Recent Advances in Dental Zirconia: 15 Years of Material and Processing Evolution

Abstract

Objectives.

The objective was to discuss the research on zirconia published in the past 15 years to help the dental materials community understand the key properties of the types of zirconia and their clinical applications.

Methods.

A literature search was performed in May/2023 using Web of Science Core Collection with the term “dental zirconia”. The search returned 5102 articles, which were categorized into 31 groups according to the research topic.

Results.

The current approach to improving the translucency of zirconia is to decrease the alumina content while increasing the yttria content. The resulting materials (4Y-, 5Y-, and above 5 mol% PSZs) may contain more than 50% of cubic phase, with a decrease in mechanical properties. The market trend for zirconia is the production of CAD/CAM disks containing more fracture resistant 3Y-TZP at the bottom layers and more translucent 5Y-PSZ at the top. Although flaws located between layers in multilayered blocks might represent a problem, newer generations of zirconia layered blocks appear to have solved this problem with novel powder compaction technology. Significant advancements in zirconia processing technologies have been made, but there is still plenty of room for improvement, especially in the fields of high-speed sintering and additive manufacturing.

Significance.

The wide range of zirconia materials currently available in the market may cause confusion in materials selection. It is therefore mandatory for laboratories as well as for clinicians to get the needed knowledge on zirconia material science, to follow manufacturers’ instructions, and to optimize the design of the prosthetic restoration with a good understanding where to reinforce the structure with a tough and strong zirconia.

Graphical Abstract

Introduction

Fifteen years have passed since professors Isabelly Denry and Robert Kelly published two seminal literature reviews on zirconia for dental applications [1, 2] in Dental Materials (2008), based on their outstanding oral presentations at the 2006 Annual Meeting of the Academy of Dental Materials (ADM) held in São Paulo, Brazil. As of January, 2024, these two papers had received a total of 2369 citations on Scopus, an indication that these manuscripts were important starting points for a significant number of researchers interested in developing their research projects on dental zirconia.

From 2008 to 2023, dental zirconia ceramics have evolved from a material with practically just one type of composition and microstructure, namely 3 mol.% yttria-stabilized tetragonal zirconia polycrystals (Y-TZP), to a relatively complex, sometimes graded material with great compositional and microstructural variations in terms of yttria and alumina contents and grain types/sizes, which resulted in a wide range of zirconia products currently available in the market with stark differences in mechanical and optical properties.

Despite exceptional mechanical properties, the Y-TZP available in 2008 had limited clinical indications and was used mostly as a core material for single crowns and fixed partial dentures (FPD), as well as implants and abutments. Due to the relatively high opacity, this first generation of materials needed to be veneered with a feldspathic porcelain or a glass-ceramic in order to achieve acceptable aesthetic properties of the final prosthesis. The next generations of zirconia materials expanded their indications to almost all types of prosthetic restorations and can be used both in a monolithic or layered structure, due to the wide range of translucency levels and mechanical properties.

Currently, manufacturers claim that depending of the material generation, zirconia can be used to produce inlays/onlays, veneers, crowns, fixed partial dentures with short or long spans, single implant post and abutment, resin-bonded anterior all-ceramic FPDs, and full rehabilitations over implants (Brånemark protocols) [3-7]. The objective of the current literature review was to assess the articles published in the past 15 years to give an overview of the evolution of zirconia as a dental biomaterial and serve as a basis for the authors’ oral presentation at the 2023 Annual Meeting of the Academy of Dental Materials in San Diego, USA.

Analysis of a literature search using “dental” and “zirconia” as keywords

A literature search was performed in May/2023 using Endnote 20 in conjunction with the online search tool Web of Science Core Collection (Clarivate) in order to include not only dental journals but also those from other important fields like materials science and engineering. The search term selected and inserted in the “Title/Keywords/Abstract” field was “dental zirconia” with the objective of retrieving a wide range of papers dealing with zirconia for dental applications. The search returned a total of 5102 articles which were analyzed individually and categorized into 31 groups according to the study topic (Figure 1).

[Figure 1] –

Distribution by topics of zirconia papers published between 2008 and 2023.

The creation of the above-mentioned groups did not take into consideration papers that fell into the so-called “discarded” or “other topics” groups. Since the search terms were relatively broad, 778 papers were placed in the “discarded” group and were not considered for this review because zirconia was not the main object of the study or it was used for applications that were far from the oral rehabilitation field or dental materials science. Examples of those papers is the one entitled “Fracture toughness comparison of six resin composites” by Watanabe et al. [8] in which zirconia is just an additive of the glass fillers used in the resin composite. Another example was the paper entitled “Translucency and Wear of Pressable Lithium Disilicate and Zirconia-reinforced Lithium Silicate Glass-ceramics: An In-vitro Study” by Potdukhe et al. [9] which deals with zirconia-containing lithium silicate glass-ceramics. In addition, 336 papers were placed in the “other topics” group, as they were dealing with very specific aspects of this ceramic material that did not fall into the most common groups depicted in Figure 1. An example of papers categorized as “other topics” is “Reuse of yttria stabilized zirconia arising from making dental implant - characterization of materials” by Assis et al. [10], which deals the reuse of zirconia powders generated after milling CAD/CAM blocks. Another example is a paper entitled “Study on PCD tool wear in hard milling of fully-sintered 3Y-TZP ceramics” by Xu et al [11], which evaluated the wear of polycrystalline diamond (PCD) tools used to mill fully sintered zirconia blocks.

After disregarding the papers categorized as “discarded” and “other topics”, the remaining 3902 papers were sorted into the 31 categories as shown in Figure 1. In Figure 1a, it is possible to see the topics with the highest number of published papers. This information is relevant for researchers, clinicians and for the dental industry as it reveals the topics related to zirconia that were the focus of a significant number of research papers published in the past 15 years. As expected, translucent zirconia represented a main research focus of the dental community with 638 papers published in that period. In fact, since Y-TZP started to be used as a dental biomaterial, there was a claim by clinicians that it should be more translucent in order to be used in a monolithic manner and to avoid chipping of the veneering materials that needed to be applied over the zirconia core to improve the aesthetic aspect of the final restoration [12]. The microstructural evolution of dental zirconia from an opaque, single layer and core material into a translucent, multi-layer and monolithic (full-contour) material is the main focus of the present literature review. The final goal was to discuss the most relevant research on zirconia published in the past 15 years in order to help the dental materials community understand the key properties of the different types of zirconia and their clinical applications.

Historical aspects of dental zirconia

One of the first zirconia-based materials available for dental prostheses was a glass-infiltrated alumina-zirconia composite (In-Ceram Zirconia, Vita Zahnfabrik) [13]. This material was developed from a glass-infiltrated alumina (In-Ceram Alumina) by means of the addition of 33 vol% of ceria-stabilized zirconia to the alumina matrix, which could be fabricated by means of slip-casting or soft machining of pre-sintered CAD/CAM blocks [1, 14]. Both processing techniques required a final glass-infiltration step, which unfortunately resulted in significant porosity in the final structure [15]. The zirconia grains in In-Ceram Zirconia were, to some extent, protected from the ambient moisture by the surrounding alumina and glass phases, therefore avoiding significant low temperature degradation (LTD) [2]. However, this material ended up being discontinued by the manufacturer due to the development of stronger and tougher versions of zirconia, notably yttria-stabilized tetragonal zirconia polycrystals (Y-TZP).

Another type of zirconia-based material that was proposed for dental applications and was later discontinued was magnesia (MgO) partially stabilized zirconia (Mg-PSZ) for hard machining [1]. This material presented a microstructure of tetragonal precipitates in a stabilized cubic zirconia matrix (Denzir-M^®, Dentronic AB) [16]. However, its relatively high sintering temperatures and prolonged dwell times, along with intricate problems related to porosity and large grain sizes resulted in a final material with worse mechanical and optical properties compared to the zirconia-based materials that were developed later [1].

Ordinarily, the tetragonal phase of zirconia only exists between 1170°C and 2370°C, but can be retained in a metastable state via suitable metal cation doping at room temperature, as eloquently demonstrated in a groundbreaking publication by Garvie et al. in 1975, where CaO-doped zirconia retains the tetragonal crystallographic form at room temperature [17]. This metastable tetragonal phase is intriguing as it has the capability of transforming into the more stable monoclinic phase in the presence of stimuli like grinding, sandblasting, or excessive occlusal forces that generate localized stresses (tensile, shear), which concentrate at existing cracks tips or defects [18]. Since the monoclinic crystallographic form is approximately 4.5 vol% larger than the tetragonal form, such transformation results in the beneficial compressive stresses that increase crack propagation resistance [17-19]. In addition, “twinning” and/or microcracking may form to accommodate the volume-and-shape change associated with the transformation, reducing the constraint imposed on the material [20]. Although many dopants like CaO, MgO, CeO₂ at varied concentrations have been proposed, the most prevalent dental zirconia is the one doped with 3 mol% of yttria [21].

The use of zirconia in Dentistry has become increasingly popular, since the initial introduction of 3Y-TZP and nowadays with increased yttria content for more translucent zirconia [22]. The t-m phase transformation toughening effect is maximized by the amount of tetragonal phase. As predicted by the phase diagram of the yttria-zirconia system, commercially available 3Y-TZPs may also contain up to 15% of cubic phase [3], something that will be further discussed in this review.

The presence of this unique toughening mechanism makes Y-TZP a smart material, which by definition is a material having one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, electric or magnetic fields [23]. According to Chevalier et al. [19], Y-TZP ceramics have the best combination of toughness and strength compared to other versions of the stabilized zirconia. In addition, due to the inert nature of zirconia grains, this material presents very high biocompatibility, an attractive feature for biomaterials used in oral rehabilitation [24].

Powder technology and compaction/consolidation methods

Most dental Y-TZPs are produced from high-purity powders usually obtained by co-precipitation techniques that guarantee homogeneous yttria distributions throughout the starting powder [25]. Uniform submicrometer particle size powders are currently produced by companies like Tosoh and Noritake in Japan, and Metoxit AG in Switzerland [19, 26]. These powders have high sinterability, resulting in final components with density near the theoretical value [25]. Powder synthesis results in the formation of granules consisting polycrystalline primary particles which are the smallest units in the powder with a clearly defined surface. These granules contain pores located inside the primary particles (isolated) and among different primary particles (continuous) or agglomerates [27].

The processing steps to produce a zirconia restoration are directed to eliminate the powder porosity to achieve the highest density possible. These steps involve powder mixing, addition of binders, consolidation (by dry-pressing, axial or isostatic), debinding/green body formation (pre-sintering heating cycle), and finally sintering, which will result in the final microstructure [28]. A wide variety of Y-TZP powders have been made available by manufacturers with different particle sizes, which are usually consolidated into the shape of CAD/CAM block or disc (puck) that are pre-sintered, milled, and then fully sintered to form the final dental prosthesis [29].

The compaction/consolidation step of the powder is very critical and may result in the creation of flaws that, if located in areas of high tensile stresses in the final restoration, may result in premature failure [30]. Figure 2 shows a partial view of a fracture surface of the distal connector of a three-unit monolithic zirconia FPD (Katana STML, Kuraray Noritake Dental Inc, Japan). A ‘large’ bulk defect is clearly evident (Figs. 2a and b). Detailed SEM examination revealed free-air sintered grains on the side walls of the open crack (Fig. 2c), indicating that the crack was there in the green state, presumably induced from the powder compaction and biscuit firing processes. This bulk defect was, however, not related to the fracture origin. Figure 3 shows a much smaller defect on another location of the same fracture surface of the distal connector, in which the free-air sintered zirconia grains are clearly visible. Again, the location of this powder compact defect is not related to the fracture origin.

[Figure 2]:

Scanning electron microscopy (SEM) views of a bulk defect in the fracture surface of the distal connector of a three-unit monolithic zirconia bridge (Katana STML, Kuraray Noritake). (a) and (b) Lower magnification overviews of the bulk defect, and (c) a higher magnification image of the defect, showing free-air sintered zirconia grains inside the open crack. Note that this bulk defect was not related to the fracture origin.

[Figure 3]:

Same Katana STML three-unit bridge distal connector fracture surface at a different site. (a) A very small bulk defect, and (b-c) detailed SEM analysis revealed free-air sintered zirconia grains inside the defect. Again, this defect is not related to the fracture origin.

The compaction behavior of the powder is influenced by granule morphology, particle size, type and amount of binders [28]. Cold isostatic pressing (CIP) at approximately 200 - 250 MPa is the most commonly used method by dental companies to compact zirconia powders. In the CIP process, a certain amount of zirconia powder is sealed in a flexible rubber mold and isostatically pressed in hydraulic oil in a pressure vessel [31]. One should keep in mind that although most companies obtain the zirconia powder from the same manufacturer, each company is responsible for compacting the powders and producing their own blanks. Thus, variations in the quality of the green compacts should be expected [32].

Unfortunately, zirconia powders are prone to agglomeration because of the high surface energy generated from the large specific surface area of the individual particles [33]. If the powder agglomerates are not completely compacted during CIP, voids will remain in the green body and may not be eliminated by sintering [34], resulting in poorly densified areas in the microstructure of the sintered material (Figure 4).

[Figure 4]:

Examples of powder compact related defects resulting in (a) poorly densified areas, and (b) flaws in the microstructure of sintered zirconia, which ultimately acted as fracture origins in fatigue fractured specimens [34].

The pressure used during CIP is a key factor to achieve the maximum density in sintered zirconia specimens [31]. Previous investigations have shown that homogenous packing of zirconia specimens cannot be easily achieved due to problems in powder flowability, and once the voids are formed during the pressing step, they may be difficult to heal completely from sintering [35]. At a manufacturing plant, agglomeration of zirconia powders can be improved by means of dispersion of primary material particles through specific processing involving chlorination, hydrolysis, drying, calcination, milling, and spray drying to form the granules, and the addition of binders to improve powder compatibility in a dry state. In addition, dense zirconia blocks can be fabricated through colloidal processing (dispersion techniques), such as: pH modification, optimizing the dispersant amounts, and applying mechanical treatments (ultrasonication, ball milling, and centrifugation) [36].

Sintering techniques

The sintering step of dental zirconia restorations takes place in furnaces where densification of the piece occurs via reduction of the porosity and junction of the powder particles [37]. Manufacturers will recommend specific sintering temperatures and times according to the powder used for block fabrication, and these parameters will ultimately determine the distribution of grain sizes in the final microstructure, with grains becoming larger at higher sintering temperatures and longer dwelling times [38]. Excessive grain growth can also occur in the presence of small amounts (traces) of CaO as have been evidenced by EDS on the microstructure of dental 3Y-TZP [30, 34].

The sintering step of Y-TZP is a relatively time-consuming process with slow heating and cooling rates of 5°C to 10°C per min and a long dwell time, resulting in a sintering time of several hours (8 to 12 hours) [1, 39, 40]. Dental restorations made of Y-TZP are usually subject to maximum sintering temperatures varying from 1350 and 1600°C and dwell times between two and five hours. Therefore, chair-side, one-visit restorations are not feasible using these schedules [41].

Shorter sintering schedules have been made available in specific furnaces (for example, SpeedFire, Dentsply Sirona) in order to allow for the production of chair-side zirconia restorations in one session [42]. Total sintering times were reduced to 25 minutes (holding time at maximum temperature of 2 minutes), with an additional 9 minutes firing cycle for the glaze layer, whereas the maximum sintering temperature was increased by 70°C compared to longer sintering cycles [41]. The literature reports promising results in terms of mechanical properties for dental zirconias processed via speed sintering. For example, the work by Ersoy et al. [40] reported significantly higher flexural strength values for a 25 minutes sintering cycle compared to the strength obtained used the 120 min cycle. The better mechanical results for speed firing were associated with smaller grain sizes and the fact that longer dwell times favor grain growth. Another work by Kaizer et al. [43] also showed smaller grain sizes for speed sintering relative to the longer firing schedules, which yielded promising results in terms of mechanical and wear properties of monolithic zirconia restorations. However, high-speed sintering protocols have a tendency of leaving isolated tiny pores in the microstructure, resulting in light scattering and thus compromising zirconia translucency but not the strength of zirconia with various yttria concentrations [44].

Microwave sintering has been proposed as an alternative to conventional sintering of zirconia as it results in uniform heat distribution in the prosthetic piece and lower production cost due to lower energy consumption associated with the fact that the great majority of the electromagnetic energy is converted into heat within the zirconia body [45-48]. In fact, there is evidence that microwave sintering results in significant sintering time reduction (6 times faster) relative to conventional sintering without compromising the structural reliability of the specimen [47, 48]. However, a decrease in the translucency parameter from 29 to 13 was reported when the zirconia specimens were microwave sintered and was attributed to slightly higher porosity leading to increased light scattering within the material [47].

Post-sintering treatments to improve densification

After a zirconia prosthesis is sintered, additional heat treatments still can be carried out with the objective of improving its mechanical reliability. Post-sintering hot-isostatic pressing (HIP) has been proposed as a healing treatment for process related defects and laboratory grinding damage of zirconia prostheses [49]. HIP is often used in the ceramic industry and involves heating the sintered piece to temperatures 100 to 200°C lower than the maximum sintering temperature using isostatic gas (argon or nitrogen) pressure (typically 100 to 300 MPa). Such pressurized heat treatment will trigger mass transport mechanisms such as grain boundary sliding, plastic deformation and diffusion-controlled creep to improve densification, crack healing, and pore shrinkage [50].

There are reports in the literature showing improvements in ceramic pieces after HIP was applied, like significant increases in the fracture resistance of 3Y-TZP intracanal posts [51], increase in the fracture toughness and modulus of elasticity of zirconia-toughened alumina ceramics [52], and increase in the flexural strength and thermal shock resistance of a ceria-doped tetragonal zirconia and alumina composites [53]. However, the most recent findings regarding HIP of dental zirconia showed that, though a small additional densification of 0.1% was obtained after post-sintering HIP of a 3Y-TZP, this procedure was not able to close any existing critical processing subsurface flaws in the size range of 10 to 60 μm, which resulted from incompletely fractured agglomerates pressed by CIP of the green zirconia body. No healing effect was observed regardless of the temperature or pressure used (1350°C or 1400°C at 195 MPa) [49].

Microstructure of Yttria-stabilized Tetragonal Zirconia Polycrystals with high alumina content

The usual microstructure of dental Y-TZP is composed of equiaxed grains of tetragonal phase sintered to >96% of their theoretical density [2]. Grain sizes are kept in the range of 0.2 to 1 μm [26], as larger grains result in instability of the tetragonal phase [54]. Another important component present in dental Y-TZP is alumina, which is usually added to the starting powder at a concentration of 0.25 wt% [3]. The presence of small amounts of alumina results in increased sinterability and therefore higher final density of the component [55]. In addition, alumina has been proven to effectively prevent ageing in Y-TZPs [19, 56]. Coloring of Y-TZP is usually adjusted by doping with small amounts of iron (e.g., Fe₂O₃) or rare-earth oxides like Er₂O₃ and the final total transmittance of dental Y-TZPs is usually in the range of 12% to 15%, which is considered relatively low in comparison to other aesthetic ceramic dental restorative materials like feldspathic ceramics and glass-ceramics [26, 55].

Decreasing the grain sizes of Y-TZPs to the nanometric level is a means of improving both the mechanical properties and the translucency of these materials. Although zirconia-based ceramic materials with grain sizes greater than 100 nm (sometimes equal to 500–600 nm) are incorrectly defined as nanomaterials, few studies [57, 58] were successful in obtaining fully dense nanostructured zirconia ceramics (grain sizes below 100 nm) since it is very difficult to retain the microstructure on a real nanometric level [59].

In fact, the high surface area of the powder results in very high reactivity, which is responsible for the agglomeration of nanoparticles that causes the emergence of large pores with inhomogeneous particle distribution, leading to low density in the green compacts and in the final product. Therefore, the larger the agglomerate size, the higher the sintering temperature required to obtain fully dense materials. As previously mentioned, increasing the sintering temperature leads to grain growth, and again the final microstructure goes back to a micrometric level once the full density is reached [60]. According to Arena et al. [59], retaining the microstructure at a nanometric level and, at the same time, avoiding grain coarsening, is a bottle neck encountered in the processing of this class of materials.

One commercial example of zirconia/alumina nanocomposite is the ceria-stabilized material called NanoZR produced by Panasonic Healthcare (Japan), launched at the end of the 90s. This material was proposed as an alternative to Y-TZP to construct dental prostheses [61] and was composed of a ceria-stabilized zirconia matrix (average grain size of 490 nm) and 30 vol% of alumina as a second phase. In this unique microstructure, nanoscale alumina particles (10 to 100 nm) are trapped within the zirconia grains, and several 10 nm-sized zirconia particles are also trapped within the alumina grains [59, 62, 63]. This microstructure has been claimed to have better mechanical properties [63] and higher LTD resistance in comparison to Y-TZP. Clinical trials showed good survival rates for prostheses produced with this material [64, 65]. Another nanometric zirconia available in the market is ZrHP-nano (ProtMat, Brazil), which is a 3Y-TZP with average grain size of 150 nm [59].

3Y-TZPs with low alumina content for improved light transmittance

One of the most important microstructural evolutions of dental zirconia was the development of novel zirconia-based materials with higher translucency for monolithic applications [12]. In the current literature review, the search on Web of Science using the terms “dental zirconia” identified as the first paper dealing with the increase in light transmittance of Y-TZP the one entitled “The Technology of Improving the Optical Property for the Zirconia Dental Ceramic” by Jiang et al [66]. These authors dealt with the difficult task of increasing the transmittance of dental Y-TZP by means of increasing the sintering temperature. In the following years, the industry focused in developing more translucent zirconia materials, what is reflected in the significant number of papers published in past 15 years on this topic (638).

A major driving force that led to the development monolithic/translucent zirconia was the relatively high chipping rates of the veneering ceramic layer applied over Y-TZP frameworks [67, 68]. This problem has been associated with the inherently low fracture toughness of the veneering ceramic materials in conjunction with residual tensile stresses generated during cooling of these bilayers [69, 70]. Another drawback of bilayered restorations is the need of excessive tooth reduction during preparation in order to create enough space for two different ceramic materials [71].

Light scattering within the microstructure is responsible for diminishing the translucency of the first-generation dental zirconia. In the case of Y-TZP, several factors cause light scattering, like pores, defects, additives, grain boundaries, and the birefringence nature of tetragonal crystals, which have anisotropic refractive indices in different crystallographic directions [72].

One possible way to increase the translucency of Y-TZP is decreasing the grain sizes to the nanometric level, which will increase the total transmittance of the material [59]. An elegant analysis carried out by Zhang [72] indicated that transmittance levels similar to those observed in glass-ceramics could be achieved by zirconia microstructures with mean grain sizes around 82 nm (for 1.3 mm thickness), 77 nm (for 1.5 mm), and 70 nm (for 2 mm). Nevertheless, as previously mentioned, developing Y-TZP with nanometric microstructure is both technologically challenging and economically unsustainable. Thus, very few manufacturers have tried to pursue this route.

A traditional way in materials science to decrease grain-boundary light scattering is to increase grain sizes, which in turn, diminishes light scattering due to the fact that the light beam encounters fewer grain boundaries when travelling through the material. However, it is also well-known that larger grains will also decrease the strength of the ceramic material [73]. In addition to that, in the specific case of Y-TZP, grains that are larger than 1 μm could transform spontaneously from tetragonal to monoclinic during cooling from the sintering temperature, generating internal cracks that can seriously comprise the strength of the material [38]. So, the industry has also avoided this type of approach to increase the light transmittance in dental Y-TZPs.

Given that the main light scattering centers in Y-TZP are the alumina particles deposited in the zirconia grain boundaries [18], the initial approaches aiming at increasing light transmittance in Y-TZPs included reduction of the alumina content from 0.25% to below 0.05% and increasing the sintering temperatures to eliminate porosity. The powder developed by Tosoh with reduced alumina content was called Zpex and kept the yttria content at 3 mol%. In this way, zirconia blocks produced with this powder showed an improvement of up to 28% in light transmittance for specimens with thickness of 1 mm in comparison to the first generation of Y-TZP with higher alumina content [3, 7, 74, 75]. On the other hand, the high mechanical strength of Y-TZP remained the same (around 1000 - 1200 MPa) in this new and more translucent material [74].

The modest improvement in light transmittance was not considered enough by clinicians in order for them to trust this material for highly aesthetics indications using monolithic structures [6, 76], and therefore further developments in the microstructure of Y-TZP were considered necessary to achieve materials with even higher translucency. Examples of commercial brands of this generation of 3Y-TZP with lower alumina content are: Lava Plus (3M, USA), Cercon ht (Dentsply Sirona, Germany), Vita YZ HT (Vita Zahnfabrik, Germany), Prettau Zirconia (Zirkonzahn, Italy), Aadva EI (GC, Japan), Ceramill Zolid (Amann Girrbach, Austria), IPS e.max ZirCAD MO/LT (Ivoclar Vivadent AG, Lichtenstein), Zenostar (Wieland Dental, Germany) [3, 77].

Partially Stabilized Zirconia: 4Y- and 5Y-PSZs

Since changing the alumina content and reducing porosity was not sufficient to fulfil the demands of clinicians in terms of translucency for Y-TZPs, the industry tried other approaches to obtain improvements in the transmittance of dental zirconia [78]. Additional increases in the size of tetragonal grains to increase light transmittance were not possible as well because these would become unstable [54]. So, the solution was to play with the phase diagram of the zirconia-yttria system, which indicates that increasing the yttria content in the range between 3 and 8% results in a material with tetragonal and cubic phases present at room temperature [19]. Unlike to the anisotropic tetragonal crystals, the cubic ones are optically isotropic and therefore do not display high scattering at grain boundaries. On the other hand, cubic grains cannot deliver the same transformation toughening mechanism seen in tetragonal grains and therefore the final mechanical properties of this new generation of translucent monolithic zirconia are significantly reduced [3].

Doping zirconia with yttria at small concentrations, i.e., around 3 mol%, will result in stabilization of the tetragonal crystals, and also of a low percentage of cubic crystals (around 6-15%) at room temperature. Such stabilization has been explained by mechanisms related to the fact that the Zr⁴⁺ ions with smaller radius (0.82 Å) are replaced by slightly larger Y³⁺ ions (0.96 Å), which will disfavor the monoclinic phase, stabilizing more symmetric structures with cubic and tetragonal symmetry. The stabilization mechanism occurs via a decrease in “oxygen overcrowding” around zirconia cations, either through the introduction of oxygen vacancies in the unit cell or through the expansion of the cations lattice [79]. The oxygen vacancies are created within the unit cell for charge compensation [80, 81], i.e., to maintain charge neutrality, an oxygen vacancy needs to be created to compensate the negative charge induced by every two Y³⁺ ions substituting for a pair of Zr⁴⁺ ions [75].

Increasing the yttria concentration between 3 and 8 mol% will cause significant changes to the microstructure of zirconia. Above the 3 mol% level, more cubic phase crystals start to be stabilized in relation to the tetragonal phase. In addition to that, the increased yttria content causes important changes to the lattice parameters of the tetragonal grains, which show a decrease in their tetragonality ratio (c/a ratio) [75]. These parameters become then closer to that of cubic zirconia. There are reports indicating that some of these grains lose their ability to transform into the monoclinic form upon stress concentration; therefore, they are named non-transformable tetragonal crystals [82]. As for the volume of the cubic and tetragonal unit cells, both increase with the increase in yttria up to 8 mol% and, after this threshold, the material becomes entirely cubic and the volume of the cubic unit cell keeps increasing with the increase in yttria content [75].

Zirconia ceramics with increased yttria content and a microstructure displaying both the tetragonal and the cubic grains are called Partially Stabilized Zirconia (PSZ) and considered by many authors as the third generation of dental zirconia [3, 6]. Currently, the yttria percentage seen in commercial materials varies from 4 to 6 mol% and correlates with the percentage of cubic grains present in the final microstructure [77].

The industry started commercializing 4Y- and 5Y-PSZ materials between 2014 and 2016. The powder containing 4 mol% of yttria was called Zpex4 (Tosoh, Japan), displayed a low amount of alumina (0.05 wt%) and had a significantly larger content of cubic grains (25 to 30%), with the remaining microstructure being composed of tetragonal grains [82, 83]. The exchange of tetragonal grains to the cubic results in an approximately 10% increase in translucency in comparison to Y-TZPs with low alumina content, and the mechanical strength decreased from 1000-1400 MPa in Y-TZPs to 900-1000 MPa in 4Y-PSZs. Commercial examples of 4Y-PSZs are: Katana Zirconia HT (Kuraray Noritake, Japan), ceramill zolid HT+ (Amann Girrbach, Austria), IPS e.max ZirCAD MT (Ivoclar Vivadent, Lichtenstein), Zenostar MT (Wieland Dental, Germany), and Vita YZ ST (Vita Zahnfabrik, Germany) [3, 6, 7, 77].

5Y-PSZs are even more translucent materials due to the addition of 5 mol% of yttria, resulting the powder called Zpex Smile by Tosoh. 5Y-PSZ still have low alumina content, and around 50 – 80% of cubic grains, with the remaining composed of tetragonal crystals. The changes in optical and mechanical properties for 5Y-PSZ in comparison to Y-TZPs with low alumina content were more drastic with an increase in translucency of almost 30% but a significant drop in strength of 50%, with flexural strength values reported for these materials in the range of 600 to 800 MPa [3, 6, 7, 72, 77]. Commercial examples of this class of material are: Aadva NT (GC, Japan), Prettau Anterior (Zirkonzahn, Italy), Vita YZ XT (Vita Zahnfabrik, (Germany), Ceramill Zolid FX (Amann Girrbach, Austria), and Cercon XT (Dentsply Sirona, USA). It is important to note that all above-mentioned ranges of flexural strength values are taking into account the fact that these mean values were obtained from studies using different starting compositions, sintering temperatures, testing methodologies and varied surface states of the specimens. Therefore, significant variations in the strength values are expected among works.

A least two studies [75, 84] compared varied properties of 3Y-TZP with low alumina content, 4Y-PSZ and 5Y-PSZ in a controlled manner, using the corresponding powders produced by Tosoh (respectively Zpex, Zpex4, and Zpex Smile). The results are interesting and helpful in understanding the evolution of these materials in terms of mechanical and optical properties. The work by Zhang et al [84] reported tetragonal/cubic ratio of 87/13, 63/37, and 42/58 for specimens produced with Zpex, Zpex4, and Zpex Smile, respectively. Mean grain sizes were larger for Zpex Smile (0.53 μm) and decreased with the yttria content: 0.36 μm and 0.30 μm for Zpex4 and Zpex, respectively. Hardness (around 12.8 GPa) and elastic moduli (around 205 GPa) did no vary significantly among these three materials. Flexural strength (MPa, 4-point bending) and fracture toughness (MPa.m^1/2, double torsion method) values were: 908 and 5.1 for Zpex, 928 and 4.1 for Zpex4, and 534 and 3.2 for Zpex Smile, respectively. The Weibull moduli of the specimens decreased significantly with the increase in yttria content from 9.2 for Zpex to 4.9 for Zpex Smile, with Zpex4 showing an intermediate value of 6.3. In terms of the translucency and contrast ratio varied significantly among the compositions regardless of the thickness (0.5 to 1.0 mm). For 1.0 mm-thick specimens, the contrast ratio increased from 0.45 (Zpex Smile) to 0.65 (Zpex), with Zpex 4 showing CR of 0.59. For 0.5 mm-thick specimens the contrast ratio increased from 0.36 (Zpex Smile) to 0.54 (Zpex), with Zpex4 showing CR of 0.47. Another study [82] testing commercial zirconias with varied yttria contents (from 3 to 5 mol%) found mechanical properties in the same range reported by the above-mentioned study for the corresponding yttria contents.

The work by Lim et al. [75], besides characterizing the same powders described above, added information regarding the effect of variations in the sintering temperature on the material microstructure. The results of this investigation showed that near-full density (~99%) was achieved for all three powders (Zpex, Zpex4, and Zpex Smile) over 1350°C. All three powder compositions showed a significant increase in cubic phase after the temperature of 1350°C was reached. As for grain sizes, above 1300°C, an accelerate growth is noted up to 1550°C, with grains approaching the size of 1 μm at the highest temperature. Only for Zpex smile, sintering at 1550°C caused grain growth up to 3 μm. Flexural strength (biaxial) did not show a significant dependence on temperature over 1350°C, however the sintering temperatures affected significantly the translucency parameter for all compositions. In fact, a TP above 20 could only be reached over temperatures of 1400°C. Moreover, TP continued to increase with sintering temperatures up to 1550°C, especially in Zpex Smile specimens. This is consistent with the recommendations of dental manufacturers, that their zirconia products should be sintered above 1400°C.

Partially Stabilized Zirconia above 5 mol% of Yttria

Some zirconia manufacturers have their own in-house production of zirconia powders, such as Kuraray Noritake (Japan), and took PSZ one step further, developing materials above 5 mol% of yttria (commercial name: Katana Zirconia UT), resulting in prosthetic devices with up to 60% increase in translucency in comparison to Y-TZPs with low alumina content, but a higher drop in strength, reported to be in the range of 500 to 600 MPa [77]. This class of material has been marketed as the first zirconia that has comparable optical and higher mechanical properties relative to low translucency lithium disilicate glass-ceramics (for example, e.max CAD - LT) [85].

In 2021, an ultra-high translucent above 5 mol% was released to the market (Shofu Disk ZR Lucent Ultra, Shofu, Japan) with higher translucency compared to the previous materials above 5 mol%, but with flexural strength around 750 MPa, i.e., at same level as that reported for 5Y-PSZs [85]. The high amount of larger cubic phase in this new material is responsible for the higher transmittance, and the higher strength in comparison to previous materials above 5 mol% is due to two distinct processing methods used to fabricate these CAD/CAM blanks. One of the changes in the processing method is the use of cyclic cold isostatic pressure methodology, in which the pressure applied to the powder varies over time, guaranteeing denser and more homogeneous compacts [86]. The other technology is called “Plus Y” and consists of immersing the semi-sintered blocks in an yttrium-containing solution, increasing the total yttria concentration at the particles’ surface with subsequent formation of tetragonal grains in the same amount as observed for 5Y-PSZs and cubic grains that are larger than those overserved for other materials above 5 mol%. This combination results in a material above 5 mol% with higher translucency than that observed for previous versions, and similar strength to that reported for 5Y-PSZ materials [85].

Adding color gradients to Y-TZPs and PSZs to produce polychromic materials with uniform composition

All previously described translucent zirconia materials (low alumina 3Y-TZP, 4-, 5-, and above 5 mol%) were initially marketed as monochromatic CAD/CAM blocks and disks with uniform composition. Due to the large acceptance by clinicians of these translucent zirconia materials for the production of monolithic dental prostheses, the next step of the dental industry was the development of blocks with uniform composition, but a color gradient by means of adding varied amounts of pigments to the different layers of the CAD/CAM blocks [87-89]. These multi-shaded materials transformed the use of zirconia from just a core material that needed to be veneered into a multi-application material that could be used in a monolithic manner with the help of a glaze layer and staining to further improve the final aesthetic result [7]. Glaze layers, however, tend to wear out over time and expose the underlying zirconia grains, as shown in Figure 5.

[Figure 5]:

SEM micrographs of the occlusal surface of a monolithic zirconia molar (Katana STML) bonded to an implant-supported titanium base after 10 months of intraoral chewing. (a) shows the worn surface with the glaze microcracking away exposing the zirconia grains (b).

One should be aware, however, that adding coloring agents may affect the mechanical properties of the created layers, since they may act as impurities in the sintered material. Zirconia can be colored by means of mixing metal oxides to the starting powder or by infiltrating the machined restoration at the pre-sintered stage with chloride solutions of rare earth elements [90]. Oxides like CeO₂, Fe₂O₃, and Bi₂O₃ are added to the coloring liquids by the manufacturers to obtain different shades [91].

Using coloring metal oxides to obtain various shades in zirconia restorations has the potential to change the crystallography and microstructure of the material, leading to changes also in the mechanical properties. In the work by Shah et al. [92], 3Y-TZP specimens were colored by immersion in cerium acetate (CA), cerium chloride (CC), or bismuth chloride (BC) solutions at 1, 5, or 10 wt%. Their results indicated significant increases in grain size and decrease in strength for the highest concentration of colorants, and are in accordance with other studies that showed the same trend [91]. It also became evident that the chloride solutions evaporate during sintering resulting in significantly higher porosity levels, with some of the specimens showing 10 times more porosity compared to the control group. Another work [93] showed that immersing zirconia bridges in a coloring liquid for prolonged time (e.g., 6 minutes) significantly reduced the measured fracture load compared to the control group. There are, however, other evidences indicating that coloring oxides have no effect on the flexural strength of zirconia specimens [94, 95].

Examples of commercial brands of these polychromic zirconia with uniform composition are: a) high alumina 3Y-TZP (Prettau 2 Dispersive, Zirkonzahn); b) 4Y-PSZ (Katana Zirconia ML, Kuraray Noritake; Vita YZ ST Multicolor, Vita Zahnfabrik; Ceramill zolid gen-x, Amann Girrbach; Shofu Block Zr Lucent, Shofu); c) 5Y-PSZ (Katana STML, Kuraray Noritake; Ceramill zolid fx multilayer, Amann Girrbach; Cercon xt ML, Dentsply Sirona; Vita YZ XT Multicolor, Vita Zahnfabrik; Prettau 4 Anterior Dispersive, Zirkonzahn; Lucent FA, Shofu); and d) Above 5mol% (Katana UTML, Kuraray Noritake) [77]. To illustrate the microstructural differences with increasing mol% of Yttria, Figure 6 shows the increase in grain sizes of the microstructure as seen under a SEM at 10000 x of Katana ML (a), Katana STML (b), and Katana UTML (c).

[Figure 6]:

SEM images at 10000 x of commercial polychromic zirconia with uniform composition from Kuraray Noritake. (a): Katana ML (3.7 mol% yttria); (b) Katana STML (4.8 mol% yttria); and (c) Katana UTML (5.4 mol% yttria). The exact mol% were obtained from a previous study [89].

PSZs with uniform composition either mono or polychromatic have varied clinical indications. 5Y and above 5 mol% PSZs, due to the significant drop in strength have been indicated for single crowns in both the anterior and posterior areas in a monolithic form, where the aesthetic result is mandatory. 4Y-PSZs have a better compromise in terms of strength and optical properties and therefore have been indicated for both crowns and bridges [6, 7, 77].

Composition-gradient zirconia, multilayered PSZs

The latest developments in the area of dental zirconia brought truly graded materials to the market. This evolution was achieved means of creating gradients, transition zones or layers of zirconia powders with different mol% of yttria in order to obtain a final material with variations in both translucency and strength [96]. This approach is significantly different from the one used in the previously mentioned multi-shaded blocks, which have polychromic layers but uniform composition [87].

The so-called fourth generation of zirconia do have polychromic multiple layers, but the layers also have different microstructure, transmittance, and mechanical properties. So, on one side of the zirconia blank, the top layer is composed of more translucent portions for incisal edges and cusps, whereas across the CAD/CAM block, the zones in the middle and at the bottom become gradually more opaque as they move to 3 mol% of yttria to provide higher strength [6]. Often three layer combinations or a composition-gradient blank with three zones have been made available in commercialized CAD/CAM blocks/disks. Hence, a bottom layer of 3Y-TZP, a middle layer of 4Y-PSZ, and a top layer of 5Y-PSZ is described for Prettau 3 Dispersive (Zirkonzahn), Lucent Supra (Shofu), Cercon xt ML (Dentsply Sirona), Katana YML (Kuraray Noritake). Other options are two layers with a bottom layer of 3Y-TZP and top layer of 4Y-PSZ (Sakura ZR, Straumann), or a bottom layer of 4Y-PSZ and a top layer of 5Y-PSZ (IPS e.max ZirCAD MT Multi, Ivoclar). However, a true gradient technology (and not a layering technology) is provided by Ivoclar with a product called IPS e.max ZirCAD Prime for which the bottom to middle of the block is a 3Y-TZP composition, followed by a gradient transition zone with yttria content moving gradually with a small transition zone, at 2/3 of the block height, to a composition of 5Y at the top providing higher translucency, but lower strength [85]. Figure 7 is an illustration of such a gradient disk (IPS e.max ZirCAD Prime) for which the bottom portion is a 3Y (Fig. 7a) and the top part, a 5Y (Fig. 7b) containing much larger (i.e., more translucent) zirconia grains.

[Figure 7]:

SEM images of the microstructure at 10000 x of a composition-gradient 3-5Y zirconia (IPS e.max ZirCAD Prime, Ivoclar) block. The bottom part illustrates the 3Y (a) and the top part shows the 5Y zirconia with large translucent zirconia grains (b).

In these multi-layer materials, the bottom layer is meant to give higher strength (and toughness) to the prosthetic device, which usually experiences tensile stresses at the bottom near the gingival area due to bending forces. Therefore, the clinical design and positioning of the part inside the block should be made with a good knowledge of the chosen zirconia material and follow manufacturer’s recommendations. Nevertheless, it is important to understand that the failure due to fracture of zirconia prostheses is governed by critical flaws (which may act as crack origins). These may be created by the machining or by post-processing (often grinding adjustments) and may be located in different areas depending on the processing method. In addition, the stress concentration on these prosthetic devices will depend on the occlusal loading, the design of the prostheses, the population of defects (intrinsic or extrinsic) and their location as well as the type of zirconia at the fracture origin (3, 4 or 5Y). Therefore, predicting the clinical outcome in terms of fracture is much more complex for these new graded materials.

Recent publications have shown that the position of the prostheses within the multilayer blocks or disks in the CAD software may affect the fracture force of the final restoration [97, 98]. An important consideration regarding these graded CAD/CAM blanks is the fact that the thickness of each layer varies significantly among the different manufacturers. For example, for Cercon ht ML, the stronger dentin/intermediate layers have 16 mm in thickness versus 1.3 mm for weaker enamel layer and for Katana YML, the dentin/intermediate layer has 12 mm in thickness versus 6.0 mm for the enamel layer. Therefore, positioning of the prosthetic piece into the block design is a very critical step [87].

For all polychromic zirconia ceramics, having either uniform or hybrid compositions, it is important to be aware that their layered manufacturing technique compromises the strength of the final prosthetic piece produced, with a reported reduction in strength of ~30% in some cases due to weak interfaces, in comparison to the strength of the individual layers. Multi-layer CAD/CAM blocks are fabricated by pressing in increments of different zirconia powders into one metallic die. Such pressing technique may result in interfacial defects, such as large impurities and/or poorly sintered agglomerates [99].

Additive manufacturing of zirconia restorations

More recently, additive manufacturing (AM) of zirconia restorations, also referred to as solid freeform fabrication, rapid prototyping or 3D printing, has been proposed as a promising alternative to subtractive manufacturing due to the significant reduction in material waste and energy consumption. 3D printing is based on a computer generated design file (usually, standard tessellation language, STL), which is virtually sliced into many horizontal two-dimensional layers for printing [100]. Then, an AM machine (3D printer) generates the tool-path along the x and y directions to build the prosthetic restoration by means of a layer-by-layer technique, in which each layer is deposited one on top of the previous one, until the final object is fully constructed [101].

Of the AM techniques available for printing ceramic materials, vat polymerization methods (Stereolitography, SLA, and Direct Light Processing, DLP) are by far the most popular ones because they result in final products with higher degree of consolidation and lower risk of cracking. In addition, these indirect methods use gradual heating protocols that avoid thermal shock [102]. SLA and DLP methods involve the production of preliminary structures in a green state that are built in the 3D printer from a mixture of zirconia powders with organic binders. Binders are necessary to make the ceramic slurry susceptible to light curing of each layer deposited by the printer. Therefore, the following step after the printer produces the object is a debinding firing cycle at low temperatures to eliminate all the binder. The temperature used for debinding is usually in the range of 200 to 500°C, and involves low heating rates, which lead to long debinding times. This step is very sensitive and all parameters must be chosen carefully, as cracks or delamination may occur due to the vaporization of organic products when shorter times are used. Plasticizing agents have been added to the initial slurry to avoid these problems [103]. The following step is the sintering at temperatures that produce high densities (1450 to 1600°C).

The current review found 68 papers dealing with 3D printing of zirconia, and not surprisingly, the majority of work reported on SLA or DLP processing, which involved curing of consecutive layers of a photosensitive polymeric binder mixed with zirconia powder. The printable slurry is usually composed of multiple acrylates and methacrylates, zirconia micro or nanoparticles, a suitable photo-initiator and dispersant agents, resulting in an homogenous hybrid sol that can be photocured by a digital light processing 3D printer [104].

There are literature reviews indicating that both SLA and DLP techniques are already able to produce prosthetic devices with high accuracy, relatively smooth surfaces, and fine details [102]. However, most of the works that evaluated flexural strength found significantly lower values for DLP and SLA compared to traditional subtractive techniques, which was associated with the observed defects in the 3D printed zirconia specimens, like pores, cracks, fractures at layer interfaces and surface defects resultant from the process of separating the prosthetic device from the building platform [104-109]. Also, scattering at the interface between layers has been reported as a factor leading to higher opacity for 3D printed zirconia prostheses in comparison to their milled counterparts [109, 110]. Therefore, most authors have concluded that the final products obtained currently by additive manufacturing still fall behind those produced by the subtractive methods and more research and technological developments are necessary to improve this technology and make it available for clinical use. An alternative to the previously mentioned methodologies is Selective Laser Sintering (SLS). A previous work used SLS associated with pressure slurry infiltration (PI) and warm-isostatic pressing (WIP) to improve both the green composite and the sintered ceramic densities. The combination of PI/WIP allowed the production of complex and crack-free Y-TZP pieces with a density of 85% [111]. However, a major limitation of SLS is poor shape accuracy [112]

Concluding remarks

Based on the discussions presented in this paper on Recent Advances in Dental Zirconia: 15 Years of Material and Processing Evolution we can conclude that:

• The current approach to improve translucency of zirconia is to decrease the alumina content while increasing the yttria content.

• The resulting materials (4Y-, 5Y-, and above 5 mol% PSZs) may contain more than 50% of cubic phase, with significant decrease in mechanical properties.

• The market trend for zirconia materials is the production of composition-gradient materials with CAD/CAM disks containing more fracture resistant 3Y-TZP at the bottom layers and more translucent 5Y-PSZ at the top.

• Although flaws located between layers in multilayered blocks might represent a problem, newer generations of zirconia layered blocks appear to have solved this problem with novel powder compaction technology

• The wide range of zirconia materials currently available in the market may cause confusion in materials selection. It is therefore mandatory for laboratories as well as for clinicians to get the needed knowledge on zirconia material science, to follow manufacturers’ instructions, and to optimize the design of the prosthetic restoration with a good understanding where to reinforce the structure with a tough and strong zirconia.

• Significant advancements in zirconia processing technologies have been made, but there is still plenty of room for improvement, especially in the field of high-speed sintering and additive manufacturing.

• Effectiveness of speed sintering for chair-side, one-visit treatments is supported by scientific evidences, but further laboratorial and clinical studies are necessary to confirm that.

Highlights.

The trend for dental zirconia is the production of composition-gradient materials

More translucent dental zirconia results from low alumina and high yttria contents

Flaws between layers in multilayered blocks decrease the mechanical properties

Improvements regarding high-speed sintering and additive manufacturing are needed