Abstract
Objective
To investigate the microstructural properties and reliability of zirconia partially stabilized by yttrium oxide (Y-TZP) for use in monolithic form (Vita YZ HT, Vita Zahnfabrik, Germany) after different clinical adjustment protocols simulation. One-hundred and sixty two Y-TZP discs of high translucency were divided into 6 groups (n = 27), according to simulated adjustments: S-Y-TZP as sintered; G-glazed Y-TZP; DbG-Y-TZP with diamond bur adjustment simulation followed by glaze application; GDb-glazed Y-TZP adjusted with diamond bur; DbDrG-Y-TZP with adjustment simulation with diamond bur and polishing kit (diamond rubber), followed by glaze; GDbDr-glazed Y-TZP with adjustment simulation with diamond bur followed by polishing kit.
Methods
After monotonic testing under biaxial flexure (n = 3), remaining specimens were distributed into 3:2:3 ratio and tested under step-stress accelerated life testing. Specimens were evaluated in a stereomicroscope and SEM. Microstructure was analyzed by X-ray diffraction and mean surface roughness (Ra). Survival data were used to calculate Weibull’s beta value (β) and reliability for missions of 300,000 and 600,000 cycles at 200 N. Ra data were statistically analyzed by 1-way ANOVA and Tukey’s test (both, a = 5%).
Results
The untreated HT zirconia (S) presented a lower probability of survival after 600,000 cycles at 200 N, compared to HT zirconia adjusted with diamond bur and glazed (DbG), and after final polishing with polishing kit (GDbDr). Final polishing with diamond rubber (GDbDr) resulted in a more homogeneous surface compared to glazed samples (G, DbG and DbDrG).
Significance
HT zirconia is more likely to present increased survival when glazed after diamond adjustments (DbG) or when it is finished with polishing kit (GDbDr).
1
Introduction
Minimal occlusal thickness and tooth preservation lead to the necessity of using restorations thinner than conventionally indicated. In posterior regions where masticatory loads are higher, polycrystalline ceramics may be the first option due to their transformation toughening properties that are not present in other all-ceramic materials .
Zirconia is a polymorphic material presenting three phases, according to temperature: monoclinic (from room temperature to 1700 °C), tetragonal (between 1700 °C and 2370 °C), and cubic (above 2370 °C until its melting). To be able to maintain the zirconia in the tetragonal phase at room temperature, stabilizing oxides are used, such as yttrium oxide. A tetragonal zirconia partially stabilized by yttrium (Y-TZP) offers high mechanical strength due to its ability to resist crack propagation (fracture toughness) .
Y-TZP may be engineered as a high translucent ceramic (Y-TZP HT) to be fabricated by computer-aided design and computer-aided manufacturing (CAD/CAM), with indications for monolithic use. During prostheses installation, occlusal, proximal and cementation surface adjustments are often required by means of diamond rubbers with different grits, as per manufacturers instructions.
Clinical adjustment of the ceramic surface can lead to crack initiation tending to propagate toward surfaces under tensile stress. The damage can also be accompanied by phase transformation. Appropriate polishing with diamond rubbers can reduce the surface roughness of the Y-TZP, though roughness seems to not influence the phase change in the material . However, the surface roughness plays an important role in the relationship with the opposing tooth, since polished and glazed ceramics have better mechanical behavior .
Whereas single load to failure testing lead to abrupt and catastrophic fractures of dental ceramics, failure usually occurs by accumulated fatigue after prolonged low load cycling . There are limited data in the literature on the probability of survival of Y-TZP HT ceramics after clinical adjustment procedures. The purpose of this in vitro study was to evaluate the effect of different occlusal adjustment techniques on monolithic zirconia, before or after the glaze layer application, on the reliability and microstructural characteristics.
2
Materials and methods
2.1
Specimen preparation
One hundred and sixty two discs were prepared. A ring device was glued onto the top surfaces of the CAD/CAM blocks (Vita YZ HT, Vita Zahnfabrik, Germany) to round them until 14 mm diameter cylinders were obtained. The cylinders were then cut into several disks in a cutting machine (ISOMET 1000, Buehler, Lake Bluff, USA). Specimens attained final dimensions of 12 mm in diameter and 0,8 mm thick after sintering. Ceramic discs were divided into 6 groups (n = 27), according to simulated adjustments as follows: S- Y-TZP as sintered; G- glazed Y-TZP; DbG- Y-TZP with diamond bur adjustment simulation followed by glaze application; GDb- glazed Y-TZP adjusted with diamond bur; DbDrG- Y-TZP with adjustment simulation with diamond bur and polishing kit (diamond rubber), followed by glaze; GDbDr- glazed Y-TZP with adjustment simulation with diamond bur followed by polishing kit.
For glazed groups, the Plus Glaze Body Spray (Vita Zahnfabrik, Germany) was applied on a 12 mm diameter surface.
2.2
Grinding and polishing on the surface
To simulate the clinical adjustment procedure with diamond burs, a 120-grit resin-bonded diamond disk, which corresponds to a 151 μm diamond bur (blue) , was used to polish the 12 mm diameter surface at a 500 rpm speed for 20 s mounted in a polishing machine (Automet/Ecomet 250, Buehler, Lake Bluff, USA). For diamond rubber polishing, the Step 1 Pre-polishing Kit (Suprinity Polishing Set, Vita Zahnfabrik, Germany) was used at 7000–12,000 rpm speed for 15 s, followed by Step 2 High Brightness with 4000–8000 rpm for 15 s, both mounted on a laboratory motor handpiece (MF Perfecta 9975, W&H, Laufen, Germany).
2.3
Surface roughness
Six specimens of each group were evaluated for surface roughness obtained by the simulations with a contact rugosimeter (Mitutoyo Corporation, Tokyo, Japan). Three parallel readings per sample were performed, and the Ra parameter was evaluated. A representative sample of each group was also evaluated in digital optical profilometer (Wyko NT model 1100, Veeco, Tucson, USA). The information obtained from profilometer was transferred to the computer through the Wyko software Vision 32 (Veeco, Tucson, USA) to generate three-dimensional images.
After confirming the normality assumptions, the roughness data (Ra parameter) were tabulated for descriptive statistics of each group and then submitted to 1-way analysis of variance (ANOVA), followed by means test (Tukey), both with α = 0.05, using Statistix 8.0 software (Analytical Software, Tallahasse, USA).
2.4
Mechanical testing and reliability analysis
Three specimens of each group underwent single load-to-failure (SLF) testing at a cross-head speed of 1 mm/min in an universal testing machine (EMIC DL 100, Sao Jose dos Pinhais, Brazil), according to ISO 6872:2015 , under water. To accelerate specimens’ failure, step-stress accelerated life testing (SSALT) was employed using an electrodynamic testing machine. Specimens of all groups were assigned to three step-stress profiles (based on mean SLF values), named based on the load increase in which a specimen is fatigued to reach a certain level of load, following the 3:2:3 ratio and designated as mild (n = 9), moderate (n = 6), and aggressive (n = 9). Twenty-four specimens of each group were cycled in a fatigue testing equipment (Biocycle, Biopdi, Sao Carlos, Brazil) with the samples immersed in distilled water, positioned in a device identical to that used in the monotonic test and with the treated surface facing the tensile side. Considering the 4 Hz frequency used, each period of 8 h of cycling corresponded to 115,200 cycles and each period of 12 h equals 172,800 cycles ( Fig. 1 ). All mechanical tests were performed using a piston-on-three-ball device , with a flat punch stainless steel load applicator of 1.4 mm diameter. Ground surfaces were always positioned on the tensile stresses side.
Based upon the step-stress distribution of failures, use level probability Weibull curves were calculated (Synthesis 9, Alta Pro, ReliaSoft, Tucson, USA) using a power law relationship for damage accumulation. Reliability for a mission of 300,000 cycles and 600,000 at 200 N (90% two-sided confidence interval) was calculated for comparison between the groups.
2.5
Scanning electron microscopy (SEM) / fractographic analysis
All samples were first inspected in light-polarized stereomicroscopy (Discovery V20 Zeiss, Jena, Germany) and representative samples were then gold sputtered (Emitech SC7620 Sputter Coater, East Sussex, Great Britain) for qualitative fractographic analysis under scanning electron microscopy (SEM Inspect S50, FEI Company, Brno, Czech Republic).
The images obtained by stereomicroscope, optical profilometer and SEM were qualitatively analyzed and described.
2.6
X-ray diffraction (XRD) phase analysis
To evaluate potential phase transformation due to simulated adjustments and fatigue, two fractured specimens from each group were evaluated by X-ray difractometry (XRD; Philips X’pert PRO MRD, Almelo, Netherlands). The analysis was carried out in partnership with the LAS/INPE laboratory (Associated Laboratory of Sensors and Materials of the National Institute of Space Research, Sao Jose dos Campos, Brazil). CuK radiation with Cu = 0.1541 8 nm wavelength was used in a range (−2) between 10° and 100°, scanning speed of 2°min −1 , voltage of 40 kV and current of 20 mA (Philips PW 1830/1840, Almelo, Netherlands).
The XRD data were evaluated by identifying the crystalline phases after comparing the experimental spectra with standard diffraction spectra of the JCPDS (Joint Committee on Powder Diffraction Standards) and ICSD (Inorganic Crystal Structure). HighScore software (Philips X’pert PANalytical, Almelo, The Netherlands) helped with the attributions of the spectra.
2
Materials and methods
2.1
Specimen preparation
One hundred and sixty two discs were prepared. A ring device was glued onto the top surfaces of the CAD/CAM blocks (Vita YZ HT, Vita Zahnfabrik, Germany) to round them until 14 mm diameter cylinders were obtained. The cylinders were then cut into several disks in a cutting machine (ISOMET 1000, Buehler, Lake Bluff, USA). Specimens attained final dimensions of 12 mm in diameter and 0,8 mm thick after sintering. Ceramic discs were divided into 6 groups (n = 27), according to simulated adjustments as follows: S- Y-TZP as sintered; G- glazed Y-TZP; DbG- Y-TZP with diamond bur adjustment simulation followed by glaze application; GDb- glazed Y-TZP adjusted with diamond bur; DbDrG- Y-TZP with adjustment simulation with diamond bur and polishing kit (diamond rubber), followed by glaze; GDbDr- glazed Y-TZP with adjustment simulation with diamond bur followed by polishing kit.
For glazed groups, the Plus Glaze Body Spray (Vita Zahnfabrik, Germany) was applied on a 12 mm diameter surface.
2.2
Grinding and polishing on the surface
To simulate the clinical adjustment procedure with diamond burs, a 120-grit resin-bonded diamond disk, which corresponds to a 151 μm diamond bur (blue) , was used to polish the 12 mm diameter surface at a 500 rpm speed for 20 s mounted in a polishing machine (Automet/Ecomet 250, Buehler, Lake Bluff, USA). For diamond rubber polishing, the Step 1 Pre-polishing Kit (Suprinity Polishing Set, Vita Zahnfabrik, Germany) was used at 7000–12,000 rpm speed for 15 s, followed by Step 2 High Brightness with 4000–8000 rpm for 15 s, both mounted on a laboratory motor handpiece (MF Perfecta 9975, W&H, Laufen, Germany).
2.3
Surface roughness
Six specimens of each group were evaluated for surface roughness obtained by the simulations with a contact rugosimeter (Mitutoyo Corporation, Tokyo, Japan). Three parallel readings per sample were performed, and the Ra parameter was evaluated. A representative sample of each group was also evaluated in digital optical profilometer (Wyko NT model 1100, Veeco, Tucson, USA). The information obtained from profilometer was transferred to the computer through the Wyko software Vision 32 (Veeco, Tucson, USA) to generate three-dimensional images.
After confirming the normality assumptions, the roughness data (Ra parameter) were tabulated for descriptive statistics of each group and then submitted to 1-way analysis of variance (ANOVA), followed by means test (Tukey), both with α = 0.05, using Statistix 8.0 software (Analytical Software, Tallahasse, USA).
2.4
Mechanical testing and reliability analysis
Three specimens of each group underwent single load-to-failure (SLF) testing at a cross-head speed of 1 mm/min in an universal testing machine (EMIC DL 100, Sao Jose dos Pinhais, Brazil), according to ISO 6872:2015 , under water. To accelerate specimens’ failure, step-stress accelerated life testing (SSALT) was employed using an electrodynamic testing machine. Specimens of all groups were assigned to three step-stress profiles (based on mean SLF values), named based on the load increase in which a specimen is fatigued to reach a certain level of load, following the 3:2:3 ratio and designated as mild (n = 9), moderate (n = 6), and aggressive (n = 9). Twenty-four specimens of each group were cycled in a fatigue testing equipment (Biocycle, Biopdi, Sao Carlos, Brazil) with the samples immersed in distilled water, positioned in a device identical to that used in the monotonic test and with the treated surface facing the tensile side. Considering the 4 Hz frequency used, each period of 8 h of cycling corresponded to 115,200 cycles and each period of 12 h equals 172,800 cycles ( Fig. 1 ). All mechanical tests were performed using a piston-on-three-ball device , with a flat punch stainless steel load applicator of 1.4 mm diameter. Ground surfaces were always positioned on the tensile stresses side.
Based upon the step-stress distribution of failures, use level probability Weibull curves were calculated (Synthesis 9, Alta Pro, ReliaSoft, Tucson, USA) using a power law relationship for damage accumulation. Reliability for a mission of 300,000 cycles and 600,000 at 200 N (90% two-sided confidence interval) was calculated for comparison between the groups.
2.5
Scanning electron microscopy (SEM) / fractographic analysis
All samples were first inspected in light-polarized stereomicroscopy (Discovery V20 Zeiss, Jena, Germany) and representative samples were then gold sputtered (Emitech SC7620 Sputter Coater, East Sussex, Great Britain) for qualitative fractographic analysis under scanning electron microscopy (SEM Inspect S50, FEI Company, Brno, Czech Republic).
The images obtained by stereomicroscope, optical profilometer and SEM were qualitatively analyzed and described.
2.6
X-ray diffraction (XRD) phase analysis
To evaluate potential phase transformation due to simulated adjustments and fatigue, two fractured specimens from each group were evaluated by X-ray difractometry (XRD; Philips X’pert PRO MRD, Almelo, Netherlands). The analysis was carried out in partnership with the LAS/INPE laboratory (Associated Laboratory of Sensors and Materials of the National Institute of Space Research, Sao Jose dos Campos, Brazil). CuK radiation with Cu = 0.1541 8 nm wavelength was used in a range (−2) between 10° and 100°, scanning speed of 2°min −1 , voltage of 40 kV and current of 20 mA (Philips PW 1830/1840, Almelo, Netherlands).
The XRD data were evaluated by identifying the crystalline phases after comparing the experimental spectra with standard diffraction spectra of the JCPDS (Joint Committee on Powder Diffraction Standards) and ICSD (Inorganic Crystal Structure). HighScore software (Philips X’pert PANalytical, Almelo, The Netherlands) helped with the attributions of the spectra.
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