- Email: [email protected]
tetragonal zirconia materials
RESEARCH AND EDUCATION
Flexural strength, fracture toughness and translucency of cubic/tetragonal zirconia materials Parissa Nassary Zadeh, Med Dent,a Nina Lümkemann, MSc,b Beatrice Sener, MTA,c Marlis Eichberger, CDT,d and Bogna Stawarczyk, PD Dr Dipl Ing (FH) MSce Zirconia dental restorations ABSTRACT are attracting considerable Statement of problem. The development of zirconia materials with optimized properties has been attention because of their rapid, and studies comparing the mechanical and optical properties of recently introduced zirconia esthetic properties and also with lithium disilicate materials are limited. because of their excellent Purpose. The purpose of this in vitro study was to compare the mechanical and optical properties biocompatibility1 and meof cubic/tetragonal zirconia materials with those of a lithium disilicate ceramic. chanical properties.2-4 TypiMaterial and methods. Specimens were fabricated from 6 different noncolored zirconia materials: cally, highly stable monolithic Ceramill Zolid FX (CZ), CopraSmile (CS), DD cubeX2 (DD), NOVAZIR MaxT (NZ), priti multidisc ZrO₂ (PD), zirconia has been used in the and StarCeram Z-Smile (SC), and 1 lithium disilicate ceramic as a control, IPS e.max Press LT A2 (CG). Fourposterior region; however, point flexural strength (N=105/n=15) and fracture toughness using the single-edge V-notched beam for anterior teeth, monolithic (N=105/n=15) were examined according to International Organization for Standardization standard lithium disilicate has been 6872:2015. Translucency (N=70/n=10) was evaluated with an ultraviolet spectrophotometer. Grain size (N=6/n=1) of zirconia was investigated by using scanning electron microscopy. Data were favored.5 This may change analyzed using the Kolmogorov-Smirnov test, multivariate analysis, 1-way analysis of variance, with the introduction of followed by the post hoc Scheffé test and Kruskal-Wallis and Mann-Whitney U tests, and Weibull esthetic, highly translucent analysis, using the maximum likelihood estimation method at 95% confidence level (a=.05). cubic/tetragonal zirconia and Results. Zirconia materials showed higher mechanical and lower optical properties than CG ideally result in a straightfor(P<.001). No differences were observed among the zirconia materials with respect to flexural ward and efficiently manufacstrength (P=.259) or fracture toughness (P=.408). CG and CS showed significantly higher Weibull tured restoration. In addition, modulus than SC and PD. The lowest translucency values were measured for NZ and SC, followed zirconia has greater fracture by CS, DD, and PD (P<.001). CZ showed the highest translucency values (P<.001). The lowest grain resistance than lithium dissizes were found for NZ, DD, and SC; the largest were shown for CS (P<.001). ilicate ceramic.6-9 Zirconia is Conclusions. Cubic/tetragonal zirconia showed better mechanical properties than lithium disilicate polymorphous, with 3 crystal ceramic. However, the optical properties and the reliability of zirconia are lower than those of lattices, monoclinic (room lithium disilicate ceramic. (J Prosthet Dent 2018;-:---) temperature up to 1170 C), tetragonal (between 1170 C and 2360 C), and cubic approximately 3% and 4% from the tetragonal to the (approximately 2360 C to the melting point of 2680 C).10 monoclinic phase takes place, followed by cracking.1 Without the addition of stabilizing oxides (MgO, CeO2, The first dental zirconia ceramic (3Y-TZP) was a or Y2O), a transformation expansion of between partially stabilized tetragonal zirconia with low
a
Assistant Professor, Department of Prosthetic Dentistry, Hospital of the University of Munich, Ludwig Maximilian University of Munich, Munich, Germany. Materials Engineer, Department of Prosthetic Dentistry, Hospital of the University of Munich, Ludwig Maximilian University of Munich, Munich, Germany. Medical Technologist Assistant, Clinic for Preventive Dentistry, Periodontics and Cardiology, University of Zurich, Zurich, Switzerland. d Certified Dental Technician, Department of Prosthetic Dentistry, Hospital of the University of Munich, Ludwig Maximilian University of Munich, Munich, Germany. e Head of Dental Materials Unit, Department of Prosthetic Dentistry, Hospital of the University of Munich, Ludwig Maximilian University of Munich, Munich, Germany. b c
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Table 1. Summary of materials used
Clinical Implications Cubic/tetragonal zirconia and lithium disilicate ceramic are characterized by specific advantages for different indications. Dentists should be aware of relevant differences among the mechanical and optical properties of the different formulations of zirconia materials.
translucency and high flexural strength.3,11 Its translucency was similar to that of dentin and so was not suitable for monolithic restorations; however, it was used as a framework veneered with esthetic ceramics.12 Monolithic zirconia restorations without veneering reduce production costs, avoid chipping problems, and require less tooth preparation6,7; however, they require zirconia with improved optical properties. This was achieved by increasing the sintering temperature, leading to microstructural alteration and higher translucency.13 The holding time, temperature increase rate, and sintering cooling rate also affected translucency.14 Sintering temperatures above 1600 C led to a decrease in flexural strength.13 Improvements were made by reducing the quantity and grain size of alumina together with repositioning them on the grain boundaries of zirconia.6,7 This resulted in increased light transmission and good long-term stability and high strength.11 However, these zirconia restorations were less translucent and thus had poorer esthetics than lithium disilicate ceramics. Recently, highly translucent fully stabilized cubic/tetragonal zirconia materials have been introduced. The cubic phase consists of approximately 50% and has been achieved by increasing the addition of stabilizing oxides. This resulted in higher translucency due to the more voluminous and more isotropic cubic crystals,14-16 maintaining the high strength of the material.17 These new zirconia materials are marketed for both posterior and anterior restorations. As limited information is available for these materials, however, the purpose of this in vitro study was to compare the properties of this zirconia to a glass-ceramic. The null hypotheses were that no differences in flexural strength, fracture toughness, or translucency would be found among 6 cubic/ tetragonal zirconia materials and a lithium disilicate ceramic. MATERIAL AND METHODS The flexural strength, fracture toughness, translucency, and grain size of 6 cubic/tetragonal zirconia materials (Ceramill Zolid FX, CopraSmile, DD cubeX2, NOVAZIR MaxT, priti multidisc ZrO2, and StarCeram Z-Smile) were analyzed. A lithium disilicate ceramic (IPS e.max Press;
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Material
Abbreviation
Manufacturer
Lot No.
1512004-114
Cubic/tetragonal zirconia Ceramill Zolid FX
CZ
Amann Girrbach AG
CopraSmile
CS
Whitepeaks Dental Solutions
IS2114
DD cubeX2
DD
Dental Direkt
8041632001
NOVAZIR MaxT
NZ
Novadent Dentaltechnik
L2251030007-6
Priti multidisc ZrO2
PD
Pritidenta
Y65 16HT
StarCeram Z-Smile
SC
H.C. Starck
50586461
CG
Ivoclar Vivadent AG
Lithium disilicate IPS e.max Press LT (A2)
U04298
Ivoclar Vivadent AG) was used as a control group (Table 1). A total of 280 specimens were produced. Each material contained 15 specimens for flexural strength, 15 for fracture toughness, and 10 for translucency including 3 for grain size analysis. Specimens were milled from partially sintered zirconia blanks by using a computeraided manufacture (CAM) machine (Ceramill motion 2; Amann Girrbach AG) (Fig. 1A, B). After specimens were ground with SiC abrasive paper up to P2500 (Buehler), they were sintered (LHT 02/16; Nabertherm GmbH) (Table 2). For flexural strength and fracture toughness, the final dimensions of the specimens were 45.0 mm (length) ×4.0 ±0.2 mm (width) ×3.0 ±0.2 mm (thickness) and 16.0 mm (diameter) ×1.0 ±0.05 mm (thickness) for the translucency specimens. The lithium disilicate ceramic specimens were produced with the press technique18 (Austromat 654 press-i-dent; Dekema DentalKeramiköfen) with 3 wax specimens in each muffle at 930 C. After the sprues were deflasked and sectioned, we polished the specimens by using a water-cooled polishing machine (Abramin; Struers) with a polishing plate. For flexural strength and fracture toughness, the final dimension of the specimens was 30.0 mm (length)×4.0 ±0.2 mm (width)×3.0 ±0.2 mm (thickness) and 16.0 mm (diameter)×1.0 ±0.05 mm (thickness) for the translucency specimens. For translucency, the specimens were polished with diamond pads (40 and 20 mm), magnetic supporting grinding disks (9 and 3 mm) plus a polishing pad (1 mm) with diamond suspensions (Struers). For lithium disilicate flexural strength and fracture toughness, the specimens were polished by using SiC up to P4000 (Struers) for wet grinding of the ceramic. The thickness of the specimens was measured at 3 points to a precision of 0.01 mm by using a digital micrometer (Mitutoyo). Additionally, the parallelism of the specimens was verified. Four-point flexural strength was measured according to International Organization for Standardization standard Nassary Zadeh et al
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Figure 1. Fracture toughness specimen preparation. A, Zirconia blank placed in CAM machine. B, Milled specimens for 4-point flexural strength and 4-point fracture toughness. C, Sectioning specimens. D, Notching specimens. E, Notched zirconia specimen (original magnification ×5). F, Specimen loaded in universal testing machine.
6872:2015.19 The long side of the specimen (4.0 ±0.2 mm) was placed in an adapted specimen holder on 2 steel rollers at a span of 40 mm. Specimens were loaded in a universal testing machine (1445 Zwick/Roell; Zwick) at a crosshead speed of 1 mm/min until fracture occurred. The force was applied with a loading apparatus with 2 steel rollers at a span of 20 mm. For the smaller lithium disilicate specimens, a modified specimen holder was used, with the 2 steel rollers at a 10-mm span and the loading rollers with a 5-mm span. The following equation was used to calculate 3Fd flexural strength: s = 4bh 2 ’, where s is flexural strength in MPa; F is fracture load in N; d is difference in span of steel rollers (for support and loading) in mm; b is the width of specimen in mm; and h is the height of specimen in mm. Fracture toughness was measured according to ISO standard 6872:2015 single-edge V-notched beam (SEVNB) method.19,20 Five specimens were placed on the narrow side (3.0 ±0.2 mm), fixed upright, and centered in an adapted specimen holder. A centered saw cut was made by using a universal cutting machine (Secotom-50; Struers) with a diamond cutoff wheel (127 mm diameter×0.4 mm model M1D13; Struers) (Fig. 1C). The depths of the saw cuts were more than 0.5 mm. The specimen holder was then placed in a notching machine (SD Mechatronik), where the specimens were notched and sharpened with a razor blade (0.3 mm blades; David Combi and Finisher) with polishing diamond paste (9 and 3 mm, MetaDi diamond paste; Buehler) (Fig. 1D). The depth of the saw cut together with the depth of the notching was between 0.8 mm and 1.2 mm. The cycles of the machine varied, and the pressing force was controlled with weights. Specimens were ultrasonically cleaned (Sonorex RK102H; Bandelin Electronic GmbH) in 80% Nassary Zadeh et al
alcohol (Alkopharm 80; Brüggemann Alcohol). The saws and notches were measured by using a microscope (Zwick/Roell Z 2.5; Zwick) (Fig. 1E). The specimens were measured with a digital micrometer and singly placed into the same modified specimen holder as for the4-point flexural strength, but they were placed on the narrow side (3.0 ±0.2 mm) with the notched surface pointing downward. Specimens were loaded in a universal testing machine (1445 Zwick/Roell) at a crosshead speed of 0.5 mm/min until fracture occurred (Fig. 1F). The following equation was used to calculate the fracture toughness: pffiffiffi F s1 −s2 3 a KIc = , Y; , b√w w 2ð1−aÞ1:5 where KIc is fracture toughness in MPa ∙ m1/2; F is fracture load in N; b is thickness of specimen in m; ѡ is width of specimen in m; s1 is support span in m; s2 is loading span in m; a is relative depth of the V notch; and Y is a form factor for stress intensity. Translucency was analyzed by using an ultraviolet/ visible light spectrophotometer (Lambda 35; PerkinElmer LAS). All specimens were cleaned with 80% alcohol (Alkopharm 80) and singly fixed in an appropriate specimen holder with barium sulfate. Specimens were placed in the spectrophotometer at the inlet hole of the integrating sphere. Translucency was quantitively measured by analyzing the definite transmission of light through each specimen. The spectrophotometer recorded the light transmission with a sensor, comparing light intensities from a split beam. The light source provided a wavelength varying between 400 and 700 nm. Initial
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Table 2. Sintering parameters used for zirconia materials Group
Heating Rate From Room Temperature and Eventual Heating Steps
CZ
8 C/min
Final Temperature ( C)
Holding Time (min)
Cooling Rate up to 25 C
1450
120
15 C/min
CS
10 C/min until 950 C is attained, then 6 C/min
1500
90
Unregulated in closed furnace
DD
8 C/min until 900 C is attained; after holding for 30 min, heating rate of 3 C/min
1450
120
10 C/min
NZ
4 C/min until 500 C is attained, then 8 C/min until 1150 C is attained; after holding for 30 min, 2 C/min to 1300 C, then 4 C/min
1450
120
8 C/min to 800 C, then 0 C/min to 100 C
PD
10 C/min
1450
120
10 C/min
SC
5 C/min until 900 C is attained; after holding for 30 min, 2.5 C/min
1450
120
5 C/min to 900 C, then unregulated
CS, CopraSmile; CZ, Ceramill Zolid FX; DD, DD cubeX2; NZ, NOVAZIR MaxT; PD, priti multidisc ZrO2; SC, StarCeram Z-Smile.
translucency was calculated by the intensity of the monochromatic light I0 and the light I, transmitted through the specimen. The transmission coefficient, tc(%), was calculated by using I/I0=tcx. The overall light transmission for each specimen (T) was calculated as the integration (tc[l] dl [10−5]) of all tc values. To analyze light transmission, the T value of each material was divided by the T value with no specimen in the spectrophotometer (baseline) to receive light transmission as a percentage. All tests were conducted at room temperature. Grain size was analyzed with scanning electron microscopy (Fig. 2) (Carl Zeiss Supra V50, Cathode: field emission; Carl Zeiss Microscopy GmbH). Specimen preparation involved thermal etching (LTH 02/16; Nabertherm) to a final temperature of 1450 C and holding at that temperature for 30 minutes. Subsequently, the specimens were cleaned with 80% alcohol (Alkopharm 80) and glued to a holder. Each specimen was sputtered with a 2-nm layer of gold for 45 seconds (Safematic CCU-010; Safematic). Scanning electron microscopy was operated at an acceleration voltage of 5 kV at a working distance of 8.4 to 10.0 mm. Grain sizes were determined at 3 different locations on the same specimen. Measured data were analyzed statistically by using statistical software (IBM SPSS Statistics v24.0; IBM Corp). Parametric and nonparametric descriptive statistics were computed. For quantitative variables, the assumption of normality was tested using the Kolmogorov-Smirnov test. The general linear model (multivariate) analysis was performed. Flexural strength and grain size data were analyzed with 1-way analysis of variance (ANOVA), followed by the Scheffé post hoc test. Translucency and fracture toughness were tested with nonparametric tests, including the Kruskal-Wallis and Mann-Whitney U tests. Statistical analyses were performed for all materials, whereas zirconia materials were considered separately (a=.05). Weibull distribution parameters (Weibull modulus, characteristic strength) for flexural strength values were calculated by using the
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maximum likelihood estimation method at a 95% confidence interval.21 RESULTS According to the multifactorial analyses, the tested materials showed significant differences for the parameters, namely flexural strength, fracture toughness, translucency, and grain size. The greatest influence on the ceramic materials was for translucency (partial eta squared [hP2]=0.979; P<.001), followed by flexural strength (hP2=0.524; P<.001), fracture toughness (hP2=0.430; P<.001), and grain size (hP2=0.383; P<.001). The Kolmogorov-Smirnov test indicated a higher rate of violations of the normality assumption for translucency (28%) and fracture toughness data (14%) (Table 3). The violation of the normality assumption was not caused by outliers but rather by measurement rounding, leading to increased coarseness of the observations in each test group. Consequently, translucency and fracture toughness data were analyzed nonparametrically. Within the flexural strength and grain size data, all groups were normally distributed and analyzed using parametric tests (Table 3). The control group, IPS e.max Press, showed the lowest flexural strength (P<.001) and fracture toughness (P<.001), but the highest translucency (P<.001) compared with the zirconia materials. No statistical differences between the zirconia materials were observed with respect to flexural strength (P=.259) or fracture toughness (P=.408). IPS e.max Press (m=8.8), and CopraSmile (m=8.4) showed significantly higher Weibull modulus than StarCeram Z-Smile (m=4.9) and priti multidisc ZrO2 (m=4.4) (Table 4). For the zirconia materials, CopraSmile (m=8.4) showed a significantly higher Weibull modulus compared with that of priti multidisc ZrO2 (m=4.4) and StarCeram Z-Smile (m=4.9). With respect to translucency, the lowest values were found for NOVAZIR MaxT and StarCeram Z-Smile, followed by CopraSmile, DD cubeX2, and priti multidisc
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Figure 2. Scanning electron micrographs showing grain size of polished zirconia translucency specimens (original magnification ×50 000). A, Ceramill Zolid FX (CZ, Amann Girrbach). B, CopraSmile (CS, Whitepeaks). C, DD cubeX2 (DD, Dental Direkt). D, NOVAZIR MaxT (NZ, Novadent). E, priti multidisc ZrO2 (PD, Pritidenta). F, StarCeram Z-Smile (SC, H.C. Starck).
ZrO2 (P<.001). Ceramill Zolid FX had the highest translucency (P<.001). The lowest mean grain size was found for NOVAZIR MaxT (Fig. 2D), DD cubeX2 (Fig. 2C), and StarCeram ZSmile (Fig. 2F) (P<.001) (Table 3). The significantly largest value was for CopraSmile (Fig. 2B) (P<.001). DISCUSSION The null hypothesis, that the flexural strength, fracture toughness, and translucency of the tested cubic/tetragonal zirconia are comparable with those of lithium disilicate ceramic was rejected. The flexural strength values varied between 490 MPa (DD cubeX2) and 557 MPa (Ceramill Zolid), depending on the zirconia material. Comparable flexural strength values have been reported.17 The lithium disilicate ceramic (IPS e.max Press) showed lower mean values (296 MPa). The flexural strength data were analyzed with the Weibull distribution, predicting failure chance at any level of stress and assessing the reliability of the materials. The Weibull modulus of the zirconia materials varied greatly, which might be attributable to their different sintering. CopraSmile showed the highest Weibull modulus and was sintered at 1500 C final temperature, while the remaining zirconia materials, sintered at 1450 C, had lower Weibull moduli. CopraSmile also had the largest mean grain size. A correlation between grain size and translucency, as well as flexural strength, was reported for the first dental zirconias.13 Nassary Zadeh et al
The Weibull modulus of lithium disilicate ceramic was similar to that of CopraSmile. The Weibull modulus values in the control group of the present study were almost twice those reported in a previous study.18 The difference might have been caused by the measuring method, a 3-point flexural strength in the previous study and a 4-point flexural strength in the present study. The 4-point bend test in the present study used different support and load spans according to the dimension of the specimens, smaller for the lithium disilicate ceramic specimens. This difference might have affected the values because of differences in the stress distribution that result in overestimated values of flexural strength for the lithium disilicate ceramic. This should be considered in further investigations. For fracture toughness, the 4-point SEVNB method was applied. This test requires a specific sharp notch root radius, which was difficult to achieve for the zirconia materials with fine-grained microstructure.15 The results for fracture toughness measured by SEVNB are an overestimate if the notch root radius is above a critical value of approximately 1.5 to 3 times the mean grain size.15,20 The mean grain size of zirconia was between 594 nm and 903 nm. Thus, a maximum notch root radius between 1 and 2 mm is needed to record the true fracture toughness. This condition was not satisfied for the zirconia materials. Although the fracture toughness values might be an overestimate, they should be comparable among the tested materials. The values were similar to
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those previously reported,15 and the mean grain size of 5 different zirconia materials varied between 250 and 700 nm, involving a maximum root radius between 1 and 2 mm, whereas the notch root radius was approximately 5 to 10 mm. In this study, the zirconia materials resulted in significantly higher fracture toughness than the lithium disilicate ceramic. All zirconia materials fell within the same value range (3.34 to 3.77 MPa ∙ m1/2), whereas the lithium disilicate ceramic showed values of 2.10 MPa ∙ m1/2. Lithium disilicate ceramic had higher translucency than zirconia, consistent with a previous study.16 In that study, IPS e.max CAD LT had higher translucency than that of various zirconia materials marketed for monolithic restorations. In the present study, the statistical differences determined among all tested zirconia materials found that Ceramill Zolid had the highest translucency. According to the manufacturers’ recommendations, the investigated materials were sintered with different parameters of heating rate, holding time, and cooling rate (Table 2). Therefore, the sintering parameters are suspected of having an impact on translucency. The translucency of all specimens was tested for the same thicknesses (1.0 ±0.05 mm). However, lithium disilicate restorations require additional thickness (1.5 mm to 2.0 mm occlusal) to withstand mechanical stress in the oral cavity. Cubic/tetragonal zirconia may be used successfully for monolithic tooth restorations with less occlusal thickness and therefore less tooth reduction.8,9,16 Clinical studies are necessary to confirm this advantage. Increasing the volume of stabilizing oxides resulted in approximately 50% of the cubic phase in these recently introduced zirconias.15 This increased their translucency, but flexural strength and fracture toughness were reduced. Varying the quantities of Y2O3 and Al2O3 may result in new materials with properties between those of conventional zirconia and lithium disilicate ceramic.17 A limitation of this study was the lack of a priori power analysis. The number of specimens for fracture toughness and flexural strength were based on a previous study.10 For contrast ratio and translucency, 10 specimens were chosen, and for grain size, 3 specimens were chosen, also based on previous work.13,14 Clinical studies are necessary to assess the optical and mechanical properties of monolithic restorations with cubic/tetragonal zirconia. In addition, the thermodynamic stability of this zirconia generation has not yet been adequately tested and should be investigated. CONCLUSIONS Based on the findings of this in vitro study, the following conclusions were drawn:
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Table 3. Parametric and nonparametric descriptive statistics for generated data Parametric Analysis Material Group
Material
Mean ±SD
Nonparametric Analysis
95% CI
Minimum Median Maximum
Flexural strength (MPa) Zirconia CZ
557 ±88b
507-606
361
564
673
CS
507 ±69b
467-545
383
508
630
DD
490 ±83b
443-536
357
495
654
NZ
540 ±86b
491-588
317
551
658
PD
493 ±119b
425-559
208
481
731
SC
498 ±104b
439-556
260
504
629
LiSi (CG) CG
296 ±39a
274-318
229
294
361
Fracture toughness (MPa ∙ m1/2) Zirconia CZ
3.56 ±0.47b
3.1-3.9
3.00
3.38
4.50
CS
3.34 ±0.56*b
2.9-3.7
2.25
3.32
4.62
DD
3.64 ±0.71b
3.1-4.1
2.87
3.52
5.39
NZ
3.69 ±0.88b
3.1-4.2
2.64
3.41
5.31
PD
3.34 ±0.72*b
2.8-3.8
2.73
3.10
5.01
SC
3.77 ±0.72b
3.2-4.2
2.86
3.57
5.11
CG
2.10 ±0.14a
1.9-2.2
1.85
2.09
2.47
Zirconia CZ
38.3 ±0.3b
37-39
37.8
38.3
38.7
CS
37.1 ±0.3c
35-38
36.4
37.2
37.5
DD
37.3 ±0.3c
36-38
36.7
37.3
37.6
NZ
33.1 ±0.5d
31-34
32.3
33.3
33.5
PD
37.6 ±0.5*c
36-38
36.3
37.6
38.2
SC
33.6 ±0.2d
32-34
33.4
33.6
34.0
LiSi (CG) CG
40.4 ±0.4a
39-41
39.9
40.5
41.0
LiSi
Translucency (%)
Grain size (mm2) Zirconia CZ
0.515 ±0.49bc 0.393-0.637
0.460
0.534
0.552
CS
0.817 ±0.048d 0.696-0.936
0.789
0.789
0.872
DD
0.373 ±0.038ab 0.278-0.468
0.331
0.385
0.404
NZ
0.353 ±0.015a 0.314-0.390
0.338
0.352
0.368
PD
0.580 ±0.041c 0.476-0.683
0.534
0.592
0.614
SC
0.462 ±0.063abc 0.304-0.618
0.394
0.473
0.518
CI, confidence interval; CS, CopraSmile; CZ, Ceramill Zolid FX; DD, DD cubeX2; NZ, NOVAZIR MaxT; PD, priti multidisc ZrO2; SC, StarCeram Z-Smile; SD = standard deviation. *Not normally distributed. Different superscript letters represent significant differences between materials within 1 test parameter.
Table 4. Weibull statistics for flexural strength values Characteristic Strength (v) (MPa)
95% CI
Weibull Modulus (m)
95% CI
CZ
594
548-644
6.9
3.9-11.8
CS
536
501-572
8.4
4.8-11.4
DD
523
481-567
6.9
3.9-11.7
NZ
577
530-626
6.7
3.8-11.4
PD
538
474-610
4.4
2.5-7.6
SC
542
483-607
4.9
2.7-8.3
313
292-332
8.8
5.0-15.0
Material Zirconia
LiSi (CG) CG
2
CS, CopraSmile; CZ, Ceramill Zolid FX; DD, DD cubeX ; NZ, NOVAZIR MaxT; PD, priti multidisc ZrO2; SC, StarCeram Z-Smile.
1. Recently introduced cubic/tetragonal zirconia can replace lithium disilicate ceramic in clinical applications regarding mechanical properties.
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2. The esthetic properties of lithium disilicate ceramic are still better than those of cubic/tetragonal zirconia. 3. The reliability of the zirconia materials was similar to that of lithium disilicate ceramics. REFERENCES 1. Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials 1999;20:1-25. 2. Filser F, Kocher P, Weibel F, Lüthy H, Schärer P, Gauckler LJ. Reliability and strength of all-ceramic dental restorations fabricated by direct ceramic machining (DCM). Int J Comput Dent 2001;4:89-106. 3. Lüthy H, Filser F, Loeffel O, Schumacher M, Gauckler LJ, Matsuzaki F, et al. Translucency and flexural strength of monolithic translucent zirconia and porcelain-layered zirconia. Dent Mater J 2015;34:910-7. 4. Stawarczyk B, Özcan M, Trottmann A, Hämmerle CHF, Roos M. Evaluation of flexural strength of hipped and presintered zirconia using different estimation methods of Weibull statistics. J Mech Behav Biomed Mater 2012;10: 227-34. 5. Awad D, Stawarczyk B, Liebermann A, Ilie N. Translucency of esthetic dental restorative CAD/CAM materials and composite resins with respect to thickness and surface roughness. J Prosthet Dent 2015;113:534-40. 6. Stawarczyk B, Keul C, Eichberger M, Figge D, Edelhoff D, Lümkemann N. Material science update: three generations of zirconiadfrom veneered to monolithic: Part I. Quintessence Int 2017a;48:369-80. 7. Stawarczyk B, Keul C, Eichberger M, Figge D, Edelhoff D, Lümkemann N. Material science update: three generations of zirconiadfrom veneered to monolithic: Part II. Quintessence Int 2017;48:441-50. 8. Nordahl N, Vult von Steyern P, Larsson C. Fracture strength of ceramic monolithic crown systems of different thickness. J Oral Sci 2015;53: 255-61. 9. Gehrt M, Wolfart S, Rafai N, Reich S, Edehoff D. Clinical results of lithiumdisilicate crowns after up to 9 years of service. Clin Oral Investig 2013;17: 275-84. 10. Chevalier J, Gremillard L. Zirconia ceramics. Bioceramics and their Clinical Applications. INSA Lyon; 2008. p.243-65. 11. Stawarczyk B, Frevert K, Ender A, Roos M, Sener B, Wimmer T. Comparison of four monolithic zirconia materials with conventional ones: contrast ratio, grain size, four-point flexural strength and two-body wear. J Mech Behav Biomed Mater 2016;59:128-38.
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12. Pecho OE, Ghinea R, Ionescu AM, Cardona Jde L, Paravina RD, Perez Mdel M. Color and translucency of zirconia ceramics, human dentine and bovine dentine. J Dent Suppl 2012;2:e34-40. 13. Stawarczyk B, Özcan M, Hallmann L, Ender A, Mehl A, Hammerlet CH. The effect of zirconia sintering temperature on flexural strength, grain size, and contrast ratio. Clin Oral Investig 2013;17:269-74. 14. Stawarczyk B, Emslander A, Roos M, Sener B, Noack F, Keul C. Zirconia ceramics, their contrast ratio and grain size depending on sintering parameters. Dent Mater J 2014;33:591-8. 15. 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:e327-37. 16. 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. 17. Carrabba M, Keeling AJ, Aziz A, Vichi A, Fabian Fonzar R, Wood D, et al. Translucent zirconia in the ceramic scenario for monolithic restorations: a flexural strength and translucency comparison test. J Dent 2017;60:70-6. 18. Emslander A, Reise M, Eichberger M, Uhrenbacher J, Edelhoff D, Stawarczyk B. Impact of surface treatment of different reinforced glassceramic anterior crowns on load bearing capacity. Dent Mater J 2015;34: 595-604. 19. International Organization for Standardization. ISO 6872:2015. Dentistryd ceramic materials. Geneva: International Organization for Standardization. 2015. Available at: https://www.iso.org/standard/59936.html. 20. Fischer H, Waindich A, Telle R. Influence of preparation of ceramic SEVNB specimens on fracture toughness testing results. Dent Mater 2008;24:618-22. 21. Butikofer L, Stawarczyk B, Roos M. Two regression methods for estimation of a two-parameter Weibull distribution for reliability of dental materials. Dent Mater 2015;31:e33-50. Corresponding author: Dr Bogna Stawarczyk Department of Prosthetic Dentistry University Hospital at Munich LMU Munich Goethestrasse 70, 80336 Munich GERMANY Email: [email protected] Acknowledgments The authors thank all companies for supporting this study with zirconia materials. Copyright © 2018 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Flexural strength, fracture toughness and translucency of cubic/tetragonal zirconia materials Parissa Nassary Zadeh, Med Dent,a Nina Lümkemann, MSc,b Beatrice Sener, MTA,c Marlis Eichberger, CDT,d and Bogna Stawarczyk, PD Dr Dipl Ing (FH) MSce Zirconia dental restorations ABSTRACT are attracting considerable Statement of problem. The development of zirconia materials with optimized properties has been attention because of their rapid, and studies comparing the mechanical and optical properties of recently introduced zirconia esthetic properties and also with lithium disilicate materials are limited. because of their excellent Purpose. The purpose of this in vitro study was to compare the mechanical and optical properties biocompatibility1 and meof cubic/tetragonal zirconia materials with those of a lithium disilicate ceramic. chanical properties.2-4 TypiMaterial and methods. Specimens were fabricated from 6 different noncolored zirconia materials: cally, highly stable monolithic Ceramill Zolid FX (CZ), CopraSmile (CS), DD cubeX2 (DD), NOVAZIR MaxT (NZ), priti multidisc ZrO₂ (PD), zirconia has been used in the and StarCeram Z-Smile (SC), and 1 lithium disilicate ceramic as a control, IPS e.max Press LT A2 (CG). Fourposterior region; however, point flexural strength (N=105/n=15) and fracture toughness using the single-edge V-notched beam for anterior teeth, monolithic (N=105/n=15) were examined according to International Organization for Standardization standard lithium disilicate has been 6872:2015. Translucency (N=70/n=10) was evaluated with an ultraviolet spectrophotometer. Grain size (N=6/n=1) of zirconia was investigated by using scanning electron microscopy. Data were favored.5 This may change analyzed using the Kolmogorov-Smirnov test, multivariate analysis, 1-way analysis of variance, with the introduction of followed by the post hoc Scheffé test and Kruskal-Wallis and Mann-Whitney U tests, and Weibull esthetic, highly translucent analysis, using the maximum likelihood estimation method at 95% confidence level (a=.05). cubic/tetragonal zirconia and Results. Zirconia materials showed higher mechanical and lower optical properties than CG ideally result in a straightfor(P<.001). No differences were observed among the zirconia materials with respect to flexural ward and efficiently manufacstrength (P=.259) or fracture toughness (P=.408). CG and CS showed significantly higher Weibull tured restoration. In addition, modulus than SC and PD. The lowest translucency values were measured for NZ and SC, followed zirconia has greater fracture by CS, DD, and PD (P<.001). CZ showed the highest translucency values (P<.001). The lowest grain resistance than lithium dissizes were found for NZ, DD, and SC; the largest were shown for CS (P<.001). ilicate ceramic.6-9 Zirconia is Conclusions. Cubic/tetragonal zirconia showed better mechanical properties than lithium disilicate polymorphous, with 3 crystal ceramic. However, the optical properties and the reliability of zirconia are lower than those of lattices, monoclinic (room lithium disilicate ceramic. (J Prosthet Dent 2018;-:---) temperature up to 1170 C), tetragonal (between 1170 C and 2360 C), and cubic approximately 3% and 4% from the tetragonal to the (approximately 2360 C to the melting point of 2680 C).10 monoclinic phase takes place, followed by cracking.1 Without the addition of stabilizing oxides (MgO, CeO2, The first dental zirconia ceramic (3Y-TZP) was a or Y2O), a transformation expansion of between partially stabilized tetragonal zirconia with low
a
Assistant Professor, Department of Prosthetic Dentistry, Hospital of the University of Munich, Ludwig Maximilian University of Munich, Munich, Germany. Materials Engineer, Department of Prosthetic Dentistry, Hospital of the University of Munich, Ludwig Maximilian University of Munich, Munich, Germany. Medical Technologist Assistant, Clinic for Preventive Dentistry, Periodontics and Cardiology, University of Zurich, Zurich, Switzerland. d Certified Dental Technician, Department of Prosthetic Dentistry, Hospital of the University of Munich, Ludwig Maximilian University of Munich, Munich, Germany. e Head of Dental Materials Unit, Department of Prosthetic Dentistry, Hospital of the University of Munich, Ludwig Maximilian University of Munich, Munich, Germany. b c
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Table 1. Summary of materials used
Clinical Implications Cubic/tetragonal zirconia and lithium disilicate ceramic are characterized by specific advantages for different indications. Dentists should be aware of relevant differences among the mechanical and optical properties of the different formulations of zirconia materials.
translucency and high flexural strength.3,11 Its translucency was similar to that of dentin and so was not suitable for monolithic restorations; however, it was used as a framework veneered with esthetic ceramics.12 Monolithic zirconia restorations without veneering reduce production costs, avoid chipping problems, and require less tooth preparation6,7; however, they require zirconia with improved optical properties. This was achieved by increasing the sintering temperature, leading to microstructural alteration and higher translucency.13 The holding time, temperature increase rate, and sintering cooling rate also affected translucency.14 Sintering temperatures above 1600 C led to a decrease in flexural strength.13 Improvements were made by reducing the quantity and grain size of alumina together with repositioning them on the grain boundaries of zirconia.6,7 This resulted in increased light transmission and good long-term stability and high strength.11 However, these zirconia restorations were less translucent and thus had poorer esthetics than lithium disilicate ceramics. Recently, highly translucent fully stabilized cubic/tetragonal zirconia materials have been introduced. The cubic phase consists of approximately 50% and has been achieved by increasing the addition of stabilizing oxides. This resulted in higher translucency due to the more voluminous and more isotropic cubic crystals,14-16 maintaining the high strength of the material.17 These new zirconia materials are marketed for both posterior and anterior restorations. As limited information is available for these materials, however, the purpose of this in vitro study was to compare the properties of this zirconia to a glass-ceramic. The null hypotheses were that no differences in flexural strength, fracture toughness, or translucency would be found among 6 cubic/ tetragonal zirconia materials and a lithium disilicate ceramic. MATERIAL AND METHODS The flexural strength, fracture toughness, translucency, and grain size of 6 cubic/tetragonal zirconia materials (Ceramill Zolid FX, CopraSmile, DD cubeX2, NOVAZIR MaxT, priti multidisc ZrO2, and StarCeram Z-Smile) were analyzed. A lithium disilicate ceramic (IPS e.max Press;
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Material
Abbreviation
Manufacturer
Lot No.
1512004-114
Cubic/tetragonal zirconia Ceramill Zolid FX
CZ
Amann Girrbach AG
CopraSmile
CS
Whitepeaks Dental Solutions
IS2114
DD cubeX2
DD
Dental Direkt
8041632001
NOVAZIR MaxT
NZ
Novadent Dentaltechnik
L2251030007-6
Priti multidisc ZrO2
PD
Pritidenta
Y65 16HT
StarCeram Z-Smile
SC
H.C. Starck
50586461
CG
Ivoclar Vivadent AG
Lithium disilicate IPS e.max Press LT (A2)
U04298
Ivoclar Vivadent AG) was used as a control group (Table 1). A total of 280 specimens were produced. Each material contained 15 specimens for flexural strength, 15 for fracture toughness, and 10 for translucency including 3 for grain size analysis. Specimens were milled from partially sintered zirconia blanks by using a computeraided manufacture (CAM) machine (Ceramill motion 2; Amann Girrbach AG) (Fig. 1A, B). After specimens were ground with SiC abrasive paper up to P2500 (Buehler), they were sintered (LHT 02/16; Nabertherm GmbH) (Table 2). For flexural strength and fracture toughness, the final dimensions of the specimens were 45.0 mm (length) ×4.0 ±0.2 mm (width) ×3.0 ±0.2 mm (thickness) and 16.0 mm (diameter) ×1.0 ±0.05 mm (thickness) for the translucency specimens. The lithium disilicate ceramic specimens were produced with the press technique18 (Austromat 654 press-i-dent; Dekema DentalKeramiköfen) with 3 wax specimens in each muffle at 930 C. After the sprues were deflasked and sectioned, we polished the specimens by using a water-cooled polishing machine (Abramin; Struers) with a polishing plate. For flexural strength and fracture toughness, the final dimension of the specimens was 30.0 mm (length)×4.0 ±0.2 mm (width)×3.0 ±0.2 mm (thickness) and 16.0 mm (diameter)×1.0 ±0.05 mm (thickness) for the translucency specimens. For translucency, the specimens were polished with diamond pads (40 and 20 mm), magnetic supporting grinding disks (9 and 3 mm) plus a polishing pad (1 mm) with diamond suspensions (Struers). For lithium disilicate flexural strength and fracture toughness, the specimens were polished by using SiC up to P4000 (Struers) for wet grinding of the ceramic. The thickness of the specimens was measured at 3 points to a precision of 0.01 mm by using a digital micrometer (Mitutoyo). Additionally, the parallelism of the specimens was verified. Four-point flexural strength was measured according to International Organization for Standardization standard Nassary Zadeh et al
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Figure 1. Fracture toughness specimen preparation. A, Zirconia blank placed in CAM machine. B, Milled specimens for 4-point flexural strength and 4-point fracture toughness. C, Sectioning specimens. D, Notching specimens. E, Notched zirconia specimen (original magnification ×5). F, Specimen loaded in universal testing machine.
6872:2015.19 The long side of the specimen (4.0 ±0.2 mm) was placed in an adapted specimen holder on 2 steel rollers at a span of 40 mm. Specimens were loaded in a universal testing machine (1445 Zwick/Roell; Zwick) at a crosshead speed of 1 mm/min until fracture occurred. The force was applied with a loading apparatus with 2 steel rollers at a span of 20 mm. For the smaller lithium disilicate specimens, a modified specimen holder was used, with the 2 steel rollers at a 10-mm span and the loading rollers with a 5-mm span. The following equation was used to calculate 3Fd flexural strength: s = 4bh 2 ’, where s is flexural strength in MPa; F is fracture load in N; d is difference in span of steel rollers (for support and loading) in mm; b is the width of specimen in mm; and h is the height of specimen in mm. Fracture toughness was measured according to ISO standard 6872:2015 single-edge V-notched beam (SEVNB) method.19,20 Five specimens were placed on the narrow side (3.0 ±0.2 mm), fixed upright, and centered in an adapted specimen holder. A centered saw cut was made by using a universal cutting machine (Secotom-50; Struers) with a diamond cutoff wheel (127 mm diameter×0.4 mm model M1D13; Struers) (Fig. 1C). The depths of the saw cuts were more than 0.5 mm. The specimen holder was then placed in a notching machine (SD Mechatronik), where the specimens were notched and sharpened with a razor blade (0.3 mm blades; David Combi and Finisher) with polishing diamond paste (9 and 3 mm, MetaDi diamond paste; Buehler) (Fig. 1D). The depth of the saw cut together with the depth of the notching was between 0.8 mm and 1.2 mm. The cycles of the machine varied, and the pressing force was controlled with weights. Specimens were ultrasonically cleaned (Sonorex RK102H; Bandelin Electronic GmbH) in 80% Nassary Zadeh et al
alcohol (Alkopharm 80; Brüggemann Alcohol). The saws and notches were measured by using a microscope (Zwick/Roell Z 2.5; Zwick) (Fig. 1E). The specimens were measured with a digital micrometer and singly placed into the same modified specimen holder as for the4-point flexural strength, but they were placed on the narrow side (3.0 ±0.2 mm) with the notched surface pointing downward. Specimens were loaded in a universal testing machine (1445 Zwick/Roell) at a crosshead speed of 0.5 mm/min until fracture occurred (Fig. 1F). The following equation was used to calculate the fracture toughness: pffiffiffi F s1 −s2 3 a KIc = , Y; , b√w w 2ð1−aÞ1:5 where KIc is fracture toughness in MPa ∙ m1/2; F is fracture load in N; b is thickness of specimen in m; ѡ is width of specimen in m; s1 is support span in m; s2 is loading span in m; a is relative depth of the V notch; and Y is a form factor for stress intensity. Translucency was analyzed by using an ultraviolet/ visible light spectrophotometer (Lambda 35; PerkinElmer LAS). All specimens were cleaned with 80% alcohol (Alkopharm 80) and singly fixed in an appropriate specimen holder with barium sulfate. Specimens were placed in the spectrophotometer at the inlet hole of the integrating sphere. Translucency was quantitively measured by analyzing the definite transmission of light through each specimen. The spectrophotometer recorded the light transmission with a sensor, comparing light intensities from a split beam. The light source provided a wavelength varying between 400 and 700 nm. Initial
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Table 2. Sintering parameters used for zirconia materials Group
Heating Rate From Room Temperature and Eventual Heating Steps
CZ
8 C/min
Final Temperature ( C)
Holding Time (min)
Cooling Rate up to 25 C
1450
120
15 C/min
CS
10 C/min until 950 C is attained, then 6 C/min
1500
90
Unregulated in closed furnace
DD
8 C/min until 900 C is attained; after holding for 30 min, heating rate of 3 C/min
1450
120
10 C/min
NZ
4 C/min until 500 C is attained, then 8 C/min until 1150 C is attained; after holding for 30 min, 2 C/min to 1300 C, then 4 C/min
1450
120
8 C/min to 800 C, then 0 C/min to 100 C
PD
10 C/min
1450
120
10 C/min
SC
5 C/min until 900 C is attained; after holding for 30 min, 2.5 C/min
1450
120
5 C/min to 900 C, then unregulated
CS, CopraSmile; CZ, Ceramill Zolid FX; DD, DD cubeX2; NZ, NOVAZIR MaxT; PD, priti multidisc ZrO2; SC, StarCeram Z-Smile.
translucency was calculated by the intensity of the monochromatic light I0 and the light I, transmitted through the specimen. The transmission coefficient, tc(%), was calculated by using I/I0=tcx. The overall light transmission for each specimen (T) was calculated as the integration (tc[l] dl [10−5]) of all tc values. To analyze light transmission, the T value of each material was divided by the T value with no specimen in the spectrophotometer (baseline) to receive light transmission as a percentage. All tests were conducted at room temperature. Grain size was analyzed with scanning electron microscopy (Fig. 2) (Carl Zeiss Supra V50, Cathode: field emission; Carl Zeiss Microscopy GmbH). Specimen preparation involved thermal etching (LTH 02/16; Nabertherm) to a final temperature of 1450 C and holding at that temperature for 30 minutes. Subsequently, the specimens were cleaned with 80% alcohol (Alkopharm 80) and glued to a holder. Each specimen was sputtered with a 2-nm layer of gold for 45 seconds (Safematic CCU-010; Safematic). Scanning electron microscopy was operated at an acceleration voltage of 5 kV at a working distance of 8.4 to 10.0 mm. Grain sizes were determined at 3 different locations on the same specimen. Measured data were analyzed statistically by using statistical software (IBM SPSS Statistics v24.0; IBM Corp). Parametric and nonparametric descriptive statistics were computed. For quantitative variables, the assumption of normality was tested using the Kolmogorov-Smirnov test. The general linear model (multivariate) analysis was performed. Flexural strength and grain size data were analyzed with 1-way analysis of variance (ANOVA), followed by the Scheffé post hoc test. Translucency and fracture toughness were tested with nonparametric tests, including the Kruskal-Wallis and Mann-Whitney U tests. Statistical analyses were performed for all materials, whereas zirconia materials were considered separately (a=.05). Weibull distribution parameters (Weibull modulus, characteristic strength) for flexural strength values were calculated by using the
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maximum likelihood estimation method at a 95% confidence interval.21 RESULTS According to the multifactorial analyses, the tested materials showed significant differences for the parameters, namely flexural strength, fracture toughness, translucency, and grain size. The greatest influence on the ceramic materials was for translucency (partial eta squared [hP2]=0.979; P<.001), followed by flexural strength (hP2=0.524; P<.001), fracture toughness (hP2=0.430; P<.001), and grain size (hP2=0.383; P<.001). The Kolmogorov-Smirnov test indicated a higher rate of violations of the normality assumption for translucency (28%) and fracture toughness data (14%) (Table 3). The violation of the normality assumption was not caused by outliers but rather by measurement rounding, leading to increased coarseness of the observations in each test group. Consequently, translucency and fracture toughness data were analyzed nonparametrically. Within the flexural strength and grain size data, all groups were normally distributed and analyzed using parametric tests (Table 3). The control group, IPS e.max Press, showed the lowest flexural strength (P<.001) and fracture toughness (P<.001), but the highest translucency (P<.001) compared with the zirconia materials. No statistical differences between the zirconia materials were observed with respect to flexural strength (P=.259) or fracture toughness (P=.408). IPS e.max Press (m=8.8), and CopraSmile (m=8.4) showed significantly higher Weibull modulus than StarCeram Z-Smile (m=4.9) and priti multidisc ZrO2 (m=4.4) (Table 4). For the zirconia materials, CopraSmile (m=8.4) showed a significantly higher Weibull modulus compared with that of priti multidisc ZrO2 (m=4.4) and StarCeram Z-Smile (m=4.9). With respect to translucency, the lowest values were found for NOVAZIR MaxT and StarCeram Z-Smile, followed by CopraSmile, DD cubeX2, and priti multidisc
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Figure 2. Scanning electron micrographs showing grain size of polished zirconia translucency specimens (original magnification ×50 000). A, Ceramill Zolid FX (CZ, Amann Girrbach). B, CopraSmile (CS, Whitepeaks). C, DD cubeX2 (DD, Dental Direkt). D, NOVAZIR MaxT (NZ, Novadent). E, priti multidisc ZrO2 (PD, Pritidenta). F, StarCeram Z-Smile (SC, H.C. Starck).
ZrO2 (P<.001). Ceramill Zolid FX had the highest translucency (P<.001). The lowest mean grain size was found for NOVAZIR MaxT (Fig. 2D), DD cubeX2 (Fig. 2C), and StarCeram ZSmile (Fig. 2F) (P<.001) (Table 3). The significantly largest value was for CopraSmile (Fig. 2B) (P<.001). DISCUSSION The null hypothesis, that the flexural strength, fracture toughness, and translucency of the tested cubic/tetragonal zirconia are comparable with those of lithium disilicate ceramic was rejected. The flexural strength values varied between 490 MPa (DD cubeX2) and 557 MPa (Ceramill Zolid), depending on the zirconia material. Comparable flexural strength values have been reported.17 The lithium disilicate ceramic (IPS e.max Press) showed lower mean values (296 MPa). The flexural strength data were analyzed with the Weibull distribution, predicting failure chance at any level of stress and assessing the reliability of the materials. The Weibull modulus of the zirconia materials varied greatly, which might be attributable to their different sintering. CopraSmile showed the highest Weibull modulus and was sintered at 1500 C final temperature, while the remaining zirconia materials, sintered at 1450 C, had lower Weibull moduli. CopraSmile also had the largest mean grain size. A correlation between grain size and translucency, as well as flexural strength, was reported for the first dental zirconias.13 Nassary Zadeh et al
The Weibull modulus of lithium disilicate ceramic was similar to that of CopraSmile. The Weibull modulus values in the control group of the present study were almost twice those reported in a previous study.18 The difference might have been caused by the measuring method, a 3-point flexural strength in the previous study and a 4-point flexural strength in the present study. The 4-point bend test in the present study used different support and load spans according to the dimension of the specimens, smaller for the lithium disilicate ceramic specimens. This difference might have affected the values because of differences in the stress distribution that result in overestimated values of flexural strength for the lithium disilicate ceramic. This should be considered in further investigations. For fracture toughness, the 4-point SEVNB method was applied. This test requires a specific sharp notch root radius, which was difficult to achieve for the zirconia materials with fine-grained microstructure.15 The results for fracture toughness measured by SEVNB are an overestimate if the notch root radius is above a critical value of approximately 1.5 to 3 times the mean grain size.15,20 The mean grain size of zirconia was between 594 nm and 903 nm. Thus, a maximum notch root radius between 1 and 2 mm is needed to record the true fracture toughness. This condition was not satisfied for the zirconia materials. Although the fracture toughness values might be an overestimate, they should be comparable among the tested materials. The values were similar to
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those previously reported,15 and the mean grain size of 5 different zirconia materials varied between 250 and 700 nm, involving a maximum root radius between 1 and 2 mm, whereas the notch root radius was approximately 5 to 10 mm. In this study, the zirconia materials resulted in significantly higher fracture toughness than the lithium disilicate ceramic. All zirconia materials fell within the same value range (3.34 to 3.77 MPa ∙ m1/2), whereas the lithium disilicate ceramic showed values of 2.10 MPa ∙ m1/2. Lithium disilicate ceramic had higher translucency than zirconia, consistent with a previous study.16 In that study, IPS e.max CAD LT had higher translucency than that of various zirconia materials marketed for monolithic restorations. In the present study, the statistical differences determined among all tested zirconia materials found that Ceramill Zolid had the highest translucency. According to the manufacturers’ recommendations, the investigated materials were sintered with different parameters of heating rate, holding time, and cooling rate (Table 2). Therefore, the sintering parameters are suspected of having an impact on translucency. The translucency of all specimens was tested for the same thicknesses (1.0 ±0.05 mm). However, lithium disilicate restorations require additional thickness (1.5 mm to 2.0 mm occlusal) to withstand mechanical stress in the oral cavity. Cubic/tetragonal zirconia may be used successfully for monolithic tooth restorations with less occlusal thickness and therefore less tooth reduction.8,9,16 Clinical studies are necessary to confirm this advantage. Increasing the volume of stabilizing oxides resulted in approximately 50% of the cubic phase in these recently introduced zirconias.15 This increased their translucency, but flexural strength and fracture toughness were reduced. Varying the quantities of Y2O3 and Al2O3 may result in new materials with properties between those of conventional zirconia and lithium disilicate ceramic.17 A limitation of this study was the lack of a priori power analysis. The number of specimens for fracture toughness and flexural strength were based on a previous study.10 For contrast ratio and translucency, 10 specimens were chosen, and for grain size, 3 specimens were chosen, also based on previous work.13,14 Clinical studies are necessary to assess the optical and mechanical properties of monolithic restorations with cubic/tetragonal zirconia. In addition, the thermodynamic stability of this zirconia generation has not yet been adequately tested and should be investigated. CONCLUSIONS Based on the findings of this in vitro study, the following conclusions were drawn:
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Table 3. Parametric and nonparametric descriptive statistics for generated data Parametric Analysis Material Group
Material
Mean ±SD
Nonparametric Analysis
95% CI
Minimum Median Maximum
Flexural strength (MPa) Zirconia CZ
557 ±88b
507-606
361
564
673
CS
507 ±69b
467-545
383
508
630
DD
490 ±83b
443-536
357
495
654
NZ
540 ±86b
491-588
317
551
658
PD
493 ±119b
425-559
208
481
731
SC
498 ±104b
439-556
260
504
629
LiSi (CG) CG
296 ±39a
274-318
229
294
361
Fracture toughness (MPa ∙ m1/2) Zirconia CZ
3.56 ±0.47b
3.1-3.9
3.00
3.38
4.50
CS
3.34 ±0.56*b
2.9-3.7
2.25
3.32
4.62
DD
3.64 ±0.71b
3.1-4.1
2.87
3.52
5.39
NZ
3.69 ±0.88b
3.1-4.2
2.64
3.41
5.31
PD
3.34 ±0.72*b
2.8-3.8
2.73
3.10
5.01
SC
3.77 ±0.72b
3.2-4.2
2.86
3.57
5.11
CG
2.10 ±0.14a
1.9-2.2
1.85
2.09
2.47
Zirconia CZ
38.3 ±0.3b
37-39
37.8
38.3
38.7
CS
37.1 ±0.3c
35-38
36.4
37.2
37.5
DD
37.3 ±0.3c
36-38
36.7
37.3
37.6
NZ
33.1 ±0.5d
31-34
32.3
33.3
33.5
PD
37.6 ±0.5*c
36-38
36.3
37.6
38.2
SC
33.6 ±0.2d
32-34
33.4
33.6
34.0
LiSi (CG) CG
40.4 ±0.4a
39-41
39.9
40.5
41.0
LiSi
Translucency (%)
Grain size (mm2) Zirconia CZ
0.515 ±0.49bc 0.393-0.637
0.460
0.534
0.552
CS
0.817 ±0.048d 0.696-0.936
0.789
0.789
0.872
DD
0.373 ±0.038ab 0.278-0.468
0.331
0.385
0.404
NZ
0.353 ±0.015a 0.314-0.390
0.338
0.352
0.368
PD
0.580 ±0.041c 0.476-0.683
0.534
0.592
0.614
SC
0.462 ±0.063abc 0.304-0.618
0.394
0.473
0.518
CI, confidence interval; CS, CopraSmile; CZ, Ceramill Zolid FX; DD, DD cubeX2; NZ, NOVAZIR MaxT; PD, priti multidisc ZrO2; SC, StarCeram Z-Smile; SD = standard deviation. *Not normally distributed. Different superscript letters represent significant differences between materials within 1 test parameter.
Table 4. Weibull statistics for flexural strength values Characteristic Strength (v) (MPa)
95% CI
Weibull Modulus (m)
95% CI
CZ
594
548-644
6.9
3.9-11.8
CS
536
501-572
8.4
4.8-11.4
DD
523
481-567
6.9
3.9-11.7
NZ
577
530-626
6.7
3.8-11.4
PD
538
474-610
4.4
2.5-7.6
SC
542
483-607
4.9
2.7-8.3
313
292-332
8.8
5.0-15.0
Material Zirconia
LiSi (CG) CG
2
CS, CopraSmile; CZ, Ceramill Zolid FX; DD, DD cubeX ; NZ, NOVAZIR MaxT; PD, priti multidisc ZrO2; SC, StarCeram Z-Smile.
1. Recently introduced cubic/tetragonal zirconia can replace lithium disilicate ceramic in clinical applications regarding mechanical properties.
Nassary Zadeh et al
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2018
2. The esthetic properties of lithium disilicate ceramic are still better than those of cubic/tetragonal zirconia. 3. The reliability of the zirconia materials was similar to that of lithium disilicate ceramics. REFERENCES 1. Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials 1999;20:1-25. 2. Filser F, Kocher P, Weibel F, Lüthy H, Schärer P, Gauckler LJ. Reliability and strength of all-ceramic dental restorations fabricated by direct ceramic machining (DCM). Int J Comput Dent 2001;4:89-106. 3. Lüthy H, Filser F, Loeffel O, Schumacher M, Gauckler LJ, Matsuzaki F, et al. Translucency and flexural strength of monolithic translucent zirconia and porcelain-layered zirconia. Dent Mater J 2015;34:910-7. 4. Stawarczyk B, Özcan M, Trottmann A, Hämmerle CHF, Roos M. Evaluation of flexural strength of hipped and presintered zirconia using different estimation methods of Weibull statistics. J Mech Behav Biomed Mater 2012;10: 227-34. 5. Awad D, Stawarczyk B, Liebermann A, Ilie N. Translucency of esthetic dental restorative CAD/CAM materials and composite resins with respect to thickness and surface roughness. J Prosthet Dent 2015;113:534-40. 6. Stawarczyk B, Keul C, Eichberger M, Figge D, Edelhoff D, Lümkemann N. Material science update: three generations of zirconiadfrom veneered to monolithic: Part I. Quintessence Int 2017a;48:369-80. 7. Stawarczyk B, Keul C, Eichberger M, Figge D, Edelhoff D, Lümkemann N. Material science update: three generations of zirconiadfrom veneered to monolithic: Part II. Quintessence Int 2017;48:441-50. 8. Nordahl N, Vult von Steyern P, Larsson C. Fracture strength of ceramic monolithic crown systems of different thickness. J Oral Sci 2015;53: 255-61. 9. Gehrt M, Wolfart S, Rafai N, Reich S, Edehoff D. Clinical results of lithiumdisilicate crowns after up to 9 years of service. Clin Oral Investig 2013;17: 275-84. 10. Chevalier J, Gremillard L. Zirconia ceramics. Bioceramics and their Clinical Applications. INSA Lyon; 2008. p.243-65. 11. Stawarczyk B, Frevert K, Ender A, Roos M, Sener B, Wimmer T. Comparison of four monolithic zirconia materials with conventional ones: contrast ratio, grain size, four-point flexural strength and two-body wear. J Mech Behav Biomed Mater 2016;59:128-38.
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