Abstract
Objective
The aim of this in vitro study was to evaluate the effect of core ceramic grinding on the fracture behaviour of bilayered zirconia under two loading schemes.
Methods
Interfacial surfaces of sandblasted zirconia disks (A) were ground with 80 (B), 120 (C) and 220 (D) grit diamond discs, respectively. Surface roughness and topographic analysis were performed using a confocal scanning laser microscope (CSLM) and a scanning electron microscopy (SEM). Relative monoclinic content was evaluated using X-ray diffraction analysis (XRD) then reevaluated after simulated veneer firing. Biaxial fracture strength ( σ ) and Weibull modulus ( m ) were calculated either with core in compression (subgroup Ac-Dc) or in tension (subgroup At-Dt). Facture surfaces were examined by SEM and energy dispersive X-ray spectroscopy (EDS). Maximum tensile stress at fracture was estimated by finite element analysis. Statistical data analysis was performed using Kruskal–Wallis and one-way ANOVA at a significance level of 0.05.
Results
As grit size of the diamond disc increased, zirconia surface roughness decreased ( p < 0.001). Thermal veneering treatment reversed the transformation of monoclinic phase observed after initial grinding. No difference in initial ( p = 0.519 for subgroups Ac-Dc) and final fracture strength ( p = 0.699 for subgroups Ac-Dc; p = 0.328 for subgroups At-Dt) was found among the four groups for both loading schemes. While coarse grinding slightly increased final fracture strength reliability ( m ) for subgroups Ac-Dc. Two different modes of fracture were observed according to which material was on the bottom surface. Components of the liner porcelain remained on the zirconia surface after fracture for all groups.
Significance
Technician grinding changed surface topography of zirconia ceramic material, but was not detrimental to the bilayered system strength after veneer application. Coarse grinding slightly improved the fracture strength reliability of the bilayered system tested with core in compression. It is recommended that veneering porcelain be applied directly after routine lab grinding of zirconia ceramic, and its application on rough zirconia cores may be preferred to enhance bond strength.
1
Introduction
Core veneered all-ceramic dental prostheses have become a popular restoration of choice for dental crown and fixed partial denture (FPD) applications because of their excellent chemical durability, biological compatibility and aesthetic outcomes . Both lithium disilicate glass-ceramic (LDG) and zirconium-oxide ceramic (Zirconia) are the most frequently used dental ceramics, with LDG having higher translucency and lower mechanical strength than zirconia . Chipping and delamination in the porcelain veneering layer have been suggested to be the most common modes of failure in zirconia . Recent clinical researches and systematic reviews have revealed a high rate of veneer chipping for zirconia-based fixed dental prosthesis that varies between 13% and 28% over a 3- to 10-year period , while the chipping rate of LDG is reported to be 10% during a 8-year observation period . Since the stability of composite structures is related to its weakest part, core–veneer bond can essentialy affect the crown reliability. An in vitro study showed that the zirconia–veneer bond strength was inferior to LDG systems suggesting that the layered zirconia frameworks are more susceptible to delamination and chipping under function . Delamination failures in all-ceramic restorations either originate from the veneer and propagate to the interface, or from the ceramic/veneer interface . A study on failure behavior of ceramic FPDs has reported that approximately 70–78% of connector fractures originate from the core/veneer interface . Accordingly, the interface of core and veneer ceramics is deemed to be linked for the most part to the successful performance of core/veneer bilayered restorations.
The success of veneered zirconia restorations depends on mechanical adhesion between the veneering ceramics and zirconia core . Grinding of zirconia ceramic is an inevitable lab procedure which enables adjustment of the marginal fit and the framework occlusion before applying veneering ceramics . Grinding results in the development of an interface with complex topography, roughness and phase transformation of zirconia crystals. These changes may affect the interfacial adhesion as well as the mechanical integrity.
Surface roughness is related to zirconia/veneer interfacial failure and surface residual stresses , but the effect of interfacial roughness on fracture behavior of bilayered ceramic structures remains a topic with divergent viewpoints. From one perspective it was assumed that a rough interface may transfer localized high tensile stresses from core to veneer layer and lower the strength of the veneering porcelain, and consequently promote fracture at the interface . In comparison, a smooth core-veneer interface was suggested to increase fracture strength of bilayered specimens of In-Ceram Alumina . On the other hand, surface topography change of LDG-ceramic materials after technician grinding was deemed not to be detrimental to the strength of a bilayered system after veneer application . The above findings, however, were only for glass infiltrated alumina and LDG ceramic . In comparison with the present study methods, those studies only considered air-abrasion , liner application and surface agents on bilayered zirconia systems tested with veneer in tension.
The aim of this study was to assess the effect of core ceramic grinding on fracture behaviour of a veneered zirconia system. Two different loading schemes were used to represent schematic clinical stress distributions in bilayered crowns and connector areas of FPDs, respectively. The primary null hypothesis was that grinding of zirconia ceramic had no influence on the fracture behavior of the bilayered structure.
2
Materials and methods
2.1
Sample preparation
152 zirconia core specimens disks (1.2 mm in height, 15 mm in diameter) were fabricated with the Katana system (Kuraray Noritake Dental Inc., Tokyo, Japan) and then were airborne-particle abraded (Silfradent SRL, Sofia, Italy) with 50 μm Al 2 O 3 at 0.2 MPa pressure for 20 s with a distance of 10 mm according to the manufacturer instructions. All specimens were cleaned in an ultrasonic cleaner with distilled water for 5 min prior to further treatments.
2.2
Treatment of core interfacial surface
To generate a similar core–veneer interfacial topography to that ground with diamond burs in the clinical situation and form a comparable flat surface of the core specimens, a series of MD-Piano diamond discs (Struers, Ballerup, Denmark) were used for grinding on an automatic metallographic lapping machine (Tegramin-30, Struers, Ballerup, Denmark).
The prepared core specimens were randomly divided into 4 groups ( n = 41) according to the grinding procedures as follows: Group A without any grinding as control; Group B ground with MD-Piano diamond disc of 80 grit (equivalent coarse-grit diamond burs); Group C ground with MD-Piano diamond disc of 80 grit and then 120 grit (equivalent medium-grit diamond burs); Group D ground successively with 80, 120 and 220 grit MD-Piano diamond discs (equivalent fine-grit diamond burs). Grinding was performed at a speed of 150 rmp with a constant force of 10 N under water cooling until 0.15 mm reduction in height was achieved. The opposite surfaces of all specimens were ground with 80 grit MD-Piano diamond grinding disc and then polished successively with 100, 220, 500, 800 and 1000 grit silicon carbide abrasive papers until the height of specimen obtained 1.0 mm. The final size of 15.0 mm in diameter × 1.0 mm in height with the parallelism tolerance less than 0.02 mm in 10 mm was determined by a micrometer.
2.3
Roughness measurement and X-ray diffraction
The interfacial surface roughness of the core ceramic was performed on randomly chosen specimens ( n = 16) using a confocal scanning laser microscope (CSLM) (LSM 700, Zeiss, Jena, Germany). Representative roughness parameters R a (arithmetical mean deviation of roughness profile) and R z (maximum height of the profile) were recorded using image analysis and measurement software ZEN 2010 (Zeiss, Jena, Germany) according to the ISO 4287:1997 . Three extra specimens of each group were sputter coated with gold for topography analysis with a scanning electron microscopy (SEM) (Quanta 200, FEI, Eindhoven, The Netherlands).
The phase compositions before and after the veneering thermal treatment were characterized by X-ray diffraction (XRD) (X’Pert PRO, PANalytical, Almelo, The Netherlands). Three extra specimens of each group were scanned using Cu-Kα X-ray from 20 to 60° 2 θ degrees with a step size of 0.04° and 5 s step interval . The phase identification from the phase-check data was performed using the JCPDS reference-card database and the ‘Search-Match’ utility of the software PANalytical X’Pert HighScore Plus. An approximate estimate of relative amount of the monoclinic was obtained using a semi-quantitative analysis proposed by Garvie and Nicholson .
The relative amounts of monoclinic were calculated as the following Eq. (1) :
where m (1 1 1) corresponds to the monoclinic phase referring to the crystallographic plane (1 1 1), m (−1 1 1) corresponds to the monoclinic phase referring to crystallographic plane (−1 1 1), t (1 1 1) corresponds to the tetragonal phase referring to crystallographic plane (1 1 1) and c (1 1 1) corresponds to the cubic phase referring to crystallographic plane (1 1 1). In all instances I refers to the integrated intensity under the respective peaks.
2.4
Bilayered sample preparation
All specimens were ultrasonically cleaned for 5 min and air dried afterwards. Prior to veneer application, a thin feldspathic liner porcelain (Cerabien ZR SBA2, Kuraray Noritake Dental Inc., Tokyo, Japan) was coated on the core ceramic fired as the manufacturer recommends. Dentin porcelain (Cerabien ZR A2B, Kuraray Noritake Dental Inc., Tokyo, Japan) (CZR) slurry was brushed on the top surface of core specimens utilizing a stainless steel set-up mould, and excess slurry was removed using a sharp blade. Firing shrinkage was compensated for with a second application of porcelain to achieve the adequate shape and thickness. The final size of bilayered disks was 15.0 mm in diameter × 2.0 mm in height with core to veneer thickness ratios of ca. 1:1.
2.5
Biaxial loading testing and Weibull analysis
All specimens ( n = 32) were submitted to a piston-on-three-ball biaxial flexural strength test. Specimens in each group were divided randomly into two subgroups ( n = 16): subgroups Ac-Dc were loaded with core ceramic in compression (i.e., veneer porcelain in tension); while subgroups At-Dt were tested with core in tension. The load was applied by a universal testing machine (Instron E3000, Instron Ltd., MA, USA) with a cross-head speed of 0.5 mm/min at a room temperature of 25 °C. A thin plastic sheet (0.05 mm in thickness) was inserted between the loading piston and the disks to evenly distribute pressures and minimize stress concentration . At the first sign of failure indicated by sound emission and a change in the load versus time curve, the test was stopped and the specimens were examined using stereomicroscope (M205A, Leica, Heerbrugg, Germany). The flexural strength ( σ , in MPa) was calculated using the Eqs. (2a–2f) :
where σ T , σ B , σ 1 and σ 2 are stresses at the top, at the bottom surfaces and of both materials at the interface of layered specimens; t 1 and t 2 are thickness of bottom and top layer, respectively; v 1 and v 2 are the Poisson’s ratio of bottom and top layer, respectively, i.e., v 1 = 0.31 for Katana zirconia and v 2 = 0.21 for CZR; E 1 and E 2 are the Young’s modulus of bottom and top materials, i.e., E 1 = 214 GPa for Katana zirconia and E 2 = 75 GPa for CZR. The above data are provided by the manufacturer, while the Young’s modulus of CZR is quoted according to ISO standard 6872 for dental ceramics as the manufacturer’s value is not provided; M is the biaxial bending moment for piston-on-three ball loadings,
where P is the load to fracture; a , c and R are the radii of the supporting circle, the piston and the specimen disk, respectively; r is radial distance from the center of the disk; v is the average Poisson’s ratio of the bilayer,
Weibull analysis was performed based on the data of flexural strength to evaluate the structural reliability of bilayered ceramic structures. Weibull modulus ( m ) and its 95% confidence interval were calculated as the following Eq. (3) :
where P f is the fracture probability, σ is the fracture strength at a given P f , σ 0 is the characteristic strength.
2.6
Finite element analysis
Finite element analysis (FEA) of the disc specimens was made using a software package ANSYS 12.0 (ANSYS Inc., TX, USA) to reveal the stress distribution in bilayered specimens. The modeling of symmetric specimens was processed as our previous report .
2.7
Analysis of failure modes and chemistry analysis
Fracture specimens were examined using a stereomicroscope (M205A, Leica, Heerbrugg, Germany) and SEM (Quanta 200, FEI, Eindhoven, The Netherlands) to determine the mode of failure based on the fracture origin and using fractographic principles .
Elemental composition of fractured interface was also analyzed by energy dispersive X-ray spectroscopy (EDS) and the mass fraction of each element was measured.
2.8
Statistical analysis
Data for R a and R z among the four groups were analyzed respectively, by Kruskal–Wallis and Dunnett T3 test. One-way ANOVA and Bonferroni test was used for σ results. Statistical analyses were performed using SPSS 15.0 (IBM Corp.). Differences were considered statistically significant at the level of 0.05.
2
Materials and methods
2.1
Sample preparation
152 zirconia core specimens disks (1.2 mm in height, 15 mm in diameter) were fabricated with the Katana system (Kuraray Noritake Dental Inc., Tokyo, Japan) and then were airborne-particle abraded (Silfradent SRL, Sofia, Italy) with 50 μm Al 2 O 3 at 0.2 MPa pressure for 20 s with a distance of 10 mm according to the manufacturer instructions. All specimens were cleaned in an ultrasonic cleaner with distilled water for 5 min prior to further treatments.
2.2
Treatment of core interfacial surface
To generate a similar core–veneer interfacial topography to that ground with diamond burs in the clinical situation and form a comparable flat surface of the core specimens, a series of MD-Piano diamond discs (Struers, Ballerup, Denmark) were used for grinding on an automatic metallographic lapping machine (Tegramin-30, Struers, Ballerup, Denmark).
The prepared core specimens were randomly divided into 4 groups ( n = 41) according to the grinding procedures as follows: Group A without any grinding as control; Group B ground with MD-Piano diamond disc of 80 grit (equivalent coarse-grit diamond burs); Group C ground with MD-Piano diamond disc of 80 grit and then 120 grit (equivalent medium-grit diamond burs); Group D ground successively with 80, 120 and 220 grit MD-Piano diamond discs (equivalent fine-grit diamond burs). Grinding was performed at a speed of 150 rmp with a constant force of 10 N under water cooling until 0.15 mm reduction in height was achieved. The opposite surfaces of all specimens were ground with 80 grit MD-Piano diamond grinding disc and then polished successively with 100, 220, 500, 800 and 1000 grit silicon carbide abrasive papers until the height of specimen obtained 1.0 mm. The final size of 15.0 mm in diameter × 1.0 mm in height with the parallelism tolerance less than 0.02 mm in 10 mm was determined by a micrometer.
2.3
Roughness measurement and X-ray diffraction
The interfacial surface roughness of the core ceramic was performed on randomly chosen specimens ( n = 16) using a confocal scanning laser microscope (CSLM) (LSM 700, Zeiss, Jena, Germany). Representative roughness parameters R a (arithmetical mean deviation of roughness profile) and R z (maximum height of the profile) were recorded using image analysis and measurement software ZEN 2010 (Zeiss, Jena, Germany) according to the ISO 4287:1997 . Three extra specimens of each group were sputter coated with gold for topography analysis with a scanning electron microscopy (SEM) (Quanta 200, FEI, Eindhoven, The Netherlands).
The phase compositions before and after the veneering thermal treatment were characterized by X-ray diffraction (XRD) (X’Pert PRO, PANalytical, Almelo, The Netherlands). Three extra specimens of each group were scanned using Cu-Kα X-ray from 20 to 60° 2 θ degrees with a step size of 0.04° and 5 s step interval . The phase identification from the phase-check data was performed using the JCPDS reference-card database and the ‘Search-Match’ utility of the software PANalytical X’Pert HighScore Plus. An approximate estimate of relative amount of the monoclinic was obtained using a semi-quantitative analysis proposed by Garvie and Nicholson .
The relative amounts of monoclinic were calculated as the following Eq. (1) :
where m (1 1 1) corresponds to the monoclinic phase referring to the crystallographic plane (1 1 1), m (−1 1 1) corresponds to the monoclinic phase referring to crystallographic plane (−1 1 1), t (1 1 1) corresponds to the tetragonal phase referring to crystallographic plane (1 1 1) and c (1 1 1) corresponds to the cubic phase referring to crystallographic plane (1 1 1). In all instances I refers to the integrated intensity under the respective peaks.
2.4
Bilayered sample preparation
All specimens were ultrasonically cleaned for 5 min and air dried afterwards. Prior to veneer application, a thin feldspathic liner porcelain (Cerabien ZR SBA2, Kuraray Noritake Dental Inc., Tokyo, Japan) was coated on the core ceramic fired as the manufacturer recommends. Dentin porcelain (Cerabien ZR A2B, Kuraray Noritake Dental Inc., Tokyo, Japan) (CZR) slurry was brushed on the top surface of core specimens utilizing a stainless steel set-up mould, and excess slurry was removed using a sharp blade. Firing shrinkage was compensated for with a second application of porcelain to achieve the adequate shape and thickness. The final size of bilayered disks was 15.0 mm in diameter × 2.0 mm in height with core to veneer thickness ratios of ca. 1:1.
2.5
Biaxial loading testing and Weibull analysis
All specimens ( n = 32) were submitted to a piston-on-three-ball biaxial flexural strength test. Specimens in each group were divided randomly into two subgroups ( n = 16): subgroups Ac-Dc were loaded with core ceramic in compression (i.e., veneer porcelain in tension); while subgroups At-Dt were tested with core in tension. The load was applied by a universal testing machine (Instron E3000, Instron Ltd., MA, USA) with a cross-head speed of 0.5 mm/min at a room temperature of 25 °C. A thin plastic sheet (0.05 mm in thickness) was inserted between the loading piston and the disks to evenly distribute pressures and minimize stress concentration . At the first sign of failure indicated by sound emission and a change in the load versus time curve, the test was stopped and the specimens were examined using stereomicroscope (M205A, Leica, Heerbrugg, Germany). The flexural strength ( σ , in MPa) was calculated using the Eqs. (2a–2f) :
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