## 4 Discussion
The use of a titanium framework and a monolithic zirconia superstructure allows for both parts to be CAD-CAM designed and milled, with highly polished interfaces and an anatomically ideal transmucosal part, both in terms of form, low bacterial adhesion and good cell adhesion [19]. The titanium framework is designed to leave at least 2 mm for the zirconia superstructure and obtain the ideal stress distribution [18]. The presence of two different strain patterns in the same prosthesis, however, may expose patients to some long-term risk for both superstructure chipping and the fracture of either the framework of the superstructure [13]. Figure 1: Zirconia–titanium hybrid ISFCDPs (**A**) Titanium bar acting as framework. (**B**) Monolithic zirconia superstructure with titanium bar being lowered inside. (**C**) Assembled ISFCDP. (**D**) Clinical view. Figure 2: Progressive (or multi-layer) Zirconia ISFCDPs. (**A**) A digital scan of the analog impression. (**B**) The analog impression with visible abutments. (**B**) The digital design of the ISFCDPs. (**C**) Rry-in ISFCDPs produced in resin through 3D printing. (**D**) Final monolithic progressive zirconia ISFCDPs, on which the gingival portion of the protheses was stratified with esthetic pink ceramic. (**E**) Clinical view. Progressive (or multi-layer) monolithic zirconia ISFCDPs have shown promising clinical outcomes, eliminating the possibility of chipping of layered ceramic and providing a homogenous structure capable of distributing the occlusal load properly [13]. Compared to translucent monolithic zirconia, progressive monolithic zirconia presents different strengths depending on which depth of the prosthesis volume is considered: in the inner part of the prosthesis, or core, a tougher and more opaque layer of zirconia is present; on the outer part of the prosthesis, a less resistant but more translucent layer of zirconia is present [13]. The presence of a stronger inner core and a more esthetic outer layer potentially avoids the main issue of full-arch esthetic monolithic zirconia ISFCDPs, which exhibit a higher incidence of prosthetic fracture compared to high-strength opaque monolithic zirconia [19; 20]. The production of a monolithic zirconia ISFCDP requires a digital design, which is then milled: prosthetic components are then glued to the prosthesis, which is screwed directly onto the implant platforms. Prosthetic design must allow for an optimal distribution of forces and adequate gingival adaptation, allowing effective cleaning procedures and avoiding food impaction. However, the literature regarding this technique is scarce: more long-term studies on the performance of progressive (or multi-layer) zirconia ISFCDPs are still needed [13]. **Zirconia-on-zirconia (zirc-on-zirc) ISFCDPs (Figures 3-5)**  
The protocol adopted for the present zirc-on-zirc ISFCDPs was designed by one of the authors (M.S). It consists of a digitally designed and 3D-printed wax up of the ISFCDP, which is then adapted on the master model obtained by impressions of the patient's arch. A temporary ISFCDP is then produced and used for the provisional phase of the restoration. After adjustments, if any, the temporary restoration is scanned intra-orally and refitted to the master model. Figure 3: A zirconia-on-zirconia ISFCDP. (**A**) 1124 MPa white zirconia framework with the multi layered 750 Mpa ceramic functional portion, divided in three sections. (**B**) The functional portion is cemented to the white zirconia framework using a dedicated ceramic fused at 90 \({}^{\circ}\)C. (**C**) Aesthetic pink ceramic and characterizations are added. (**D**) Final result. Based on the digital model of the temporary ISFCDP, the framework is milled from a hard-strength white zirconia disk. The framework shows abutments for crown cementation corresponding to the final teeth position, and pink ceramic is stratified and baked on the lowest part of the ISFCDP, which will be in contact with the patient's soft tissues.