- Email: [email protected]
Effects of an etching solution on the adhesive properties and surface microhardness of zirconia dental ceramics
RESEARCH AND EDUCATION
Effects of an etching solution on the adhesive properties and surface microhardness of zirconia dental ceramics Sahar Ansari, MSc, PhD,a Nozhan Jahedmanesh, DDS,b Domenico Cascione, BS, CDT,c Pouya Zafarnia, DDS,d Kumar C. Shah, DDS, MS,e Benjamin M. Wu, DDS, PhD,f and Alireza Moshaverinia, DDS, MS, PhDg
ABSTRACT Statement of problem. Conventional approaches to adhesive bonding are not applicable to zirconia restorations. Recently, an etching solution, Zeta Etching Solution (ZES), has been introduced for etching the surface of zirconia. The effects of this etching solution on the bond strength and mechanical properties of zirconia are unknown. Purpose. The purpose of this in vitro study was to examine the effects of ZES on the bond strength and surface hardness of zirconia. Material and methods. Two different types of partially stabilized tetragonal polycrystalline zirconia (TZP), Prettau zirconia (group P) and anterior Prettau (group AP), were evaluated with and without ZES etching. Each group was bonded to a zirconia substrate by using an adhesive resin cement. After 24 hours of storage in distilled water, the bond strength of the zirconia was analyzed. Vickers hardness was determined by using a microhardness tester. Scanning electron microscopy was used to analyze the surface microstructure and determine the mode of failure for each specimen. Results were analyzed and compared using 1-way ANOVA and Student t tests ( a =.05). Results. Scanning electron microscopy analysis showed that etching the surface of zirconia with ZES etching solution for 60 minutes changed the morphological characteristics and microstructure of zirconia, making the surface more irregular. The changes were more pronounced for AP specimens. Etching with ZES significantly increased the shear bond strength of zirconia (P<.05) in AP specimens. The bond strength of Prettau (P group) specimens after ZES etching did not increase significantly (P>.05). An adhesive failure mode was observed for P zirconia specimens, whereas zirconia specimens exhibited a cohesive mode of failure. No significant decrease (P>.05) was observed in the mean Vickers hardness numbers. Conclusions. Within the limitations of this in vitro study, it was concluded that etching in ZES for 30 minutes significantly enhanced the shear bond strength of highly translucent anterior Prettau (AP) zirconia restorations. Moreover, etching with ZES did not adversely affect the surface hardness of the zirconia specimens tested. (J Prosthet Dent 2018;120:447-53)
a Lecturer, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif. b Preceptor, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif. c Lecturer, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif. d Preceptor, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif. e Associate professor and director, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif. f Professor and Chairman, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif. g Assistant Professor, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif.
THE JOURNAL OF PROSTHETIC DENTISTRY
447
448
Clinical Implications Under the testing conditions of this study, ZES may be a promising candidate for the surface treatment of zirconia, providing enhanced adhesive bond strength to other restorative materials.
Zirconia dental ceramics have become popular for toothand implant-supported restorations from single units to full-mouth fixed partial dentures because of high and favorable mechanical properties, corrosion resistance, and biocompatibility.1,2 Zirconia’s bioinertness leads to favorable corrosion resistance properties.2,3 However, unlike feldspathic porcelain or lithium disilicate ceramic, hydrofluoric acid (HF) is unable to etch zirconia.1-5 Therefore, the conventional approaches to adhesive bonding to silica-based ceramics are not applicable to zirconia. The bioinertness of zirconia has made bonding to tooth structure, titanium, or other ceramic materials challenging. As a result, surface treatments or conditioning techniques are required to achieve predictable and durable adhesion. The adhesion of resin-based luting cements to zirconia has been evaluated,6-8 but these studies report less favorable adhesive outcomes than for silicate-based dental ceramics or glass-ceramics.6-8 HF (9.5%) solution has been used to etch zirconia by extending the immersion time (up to 24 hours)9 or by elevating the temperature (80 C) for up to 30 minutes.10 Additionally, HF solutions of higher concentrations (48%) have been used.11 However, these protocols have failed to improve adhesive bonding to zirconia.9-11 Alternative surface treatments such as selective infiltration technique, laser treatment, and coating with nanostructured alumina or tribochemical silica have been proposed.12-14 However, clinically significant improvements have been lacking, and these alternative treatments have caused surface microcracks, weakening the material and making it more prone to crack propagation. As an alternative method of bonding to zirconia restorations, phosphate ester primer such as 10-methacryloyloxydecyl dihydrogen phosphate (MDP) have been used successfully to improve adhesion to zirconia restorations.14-18 Therefore, alternative surface treatments are needed to improve the adhesive bond strength of zirconia without compromising its mechanical properties.18-20 Recently, highly translucent zirconia materials have been introduced, including a fully stabilized cubic/ tetragonal zirconia. The increased translucency has been achieved by materials with a high percentage of isotropic cubic phase with a larger grain size.24,25 This material may be a promising alternative to lithium disilicate and
THE JOURNAL OF PROSTHETIC DENTISTRY
Volume 120 Issue 3
feldspathic ceramic materials if it can be reliably bonded to tooth structure or other restorative materials. Also, recently, an HF solution containing nitric acid (Zircos E etching system, ZSAT; Eunjin Chemical Co) has been introduced that can etch the surface of zirconia at room temperature.15 Cho et al15 analyzed the surface roughness of zirconia after etching with ZSAT for 3 hours at 25oC. They also measured the shear bond strength to 4 different resin cements (Panavia F 2.0, Rely X Unicem, Superbond C&B, and Hot Bond of zirconia surfaces treated with ZSAT solution). They reported that ZSAT could increase the shear bond strength of zirconia.15 Recently, a new version of the etching solution (Zeta Etching Solution; Eunjin Chemical Co) has been introduced containing HF, hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), and phosphoric acid (H3PO4). The present authors are unaware of studies of the effects of this etching solution on the bond strength and mechanical properties of zirconia. Therefore, the purpose of this in vitro study was to examine the effects of ZER etching solution on these properties. The null hypothesis is that no differences will be found between the bond strength properties and surface hardness of zirconia specimens, with or without etching with ZES. MATERIAL AND METHODS Two types of partially stabilized tetragonal polycrystalline zirconia were used: Prettau (P) zirconia (Zirkonzahn) and anterior Prettau (AP) (Zirkonzahn). The materials were divided into 4 groups as follows: ZES-etched Prettau zirconia (PE) with ZES; ZES-etched anterior Prettau zirconia (APE); unetched Prettau zirconia (PU); and unetched anterior Prettau zirconia (APU). Specimens were milled by using a 5-axis milling machine (Zirconzahn). For shear bond strength and Vickers hardness tests, cylindrical zirconia specimens (10-mm diameter×5-mm height) were fabricated. For 3-point bend testing, rectangular specimens (25×2×2 mm) were fabricated. Before we undertook mechanical testing, specimens were airborne-particle abraded by using 50-mm Al2O3 particles for 15 seconds and immersed in ZES for 60 minutes in an ultrasonic bath at room temperature. Subsequently, specimens were cleaned with orthophosphoric acid for 1 minute and placed in an alcohol ultrasonic bath for 10 minutes. Silane was not used. A resin-based, dual-polymerizable, hydrophilic cement (URC; Bioloren) was used to bond the specimens (zirconiaezirconia). Unetched specimens were only airborne-particle abraded before bonding procedures. After 24 hours of storage in distilled water, a shear load was applied to the zirconiaezirconia interface by using a mechanical testing machine (model 5564; Instron
Ansari et al
September 2018
449
Figure 1. Scanning electron microscopy images of anterior Prettau zirconia specimens. A, B, Etched APE. C, D, Unetched APU. AP zirconia showed combination of tetragonal and cubic structure. Upon etching, pronounced surface roughness and etching were apparent in AP zirconia specimens. A, C, original magnification ×750; B, D, original magnification ×3300. APE, anterior Prettau zirconia; APU, anterior Prettau zirconia.
Corp) with a knife-edged rod at a crosshead speed of 0.5 mm/min.16 Seven specimens were used for each group. All mechanical testing machines were calibrated before starting measurements. The shear force required to separate the zirconia cylinder from the substrate zirconia was recorded in Newtons and divided by the contact surface area to determine the shear bond strength value in megapascals. Subsequently, the failure modes of the debonded zirconia specimens were analyzed by scanning electron microscopy (SEM) (Supra 40/40 VP; Carl Zeiss SMT). To further analyze the effect of ZES on the adhesive bonding of zirconia and its interfacial properties, a 3-point bend test was carried out according to International Organization for Standardization standard 9693:1999.26 Ten specimens were used in each group. Specimens were placed horizontally on a 20-mm-diameter annular, knife-edged support ring, and a knife-edged indenter was placed exactly in the center of the specimens. Force was applied at a crosshead speed of 0.5 mm/min until the specimens debonded. Zirconia debonding was
Ansari et al
determined by an abrupt decrease in the stress-strain graphs. The zirconiaezirconia bond strength was then calculated.27 Vickers hardness tests were conducted using a microhardness tester (model 1600-4963; Buehler), using previously reported protocols.17 Briefly, a 3-N force was applied to the specimen with a diamond indenter for 15 seconds. Each group contained 7 specimens, which were conditioned for 24 hours at 37 C. The mechanical testing machine was calibrated before taking measurements. All results were generated and reported in Vickers hardness units by the microhardness tester. To analyze the effects of ZES on the surface morphology of the zirconia specimens, SEM was used. SEM was also used to determine the mode of failure for each specimen. The specimens were coated with iridium (Ion Beam Sputtering; South Bay Technology) before EM analysis. One-way ANOVA and Student t tests were used to determine any significant differences between the values of shear bond strength and those of 3-point bend testing
THE JOURNAL OF PROSTHETIC DENTISTRY
450
Volume 120 Issue 3
Figure 2. Scanning electron microscopy images of Prettau zirconia specimens. A, B, Etched PE. C, D, Unetched PU. PU zirconia specimens showed tetragonal structure. Upon etching, modest changes in surface roughness were found. A, C, original magnification ×750; B, D, original magnification ×3300. PE. C, D, Unetched PU. PE, ZES-etched Prettau zirconia; PU, unetched Prettau zirconia.
with statistical software (SPSS for Windows v16.0; SPSS Inc) (a=.05). RESULTS SEM analysis (Figs. 1, 2) showed that etching the surface of zirconia with ZES for 30 minutes changed its shape, making the surface more irregular. These changes were more pronounced for APE (etched anterior Prettau) than PE (etched Prettau) specimens, which may be related to the presence of the cubic phase. Tetragonal structures were present in PU (unetched Prettau) specimens and tetragonal/cubic in the AP specimens. Etching with ZES changed the surface layer, as was apparent from the SEM images (Figs. 1, 2). Shear bond strength results are presented in Figure 3A. Etching with ZES significantly increased the shear bond strength of zirconiaezirconia (P<.05) in AP specimens, but the difference for group P was not significant (P>.05). Results of the 3-point bend test showed that etching in ZES significantly increased (P<.05) the bond strength of AP zirconia specimens (Fig. 3B).
THE JOURNAL OF PROSTHETIC DENTISTRY
However, the bond strength of the group P zirconia specimens was not significantly (P>.05) affected. SEM analysis of the mode of failure after shear bond strength testing revealed a partial cohesive mode of failure for APE specimens, whereas the PE specimens showed an adhesive mode of failure (Figs. 4, 5). Furthermore, as presented in the SEM images (Figs. 4, 5), etching with ZES resulted in a completely intermingled cement-zirconia layer for the APE and PE specimens. In contrast, 2 distinct layers of cement and zirconia were observed for the APU and PU specimens (Fig. 6). The microhardness measurements showed no significant decrease (P>.05) in the surface hardness of zirconia specimens after 30 minutes of etching in ZES (Fig. 7). DISCUSSION In the current study, a recently introduced acid etching solution (ZES) was used to condition the surface of zirconia to increase the adhesive bond strength of zirconia ceramic. Based on the results of this study, the
Ansari et al
Shear Bond Strength (MPa)
September 2018
451
NS
*
16 12 8 4 0
Unetched Prettau (PU) zirconia
Etched Prettau (PE) zirconia
Etched Unetched Anterior Anterior Prettau (APU) Prettau (APE) zirconia zirconia
A
Bond Strength (MPa)
Group *
NS
30 23 15 8 0
Unetched Prettau (PU) zirconia
Etched Prettau (PE) zirconia
Unetched Etched Anterior Anterior Prettau (APU) Prettau (APE) zirconia zirconia
Group
B
Figure 3. Etched and unetched AP zirconia and P zirconia specimens. A, Shear bond strength. B, Bond strength measurements *P<.05. NS, not significant.
null hypothesis that ZES would not cause any significant changes to the measured properties was partially rejected. The shear bond strength and 3-point bend test results showed that etching with ZES significantly increased the bond strength of anterior Prettau (AP) zirconia (P<.05). However, no significant increase in these values was observed for Prettau (P) specimens (P>.05). Furthermore, the etching procedure did not compromise the surface hardness of the tested materials; no significant decrease in the surface hardness of the tested zirconia specimens was observed after etching with ZES (P>.05). However, for a more comprehensive analysis of the effects of this etching material, additional types of resin cement should be evaluated. Moreover, the effects of ZES etching on the bonding of zirconia to different substrates needs to be analyzed further. Among the current ceramic materials, zirconia ceramics have attracted attention because of their extremely high strength and favorable esthetics.18 Additionally, zirconia biomaterials are bio-inert, meaning that they are resistant to corrosion and do not go through any composition or chemical changes when exposed to the oral environment. However, this bio-inertness makes bonding zirconia to tooth structure and other restorative dental materials challenging.18,19 Unlike silica-based ceramics, zirconia dental restorations cannot be etched
Ansari et al
with HF. Therefore, different methods have been proposed to increase the adhesive properties of zirconia ceramics.20,21 These surface modifications, however, reportedly compromise the strength of the materials and their long-term bond strength properties.22,23 The adhesive bonding properties of zirconia are even more important for the recently introduced fully stabilized cubic/tetragonal zirconia, which is highly translucent because of the presence of a high percentage of isotropic cubic phase with larger grain size.24,25 This material can be considered a promising alternative to lithium disilicate and feldspathic ceramic materials if it can be properly bonded to tooth structure or other restorative materials. The etching solution studied (Zeta etching solution; Eunjin Chemical Co) contained HF, hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), and phosphoric acid (H3PO4) and was capable of etching the surface of zirconia restorations. The authors are unaware of prior studies of the effects of this etching solution on the bond strength and mechanical properties of zirconia. The shear bond strength data in the current study correlated well with previously reported values.15 In addition, SEM analysis showed that after 30 minutes of etching with ZES, the surfaces of the zirconia specimens were completely etched, and surface irregularities were apparent. The anterior Prettau (AP) specimens were affected significantly more than the Prettau (P) specimens. This phenomenon can be attributed to the phasic structure of the highly translucent AP specimens. Etching with ZES for 30 minutes significantly increased the shear bond strength of the AP specimens. This result was also expected, as more changes to the surface morphological features of the AP zirconia were observed after etching. To further analyze the effect of ZES on the adhesive bonding strength of the zirconia materials, a modified 3-point bend test was used. The data obtained in this test correlated well with the shear bond strength test outcomes, confirming that etching in ZES significantly increased the amount of force require to detach the AP zirconia specimens. From the SEM analysis an adhesive failure mode was observed in the Prettau specimens, due to the inability of the ZES to etch the surface. In contrast, etching with ZES for 30 minutes changed the shape of the highly translucent AP zirconia specimens, and the SEM analysis found a cohesive mode of failure. The differences in the surface patterns after etching may be attributed to the composition differences between the AP and P zirconia groups and the presence of the cubic phase in more translucent zirconia ceramics.
THE JOURNAL OF PROSTHETIC DENTISTRY
452
Volume 120 Issue 3
Figure 4. Scanning electron microscopy analysis of zirconiaezirconia interface after shear bond strength test showed partial cohesive mode of failure for AP specimens for etched groups. Note intermingled interface of cement and etched zirconia layer for AP/AP specimens. Original magnification A, ×750; B, ×1800; C, ×3300. AP, anterior Prettau group; C, cement Zir, zirconia restoration. Red box, area of interest.
Figure 5. Scanning electron microscopy analysis of zirconiaezirconia interface after shear bond strength test showed adhesive mode of failure for P specimens for etched groups. Note modest interlocked interface of cement and etched zirconia layer for P/P specimens. Original magnification A, ×750; B, ×1800; C, ×3300. C, cement; P, prettau zirconia group; Zir, zirconia restoration. Red box, area of interest.
Figure 6. Scanning electron microscopy analysis of zirconiaezirconia interface after shear bond strength of unetched specimens. Two distinct layers of cement and zirconia were observed. A, APU. B, PU. Original magnification, A, B, ×750; inset original magnification ×1200. APU. B, PU. APU, anterior Prettau zirconia; PU, unetched Prettau zirconia. Red box, area of interest magnified.
The surface hardness analysis found that etching in ZES for 30 minutes did not compromise the surface hardness of the materials. Further analyses are required to examine the effects of the etching technique on the bond strength of zirconia to tooth structure and other restorative materials. Limitations of the current study are that only 1 type of cement was used and that no silane or MDP monomer was applied. Additionally, the effects of the etching solution on the bonding of zirconia to only 1 THE JOURNAL OF PROSTHETIC DENTISTRY
substrate were analyzed. In future work, different types of commercially available adhesive resin cements will be tested and the effects of ZES etching solution on bonding of zirconia to other substrates such as lithium disilicate, titanium, and feldspathic porcelain. Results of the current study demonstrate that ZES is capable of changing the morphology of zirconia, leading to a significant increase in bond strength without any adverse effect on the surface hardness of the material. Ansari et al
September 2018
453
NS NS
NS
1500
VHN
1125 750 375 0
Unetched Prettau (PU) zirconia
Etched Unetched Etched Prettau (PE) Anterior Anterior zirconia Prettau (APU) Prettau (APE) zirconia zirconia
Group Figure 7. VHN of the etched and unetched AP and regular P specimens after 24 hours of storage in distilled water at 37 C, showing slight decrease in surface hardness values. NS, not significant; VHN, Vickers hardness number.
CONCLUSIONS Based on the findings of this in vitro study, the following conclusions were drawn: 1. Etching in ZES for 30 minutes significantly increased the bond strength of translucent anterior Prettau zirconia restorations. 2. Etching with ZES did not adversely affect the surface hardness of tested zirconia specimens. REFERENCES 1. Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater 2008;24:299-307. 2. Kelly JR, Denry I. Stabilized zirconia as a structural ceramic: an overview. Dent Mater 2008;24:289-98. 3. Denry I, Kelly JR. Emerging ceramic-based materials for dentistry. J Dent Res 2014;93:1235-42. 4. Blatz MB, Sadan A, Kern M. Resin-ceramic bonding: a review of the literature. J Prosthet Dent 2003;89:268-74. 5. Sahafi A, Peutzfeldt A, Asmussen E, Gotfredsen K. Bond strength of resin cement to dentin and to surface-treated posts of titanium alloy, glass fiber, and zirconia. J Adhes Dent 2003;5:153-62. 6. Guazzato M, Proos K, Quach L, Swain M. Strength, reliability and mode of fracture of bilayered porcelain/zirconia (Y-TZP) dental ceramics. Biomaterials 2004;25:5045-52. 7. 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. 8. Kosmac T, Oblak C, Jevnikar P, Funduk N, Marion L. Strength and reliability of surface treated Y-TZP dental ceramics. J Biomed Mater Res Part B Appl Biomater 2000;53:304-13. 9. Sriamporn T, Thamrongananskul N, Busabok C, Poolthong S, Uo M, Tagami J. Dental zirconia can be etched by hydrofluoric acid. Dent Mater J 2014;33:79-85.
Ansari et al
10. Smielak B, Klimek L. Effect of hydrofluoric acid concentration and etching duration on select surface roughness parameters for zirconia. J Prosthet Dent 2015;113:596-602. 11. Chaiyabutr Y, McGowan S, Phillips KM, Kois JC, Giordano RA. The effect of hydrofluoric acid surface treatment and bond strength of a zirconiaveneering ceramic. J Prosthet Dent 2008;100:194-202. 12. Casucci A, Monticelli F, Goracci C, Mazzitelli C, Cantoro A, Papacchini F, et al. Effect of surface pre-treatments on the zirconia ceramic-resin cement microtensilebond strength. Dent Mater 2011;27:1024-30. 13. Blatz MB, Sadan A, Martin J, Lang B. In vitro evaluation of shear bond strengths of resin to densely-sintered high-purity zirconium-oxide ceramic after long-term storage and thermal cycling. J Prosthet Dent 2004;91:356-62. 14. Blatz MB, Chiche G, Holst S, Sadan A. Influence of surface treatment and simulated aging on bond strengths of luting agents to zirconia. Quintessence Int 2007;38:745-53. 15. Cho JH, Kim SJ, Shim JS, Lee KW. Effect of zirconia surface treatment using nitric acid-hydrofluoric acid on the shear bondstrengths of resin cements. J Adv Prosthodont 2017;9:77-84. 16. Moshaverinia A, Ansari S, Moshaverinia M, Roohpour N, Darr JA, Rehman I. Effects of incorporation of hydroxyapatite and fluoroapatite nanobioceramics into conventional glass ionomer cements (GIC). Acta Biomater 2008;4: 432-40. 17. Moshaverinia A, Brantley WA, Chee WW, Rohpour N, Ansari S, Zheng F, et al. Measure of microhardness, fracture toughness and flexural strength of N-vinylcaprolactam (NVC)-containing glass-ionomer dental cements. Dent Mater 2010;26:1137-43. 18. Cura C, Özcan M, Isik G, Saracoglu A. Comparison of alternative adhesive cementation concepts for zirconia ceramic: glaze layer vs zirconia primer. J Adhes Dent 2012;14:75-82. 19. Wang C, Niu L, Wang Y, Jiao K, Liu Y, Zhou W, et al. Bonding of resin cement to zirconia with high pressure primer coating. PLoS One 2014;9: 101174-7. 20. Karimipour-Saryazdi M, Sadid-Zadeh R, Givan D, Burgess JO, Ramp LC, Liu PR. Influence of surface treatment of yttrium-stabilized tetragonal zirconium oxides and cement type on crown retention after artificial aging. J Prosthet Dent 2014;111:395-403. 21. Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Selective infiltration-etching technique for a strong and durable bond of resin cements to zirconia-based materials. J Prosthet Dent 2007;98:379-88. 22. Kern M, Wegner SM. Bonding to zirconia ceramic: adhesion methods and their durability. Dent Mater 1998;14:64-71. 23. Attia A, Kern M. Long-term resin bonding to zirconia ceramic with a new universal primer. J Prosthet Dent 2001;106:319-27. 24. Zhang F, Inokoshi M, Batuk M, Hadermann J, Naert I, Van Meerbeek B, et al. Strength, toughness and aging stability of highly-translucent Y-TZP ceramics for dental restorations. Dent Mater 2016;32:327-37. 25. Harada K, Raigrodski AJ, Chung KH, Flinn BD, Dogan S, Mancl LA. A comparative evaluation of the translucency of zirconias and lithium disilicate for monolithic restorations. J Prosthet Dent 2016;116: 257-63. 26. Lenz J, Schwarz S, Schwickerath H, Sperner F, Schafer A. Bond strength of metaleceramic systems in three-point flexure bond test. J Appl Biomater 1995;6:55-64. 27. Zinelis S, Barmpagadaki X, Vergos V, Chakmakchi M, Eliades G. Bond strength and interfacial characterization of eight low fusing porcelains to cp Ti. Dent Mater 2010;26:264-73. Corresponding author: Dr Alireza Moshaverinia Weintraub Center for Reconstructive Biotechnology Division of Advanced Prosthodontics UCLA School of Dentistry 10833 Le Conte Avenue, B3-023 CHS Los Angeles, CA 90095-1668 Email: [email protected] Copyright © 2017 by the Editorial Council for The Journal of Prosthetic Dentistry.
THE JOURNAL OF PROSTHETIC DENTISTRY
Effects of an etching solution on the adhesive properties and surface microhardness of zirconia dental ceramics Sahar Ansari, MSc, PhD,a Nozhan Jahedmanesh, DDS,b Domenico Cascione, BS, CDT,c Pouya Zafarnia, DDS,d Kumar C. Shah, DDS, MS,e Benjamin M. Wu, DDS, PhD,f and Alireza Moshaverinia, DDS, MS, PhDg
ABSTRACT Statement of problem. Conventional approaches to adhesive bonding are not applicable to zirconia restorations. Recently, an etching solution, Zeta Etching Solution (ZES), has been introduced for etching the surface of zirconia. The effects of this etching solution on the bond strength and mechanical properties of zirconia are unknown. Purpose. The purpose of this in vitro study was to examine the effects of ZES on the bond strength and surface hardness of zirconia. Material and methods. Two different types of partially stabilized tetragonal polycrystalline zirconia (TZP), Prettau zirconia (group P) and anterior Prettau (group AP), were evaluated with and without ZES etching. Each group was bonded to a zirconia substrate by using an adhesive resin cement. After 24 hours of storage in distilled water, the bond strength of the zirconia was analyzed. Vickers hardness was determined by using a microhardness tester. Scanning electron microscopy was used to analyze the surface microstructure and determine the mode of failure for each specimen. Results were analyzed and compared using 1-way ANOVA and Student t tests ( a =.05). Results. Scanning electron microscopy analysis showed that etching the surface of zirconia with ZES etching solution for 60 minutes changed the morphological characteristics and microstructure of zirconia, making the surface more irregular. The changes were more pronounced for AP specimens. Etching with ZES significantly increased the shear bond strength of zirconia (P<.05) in AP specimens. The bond strength of Prettau (P group) specimens after ZES etching did not increase significantly (P>.05). An adhesive failure mode was observed for P zirconia specimens, whereas zirconia specimens exhibited a cohesive mode of failure. No significant decrease (P>.05) was observed in the mean Vickers hardness numbers. Conclusions. Within the limitations of this in vitro study, it was concluded that etching in ZES for 30 minutes significantly enhanced the shear bond strength of highly translucent anterior Prettau (AP) zirconia restorations. Moreover, etching with ZES did not adversely affect the surface hardness of the zirconia specimens tested. (J Prosthet Dent 2018;120:447-53)
a Lecturer, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif. b Preceptor, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif. c Lecturer, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif. d Preceptor, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif. e Associate professor and director, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif. f Professor and Chairman, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif. g Assistant Professor, Division of Advanced Prosthodontics, Weintraub Center for Reconstructive Biotechnology and Division of Advanced Prosthodontics, School of Dentistry, University of California Los Angeles, Los Angeles, Calif.
THE JOURNAL OF PROSTHETIC DENTISTRY
447
448
Clinical Implications Under the testing conditions of this study, ZES may be a promising candidate for the surface treatment of zirconia, providing enhanced adhesive bond strength to other restorative materials.
Zirconia dental ceramics have become popular for toothand implant-supported restorations from single units to full-mouth fixed partial dentures because of high and favorable mechanical properties, corrosion resistance, and biocompatibility.1,2 Zirconia’s bioinertness leads to favorable corrosion resistance properties.2,3 However, unlike feldspathic porcelain or lithium disilicate ceramic, hydrofluoric acid (HF) is unable to etch zirconia.1-5 Therefore, the conventional approaches to adhesive bonding to silica-based ceramics are not applicable to zirconia. The bioinertness of zirconia has made bonding to tooth structure, titanium, or other ceramic materials challenging. As a result, surface treatments or conditioning techniques are required to achieve predictable and durable adhesion. The adhesion of resin-based luting cements to zirconia has been evaluated,6-8 but these studies report less favorable adhesive outcomes than for silicate-based dental ceramics or glass-ceramics.6-8 HF (9.5%) solution has been used to etch zirconia by extending the immersion time (up to 24 hours)9 or by elevating the temperature (80 C) for up to 30 minutes.10 Additionally, HF solutions of higher concentrations (48%) have been used.11 However, these protocols have failed to improve adhesive bonding to zirconia.9-11 Alternative surface treatments such as selective infiltration technique, laser treatment, and coating with nanostructured alumina or tribochemical silica have been proposed.12-14 However, clinically significant improvements have been lacking, and these alternative treatments have caused surface microcracks, weakening the material and making it more prone to crack propagation. As an alternative method of bonding to zirconia restorations, phosphate ester primer such as 10-methacryloyloxydecyl dihydrogen phosphate (MDP) have been used successfully to improve adhesion to zirconia restorations.14-18 Therefore, alternative surface treatments are needed to improve the adhesive bond strength of zirconia without compromising its mechanical properties.18-20 Recently, highly translucent zirconia materials have been introduced, including a fully stabilized cubic/ tetragonal zirconia. The increased translucency has been achieved by materials with a high percentage of isotropic cubic phase with a larger grain size.24,25 This material may be a promising alternative to lithium disilicate and
THE JOURNAL OF PROSTHETIC DENTISTRY
Volume 120 Issue 3
feldspathic ceramic materials if it can be reliably bonded to tooth structure or other restorative materials. Also, recently, an HF solution containing nitric acid (Zircos E etching system, ZSAT; Eunjin Chemical Co) has been introduced that can etch the surface of zirconia at room temperature.15 Cho et al15 analyzed the surface roughness of zirconia after etching with ZSAT for 3 hours at 25oC. They also measured the shear bond strength to 4 different resin cements (Panavia F 2.0, Rely X Unicem, Superbond C&B, and Hot Bond of zirconia surfaces treated with ZSAT solution). They reported that ZSAT could increase the shear bond strength of zirconia.15 Recently, a new version of the etching solution (Zeta Etching Solution; Eunjin Chemical Co) has been introduced containing HF, hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), and phosphoric acid (H3PO4). The present authors are unaware of studies of the effects of this etching solution on the bond strength and mechanical properties of zirconia. Therefore, the purpose of this in vitro study was to examine the effects of ZER etching solution on these properties. The null hypothesis is that no differences will be found between the bond strength properties and surface hardness of zirconia specimens, with or without etching with ZES. MATERIAL AND METHODS Two types of partially stabilized tetragonal polycrystalline zirconia were used: Prettau (P) zirconia (Zirkonzahn) and anterior Prettau (AP) (Zirkonzahn). The materials were divided into 4 groups as follows: ZES-etched Prettau zirconia (PE) with ZES; ZES-etched anterior Prettau zirconia (APE); unetched Prettau zirconia (PU); and unetched anterior Prettau zirconia (APU). Specimens were milled by using a 5-axis milling machine (Zirconzahn). For shear bond strength and Vickers hardness tests, cylindrical zirconia specimens (10-mm diameter×5-mm height) were fabricated. For 3-point bend testing, rectangular specimens (25×2×2 mm) were fabricated. Before we undertook mechanical testing, specimens were airborne-particle abraded by using 50-mm Al2O3 particles for 15 seconds and immersed in ZES for 60 minutes in an ultrasonic bath at room temperature. Subsequently, specimens were cleaned with orthophosphoric acid for 1 minute and placed in an alcohol ultrasonic bath for 10 minutes. Silane was not used. A resin-based, dual-polymerizable, hydrophilic cement (URC; Bioloren) was used to bond the specimens (zirconiaezirconia). Unetched specimens were only airborne-particle abraded before bonding procedures. After 24 hours of storage in distilled water, a shear load was applied to the zirconiaezirconia interface by using a mechanical testing machine (model 5564; Instron
Ansari et al
September 2018
449
Figure 1. Scanning electron microscopy images of anterior Prettau zirconia specimens. A, B, Etched APE. C, D, Unetched APU. AP zirconia showed combination of tetragonal and cubic structure. Upon etching, pronounced surface roughness and etching were apparent in AP zirconia specimens. A, C, original magnification ×750; B, D, original magnification ×3300. APE, anterior Prettau zirconia; APU, anterior Prettau zirconia.
Corp) with a knife-edged rod at a crosshead speed of 0.5 mm/min.16 Seven specimens were used for each group. All mechanical testing machines were calibrated before starting measurements. The shear force required to separate the zirconia cylinder from the substrate zirconia was recorded in Newtons and divided by the contact surface area to determine the shear bond strength value in megapascals. Subsequently, the failure modes of the debonded zirconia specimens were analyzed by scanning electron microscopy (SEM) (Supra 40/40 VP; Carl Zeiss SMT). To further analyze the effect of ZES on the adhesive bonding of zirconia and its interfacial properties, a 3-point bend test was carried out according to International Organization for Standardization standard 9693:1999.26 Ten specimens were used in each group. Specimens were placed horizontally on a 20-mm-diameter annular, knife-edged support ring, and a knife-edged indenter was placed exactly in the center of the specimens. Force was applied at a crosshead speed of 0.5 mm/min until the specimens debonded. Zirconia debonding was
Ansari et al
determined by an abrupt decrease in the stress-strain graphs. The zirconiaezirconia bond strength was then calculated.27 Vickers hardness tests were conducted using a microhardness tester (model 1600-4963; Buehler), using previously reported protocols.17 Briefly, a 3-N force was applied to the specimen with a diamond indenter for 15 seconds. Each group contained 7 specimens, which were conditioned for 24 hours at 37 C. The mechanical testing machine was calibrated before taking measurements. All results were generated and reported in Vickers hardness units by the microhardness tester. To analyze the effects of ZES on the surface morphology of the zirconia specimens, SEM was used. SEM was also used to determine the mode of failure for each specimen. The specimens were coated with iridium (Ion Beam Sputtering; South Bay Technology) before EM analysis. One-way ANOVA and Student t tests were used to determine any significant differences between the values of shear bond strength and those of 3-point bend testing
THE JOURNAL OF PROSTHETIC DENTISTRY
450
Volume 120 Issue 3
Figure 2. Scanning electron microscopy images of Prettau zirconia specimens. A, B, Etched PE. C, D, Unetched PU. PU zirconia specimens showed tetragonal structure. Upon etching, modest changes in surface roughness were found. A, C, original magnification ×750; B, D, original magnification ×3300. PE. C, D, Unetched PU. PE, ZES-etched Prettau zirconia; PU, unetched Prettau zirconia.
with statistical software (SPSS for Windows v16.0; SPSS Inc) (a=.05). RESULTS SEM analysis (Figs. 1, 2) showed that etching the surface of zirconia with ZES for 30 minutes changed its shape, making the surface more irregular. These changes were more pronounced for APE (etched anterior Prettau) than PE (etched Prettau) specimens, which may be related to the presence of the cubic phase. Tetragonal structures were present in PU (unetched Prettau) specimens and tetragonal/cubic in the AP specimens. Etching with ZES changed the surface layer, as was apparent from the SEM images (Figs. 1, 2). Shear bond strength results are presented in Figure 3A. Etching with ZES significantly increased the shear bond strength of zirconiaezirconia (P<.05) in AP specimens, but the difference for group P was not significant (P>.05). Results of the 3-point bend test showed that etching in ZES significantly increased (P<.05) the bond strength of AP zirconia specimens (Fig. 3B).
THE JOURNAL OF PROSTHETIC DENTISTRY
However, the bond strength of the group P zirconia specimens was not significantly (P>.05) affected. SEM analysis of the mode of failure after shear bond strength testing revealed a partial cohesive mode of failure for APE specimens, whereas the PE specimens showed an adhesive mode of failure (Figs. 4, 5). Furthermore, as presented in the SEM images (Figs. 4, 5), etching with ZES resulted in a completely intermingled cement-zirconia layer for the APE and PE specimens. In contrast, 2 distinct layers of cement and zirconia were observed for the APU and PU specimens (Fig. 6). The microhardness measurements showed no significant decrease (P>.05) in the surface hardness of zirconia specimens after 30 minutes of etching in ZES (Fig. 7). DISCUSSION In the current study, a recently introduced acid etching solution (ZES) was used to condition the surface of zirconia to increase the adhesive bond strength of zirconia ceramic. Based on the results of this study, the
Ansari et al
Shear Bond Strength (MPa)
September 2018
451
NS
*
16 12 8 4 0
Unetched Prettau (PU) zirconia
Etched Prettau (PE) zirconia
Etched Unetched Anterior Anterior Prettau (APU) Prettau (APE) zirconia zirconia
A
Bond Strength (MPa)
Group *
NS
30 23 15 8 0
Unetched Prettau (PU) zirconia
Etched Prettau (PE) zirconia
Unetched Etched Anterior Anterior Prettau (APU) Prettau (APE) zirconia zirconia
Group
B
Figure 3. Etched and unetched AP zirconia and P zirconia specimens. A, Shear bond strength. B, Bond strength measurements *P<.05. NS, not significant.
null hypothesis that ZES would not cause any significant changes to the measured properties was partially rejected. The shear bond strength and 3-point bend test results showed that etching with ZES significantly increased the bond strength of anterior Prettau (AP) zirconia (P<.05). However, no significant increase in these values was observed for Prettau (P) specimens (P>.05). Furthermore, the etching procedure did not compromise the surface hardness of the tested materials; no significant decrease in the surface hardness of the tested zirconia specimens was observed after etching with ZES (P>.05). However, for a more comprehensive analysis of the effects of this etching material, additional types of resin cement should be evaluated. Moreover, the effects of ZES etching on the bonding of zirconia to different substrates needs to be analyzed further. Among the current ceramic materials, zirconia ceramics have attracted attention because of their extremely high strength and favorable esthetics.18 Additionally, zirconia biomaterials are bio-inert, meaning that they are resistant to corrosion and do not go through any composition or chemical changes when exposed to the oral environment. However, this bio-inertness makes bonding zirconia to tooth structure and other restorative dental materials challenging.18,19 Unlike silica-based ceramics, zirconia dental restorations cannot be etched
Ansari et al
with HF. Therefore, different methods have been proposed to increase the adhesive properties of zirconia ceramics.20,21 These surface modifications, however, reportedly compromise the strength of the materials and their long-term bond strength properties.22,23 The adhesive bonding properties of zirconia are even more important for the recently introduced fully stabilized cubic/tetragonal zirconia, which is highly translucent because of the presence of a high percentage of isotropic cubic phase with larger grain size.24,25 This material can be considered a promising alternative to lithium disilicate and feldspathic ceramic materials if it can be properly bonded to tooth structure or other restorative materials. The etching solution studied (Zeta etching solution; Eunjin Chemical Co) contained HF, hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), and phosphoric acid (H3PO4) and was capable of etching the surface of zirconia restorations. The authors are unaware of prior studies of the effects of this etching solution on the bond strength and mechanical properties of zirconia. The shear bond strength data in the current study correlated well with previously reported values.15 In addition, SEM analysis showed that after 30 minutes of etching with ZES, the surfaces of the zirconia specimens were completely etched, and surface irregularities were apparent. The anterior Prettau (AP) specimens were affected significantly more than the Prettau (P) specimens. This phenomenon can be attributed to the phasic structure of the highly translucent AP specimens. Etching with ZES for 30 minutes significantly increased the shear bond strength of the AP specimens. This result was also expected, as more changes to the surface morphological features of the AP zirconia were observed after etching. To further analyze the effect of ZES on the adhesive bonding strength of the zirconia materials, a modified 3-point bend test was used. The data obtained in this test correlated well with the shear bond strength test outcomes, confirming that etching in ZES significantly increased the amount of force require to detach the AP zirconia specimens. From the SEM analysis an adhesive failure mode was observed in the Prettau specimens, due to the inability of the ZES to etch the surface. In contrast, etching with ZES for 30 minutes changed the shape of the highly translucent AP zirconia specimens, and the SEM analysis found a cohesive mode of failure. The differences in the surface patterns after etching may be attributed to the composition differences between the AP and P zirconia groups and the presence of the cubic phase in more translucent zirconia ceramics.
THE JOURNAL OF PROSTHETIC DENTISTRY
452
Volume 120 Issue 3
Figure 4. Scanning electron microscopy analysis of zirconiaezirconia interface after shear bond strength test showed partial cohesive mode of failure for AP specimens for etched groups. Note intermingled interface of cement and etched zirconia layer for AP/AP specimens. Original magnification A, ×750; B, ×1800; C, ×3300. AP, anterior Prettau group; C, cement Zir, zirconia restoration. Red box, area of interest.
Figure 5. Scanning electron microscopy analysis of zirconiaezirconia interface after shear bond strength test showed adhesive mode of failure for P specimens for etched groups. Note modest interlocked interface of cement and etched zirconia layer for P/P specimens. Original magnification A, ×750; B, ×1800; C, ×3300. C, cement; P, prettau zirconia group; Zir, zirconia restoration. Red box, area of interest.
Figure 6. Scanning electron microscopy analysis of zirconiaezirconia interface after shear bond strength of unetched specimens. Two distinct layers of cement and zirconia were observed. A, APU. B, PU. Original magnification, A, B, ×750; inset original magnification ×1200. APU. B, PU. APU, anterior Prettau zirconia; PU, unetched Prettau zirconia. Red box, area of interest magnified.
The surface hardness analysis found that etching in ZES for 30 minutes did not compromise the surface hardness of the materials. Further analyses are required to examine the effects of the etching technique on the bond strength of zirconia to tooth structure and other restorative materials. Limitations of the current study are that only 1 type of cement was used and that no silane or MDP monomer was applied. Additionally, the effects of the etching solution on the bonding of zirconia to only 1 THE JOURNAL OF PROSTHETIC DENTISTRY
substrate were analyzed. In future work, different types of commercially available adhesive resin cements will be tested and the effects of ZES etching solution on bonding of zirconia to other substrates such as lithium disilicate, titanium, and feldspathic porcelain. Results of the current study demonstrate that ZES is capable of changing the morphology of zirconia, leading to a significant increase in bond strength without any adverse effect on the surface hardness of the material. Ansari et al
September 2018
453
NS NS
NS
1500
VHN
1125 750 375 0
Unetched Prettau (PU) zirconia
Etched Unetched Etched Prettau (PE) Anterior Anterior zirconia Prettau (APU) Prettau (APE) zirconia zirconia
Group Figure 7. VHN of the etched and unetched AP and regular P specimens after 24 hours of storage in distilled water at 37 C, showing slight decrease in surface hardness values. NS, not significant; VHN, Vickers hardness number.
CONCLUSIONS Based on the findings of this in vitro study, the following conclusions were drawn: 1. Etching in ZES for 30 minutes significantly increased the bond strength of translucent anterior Prettau zirconia restorations. 2. Etching with ZES did not adversely affect the surface hardness of tested zirconia specimens. REFERENCES 1. Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater 2008;24:299-307. 2. Kelly JR, Denry I. Stabilized zirconia as a structural ceramic: an overview. Dent Mater 2008;24:289-98. 3. Denry I, Kelly JR. Emerging ceramic-based materials for dentistry. J Dent Res 2014;93:1235-42. 4. Blatz MB, Sadan A, Kern M. Resin-ceramic bonding: a review of the literature. J Prosthet Dent 2003;89:268-74. 5. Sahafi A, Peutzfeldt A, Asmussen E, Gotfredsen K. Bond strength of resin cement to dentin and to surface-treated posts of titanium alloy, glass fiber, and zirconia. J Adhes Dent 2003;5:153-62. 6. Guazzato M, Proos K, Quach L, Swain M. Strength, reliability and mode of fracture of bilayered porcelain/zirconia (Y-TZP) dental ceramics. Biomaterials 2004;25:5045-52. 7. 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. 8. Kosmac T, Oblak C, Jevnikar P, Funduk N, Marion L. Strength and reliability of surface treated Y-TZP dental ceramics. J Biomed Mater Res Part B Appl Biomater 2000;53:304-13. 9. Sriamporn T, Thamrongananskul N, Busabok C, Poolthong S, Uo M, Tagami J. Dental zirconia can be etched by hydrofluoric acid. Dent Mater J 2014;33:79-85.
Ansari et al
10. Smielak B, Klimek L. Effect of hydrofluoric acid concentration and etching duration on select surface roughness parameters for zirconia. J Prosthet Dent 2015;113:596-602. 11. Chaiyabutr Y, McGowan S, Phillips KM, Kois JC, Giordano RA. The effect of hydrofluoric acid surface treatment and bond strength of a zirconiaveneering ceramic. J Prosthet Dent 2008;100:194-202. 12. Casucci A, Monticelli F, Goracci C, Mazzitelli C, Cantoro A, Papacchini F, et al. Effect of surface pre-treatments on the zirconia ceramic-resin cement microtensilebond strength. Dent Mater 2011;27:1024-30. 13. Blatz MB, Sadan A, Martin J, Lang B. In vitro evaluation of shear bond strengths of resin to densely-sintered high-purity zirconium-oxide ceramic after long-term storage and thermal cycling. J Prosthet Dent 2004;91:356-62. 14. Blatz MB, Chiche G, Holst S, Sadan A. Influence of surface treatment and simulated aging on bond strengths of luting agents to zirconia. Quintessence Int 2007;38:745-53. 15. Cho JH, Kim SJ, Shim JS, Lee KW. Effect of zirconia surface treatment using nitric acid-hydrofluoric acid on the shear bondstrengths of resin cements. J Adv Prosthodont 2017;9:77-84. 16. Moshaverinia A, Ansari S, Moshaverinia M, Roohpour N, Darr JA, Rehman I. Effects of incorporation of hydroxyapatite and fluoroapatite nanobioceramics into conventional glass ionomer cements (GIC). Acta Biomater 2008;4: 432-40. 17. Moshaverinia A, Brantley WA, Chee WW, Rohpour N, Ansari S, Zheng F, et al. Measure of microhardness, fracture toughness and flexural strength of N-vinylcaprolactam (NVC)-containing glass-ionomer dental cements. Dent Mater 2010;26:1137-43. 18. Cura C, Özcan M, Isik G, Saracoglu A. Comparison of alternative adhesive cementation concepts for zirconia ceramic: glaze layer vs zirconia primer. J Adhes Dent 2012;14:75-82. 19. Wang C, Niu L, Wang Y, Jiao K, Liu Y, Zhou W, et al. Bonding of resin cement to zirconia with high pressure primer coating. PLoS One 2014;9: 101174-7. 20. Karimipour-Saryazdi M, Sadid-Zadeh R, Givan D, Burgess JO, Ramp LC, Liu PR. Influence of surface treatment of yttrium-stabilized tetragonal zirconium oxides and cement type on crown retention after artificial aging. J Prosthet Dent 2014;111:395-403. 21. Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Selective infiltration-etching technique for a strong and durable bond of resin cements to zirconia-based materials. J Prosthet Dent 2007;98:379-88. 22. Kern M, Wegner SM. Bonding to zirconia ceramic: adhesion methods and their durability. Dent Mater 1998;14:64-71. 23. Attia A, Kern M. Long-term resin bonding to zirconia ceramic with a new universal primer. J Prosthet Dent 2001;106:319-27. 24. Zhang F, Inokoshi M, Batuk M, Hadermann J, Naert I, Van Meerbeek B, et al. Strength, toughness and aging stability of highly-translucent Y-TZP ceramics for dental restorations. Dent Mater 2016;32:327-37. 25. Harada K, Raigrodski AJ, Chung KH, Flinn BD, Dogan S, Mancl LA. A comparative evaluation of the translucency of zirconias and lithium disilicate for monolithic restorations. J Prosthet Dent 2016;116: 257-63. 26. Lenz J, Schwarz S, Schwickerath H, Sperner F, Schafer A. Bond strength of metaleceramic systems in three-point flexure bond test. J Appl Biomater 1995;6:55-64. 27. Zinelis S, Barmpagadaki X, Vergos V, Chakmakchi M, Eliades G. Bond strength and interfacial characterization of eight low fusing porcelains to cp Ti. Dent Mater 2010;26:264-73. Corresponding author: Dr Alireza Moshaverinia Weintraub Center for Reconstructive Biotechnology Division of Advanced Prosthodontics UCLA School of Dentistry 10833 Le Conte Avenue, B3-023 CHS Los Angeles, CA 90095-1668 Email: [email protected] Copyright © 2017 by the Editorial Council for The Journal of Prosthetic Dentistry.
THE JOURNAL OF PROSTHETIC DENTISTRY