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Survey of the mechanical and physical behaviors of yttria-stabilized zirconia from multiple dental laboratories

Open AccessPublished:January 12, 2023DOI:https://doi.org/10.1016/j.jfscie.2022.100018

      Abstract

      Background

      When selecting zirconia for a dental restoration, laboratory prescriptions often refer to high strength and high translucency. In this survey, zirconia specimens were ordered from various dental laboratories for posterior (high strength) and anterior (high translucency) clinical indications. The specimens were then tested and evaluated for their mechanical and physical behaviors.

      Methods

      In a double-blinded manner, 9 laboratories provided 32 specimens from 17 different zirconia blanks, which were tested in the American Dental Association laboratory. Flexural strength tests were performed on standard specimens, and fracture surfaces were examined using both optical and scanning electron microscopy. Chemical composition, Vickers hardness, and absolute transmittance measurements were also performed.

      Results

      A large scatter in the strength values was observed. The zirconia intended for posterior applications displayed strengths (SD) from 195 through 783 MPa (490 [183] MPa), which overlapped greatly with the strengths (SD) of the zirconia intended for anterior applications, 320 through 768 MPa (581 [136] MPa). However, when the strength values were recalculated on the basis of yttria content, the strengths (SD) were 584 (158) MPa for 3 mol% yttria and 373 (104) MPa for 5 mol% yttria.

      Conclusions

      When a prescription was given to dental laboratories to request zirconia on the basis of clinical requirements, there was a large scatter and no consistency in the resulting strength values partly because of mixed use of 3 mol% and 5 mol% yttria zirconia. The 3 mol% materials had much higher strength when the strength values were grouped according to yttria content. Strength was also highly dependent on processing and finishing at the dental laboratories.

      Graphical abstract

      Key Words

      Dental laboratories fill prescriptions for dental restorations on the basis of the patient’s clinical indications. Their knowledge of the available products is important when zirconia material is indicated. In this survey, a dental materials research group worked with a practicing dentist and a dental laboratory owner to fill a prescription for zirconia specimens from various dental laboratories ranging from posterior, high strength to anterior, high-translucency clinical indications. The consistency of the received materials was evaluated with respect to their mechanical and physical behaviors. The large scatter in the flexural strength data prompted the authors to analyze the composition of the specimens, which showed a mixed use of 3 mol% and 5 mol% yttria. Typically, 3 mol% yttria is added to stabilize zirconia at room temperature, resulting in high strength and toughness; however, increasing yttria increases translucency but decreases strength. Grouping of the strength data based on yttria content showed that some laboratories provided zirconia with 5 mol% yttria for posterior applications, although this material is generally indicated for anterior applications except for single-unit posterior prostheses. Good communication between the dentist and the laboratory plays a vital role in ensuring the patient receives the appropriate type of zirconia material for the clinical indication.

      Introduction

      Since its introduction as a dental material, zirconia has attracted considerable interest from dentists and patients.
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      Among various possible contributing factors of such failures, the processing procedures for zirconia devices, before they are sent to dentists, deserve thorough consideration. For instance, zirconia milling blank manufacturers press the raw material, usually in powder form, into approximately 100-mm–diameter disks, which are then presintered to remove polymeric binders. Typically, the exact formulations of the blocks, including binder compositions and additives (besides estimates of oxide stabilizers and sintering aids, such as yttria and alumina), are proprietary information and not readily available.
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      Dental laboratories further process the blanks by milling them into final shapes, according to the 3-dimensional profiles provided by dentists, and then sintering the dental products above 1,400 °C to achieve the desired final phase. Each milling block has a designated enlargement factor, which is taken into account during the milling process, to compensate for shrinkage during sintering.
      Similarly, detailed processing procedures, such as grinding and finishing procedures, are also commonly unavailable. Therefore, the exact chemical composition, microstructure, and processing of zirconia materials for dental applications can vary significantly, depending on the milling blank manufacturer and dental laboratory. There are several reports comparing different brands of zirconia milling blanks,
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      but we have found limited information about the influence of dental laboratories. Because dental laboratories play a critical role in supplying zirconia products to dentists and, subsequently, patients, it is necessary to gain a better understanding of the final consistency of the zirconia materials they supply. Therefore, to gain an insight into the consistency of zirconia materials supplied by dental laboratories, a prescription was given to multiple dental laboratories to supply zirconia specimens for posterior (HS) and anterior (high translucency [HT]) clinical indications. We then evaluated the consistency of the supplied specimens with respect to their mechanical (flexural strength and hardness) and physical (translucency) behaviors.

      Methods

      A form letter requesting zirconia specimens was emailed to multiple dental laboratories by a dental laboratory owner. The letter requested that the dental laboratory provide standard zirconia specimens covering the full range of clinical indications for which they supply zirconia products from anterior (HT) to posterior (HS) applications. The letter included instructions to provide 2 specimens of each zirconia product in shade A3 to the finished size of 3 × 4 × 45 mm using the methodologies for final finished products sent to clinicians. The request included a computer-aided design model in the stereolithography (STL) file format to be used on their equipment and the instructions that the sender of the email should be contacted for any queries.
      The identity of our testing laboratory was not disclosed to the dental laboratories. Furthermore, the authors performing the evaluations were blinded to the identity of the laboratories that accepted the work, along with the brands of the zirconia products supplied by the participating laboratories. Nine laboratories responded to the email and provided specimens from 17 different dental zirconia milling blank products. For 15 of those products, 2 flexural strength specimens were provided per zirconia milling blank; for the remaining 2 products, only 1 flexural strength specimen was provided per zirconia milling blank. Of the total 32 specimens, 20 were specified by the dental laboratories for posterior restorative applications and 12 for anterior applications.
      Flexural strength tests were conducted using the procedures outlined in ASTM C1161 as a guide.
      ASTM C1161: standard test method for flexural strength of advanced ceramics at ambient temperatures. 2013. American Society for Testing and Materials International. Accessed December 13, 2002.
      A fully articulating 4-point loading fixture with a 40-mm support span and a 20-mm loading span was used for the tests. An exception to the standard was that the cross-sectional specimen size requirements were not strictly followed because a factor in the survey was the consistency of the supplied specimens, including adherence to the requested specimen dimensions. However, when the specimen parallelism requirements cannot be met, a fully articulating fixture is required by ASTM C1161, which was used.
      ASTM C1161: standard test method for flexural strength of advanced ceramics at ambient temperatures. 2013. American Society for Testing and Materials International. Accessed December 13, 2002.
      Furthermore, in addition to the standard procedure for finishing specimens, ASTM C1161 allows for application-matched machining to be used, that is the same surface preparation as for its given application,
      ASTM C1161: standard test method for flexural strength of advanced ceramics at ambient temperatures. 2013. American Society for Testing and Materials International. Accessed December 13, 2002.
      which is what was requested of the dental laboratories supplying the specimens. An Instron 5582 screw-driven universal testing machine was used to load each test specimen at a cross-head speed of 0.5 mm per min. The width and thickness of each specimen were measured using a Nikon profile projector, and the surface morphology was examined using a Zygo NewView 8000 white light interferometer.
      Vickers hardness (VH) testing was performed per ASTM C1327

      Standard Test Method for Vickers Indentation Hardness of Advanced Ceramics. American Society for Testing and Materials International; 2008.

      on specimens mounted in resin and sequentially ground and polished, starting at 80 grit and finishing with a 0.3 μm alumina suspension.
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      Mechanical behavioral assessment of zirconia ceramics using Vickers indentation hardness.
      Five indents were made on each specimen using a 1 kg load and 15-second dwell time. Indentation images were acquired via reflected light microscopy. The dimensions of the indents and the consequential cracks emanating from the corners were measured to calculate a VH number and total crack length (TCL), respectively. The TCL is the sum of each crack emanating from an indent, with each crack being measured from the corner of the indent to the crack tip.
      After the flexural strength tests were performed, the regions near the ends of the specimens were sectioned into 0.7-mm–thick pieces and polished for absolute light transmittance measurements.
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      For the measurements, a tungsten halogen light source was connected to a custom-fabricated specimen holder attached to a port in an integrating sphere (Labsphere) that was in turn connected to a spectrometer (USB 2000+, Ocean Optics) to create a spectrometer:integrating sphere system. After obtaining a background light reading without a specimen in the holder, a 3- × 4- × 0.7-mm specimen was mounted in the holder, and another reading was taken with the light transmitted through the 0.7-mm thick specimen into the spectrometer:integrating sphere. Three measurements were performed for each specimen, and the average percent light transmittance was calculated for wavelengths from 380 through 780 nm using the OceanView software (Ocean Optics).
      The fracture surfaces from the flexural strength specimens were examined using a JEOL JCM-6000 scanning electron microscope (SEM). Each fractured surface was coated with an approximately 12-nm–thick gold layer to ensure conductivity. At the Advanced Photon Source of Argonne National Laboratory, x-ray fluorescence (XRF) microscopy was performed at both the 2-ID-D beamline and the 9-ID-B beamline (using Bionanoprobe
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      ) to obtain the chemical composition. The incident x-ray energy was tuned to 20 keV using a double-crystal Si (111) monochrometer. A silicon drift energy dispersive detector was placed at 90° with respect to the incident beam to collect the XRF signal. A 250- × 250-μm area was surveyed for each specimen using a 25-second exposure time, yielding an average chemical composition. Spectrum fitting and quantification were performed using the MAPS software.
      • Vogt S.
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      Results

      Flexural strength and fractography, as clinically indicated by the dental laboratories

      The specimens received from the different laboratories were rectangular with little distortion. Raised imperfections were frequently observed on the surfaces of many of the specimens, presumably resulting from the milling process. For instance, Figure 1A shows a 40 μm × 60 μm protrusion raised approximately 8 μm from the surface. The nominal grain size of the specimens made of 3Y-TZP (as determined by XRF) was approximately 300 nm, as shown in the SEM image of product K in Figure 1B. Figure 1C shows a SEM image of product S, a 5Y-PSZ material (as determined by XRF). The materials that were ultimately identified as being 5Y-PSZ products by XRF had a larger grain size ranging from about 700 through 1,000 nm.
      Figure thumbnail gr1
      Figure 1A. A 40 μm × 60 μm protrusion raised approximately 8 μm from the surface. B. Scanning electron microscope micrograph of specimen number 1 from product K, a 3Y-tetragonal zirconia polycrystal material, showing that the grain size was approximately 300 nm after sintering. C. Scanning electron microscope micrograph of specimen number 1 from product S, a 5Y-partially stabilized zirconia. The grain size was approximately 1 μm.
      Figures 2A and 2B show the average thickness and width values, respectively, for the specimens received from the various dental laboratories grouped according to the zirconia milling blank product. The average thickness and width measurements for the specimens were all slightly larger than the specified values. Some specimens were close to the specified 3-mm thickness, whereas others were considerably larger, with the largest error being 32% more than the specified thickness. The largest width was 4.3 mm, 7.5% more than the specified width of 4.0 mm. The laboratory prescription requested that the laboratories fabricate fully processed and finished specimens to the dimensions provided in the stereolithography file. Although some laboratories met the final sizes requested by the form letter, others deviated appreciably from what was requested.
      Figure thumbnail gr2
      Figure 2The average thickness (A) and width (B) values for the specimens received from the participating laboratories. The requested dimensions were 3 mm × 4 mm × 45 mm, complying with ASTM C1161. A letter specifying the dimensions was provided to each dental laboratory along with a stereolithography file. The letters on the x-axes refer to the different zirconia milling blank products.
      All the specimens procured from the laboratories were fractured within the inner span of the 4-point loading configuration and away from the loading rollers. The specimens identified as intended for posterior restorative applications (abbreviated as HS) exhibited flexural strengths in the range of 195 through 783 MPa, with an average (SD) strength of 490 (183) MPa. Some laboratories had specimens with much lower strength values for some of their products than others. For instance, Laboratory 6 had values of 195 and 196 MPa for product N and 205 and 239 MPa for product O compared with 588 and 732 MPa for product P. Even specimens fabricated by a single laboratory from the same product sometimes yielded different strength values. For example, for Laboratory 3, specimen number 2, which was made from product G, had a strength of 783 MPa (the highest among all of the specimens from all of the laboratories), whereas specimen number 1 from the same product G of Laboratory 3 had a strength value of only 445 MPa, approximately 57% of the strength of specimen number 2. Because the specimens were made from the same milling blank, the large difference in strengths between the 2 specimens can presumably be attributed to flaws caused by milling or processing.
      Those specimens identified by the dental laboratories as intended for anterior restorative applications (abbreviated as HT) had flexural strength values ranging from 320 through 768 MPa, with an average (SD) strength of 581 (136) MPa. The highest strength (768 MPa for specimen number 1 from product E) is comparable with the high end of the range for the specimens meant for posterior applications (783 MPa). Figures 3A and 3B show bar charts of the flexural strength values for the HS and HT specimens, respectively. A remarkable spread in the flexural strength values across the products from the different dental laboratories is evident in the figures.
      Figure thumbnail gr3
      Figure 3Flexural strength values for specimens indicated by the participating laboratories are for posterior (A) and anterior (B) restorative applications, respectively. The specimens were tested using a 4-point bending configuration complying with ASTM C-1161. The letters on the x-axes refer to the different zirconia milling blank products. For both products T and X, only 1 specimen was provided.
      Fractography analysis of a specimen from product H is shown in Figures 4A and 4B . The arrow in Figure 4A points to a milling mark on the bottom surface that can be followed to the crack-initiation site on the fracture surface, as indicated by the arrow in Figure 4B. For all specimens, fracture analysis similarly showed that the cracks originated from the milling marks.
      Figure thumbnail gr4
      Figure 4Optical images of specimen number 1 from product H. A. Bottom surface (in tension) showing straight milling marks. B. Fracture surface showing that the fracture initiated from the milling marks on the bottom surface.
      The fracture surfaces of selected specimens from the products submitted by the laboratories meant for both posterior (HS) and anterior (HT) applications were further examined using the SEM. Figure 5 shows the SEM micrographs of specimen number 1 from products G, N, and E and specimen number 2 from product S. The specimens yielding the highest strengths in both the HS (product G) and HT (product E) categories exhibited similar surface features, such as several dimples and spheres, as shown in Figures 5A and 5C. The dimple and sphere features are approximately 300 nm in size, indicating intergranular fracture. In contrast, specimens yielding the lowest strengths in both the HS (product N) and HT (product S) categories exhibited relatively smooth cleavage steps, as shown in Figures 5B and 5D. This brittle, transgranular fracture suggests that cracks propagated through the grains. These surface tomography results are consistent with the measured flexural strength results.
      Figure thumbnail gr5
      Figure 5Scanning electron microscope micrographs of fracture surfaces at 10 kV, ×4,000 A. Specimen number 1 of product G with the highest strength of 783 MPa in the high strength category. B. Specimen number 1 of product N with the lowest flexural strength of 195 MPa in the high strength category. C. Specimen number 1 of product E with the highest flexural strength of 768 MPa in the high translucency category. D. Specimen number 2 of product S with the lowest flexural strength of 320 MPa in the high translucency category. The x-ray fluorescence analysis showed that product N is 5Y-partially stabilized zirconia, typically meant for anterior applications, and product E is 3Y-tetragonal zirconia polycrystal, typically for posterior applications. After compositional identification through x-ray fluorescence analysis, it was evident that the 3Y-tetragonal zirconia polycrystal specimens exhibited intergranular fracture (A and C), whereas the 5Y-partially stabilized zirconia specimens exhibited transgranular fracture (B and D).

      Composition

      Synchrotron XRF microscopy was used to examine the composition of the zirconia specimens. All specimens were primarily composed of zirconia, hafnium, yttria, and impurities such as calcium, iron, and cobalt (note that aluminum cannot be measured using synchrotron XRF). The Table shows the mol% of yttria and other mechanical and physical characteristics of the zirconia products. Products B, D, H, N, S, and T consist of approximately 5 mol% yttria, and the remaining products have approximately 3 mol% yttria. From the Table, it can be seen that 3 of the products (D, H, and N) that were identified by the dental laboratories as intended for posterior applications have approximately 5 mol% yttria, which is typically meant for anterior applications except for single-unit posterior prostheses for both monolithic ceramic and partially or fully covered substructure. Similarly, 4 of the products (E, M, Q, and X) identified by the dental laboratories as intended for anterior applications have approximately 3 mol% yttria, typically meant for posterior applications. As part of our compositional analysis, we recalculated the average values for the mechanical and physical characteristics of the zirconia products on the basis of the yttria content and compared them to the average values on the basis of the clinical indications identified by the dental laboratories as follows.
      TableYttria concentration, flexural strength, total crack length, hardness, and percent light transmittance of the zirconia specimens.
      LaboratoryProductApplicationYttria mol%Flexural strength, MPa (SD)Total crack length, mm (SD)Vickers hardness number using a 1 kg load (SD)Percent light transmittance, %
      1APosterior3.0656 (5)0.030 (0.001)1415 (18)7.4
      2CPosterior3.1556 (51)0.032 (0.003)1464 (37)7.8
      2DPosterior5.3377 (7)0.100 (0.005)1502 (29)12.6
      3GPosterior3.0614 (169)0.021 (0.003)1412 (15)10.8
      4HPosterior5.2476 (2)0.106 (0.007)1411 (45)11.2
      4JPosterior3.1695 (17)0.019 (0.006)1389 (5)7.3
      5KPosterior3.0448 (86)0.020 (0.003)1396 (14)8.4
      6NPosterior5.0195 (1)0.092 (0.008)1465 (12)11.7
      6OPosterior2.9222 (17)0.094 (0.005)1445 (18)7.4
      6PPosterior3.0660 (72)0.027 (0.004)1405 (14)8.0
      6BAnterior5.1574 (37)0.034 (0.003)1385 (6)8.6
      7EAnterior3.1719 (49)0.020 (0.001)1422 (18)7.6
      5MAnterior3.1675 (27)0.027 (0.005)1377 (13)8.3
      8QAnterior2.9597 (33)0.027 (0.003)1403 (14)9.5
      8SAnterior5.2339 (19)0.093 (0.007)1431 (14)12.4
      8TAnterior5.32600.107 (0.005)1384 (17)14.0
      9XAnterior3.05010.022 (0.004)1393 (48)9.6

      Flexural strength and fractography, as grouped by yttria content

      After performing the synchrotron XRF analysis, we recalculated average flexural strength values for the zirconia products on the basis of the yttria content. Consequently, the average flexural strength for the zirconia products with approximately 3 mol% yttria was calculated to be 584 (158) MPa, and the average (SD) flexural strength for the zirconia products with approximately 5 mol% yttria was 373 (104) MPa. Similarly, after compositional identification through XRF analysis, it was evident that the 3Y-TZP specimens exhibited intergranular fracture (Figures 5A and 5C), whereas the 5Y-PSZ specimens exhibited transgranular fracture (Figures 5B and 5D).

      VH, both as clinically indicated by the dental laboratories and grouped by yttria content

      The average (SD) VH values were measured to be 1430 (37) HV1 for the products intended for posterior applications and 1399 (20) HV1 for the products meant for anterior applications. After XRF compositional analysis, for comparison, we recalculated the average hardness values with respect to yttria content, resulting in an average (SD) VH value for the 3Y-TZP specimens of 1411 (25) HV1 and an average value for the 5Y-PSZ specimens of 1430 (47) HV1.
      All of the indents had cracks extending from their corners because of the general brittleness of the zirconia materials. Figures 6A and 6B display the optical images of the indents of product J (3 mol% yttria) and product N (5 mol% yttria), respectively, showing that the indentation cracks on the 5Y-PSZ specimen are much longer than for the 3Y-TZP specimen. This trend generally held true for most of the specimens. That is, when the results were reported with respect to yttria content, the TCLs for the 3Y-TZP specimens fell between 0.019 mm and 0.032 mm, except for product O, which was 0.094 mm. Similarly, the 5Y-PSZ specimens had distinctly longer TCLs, ranging from 0.092 through 0.107 mm, except for product B, which was 0.034 mm.
      Figure thumbnail gr6
      Figure 6Images of Vickers indents performed using a 1 kg load and 15-second dwell time. A. Specimen number 2 of product J with 3 mol% yttria. B. Specimen number 2 of product N with 5 mol% yttria. All indents had at least 1 crack present. C. Total crack length values as a function of the yttria content. Most 3Y-tetragonal zirconia polycrystal specimens had total crack length values of approximately 0.03 mm, whereas most 5Y-partially stabilized zirconia specimens exhibited larger total crack length values of approximately 0.10 mm. One outlier was observed for each category.

      Percent light transmittance, both as clinically indicated by the dental laboratories and grouped by yttria content

      The Table shows the percent light transmittance values for each zirconia product. The percent light transmittance (SD) of the products identified by the dental laboratories as intended for posterior applications was measured to be 9.3% (2.1%) and just slightly higher at 10.0% (2.3%) for anterior applications. When we regrouped the results by the yttria content, the percent light transmittance (SD) for the 3Y-TZP specimens was calculated to be 8.4% (1.1%), whereas for the 5Y-PSZ specimens it was noticeably higher at 11.8% (1.8%). Figure 7 shows a plot of flexural strength as a function of percent light transmittance with a line of best fit through the data, and the shaded region indicating the 95% CI. Generally, the plot in Figure 7 shows that as the flexural strength of the specimens increased, their translucency decreased.
      Figure thumbnail gr7
      Figure 7Flexural strength as a function of percent light transmittance with a line of best-fit through the data and a shaded region indicating the 95% CI.

      Discussion

      Despite the relatively small sample size of the products from the participating dental laboratories and the lack of statistical data, useful information was gathered from this research. The zirconia products from the laboratories exhibited large spreads in flexural strength values and final finished dimensions. The production of a zirconia dental restoration is highly dependent on processing parameters, including milling, sintering, and finishing. All the laboratories were provided the same instructions, requesting them to fabricate specimens that were fully processed and finished for anterior and posterior applications in the form of bars. From the dimensional accuracy and flexural strength data alone, it is apparent that some of the laboratories failed to process and finish the specimens properly. During a 4-point bending test, the bottom surface of the specimen is subjected to high tension, and fracture often originates from defects on that surface. Fractography performed on the flexural strength specimens showed that all the fractures began from the machining flaws associated with the travel paths of the milling instruments, as shown in Figure 4. This fractography analysis is in agreement with a previous study on the fatigue behavior of 3Y-TZP in which preexisting processing flaws were pinpointed as the fracture origin in all cases.
      • Denry I.
      • Kelly J.R.
      State of the art of zirconia for dental applications.
      In fact, a series of studies on the effect of sharp indentation damage on the durability of 3Y-TZP showed that sandblasting and sharp indentations (even at very low loads) were detrimental to the long-term performance of the material when tested under cyclic loading.
      • Denry I.
      • Kelly J.R.
      State of the art of zirconia for dental applications.
      ,
      • Liu S.Y.
      • Chen I.W.
      Fatigue of yttria-stabilized zirconia: I, fatigue damage, fracture origins, and lifetime prediction.
      • Zhang Y.
      • Lawn B.R.
      Fatigue sensitivity of Y-TZP to microscale sharp-contact flaws.
      • Zhang Y.
      • Pajares A.
      • Lawn B.R.
      Fatigue and damage tolerance of Y-TZP ceramics in layered biomechanical systems.
      • Zhang Y.
      • Lawn B.R.
      • Rekow E.D.
      • Thompson V.P.
      Effect of sandblasting on the long-term performance of dental ceramics.
      Although the bar specimens tested in this survey were not finished restorations, the dental laboratories were requested to follow the procedures they normally used. Therefore, the reduction in strength observed for zirconia materials when dimensional accuracy and surface finish are not properly considered may occur for actual restorative products when proper care is not taken by the dental laboratory.
      The fractography analysis also indicated that some of the zirconia products supplied by the laboratories were not appropriate for the requested applications. Figure 5 shows the fracture surfaces of 4 specimens exhibiting 2 distinct fracture patterns. The specimens with the lowest measured flexural strength values, identified by the laboratories as indicated for both posterior and anterior applications, exhibited similar flat, smeared cleavages with steps characteristic of brittle, transgranular fracture of ceramic materials. However, the synchrotron XRF results showed that product N in Figure 5B, specified by the laboratory for posterior applications, was zirconia with 5 mol% yttria, which is more suitable for anterior applications, and product E in Figure 5C, specified by the laboratory for anterior applications, was zirconia with 3 mol% yttria, which is more suitable for posterior applications. Therefore, the 2 distinct fracture patterns shown in Figure 5 are related to the yttria content. The flat, smeared cleavages, indicating transgranular fracture, shown in Figures 5B and 5D, are representative of the specimens with 5 mol% yttria. In contrast, the dimpled and spherical features, suggesting intergranular fracture, are characteristic of the 3 mol% yttria specimens. Using 5Y-PSZ, which exhibits brittle, transgranular fracture, for HS applications can lead to sudden failure of a restoration.
      When considering the appropriateness of zirconia for a clinical application, it is necessary to reference the classification systems specified in the standards for dental ceramic materials: American National Standards Institute/American Dental Association (ANSI/ADA) Standard No. 69 Dental Ceramic
      American National Standards Institute. ANSI/ADA Specification No. 69-1999: Dental Ceramic. 2020.
      or International Organization for Standardization (ISO) 6872 Dentistry—ceramic materials.

      International Organization for Standardization. 6872: Dentistry-Ceramic Materials. 2015.

      These documents are harmonized, so they have the same classification system. Besides dental zirconia, the scope of ISO 6872 applies to all dental ceramic materials for fixed all-ceramic and metal-ceramic restorations and appliances, including dental porcelain, glass ceramic, glass-infiltrated dental ceramic, and other dental ceramic materials. Therefore, the classification system reflects many clinical indications and strength requirements. Of particular relevance to HS zirconia materials for posterior applications are Classes 4 and 5. Specifically, the recommended clinical indications for Class 4 dental ceramics include a monolithic ceramic for 3-unit prostheses that involve molar restoration, along with a partially or fully covered substructure for 3-unit prostheses involving molar restoration. Class 4 dental ceramics have a minimum requirement of a mean flexural strength of 500 MPa, whereas Class 5 dental ceramics have a highest mean flexural strength requirement of 800 MPa. The recommended clinical indications for Class 5 dental ceramics include monolithic ceramic for prostheses that involve a partially or fully covered substructure for 4 or more units, together with a fully covered substructure for prostheses that involve 4 or more units. Using this classification system, the mean (SD) flexural strength (490 [183] MPa) of the zirconia materials clinically indicated by the dental laboratories for posterior applications did not meet the minimum requirements for either Class 4 or 5 dental ceramics. Therefore, the only posterior application that this average flexural strength value meets is the limited indication of single-unit posterior prostheses for both monolithic ceramic and partially or fully covered substructures listed for Class 3 dental ceramics as detailed below. In contrast, the mean (SD) flexural strength (581 [136] MPa) of the zirconia materials supplied by the dental laboratories for anterior applications exceeds the minimum Class 4 requirement but fails to meet the Class 5 requirement.
      It is well known from the literature that the 3Y-TZP material is generally stronger than 5Y-PSZ.
      • Denry I.
      • Kelly J.R.
      State of the art of zirconia for dental applications.
      ,
      • Kelly JR Denry I.
      Stabilized zirconia as a structural ceramic: an overview.
      After performing the synchrotron XRF analysis, we rearranged the products into groups on the basis of the yttria content. The average flexural strength (SD) for specimens with approximately 3 mol% yttria was calculated to be 584 (158) MPa, and that for the specimens with approximately 5 mol% yttria was 373 (104) MPa. Furthermore, when we regrouped the products by yttria content, the average flexural strength of the 3Y-TZP products exceeded the minimum Class 4 requirement for dental ceramics, which is expected, whereas the average flexural strength for the 5Y-PSZ products fell below the 500 MPa requirement. This indicates that, generally, the 5Y-PSZ products evaluated in this survey should not be used for Class 4 or Class 5 types of clinical indications because their average flexural strength is insufficient.
      However, the 5Y-PSZ products meet the minimum requirement of 300 MPa for a Class 3 dental ceramic. The recommended clinical indications for a Class 3 ceramic include anterior and limited posterior applications. For monolithic ceramic, Class 3 includes single-unit anterior or posterior prostheses and 3-unit prostheses that do not involve molar restoration and that are adhesively or nonadhesively cemented. Similarly, for partially or fully covered substructures, Class 3 includes single-unit anterior or posterior prostheses and 3-unit prostheses that do not involve molar restoration and that are adhesively or nonadhesively cemented. Because of the overlap of clinical indications for anterior and some posterior applications for a Class 3 dental ceramic, some dental laboratories may have included 5Y-PSZ products when asked to supply posterior zirconia material. However, it is worth emphasizing that the classification system for Class 4 and Class 5 dental ceramics does not include products with mean flexural strength values less than 500 MPa and those indicated for anterior applications. Therefore, there must be a clear understanding between the dentist and the laboratory as to the specifics of the anticipated restoration, along with the minimum strength requirements, so that the appropriate type of zirconia is supplied.
      There are no percent light transmittance (translucency) or hardness requirements in the ANSI/ADA and ISO dental ceramic standards.
      American National Standards Institute. ANSI/ADA Specification No. 69-1999: Dental Ceramic. 2020.
      ,

      International Organization for Standardization. 6872: Dentistry-Ceramic Materials. 2015.

      However, as noted above, zirconia dental products are often marketed as being more or less translucent than competing brands. Figure 7 shows a general trend of decreased flexural strength with increased percent light transmittance for the different laboratory products. This is in agreement with a study by Zhang et al
      • Zhang F.
      • Inokoshi M.
      • Batuk M.
      • et al.
      Strength, toughness and aging stability of highly-translucent Y-TZP ceramics for dental restorations.
      on Tosoh Zpex (∼3 mol% yttria) and Zpex Smile (∼5 mol% yttria) materials. In their study, the researchers showed that increasing the yttria content from about 3 mol% through 5 mol% increased the average grain size of the zirconia from 304 nm to 713 nm, with a corresponding increase in cubic phase from 9.6 wt% through 53.7 wt%, resulting in the translucency parameter being increased by 54%; however, this increase in translucency was accompanied by a 43% decrease in flexural strength. In our survey, after grouping the products by yttria content after XRF compositional analysis, the average percent light transmittance for the approximately 3 mol% yttria products was 8.4%, and this value increased by about 40% through 11.8% for the products with approximately 5 mol% yttria. This translucency increase was also accompanied by a 36% decrease in average flexural strength.
      Unlike the flexural strength and percent light transmittance values, the hardness values did not show a strong relationship with yttria content. However, the Vickers indentation testing did show a relationship between TCL values extending from the indentations and yttria content. Figure 6C shows that when the zirconia products from the different laboratories are grouped according to the yttria content, the TCL values for most of the 3 mol% yttria zirconia products are approximately 0.03 mm, whereas the TCL values for most of the 5 mol% yttria zirconia products are approximately 0.10 mm. Only 1 exception was observed for each category, and the reason is yet to be understood. In general, for the same applied load, the 5Y-PSZ products had TCLs emanating from a given indentation that were about 3 to 5 times longer than for the 3Y-TZP products because of the brittle nature of the former material, and it is shown that the TCL values generally can be used to differentiate the 2 products.
      For comparison purposes, we performed additional flexural strength testing, using the same ASTM C1161 4-point loading method previously described as a guide, on 2 additional zirconia milled blank products selected on the basis of yttria content. BruxZir Solid (3Y-TZP, lot B1 464951) and BruxZir Anterior White (5Y-PSZ, lots Z0872867 and Z0812868) from Prismatik Dentalcraft (Glidewell Direct) were purchased by the authors (Y.L., S.M.) and milled at 1 dental laboratory, which was not one of the blinded laboratories in the dental laboratory survey. The purpose of the experiment and the identity of the authors were disclosed to this latter laboratory. For each of the BruxZir products, 27 specimens were milled from 3 milling blanks by the chosen laboratory to the same dimensions (3 × 4 × 45 mm) and processed as per the processing and finishing instructions specified in the form letter sent to the other laboratories in this survey. The average (SD) flexural strength results were as follows: the BruxZir Solid 3Y-TZP material was 735 (135) MPa (range, 430-967 MPa), and the BruxZir Anterior White 5Y-PSZ material was 384 (64) MPa (range, 268-529 MPa). It can be seen that the average (SD) flexural strength for the BruxZir Anterior White 5Y-PSZ (384 [64] MPa) is comparable with the average (SD) flexural strength for the 5 mol% yttria content zirconia products (373 [104] MPa) supplied by the multiple blinded laboratories. However, for the 3Y-TZP materials, the 735 (135) MPa average (SD) flexural strength for the BruxZir Solid is higher than the 584 (158) MPa average (SD) value for the 3 mol% yttria content zirconia products supplied by the multiple blinded laboratories, with both average values being lower than the 800 MPa required for Class 5 dental ceramics. In addition, the average flexural strength of the BruxZir Solid is lower than the 800 MPa minimum value stated in the manufacturer’s instructions for use that accompanied the milling blocks. To put these values into perspective, a large international collaboration for prestandardization research (the Versailles Advanced Materials and Standards project) reported flexural strength values for specimens made from Tosoh tetragonal-zirconia-3Y powder, which is very fine-grained zirconia with 3 mol% yttria, that was cold-pressed; sintered at 1,510 °C for 2 hours; and then posthipped at 1,450 °C for 1 hour at 1,100 bar before being machined into 3- × 4- × 45-mm bars and tested using a 4-point testing configuration. The average flexural strength for this well-characterized 3 mol% yttria zirconia material was 774 MPa, which compares well with the average flexural strength value we obtained for the BruxZir Solid 3Y-TZP material of 735 MPa.
      • Quinn G.D.
      • Kübler J.J.
      • Gettings R.J.
      Fracture Toughness of Advanced Ceramics by the Surface Crack in Flexure (SCF) Method: a VAMAS Round Robin.
      Furthermore, for the previously mentioned Zhang et al
      • Zhang F.
      • Inokoshi M.
      • Batuk M.
      • et al.
      Strength, toughness and aging stability of highly-translucent Y-TZP ceramics for dental restorations.
      study on the Tosoh Zpex (∼3 mol% yttria) and Zpex Smile (∼5 mol% yttria) materials, which used 3- × 4- × 45-mm specimens tested using a 4-point testing configuration, the researchers reported average values of 854 and 485 MPa, respectively. However, Tosoh Corporation reports typical values of 1,100 and 600 MPa for Zpex and Zpex Smile, respectively, when tested using a 3-point configuration with 3- × 4- × 30-mm bend specimens. Therefore, on closer examination of both the product and the published literature, there are some rational explanations for the differences in reported values, such as the size of the test specimens and the test configuration,
      ASTM C1161: standard test method for flexural strength of advanced ceramics at ambient temperatures. 2013. American Society for Testing and Materials International. Accessed December 13, 2002.
      which are a topic of ongoing research.

      Conclusions

      This survey showed that dental laboratories did not always adhere to the details requested in a prescription for a zirconia-based restorative. Fractography performed on the flexural strength specimens showed that all fractures originated from machining flaws. Furthermore, there was a large scatter in the mechanical and physical behaviors of the zirconia specimens due in part to the mixed use of 3 mol% and 5 mol% yttria containing zirconia. Based on the ANSI/ADA and ISO classification systems for dental ceramics, the mean flexural strength value of the zirconia materials supplied by the participating laboratories for posterior applications did not meet the minimum requirements for either Class 4 or 5 dental ceramics. As far as posterior applications are concerned, this mean flexural strength value only meets the limited clinical indication of single-unit posterior prostheses for both monolithic ceramic and partially or fully covered substructures listed for Class 3 dental ceramics. However, when the same specimens were grouped according to their yttria content, they were clearly distinguished on the basis of the mechanical (flexural strength, TCL) and physical (percent light transmittance) behaviors.
      Furthermore, the 3Y-TZP products in this survey exceeded the minimum Class 4 requirement for a dental ceramic of 500 MPa, and the 5Y-PSZ products met the minimum requirement of 300 MPa for a Class 3 dental ceramic. In addition, a clear transition from intergranular to transgranular fracture was observed between the 3Y-TZP and 5Y-PSZ products, respectively. Incorrectly using 5Y-PSZ material, which exhibits brittle, transgranular fracture, for HS applications can lead to sudden failure of a restoration. Good communication between the dentist and the laboratory plays a vital role in ensuring that the patient receives the appropriate type of zirconia material for the clinical indication.

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