**2. Dental restorative materials**

## **2.1 Resin**

Most dental ceramics and hybrid resin composites have the potential to mimic the enamel and dentin, respectively. However, it has been suggested that moderate damage to teeth could be restored with resin composites. For the resin composites restorations, minimal preparation of teeth is required, reducing the likelihood of pulpal involvement and tooth fracture [11].

The filling resin composite can strengthen the remaining tooth structure in some cases. For example, cemented porcelain restorations are recommended for severely damaged, worn, or broken teeth in dental clinics. Besides, alumina and nanohydroxyapatite are also widely used in dentistry [12]. Alumina is recommended because it has good fracture resistance, abrasion resistance, and high compressive strength. In addition, nano-hydroxyapatite is an essential part of teeth and bones. Therefore, it should achieve biomimetic properties in the restoration. Glass ionomer cement has a bactericidal effect because it releases fluoride and can stimulate hardened dentin [13]. In addition, these cements have properties comparable to dentin, thus realizing the concept of bionics. Glass-ionomer cements are used as restorative materials in deep class I or II cavities in pedodontics and restoration of class V cavities [14]. Glass-ionomer cements are not generally recommended in load-bearing posterior dentition due to low tensile strength. Therefore, in the context of mismatched elastic modulus between enamel and the direct restorative materials, more stresses may be transferred to teeth, leading to either tooth damage or failure of the restoration.

Nowadays, many clinicians take direct resin composite posterior restoration as their first choice in treating carious lesions or other tooth defects, including restoration of large cavities [15]. However, partial indirect restorations (inlay, onlay, and overlay) for excessive posterior tooth defects have started to replace direct resin composite restorations since the development of modern chairside computer-aided design/computer-aided manufacturing (CAD/CAM) systems [16].

One of the most important advancements in chairside CAD/CAM systems is the production of resin composite blocks [17, 18]. Paradigm MZ100, as an industrial polymerized version of direct resin composite (Z100), is the first product in this field [19]. Paradigm MZ100 contains bisphenol A-glycidyl methacrylate (Bis-GMA), triethylene glycol dimethacrylate, and 85% (by weight) zirconia-silica filler. Therefore, its degree of polymerization and mechanical properties are better than Z100 [20]. Later, a new resin composite block containing urethane dimethacrylate instead of Bis-GMA was produced under high temperature and pressure. This kind of resin was developed with the ambition of increasing the degree of polymerization [21, 22]. Recently, the flexibility and convenience of CAD/CAM resin composite are similar to resin composites, combined with durability, and surface finish characteristics are identical to ceramics. In addition, compared with glass ceramics, the resin composite block has minor wear on the relative teeth and can maintain its gloss for a longer time. Their non-fusion and composite-like properties make them easier to grind, polish, and adapt. Due to the less brittleness, the resin composite block has better edge characteristics. Furthermore, these materials produced less blunting on the drills during milling. They can also be repaired using resin composites with cutback or adding techniques. Besides, physical (color stability, water sorption, and water solubility) and mechanical (fracture-resistant, wear, compressive strength, hardness, and elastic modulus) properties of resin composite blocks were found better than that of conventional resin composite because of their higher degree of polymerization [23, 24].

Tunac et al. evaluate the 2 year clinical performance of computer-aided design/ computer-aided manufacturing (CAD/CAM) resin composite inlay restorations in comparison with direct resin composite restorations. According to FDI standards, the results show that the 2 year clinical performance of CAD/CAM resin composite inlay restorations is similar to that of direct resin composite restorations. After 2 years of clinical trials, CAD/CAM resin composite inlays have shown exemplary performance in class II cavities and meet clinical needs [25].

Despite the above advantages, due to the high degree of polymerization, discoloration, tarnishing, and fracture of the resin composite block overtime after the repair, the adhesion failure of the cement interface is a problem that may need to be considered for its long-term clinical performance [26]. However, data on CAD/ CAM resin composite partial crowns (inlays, onlays, and overlying) restorations are limited. Therefore, more clinical trials are needed to draw further conclusions about its clinical behavior.

#### **2.2 Alloy**

Porcelain fused to metal (PFM) restoration comprises a metal coping that supports overlying ceramic (**Figure 2**) [27]. PFM restorations have a long clinical track record. However, the PFM fixed partial denture (FPD) failure rates were 4% after 5 years, 12% after 10 years, and 32% after 15 years [28].

To date, PFM restorations remain the most widely and successfully used option for FPDs because their failure rates are often low (8–10% within 10 years) [29]. It was reported that clinical survival rates of FPDs are between 72% and 87% after 10 years, between 69% and 74% after 15 years, and 53% after 30 years [30, 31]. However, as is well-known, the metals used in PFM restorations can cause allergic or toxic reactions within soft or hard tissue [32]. Besides, PFM is known to cause graying of the gingival margin because of metal show-through [33].

Compatibility between the ceramic and the metal alloy is of paramount importance. PFM ceramic veneers consist of an opaque ceramic (e.g., a titanium oxide glass) that is required to mask the color of the underlying metal and provides the bond with the metal alloy [34, 35].

Opaque ceramics are combined with metal alloys through an oxide layer formed on the metal surface. This process is called degassing [36]. The degassing process can also remove contaminants on the surface of the alloy—coating dentin/body ceramics on opaque ceramics. Dentin ceramics can mimic natural dentin. Then apply the incisal ceramic to the dentin/body ceramic on the incisal third. The restoration can also be polished by using low-melting glazed ceramics or self-glazing.

#### **Figure 2.**

*Porcelain fused to metal (PFM). Porcelain fused to metal (PFM) restoration comprises a metal coping that supports overlying ceramic.*

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

One of PFM restoration's main disadvantages is its inability to transmit light, thus having a negative effect on the aesthetic outcome of the restoration because it may appear dark in color [37]. This drawback is noticeable at the restoration's cervical area, where it is sometimes difficult to get enough room. Therefore, a sufficient tooth structure should be removed to accommodate the ceramic material, mask the underlying metal without overly modifying the restoration. In addition, the metal braces should stop 1 mm from the buccal finish line, and ceramic edges (shoulder ceramics) are recommended. Another disadvantage of a PFM restoration is allergic reactions in some patients to metal elements such as nickel in the metal alloy [32].
