**2.3 Ceramic**

All-ceramic restorations refer to ceramic restorations made entirely of ceramic materials [38]. There are two kinds of all-ceramic restorations. One is monolithic (single layer), which composes of a single ceramic material. The other is a two-layer all-ceramic restoration which consists of a ceramic core material covered with a ceramic veneer [39, 40]. In the bi-layered, all-ceramic restoration, the ceramic core supports the restoration and gives it strength, while the veneer provides the restoration with its final shape, shade, and aesthetic [41]. Nevertheless, the veneer-core bond strength is considered one of the weakest links of the bi-layered all-ceramic restorations because they are prone to delamination and fracture [39]. Nowadays, with the increasing interest in aesthetics, a bi-layered all-ceramic restoration is widely applied in dentistry. However, the main disadvantages associated with this repair include delamination and fracture of the veneer [42]. In addition, it is sometimes difficult to achieve excellent occlusal contact with the structure of the opposing tooth. Finally, to achieve a lasting repair, the compatibility of the core and veneer materials is crucial [43].

When aesthetics is the priority, dental ceramics are the material of choice because they can successfully mimic the tooth substance's character (**Figure 3**) [44]. Ceramics can successfully simulate the visual characteristics of the tooth substance. Ceramics are biocompatible and inert material and have a high degree of intra-oral stability. Therefore, they can be safely used in the oral cavity. For example, the use of all-ceramic restorations has increased in recent years [45]. However, there are many ceramic materials and systems on the market that can be used in dentistry. The increased use of ceramics for restorative procedures and the need to improve clinical performance has led to the development and introduction of several new ceramic restorative materials and techniques [46].

#### **Figure 3.**

*Ceramics. Ceramics can successfully simulate the visual characteristics of the tooth substance. Ceramics are biocompatible and inert material and have a high degree of intra-oral stability.*

The all-ceramic restorations can be used as a bi-layered restoration, in which the more aesthetic ceramic veneer is the core or framework. They can also be used as full-contour (monolithic) restorations, which can be stained when required [47].

In the past decade, countless types of all-ceramic crown systems have been introduced unprecedentedly. Many of these systems have been criticized for their failure in restorations. It was reported that the survival rate of all-ceramic restorations ranges from 88–100% after 2–5 years of use and can still reach 97% after 5–15 years of use [48]. Although all-ceramic restorations have been greatly improved, zirconia is still the best all-ceramic restoration currently available. Since the end of the 1990s, due to many clinical and basic scientific research, this form of partially stabilized zirconia has been popularized for application in dentistry due to its excellent strength and excellent fracture resistance [49]. Currently, two main types of all-ceramic FDP systems have been proposed. The first of these systems involves the use of a single material to make full-contour crowns. For instance, a single crown in anterior teeth and premolars is made by reinforced glass materials successfully [50]. Further, a full-contour crown in the molar region is prepared with polycrystalline zirconia with improved translucency [51]. For the second system, porcelain and other glass materials are fused into a frame made of high-strength ceramics [52]. Dense sintered polycrystalline zirconia-based materials are expected to be used in FDP frameworks [53].

Yttrium partially stabilized tetragonal zirconia polycrystalline (Y-TZP), due to its superior mechanical properties and excellent fracture resistance, has drawn lots of attention in clinical applications. For instance, the fracture toughness of Y-TZP ranges from 5 to 10 MPa m1/2, and bending strength varies from 900 to 1400 MPa [54]. Y-TZP-based systems are a recent addition to the high-strength, all-ceramic systems used for crowns and fixed partial dentures.

Zirconia is a white crystalline oxide of zirconium with high mechanical strength, toughness, and corrosion resistance. Besides, zirconia has excellent biocompatibility, which can significantly reduce dental plaque [55]. However, zirconia is degradable at low temperatures, and this is a gradual, spontaneous phenomenon. Recently, the introduction of stabilized zirconia is supposed to overcome this drawback and promote the application of zirconia in dental restorations [56].

Marchack et al. proposed a custom-designed powerful grinding ceramic core technology for all-ceramic crowns [57]. This technique can eliminate the porcelain covering of the zirconia inner crown and frame to reduce the incidence of chipping or cracking of the porcelain veneer. The fracture of veneering ceramic is the most common complication for zirconia restorations. Thus, some suggestions for optimizing the manufacturing process of zirconia-based FPDs have been issued, including changes to the firing protocol. It was recommended because it can reduce the chipping rate. In addition, zirconia-ceramic FDP shows more clinical problems like prolonged fracture of the veneer ceramic [58]. Therefore, dentists should pay more attention to zirconia-ceramic FDP generated by CAD/CAM system before all treatment procedures [29]. On the other hand, with the development of ceramics on zirconia, people invented the framework of lithium disilicate glass-ceramics.

Cercon ht (Dentsply Intl., York, PA, USA) is developed from a clinically proven Cercon-based yttria-stabilized zirconia material formulation. It represents a new generation of zirconia with excellent transparency and can be used for esthetic restorations without build-up porcelain [29]. In order to better reproduce the color of natural teeth, some zirconia-based materials have been developed as translucent [59]. Among them, zirconia is widely applied as crown and FDP without veneer or pressed ceramics. Zirconia has a high flexural strength of more than 1200 MPa and has excellent veneer properties [60]. In the dental clinic, zirconia has proven to be a durable and reliable frame material that can inhibit crack propagation and prevent

#### *The Application of Zirconia in Tooth Defects DOI: http://dx.doi.org/10.5772/intechopen.101230*

catastrophic failure. However, there are clinical studies show that zirconia has an abrasive effect on the dentition, leading to excessive wear of the tooth structure [61]. The in vivo studies indicated that polished zirconia has higher wear resistance and lower resistance to wear than porcelain [62]. Currently, the new zirconia materials make the surface of the antagonist smooth, just like natural tooth enamel [63]. Although more and more research is focused on zirconia, there is still much to be understood about the production of zirconia and the production of zirconia inner crowns and frames. Dentists and researchers need further studies with larger sample sizes and extended follow-up periods to investigate the possible influencing factors of technical failures.

Ceria-stabilized tetragonal zirconia polycrystalline (Ce-TZP) is a newly developed ceramic material, which has not yet been used in the dental field. Its fracture toughness is 19 MPa m1/2, which is significantly better than Y-TZP. However, Ce-TZP has lower bending strength and hardness than Y-TZP [64]. Then, Ce-TZP/alumina nanocomposite (Ce-TZP/A) was developed to improve the property of Ce-TZP [65]. Ce-TZP/A contains nano-sized Al2O3 particles and Ce-TZP particles dispersed in Ce-TZP grains and grain boundaries [66]. This uniform dispersion of alumina in the Ce-TZP matrix plays a positive role in grain growth. However, it also negatively affects flexural strength, hardness, and hydrothermal stability of tetragonal zirconia. As reported, Ce-TZP/A is currently the toughest zirconia material available, and its fracture toughness reaches 19 MPa m1/2, and the flexural strength is high as 1400 MPa [65]. More importantly, Ce-TZP/A is entirely resistant to low-temperature aging degradation (LTAD), a critical drawback of Y-TZP [67]. The tremendous improvement of these characteristics is expected to extend the clinical application of dental ceramics to all-ceramic restorations and other areas, such as implant abutments, implants, removable denture bases, and components.
