**2. Zirconia**

#### **2.1. Mechanical aspects**

Zirconia (zirconium dioxide, ZrO2) is a white crystalline oxide of zirconium. It is polymorphic in nature, transforming its crystalline reticule from monoclinic (at room temperature) to tetragonal to cubic at increasing temperatures. By adding some oxides to zirconia, it is possible to stabilize the tetragonal and/or cubic phases. The so-called *partially stabilized zirconia (PSZ)* consists mainly of a cubic phase, with monoclinic and tetragonal zirconia as minor phases. By adding 2–3% of yttria (yttrium oxide, Y2O3), it is possible to obtain a completely tetragonal zirconia, the so-called *yttria-stabilized tetragonal zirconia polycrystal (Y-TZP)*. The Y-TZP is the most performing zirconia from a mechanical point of view and the most used in dentistry to produce implants, implant abutments and frameworks for crowns and bridges.

Its interesting and in some cases unique mechanical properties are the reasons why zirconia is often called "ceramic steel": a high corrosion and wear resistance, high Young's modulus (200 GPa), a very high flexural strength (up to 1200 MPa), a high fracture toughness and a polymorphic behavior [10]. The latter is probably the most interesting aspect: zirconia may adapt the three-dimensional disposition of the structure when some energy is provided, that is what happens in a crack initiation. In proximity of the crack, the energy changes the phase locally, turning the reticule from tetragonal to monoclinic. This phase transformation happens with an increase in volume (3–4%): the expansion of the crystals opposes to crack propagation and prevents macroscopic failure, enhancing fracture toughness. This mechanism is known as *transformation toughening* [11, 12].

Such a phenomenal mechanism of action against crack propagation has been questioned because of the so-called *low-temperature degradation process*, a sort of aging of zirconia. It seems that in the presence of water, the yttrium ions can be leached, and their stabilizing effect can be lost [13]. In that case, a spontaneous irreversible transformation from the metastable tetragonal phase to the stable monoclinic phase can occur on the surface of zirconia. Such a stabilized monoclinic phase does not have the capacity anymore to rearrange the crystalline reticule and so to oppose to an incoming fracture. However, the impact of this issue on the long-term clinical behavior of zirconia prosthetic components and implants is still unclear [5].

#### **2.2. Biological aspects**

mucosa recedes over time. The availability of a "white" implant may be crucial in those clinical

Furthermore, titanium particles due to wear and corrosion products may be released in tissues close to implants, and they were found in regional lymph nodes [6]. In some cases, this may lead to host reaction or sensitization [7]. Some cases of allergic reaction to titanium are documented, even if rare [8, 9]. So, using some nonmetallic material as an alternative to the titanium implant may be useful and, in some cases, critical. Last but not least, always more

Ceramic implants were introduced to overwhelm some esthetic and biological problems that can arise from titanium. The first ceramic dental implant was made from alumina (i.e., aluminum oxide, Al2O3) between 1960s and 1970s, and that was the only ceramic material used until recently. However, alumina presented some biomechanical problems (like low fracture toughness), and it was then completely abandoned and replaced with zirconia that is nowa‐ days the only alternative ceramic material to titanium for dental implants (**Figure 1**) [5].

The aim of this chapter is to review the existing literature regarding zirconia dental implants,

cases in which esthetic result is mandatory.

92 Dental Implantology and Biomaterial

patients request completely metal-free prosthetic reconstructions.

highlighting the strong points and stressing the so far unclear aspects.

**Figure 1.** A one-piece zirconia implant (courtesy of Prof. Andrea Enrico Borgonovo, University of Milan).

Zirconia (zirconium dioxide, ZrO2) is a white crystalline oxide of zirconium. It is polymorphic in nature, transforming its crystalline reticule from monoclinic (at room temperature) to

**2. Zirconia**

**2.1. Mechanical aspects**

The biocompatibility of zirconia is well established from both in vitro and in vivo studies [14]. In-vitro tests were conducted on various cellular lines, such as osteoblasts, fibroblasts, lymphocytes, monocytes, and macrophages, showing no cytotoxic effects. In vivo tests also showed no cytotoxicity in soft (connective) or hard (bone) tissues [12]. For this reason, its use as a biomedical implant (e.g., in orthopedic surgery) is widespread [15].
