In Vitro Fracture Toughness of Commercial Y-TZP Ceramics: A Systematic Review Sheila Pestana Passos, DDS, MSc, PhD,1 John A. Nychka, MSc, PhD,1,2 Paul Major, DDS, MSc,1 Bernie Linke, DDS, MSD FRCD(C),1 & Carlos Flores-Mir, DDS, MSc, PhD1 1 2
School of Dentistry, University of Alberta, Edmonton, Canada Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Canada
Keywords Fracture toughness; Y-TZP; zirconia; systematic review. Correspondence Dr. Sheila P. Passos, School of Dentistry, University of Alberta, 11405–87 Ave NW, Room #5083C, Edmonton Clinic Health Academy (ECHA), Edmonton, AB, Canada T6G 1C9. E-mail: [email protected] The authors deny any conflicts of interest. Accepted December 13, 2013 doi: 10.1111/jopr.12179
Abstract Purpose: The aim of this review was to assess research methods used to determine the fracture toughness of Y-TZP ceramics in order to systematically evaluate the accuracy of each method with regard to potential influencing factors. Materials and Methods: Six databases were searched for studies up to April 2013. The terms “tough*,” “critical stress intensity factor,” “zirconi*,” “yttri*,” “dent*,” “zirconia,” “zirconium,” and “stress” were searched. Titles and abstracts were screened, and literature that fulfilled the inclusion criteria was selected for a full-text reading. Test conditions with potential influence on fracture toughness were extracted from each study. Results: Ten laboratory studies met the inclusion criteria. There was a significant variation in relation to test method, ambient conditions, applied/indentation load, number of specimens, and geometry and dimension of the specimen. The results were incomparable due to high variability and missing information. Therefore, 10 parameters were listed to be followed to standardize future studies. Conclusions: A wide variation in research methods affected the fracture toughness reported for Y-TZP ceramics among the selected studies; single-edge-precracked beam and chevron-notched-beam seem to be the most recommended methods to determine Y-TZP fracture toughness; the indentation methods have several limitations. Clinical significance: The accurate calculation of toughness values is fundamental because overestimating toughness data in a clinical situation can negatively affect the lifetime of the restoration.
In fixed prosthodontics, several all-ceramic materials are extensively used due to their biocompatibility and excellent esthetic characteristics. The addition of yttria to zirconia stabilizes the crystalline structure of zirconium oxide at ambient temperature creating a yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramic, a toughened oxide ceramic. The tetragonal to monoclinic (t→m) transformation in the ceramic structure improves the toughness of Y-TZP ceramics.1,2 Such a transformation prevents further propagation of cracks by acting at the crack tip and around the crack by localized compressive stresses,3 which are induced as a result of the large shear strain and a positive volumetric change associated with the phase transformation.4 However, the presence of flaws and cracks of different sizes in the ceramic material should be considered. Preexisting flaws/cracks on the ceramic surface or in the ceramic material act as stress concentrations. Crack initiation sites under stress can accelerate the crack growth rate to the point of failure at seemingly low applied stresses.5
The long-term stability of Y-TZP in the presence of water is limited by the continuing transformation from the tetragonal to monoclinic phase via a process of low temperature degradation (LTD) and subcritical crack growth (growth of cracks below the stress required to lead to wholesale fracture). All these factors might affect the lifetime of Y-TZP dental restorations.6,7 Other factors that affect the long-term behavior of zirconia ceramics in dental applications remain to be better explored.8-11 The fracture toughness or critical stress intensity (KIC ) defines the material behavior in the presence of a preexisting flaw characterizing the resistance of the material12,13 against crack propagation. Fracture toughness of dental ceramics has been evaluated using different test methods and different specimen shapes.14-16 Several previous studies evaluated the fracture toughness of dental ceramics by the indentation fracture (IF) method.12,14,15,17-26 The IF technique is simple, and the required number of specimens per test is small; however, high dispersion has been observed due to inaccurate measurement of crack length and slow crack growth (SCG).14,22,27 Kies and Clark28
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proposed the double torsion test setup to determine toughness and SCG. It has been considered a valuable method,28,29 and it has been recently used.30 To calculate the toughness values, other methods are also used, such as single edge v-notched beam (SEVNB), fractography (FR), indentation strength (IS), single-edge-precracked beam (SEPB), and chevron-notched-beam (CNB). The SEVNB method requires creating a sharp V-notch, and then the flexural strength test is performed. FR can be used measuring the critical crack size and the flexural strength of the material. For IS, a Vickers microindentation is performed before the flexural strength test, and the hardness is measured as well. SEPB is performed creating a straight-through precrack, and then a flexural strength test is conducted. CNB is executed preparing a chevron-notch, and then flexural strength is calculated. In vitro tests have limitations and do not necessarily determine the clinical success of all-ceramic core materials; however, they do represent the most available information about Y-TZP dental restorations. The possibility of collecting in vivo data has been limited so far due to ethical and practical reasons. It is believed that better clinical performance and a longer lifetime can be achieved with improved fracture toughness of the material structure.31 To predict the lifetime of zirconia restorations, it is important to know the toughening mechanisms. Thus, this review will focus on systematically evaluating the accuracy of each fracture toughness testing method with regard to potential influencing factors based on in vitro studies.
Table 1 Search strategies used for each database Database
MEDLINE (OvidSP)
PubMed (NLM) EMBASE (OvidSP) Cochrane (Wiley) Scopus (Elsevier)
LILACS (Virtual Health Library)
Dates of coverage
Keywordcs used
1946 – April/2013 “tough∗ ” OR “critical stress intensity factor” AND “zirconi∗ ” 1950 – April/2013 Same search strategy as MEDLINE (OvidSP) 1980 – April/2013 Same search strategy as MEDLINE (OvidSP) to the fourth Same search strategy as quarter of 2011 MEDLINE (OvidSP) 1960 – April/2013 “tough∗ ” OR “critical stress intensity factor” AND “zirconi∗ .” Within results other terms were added: “yttri∗ ” AND “dent∗ ” 1982 – April/2013 “toughness” AND “zirconia” “toughness” AND “zirconium” “stress” AND “zirconia” “stress” AND “zirconium” “tenacidade” AND ˆ “zirconia” “tenacidade” AND ˆ “zirconio” ˆ “estresse” AND “zirconia” ˆ “estresse” AND “zirconio”
Materials and methods The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) statement32 was followed as much as possible. Data collection
Multiple online databases were systematically searched: MEDLINE (OvidSP), PubMed (NLM—National Library of Medicine), EMBASE (OvidSP), Cochrane Library (Wiley), Scopus (Elsevier), and LILACS (Virtual Health Library). The search terms “tough*,” “critical stress intensity factor,” “zirconi*,” “yttri*,” “dent*,” “zirconia,” “zirconium,” and “stress” were used to search in vitro studies up to April 2013. Search strategies and dates of coverage of each database are outlined in Table 1, which also shows the specific combination and truncation of the keywords. The electronic search was not limited by any means. The final search strategy was fine tuned with the assistance of a health sciences librarian.
1. No veneered specimens.
Criteria for selection of studies
From the search results according to titles/abstracts relevant to topic, studies were selected based on the following inclusion criteria: 1. In vitro studies. 2. Dental application of commercial Y-TZP ceramics obtained from presintered blocks. 2
Where a relevant title without a listed abstract was available, a full copy of the study was assessed for inclusion as explained below. The studies’ titles and abstracts of potential studies were evaluated by two independent reviewers. All abstracts that appeared to meet inclusion criteria were selected based on a consensus agreement between two reviewers, and full articles or full theses were obtained. Conference abstracts, such as International Association for Dental Research (IADR) abstracts, were not selected because they do not present the detailed test method, but an attempt was made to retrieve the full articles or thesis if they were already published. The literature search was completed by manual search within the reference lists of selected full-text studies. Only articles that complied with the initial inclusion criteria were further reviewed. Full copies of articles were then reviewed independently by two reviewers to determine whether the exclusion criteria applied. In addition to the initial selection criteria the following exclusion criterion was only considered at this stage:
Data extraction
The toughness method was extracted, as were the experimental conditions (Table 2). In addition, the authors of these articles were contacted to obtain any incomplete/missing information. Assessment of study risk of bias was accomplished with individual information collected in Table 2.
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Vita Zahnfabrik (Bad Sackingen, Germany) 0.7 µm
2. Manufacturer
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25 × 2 ± 0.2 × 2.5 ± 0.2 mm
2 × 4 × 16 mm
Diamond disc
275 rpm
7. Specimen dimensions
8. Sectioning procedure
9. Sectioning speed
Slow speed
100-µm- thick diamond
3
4. Yttria content 5. Toughness method 6. Number of specimens
5
0.383 µm (0.14-0.8 µm)62
0.383 µm (0.14-0.8 µm)62
4 × 3 x 32 mm
2 × 5 × 25 mm
Not reported
Milling procedure
KaVo Everest engine
16
Diamond disc
Nemli et al18
Not reported
Not reported
15-mm diameter x 1-mm thick
10
R Cercon Everest ZS, LavaTM , and Zirconia and Crystal HS LavaTM Dentsply Kavo, 3M (York, PA) (Seefeld, and 3M Germany) and DLMS (Scottsdale, AZ) Cercon: Lava: 0.1 µm63 0.2-0.3 Crystal µm64 HS:0.07-0.3 µm Lava: 0.1 µm63 3 mol% 3 mol% SEVNB IF
Marinis et al34
10
3 mol% IF
Kavo
Kavo (Biberach, Germany)
3 mol% IF
Everest ZS
Harada et al26
Everest ZS
Mitov et al17
5 mol%∗∗ FR
3. Zirconia grain size
Vita In-Ceram R YZ
Kosmac et al6
1. Y-TZP ceramic system
Experimental conditions
Table 2 Experimental conditions assessed in each selected article
Diamondcoated disc Not reported
IS: 20 SEVNB: 20 FR: 8 1.5 × 2.5 × 25 mm
3 mol% IS, SEVNB, FR
Not reported
Diamond disc∗∗∗
15-mm diameter, 1.2 ± 1.2-mm thick Sp provided by the manufacturer Not reported
10
1∗∗∗
2 × 4 × 50 mm
3 mol% IF
0.2-0.3 µm64
Dentsply
R Cercon Zirconia
Yilmaz et al25
3 mol% IF
0.1-0.3 µm
Dentsply
Dentsply
0.3-0.5 µm
R Cercon Zirconia
Studart et al24
R Cercon Zirconia
Aboushelib et al23
Not reported
(Continued)
Sp provided by the manufacturer Not reported
3 × 4 × 25 mm
0.9 × 19.65 × 56 mm
Not reported
5
2 mol% SEPB
0.6 µm
Norton-St. Gobain (Raleigh, NC)
Prozyr
Quinn et al33
5
3 mol% SEVNB
0.25-1.4 µm
Dentsply
R Cercon Zirconia
Aboushelib et al30
Passos et al Fracture Toughness of Dental Y-TZP Ceramics
3
4 VI: 196 N
Pre-load: 50% of the fracture load 0.01 MPa/s
Until failure
19. Applied/ Indentation load
20. Duration of load applied
15 s
Not reported
17. Number of indentations 18. Indentation location
NA
NA
NA
NA
NA
10
NA
NA
NA
x
15 s
Not reported
Not reported
NA
Center∗∗∗
4-point bending
1 mm V-shaped
Not reported
NA
VI: 9.8, 49, 98, 196, and 294 N
x
x
√
Marinis et al34
Diamond disc
1∗∗∗
NA
NA
NA
NA
x
√ √
√ √
√46 √
√46
Harada et al26
Mitov et al17
Kosmac et al6
3-point bending NA
16. Flexural test
15. Notch dimensions∗
14. Notch length∗
12. Cleaning of specimens 13. Notch procedure∗
10. Water cooling 11. Finishing of specimens
Experimental conditions
Table 2 Continued
15 s
VI: 4.5 mm away from the center VI: 294 N
3
NA
NA
NA
NA
√
x √
Nemli et al18
Not reported
VI: center SEVNB: NA FR: NA VI: 19.6 N and 196 N
5
4-point bending
0.3-mm thick, 300-µm deep
Diamondcoated disc Not reported
x
√ √
Aboushelib et al23
Not reported
VI: 49, 98 and 196 N∗∗∗
VI: Center∗∗∗
5∗∗∗
3
Not reported
VI: 4.5 mm away from the center VI: 588 N
NA
NA
NA
NA
NA
NA
NA
NA
x
x x
Yilmaz et al25
Sp were ground with SiC paper #80 x
√ ∗∗∗
Studart et al24
Not reported
Not reported
NA
NA
3.5-mm diameter, 300-µm deep Not reported
16 mm
Not reported
x
x √
Aboushelib et al30
(Continued)
VI: 78 N Compressive load: between 6000 and 14,000 N VI: not reported Compressive load: as soon as precrack was heard to pop in
VI: center
4-point bending 1
Pre-crack: 0.4-0.6 of the sp deep NA
Pre-crack (VI)
√ ∗∗∗
x √
Quinn et al33
Fracture Toughness of Dental Y-TZP Ceramics Passos et al
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√
6.5 (0.3)
37°C In artificial saliva √
3.17
x
x
98 N: 8.0 (0.5) 196 N: 7.1 (0.5) 294 N: 6.8 (0.2)
√
23°C∗∗∗
Not reported √
Vickers hardness
√ ∗∗∗
Harada et al26
Vickers hardness
x
x
3-point: Universal testing machine
Mitov et al17
Kosmac et al6
Cercon: 6.8 (0.1) Lava: 6.6 (0.13) After fatigue: Cercon: 6.9 (0.11) Lava: 7.3 (0.1) x
Everest: 5.8 (1.4) Lava: 5.2 (0.5) Crystal HS: 4.9 (0.7)
x
√
Not reported
Vickers hardness
√
Nemli et al18
√
Room temperature
4-point: Universal testing machine
x
Marinis et al34
√
√ IS √ SEVNB FR: not reported Similar values for all techiques: 7.4 (0.32)
IF: Vickers hardness 4-point: Universal testing machine 22°C In silicone oil
√
Aboushelib et al23
5.6
x
23°C∗∗∗ Ambient air conditions∗∗∗ √ ∗∗∗
Vickers hardness
√ ∗∗∗
Studart et al24
x
6.27 (0.05)
√
Not reported
Vickers hardness
x
Yilmaz et al25
x
5.06 (0.97)
√
Not reported
4-point: Universal testing machine
x
Aboushelib et al30
SEVNB, single edge notched beam; IF, indentation fracture; FR, fractography; IS, indentation strength; SEPB, single-edge-precracked beam; NA, not applicable; VI, Vickers indentation. *Applicable for the fracture toughness using SEVNB and SEPB. **Information obtained from the manufacturer. ***Information obtained from the authors (not reported in the article). 46 Reference of the assessed information.
26. Fractographic assessment
25. Toughness in MPam (sd)
24. Equation
23. Ambient conditions
21. Specimen tested immediately after notching or indentation 22. Testing machine
Experimental conditions
Table 2 Continued
√
4.9 (0.2)
√
Ambient air conditions
Vickers hardness + hydraulic testing machine
√ ∗∗∗
Quinn et al33
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MEDLINE
PubMed
EMBASE
Passos et al
Cochrane
206 studies 208 studies 210 studies
0
Scopus
LILACS
217 studies 19 studies
After removing duplicates (268) Studies excluded from the first selection stage (213) Potentially relevant studies (55)
Studies excluded from the second selection stage (45)
Studies relevant to the topic
Article from selected thesis (1)
(9 articles + 1 thesis)
TOTAL: 11 studies Duplicate study excluded (1 thesis) Studies relevant to the topic (10) References of selected full-text articles (0) Figure 1 Schematic study selection procedure.
Studies included (10)
Results Ten6,17,18,23-26,30,33,34 laboratory studies were included in the systematic review (Fig 1). Among the studies considered in the second selection stage, a few were eliminated after inclusion and exclusion criteria were applied. One was excluded due to the absence of toughness value.35 In vitro studies that evaluated the fracture toughness of veneered zirconia,36-38 experimental Y-TZP ceramic,39,40 or Y-TZP ceramics not obtained from partially sintered blocks,20,21,41-43 and articles which evaluated HIPped Y-TZP ceramic44 were excluded. In addition, one article was excluded because it did not have an English translation.45 Because a thesis paper46 was selected from the second selection stage, the article6 from the thesis was searched and included. For that reason, the thesis was excluded as a duplicate study. The online search was completed by manual search through the references of the selected articles; however, no additional study was found. The selected studies were published between 2003 and 2013. They differed widely with respect to experimental methodology, manufacturing company, and study design (number of specimens, geometry and dimension of the specimen, load applied/indentation load, ambient conditions). Such variation precluded the possibility of attempting a meta-analysis. 6
Five methods to determine the fracture toughness were identified among the ten articles, with IF being the most investigated technique (Table 2). One study23 compared three techniques (IS, SEVNB, FR), and statistically similar values were observed for the Y-TZP ceramic. The SEPB technique was also applied by one study.33 Among the six brands identified from the ten studies, R Zirconia (Dentsply, York, PA) was the most investiCercon gated (Table 2). The preparation procedure was not mentioned for most of the selected articles.17,18,23,24,30,34 Only two studies clearly stated processing conditions in accordance with manufacturer’s recommendation,6,26 and the remaining two articles reported that the specimens were provided by the manufacturers,25,33 presumably indicating that manufacturerrecommended preparation was followed. Regarding aging conditions, of the included studies, only one article18 considered the influence of aging on two differR ent ceramic systems (Cercon Zirconia, LavaTM ) and found a difference between before and after fatigue for the LavaTM system at ambient conditions (22 ± 1°C with 60 ± 5% relative humidity). Fractographic analysis was used as a technique to determine the fracture toughness,6 for further detection of chipping or fractures,23,33 or to validate the data obtained from the SEVNB test.23
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Table 3 Recommended methods and procedures to determine toughness fracture for zirconia according to the findings of the current study, ISO 24370 (CNB),47 ISO 15732 (SEPB),48 and ASTM C1421 (CNB, SEPB, SCF)49 Experimental design
Chevron-notched-beam (CNB)47,49
Single-edge-precracked beam (SEPB)48,49
4. Specimen edges
Not less than 5 L: min 45 mm, Th: 3 ± 0.2 mm, W: 4 ± 0.2 mm, Finish grinding with a diamond-grit wheel of 320 grit or finer Not chamfered or beveled
5. Type of notch
Chevron-notched
Not less than 5 L: 18 mm, Th: 3 ± 0.2 mm, W: 4 ± 0.2 mm Ra : not more than 0.2 µm Ra has to be determined and reported The four long edges have to be chamfered uniformly at 45° ± 5°. Chamfered edge length: 0.12 mm ± 0.03 mm Saw notch or indentation
6. Notch length∗∗ 7. Notch dimensions
16 mm V-shaped W: min 3.8 mm, Th: less than 0.3 mm
8. Ambient conditions
Laboratory ambient conditions. Record the test temperature and relative humidity
9. Crack measurement
NA
10. Fracture test
3- or 4-point bending, fracture the test specimens in distilled water or artificial saliva
1. Number of specimens 2. Specimen dimensions∗ 3. Finishing of specimens
16 mm W: 0.1 mm, Deep: between 0.4 and 0.6 mm,44 Deep: between 0.2 and 0.3 mm45 Laboratory ambient conditions. Record the test temperature and relative humidity A drop of dye penetrant can be placed on saw notch. Prior to fracture testing, dye penetrants have to be dry 3- or 4-point bending, fracture the test specimens in distilled water or artificial saliva
Our recommendations Power calculation is required See note below∗ SEPB recommendation for CNB as well SEPB recommendation for CNB as well
CNB – no change. Indentation technique is not recommended. No change No change
No change
No change
4-point bending, fracture the test specimens in distilled water or artificial saliva
*Alternative geometries and basic dimensions (e.g., L, W, B) can generally be used. **The crack does not alter itself with this length.65
A significant variation among the fracture toughness values was obtained by different studies, even for the same system and test method. The values ranged from 3.17 to 8 MPam. Not all articles reported all experimental conditions that might affect the toughness, such as number of specimens, sectioning procedure, number of indentations, ambient conditions, equation(s) used for calculation of fracture toughness, indentation load, and location, as well as if the specimen was tested immediately after indentation. The authors were contacted, and only three replied to provide missing/incomplete information. Recommended method designs to measure zirconia fracture toughness are presented in Table 3.
Discussion This systematic review aimed to evaluate possible factors influencing the accuracy of fracture toughness values on Y-TZP ceramics. There was a significant variation in the experimental conditions to determine the fracture toughness of Y-TZP ceramics reported. The wide heterogeneity in the test method, temperature and moisture, specimen thickness, and applied
load were identified as potential factors that affected the difference in the toughness values among the studies. Such variation occurred even for the same commercial zirconia ceramic system. For that reason, guidelines to determine toughness fracture for zirconia regarding the method and method design were developed. The recommended method designs based on the results of this study, International Organization for Standardization (ISO) and American Society for Testing Materials (ASTM)47-49 are shown in Table 3; however, for specimen dimensions, some alternatives can be used according to each investigation. According to the manufacturer and the literature review, Y-TZP toughness values range from 3.17 to 10.5 MPam. Among the articles included in this systematic review, the fracture toughness values reported were from 3.17 to 8 MPam, and such a range is substantial. According to ISO 6872,50 the minimum toughness values for substructure ceramic for nonadhesively cemented anterior or posterior crowns and threeunit substructure ceramic not involving molars should be 3 MPam; three-unit substructure ceramic involving molars, 3.5 MPam; and substructure ceramic involving four or more units, 5 MPam.
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Among the ten selected studies, six brands of Y-TZP ceramic systems were used, including Everest ZS, LavaTM , Crystal HS, R R Zirconia, Vita In-Ceram YZ, and Prozyr. Although Cercon all the ceramic material investigated was Y-TZP ceramic, different processing methods and material formulation for each brand can affect the toughness of the ceramic specimens. The observed differences among the toughness values of the studies could be related to the differences in the ceramic system, method, and study design. Nevertheless, different toughness values were also observed when the same method (IF) and ceR ramic system (Cercon Zirconia) were used (Table 2).18,24,25 Such differences can mainly be explained due to errors in measuring critical crack dimensions, ambient conditions, thickness, load applied, and inadequate estimation of the tested specimen’s material properties. It is fundamental to understand the material properties and toughening mechanisms, the latter of which can be classified as intrinsic and extrinsic toughening mechanisms.51 Intrinsic toughness mechanisms are influenced by inherent material responses, which affect crack initiation and crack propagation (e.g., responses ahead of the crack tip, such as plastic deformation response, phase transformations, microvoid coalescence, cleavage fracture). On the other hand, extrinsic toughening mechanisms exist behind the crack tip and act to impede crack growth (e.g., grain bridging, fiber bridging, oxide wedging), and thus have little effect on crack initiation. Both mechanisms involve increasing the microstructural resistance to crack growth; however, for Y-TZP ceramics, the addition of yttria to zirconia enhances stress-induced phase transformation toughening (t→m), which occurs at the crack tip and around the crack and acts to provide closure forces on the crack and thus increases crack growth resistance. Thus, catastrophic fractures take place by the combination of crack initiation and instability, in the absence of extrinsic mechanisms.52 Many test methods are available for the measurement of fracture toughness. IF technique is the most often reported technique used to determine fracture toughness of dental materials,14 which might be explained because the technique is simple to perform. Most studies in this systematic review also determined the zirconia toughness by IF. SEVNB, FR, IS, and SEPB were also used to determine the toughness by some included studies. In addition, CNB is used to calculate fracture toughness.47 The major advantage of the CNB method is that it is not necessary to measure the crack size. Yet, surface crack in flexure (SCF) can be used to determine the fracture toughness of ceramics; however, fractographic equipment and techniques are required to measure the small precracks.49,53 Comparing different techniques (IS, SEVNB, FR), no discrepancies were observed in the fracture toughness values.23 In contrast, previous studies22,54 reported significant discrepancies between IF and the other techniques. Also, significant differences between the toughness data obtained from SEVNB and the IF methods were reported.55 SEVNB is not recommended for polycrystal materials (Y-TZP) because a sharp crack is not created at the root of the V-notch, according to ISO 23146:2012. Also, a sufficiently small notch-root radius is difficult to obtain for materials with grain sizes of less than 1 µm.56 For those materials, greater toughness results were obtained compared with SCF. Therefore, notch dimensions are critical in deter8
mining toughness fracture. For that reason, SEVNB was not listed in this review as one of the recommended tests; however, a few studies included in this review calculated toughness by mean SEVNB. Thus, more comparative studies are needed to validate this method as an accurate test to determine Y-TZP fracture toughness. The different methods used in the selected studies presented different standard deviation ranges. From two studies,17,24 the standard deviations were not reported. SEVNB exhibited the highest standard deviation among the included studies, from 0.1 to 1.4. IF, SEPB, FR, and IS presented similar standard deviation, varying from 0.05 to 0.5; however, regarding IF, the IF results usually overestimate toughness values. This method is even more critical for polycrystalline zirconia once very high loads are required to generate cracks. Difficulty in using IF to determine the fracture toughness31,57 is attributed to the difficulty in obtaining an accurate measurement of crack length of dental ceramics (typically an underestimation of the crack length) and subcritical SCG.58 Therefore, indentation techniques are not reliable to calculate fracture toughness for ceramics or for other brittle materials.50,59 For that reason, indentation techniques (IF and IS) were not listed in this review as recommended methods to determine fracture toughness of Y-TZP ceramics. SCG takes place immediately after notching or indentation,22 which indicates that the fracture toughness measurement could be less than the critical value in cases where it is not performed immediately after notching or indentation. SCG can still be significant when short test times are used. The majority of selected studies did not report when the notch or indentation was measured. SCG and R-curve behavior are also affected by room conditions. Among the selected studies, only three reported the ambient conditions, and in one study the data were obtained by contacting the authors. The highest toughness values were observed in the study that used the thickest specimens (5 mm) among the included studies, at 98 N indentation load, 8 (0.5) MPam26 and in the study which used silicone oil, 7.4 (0.32) MPam, at 22°C.23 The oil acts as a moisture diffusion barrier, and moisture migration into the crack tip is minimized, hence any LTD is minimized if not altogether halted. In the presence of body fluid or water, either in the form of liquid or as humidity in the air, the crack growth takes place faster. As a result, the fracture toughness of zirconia is reduced.24,60 Therefore, to assess the SCG, it is recommended that zirconia specimens be tested at two different test environments (air, water, or inert), or in appropriately different environments (i.e., inert environment—dry nitrogen, argon, vacuum, or silicone oil). According to ASTM C1421,49 if different results were obtained, select the fracture toughness values determined in the inert environment or at the higher test rates; however, although fracture toughness value should be the fracture toughness for which environmental effects are eliminated or minimized, for dental zirconia the test shall not be performed in the absence of environmental effects. Therefore, artificial saliva6 or distilled water should be used during the fracture test to obtain significant clinical fracture toughness results. A bending test provides fundamental data to calculate fracture toughness. According to the standards, 3- or 4-point
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bending methods can be performed to determine the material toughness using SEPB48,49 or CNB47,49 methods; however, for the 3-point bending method, the stress is located under a loading nose, while in the 4-point bending test the stress is distributed uniformly between two loading noses. The 4-point bend test is recommended as it is more conservative. The uniform stress across the sample surface in 4-point bending effectively probes a larger sample volume, and thus the probability of finding a critical flaw increases; hence, the test is more representative of the statistical nature of brittle fracture in ceramics in that a larger fraction of the flaw population is tested. The thickness of the specimen and applied load are important factors that also affect the reported toughness values. Dimension and load presented wide variation among the included studies. The thicknesses of the specimens were from 0.9 to 5 mm. In addition, there was a huge variation in relation to the Vickers indentation load (from 9.8 to 588 N). The crack length used to calculate the fracture toughness is related to the indentation load applied. One study investigated different indentation loads, and the indentations at 9.8- and 49-N loads created small cracks that were not measurable with the measuring microscope of the Vickers hardness tester.26 The crack size immediately measured after greater loads is higher than cracks caused by lower indentation loads. Overestimating toughness data in a clinical situation can negatively affect the restoration long term. Regarding specimen preparation, different specimen geometries were used to determine toughness, even when the same method was applied.17,18,24-26 The four long edges have to be chamfered uniformly according to the international standard specifications for flexural strength test.61 That procedure is necessary because the edges of the bars might have an irregular surface or residual stress from the cutting procedure, which would affect the results. For that reason, the edges play an important role on the fracture behavior. Another aspect is the surface preparation via polishing. In one study24 the specimens were not polished but only ground with SiC paper #80. In the other two studies,25,34 no surface preparation procedure was reported. The purpose of polishing is to remove all surface residual stresses, to round chipped corners of the specimens, and to eliminate superficial flaws and cracks on the material surface. The abovementioned studies24,25 presented lower toughness compared to the study that used the same method (IF) and ceramic sysR Zirconia).18 Therefore, it is important to polish tem (Cercon the specimens to be able to compare the results with other studies, and to avoid external effects on toughness values that are not representative of how the ceramics will be clinically used. With regard to aging, only one study18 determined the toughness of Y-TZP after fatigue. This is the only study in the literature reporting the effect of mechanical cycling on the toughness of Y-TZP. In that study, a fatigue test was performed using 20,000 cycles (200 N loads with a 2 Hz frequency) at ambient conditions (22 ± 1°C with 60 ± 5% relative humidity). R Cercon Zirconia and LavaTM ceramic were evaluated, and toughness values significantly increased after fatigue only for the LavaTM system (Table 2). This is important, as it could reflect the reliability of the material in the cyclic oral environment.
Fracture Toughness of Dental Y-TZP Ceramics
Among other factors, development of ceramic dental materials has focused on improving their fracture toughness and strength, consequently increasing their applicability in clinical situations. Therefore, recommendations concerning a consistent methodology to determine fracture toughness are presented in Table 3 in order to obtain valid data and to be able to compare the results from different studies. The appropriate method is important to optimize the reliability of the results.
Conclusions Based on the findings of this systematic review, the following conclusions can be drawn: 1. Wide variation in research methods affected the fracture toughness reported for Y -TZP ceramics among the selected studies. 2. The most used method to determine the fracture toughness among the selected studies was IF; however, IF has several limitations, such as crack size measurement, and a bending test that provides fundamental data to calculate fracture toughness is not performed. 3. Results of indentation-based methods may only be compared within the same experiment. For that reason, CNB and SEPB are the most recommended methods to measure fracture toughness of dental zirconia.
Acknowledgment The authors thank the Health Science Librarian at the University of Alberta, Linda Seale, for assistance with the search strategy.
References 1. Hannink RHJ, Kelly PM, Muddle BC: Transformation toughening in zirconia-containing ceramics. J Am Ceram Soc 2000;83:461-487 2. Porter DL, Heuer AH: Mechanisms of toughening partially stabilized zirconia (PSZ). J Am Ceram Soc 1977;60:183-184 3. Christel P, Meunier A, Heller M, et al: Mechanical properties and short-term in-vivo evaluation of yttrium-oxide-partially-stabilized zirconia. J Biomed Mater Res 1989;23:45-61 4. Chevalier J, Gremillard L, Virkar AV, et al: The tetragonal-monoclinic transformation in zirconia: lessons learned and future trends. J Am Ceram Soc 2009;92:1901-1920 5. Kosmac T, Oblak C, Jevnikar P, et al: The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic. Dent Mater 1999;15:426-433 6. Borba M, de Ara´ujo MD, Fukushima KA, et al: Effect of the microstructure on the lifetime of dental ceramics. Dent Mater 2011;27:710-721 7. Gonzaga CC, Cesar PF, Miranda WG Jr, et al: Slow crack growth and reliability of dental ceramics. Dent Mater 2011;27:394-406 8. Hisbergues M, Vendeville S, Vendeville P: Review zirconia: established facts and perspectives for a biomaterial in dental implantology. J Biomed Mater Res: Appl Biomater 2009;88B:519-529 9. Denry I, Kelly JR: State of art of zirconia for dental applications. Dent Mater 2008;24:299-307 10. Kelly JR, Denry I: Stabilized zirconia as structural ceramic: an overview. Dent Mater 2008;24:289-298
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11. Wenz HJ, Bartsch J, Wolfart S, et al: Osseointegration and clinical success of zirconia dental implants: a systematic review. Int J Prosthodont 2008;21:27-36 12. Albakry M, Guazzato M, Swain MV: Influence of hot pressing on the microstructure and fracture toughness of two pressable dental glass-ceramics. J Biomed Mater Res B Appl Biomater 2004;71:99-107 13. Guazzato M, Albakry M, Swain MV, et al: Mechanical properties of In-Ceram Alumina and In-Ceram Zirconia. Int J Prosthodont 2002;15:339-346 14. Scherrer SS, Denry IL, Wiskott HW: Comparison of three fracture toughness testing techniques using a dental glass and a dental ceramic. Dent Mater 1998;14:246-255 15. Sakar-Deliormanli A, Guden M: Microhardness and fracture toughness of dental materials by indentation method. J Biomed Mater Res B Appl Biomater 2006;76:257-264 16. Chantikul P, Anstis GR, Lawn BR, et al: Critical evaluation of indentation techniques for measuring fracture toughness. II. Strength method. J Am Ceram Soc 1981;64:539-543 17. Mitov G, Gessner J, Lohbauer U, et al: Subcritical crack growth behavior and life data analysis of two types of dental Y-TZP ceramics. Dent Mater 2011;27:684-691 18. Nemli SK, Yilmaz H, Aydin C, et al: Effect of fatigue on fracture toughness and phase transformation of Y-TZP ceramics by X-ray diffraction and Raman spectroscopy. J Biomed Mater Res B Appl Biomater 2012:100B:416-424 19. Benzaid R, Chevalier J, Saˆadaoui M, et al: Fracture toughness, strength and slow crack growth in a ceria stabilized zirconia-alumina nanocomposite for medical applications. Biomaterials 2008;29:3636-3641 20. Guazzato M, Albakry M, Ringer SP, et al: Strength, fracture toughness and microstructure of a selection of all-ceramic materials. Part II. Zirconia-based dental ceramics. Dent Mater 2004;20:449-456 21. Luthardt RG, Holzhuter M, Sandkuhl O, et al: Reliability and properties of ground Y-TZP zirconia ceramics. J Dent Res 2002;81:487-491 22. Anstis GR, Chantikul P, Lawn BR, et al: A critical evaluation of indentation techniques for measuring fracture toughness: I, direct crack measurements. J Am Ceram Soc 1981;64:533-538 23. Aboushelib MN, Kleverlaan CJ, Feilzer AJ: Evaluation of a high fracture toughness composite ceramic for dental applications. J Prosthodont 2008;17:538-544 24. Studart AR, Filser F, Kocher P, et al: Fatigue of zirconia under cyclic loading in water and its implications for the design of dental bridges. Dent Mater 2007;23:106-114 25. Yilmaz H, Aydin C, Gul BE: Flexural strength and fracture toughness of dental core ceramics. J Prosthet Dent 2007;98:120-128 26. Harada K, Shinya A, Yokoyama D, et al: Effect of loading conditions on the fracture toughness of zirconia. J Prosthodont Res 2013;57:82-87 27. Albakry M, Guazzato M, Swain MV: Fracture toughness and hardness evaluation of three pressable all-ceramic dental materials. J Dent 2003;31:181-188 28. Kies JA, Clark ABJ: Propagation rates and times to fail following proof stress in bulk glass fracture. Proceedings of the 2nd International Conference on Fracture. 1969, pp. 483–491, Paper 42. 29. Fujishima A, Ferracane JL: Comparison of four modes of fracture toughness testing for dental composites. Dent Mater 1996;12:38-43
10
30. Aboushelib MN, de Jager N, Kleverlaan CJ, et al: The influence of pigments on the slow crack growth in dental zirconia. Dent Mater 2012;28:410-415 31. Fischer H, Marx R: Fracture toughness of dental ceramics: comparison of bending and indentation method. Dent Mater 2002;18:12-19 32. Moher D, Liberati A, Tetzlaff J, et al: Preferred reporting Items for systematic reviews and meta-analyses: the PRISMA statement. BMJ 2009;339:b2535, 332-336. 33. Quinn JB, Sundar V, Lloyd IK: Influence of microstructure and chemistry on the fracture toughness of dental ceramics. Dent Mater 2003;19:603-611 34. Marinis A, Aquilino SA, Lund PS, et al: Fracture toughness of yttria-stabilized zirconia sintered in conventional and microwave ovens. J Prosthet Dent 2013;109:165-171 35. Xiao R, Chu BF, Zhang L, et al: Aging performances for resisting low-temperature of three dental Yttria-stabilized zirconia ceramic core materials. Chin Med J (Engl) 2012;125:1999-2003 36. Saied MA, Lloyd IK, Haller WK, et al: Joining dental ceramic layers with glass. Dent Mater 2011;27:1011-1016 37. Bulpakdi P, Taskonak B, Yan J, et al: Failure analysis of clinically failed all-ceramic fixed partial dentures using fractal geometry. Dent Mater 2009;25:634-640 38. Turp V, Tuncelli B, Sen D, et al: Evaluation of hardness and fracture toughness, coupled with microstructural analysis, of zirconia ceramics stored in environments with different pH values. Dent Mater J 2012;31:891-902 39. Lazar DR, Bottino MC, Ozcan M, et al: Y-TZP ceramic processing from coprecipitated powders: a comparative study with three commercial dental ceramics. Dent Mater 2008;24:1676-1685 40. Ebert J, Ozkol E, Zeichner A, et al: Direct inkjet printing of dental prostheses made of zirconia. J Dent Res 2009;88: 673-676 41. Manawi M, Ozcan M, Madina M, et al: Impact of surface finishes on the flexural strength and fracture toughness of In-Ceram Zirconia. Gen Dent 2012;60:138-142 42. Yin L: Property-process relations in simulated clinical abrasive adjusting of dental ceramics. J Mech Behav Biomed Mater 2012;16:55-65 43. Triwatana P, Srinuan P, Suputtamongkol K: Comparison of two fracture toughness testing methods using a glass-infiltrated and a zirconia dental ceramic. J Adv Prosthodont 2013;5:36-43 44. Tinschert J, Natt G, Mohrbotter N, et al: Lifetime of aluminaand zirconia ceramics used for crown and bridge restorations. J Biomed Mater Res B Appl Biomater 2007;80:317-321 45. Mirkovi´c N, Gostovi´c AS, Lazi´c Z, et al: Fracture toughness of zirconia ceramic crowns made by feather-edge tooth preparation design. Vojnosanit Pregl 2012;69:562-568 46. Borba M. Mechanical Behavior of Ceramic Materials Used for Fixed Partial Dentures. S˜ao Paulo, University of S˜ao Paulo, 2010, p. 170. 47. ISO. Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics) Test Method for Fracture Toughness of Monolithic Ceramics at Room Temperature by Chevron-Notched Beam (CNB) Method (ed 1). ISO 24370. International Organization for Standardization, 2005. 48. ISO. Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics) Test Method for Fracture Toughness of Monolithic Ceramics at Room Temperature by Single Edge Precracked Beam (SEPB) Method. ISO 15732. International Organization for Standardization, 2003.
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Passos et al
49. ASTM. Standard Test Methods for Determination of Fracture Toughness of Advanced Ceramics at Ambient Temperature. ASTM C1421. American Society for Testing Materials, 2010. 50. ISO. Dentistry—Ceramic Materials. ISO 6872. International Organization for Standardization, 2008. 51. Ritchie RO: Mechanisms of fatigue-crack propagation in ductile and brittle solids. Int J Fracture 1999;100:55-60 52. Launey M, Ritchie RO: On the fracture toughness of advanced materials. Adv. Mater 2009;21:2103-2110 53. ISO. Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics) Determination of Fracture Toughness of Monolithic Ceramics at Room Temperature by the Surface Crack in Flexure (SCF) Method. ISO 18756. International Organization for Standardization, 2003. 54. Quinn GD, Salem J, Bar-On I, et al: Fracture toughness of advanced ceramics at room temperature. J Res Natl Inst Stand Technol 1992;97:579-607 55. Hornberger H, Marquis PM: Mechanical properties and microstructure of In- Ceram, a ceramic-glass composite for dental crowns. Glastech Ber Glass Sci Technol 1995;68: 188-194 56. ISO. Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics)—Test Methods for Fracture Toughness of Monolithic Ceramics—Single-Edge V-Notch Beam (SEVNB) Method. ISO 23146. International Organization for Standardization, 2012.
Fracture Toughness of Dental Y-TZP Ceramics
57. Mahoney E, Holt A, Swain M, et al: The hardness and modulus of elasticity of primary molar teeth: an ultra-microindentation study. J Dent 2000;28:589-594 58. Xu HHK, Smith DT, Jahanmir S, et al: Indentation damage and mechanical properties of human enamel and dentin. J Dent Res 1998;77:472-480 59. Quinn GD, Bradt RC: On the Vickers indentation fracture toughness test. J Am Ceram Soc 2007;90:673-680 60. De Aza AH, Chevalier J, Fantozzi G, et al: Crack growth resistance of alumina, zirconia and zirconia toughened alumina ceramics for joint prostheses. Biomaterials 2002;23:937-945 61. ISO. Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics)—Test Method for Flexural Strength of Monolithic Ceramics at Room Temperature. ISO 14704. International Organization for Standardization, 2008. 62. Scherrer SS, Cattani-Lorente M, Vittecoq E, et al: Fatigue behavior in water of Y-TZP zirconia ceramics after abrasion with 30 µm silica-coated alumina particles. Dent Mater 2011;27:e28-e42 63. Singh R, Gill C, Lawson S, et al: Sintering, microstructure and mechanical properties of commercial Y-TZPs. J Mater Sci 1996;31:6055-6062 64. Studart AR, Filser F, Kocher P,et al: In vitro lifetime of dental ceramics under cyclic loading in water. Biomaterials 2007;28:2695-2705 65. Chevalier J, Saadaoui M, Olagnon C, et al: Double-torsion testing a 3Y-TZP ceramic. Ceram Int 1996;22:171-177
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School of Dentistry, University of Alberta, Edmonton, Canada Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Canada
Keywords Fracture toughness; Y-TZP; zirconia; systematic review. Correspondence Dr. Sheila P. Passos, School of Dentistry, University of Alberta, 11405–87 Ave NW, Room #5083C, Edmonton Clinic Health Academy (ECHA), Edmonton, AB, Canada T6G 1C9. E-mail: [email protected] The authors deny any conflicts of interest. Accepted December 13, 2013 doi: 10.1111/jopr.12179
Abstract Purpose: The aim of this review was to assess research methods used to determine the fracture toughness of Y-TZP ceramics in order to systematically evaluate the accuracy of each method with regard to potential influencing factors. Materials and Methods: Six databases were searched for studies up to April 2013. The terms “tough*,” “critical stress intensity factor,” “zirconi*,” “yttri*,” “dent*,” “zirconia,” “zirconium,” and “stress” were searched. Titles and abstracts were screened, and literature that fulfilled the inclusion criteria was selected for a full-text reading. Test conditions with potential influence on fracture toughness were extracted from each study. Results: Ten laboratory studies met the inclusion criteria. There was a significant variation in relation to test method, ambient conditions, applied/indentation load, number of specimens, and geometry and dimension of the specimen. The results were incomparable due to high variability and missing information. Therefore, 10 parameters were listed to be followed to standardize future studies. Conclusions: A wide variation in research methods affected the fracture toughness reported for Y-TZP ceramics among the selected studies; single-edge-precracked beam and chevron-notched-beam seem to be the most recommended methods to determine Y-TZP fracture toughness; the indentation methods have several limitations. Clinical significance: The accurate calculation of toughness values is fundamental because overestimating toughness data in a clinical situation can negatively affect the lifetime of the restoration.
In fixed prosthodontics, several all-ceramic materials are extensively used due to their biocompatibility and excellent esthetic characteristics. The addition of yttria to zirconia stabilizes the crystalline structure of zirconium oxide at ambient temperature creating a yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramic, a toughened oxide ceramic. The tetragonal to monoclinic (t→m) transformation in the ceramic structure improves the toughness of Y-TZP ceramics.1,2 Such a transformation prevents further propagation of cracks by acting at the crack tip and around the crack by localized compressive stresses,3 which are induced as a result of the large shear strain and a positive volumetric change associated with the phase transformation.4 However, the presence of flaws and cracks of different sizes in the ceramic material should be considered. Preexisting flaws/cracks on the ceramic surface or in the ceramic material act as stress concentrations. Crack initiation sites under stress can accelerate the crack growth rate to the point of failure at seemingly low applied stresses.5
The long-term stability of Y-TZP in the presence of water is limited by the continuing transformation from the tetragonal to monoclinic phase via a process of low temperature degradation (LTD) and subcritical crack growth (growth of cracks below the stress required to lead to wholesale fracture). All these factors might affect the lifetime of Y-TZP dental restorations.6,7 Other factors that affect the long-term behavior of zirconia ceramics in dental applications remain to be better explored.8-11 The fracture toughness or critical stress intensity (KIC ) defines the material behavior in the presence of a preexisting flaw characterizing the resistance of the material12,13 against crack propagation. Fracture toughness of dental ceramics has been evaluated using different test methods and different specimen shapes.14-16 Several previous studies evaluated the fracture toughness of dental ceramics by the indentation fracture (IF) method.12,14,15,17-26 The IF technique is simple, and the required number of specimens per test is small; however, high dispersion has been observed due to inaccurate measurement of crack length and slow crack growth (SCG).14,22,27 Kies and Clark28
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proposed the double torsion test setup to determine toughness and SCG. It has been considered a valuable method,28,29 and it has been recently used.30 To calculate the toughness values, other methods are also used, such as single edge v-notched beam (SEVNB), fractography (FR), indentation strength (IS), single-edge-precracked beam (SEPB), and chevron-notched-beam (CNB). The SEVNB method requires creating a sharp V-notch, and then the flexural strength test is performed. FR can be used measuring the critical crack size and the flexural strength of the material. For IS, a Vickers microindentation is performed before the flexural strength test, and the hardness is measured as well. SEPB is performed creating a straight-through precrack, and then a flexural strength test is conducted. CNB is executed preparing a chevron-notch, and then flexural strength is calculated. In vitro tests have limitations and do not necessarily determine the clinical success of all-ceramic core materials; however, they do represent the most available information about Y-TZP dental restorations. The possibility of collecting in vivo data has been limited so far due to ethical and practical reasons. It is believed that better clinical performance and a longer lifetime can be achieved with improved fracture toughness of the material structure.31 To predict the lifetime of zirconia restorations, it is important to know the toughening mechanisms. Thus, this review will focus on systematically evaluating the accuracy of each fracture toughness testing method with regard to potential influencing factors based on in vitro studies.
Table 1 Search strategies used for each database Database
MEDLINE (OvidSP)
PubMed (NLM) EMBASE (OvidSP) Cochrane (Wiley) Scopus (Elsevier)
LILACS (Virtual Health Library)
Dates of coverage
Keywordcs used
1946 – April/2013 “tough∗ ” OR “critical stress intensity factor” AND “zirconi∗ ” 1950 – April/2013 Same search strategy as MEDLINE (OvidSP) 1980 – April/2013 Same search strategy as MEDLINE (OvidSP) to the fourth Same search strategy as quarter of 2011 MEDLINE (OvidSP) 1960 – April/2013 “tough∗ ” OR “critical stress intensity factor” AND “zirconi∗ .” Within results other terms were added: “yttri∗ ” AND “dent∗ ” 1982 – April/2013 “toughness” AND “zirconia” “toughness” AND “zirconium” “stress” AND “zirconia” “stress” AND “zirconium” “tenacidade” AND ˆ “zirconia” “tenacidade” AND ˆ “zirconio” ˆ “estresse” AND “zirconia” ˆ “estresse” AND “zirconio”
Materials and methods The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) statement32 was followed as much as possible. Data collection
Multiple online databases were systematically searched: MEDLINE (OvidSP), PubMed (NLM—National Library of Medicine), EMBASE (OvidSP), Cochrane Library (Wiley), Scopus (Elsevier), and LILACS (Virtual Health Library). The search terms “tough*,” “critical stress intensity factor,” “zirconi*,” “yttri*,” “dent*,” “zirconia,” “zirconium,” and “stress” were used to search in vitro studies up to April 2013. Search strategies and dates of coverage of each database are outlined in Table 1, which also shows the specific combination and truncation of the keywords. The electronic search was not limited by any means. The final search strategy was fine tuned with the assistance of a health sciences librarian.
1. No veneered specimens.
Criteria for selection of studies
From the search results according to titles/abstracts relevant to topic, studies were selected based on the following inclusion criteria: 1. In vitro studies. 2. Dental application of commercial Y-TZP ceramics obtained from presintered blocks. 2
Where a relevant title without a listed abstract was available, a full copy of the study was assessed for inclusion as explained below. The studies’ titles and abstracts of potential studies were evaluated by two independent reviewers. All abstracts that appeared to meet inclusion criteria were selected based on a consensus agreement between two reviewers, and full articles or full theses were obtained. Conference abstracts, such as International Association for Dental Research (IADR) abstracts, were not selected because they do not present the detailed test method, but an attempt was made to retrieve the full articles or thesis if they were already published. The literature search was completed by manual search within the reference lists of selected full-text studies. Only articles that complied with the initial inclusion criteria were further reviewed. Full copies of articles were then reviewed independently by two reviewers to determine whether the exclusion criteria applied. In addition to the initial selection criteria the following exclusion criterion was only considered at this stage:
Data extraction
The toughness method was extracted, as were the experimental conditions (Table 2). In addition, the authors of these articles were contacted to obtain any incomplete/missing information. Assessment of study risk of bias was accomplished with individual information collected in Table 2.
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Vita Zahnfabrik (Bad Sackingen, Germany) 0.7 µm
2. Manufacturer
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25 × 2 ± 0.2 × 2.5 ± 0.2 mm
2 × 4 × 16 mm
Diamond disc
275 rpm
7. Specimen dimensions
8. Sectioning procedure
9. Sectioning speed
Slow speed
100-µm- thick diamond
3
4. Yttria content 5. Toughness method 6. Number of specimens
5
0.383 µm (0.14-0.8 µm)62
0.383 µm (0.14-0.8 µm)62
4 × 3 x 32 mm
2 × 5 × 25 mm
Not reported
Milling procedure
KaVo Everest engine
16
Diamond disc
Nemli et al18
Not reported
Not reported
15-mm diameter x 1-mm thick
10
R Cercon Everest ZS, LavaTM , and Zirconia and Crystal HS LavaTM Dentsply Kavo, 3M (York, PA) (Seefeld, and 3M Germany) and DLMS (Scottsdale, AZ) Cercon: Lava: 0.1 µm63 0.2-0.3 Crystal µm64 HS:0.07-0.3 µm Lava: 0.1 µm63 3 mol% 3 mol% SEVNB IF
Marinis et al34
10
3 mol% IF
Kavo
Kavo (Biberach, Germany)
3 mol% IF
Everest ZS
Harada et al26
Everest ZS
Mitov et al17
5 mol%∗∗ FR
3. Zirconia grain size
Vita In-Ceram R YZ
Kosmac et al6
1. Y-TZP ceramic system
Experimental conditions
Table 2 Experimental conditions assessed in each selected article
Diamondcoated disc Not reported
IS: 20 SEVNB: 20 FR: 8 1.5 × 2.5 × 25 mm
3 mol% IS, SEVNB, FR
Not reported
Diamond disc∗∗∗
15-mm diameter, 1.2 ± 1.2-mm thick Sp provided by the manufacturer Not reported
10
1∗∗∗
2 × 4 × 50 mm
3 mol% IF
0.2-0.3 µm64
Dentsply
R Cercon Zirconia
Yilmaz et al25
3 mol% IF
0.1-0.3 µm
Dentsply
Dentsply
0.3-0.5 µm
R Cercon Zirconia
Studart et al24
R Cercon Zirconia
Aboushelib et al23
Not reported
(Continued)
Sp provided by the manufacturer Not reported
3 × 4 × 25 mm
0.9 × 19.65 × 56 mm
Not reported
5
2 mol% SEPB
0.6 µm
Norton-St. Gobain (Raleigh, NC)
Prozyr
Quinn et al33
5
3 mol% SEVNB
0.25-1.4 µm
Dentsply
R Cercon Zirconia
Aboushelib et al30
Passos et al Fracture Toughness of Dental Y-TZP Ceramics
3
4 VI: 196 N
Pre-load: 50% of the fracture load 0.01 MPa/s
Until failure
19. Applied/ Indentation load
20. Duration of load applied
15 s
Not reported
17. Number of indentations 18. Indentation location
NA
NA
NA
NA
NA
10
NA
NA
NA
x
15 s
Not reported
Not reported
NA
Center∗∗∗
4-point bending
1 mm V-shaped
Not reported
NA
VI: 9.8, 49, 98, 196, and 294 N
x
x
√
Marinis et al34
Diamond disc
1∗∗∗
NA
NA
NA
NA
x
√ √
√ √
√46 √
√46
Harada et al26
Mitov et al17
Kosmac et al6
3-point bending NA
16. Flexural test
15. Notch dimensions∗
14. Notch length∗
12. Cleaning of specimens 13. Notch procedure∗
10. Water cooling 11. Finishing of specimens
Experimental conditions
Table 2 Continued
15 s
VI: 4.5 mm away from the center VI: 294 N
3
NA
NA
NA
NA
√
x √
Nemli et al18
Not reported
VI: center SEVNB: NA FR: NA VI: 19.6 N and 196 N
5
4-point bending
0.3-mm thick, 300-µm deep
Diamondcoated disc Not reported
x
√ √
Aboushelib et al23
Not reported
VI: 49, 98 and 196 N∗∗∗
VI: Center∗∗∗
5∗∗∗
3
Not reported
VI: 4.5 mm away from the center VI: 588 N
NA
NA
NA
NA
NA
NA
NA
NA
x
x x
Yilmaz et al25
Sp were ground with SiC paper #80 x
√ ∗∗∗
Studart et al24
Not reported
Not reported
NA
NA
3.5-mm diameter, 300-µm deep Not reported
16 mm
Not reported
x
x √
Aboushelib et al30
(Continued)
VI: 78 N Compressive load: between 6000 and 14,000 N VI: not reported Compressive load: as soon as precrack was heard to pop in
VI: center
4-point bending 1
Pre-crack: 0.4-0.6 of the sp deep NA
Pre-crack (VI)
√ ∗∗∗
x √
Quinn et al33
Fracture Toughness of Dental Y-TZP Ceramics Passos et al
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√
6.5 (0.3)
37°C In artificial saliva √
3.17
x
x
98 N: 8.0 (0.5) 196 N: 7.1 (0.5) 294 N: 6.8 (0.2)
√
23°C∗∗∗
Not reported √
Vickers hardness
√ ∗∗∗
Harada et al26
Vickers hardness
x
x
3-point: Universal testing machine
Mitov et al17
Kosmac et al6
Cercon: 6.8 (0.1) Lava: 6.6 (0.13) After fatigue: Cercon: 6.9 (0.11) Lava: 7.3 (0.1) x
Everest: 5.8 (1.4) Lava: 5.2 (0.5) Crystal HS: 4.9 (0.7)
x
√
Not reported
Vickers hardness
√
Nemli et al18
√
Room temperature
4-point: Universal testing machine
x
Marinis et al34
√
√ IS √ SEVNB FR: not reported Similar values for all techiques: 7.4 (0.32)
IF: Vickers hardness 4-point: Universal testing machine 22°C In silicone oil
√
Aboushelib et al23
5.6
x
23°C∗∗∗ Ambient air conditions∗∗∗ √ ∗∗∗
Vickers hardness
√ ∗∗∗
Studart et al24
x
6.27 (0.05)
√
Not reported
Vickers hardness
x
Yilmaz et al25
x
5.06 (0.97)
√
Not reported
4-point: Universal testing machine
x
Aboushelib et al30
SEVNB, single edge notched beam; IF, indentation fracture; FR, fractography; IS, indentation strength; SEPB, single-edge-precracked beam; NA, not applicable; VI, Vickers indentation. *Applicable for the fracture toughness using SEVNB and SEPB. **Information obtained from the manufacturer. ***Information obtained from the authors (not reported in the article). 46 Reference of the assessed information.
26. Fractographic assessment
25. Toughness in MPam (sd)
24. Equation
23. Ambient conditions
21. Specimen tested immediately after notching or indentation 22. Testing machine
Experimental conditions
Table 2 Continued
√
4.9 (0.2)
√
Ambient air conditions
Vickers hardness + hydraulic testing machine
√ ∗∗∗
Quinn et al33
Passos et al Fracture Toughness of Dental Y-TZP Ceramics
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Fracture Toughness of Dental Y-TZP Ceramics
MEDLINE
PubMed
EMBASE
Passos et al
Cochrane
206 studies 208 studies 210 studies
0
Scopus
LILACS
217 studies 19 studies
After removing duplicates (268) Studies excluded from the first selection stage (213) Potentially relevant studies (55)
Studies excluded from the second selection stage (45)
Studies relevant to the topic
Article from selected thesis (1)
(9 articles + 1 thesis)
TOTAL: 11 studies Duplicate study excluded (1 thesis) Studies relevant to the topic (10) References of selected full-text articles (0) Figure 1 Schematic study selection procedure.
Studies included (10)
Results Ten6,17,18,23-26,30,33,34 laboratory studies were included in the systematic review (Fig 1). Among the studies considered in the second selection stage, a few were eliminated after inclusion and exclusion criteria were applied. One was excluded due to the absence of toughness value.35 In vitro studies that evaluated the fracture toughness of veneered zirconia,36-38 experimental Y-TZP ceramic,39,40 or Y-TZP ceramics not obtained from partially sintered blocks,20,21,41-43 and articles which evaluated HIPped Y-TZP ceramic44 were excluded. In addition, one article was excluded because it did not have an English translation.45 Because a thesis paper46 was selected from the second selection stage, the article6 from the thesis was searched and included. For that reason, the thesis was excluded as a duplicate study. The online search was completed by manual search through the references of the selected articles; however, no additional study was found. The selected studies were published between 2003 and 2013. They differed widely with respect to experimental methodology, manufacturing company, and study design (number of specimens, geometry and dimension of the specimen, load applied/indentation load, ambient conditions). Such variation precluded the possibility of attempting a meta-analysis. 6
Five methods to determine the fracture toughness were identified among the ten articles, with IF being the most investigated technique (Table 2). One study23 compared three techniques (IS, SEVNB, FR), and statistically similar values were observed for the Y-TZP ceramic. The SEPB technique was also applied by one study.33 Among the six brands identified from the ten studies, R Zirconia (Dentsply, York, PA) was the most investiCercon gated (Table 2). The preparation procedure was not mentioned for most of the selected articles.17,18,23,24,30,34 Only two studies clearly stated processing conditions in accordance with manufacturer’s recommendation,6,26 and the remaining two articles reported that the specimens were provided by the manufacturers,25,33 presumably indicating that manufacturerrecommended preparation was followed. Regarding aging conditions, of the included studies, only one article18 considered the influence of aging on two differR ent ceramic systems (Cercon Zirconia, LavaTM ) and found a difference between before and after fatigue for the LavaTM system at ambient conditions (22 ± 1°C with 60 ± 5% relative humidity). Fractographic analysis was used as a technique to determine the fracture toughness,6 for further detection of chipping or fractures,23,33 or to validate the data obtained from the SEVNB test.23
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Fracture Toughness of Dental Y-TZP Ceramics
Table 3 Recommended methods and procedures to determine toughness fracture for zirconia according to the findings of the current study, ISO 24370 (CNB),47 ISO 15732 (SEPB),48 and ASTM C1421 (CNB, SEPB, SCF)49 Experimental design
Chevron-notched-beam (CNB)47,49
Single-edge-precracked beam (SEPB)48,49
4. Specimen edges
Not less than 5 L: min 45 mm, Th: 3 ± 0.2 mm, W: 4 ± 0.2 mm, Finish grinding with a diamond-grit wheel of 320 grit or finer Not chamfered or beveled
5. Type of notch
Chevron-notched
Not less than 5 L: 18 mm, Th: 3 ± 0.2 mm, W: 4 ± 0.2 mm Ra : not more than 0.2 µm Ra has to be determined and reported The four long edges have to be chamfered uniformly at 45° ± 5°. Chamfered edge length: 0.12 mm ± 0.03 mm Saw notch or indentation
6. Notch length∗∗ 7. Notch dimensions
16 mm V-shaped W: min 3.8 mm, Th: less than 0.3 mm
8. Ambient conditions
Laboratory ambient conditions. Record the test temperature and relative humidity
9. Crack measurement
NA
10. Fracture test
3- or 4-point bending, fracture the test specimens in distilled water or artificial saliva
1. Number of specimens 2. Specimen dimensions∗ 3. Finishing of specimens
16 mm W: 0.1 mm, Deep: between 0.4 and 0.6 mm,44 Deep: between 0.2 and 0.3 mm45 Laboratory ambient conditions. Record the test temperature and relative humidity A drop of dye penetrant can be placed on saw notch. Prior to fracture testing, dye penetrants have to be dry 3- or 4-point bending, fracture the test specimens in distilled water or artificial saliva
Our recommendations Power calculation is required See note below∗ SEPB recommendation for CNB as well SEPB recommendation for CNB as well
CNB – no change. Indentation technique is not recommended. No change No change
No change
No change
4-point bending, fracture the test specimens in distilled water or artificial saliva
*Alternative geometries and basic dimensions (e.g., L, W, B) can generally be used. **The crack does not alter itself with this length.65
A significant variation among the fracture toughness values was obtained by different studies, even for the same system and test method. The values ranged from 3.17 to 8 MPam. Not all articles reported all experimental conditions that might affect the toughness, such as number of specimens, sectioning procedure, number of indentations, ambient conditions, equation(s) used for calculation of fracture toughness, indentation load, and location, as well as if the specimen was tested immediately after indentation. The authors were contacted, and only three replied to provide missing/incomplete information. Recommended method designs to measure zirconia fracture toughness are presented in Table 3.
Discussion This systematic review aimed to evaluate possible factors influencing the accuracy of fracture toughness values on Y-TZP ceramics. There was a significant variation in the experimental conditions to determine the fracture toughness of Y-TZP ceramics reported. The wide heterogeneity in the test method, temperature and moisture, specimen thickness, and applied
load were identified as potential factors that affected the difference in the toughness values among the studies. Such variation occurred even for the same commercial zirconia ceramic system. For that reason, guidelines to determine toughness fracture for zirconia regarding the method and method design were developed. The recommended method designs based on the results of this study, International Organization for Standardization (ISO) and American Society for Testing Materials (ASTM)47-49 are shown in Table 3; however, for specimen dimensions, some alternatives can be used according to each investigation. According to the manufacturer and the literature review, Y-TZP toughness values range from 3.17 to 10.5 MPam. Among the articles included in this systematic review, the fracture toughness values reported were from 3.17 to 8 MPam, and such a range is substantial. According to ISO 6872,50 the minimum toughness values for substructure ceramic for nonadhesively cemented anterior or posterior crowns and threeunit substructure ceramic not involving molars should be 3 MPam; three-unit substructure ceramic involving molars, 3.5 MPam; and substructure ceramic involving four or more units, 5 MPam.
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Among the ten selected studies, six brands of Y-TZP ceramic systems were used, including Everest ZS, LavaTM , Crystal HS, R R Zirconia, Vita In-Ceram YZ, and Prozyr. Although Cercon all the ceramic material investigated was Y-TZP ceramic, different processing methods and material formulation for each brand can affect the toughness of the ceramic specimens. The observed differences among the toughness values of the studies could be related to the differences in the ceramic system, method, and study design. Nevertheless, different toughness values were also observed when the same method (IF) and ceR ramic system (Cercon Zirconia) were used (Table 2).18,24,25 Such differences can mainly be explained due to errors in measuring critical crack dimensions, ambient conditions, thickness, load applied, and inadequate estimation of the tested specimen’s material properties. It is fundamental to understand the material properties and toughening mechanisms, the latter of which can be classified as intrinsic and extrinsic toughening mechanisms.51 Intrinsic toughness mechanisms are influenced by inherent material responses, which affect crack initiation and crack propagation (e.g., responses ahead of the crack tip, such as plastic deformation response, phase transformations, microvoid coalescence, cleavage fracture). On the other hand, extrinsic toughening mechanisms exist behind the crack tip and act to impede crack growth (e.g., grain bridging, fiber bridging, oxide wedging), and thus have little effect on crack initiation. Both mechanisms involve increasing the microstructural resistance to crack growth; however, for Y-TZP ceramics, the addition of yttria to zirconia enhances stress-induced phase transformation toughening (t→m), which occurs at the crack tip and around the crack and acts to provide closure forces on the crack and thus increases crack growth resistance. Thus, catastrophic fractures take place by the combination of crack initiation and instability, in the absence of extrinsic mechanisms.52 Many test methods are available for the measurement of fracture toughness. IF technique is the most often reported technique used to determine fracture toughness of dental materials,14 which might be explained because the technique is simple to perform. Most studies in this systematic review also determined the zirconia toughness by IF. SEVNB, FR, IS, and SEPB were also used to determine the toughness by some included studies. In addition, CNB is used to calculate fracture toughness.47 The major advantage of the CNB method is that it is not necessary to measure the crack size. Yet, surface crack in flexure (SCF) can be used to determine the fracture toughness of ceramics; however, fractographic equipment and techniques are required to measure the small precracks.49,53 Comparing different techniques (IS, SEVNB, FR), no discrepancies were observed in the fracture toughness values.23 In contrast, previous studies22,54 reported significant discrepancies between IF and the other techniques. Also, significant differences between the toughness data obtained from SEVNB and the IF methods were reported.55 SEVNB is not recommended for polycrystal materials (Y-TZP) because a sharp crack is not created at the root of the V-notch, according to ISO 23146:2012. Also, a sufficiently small notch-root radius is difficult to obtain for materials with grain sizes of less than 1 µm.56 For those materials, greater toughness results were obtained compared with SCF. Therefore, notch dimensions are critical in deter8
mining toughness fracture. For that reason, SEVNB was not listed in this review as one of the recommended tests; however, a few studies included in this review calculated toughness by mean SEVNB. Thus, more comparative studies are needed to validate this method as an accurate test to determine Y-TZP fracture toughness. The different methods used in the selected studies presented different standard deviation ranges. From two studies,17,24 the standard deviations were not reported. SEVNB exhibited the highest standard deviation among the included studies, from 0.1 to 1.4. IF, SEPB, FR, and IS presented similar standard deviation, varying from 0.05 to 0.5; however, regarding IF, the IF results usually overestimate toughness values. This method is even more critical for polycrystalline zirconia once very high loads are required to generate cracks. Difficulty in using IF to determine the fracture toughness31,57 is attributed to the difficulty in obtaining an accurate measurement of crack length of dental ceramics (typically an underestimation of the crack length) and subcritical SCG.58 Therefore, indentation techniques are not reliable to calculate fracture toughness for ceramics or for other brittle materials.50,59 For that reason, indentation techniques (IF and IS) were not listed in this review as recommended methods to determine fracture toughness of Y-TZP ceramics. SCG takes place immediately after notching or indentation,22 which indicates that the fracture toughness measurement could be less than the critical value in cases where it is not performed immediately after notching or indentation. SCG can still be significant when short test times are used. The majority of selected studies did not report when the notch or indentation was measured. SCG and R-curve behavior are also affected by room conditions. Among the selected studies, only three reported the ambient conditions, and in one study the data were obtained by contacting the authors. The highest toughness values were observed in the study that used the thickest specimens (5 mm) among the included studies, at 98 N indentation load, 8 (0.5) MPam26 and in the study which used silicone oil, 7.4 (0.32) MPam, at 22°C.23 The oil acts as a moisture diffusion barrier, and moisture migration into the crack tip is minimized, hence any LTD is minimized if not altogether halted. In the presence of body fluid or water, either in the form of liquid or as humidity in the air, the crack growth takes place faster. As a result, the fracture toughness of zirconia is reduced.24,60 Therefore, to assess the SCG, it is recommended that zirconia specimens be tested at two different test environments (air, water, or inert), or in appropriately different environments (i.e., inert environment—dry nitrogen, argon, vacuum, or silicone oil). According to ASTM C1421,49 if different results were obtained, select the fracture toughness values determined in the inert environment or at the higher test rates; however, although fracture toughness value should be the fracture toughness for which environmental effects are eliminated or minimized, for dental zirconia the test shall not be performed in the absence of environmental effects. Therefore, artificial saliva6 or distilled water should be used during the fracture test to obtain significant clinical fracture toughness results. A bending test provides fundamental data to calculate fracture toughness. According to the standards, 3- or 4-point
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bending methods can be performed to determine the material toughness using SEPB48,49 or CNB47,49 methods; however, for the 3-point bending method, the stress is located under a loading nose, while in the 4-point bending test the stress is distributed uniformly between two loading noses. The 4-point bend test is recommended as it is more conservative. The uniform stress across the sample surface in 4-point bending effectively probes a larger sample volume, and thus the probability of finding a critical flaw increases; hence, the test is more representative of the statistical nature of brittle fracture in ceramics in that a larger fraction of the flaw population is tested. The thickness of the specimen and applied load are important factors that also affect the reported toughness values. Dimension and load presented wide variation among the included studies. The thicknesses of the specimens were from 0.9 to 5 mm. In addition, there was a huge variation in relation to the Vickers indentation load (from 9.8 to 588 N). The crack length used to calculate the fracture toughness is related to the indentation load applied. One study investigated different indentation loads, and the indentations at 9.8- and 49-N loads created small cracks that were not measurable with the measuring microscope of the Vickers hardness tester.26 The crack size immediately measured after greater loads is higher than cracks caused by lower indentation loads. Overestimating toughness data in a clinical situation can negatively affect the restoration long term. Regarding specimen preparation, different specimen geometries were used to determine toughness, even when the same method was applied.17,18,24-26 The four long edges have to be chamfered uniformly according to the international standard specifications for flexural strength test.61 That procedure is necessary because the edges of the bars might have an irregular surface or residual stress from the cutting procedure, which would affect the results. For that reason, the edges play an important role on the fracture behavior. Another aspect is the surface preparation via polishing. In one study24 the specimens were not polished but only ground with SiC paper #80. In the other two studies,25,34 no surface preparation procedure was reported. The purpose of polishing is to remove all surface residual stresses, to round chipped corners of the specimens, and to eliminate superficial flaws and cracks on the material surface. The abovementioned studies24,25 presented lower toughness compared to the study that used the same method (IF) and ceramic sysR Zirconia).18 Therefore, it is important to polish tem (Cercon the specimens to be able to compare the results with other studies, and to avoid external effects on toughness values that are not representative of how the ceramics will be clinically used. With regard to aging, only one study18 determined the toughness of Y-TZP after fatigue. This is the only study in the literature reporting the effect of mechanical cycling on the toughness of Y-TZP. In that study, a fatigue test was performed using 20,000 cycles (200 N loads with a 2 Hz frequency) at ambient conditions (22 ± 1°C with 60 ± 5% relative humidity). R Cercon Zirconia and LavaTM ceramic were evaluated, and toughness values significantly increased after fatigue only for the LavaTM system (Table 2). This is important, as it could reflect the reliability of the material in the cyclic oral environment.
Fracture Toughness of Dental Y-TZP Ceramics
Among other factors, development of ceramic dental materials has focused on improving their fracture toughness and strength, consequently increasing their applicability in clinical situations. Therefore, recommendations concerning a consistent methodology to determine fracture toughness are presented in Table 3 in order to obtain valid data and to be able to compare the results from different studies. The appropriate method is important to optimize the reliability of the results.
Conclusions Based on the findings of this systematic review, the following conclusions can be drawn: 1. Wide variation in research methods affected the fracture toughness reported for Y -TZP ceramics among the selected studies. 2. The most used method to determine the fracture toughness among the selected studies was IF; however, IF has several limitations, such as crack size measurement, and a bending test that provides fundamental data to calculate fracture toughness is not performed. 3. Results of indentation-based methods may only be compared within the same experiment. For that reason, CNB and SEPB are the most recommended methods to measure fracture toughness of dental zirconia.
Acknowledgment The authors thank the Health Science Librarian at the University of Alberta, Linda Seale, for assistance with the search strategy.
References 1. Hannink RHJ, Kelly PM, Muddle BC: Transformation toughening in zirconia-containing ceramics. J Am Ceram Soc 2000;83:461-487 2. Porter DL, Heuer AH: Mechanisms of toughening partially stabilized zirconia (PSZ). J Am Ceram Soc 1977;60:183-184 3. Christel P, Meunier A, Heller M, et al: Mechanical properties and short-term in-vivo evaluation of yttrium-oxide-partially-stabilized zirconia. J Biomed Mater Res 1989;23:45-61 4. Chevalier J, Gremillard L, Virkar AV, et al: The tetragonal-monoclinic transformation in zirconia: lessons learned and future trends. J Am Ceram Soc 2009;92:1901-1920 5. Kosmac T, Oblak C, Jevnikar P, et al: The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic. Dent Mater 1999;15:426-433 6. Borba M, de Ara´ujo MD, Fukushima KA, et al: Effect of the microstructure on the lifetime of dental ceramics. Dent Mater 2011;27:710-721 7. Gonzaga CC, Cesar PF, Miranda WG Jr, et al: Slow crack growth and reliability of dental ceramics. Dent Mater 2011;27:394-406 8. Hisbergues M, Vendeville S, Vendeville P: Review zirconia: established facts and perspectives for a biomaterial in dental implantology. J Biomed Mater Res: Appl Biomater 2009;88B:519-529 9. Denry I, Kelly JR: State of art of zirconia for dental applications. Dent Mater 2008;24:299-307 10. Kelly JR, Denry I: Stabilized zirconia as structural ceramic: an overview. Dent Mater 2008;24:289-298
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9
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Passos et al
11. Wenz HJ, Bartsch J, Wolfart S, et al: Osseointegration and clinical success of zirconia dental implants: a systematic review. Int J Prosthodont 2008;21:27-36 12. Albakry M, Guazzato M, Swain MV: Influence of hot pressing on the microstructure and fracture toughness of two pressable dental glass-ceramics. J Biomed Mater Res B Appl Biomater 2004;71:99-107 13. Guazzato M, Albakry M, Swain MV, et al: Mechanical properties of In-Ceram Alumina and In-Ceram Zirconia. Int J Prosthodont 2002;15:339-346 14. Scherrer SS, Denry IL, Wiskott HW: Comparison of three fracture toughness testing techniques using a dental glass and a dental ceramic. Dent Mater 1998;14:246-255 15. Sakar-Deliormanli A, Guden M: Microhardness and fracture toughness of dental materials by indentation method. J Biomed Mater Res B Appl Biomater 2006;76:257-264 16. Chantikul P, Anstis GR, Lawn BR, et al: Critical evaluation of indentation techniques for measuring fracture toughness. II. Strength method. J Am Ceram Soc 1981;64:539-543 17. Mitov G, Gessner J, Lohbauer U, et al: Subcritical crack growth behavior and life data analysis of two types of dental Y-TZP ceramics. Dent Mater 2011;27:684-691 18. Nemli SK, Yilmaz H, Aydin C, et al: Effect of fatigue on fracture toughness and phase transformation of Y-TZP ceramics by X-ray diffraction and Raman spectroscopy. J Biomed Mater Res B Appl Biomater 2012:100B:416-424 19. Benzaid R, Chevalier J, Saˆadaoui M, et al: Fracture toughness, strength and slow crack growth in a ceria stabilized zirconia-alumina nanocomposite for medical applications. Biomaterials 2008;29:3636-3641 20. Guazzato M, Albakry M, Ringer SP, et al: Strength, fracture toughness and microstructure of a selection of all-ceramic materials. Part II. Zirconia-based dental ceramics. Dent Mater 2004;20:449-456 21. Luthardt RG, Holzhuter M, Sandkuhl O, et al: Reliability and properties of ground Y-TZP zirconia ceramics. J Dent Res 2002;81:487-491 22. Anstis GR, Chantikul P, Lawn BR, et al: A critical evaluation of indentation techniques for measuring fracture toughness: I, direct crack measurements. J Am Ceram Soc 1981;64:533-538 23. Aboushelib MN, Kleverlaan CJ, Feilzer AJ: Evaluation of a high fracture toughness composite ceramic for dental applications. J Prosthodont 2008;17:538-544 24. Studart AR, Filser F, Kocher P, et al: Fatigue of zirconia under cyclic loading in water and its implications for the design of dental bridges. Dent Mater 2007;23:106-114 25. Yilmaz H, Aydin C, Gul BE: Flexural strength and fracture toughness of dental core ceramics. J Prosthet Dent 2007;98:120-128 26. Harada K, Shinya A, Yokoyama D, et al: Effect of loading conditions on the fracture toughness of zirconia. J Prosthodont Res 2013;57:82-87 27. Albakry M, Guazzato M, Swain MV: Fracture toughness and hardness evaluation of three pressable all-ceramic dental materials. J Dent 2003;31:181-188 28. Kies JA, Clark ABJ: Propagation rates and times to fail following proof stress in bulk glass fracture. Proceedings of the 2nd International Conference on Fracture. 1969, pp. 483–491, Paper 42. 29. Fujishima A, Ferracane JL: Comparison of four modes of fracture toughness testing for dental composites. Dent Mater 1996;12:38-43
10
30. Aboushelib MN, de Jager N, Kleverlaan CJ, et al: The influence of pigments on the slow crack growth in dental zirconia. Dent Mater 2012;28:410-415 31. Fischer H, Marx R: Fracture toughness of dental ceramics: comparison of bending and indentation method. Dent Mater 2002;18:12-19 32. Moher D, Liberati A, Tetzlaff J, et al: Preferred reporting Items for systematic reviews and meta-analyses: the PRISMA statement. BMJ 2009;339:b2535, 332-336. 33. Quinn JB, Sundar V, Lloyd IK: Influence of microstructure and chemistry on the fracture toughness of dental ceramics. Dent Mater 2003;19:603-611 34. Marinis A, Aquilino SA, Lund PS, et al: Fracture toughness of yttria-stabilized zirconia sintered in conventional and microwave ovens. J Prosthet Dent 2013;109:165-171 35. Xiao R, Chu BF, Zhang L, et al: Aging performances for resisting low-temperature of three dental Yttria-stabilized zirconia ceramic core materials. Chin Med J (Engl) 2012;125:1999-2003 36. Saied MA, Lloyd IK, Haller WK, et al: Joining dental ceramic layers with glass. Dent Mater 2011;27:1011-1016 37. Bulpakdi P, Taskonak B, Yan J, et al: Failure analysis of clinically failed all-ceramic fixed partial dentures using fractal geometry. Dent Mater 2009;25:634-640 38. Turp V, Tuncelli B, Sen D, et al: Evaluation of hardness and fracture toughness, coupled with microstructural analysis, of zirconia ceramics stored in environments with different pH values. Dent Mater J 2012;31:891-902 39. Lazar DR, Bottino MC, Ozcan M, et al: Y-TZP ceramic processing from coprecipitated powders: a comparative study with three commercial dental ceramics. Dent Mater 2008;24:1676-1685 40. Ebert J, Ozkol E, Zeichner A, et al: Direct inkjet printing of dental prostheses made of zirconia. J Dent Res 2009;88: 673-676 41. Manawi M, Ozcan M, Madina M, et al: Impact of surface finishes on the flexural strength and fracture toughness of In-Ceram Zirconia. Gen Dent 2012;60:138-142 42. Yin L: Property-process relations in simulated clinical abrasive adjusting of dental ceramics. J Mech Behav Biomed Mater 2012;16:55-65 43. Triwatana P, Srinuan P, Suputtamongkol K: Comparison of two fracture toughness testing methods using a glass-infiltrated and a zirconia dental ceramic. J Adv Prosthodont 2013;5:36-43 44. Tinschert J, Natt G, Mohrbotter N, et al: Lifetime of aluminaand zirconia ceramics used for crown and bridge restorations. J Biomed Mater Res B Appl Biomater 2007;80:317-321 45. Mirkovi´c N, Gostovi´c AS, Lazi´c Z, et al: Fracture toughness of zirconia ceramic crowns made by feather-edge tooth preparation design. Vojnosanit Pregl 2012;69:562-568 46. Borba M. Mechanical Behavior of Ceramic Materials Used for Fixed Partial Dentures. S˜ao Paulo, University of S˜ao Paulo, 2010, p. 170. 47. ISO. Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics) Test Method for Fracture Toughness of Monolithic Ceramics at Room Temperature by Chevron-Notched Beam (CNB) Method (ed 1). ISO 24370. International Organization for Standardization, 2005. 48. ISO. Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics) Test Method for Fracture Toughness of Monolithic Ceramics at Room Temperature by Single Edge Precracked Beam (SEPB) Method. ISO 15732. International Organization for Standardization, 2003.
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Passos et al
49. ASTM. Standard Test Methods for Determination of Fracture Toughness of Advanced Ceramics at Ambient Temperature. ASTM C1421. American Society for Testing Materials, 2010. 50. ISO. Dentistry—Ceramic Materials. ISO 6872. International Organization for Standardization, 2008. 51. Ritchie RO: Mechanisms of fatigue-crack propagation in ductile and brittle solids. Int J Fracture 1999;100:55-60 52. Launey M, Ritchie RO: On the fracture toughness of advanced materials. Adv. Mater 2009;21:2103-2110 53. ISO. Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics) Determination of Fracture Toughness of Monolithic Ceramics at Room Temperature by the Surface Crack in Flexure (SCF) Method. ISO 18756. International Organization for Standardization, 2003. 54. Quinn GD, Salem J, Bar-On I, et al: Fracture toughness of advanced ceramics at room temperature. J Res Natl Inst Stand Technol 1992;97:579-607 55. Hornberger H, Marquis PM: Mechanical properties and microstructure of In- Ceram, a ceramic-glass composite for dental crowns. Glastech Ber Glass Sci Technol 1995;68: 188-194 56. ISO. Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics)—Test Methods for Fracture Toughness of Monolithic Ceramics—Single-Edge V-Notch Beam (SEVNB) Method. ISO 23146. International Organization for Standardization, 2012.
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57. Mahoney E, Holt A, Swain M, et al: The hardness and modulus of elasticity of primary molar teeth: an ultra-microindentation study. J Dent 2000;28:589-594 58. Xu HHK, Smith DT, Jahanmir S, et al: Indentation damage and mechanical properties of human enamel and dentin. J Dent Res 1998;77:472-480 59. Quinn GD, Bradt RC: On the Vickers indentation fracture toughness test. J Am Ceram Soc 2007;90:673-680 60. De Aza AH, Chevalier J, Fantozzi G, et al: Crack growth resistance of alumina, zirconia and zirconia toughened alumina ceramics for joint prostheses. Biomaterials 2002;23:937-945 61. ISO. Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics)—Test Method for Flexural Strength of Monolithic Ceramics at Room Temperature. ISO 14704. International Organization for Standardization, 2008. 62. Scherrer SS, Cattani-Lorente M, Vittecoq E, et al: Fatigue behavior in water of Y-TZP zirconia ceramics after abrasion with 30 µm silica-coated alumina particles. Dent Mater 2011;27:e28-e42 63. Singh R, Gill C, Lawson S, et al: Sintering, microstructure and mechanical properties of commercial Y-TZPs. J Mater Sci 1996;31:6055-6062 64. Studart AR, Filser F, Kocher P,et al: In vitro lifetime of dental ceramics under cyclic loading in water. Biomaterials 2007;28:2695-2705 65. Chevalier J, Saadaoui M, Olagnon C, et al: Double-torsion testing a 3Y-TZP ceramic. Ceram Int 1996;22:171-177
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