Classification and Generations of Dental Zirconia

Abstract

Zirconium oxide (ZrO2) is polymorphic (temperature dependent) structure; zirconia can take three crystallographic forms at ambient pressure. Under normal conditions, pure zirconia is monoclinic (m). At (1170°C), the substance converts to a tetragonal crystal structure (t), then to a cubic crystal structure (c) at (2370°C), and finally to a fluorite structure above (2370°C), melting at (2716°C). During the heating and cooling cycles, the Zirconium oxide ceramic undergoes a hysteretic, martensitic t- m transformation, which is reversible at 950°C upon cooling. For dental applications, various types of zirconium-dioxide (zirconia) materials are available. These materials have a variety of chemical compositions, crystal configurations, manufacturing processes, and important variations in their mechanical and optical properties. Numerous generations of zirconia materials have been developed, ranging from the use of zirconia crystals as reinforcement elements in zirconia toughened alumina (ZTA) to partially stabilized zirconia (PSZ) and the conventional (3Y-TZP) to the appearance of new translucent zirconia materials such as cubic stabilized zirconia (CSZ).

1. Introduction

Zirconium (Zr) is a chemical element whose name Zr is taken from the name of mineral zircon and the name zircon comes from the Persian word “Zar-Gun,” which means “golden color”.Zr is a transition metal with an atomic mass of (91.224 g/mol) and an atomic number of (40). The melting point of Zr is (1855°C), while the boiling point is (4371°C). Zr was discovered in 1789 by Martin Heinrich Klaproth, a German scientist and isolated in 1824 by Swedish chemist Jöns Jacob Berzelius (Gautam et al., 2016; Nistico, 2021). Helmer and Driskell reported the first biomedical application of Zr in 1969; however Christel (1988) was the first to use Zr to make a ball head for a complete hip replacement [1, 2].

Zr is never found in nature as a naturally occurring metal. It occurs naturally in igneous rocks in combination with other elements such as iron, titanium, and silicon oxide. The most abundant source of Zr is zircon (ZrSiO4), which is found mostly in Australia, South Africa, Brazil, India, Russia, and the United States. Many other mineral species contain Zr, including baddeleyite. Despite the fact that zirconia was first used in orthopedics in 1969 for hip head replacement, it was not used in dentistry until the 1990s. Zirconia-based ceramics such as tetragonal zirconia which is partially stabilized by yttria have been successfully incorporated into daily dental work to construct fixed dental prostheses (FDPs) and dental (CAD/CAM)system based on their excellent biological, mechanical, and physical properties. Many endeavors, on the other hand, have undergone numerous improvements in composition and microstructure in order to improve their optical properties without minimizing their mechanical properties [3, 4, 5].

Zirconia is a non-etchable polycrystalline material as it has no glassy phase within its structure that can be bonded to tooth structures using both traditional and resin cements; however, resin cements are favored because they have a great marginal seal, better retention, and improved ceramic fracture resistance. Zirconia –based ceramics are conventionally used as substructure materials that require a veneer facing for clinically acceptable appearance due to their high opacity and whitish optical appearance with excellent biocompatibility and lower the risk of pulp irritation that may occur due to the lower thermal conductivity. Ceramic facing, on the other hand, has some drawbacks, such as poor tensile strength and crack toughness. Porcelain is vulnerable to cracking when stressed due to intrinsic flaws in the crystal structure and the presence of voids. As a result, veneer chipping has been described as a major cause of failure and the most common complication in all-ceramic crowns [6, 7].

A new generation of zirconia, known as “monolithic zirconia,” has recently been introduced. Monolithic zirconia restorations, according to the manufacturers, has a higher translucency than conventional zirconia and therefore does not need a veneer layer. Monolithic zirconia restorations can have a number of clinical advantages. Without the use of a veneer, the amount of tooth loss and the risk of chipping are reduced, as well as the restorations’ actual strength will be increased [8, 9].

2. Classification of zirconia

Over the past 20 years, there have been more varieties of dental zirconia, making it occasionally challenging to select the right kind for each restoration. Utilizing CAD/CAM technologies, it is now possible to create dental restoratives with extremely precise fitting owing to advancements in digital technology. Metal-free restorations are also appealing for biological and cosmetic reasons. For dental applications, various types of zirconium-dioxide (zirconia) materials are available. These materials have a variety of chemical compositions, crystal configurations, manufacturing processes, and important variations in their mechanical and optical properties. Numerous generations of zirconia materials have been developed, ranging from the use of zirconia crystals as reinforcement elements in zirconia toughened alumina (ZTA) to partially stabilized zirconia (PSZ) and the conventional (3Y-TZP) to the appearance of new translucent zirconia materials such as cubic stabilized zirconia [10, 11]. Figure 1 is showing recent classification of yttria stabilized zirconia.

Figure 1.

Structural schematic diagram and classification of yttria-stabilized dental zirconia [10].

Zirconia can be classified according to the following criteria into:

2.1 Zirconia microstructure

The monoclinic phase of pure zirconia (ZrO2) is stable at ambient temperature, while the tetragonal and cubic crystal phase systems change with temperature. When zirconia is solidly dissolved in yttrium(Y), calcium (Ca), magnesium (Mg), cerium (Ce), or other ions with an ionic radius larger than that of zirconium (Zr), and accordingly divided into [12]:

2.1.1 Partially stabilized zirconia (PSZ)

The tetragonal and cubic phase systems become stable at room temperature when zirconia is solidly dissolved in yttrium (Y), calcium (Ca), magnesium (Mg), cerium (Ce), or other ions with a greater ionic radius than that of zirconium (Zr) [1, 13]. Cubic-stabilized zirconia (CSZ) is the cubic phase zirconia that is stable at room temperature when yttria (Y2O3) is introduced in amounts greater than 8 mol%. Tetragonal and cubic phases are mixed at room temperature when yttria is 3 to 8 mol%, and this material is known as partially stabilized zirconia (PSZ). Tetragonal zirconia polycrystal (TZP), also known as toughened zirconia, is zirconia that is close to 100% tetragonal at room temperature when yttria is approximately 3 mol%. This 3 mol% yttria tetragonal zirconia polycrystal (3Y-TZP) was one of the earliest zirconia used as “white metal” in dentistry [10].

2.1.2 Fully stabilized zirconia (FSZ)

It is a cubic zirconia that includes no less than 8% yttrium oxide(Y2O3), hence the term “fully” stabilized zirconia (FSZ). presenting tetragonal and cubic crystals on its microstructure, being totally inert to the aging in autoclave because cubic zirconia does not exhibit the t/m transformation toughening phenomenon. The volume of cubic crystals is greater. They are less firmly bonded and, as a result, have increased light scattering at the grain boundaries due to reduced residual porosity. Additionally, incident light is emitted more uniformly in all spatial directions because the cubic crystal structures are more isotropic than the tetragonal structures. To increase the translucency of dental zirconia, several producers have recently started producing it in its full cubic stabilized form (due to increased cubic phase) [14, 15, 16, 17].

2.2 Zirconia used with porcelain facing or not

It can be divided into:

2.2.1 Monolithic (full contour restoration)

The terms “monolithic” is Greek word that means: mono is “single” and “lithos” is “stone,” This indicates that the materials have a consistent appearance. As microstructures, monolithic materials consist of two or more phases such as in zirconia it has three phases monoclinic, tetragonal and cubic phases. With the continuous development of new and more translucent Y-TZP and the advancement of CAD-CAM technology that facilitate the fabrication of monolithic Y-TZP crowns and FDPs. These systems aim to eliminate the problem of veneering porcelain chipping and provide acceptable esthetics with characterization, which may be an esthetic option in the molar area without a reduction in strength. Full-contour zirconia restorations, however, do not have adequate translucency because the matrix and zirconia particles have different refractive indices. To produce restorations with great translucency and to achieve zirconia’s mechanical qualities, a number of brands of monolithic zirconia have been introduced. To increase zirconia’s translucency, a full cubic stabilized monolithic zirconia (FSZ) has been produced. Comparing newly developed Y-TZP monolithic materials to traditional zirconia, the translucency has enhanced and low temperature degradation has been limited due to the materials’ lower alumina content, relatively fine grain size, and presence of optically isotropic cubic zirconia particles. A monolithic multilayer zirconia has been developed. It is a polychromatic, translucent zirconia with combined shade and translucency gradient [18, 19, 20].

2.2.2 Core build up (veneered with porcelain facing)

A high strength ceramic substructure made of zirconia or alumina is the foundation of bi-layered crowns, which are then veneered with ceramic or dental porcelain such as feldspathic porcelain. Despite having good esthetic qualities, the resulting restorations are prone to failure, such as chipping of the veneering ceramic. An alternative approach is to eliminate the veneer and produce a full contour monolithic zirconia crown but the possibility of wearing the opposing natural teeth is still a concern. Furthermore, the potential loss of strength brought on by low temperature aging or degradation (LTD), which may be induced in an aqueous environment. By completely covering the zirconia restoration with a ceramic veneer, it can prevent it from coming into direct contact with the oral cavity and potentially prevent this occurrence [20, 21].

2.3 Colored and non-colored zirconia

Dental zirconia has changed over the past 20 years from its original white, opaque appearance to translucent and chromatic as well as polychromatic (multi-layered) forms. Zirconia has become as the most versatile restorative material, offering a wide spectrum of translucency and colors.

2.3.1 Non dyed (monochromatic white)

Non dyed blanks have a hard-white monochrome color, which can be an esthetic disadvantage in many indications. The restorations milled in the white body condition can be manually and individually colored with coloring oxides after the milling process and sintered afterwards to overcome this drawback. The form-milled open-pore framework is immersed in the suitable colored liquid for a brief period of time to dye it. Alternatively, brushes can be used to create color gradients that are comparable to various colored liquids of varying intensities. The sintering procedure is performed after removing the excess remaining color while it is still wet and drying the framework. Utilizing liner or stain is another way to make white zirconia more esthetically beautiful [17].

2.3.2 Dyed polychromatic (multilayer)

Construction of polychromatic zirconia in the form of a gradational multilayered zirconia disc with two to seven or even more layers of color, mimicking the appearance of layered porcelain in full-contour monolithic restorations, and the creation of zirconia blanks with multiple, different shaded layers make full-contour zirconia restorations more esthetically pleasing than those made with conventional monochromatic zirconia. After sintering, full-contour zirconia restorations with esthetics that outperform monochromatic restorations can be obtained. The Katana Multi-Layered zirconia by Kuraray Noritake Dental Japan was the first polychromatic zirconia to be introduced to the dental market in February 2015. They are intended for the production of full contour zirconia crowns with a greater level of esthetics. Since then, virtually every significant dental company has introduced their own version of multi-layer zirconia [20, 22].

2.4 Fully sintered or partially sintered zirconia blanks

2.4.1 Fully sintered or hot isostatic procedure (HIP)

Performed by a hot isostatic press at 1000 bar and 50°C below the sintering temperature at 1400–1500°C under high pressure and an inert gas atmosphere to reduce the material porosity and ensure high values of toughness and translucency of zirconium ceramics by such procedure. Carrying out HIP on Y-TZP results in a gray-black material that usually requires subsequent heat treatment to oxidize and restore whiteness. Then, a specially designed milling system used to machine the blanks, but with a low machinability and high hardness of a fully sintered Y-TZP, such milling system has to be particularly robust [23, 24].

2.4.2 Partially sintered or cold isostatic pressing

With cold isostatic pressing, the powders are shaped into ceramic blanks. The powder of the partially sintered blanks typically contains a binder that enhance its pressing, but during the step of a pre-sintering it will be eliminated. The most widely used procedural method for Y-TZP shaping is cold isostatic pressing, which yields stable, chalk-like non-sintered green-stage objects with a very high primary density. Sintering without pressure in the oxidizing atmosphere of a specific furnace allows the green objects to be further stabilized and condensed up to approximately 95% of their theoretical density., the pre-sintering temperature of the heat treatment effects on the machined blank roughness, a higher pre-sintering temperature gives a rougher surface, so the choice of a proper pre-sintering temperature will be thus critical. These materials remain softer than the HIP zirconia and are easier to mill [24].

2.5 Zirconia generations

2.5.1 First generation

The first generation of 3 mol% yttria (3Y-TZPs) had flexure strengths greater than 1 GPa and contained 0.25 wt% alumina (Al2O3) as a sintering aid. However, that zirconia displayed considerable opacity, due to the inherent birefringence of noncubic zirconia phases as light is scattered at the grain boundaries, pores, and additional inclusions. They were primarily used as framework materials in posterior and anterior fixed dental prostheses (FDPs) and porcelain-veneered crowns [25, 26].

2.5.2 The second generation

The alumina content of the second iteration of 3 mol% yttria (3Y-TZP), used in dentistry was reduced from 0.25 wt% to 0.05 wt%. The 3Y-TZP with 0.05 wt% alumina is more translucent than the 3Y-TZP with 0.25 wt% alumina, but because there is less alumina to stabilize the tetragonal phase, it is more prone to low-temperature degradation. Some confusion in nomenclature has occurred as both 0.05 wt% alumina- containing 3Y-TZP and 5 mol% yttria-stabilized zirconia polycrystal (5Y-ZP) have been called “translucent zirconia”; however, these zirconia materials have different mechanical and optical properties [26, 27].

2.5.3 The third generation

The third iteration of dental zirconia is doped with 5 mol% yttria, which creates a partially stabilized zirconia with around 50% cubic phase zirconia is produced. Zirconia’s cubic phase is isotropic in many crystallographic directions, which reduces the amount of light scattering at grain boundaries. The cubic zirconia thus seems more transparent. Since stabilized cubic zirconia does not change at room temperature, it will not degrade at low temperatures or undergo transformation toughening. It has diminished mechanical qualities but will not change with time [26].

2.6 Zirconia according to mol% concentration of yttria

2.6.1 3 mol% yttria-partially stabilized zirconia (3Y-PSZ)

Mechanical and optical properties of zirconia depend on the yttria mol% content. Of all the several forms of zirconia, 3Y-PSZ has the greatest values for opacity, flexural strength, and fracture toughness. The material’s high color value and opacity restrict its use to posterior restorations [26].

2.6.2 4 mol% yttria-partially stabilized zirconia (4Y-PSZ)

4Y-PSZ zirconia has translucency and mechanical properties in between 3Y-PSZ zirconia and 5Y- PSZ zirconia making it an attractive in between material for esthetic zone [26].

2.6.3 5 mol% yttria-partially stabilized zirconia (5Y-PSZ)

5Y-PSZ zirconia has enhanced translucency, but reduced mechanical properties [26]. Table 1 is showing zirconia by the yttrium content and its effect on physical and mechanical properties.

3Y-TZP: Opaque Zirconia	4Y-TZP: Some Translucency	5Y-TZP: Most Translucency
High mechanical properties	High mechanical properties	High mechanical properties
White opaque	Some Translucency	4Y-TZP: High translucency
Low temperature degradation	Decreased low temperature degradation	Little or no low temperature degradation
Mainly tetragonal phase	Tetragonal and cubic phases	More Cubic less tetragonal phases

Table 1.

Defining zirconia used in dentistry by the yttrium content and its effect on physical and mechanical properties [28].

3. Zirconia ceramics for dental applications

The following are the three most common forms of zirconia used in dentistry today [1]:

1. Tetragonal zirconia polycrystals doped with yttrium cation (3Y TZP)

Traditional 3Y-TZP zirconia is the most commonly used zirconia material in bilayered dental restorations as a substitute for the metal substructure. The 3Y-TZP is the toughest zirconia material currently available in dentistry. As a stabilizer, 3Y-TZP contains 3 mol% Yttria (Y₂O₃). The stabilizer Y³⁺ cations and Zr⁴⁺ ions are distributed randomly over the cationic sites, but electrical neutrality is attained by creating oxygen vacancies [29].

The mechanical properties of 3Y-TZP are highly influenced by the particle size. 3Y-TZP becomes less vulnerable to spontaneous tetragonal to monoclinic (t-m) transformation when the particle size is less than (1 μ). Furthermore, the transformation is not possible below a certain particle size (0.2 μ), resulting in decreased fracture toughness. A particle size greater than (1 μ), on the other hand, makes 3Y-TZP less stable and more vulnerable to spontaneous t-m transformation, resulting in huge volume increase and a decrease in fracture toughness. As a result, it appears that the critical particle size for 3Y-TZP is 1(μ) [30].

According to this, the sintering conditions that result in an increased particle size have a significant effect on the final ceramic product’s stability and strength. Larger particle sizes result from higher sintering temperatures and longer sintering times. Depending on the producer, the final sintering temperatures of 3Y-TZP range from (1350 to 1550C). Furthermore, the particle size and phase stability of 3Y-TZP used in dental applications can be affected by variations in sintering temperatures during firing [30, 31, 32].

1. Partially stabilized zirconia doped with magnesium cation (Mg-PSZ)

The quantity of magnesium oxide in commercial materials ranges from 8 to 10 mol%. To control the fracture toughness of Mg-PSZ, high sintering temperatures (1680 and 1800 C) and tightly controlled cooling cycles are required. Because of the less stable molecular composition of this material, early t-m transition may occur, leaving insufficient tetragonal zirconia for the material to transform and toughen upon further fracture formation. Lower mechanical properties are the outcome of the higher prevalence of the monoclinic form. Due to the existence of surface porosity associated with a large particle size (30–60 μ), magnesium stabilized zirconia (Mg-PSZ) has not been widely used in biomedical applications due to the risk of excessive wear [1].

1. Zirconia toughened alumina (ZTA)

Another way to take advantage of stress-induced zirconia transformation is to use zirconia toughened alumina (ZTA), which is composed of an alumina matrix and zirconia. The commercially accessible dental product In-Ceram Zirconia (VidentTM, Brea, CA) was developed by adding 33 vol.% of 12 mol % ceria-stabilized zirconia (12Ce-TZP) to In-Ceram Alumina. Slip casting or soft machining may be used to create In-Ceram Zirconia restorations. Increased porosity is likely related to In-Ceram zirconia’s lower mechanical properties as compared to 3Y-TZP dental ceramics [33].

4. Zirconia microstructure

Zirconium oxide (ZrO₂) is polymorphic (temperature dependent) structure; zirconia can take three crystallographic forms at ambient pressure. Under normal conditions, pure zirconia is monoclinic (m). At (1170°C), the substance converts to a tetragonal crystal structure (t), then to a cubic crystal structure (c) at (2370°C), and finally to a fluorite structure above (2370°C), melting at (2716°C). During the heating and cooling cycles, the Zirconium oxide ceramic undergoes a hysteretic, martensitic t- m transformation, which is reversible at 950°C upon cooling [2, 34]. Figure 2 is showing Crystallographic phases of zirconia and temperature hysteresis.

Figure 2.

Crystallographic phases of zirconia, temperature and hysteresis [2].

Monoclinic phase (m) is a deformed prism with parallelepiped sides that has lower mechanical properties and may lead to a reduction in the cohesion of ceramic particles [35].

Tetragonal phase (t): is a straight prism with rectangular sides that possesses increased mechanical properties. Precipitation of a finely dispersed tetragonal phase within a cubic matrix, capable of being changed into the monoclinic phase when the matrix’s pressure was alleviated by a crack propagated within the matrix [35].

Cubic phase (c): in the shape of a square-sided straight prism has a large grain size, which reduces light scattering and birefringence at grain boundaries. The cubic phase of zirconia is isotropic in different crystallographic directions, which reduces light scattering at grain boundaries. The cubic zirconia becomes more transparent as a result. At room temperature, stabilized cubic zirconia does not transform, so it will not undergo transformation toughening or low-temperature degradation. In other words, it has weakened mechanical properties but will not change over time [28, 35].

5. Generations of zirconia in dentistry

There are four main generation of zirconia in dentistry and new multilayer novel generation of zirconia:

1. (First generation 3Y-TZP 0.25Al₂O₃): this was introduced about two decades ago. Zirconia crystals are partly stabilized with 3 mol % yttria (4.5–6 percent by weight of Y₂O₃). Owing to the presence of alumina, it is opaque. When green-state zirconia is put in the furnace, alumina is applied as a sintering aid to help prevent the formation of pores [7, 35].

2. (Second generation 3Y-TZP 0.05 Al₂O₃): this was introduced between 2012 and 2013. The zirconia crystal structure is partly stabilized with 3 mol percent yttria (Y₂O₃), but altered by providing a lower volume of aluminum oxide (Al₂O₃) with smaller Al₂O₃ grain sizes and re-positioned in the zirconia matrix. Since there is a less alumina to stabilize the tetragonal phase, it is more prone to low-temperature degradation [28].

3. (Third generation 5Y-TZP 0.05 Al₂O₃): Between 2014 and 2015, a new category of translucent zirconia materials, also known as Anterior Zirconia, High Translucent Zirconia, or Cubic stabilized Zirconia (CSZ), was developed to overcome the translucency limitation. The translucency of the new materials is produced by increasing the yttria content from 9 to 10% by weight, which results in the production of 50% cubic phases at the expense of the metastable tetragonal phase at the microstructural level. With increasing cubic phase particle size, the substance becomes more porous, resulting in less light scattering at grain boundaries and hence increased translucency [18].

In comparison to the previous two generations of zirconia materials, those cubic zirconia materials exhibit optical qualities that are comparable to those of glass ceramics. However, on a material level, the high translucency obtained by increasing the yttria level reduces the material’s intrinsic mechanical properties to a significant degree. As a result, cubic zirconia materials have a flexural strength of around half that of standard 3Y-TZP materials (approximately 600 MPa) [18, 28].

1. (Fourth generation 4Y-TZP 0.05 Al₂O₃): as mentioned before 4Y-PSZ zirconia has a translucency and mechanical properties that fall between 3Y-PSZ zirconia and 5Y-PSZ zirconia, making it an appealing material for the esthetic region [26].

2. Novel Multilayer zirconia

Construction of zirconia blanks with several, differently-shaded layers and creation of polychromatic zirconia for more esthetic full-contour zirconia restorations than conventional monochromatic zirconia in the form of a gradational multilayered zirconia disc with two to seven or even more layers of color, simulating the appearance of layered porcelain in a full-contour monolithic restorations. Full-contour zirconia restorations with superior esthetics to the monochromatic restorations can be obtained immediately after sintering [15].

This multi-layered technology uses only pigmentation to simulate the shade-gradient of natural teeth while maintaining the same yttria percentage in the zirconia blank, resulting in a color gradient with no difference in flexural intensity between the enamel and dentin layers [23, 36].

What could be called the age of the next generation of modern multi-layered transparent zirconia materials has begun in the last years. These materials (for example, IPS e.max ZirCAD Prime and Katana Multi-Layer) comprise two to four layers of varying translucency inside the same product. This is accomplished by a novel manufacturing procedure that blends two zirconia materials (3Y-TZP and 5Y-TZP) to create a single unique material. By lowering the cubic content of the previous generation to a balanced level with 3Y-TZP, these new materials achieve a balance of strength and esthetics [36, 37].

A multi-layered technology was introduced by combining two generations of zirconia (combination of two different percentages of yttia) in one blank with the aim of combining the benefits of both generations of zirconia. This is mainly a combination of a high-flexural-strength 3Y-TZP in the dentin/body region to enhance flexibility and a high-translucency 5Y-TZP in the incisal or occlusal region to improve esthetics. Even the 5Y-TZP, which has the lowest flexural strength of the zirconia generations, possessed superior mechanical qualities and a translucency comparable to lithium disilicate ceramics [7, 38].

6. Clinical applications of zirconia in dentistry

The construction of veneers, full and partial coverage crowns, implants, fixed partial dentures (FPDs), posts and/or cores, implants, and implant abutments are among the range of zirconia’s modern clinical applications. Extra-coronal attachments, surgical drills, cutting burs, and orthodontic brackets are a few additional zirconia-based auxiliary components that are offered as commercial dental products [39].

6.1 Zirconia crown

Space, para-functional habits, malocclusion, short clinical crowns, tooth mobility, tooth inclination considerations, and basic clinical sequence are all the same as they are for other all-ceramic crowns. The tooth preparation clinical guidelines for zirconia crowns are also the same as those for metal-ceramic restorations.

An appropriately constructed diamond set is typically used to accomplish the tooth preparation for a zirconia crown, which should give a desirable distribution of the functional stresses. In general, 1.5 mm to 2.0 mm of incisal or occlusal reduction and 1.2 mm to 1.5 mm of axial reduction are needed to prepare a tooth for a zirconia restoration. All dihedral angles should be tapered, and the axial convergence angle of the crown preparation should be around 6 degrees. The preparation should end with a uniform 0.8 mm to 1.2 mm subgingival (about 0.5 mm) deep chamfer or marginal shoulder with rounded internal angles [39]. Figure 3 is showing a full coverage of 12 maxillary single zirconia crowns.

Figure 3.

A total of 12 maxillary single zirconia crowns (teeth 16 to 26). Top: full coverage preparation of the abutment teeth. Middle: Zirconia. Bottom: final clinical situation after crown adhesive cementation [39].

6.2 Zirconia fixed partial denture

In comparison to other traditional all-ceramic systems like lithium-disilicate glass ceramics and zirconia-reinforced glass-infiltrated alumina, Y-TZP FPDs were found to have a significantly higher load bearing capacity. It has also been reported that veneering further increased fracture resistance [39, 40]. The most recent framework material for the production of all-ceramic FPDs in either anterior or posterior sites is Y-TZP, which is based on the superior mechanical qualities of zirconia (such as high flexural strength and fracture resistance) [41]. Figures 4 and 5 are showing zirconia framework in anterior and posterior regions respectively.

Figure 4.

Anterior six-unit zirconia fixed partial denture restoration (teeth 13 to 23). Top: Zirconia framework in situ. Middle: Zirconia framework after laboratory completion. Bottom: Final clinical situation after cementation [39].

Figure 5.

Posterior four-unit zirconia fixed partial denture restoration (teeth 47 to 44). Top: Zirconia framework. Middle: Zirconia framework after laboratory completion. Bottom: final clinical situation after cementation [39].

6.3 Zirconia as post and core

Meyenberg et al. reported using zirconia in post-and-core systems for the first time in 1995. They found no failure after an average observation period of 11 months [42]. Since then, zirconia posts (ZPs) have emerged as a potential treatment option for teeth with reduced structural integrity and filled roots teeth, particularly for patients with high esthetic needs. Because there are different observation periods, post surface treatments, cement systems, tooth preparation designs, and post designs, the success rate of ZPs in the literature ranges greatly, from 81.3%2 to 100%18. In order to prevent rotation of the posts and cores, for instance, the length of the posts should be prepared to be longer than that of the clinical crowns [43]. Additionally, the structural integrity of the root-filled teeth must have a ferrule with a minimum height of 1.5 mm to counterbalance the lateral forces experienced during post insertion, the wedging effect of posts, and the functional lever forces [44, 45, 46]. these restorations also have some problems. For instance, the high elastic modulus may lead to a less uniform stress distribution throughout the tooth, and complications such as tooth fracture and post debonding still exist [47]. Figure 6 is showing all-zirconia post and core of a maxillary left endodontically treated lateral incisor (tooth 12)) where Laboratory work performed by Mr. F Ferraresso (Saluzzo, Italy) and Dr. SO Koutayas (Corfu, Greece).

Figure 6.

Single crown restoration of a maxillary left endodontically treated lateral incisor (tooth 12) with the use of an all-zirconia post and core: a) initial situation after endodontic treatment b) prefabricated zirconia post with core analogue model, c) two- piece all-zirconia post and core after milling of a Y-TZP core d) bonding of the post and core restoration using an adhesive resin (Panavia 21, Kuraray), e) completion of the tooth preparation, f) final clinical situation after crown placement [39].

6.4 Zirconia implants titanium

Zirconia is emerging as a promising alternative to conventional Titanium based implant system for oral rehabilitation with superior biological, esthetic, mechanical and optical properties [48]. To date, there are five commercially available zirconia implant systems on the market. One- and two-piece designs are offered by only one system (Sigma, Incermed, Lausanne, Switzerland), whereas one-piece designs are only offered by CeraRoot, CeraRoot Dental Implants, Barcelona, Spain; Z- Look3, Z-Systems, Constance, Germany; whiteSKY, Bredent Medical, Senden, Germany; and zit-z, Ziterion, Uffenheim, Germany. A customized zirconia root-analogue implant with a micro- and macro-retentive implant surface was also described in a recent clinical trial, but neither the zirconia material nor the milling machine were fully explained [49]. Zirconia implants do not have any clinical long-term data despite some encouraging preliminary clinical results. 93% of survival rate after one year, according to reports (189 one-piece implants, Z-Systems) [50] 98% (66 one-piece implants, Z-Systems), [51] and 100% (one-piece implants, CeraRoot) [52]. Figure 7 is showing: Zirconia implant supported zirconia crown (tooth 12) where laboratory work performed by Mr. W Woerner (Freiburg, Germany).

Figure 7.

Zirconia implant supported zirconia crown (tooth 12. Top: Zirconia implant placement after tooth extraction. Middle: 4 months later; placement of retraction cord prior to impression. Bottom: after final cementation of zirconia crown [39].

6.5 Zirconia implant abutment

In the most recent systematic review, published in2013, Bidra and Rungruanganunt compared the survival, mechanical, biological, and esthetic outcomes of implant abutments (Ti and Zir). Due to greater color integration, they came to the conclusion that Zir abutments were preferred from an esthetic standpoint, especially for patients with thin mucosal tissues. They had improved gingival color, according to a recent analysis of their esthetic outcomes, and Zirconia had comparable soft-tissue recession, probing depths, bleeding on probing, marginal bone level, and patient reported outcomes as Ti [53, 54]. Zirconia abutments, however, had greater mechanical issues than Ti abutments. Therefore, the main obstacle to the widespread application of Zir abutments is their lack of mechanical strength [55]. Figure 8 is showing zirconia prefabricated implant abutment of an upper right lateral incisor (tooth 12) where Clinical and laboratory work performed by Dr. SO Koutayas (Corfu, Greece) and Dr. D Charisis (Athens, Greece), respectively.

Figure 8.

Single implant all-ceramic crown restoration with the use of a zirconia prefabricated of an upper right lateral incisor (tooth 12). Top: abutment connection. Middle: Zirconia abutment after laboratory modification and Ti screw. Bottom: final clinical situation after crown adhesive cementation [39].

6.6 Zirconia dental auxiliary components

different zirconia-based auxiliary components such as cutting burs and surgical drills, extra-coronal attachments, and orthodontic brackets are also available as commercial dental product [39, 56].

6.6.1 Zirconia orthodontic brackets

Orthodontic brackets made of Y-TZP are stronger, more resistant to wear and deformation, less likely to adhere plaque, and more esthetically pleasing. Additionally, they share the same frictional qualities as polycrystalline alumina brackets and show good sliding properties with both stainless steel and nickel-titanium arch wires [56, 57].

6.6.2 Zirconia prefabricated zirconia attachments

The use of Prefabricated zirconia attachments in clinical applications is based on the material’s durability and strength. The literature on clinical performance and effectiveness, however, is nonexistent. There are currently two different types of Y-TZP attachments available on the market: an extracoronal, cylindrical, or ball attachment for removably attached partial dentures, and a ball attachment for overdentures that is a part of a zirconia post (Biosnap, Incermed) and is available in three diameters for three levels of retention (Proxisnap, Incermed) [39].

6.6.3 Zirconia cutting and surgical instruments

Newly created zirconia cutting instruments (such as drills and burs) can be employed in soft tissue trimming, maxillofacial surgery, implantology, and other fields. These tools have been shown to be resistant to chemical corrosion, and they provide maximum cutting efficiency while operating smoothly and with less vibration. Lastly, alumina-toughened zirconia (ATZ) can be used to create surgical equipment such as scalpels, tweezers, periosteal elevators, and depth gauges using injection molding (Z- Look3 Instruments, Z-Systems) [39].

7. Conclusions

It is obvious that zirconia is continuous developing and growing ceramic material for wide applications in dentistry from the traditional core or framework material to the newly developed monolithic multilayer zirconia Dental zirconia continues to increase and is classified into many species in the yttria system alone. They are classified with yttria content, monochromatic/polychromatic, uniform/hybrid composition, and monolayer/multilayer. Zirconia with a higher yttria content is more translucent and less strong mechanically. Zirconia applications seem to consolidate a well-established position in clinical dentistry, due to the improvements in CAD/CAM technology and to the material’s exceptional physical properties. Existing clinical studies demonstrated a promising survival potential regarding tooth-supported restorations therefore, a suitable zirconia should be selected depending on whether strength or esthetics are desired. Therefore, it is concluded that an adequately selected zirconia is a suitable material because of its mechanically, esthetically, and biologically excellent properties. Zirconia abutments provide a favorable bioesthetic addition to implant dentistry, however, long-term clinical assessment is needed for accurate evaluation of implant-supported zirconia restorations.

Acknowledgments

Great thanks to the college of dentistry/university of Mosul/Iraq for their continuous support and encouragement.

Conflict of interest

The authors declare no conflict of interest.

Abbreviations

3Y-HA	conventional TZP stabilized with 3-mol% yttria and added with a relatively high content of alumina (0.25–0.5 wt%)
3Y	high translucent TZP stabilized with 3-mol% yttria and added with a relatively low content of alumina (less than 0.05 wt%)
4Y	high strength PSZ stabilized with 4-mol% yttria and added with a relatively low content of alumina (less than 0.05 wt%)
5Y	high translucent PSZ stabilized with 5-mol% yttria and added with a relatively low content of alumina (less than 0.05 wt%)
6Y	super high translucent PSZ stabilized with 6-mol% yttria and added with a relatively low content of alumina (less than 0.05 wt%)
M3Y	polychromatic multilayered 3Y
M4Y	polychromatic multilayered 4Y
M5Y	polychromatic multilayered 5Y
M6Y	polychromatic multilayered 6Y
M3Y-5Y	polychromatic multilayer with hybrid composition from 3Y to 5Y
M3Y-4Y	polychromatic multilayer with hybrid composition from 3Y to 4Y
M4Y-5Y	polychromatic multilayer with hybrid composition from 4Y to 5Y