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In restorative dentistry, there are basically two requirements aspired to be fulfilled by the material of choice to be the main constituent of the restorations, those include superior mechanical characteristics and outstanding esthetic properties. Zirconia (ZrO2) attains great popularity nowadays and is considered a promising material in dental applications. The excellent tensile strength, high thermal stability, relatively low thermal conductivity, wear resistance, corrosion resistance, chemical stability, low cytotoxicity, minimal bacterial adhesion, and biocompatibility properties of zirconia adding to them its tooth-like color and esthetic appearance have promoted its introduction as a successive dental substance. It was found to be a potential alternative and favorable material in dental restorations competing with many of the previously known and employed ceramics and metals, such as titanium. Despite the excellent properties and wide use of titanium in dental applications, it still suffers from unfavorable drawbacks. However, some problems in zirconia diminish its mechanical properties, such as phase transformation and aging, which could be overcome via the utilization of dopants within the zirconia’s structure. This chapter discussed the main stabilized zirconia types, properties, dental components, manufacturing, and treatment techniques. Further modifications on zirconia with the maintenance of both mechanical and esthetic properties are still under investigation.
*Address all correspondence to: gdaghra@birzeit.edu
1. Introduction
Zirconium, a strong solid group (IV) transition metal, was isolated for the first time as an impure metal in 1824 by Berzelius and produced as a pure metal by van Arkel and de Boer in 1925 [1]. It has a hexagonal close-packed (HCP) crystalline structure at ambient conditions [2]. Zirconium is well-known for its ductility, malleability, and ease of forming stable compounds. Therefore, it occurs in nature in conjunction with other components as ores, not as a pure metal. The most common zirconium ores include the more abundant but less pure zirconium(IV) silicate (ZrSiO4), commonly known as zircon, and the relatively pure baddeleyite which contains 96.5 to 98.5 weight percent zirconium dioxide also named zirconia [3]. Some of the main chemical and physical characteristics of zirconium are presented in Table 1. The importance of zirconium manifests clearly through its compounds, which have several interesting applications. Some of these applications are associated with the nuclear field in the form of zircaloys. Moreover, zirconium in organometallic compounds, such as zirconocenes, could act as an intermediate in the synthesis of several biological organic compounds; and zirconia ceramics, which will be discussed in depth in this chapter [4, 5].
Property
Value
Atomic Symbol
Zr
Atomic Number
40
Atomic mass
91.224
Density (g/cm3)
6.49
Melting Point (°K)
2128
Boiling Point (°K)
4682
Table 1.
Some physical properties of zirconium.
Zirconia (ZrO2) is a white polycrystalline ceramic material; it is one of the oxide ceramics with superior mechanical and biomedical properties. The crystallographic form of zirconia is pressure and temperature-dependent, which means it can assume one of three phases: monoclinic (m), tetragonal (t), or cubic form (c). At ambient pressure, the monoclinic structure, with the shape of a deformed rectangular prism, is the stable form at room temperature, however, it has inferior mechanical performance when compared with other phases. Crystals assume the monoclinic structure until 1170°C, where the unusual performance occurs: shrinkage upon heating, resulting in a transition to the metastable tetragonal shape, a regular rectangular prism with improved mechanical properties. This structure is stable between 1170 and 2370°C; above this temperature, further shrinkage occurs to form the stable cubic structure, a form with moderate mechanical properties [6]. Such phase transitions are summarized in Figure 1.
Figure 1.
Phase transformations of zirconia crystalline structure as temperature changes.
The unusual performance of temperature-dependent crystalline structural transitions presents a considerable problem upon cooling. A phase transformation from the metastable tetragonal into the monoclinic phase (t → m) occurs upon cooling with volume expansion of about (3–5%), the lattice becomes rigid and unable to accommodate the rapid volume expansion. Such considerable stresses through the lattice result in a hysteresis behavior of zirconia, where catastrophic fracture and propagation of any lattice cracks ensue over time. Therefore, pure zirconia is not easily produced and employed due to spontaneous structural failures [7, 8]. In the case of zirconia-based biomaterials, this problem could be catastrophic.
For the sake of inhibiting (t → m) transformation or at least reducing the transformation rate and maintaining the metastable tetragonal lattice at room temperature, which in turn inhibits crack propagation, some significant factors could be controlled [7, 8]. The addition of stabilizers in sufficient concentrations allows the tetragonal lattice to maintain at ambient conditions.
Zirconia stabilization is a process that aims to suppress its crystalline structure during cooling; by adding specific amounts of dopants to zirconia to avoid the conversion to the monoclinic form during cooling and stabilize the tetragonal form at room temperature to a great degree. Dopants, also called stabilizers, are metallic oxides such as magnesium, calcium, cerium, and yttrium oxides.
The lattice after stabilization overcomes crack propagation through a mechanism known as “transformation toughening” in which the toughness is enhanced because of the dissipation of the energy when cracking initiates to overcome the effect of the stabilizer, a (t → m) transition occurs followed by volume expansion, which leads to close the crack and blunt any developing crack [6]. This phenomenon would increase the flexural strength of the lattice.
Stabilized zirconia takes a unique place among oxide ceramics due to its distinctive mechanical and biological properties. Its mechanical properties are comparable to those of stainless steel, for example, excellent tensile strength, high thermal stability, relatively low thermal conductivity, wear resistance, and corrosion resistance. Nevertheless, the excellent chemical stability, low cytotoxicity, minimal bacterial adhesion, biocompatibility, and tooth-like color “ivory” of zirconia make it a material of keen interest in several biomedical applications, substantially in dentistry.
Zirconia was used in orthopedic joint replacement, such as hip joint replacement and surgical implants. In dentistry, with its protruding mechanical properties, zirconia is successfully used in endosseous implants with the maintenance of the natural ivory look of the tooth. Its esthetic properties enhance its utilization in dental applications widely; since esthetic concerns become a priority, as well as medication aspects as in Figure 2 [8].
Figure 2.
Zirconia’s esthetic properties in dentistry.
Zirconia aging or low-temperature degradation (LTD) negatively affects its properties, it is a water-catalyzed phenomenon that results in a slow (t→m) transformation and does not require mechanical stress. The mechanism of zirconia aging is similar to that of transformation toughening where the transformation is prolonged with volume expansion and stress induction to the surrounding. Then, the surface could rise, allowing for water to move through the lattice and penetrate down, which would exacerbate surface degradation [9]. This problem could be triggered by surface roughness.
2. Types of stabilized zirconia ceramics available for dental and biomedical purposes
As a ceramic material, zirconia was found to be hard but brittle with inferior impact resistance; this constrained its application and dictated the modification of such material with promising characteristics to permit it to work as an alternative for several metal-based artificial joints and implants. Moreover, materials to be employed in dental aspects have to be durable and stable under harsh oral cavity conditions [10]. The stabilization processes of zirconia ceramics, in which oxides are added to zirconia, enhanced its phases’ stability and retained the preferred tetragonal structure more than before at ambient temperature, resulting in partially stabilized zirconia (PSZ), zirconia toughened ceramics, or tetragonal zirconia polycrystals (TZP) with improved properties [11]. Even though there are many types of stabilized zirconia combinations, only three of them, yttrium tetragonal zirconia polycrystals (Y-TZP), magnesium partially stabilized zirconia (Mg-PSZ), and zirconia toughened alumina (ZTA), are intensively tested and introduced in dentistry to overcome (t → m) transformation. The mechanical properties of these types are summarized in Table 2.
3Y-TZP
ZTA
Mg-PSZ
Chemical components
Y2O3, ZrO2
Al2O3, ZrO2
MgO, ZrO2
Crystalline phase
Monophase
Biphase
Biphase
Flexural strength (MPa)
800–1300
750–850
700–800
Hardness (GPa, Vicker’s)
10–12
12–15
5–16
Fracture toughness (MPam 1/2)
5–10
6–12
8–15
Table 2.
The main mechanical properties of the principal types of stabilized zirconia [8].
When Yttria (Y2O3) is the dopant, a monophasic tetragonal zirconia polycrystal (TZP) is obtained, usually containing 3 mol% of yttrium oxide (3Y-TZP). This type of low porosity and relatively high-density stabilized zirconia attracted considerable attention when compared with other combinations due to its chemical stability and better mechanical properties supporting its biocompatibility. Exhibiting the transformation toughening phenomenon makes it a favorable type in medical applications and particularly dental implementations, including implants, crowns, abutments, bridges, and fixed partial dentures. Production of dental restorations is carried out through hard or soft machining along with sintering processes (Processing techniques will be discussed in a separate section of this chapter). The key point of tetragonal phase stability at ambient temperature is the vacancies in the zirconia lattice appeared due to the presence of yttria.
Despite the features, (3Y-TZP) undergoes aging or LTD phenomenon under hydrothermal conditions at low temperatures as well as human body temperature, and as a result, its mechanical properties degrade. In fact, the vacancies in the stabilized zirconia could act as host sites for water molecules diffused from the surrounding under hydrothermal conditions, decreasing site numbers consequently, as well as the stability of the tetragonal phase. This phenomenon is induced by exposure to hydrothermal “intraoral” conditions, surface roughness, microcracking, and stresses. It is found to be sensitive to processing conditions, including mixing, distribution, and polishing techniques. Some factors can be controlled to impact the mechanical properties of stabilized zirconia, including the grain size and sintering conditions. Spontaneous (t → m) transformation is susceptible unless the grain size is small. The transformation rate is reduced when the grain size is less than 1 μm and prevented when it is less than about 0.2 μm, and thus the brittleness of zirconia will reduce. The grain size is directly affected by sintering conditions, particularly sintering time and temperature. Large grain size will be obtained after a long time or high sintering temperature affecting the phase stability [9]. To overcome the LTD problem and failure of zirconia and improve its mechanical properties, efforts focused on the production of new composites, alumina is introduced instead of yttria to have zirconia toughened alumina. But improved (3Y-TZP) is still an attractive composite especially when computer-aided manufacturing and computer-aided design (CAM/CAD) processing are applied. Some generations reduced the LTD by adding a small amount of alumina to the (3Y-TZP) composite [12].
2.2 Zirconia-toughened alumina (ZTA)
A biphasic ZTA is accomplished by combining zirconia with an alumina matrix by slip casting or soft machining processes for the sake of integration between alumina’s high stiffness with zirconia’s superior toughness [13]. This type of stabilized zirconia was found to have better fracture toughness and wear resistance, also when compared with (3Y-TZP) it has enhanced aging resistance [14]. There is no stabilizing dopant in this type. Thus, the stability of the tetragonal phase at ordinary temperatures depends mainly on the particles’ morphology and size, in addition to whether the location is intergranular or intragranular particles [9]. By adding zirconia, the grain growth of alumina is inhibited, improving the fracture toughness of the composite. An advantage of this combination is that both of these components are naturally white and thus act as an efficient mask for teeth of dark colors. However, ZTA exhibits low translucency. Therefore, it is not ideal in cases where esthetic concerns are paramount. The opalescence of ZTA precluded it from dental restorations of anterior sites [12, 15].
For better outcomes, ZTA was incorporated into new composites using several additives. Many ceramic oxides are utilized within ZTA to modify their properties by influencing the lattice parameters. For example, a newly developed bio-safe ternary composite of zirconia, alumina, and titania (TiO2) facilitates sintering while processing and attains high mechanical properties with low manufacturing costs [13]. In several studies, alumina was used as an additive with other stabilized zirconia types. Alumina nanoparticles enhanced flexural strength and fracture toughness when incorporated with ceria-stabilized TZP [10]. The addition of a specific amount of silver as a dopant to ZTA enhances the antibacterial properties of the dental parts [16].
The two possible zirconia-alumina composites: ZTA, which is obtained when the alumina matrix is reinforced with ZrO2 particles (alumina is the main component in ZTA), and alumina-toughened zirconia (ATZ), which is obtained when the zirconia matrix is reinforced with Al2O3 particles. Both composites show better toughness; ATZ displays improved mechanical stability and aging resistance, while ZTA exhibits much better-aging resistance [17, 18, 19, 20].
Magnesia is the dopant of this type, usually using (8–10 mol%) of MgO to provide a biphasic composition of precipitates of tetragonal intragranular zirconia within the cubic matrix of stabilized zirconia through the introduction of magnesium cations within the tetragonal or cubic lattice of zirconia [9, 21]. Although (Mg-PSZ) exhibits transformation toughening with good chemical and thermal resistance properties, this material had precluded from several biomedical applications, it is considered an unstable material; due to its inferior properties resulting from the large grain size, which in turn increases the residual porosity and makes it susceptible to wear. Thus, it requires high sintering temperatures using special equipment for heating. Moreover, (Mg-PSZ) is almost impossible to be obtained purely free from alumina and silica, which diminishes magnesium content and allows (t → m) transformation [22, 23, 24]. The incorporation of 8 mol% (Mg-PSZ) was confirmed to enhance the fracture toughness of (3Y-TZP) [25]. The mechanical properties of (Mg-PSZ) can be controlled by treatment with isothermal heat, controlled cooling followed by temperature-controlled sub-eutectoid aging [26]. Spark plasma sintering before sub-eutectoid aging was found to contribute to finetuning (Mg-PSZ) properties [27]. Several methods were used for (Mg-PSZ) preparation, such as solid-state reaction, sol-gel, electrospinning, precipitation, microwave, and sugar techniques [28]. In vitro studies demonstrated the enhancement of the biocompatibility of (Mg-PSZ) when coated with functionally graded bioactive glass, such results promote the fixation of this type of stabilized zirconia-based dental parts [29].
3. Investigation of zirconia in dental restorations
3.1 Zirconia dental posts
If the root of the dentine is not healthy and cannot serve teeth stability, a dental post is an option for treatment of the readily existing tooth as an alternative to the root to support and strengthen the teeth via retention of the restorations. Stabilized zirconia ceramics were introduced in dental posts for the first time in 1995 with comparable mechanical properties to other metal-based posts [30]. Many limitations in metal-based and all ceramic restorations encourage the employment of zirconia in dental posts. Corrosion activity of some metal-based posts leads to sensitization, unfavorable metallic taste, oral burnings, and pains. Furthermore, the esthetic concern has been a critical factor in recent years since metal posts usually do not fulfill this concern. This is clear when employed in anterior restorations, where unfavorable metallic grayish-blue discoloration due to the complete opacity of metals could affect the root and gingiva.
Such health and esthetic concerns dictate the development of translucent white dental posts with high chemical stability and biocompatibility with very low toxicity besides their mechanical properties. Zirconia is the favored efficient choice and confirmed as a material that attained high clinical success rates with strength comparable to all ceramic posts. Figure 3 represents the root translucency difference between titanium-based versus TZP-based posts with composite build-up. To prevent the most severe problem, restorations fractures, the root should uniformly distribute the stress caused by the occlusal load; this could be achieved through the engagement of materials of lowered Young’s modulus analogous to that of dentine.
Figure 3.
Translucency of titanium (a) vs. TZP based posts (b) with composite build-up [30].
Upon studying the features and disadvantages of zirconia-based dental posts, some drawbacks limit their application. Retreatment of the zirconia posts in dentine is complex since it is nearly impossible to remove or grind away the posts from the root canal; likewise, the root temperature will increase if ultrasonic vibration is used to remove fractured posts. Moreover, some studies proved the poor resin bonding of zirconia posts after dynamic loading. Dental resin is a bonding material for integration between the restoration parts [31]. Also, the relatively high Young’s modulus and stiffness of zirconia promote zirconia posts to transfer the applied stress to the surrounding less rigid tooth, resulting in root fractures [32, 33].
3.2 Zirconia dental implants
The dental implants are similar to posts in their function as an alternative tooth root, whilst the implant is a foundation for a missing tooth and acts as a base for other prosthetic components for a totally-artificial tooth in the mandible or maxilla, as shown in Figure 4(a). This foundation should conduct a robust functional and pleasant esthetic role. In the current century, zirconia-based implants proved to be an efficient alternative to other prominent metal-based implants used widely in the past decades, including alumina and titanium alloys.
Figure 4.
Elucidative image of endosseous implant, abutment, and restoration (a). Bi-layer crowns of zirconia cores and translucent veneers (b) [8].
Despite their high success rates, titanium alloys in dental implants had some unfavorable aspects that limited their applications, including the esthetic requirements and high wear properties, resulting in metallic particles causing allergic and toxic consequences. Alumina-based implants were limited with their high fracture susceptibility and inferior osseointegration. Stabilized zirconia, and (Y-TZP) implants in particular, are favored for their high flexural strength, tooth-like color, low-temperature conductivity, and masticatory forces bearing.
Several studies proved the biocompatibility of zirconia implants which represent a critical property as a component placed directly in contact with alveolar bone and connective tissues. The low toxicity of such implants was also confirmed when tested using the immune system cells, this is supported by zirconia’s low plaque affinity, avoiding surrounding tissues’ inflammatory risks, and lower bacterial colonization compared to titanium implants. Zirconia osseointegration was directly influenced by surface roughness, which can be controlled via manufacturing processes, other than the machining process such as coating, sandblasting, and acid etching [8, 34].
3.3 Zirconia dental abutments
Zirconia abutment, the connection between the implant and the prosthesis as illustrated in Figure 4(a), was introduced in 1996 [35]. It was developed to overcome titanium abutment esthetic limitations, with the maintenance of its mechanical strength. Zirconia could be sintered in titanium abutments to form titanium-reinforced zirconia with improved properties.
Previously, one-piece implants were designed without the introduction of the abutment as a connection, for concerns related to wear, fitting precision, and the colonization potential of bacteria and microbes in the micro-gap between the abutment and the implant from one side and the prosthetic (crown or bridge) from the other side. Such a one-piece implant system limited the versatility of the prosthesis compatibility with the implants’ position. Furthermore, preparation of this type and grinding, in particular, may trigger (t → m) transformation. Thus, a two-piece system was developed, including an implant and a separated abutment. This combination proved to achieve better osseointegration when the implant is not protruding, furthermore, better fracture resistance would result if the stress distributes over the two separate components.
An important point to be taken into account is that when a zirconia implant is fitted directly to the zirconia abutment, restoration fracture is already feasible; due to its inherent strength. Durable, nontoxic, and good adherence bonding cement is mandatory. Many types of cement were introduced, such as zinc phosphate, zinc polycarboxylate, glass, and resin ionomer. The last type is well-known for its ease of use and high retention [31]. Zirconia abutments are favored over the brittle alumina ones, on the other hand, zirconia abutments are more susceptible to fractures when compared to titanium-reinforced zirconia [12, 33, 36, 37, 38].
3.4 Zirconia in dental prosthesis
Dental prosthetics, including single prosthetic crowns (fixed dental prosthesis), bridges (multiple joined prosthesis), and dentures (Full jaw prosthesis), are used as an alternative for missing dentine. It is a major part of the restoration in which functional and esthetic considerations must be satisfied; due to its exposure to harsh conditions and bearing direct pressure and masticatory loads. Moreover, as an external part of the restoration, the esthetic appearance is a priority, especially for the anterior dentine [12]. Among the versatile types of ceramic restorations; zirconia and lithium disilicate-based prostheses showed intrinsic promising functional capability as in Figure 5. Zirconia prosthetics are preferred for their superior mechanical properties and have become a popular trend among dentists as a material that fulfills the prementioned requirements. CAD/CAM technologies’ advancement in zirconia production also facilitates its selection as the approved type.
Figure 5.
Zirconia and lithium disilicate-based prostheses.
Prosthesis preparation is a critical factor for its fitting with both the foundation and the veneering components [9]. Transformation toughening is crucial in making its mechanical properties exceed other ceramic prosthetics. (3Y-TZP) is widely preferred for its high flexural strength and fracture toughness. Versatile types could be obtained with different properties via variations of stabilized zirconia concentration, grain size, sintering conditions, coloration technique, and surface processing and treatment [39].
Despite the favored clinical performance of zirconia crowns and other prosthetics, it has some drawbacks in bi-layer prosthesis, as shown in Figure 4(b), which consists of veneering porcelain that works as a translucent mask over the opaque white zirconia, mainly for esthetic concerns, and particularly in anterior regions. The veneering porcelain in bi-layer prosthetics is susceptible to chipping and aging problems in the presence of water. Chipping is the cohesive frailer between the zirconia substructure and the veneer.
The veneering problem is attributed to many reasons, including a mismatch of thermal expansion coefficient between the two layers, vacant sites due to porosity, lower fracture toughness of the veneer structure, overloading, and inappropriate design of zirconia frameworks with insufficient support of porcelain over the framework. Chipping could be solved in some cases simply by polishing the rough sites, using composite resin for fracture treatment, or total replacement in some complicated cases [40]. Monolithic zirconia recently is preferred, in which a one-piece component is introduced instead of two separated components core and veneer.
3.5 Zirconia in esthetic brackets
Several types of brackets have been used for orthodontic treatment, such as stainless steel, titanium composites, and alumina. Zirconia brackets were introduced as the type of greatest toughness among other ceramic-based types. Its frictional coefficient is lower than that of alumina, which is a feature. Nevertheless, zirconia’s high opacity inhibits the esthetic appearance, especially with the growing trends for translucent brackets [41]. Recent advances work on developing the zirconia bracket’s structure to enhance its properties and shorten the treatment period. A treatment using zirconia brackets fabricated in a specific process with three slots within its structure, connected with nickel-titanium arch-wires was successful, in which the tooth movement occurs at a higher rate. A shorter treatment period was observed when compared to edge-wise appliances [42].
3.6 Zirconia properties
Understanding zirconia properties helps in a better conception of its performance and applications. One of the important phenomena in zirconia is low-temperature degradation (LTD) or aging described previously in the second section. Other properties will be described in this section:
Biocompatibility could be described as the ability of a biomaterial or medical component to carry out a medical therapeutic function usually for long-term contact with human body tissues, with relatively no pathogenic harmful side effects, such as inflammation, cytotoxicity, allergy, or carcinogenic effects [43]. Many researchers tested zirconia biocompatibility in dental restoration. It is proven that partially stabilized zirconia performed favorable initial fibroblast adhesion on its surface, which enhances the growth of the connective tissues when compared to titanium, polystyrene, and fully stabilized zirconia. Increasing the concentration of yttria in partially stabilized zirconia produces fully stabilized zirconia with a rougher surface which affects the adhesion behavior [44]. In vivo and in vitro studies proved that zirconia is an osseoconductive biomaterial in addition to being safe and stable without reported carcinogenic or mutagenic impacts [45]. Some studies worked on modifying zirconia surfaces, such as incorporating calcium ions, which enhanced the biocompatibility of such restorations without affecting their mechanical properties [46]. In general, no cytotoxic or damaging side reactions manifest from the introduction of zirconia in dental applications, in addition to the good bone response with relatively no inflammation or bacterial growth within acceptable levels [47].
4.1 Optical properties and translucency
Translucency is an intermediate property between transparency and total opaque. Translucent material allows the transmission of light with dispersion which obstacles a clear seen of objects through it [48]. It is a requirement for esthetic aspects in dental restorations. Zirconia opacity is a drawback despite its naturally white color, but zirconia’s translucency is required since color compatibility is essential. This material also showed relatively high X-Ray opacity obstructing diagnosis [47]. Therefore, efforts worked on the enhancement of zirconia translucency in several ways and attempted to control its optical properties and refractive index, to tong the top choice for restorative components. Many factors could affect zirconia’s translucency, such as impurities, porosity, grain size, restoration thickness [49], and processing conditions. Controlling these factors could enhance translucency [50].
4.2 Radioactivity
Zirconia was found to contain small portions of radionuclides from uranium, radium, and thorium series type. It is possible to obtain zirconia of radioactivity within acceptable limits via purification processes [9]. To understand the effect of composition on zirconia radioactivity, an in vitro study represents the radioactivity of three common types of dental zirconia composites using gamma spectrometry, Vita In-Ceram YZ, Zirkonzahn, and Zirkonzahn Prettau (Table 3). Zirkonzahn Prettaud had the highest radioactivity as Figure 6 represents. Even though, all results were within acceptable limits of 1000 Bq/kg according to the International Atomic Energy Agency (IAEA) [51].
Vita In-Ceram YZ
Zirkonzahn
Zirkonzahn Prettau
Components
Weight (%)
Components
Weight (%)
Components
Weight (%)
ZrO2
90.9–94.5
ZrO2
Main
ZrO2
Main
Y2O3
4–6
Y2O3
4–6
Y2O3
4–6
HfO2
1.5–2.5
Al2O3
<1
Al2O3
<1
AI2O3
0–0.3
Na2O
max. 0.04
Na2O
max. 0.04
Fe2O3
0–0.3
SiO2
max. 0.02
SiO2
max. 0.02
—
—
Fe2O3
max. 0.01
Fe2O3
max. 0.01
Table 3.
Chemical composition of three common zirconia types [51].
Figure 6.
Total radioactivity for each type [51].
4.3 Wear behavior
Surface roughness influences the abrasion of material and the wear with the adjacent teeth. Enamel wear is affected directly by the surface roughness and surface microstructures, in addition to the surrounding environmental conditions. The surface roughness could be raised by grinding and decreased by polishing [52].
Zirconia-based dental restorations nowadays are designed and manufactured using CAD/CAM technology. Two techniques are mainly used for the manufacturing process; soft and hard machining. Soft machining is based on the milling process of pre-sintered blanks produced by cold isostatic pressing of compact zirconia powder in the presence of a binder. Later on, those blanks will be fully sintered. High temperature is utilized for the sintering step. This technique is usually used for (3Y-TZP) manufacturing.
On the other hand, the hard machining technique uses fully sintered stabilized zirconia blocks. The pre-sintered zirconia is obtained at a temperature of less than 1500°C to maintain the required density. Then, hot isostatic pressing is obtained at temperatures between 1400 and 1500°C under inert conditions and high pressure to maintain high density, hardness, and homogeneity of the blocks. A special milling system then performs machining to obtain the required dimensions. The milling system has to be strong and robust because the fully sintered zirconia is very hard with low machinability [9, 15].
6. Surface modifications for zirconia dental implants
Many factors could affect the nature and properties of the fabricated zirconia and its quality and biocompatibility, such as its chemical composition, morphology, and surface roughness [53]. In this section, the main surface treatment methods of zirconia-based dental restorations are discussed, which are found to have a direct influence on its osseointegration as well as the mechanical properties. Various classifications for the treatment methods were used; herein the classification of these methods is presented within three categories; physical treatment, chemical treatment, and coating processes.
6.1 Physical treatment
6.1.1 Surface sandblasting
It is a surface abrasion process in which the pressure of the compressed air ejects particles strongly to acquire a surface with micro-roughness. In this process, the abrasion is performed as a homogeneous and anisotropic abrasion on the hard material [54]. Many studies confirm the advantageous role of increasing zirconia-based implant roughness with sandblasting on the amount of integration and contact with the bone tissues. The roughness is detected by a parameter known as bone-implant contact (BIC), in addition to improving the strength of the bond, detected by the removal torque parameter (RTQ) [55].
Sandblasting of zirconia with alumina was confirmed to increase its surface roughness. Thus, highly efficient initial adhesion of human osteoblasts cells was achieved [56]. However, this process has some drawbacks; sandblasting with alumina will affect the elemental chemical composition of the treated surface, in which alumina act as a contaminant. The problem could be solved by utilizing the acid etching treatment [54]. Moreover, LTD is susceptible to occurring with increased sandblasting pressure due to the impact of the mechanical forces on the surface [57].
6.1.2 Laser
Laser is a promising treatment for zirconia surface to improve its osseointegration. Unlike sandblasting, zirconia phase transformation in laser treatment is uncommon and surface contamination is avoided since there is no direct contact between the laser and the surface [54]. This type of treatment is fast, easy to operate, clean, and highly accurate; it promotes micro-grooved implant surface and thus enhances the surface roughness. Laser treatment promotes surface properties changes, including roughness, topography, and wettability. This treatment type was confirmed to improve surface micro and nano-scaled roughness; this resulted in enhanced wettability, which directly influences biocompatibility, cell adhesion, and proliferation.
The properties of the treated surface depend on the irradiation conditions, which could be optimized by controlling the irradiation frequency, intensity, and time [58]. The femtosecond laser established a consistent roughness between the surface and the bonding resin, increasing the bond strength. A fiber laser could achieve wider adequate grooves; this advances the (BIC) and (RTQ) parameters [4, 55].
6.1.3 Ultraviolet (UV) light
As mentioned before, wettability is correlated with biocompatibility and integration with bone tissues. Many studies proved that surface treatment with UV light increases wettability by lowering the surface contact angle below 20° to obtain a hydrophilic and even super-hydrophilic surface. The hydrophilicity surface character is mainly obtained due to decreasing the superficial hydrocarbons via UV light [59], and increasing the oxygen character. When the treated hydrophilic oxide binds to water in the wet environment near the tissues, the surface will be in the form of hydroxylated oxide, which induces surface reactivity toward the surrounding amino acids and proteins [54]. Thus, zirconia treated with UV radiation exhibits very good osteoblast response and proliferation. However, the influence of UV light on zirconia aging still needs further effort and study. Many studies indicated that UV treatment reduces the ability of zirconia to age, while some controversial studies found that UV light triggers crystalline transformation [55].
6.2 Chemical treatment
6.2.1 Acid etching
Acids, such as HF, HNO3, or H2SO4 usually used for zirconia surface treatment in the acid etching process to increase its roughness homogeneously, even in the case of irregular surfaces, without destroying its morphology. Acid etching is effective in overcoming the prementioned sandblasting contamination problem by removing excessive residues. Thus, acid etching is usually employed in conjunction with a previous surface sandblasting. Sandblasting of zirconia implants using large grits followed by surface etching with a strong acid provides SLA implants, an abbreviation for sandblasted, large grit, acid-etched implant surface, with an increased surface area, promoting surface bio-adhesion [60, 61]. A comparative animal study tested three different types of sandblasted zirconia implants, the three types are acid-etched (SLA), alkaline-etched, and sandblasted zirconia without etching. The results indicated the highest BIC values were attributed to the acid-etched SLA type, while the alkaline-etched implants achieved the lowest BIC values [62].
6.2.2 Electrochemical treatment
Despite the nonconductive character of zirconia, many electrochemical treatment methods were proven to enhance the properties of zirconia dental restorations. For example, electrochemical deoxidation of zirconia (ECD) improved its biocompatibility by developing a micro-porous surface and thus lowering the contact angle. This method is known to enhance surface wettability through oxygen removal from the surface of the solid zirconia using molten salt electrolysis [63].
Recent electrochemical techniques focused on producing nanostructures on the surface of zirconia such as nanotubes [64]. The electrochemical anodization technique (EA) is extensively used for the fabrication of zirconia nanopores or nanotubes since it is a cost-effective technique and can control the physical and chemical properties of the prepared nanostructures. Zirconia nanotubes were found to promote the stability of zirconia implants with better initial cell adhesion. The fabricated nanotubes could be modified via annealing to improve their corrosion resistance [65].
6.3 Coating
Several types of bioactive coatings are employed to augment the function of the osteoblasts. Some of these coating materials, such as calcium phosphate, polydopamine, bioactive glass, and biomolecular coatings are discussed in this section.
6.3.1 Calcium phosphate
Calcium phosphate is a mineral component in bones; this critical point accelerates and supports the osseointegration when zirconia implants are coated with these compounds. Furthermore, calcium deposition on such coated zirconia implants and the adhesion of proteins will be enhanced [61]. This type of coating is affected by the properties of the utilized compound since several compounds are implicated as members of the calcium phosphate family, such as β-tricalcium phosphate (β-TCP) and the most stable hydroxyapatite (HA) with the chemical formula Ca10(PO4)6(OH)2.
Several methods are employed for coatings. The plasma spraying method is favored over sol-gel, wet powder spraying, and aerosol deposition techniques since it is inexpensive and its deposition rate is high, but not suitable for complex morphologies [55, 66]. However, calcium phosphate bonding on the zirconia surface is relatively weak; several studies attempted to reinforce bonding in different ways, such as laser treatment before coating, or the application of coating along with (HA) and hydrothermal sintering after the coating process [65].
6.3.2 Polydopamine (PDA)
The good adhesion properties of marine mussels’ proteins attracted the interest to take advantage of the components in these proteins. Upon studying this property, the unusual amino acid 3,4-dihydroxy-L-phenylalanine referred to as (L-DOPA) is confirmed as the liable component. Dopamine is considered an (L-DOPA) precursor [67]. The introduction of dopamine and polydopamine was approved to augment the bioactivity of the zirconia surface and enhance cell adhesion over the surface. The enhancement is attributed to the easy adsorption processes taking place on the surfaces due to the strong anchoring of the catechol functionality [68]. This type of coating also reduces bacterial adhesion on the coated surface and improves antimicrobial activity, which aids in the rapid healing and regeneration of the soft tissues surrounding the dental implants. Moreover, PDA coating is simple and nontoxic [67].
6.3.3 Coating with biomacromolecules
Arginine-glycine-aspartate, referred to as RGD is a tripeptide biomolecule known for its proteins’ adhesion properties. It is found that RGD plays a key role in the adhesion of osteogenic cells and thus holds significant promise to achieve efficient bioactivity if incorporated with dental implants [69]. Zirconia surface coating with RGD was accomplished through the immobilization of the coating material on the zirconia implant surface by a process called surface biofunctionalization or biomimetic modification. This process aid in the enhancement of its biological character. RGD coated (Y-TZP). The ability of successful chemical bonding between RGD and (Y-TZP) was proven, and this combination performed better biocompatibility [54, 70].
6.3.4 Bioactive glass
Bio-glass, a composition of sodium, calcium, silicon, and phosphorous oxides, is considered a bioactive material, which promotes its introduction in zirconia surface coating. It forms hydroxyapatite between the implant’s surface and biological tissues when present within a biological environment [55]. This type of coating is proposed to enhance the biocompatibility of the implants and reduce the healing period. However, the success of these attempts was limited because of several drawbacks in bio-glass. Its mechanical properties are insufficient, making it a fragile component. Furthermore, its thermal expansion coefficient is relatively high, so it is unsuitable for zirconia thermal coating, and cracking of the coating is susceptible [61].
7. Challenges and future developments of zirconia application
To overcome the dental veneer drawbacks, chipping, and delamination, in addition to thickness considerations, attention has focused on the development and further modifications of the monolithic zirconia. To enhance monolithic zirconia’s properties, chemical dopants are utilized. Increasing the yttria content with a lowering in alumina content and the incorporation of (0.2 mol%) La2O as a dopant resulted in improved (3Y-TZP) translucency and aging resistance enhanced. Increased yttria concentration to (5Y-TZP) resulted in superior translucency and aging resistance properties, but the toughness was scarified. Zirconia opacity is still a drawback that prevents its participation in the anterior sites. Dopants incorporation is considered a promising technique. The opacity of stabilized zirconia could be attributed to the light scattering performed by its grain boundaries and microstructural defects. Alumina was found to increase light scattering, thus its content in zirconia should decrease, but not eliminate, to a level below (0.25 wt%). The transmittance was enhanced when the grain size of zirconia was reduced to a diameter of less than 100 nm [71].
As known, nanotechnology is spreading every day through most of the manufacturing processes and applications; due to its distinct properties and the superior development of the material’s character. For example, nano-powders with a regulated composition of stabilizing material or additives could be used for the fabrication of zirconia and has found to improve grains development during sintering and decreases porosity. But for this technique, new processing methods should be developed; since the normal fabrication method used for processing zirconia with nano-powders was very hard [52]. Some studies confirmed the promising properties and positive role of alumina-zirconia nanocomposite with better toughness and high capability for aging resistance in addition to crack propagation inhibition, which increases its reliability in medical applications [33].
The future of stabilized zirconia in the material science field is promising and requires intensive efforts and searching for new strategies to withstand the challenge of enhancing the esthetic properties while maintaining the mechanical properties of zirconia at the same time. This is a game of advancements and compromises.
Zirconia has attracted significant attention recently, especially in dental applications for mechanical and esthetic considerations. Herein, we demonstrate a general view of zirconium, zircon, and zirconia. This chapter discussed the main types of stabilized zirconia incorporated in dental restorations: Y-TZP, ZTA, and Mg-PSZ. Zirconia-based dental restorations, including posts, implants, abutments, fixed denture prostheses, and orthodontic brackets, were explained. Then, an illustration of some of the substantial zirconia properties that directly affect its mechanical and esthetic properties, such as (LTD) or aging, zirconia biocompatibility, optical properties, translucency, and radioactivity. Surface modifications of dental zirconia are also presented. The physical treatment techniques: sandblasting, laser, and UV light, in addition to the chemical treatment, including acid etching and electrochemical treatment, were discussed. Different coatings utilizing calcium phosphate, polydopamine, bio-macromolecules, and bioactive glass were introduced. Finally, we demonstrated recent developments, challenges, and directions for future research to enhance the survival rates of different zirconia-based dental restorations.
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Written By
Ghassan Albarghouti and Haneen Sadi
Submitted: 27 April 2023Reviewed: 17 May 2023Published: 26 August 2023
Open access peer-reviewed chapter
