Influence of zirconia surface treatment on veneering porcelain shear bond strength after cyclic loading Atsushi Nishigori, DDS, MSD,a Takamitsu Yoshida, DDS, PhD,b Marco C. Bottino, DDS, PhD,c and Jeffrey A. Platt, DDS, MSd Indiana University School of Dentistry, Indianapolis, Ind Statement of problem. The inﬂuence of yttria-stabilized tetragonal zirconia polycrystal surface treatment on veneering porcelain shear bond strength after cyclic loading is not fully understood. Purpose. The purpose of this study was to investigate the inﬂuence of yttria-stabilized tetragonal zirconia polycrystal surface treatment on veneering porcelain shear bond strength and cyclic loading on the shear bond strength between the 2 materials. Material and methods. A total of 48 cylinder-shaped yttria-stabilized tetragonal zirconia polycrystal specimens were fabricated with computer-aided design and computer-aided manufacturing (CAD/CAM), sintered for 8 hours at 1500 C, ground with 320-grit diamond paper, and divided into 4 groups (n¼12) according to surface treatment as follows: no treatment/control; heat treatment of 650 C to 1000 C at 55 C/min; airborne-particle abrasion with 50-mm alumina at 0.4 MPa pressure for 10 seconds; or heat treatment after abrasion. A veneering porcelain cylinder was built and ﬁred on the prepared yttria-stabilized tetragonal zirconia polycrystal specimens. The shear bond strength was tested with a universal testing machine. Six specimens from each group were subjected to cyclic loading (10 000 cycles, 1.5 Hz, 10 N load) before testing. Results. The mean SD ranged from 10.7 15.4 to 34.1 10.0. Three-way ANOVA found no statistically signiﬁcant (P>.05) effect of surface treatment and cyclic loading on shear bond strength. The Sidak multiple comparisons procedure found that cyclic loading specimens had signiﬁcantly lower shear bond strength than noncyclic loading specimens after airborne-particle abrasion without heat treatment (P¼.013). Conclusions. Within the limitations of this study, the shear bond strength between yttria-stabilized tetragonal zirconia polycrystal and veneering porcelain was not signiﬁcantly affected by surface treatment. Airborne-particle abrasion without subsequent heat treatment should be avoided as a surface treatment in fabrication methods. (J Prosthet Dent 2014;-:---)
Clinical Implications The use of airborne-particle abrasion alone on an yttria-stabilized tetragonal zirconia polycrystal core may compromise the bond strength of the overlying veneering porcelain. If used, heat treating the core before veneering should enhance restoration longevity. Numerous studies have investigated the cause of increased chipping or cracking of veneering porcelain (VP) on yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) copings compared with metal counterparts.1,2 The cause
of chipping or cracking of VP has been reported to be multifactorial.1,2 A signiﬁcant difference between the Y-TZP and metal copings is the adhesion mechanism of the coping to the VP. A recent clinical report highlighted the
This study is partially supported by a grant from Shofu Inc, Japan. a
Resident, Department of Restorative Dentistry. Visiting Assistant Professor, Department of Restorative Dentistry. c Assistant Professor, Department of Restorative Dentistry. d Associate Professor, Department of Restorative Dentistry. b
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problem of clinical delamination of the VP from the Y-TZP core material.3 Whereas mechanical and chemical bonds resulting from suitable metal oxidation and interdiffusion of ions seem to play an important role in the
Volume metal coping-VP interface, the bonding mechanism of VP to Y-TZP copings is still not yet fully understood.4 Bond strength can be compromised by residual stresses resulting from a mismatch of the coefﬁcient of thermal expansion (CTE) between the VP and coping. VP with a slightly lower CTE than that of the metal coping is used in metal ceramic restorations, because the slightly lower CTE generates compressive stresses in the VP, which helps offset low tensile strength. Likewise, this concept has been discussed in numerous studies concerning the VP and Y-TZP coping material.2,4-6 Moreover, to generate acceptable residual stresses within the VP, dental manufacturers have attempted to develop low fusing VP with similar CTE to the Y-TZP coping material. Saito et al5 concluded in an in vitro study that similar clinical behavior can be expected for Y-TZP ceramic restorations when VP with a slightly lower CTE is used. Thermal conductivity is another important factor.7,8 Whereas dental metal alloys used in metal ceramic restorations have a high thermal conductivity (in the range of 300 W m1 K1 for noble alloys), thermal conductivity of the Y-TZP coping materials is low (2-2.2 W m1K1), similar to feldspathic VP. The lower coping thermal conductivity retards the porcelain cooling rate at the interface between the VP and the Y-TZP coping, changing the effect of CTE and resulting residual thermal stresses. Guazzato et al7 concluded that crack incidence increased with increased VP thickness and a faster cooling rate in nominally compatible porcelain/zirconia systems in the geometrically conﬁgured specimens tested. Therefore, the design of the framework and the thickness of the VP should be considered as important factors in the success of Y-TZP ceramic restorations. The high initial strength and fracture toughness of Y-TZP results from a physical property known as transformation toughening.9 The transformation from a tetragonal to a monoclinic (t/m) crystalline structure
occurs when the material is subjected to stress and is accompanied by a 3% to 4% volumetric expansion of particles. This results in a complex stress state that affects crack propagation. However, according to Aboushelib et al,10 Y-TZP ceramic restorations have a high percentage of interfacial failure when there are structural defects between the core and veneer ceramic. The presence of a monoclinic phase of zirconium oxide at the core-veneer interface may be the cause of microspaces found at the interface. In addition, Guazzato et al11 supported the idea that phase transformation is accompanied by the generation of localized stresses, which may nucleate microcracks in the glass phase of the veneer. Airborne-particle abrasion is likely to trigger the (t/m) transformation of Y-TZP. The important consequence of the phase transformation is the amount of compressive stress at the surface.12 Kosmac et al13 reported that airborneparticle abrasion is more effective than grinding in inducing the phase transformation. The efﬁcacy of the phase transformation depends on the mean grain size, with larger grain size resulting in more transformation.14 However, the monoclinic phase created by the airborne-particle abrasion or grinding may lead to tensile stress in the VP because of the quite low CTE of the monoclinic phase.15 Therefore, heat treatment has been recommended to reverse the monoclinic phase to the tetragonal phase.16 Y-TZP ceramic restorations are subjected to repetitive mastication forces in the oral environment. The fatigue behavior may act on residual stresses, leading not only to crack propagation but also to the generation of tetragonal-monoclinic transformation in Y-TZP. Moreover, stress-generating surface treatments, such as grinding or airborne-particle abrasion and VP ﬁring over the Y-TZP coping, can trigger the (t/m) transformation.11 Some studies have mentioned that little monoclinic phase was detected in Y-TZP with heat treatment at approximately 900 C after airborne-particle
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abrasion.17,18 de Kler concluded18 that sintered tetragonal structure was converted to monoclinic structure up to a depth of 27 mm after airborne-particle abrasion and reversed to tetragonal structure after porcelain veneering. In other words, heat treatment and the ﬁring process caused reverse transformation and the release of the compressive stresses. In fact, Doi et al17 found signiﬁcantly higher debonding/ crack-initiation strength with heat treatment after airborne-particle abrasion than without. However, Fischer et al15 observed that heat treatment signiﬁcantly decreased the shear bond strength of both polished and airborneparticle-abraded surfaces. Failure a few years after cementation is more likely to involve subcritical crack growth (SCCG) or cyclic fatigue (or both).12 SCCG is deﬁned as occurring at stresses below the critical value until the crack reaches its critical length, leading to fast failure. Speciﬁcally, it has been reported that ceramic restorations are indeed susceptible to SCCG in the humidity of the oral environment.19,20 When subjected to cyclic fatigue, the mechanical defects created by airborne-particle abrasion may be the foci of stress concentration and residual stress and the origins of failure within the interface. Moreover, the (t/m) phase transformation due to the airborne-particle abrasion may cause the SCCG and volumetric expansion of the Y-TZP surface as a result of the fatigue loading. Several studies have reported that the bond strength between Y-TZP copings and VP is signiﬁcantly affected by some surface treatments such as airborne-particle abrasion, heat treatment, and use of liner porcelain.10,15,21-23 However, the inﬂuence of different surface treatments on the bond strength of VP to zirconia copings is not clear. The objective of this study was to increase the understanding of the inﬂuence of different core surface preparation techniques on the ability of a veneered Y-TZP to resist failure after fatigue loading. Therefore, the present study proposed to test the hypotheses that shear bond
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strength between Y-TZP and VP would be increased by Y-TZP surface treatment with heat and that airborne-particle abrasion and heat after airborneparticle abrasion and cyclic loading would decrease the shear bond strength between Y-TZP and VP.
MATERIAL AND METHODS A total of 48 cylinder-shaped Y-TZP (Lot No. P02286, Diazir; Ivoclar Vivadent) ceramic specimens were manufactured with a computer-aided design and computer-aided manufacturing (CAD/CAM) machine (Sirona InLab MC XL; Sirona Dental Systems LLC) with the required shape and dimensions (6 mm in diameter and 4 mm in height). The specimens were sintered in a hightemperature furnace (Programat S1; Ivoclar Vivadent) at 1500 C according to the manufacturer’s instructions as previously reported.24 In addition, all of the materials were wet ground on one side with a 320-grit silicon carbide paper to standardize the zirconia surface. The specimens were divided into 4 groups (n¼12) according to surface treatment. As a control (group C), no further treatment was applied to the specimens after grinding. Group H was heat treated (Cerampress QEX Furnace; Dentsply Prosthetics) with an increasing rate of 55 C/min from 650 C to 1000 C as the pretreatment consistent with manufacturer recommendations. Group S was airborne-particle abraded with 50 mm alumina (Al2O3) particles under a pressure of 0.4 MPa for 10 seconds in a direction perpendicular to the surface and at a distance of 10 mm with an airborne-particle abrasion device (Sandstorm expert; Vaniman Co). In group SH, heat treatment was performed after airborne-particle abrasion. All of the specimens were cleaned in an ultrasonic bath containing acetone for 15 minutes. On the prepared surface of each specimen, a VP cylinder was built with a custom-made split silicone mold. Porcelain powder (Vintage ZR; Shofu Inc) was mixed with the appropriate amount of distilled water, added, and
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3 condensed to form a cylinder 2.4 mm in diameter and 2 mm in height. Excess water was removed with tissue paper. Firing was performed in a calibrated porcelain furnace (Pro Press 100; Whip Mix Corp) with an increasing rate of 45 C/min from 650 C to 910 C, according to manufacturer recommendations. A second ﬁring was required to compensate for the porcelain shrinkage that occurred during the ﬁrst ﬁring (Fig. 1). Each specimen was embedded in an acrylic resin mold with Type 4 stone (Silky-Rock, Whip Mix Corp). The 12 specimens for each group were divided into 2 subgroups of 6 specimens, either with or without cyclic loading, before the shear bond strength test. The cyclic loading subgroup was subjected to cyclic fatigue loading in a mechanical cycling machine (Electropuls 3000; Instron). The specimens were securely mounted in a steel supporting vice and a custom-made acrylic resin mold. A cylindrical loading rod with 10-mm diameter was used to axially induce 10-N loads on the top of the cylinders
for 10 000 cycles with a frequency of 1.5 Hz under room conditions. All specimens were mounted and placed in a shear-testing device (Ultradent Products Inc) with a semicylindrical loading surface (2.4 mm in diameter). The shear bond strength was determined by using a screw-driven universal testing machine (Sintech ReNew 1123; MTS) at a crosshead speed of 0.5 mm/min (Fig. 2). The shear bond strength was calculated from the following formula: Shear bond strength (MPa) ¼ Load (N)/area (mm2). Failure mode was observed and classiﬁed as interfacial, cohesive, or combination. Analysis was made under an optical microscope (Measurescope UM-2; Nikon) at 40 magniﬁcation after shear bond strength testing. Randomly selected specimens representing the numerically highest (group S without cyclic loading) and lowest (group S with cyclic loading) shear bond strength groups were sputter-coated with gold and imaged with a scanning electron microscope (SEM) (JEOL 6390 LV; JEOL Ltd).
Custom Made Split Silicone Mold Y-TZP
1 Split silicone mold for veneering porcelain application. Semicylindrical Jig
Acrylic Resin Mold
2 Shear bond strength testing device.
Volume Summary statistics were calculated for the shear bond strength data for each of the 4 surface treatment groups with and without cyclic loading. The effects of heat treatment, airborne-particle abrasion, and cyclic loading on shear bond strength were evaluated with 3-way ANOVA, followed by pairwise group comparisons with the Sidak multiple comparisons procedure (a¼.05). Based on previous studies,3,4,15 the standard deviations were expected to be 4 MPa for shear bond strength. With a sample size of 6 specimens per surface treatment group, with and without cyclic loading, the study was designed to have 80% power to detect a difference of 9.8 MPa between any 2 treatment groups, assuming 2-sided tests conducted at an overall 5% signiﬁcance level.
RESULTS Adjusted mean values using ranks were used because of the non-normal distribution of the bond strength measurements (Table I). The highest mean shear bond strength was recorded for the airborne-particle abrasion group without cyclic loading (34.1 10 MPa). The lowest mean shear bond strength was for the airborne-particle abrasion group with cyclic loading (10.7 15.4 MPa). Two specimens in the airborneparticle abrasion group with cyclic loading had complete delamination of
VP during the fatigue loading, and the shear bond strength of these specimens was included as 0 MPa. The Sidak multiple comparisons procedure was performed to make all pairwise group comparisons among the 8 subgroups in the present study. The 3-way ANOVA found no statistically signiﬁcant effect of surface treatment or cyclic loading on shear bond strength (Tables I, II). No signiﬁcant effect of airborneparticle abrasion was found, either overall (P¼.39) or for any heat treatment or cyclic loading combination (P>.26). Likewise, no signiﬁcant effect of heat treatment was found, either overall (P¼.28) or for any cyclic loading or airborne-particle abrasion combination (P>.06). Cyclic loading specimens had signiﬁcantly lower shear bond strength than noncyclic loading specimens after airborne-particle abrasion without heat treatment (P¼.013). Cyclic loading had no effect for any other heat treatment or airborne-particle abrasion combination (P>.40). With regard to failure mode classiﬁcation, all of the specimens showed a combination of interfacial and cohesive failure independent of the surface treatment and with or without cyclic loading. Representative SEM micrographs of the highest mean bond strength group (group S without cyclic loading; Figs. 3, 4) and the lowest mean bond strength group (group S with cyclic loading; Figs. 5, 6) are given.
DISCUSSION In comparison with other studies1,3 using similar test designs and surface conditions to measure shear bond strength, the values obtained in the present study for the airborne-particle abrasion group and the heat treatment group without cyclic loading are in the same range. However, Kim et al21 reported that the mean shear bond strength of VP to Y-TZP zirconia copings ground with 320-grit silicon carbide paper was 32.08 MPa, which was higher than the control group shear bond strength obtained in this study. The Y-TZP specimen size and shape used were different from that of the present study. The volume of Y-TZP used in the cited study was 4 times larger than the volume used in this study. Given that the nominal Y-TZP/VP thickness ratio of ﬁxed restorations in clinical situations is
All Effects had a Num DF of 1 and a Den DF of 40.
Summary statistics of bond strength (MPa) and ranks of bond strength measurements
S H SH
y y y
n y y
y n y
Group C, control group; group S, airborne-particle abraded; group H, heat-treated; group SH, heat-treatment performed after airborne-particle abrasion; y, yes; n, no; Q1, ﬁrst quartile; Q3, third quartile; Min, minimum; Max, maximum. Statistically similar groups were determined with ranked data and are identiﬁed with superscript letters (a¼.05).
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3 Representative scanning electron microscope micrograph of debonded surface of randomly selected specimen from group S (veneering porcelain/airborne-particle-abraded zirconia, no cyclic loading) (35).
4 Scanning electron microscope micrograph of failure junction in Figure 3 (1500).
5 Representative scanning electron microscope micrograph of debonded surface of specimen from group S. Veneering porcelain/airborne-particle-abraded zirconia, with cyclic loading (35). 0.5 to 1.0 mm, the ratio used in this study may be more clinically relevant for cooling rate differences.
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In the present study, the Y-TZP (Diazir) and VP (Vintage ZR; Shofu Inc) were from different manufacturers. The
CTE of the Y-TZP (10.6106/ C) was slightly higher than that of the VP (9.4106/ C). An acceptable CTE of the VP was reported to be approximately 1.0106/ C below the CTE of the Y-TZP.25,26 In addition, Saito et al5 found no correlation of shear bond strength with the CTE mismatch between Y-TZP copings and 5 VPs for zirconia used in the range of 1.0 to 1.7106/ C. Therefore, the combination of VP and Y-TZP products used in the present study agrees with previous studies in terms of the CTE difference. However, in terms of the microstructural composition or properties, the correlation between the VP and Y-TZP used in the present study is unclear. According to a literature review by Denry,12 microstructural analyses of the interface between VP and Y-TZP copings are required to better establish the role of preexisting stresses cause by the thermal expansion of the phase transformation and of intergranular stresses on the transformability of Y-TZP. Therefore, the effect of microstructural composition and properties of Y-TZP and VP should be considered. The ﬁrst hypothesis, that the shear bond strength between Y-TZP and VP would be increased by Y-TZP surface treatment using heat treatment, airborne-particle abrasion, or heat treatment after airborne-particle abrasion, was not supported. No statistically signiﬁcant effect was found for shear bond strength according to surface treatment in this study. For the effect of airborne-particle abrasion, the mean SBS value of the airborne-particle abrasion group without cyclic loading was higher than that of the control group, but the pairwise tests found no signiﬁcant effect of airborne-particle abrasion overall. This ﬁnding was comparable to previous studies.15,22 In contrast, some studies have reported that airborne-particle abrasion was found to decrease the percentage of interfacial failure.2,21 However, the results of the previous studies cannot be compared with those in this study because of the differences in
or cracking rates of VP on Y-TZP copings, future studies should investigate microstructural properties, wetting properties, residual stress, and impact fatigue.
6 Higher-magniﬁcation scanning electron microscope micrograph of surface shown in Figure 5 (2500). Note presence of veneering porcelain remnants throughout zirconia surface of specimen that underwent cyclic loading, indicating mostly cohesive failure (Figs. 5, 6), as opposed to predominant interfacial failure for noncyclic loaded group (Figs. 3, 4). study methodology, design, and control specimen fabrication. All of the specimens had a combination of interfacial and cohesive failure regardless of surface treatment, with or without cyclic loading. The interfacial failure pattern has been related to increased stress caused by factors such as the difference of the elastic moduli, the test methodology, and the mismatch of the CTE.27-30 Moreover, airborneparticle abrasion may compromise the mechanical strength of the interface by initiating surface defects on the Y-TZP coping that concentrate stress and lead to failure.30,31 The results of the present study indicated no signiﬁcant effect of heat treatment overall, which is in agreement with a previous study by Fischer.15 He concluded that even if heat treatment can relax the compressive stresses at the surface, microcracks did not close at this temperature. In contrast, some studies have reported that heat treatment in the temperature range 850 C to 1000 C induces the reverse transformation.32,33 According to Denry,12 mechanical and thermal residual stresses have been proven to play an important role in the mechanical performance of Y-TZP restorations. The mechanical residual stress from grinding or airborne-particle abrasion and thermal residual stress develop
from the veneering process or heat treatment. The effect of mechanical retention from airborne-particle abrasion may not outweigh the inﬂuence of the mechanical and thermal residual stresses of the mechanical defects. In the present study, cyclic loading specimens had signiﬁcantly lower shear bond strength than noncyclic loading specimens after airborne-particle abrasion without heat treatment. The second hypothesis, that cyclic loading will decrease the shear bond strength between Y-TZP and VP, was partially accepted. According to Harding et al,34 cyclic loading did not affect bond strength, regardless of surface treatment, including airborne-particle abrasion. However, in the previous study, microtensile bond strength testing was completed on the Y-TZP with pressable veneer porcelain instead of layering veneer porcelain. The design of the present study has limitations with regard to clinical situations. According to Al-Dohan,1 a curved knife wrapping around the cylinder shape of VP was recommended to minimize load concentration. Here, a semicylindrical jig was used for shear bond strength testing. The potential for nonuniform interfacial stress by the jig may account for high standard deviations in the data.2,10,21-22 To further understand the high chipping
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Within the limitations of this in vitro study, the following conclusions were drawn. Shear bond strength between Y-TZP and VP is not affected statistically by surface preparation with heat treatment, airborne-particle abrasion, or heat treatment after airborneparticle abrasion. A signiﬁcant difference in shear bond strength is found with airborne-particle abrasion before and after cyclic loading. This difference suggests that airborne-particle abrasion should be avoided as a surface treatment, particularly in the absence of heat treatment. Using a cyclic fatigue methodology with bond strength testing provides better discrimination between the treatment groups.
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30. Guazzato M, Proos K, Sara G, Swain MV. Strength, reliability, and mode of fracture of bilayered porcelain/core ceramics. Int J Prosthodont 2004;17:142-9. 31. Kern M, Barloi A, Yang B. Surface conditioning inﬂuences zirconia ceramic bonding. J Dent Res 2009;88:817-22. 32. Denry IL, Peacock JJ, Holloway JA. Effect of heat treatment after accelerated aging on phase transformation in 3Y-TZP. J Biomed Mater Res B Appl Biomater 2010;93:236-43. 33. Guazzato M, Albakry M, Quach L, Swain MV. Inﬂuence of surface and heat treatments on the ﬂexural strength of a glass-inﬁltrated alumina/zirconiareinforced dental ceramic. Dent Mater 2005;21:454-63. 34. Harding AB, Norling BK, Teixeira EC. The effect of surface treatment of the interfacial surface on fatigue-related microtensile bond strength of milled zirconia to veneering porcelain. J Prosthodont 2012;21: 346-52. Corresponding author: Dr Jeffrey A. Platt DS 118 1121 W Michigan St Indianapolis, IN 46202 E-mail: [email protected]
Acknowledgments The authors thank Dr David T. Brown, Dr John A. Levon, Dr Masatoshi Ando, Mr George Eckert, and Meoghan MacPherson for their signiﬁcant support in the completion of this project. Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.