0
2
8
before
6
0
4
after
0
3
7
before
3
0
7
after
0
2
8
before
2
2
6
after
0
0
10
and the other three groups, in which Z-Prime Plus and/ or RelyX Unicem were used, presented intermediate SBS values (p < 0.01) both before and after aging. One-way ANOVA also showed that the combination of 50-day water storage and 12,000 thermal cycles exerted no statistical influence on the SBS of groups U, SSC, ZC, and ZU (p < 0.05), while post-aging SBS values were lower in group C than before aging. The failure modes are shown in Table 3. Typical SEM images of the fracture interface are shown in Fig 5. Vol 16, No 2, 2014
b
c Fig 5 SEM images of fracture interfaces. a) adhesive failure, (14X magnification); b) mixed failure (15X magnification); c) cohesive failure (15X magnification).
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DISCUSSION Original roughness produced by milling during manufacture is reportedly not sufficient to promote adhesion between Y-TZP and resin. The most commonly used method of increasing their bonding is to create a rougher surface texture through air abrasion with alumina particles.3,5,32 According to a study by Kern,15 a combination of either lower-pressure or higher-pressure air abrasion and priming improved long-term resin bonding to Y-TZP significantly, while omitting air abrasion resulted in debonding during artificial aging, independent of primer use. Although some authors reported that alumina sandblasting would negatively influence the mechanical strength, the positive effects of alumina sandblasting are widely recognized, include increasing the surface roughness, surface energy, and wettability, increasing the surface area for bonding, and even enhancing the mechanical strength caused by phase transformation toughening.3,23,33 Therefore, standardization of the Y-TZP surfaces was achieved by using the alumina air-abrasion technique instead of polishing in current study. In the SBS test, half of the specimens of group C, in which only the conventional bis-GMA-based resin cement, Choice, was applied, debonding occurred after aging. Failure mode analysis also showed exclusively adhesive failure in this group. The SEM image of sandblasted Y-TZP also showed that because of the high rigidity of Y-TZP, it was not possible to obtain a well-roughened surface through alumina sandblasting. Thus, a lack of sufficient micromechanical interlocking and chemical adhesion may be the main reason for the weaker bond strength and poor bond durability. It is generally believed that some adhesive monomers present in luting agents or zirconia/metal primers provide a possible chemical bonding mechanism with metal oxides such as alumina and zirconia.22-24,37 The chemical adhesion results through the reaction between acidic functional groups in the adhesive monomers and oxide groups on the Y-TZP surface, which is similar to the reaction between silane and silica-based ceramics.11,23,36 These kinds of adhesive monomers typically include the carboxylic acid group (-COOH) or its anhydride, and the phosphoric [-O-P(=O)(OH)2] or phosphonic acid group [-P(=O)(OH)2].11,36 Representative examples are 10-methacryloyloxydecyldihydrogenphosphate (10-MDP) and 4-methacryloyloxyethyl trimellitic acid (4-MET).11,25,28,37 According to current SBS test results, no matter which resin cement was used, application of the zirconia primer always provided higher SBS values compared to the control group. Referring to the product’s safety data sheet, the main functional components of Z-Prime Plus are biphenyl dimethacrylate (BPDM) and 2-hydroxyethyl methacrylate (HEMA). BPDM features a double aromatic ring structure.10,29 HEMA is a low-molecularweight monomer, which is used in many adhesives for its positive influence on the bond strength.9,38 One study reported that Z-Prime Plus combined both phosphate and carboxylic monomers, with the former bonding chemically with oxide groups on the Y-TZP surface, and the latter participating in the development of the chemical bonds.23 174
However, the FT-IR analysis of the current study could not detect phosphate groups in Z-Prime Plus. As shown in Fig 2, carboxylic groups might exist in Z-Prime Plus, but whether the carboxylic groups contributed mainly to the chemical bonds between Z-Prime Plus and Y-TZP needs to be addressed by further tests. The MS analysis in the present study identified several components of Z-Prime Plus. The main functions of these components are as follows: Ethanol and water are the solvents. Ethyl-4-dimethylaminobenzoate is often used in curing systems in combination with other photoinitiators, or used as an intermediate in organic synthesis. HEMA is often used in dentin adhesive systems and has been described above. Triethylamine is often used for organic synthesis of materials, or as an organic solvent and wetting agent. Therefore, there is still no evidence to suggest that Z-Prime Plus could condition zirconia surfaces via the chemical reaction between phosphate monomer and zirconia. Choosing the right resin cement has been considered to be another important factor in achieving adhesion to zirconia or alumina-based ceramics in practice, since they are difficult to condition.20,35 In the current study, one self-adhesive resin cement and one conventional bis-GMA-based resin cement were evaluated. According to the chemical formula, two functional phosphoric acid groups are contained in the functional monomer of the self-adhesive resin cement RelyX Unicem. The manufacturer claims the special molecular formula provides high reactivity and excellent mechanical properties. Theoretically, the main chain structure of a polymer decides its molecular flexibility. Higher molecular flexibility means greater molecular motion, which brings the molecules in the adhesive system closer together and increases their adsorbability.26 The main chain structure of methacrylate monomers in the RelyX Unicem resin cement consists in single C-C bonds. Because internal rotation occurs in each bond, this kind of polymer theoretically possesses higher flexibility and better adhesion. Choice is a conventional bis-GMA-based material. In the molecular formula of bis-GMA, two benzene rings exist in the main chain structure, which induces higher rigidity because of the weaker molecular motion of the heteroaromatic ring compared with the single C-C bond. The monomer of RelyX Unicem contains at least two phosphoric acid groups and a minimum of two double-bonded carbon units (C=C) per molecule; thus, it might seem logical that this self-adhesive resin cement might provide better adhesion to Y-TZP. Conforming to this, the current SBS test showed increased SBS values when this self-adhesive resin cement was used. According to the results of the failure mode analysis, cohesive and mixed failures occurred in all bond specimens of groups U, SSC, ZC, ZU, and exclusively adhesive failure occurred in almost all bonded specimens of group C, which suggested that the resin/ceramic bond interface was similar to or even stronger than the cohesive strength of the resin cement itself when Z-Prime Plus or the self-adhesive resin cement were used. The manufacturer has also claimed that the Z-Prime Plus associated with self-adhesive resin cement can provide synergistic bondimproving effects for the adhesion of Y-TZP, alumina, and metal. However, the combination of Z-Prime Plus with the The Journal of Adhesive Dentistry
Chen et al
resin cement RelyX Unicem in the current study failed to increase initial SBS values and bond durability compared with using RelyX Unicem by itself. It has been proven that strong resin bond strength to alumina- and zirconia-based ceramics was achieved with the use of tribochemical silica coating. Its principal mechanism is that the silica coating provides an outermost atomic silica surface layer to bond with the silane agent.12,13 According to the SBS results of the present study, although the groups using Z-Prime Plus or RelyX Unicem showed advantages of simplified procedures and improved SBS, a combination of tribochemical silica coating and silanization yielded both the highest initial SBS values and the highest bond strengths after aging, which suggests that further improving bond strength and bond durability needs to be realized by chemically conditioning the zirconia with adhesive monomers. Water absorption and hydrolization are conditions that influence the long-term stability of the resin bond. Aging by water storage and thermocycling is the most commonly used method to evaluate the durability of resin bonds.3,16,20 The present results showed that aging decreased the SBS values for group C, while it did not significantly affect the SBS values for groups U, SSC, ZC, or ZU, which suggests that the chemical bonding possibly conditioned Y-TZP and improved its bond durability. Similarly, other authors have reported unchanged bond strength between zirconia and resin after artificial aging: de Castro et al7 found that 60-day water storage did not significantly reduce the bond strength to Y-TZP, and Matinlinna and Lassila19 showed that 6000 thermal cycles significantly increased the SBS values of zirconia conditioned with silane monomers. When analyzing the results of studies such as these, it is important to bear the limitations of the SBS test in mind. Although it is a popular method and has been employed widely because of its convenience, comparableness, and high efficiency,3,17,18,22,28 the SBS test has been criticized for uneven stress distribution, difficulty in duplicating in vivo conditions, the influence of a variety of loading edge/point shapes for applying the shear force, etc.6,31
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5.
6. 7.
8. 9.
10.
11.
12. 13.
14.
15. 16. 17.
18.
19.
20.
CONCLUSIONS 21.
The null hypothesis was only partially accepted. Within the limitations of this study, it may be reasonably concluded that Z-Prime Plus improves the resin bond strength of Y-TZP when conventional bis-GMA-based resin cement is used. There was no evidence that Z-Prime Plus could condition zirconia surface via a chemical reaction between phosphate monomer and zirconia.
ACKNOWLEDGMENTS This study was supported by the National High Technology Research and Development Program of China (863 Program, 2012AA030309), Research Grant JH10-27 from the Education Ministry, Jiangsu, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (2011-137).
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23. 24.
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Clinical relevance: Primers or resin cements containing acidic monomers are able to improve bonding of Y-TZP through simple application procedures.
The Journal of Adhesive Dentistry
