Since the first publication referring to zirconia as the “ceramic analog of steel” in the 1970s (Garvie et al. 1975), zirconia-based materials have evolved significantly and are currently being applied in numerous clinical situations in the dental field with a relatively high success rate (Zhang & Lawn 2017). Zirconia was initially used as a biomaterial in the orthopedic field in the late 1980s when alumina femoral heads started to be replaced due to the their high brittleness (Chevalier 2006). The first ceramic dental implants used decades ago were also made of alumina, however, as with prosthetic femoral heads, alumina implants were rapidly substituted by stronger zirconia-based components (Andreiotelli et al. 2009). In the 2000s, zirconia-based materials were made widely available for dental applications, with yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) by far the most successful version used in dentistry (Reveron et al. 2017). Although other dopants like CaO, MgO, CeO₂ at varied concentrations have been proposed, the most prevalent dental zirconia is the one cation-doped with 3%mol of yttria (Chen et al. 2013).

As demonstrated in the groundbreaking publication by Garvie et al. in 1975, doping zirconia with yttria results in the retention of the tetragonal crystallographic form at room temperature (Garvie et al. 1975). Usually the tetragonal phase of zirconia only exists between 1170°C and 2370°C, and its existence via doping at room temperature makes monoclinic phase transformation possible upon stimuli-like stress concentration around a cracked tip (Piconi & Maccauro 1999). Since the monoclinic crystallographic form is approximately 4.5 vol% larger than the tetragonal form, this transformation results in the creation of beneficial compressive stresses that hinder crack propagation and ultimately avoid catastrophic failure of the component (Lughi & Sergo 2010).

The presence of this unique toughening mechanism makes Y-TZP a smart material with one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, electric or magnetic fields (Ball 1998). According to Chevalier et al. (Chevalier et al. 2009), Y-TZP ceramics have the best combination of toughness and strength compared to other versions of the stabilized zirconia, such as partially stabilized zirconias (PSZ) currently used to produce translucent monolithic prosthetic crowns (Zhang 2014). PSZ contains larger amounts of yttria (5-8 mol%) to guarantee stabilization of the tetragonal phase and high content (>50 wt%) of the weaker zirconia cubic phase. The latter is more translucent but not able to deliver the transformation toughening mechanism and therefore has much lower mechanical properties compared to Y-TZP (Zhang & Lawn 2017).

The typical microstructure of Y-TZP used in dental implants is composed of equiaxed grains of tetragonal phase sintered to 96-99.5% of their theoretical density (Kelly & Denry 2008). Grain sizes are kept in the range of 0.2 to 1 µm (Palmero et al. 2014), as larger grains result in instability of the tetragonal phase (Heuer et al. 1982). Another important component present in dental Y-TZP is alumina, which began to be added to the starting powder in small concentrations (0.25 wt%) in order to increase the sinterability and consequently the final density of the component (Zhang & Lawn 2017). The addition of alumina later proved to effectively avoid low temperature degradation in Y-TZPs (Chevalier et al. 2009; Zhang et al. 2014).

In comparison to titanium implants, which display some immune reactions when evaluated in vivo, Y-TZP does not trigger local or systemic effects when used as an oral implant, and therefore has been indicated in patients with metal sensitivity (Kohal et al. 2004; Sevilla et al. 2010). Besides the high biocompatibility, Y-TZP is also known for giving much better esthetic results in comparison to titanium, especially regarding soft tissues surrounding the implant. In fact, the dark shade of titanium implants can compromise the esthetic result in patients with thin gingival biotype or after the implant base has been exposed due to gingival retraction (Heydecke et al. 1999). On the other hand, metallic implants have much higher fracture toughness, usually 6 to 10 times higher than their ceramic counterparts, and Y-TZP is a bioinert biomaterial (Cesar et al. 2017). Bioinert materials have poor interaction with surrounding living tissues and this feature may negatively affect the osseointegration process (Siddiqi et al. 2017). Therefore, the industry has proposed varied solutions to this problem usually by means of surface modifications of the zirconia implant in order to increase integration with bone (Wenz et al. 2008).

Although Y-TZP is currently the main biomaterial used in dental implants, clinical problems with Y-TZP hip prostheses in the early 2000s demonstrated that this material is prone to aging via a low temperature degradation phenomenon, which will be further explained in this review. In order to overcome this issue, alternative polycrystalline ceramic composites were developed to try to keep the same level of strength and fracture toughness, but with increased in vivo stability (Reveron et al. 2017). According to Osman and Swain, the main ceramic composite currently available for dental implants is alumina-toughened zirconia (ATZ) (Osman & Swain 2015).

ATZ is a composite ceramic material usually consisting of a mixture of 20 wt% alumina and 80 wt% of Y-TZP. This material is claimed to have increased fracture strength and reduced aging susceptibility compared to Y-TZP. According to Pieralli et al. (Pieralli et al. 2017), to date, no clinical studies including ATZ implants have been performed, and only studies on animals are available (Kohal et al. 2016; Schierano et al. 2015) that suggest an osseointegration capability comparable to Y-TZP.

Different compositions of ATZ have been developed, and the first alternative material was 12Ce-TZP/20wt%-Al₂O₃ intergranular composite (12 mol% CeO₂, zirconia grain size of 1.5 µm, and alumina grain size of 500 nm) (Reveron et al. 2017). Ceria was chosen as an alternative stabilizer in this case due to its ability to undergo a larger amount of stress-induced phase transformation, which in turn leads to higher fracture toughness when compared to Y-TZP (Tsukuma & Shimada 1985). This material later evolved to a refined version (10Ce-TZP-based composites) composed of 10 mol% CeO₂, zirconia grain size of 1 µm containing 30 vol% of nano-sized alumina particles (10 to 100 nm) and alumina particles around 500 nm containing a few nano-sized Ce-TZP particles (Nanozr, Panasonic Electric Works, Japan) (Li et al. 2014). Further improvements are still in an experimental stage with substitution of alumina for 16 vol% of nanometric MgAl₂O₄ (Apel et al. 2012) and the addition of elongated third phases, like SrAl₁₂O₁₉ and LaAl₁₁O₁₈ platelets, which may activate further toughening mechanisms like bridging or crack-deflection (Kern 2014; Reveron et al. 2017).