## 1 Introduction  
Zirconia is a polycrystalline ceramic derived from zirconium, first produced in 1914, and implemented for medical use in 1969 for hip head replacement. Zirconia can be obtained from zirconium through calcination, producing zirconium dioxide (ZrO2). Zirconium dioxide is then stabilized in a crystalline form through yttrium oxide (Y2O3), thus obtaining yttria stabilized tetragonal zirconia polycrystal (Y-TZP) [1]. This material possesses excellent mechanical properties, and has a structure made of molecules organized in three different phases: monoclinic, tetragonal and cubic. Because of the large number of techniques and materials used to produce monolithic zirconia, the strength of monolithic zirconia is heavily dependent on its brand: the overall strength and the fracture resistance, however, exceed those of zirconia cores layered with conventional ceramic, and are reported as two thirds greater than those of lithium disilicate [2]. Higher yttria molarity enhances the translucency and esthetics of zirconia, but also lowers its strength; thus, high-yttria-molarity zirconia should not be used for more than 3 units in posterior restorations or in particularly stressful occlusal configurations [2]. For this reason, zirconia with a different yttria molarity has a number placed before the \"Y\" corresponding to \"Ytria\"; for example, 3Y-ZTP identifies a low-yttria-molarity zirconia, which is tougher but more opaque in color; 5Y-ZTP identifies a high-yttria-molarity zirconia, which is less resistant but has an higher translucency [3]. One of the main uses of zirconia in dentistry is the production of Implant-Supported Fixed Complete Dental Prostheses (ISFCDPs), which are a good therapeutic option to treat completely edentulous patients [4] as a valid alternative to removable dentures [5; 6]. Although the prevalence of edentulism is decreasing, [7] its impact on patients' quality of life remains detrimental, leading to reduced social interactions [8], difficulties in nutrition and reduced self-esteem [9]. Clinical results of ISFCDPs can be influenced by multiple factors, some related to the prosthesis design and others related to the materials used. The literature provides few clinical trials comparing the clinical performances of ISFCDPs with different materials; however, the available evidence reports good clinical results of ISFCDPs made of zirconia, with high survival rates, ranging from 88% to 100% [4; 6; 10; 11]. Zirconia has evolved over the years and, at the present time, both high-translucency/ lower-strength zirconia and low-translucency/high-strength zirconia are available on the market, allowing the development of different techniques to produce ISFCDPs. The most common ISFCDPs described in the literature are the following [10; 12]:  
* Zirconia-titanium hybrid ISFCDPs: a monolithic zirconia prosthesis (Katana YML 1050 MPa, Kuraray Noritake, Tokyo, Japan) is glued on a titanium framework and acts as the esthetic and functional portion of the prosthesis, while a titanium framework provides elasticity and force dissipation to the system. The prosthesis is usually screwed directly on the implants or on Multi-Unit Abutments (MUAs), depending on the implant orientation. * Progressive monolithic zirconia ISFCDPs: a monolithic zirconia prosthesis (Katana HTML 1200 MPa, Kuraray Noritake) is glued on dedicated components and screwed directly on the implants, without the use of a titanium framework. The progressive nature of zirconia is provided by layers with a different yttria molarity, with the more esthetic zirconia on the external section of the prosthesis and stronger zirconia at the core. * Zirconia-on-zirconia (Zirc-on-Zirc) ISFCDPs are produced with a protocol in which a high-strength and opaque zirconia framework (Katana HTML 1200 Mpa, Kuraray Noritake) is glued on dedicated components to screw it directly on the implants, while highly esthetic zirconia crowns (Katana STML 750 MPa, Kuraray Noritake) are cemented on the zirconia framework using a fluid ceramic fused at high temperatures.