## 4 Discussion  
RBFDP is an aesthetic and conservative treatment option to replace a single tooth in a younger patient; therefore, it is a technique sensitive procedure because it requires proper planning of the clinical case and choice of materials. This in vitro study investigated the different types of failure after static fracture strength tests on RBFPDPs fabricated using zirconia, lithium disilicate, and PMMA-based material with ceramic fillers. Zirconia, lithium disilicate, and PMMA-based material with ceramic fillers presented the mean values static fracture strength of 292.5 N (SD 36.6), 210 N (SD 37.6), and 133 N (SD 16.3), respectively. Moreover, different types of failure were collected according to the materials. The 80% of the RBFPDPs made in lithium disilicate and PMMA-based material with ceramic fillers presented fracture of the connector; 20% of debonding and all of the RBFPDPs in zirconia presented fracture of the abutment tooth. The results can be explained by the different elastic modulus of materials. The lithium disilicate and PMMA-based material with ceramic fillers presented lower elastic modulus and, consequently, a greater deformation until the fracture of the materials. Another possible explanation of the fracture of the connector was recognizable to the superior bond strength, with respect to the materials. The authors highlight that the lithium disilicate is not indicated to fabricate RBFPDPs by manufacturer; however, the results showed a mean fracture strength of 210 N. Therefore, clinical studies are necessary to evaluate the performance of this material used to fabricate RBFPDPs. Instead, all the RBFPDPs in zirconia presented a fracture of the abutment tooth. The reasons may be attributable to the high elastic modulus of the material or high bond strength of resin luting cement, with respect to the abutment tooth. Opposite results were observed in clinical trials [16; 17; 18; 19] compared to this in vitro study. The reasons could be several; however, the load strength of the RBFDP is different in each patient. Moreover, the pontic elements may not have static and dynamic contacts. In this in vitro study, the uniform fabrication of all the RBFPDPs with different materials by the same STL file, and the strength applied through a universal machine until failure, allowed to know the mechanic performance of these prostheses with respect to an in vivo study. In literature, the same authors in two in vitro studies evaluated the influence of framework design [14] and mode of loading on the fracture strength [26] on cantilever RBFPDPs fabricated in pre-sintered aluminum-oxide blocks (In-Ceram alumina blanks). The microscopic examination revealed that 58% of the specimens fractured at the connector only, exactly at the framework-to-veneer interface, 17% fractured at the connector, including veneered parts of the pontic, and 25% fractured at the retainer only. The possible difference of the performance should be explained that, in this research, the authors used bovine teeth. Physiologic occlusal forces for adults in the anterior region were determined to be in a range of 10 to 35 N [27]. Maximal incisive biting forces may vary up to 270 N, primarily depending on facial morphology and age [28]. Compared to the maximal incisive biting forces, only zirconia RBFPDs reached values higher than the physiologic values. Even though the mean fracture strength of lithium disilicate RBFPDPs was lower compared to the maximal incisive biting force, it is possible to use the material to fabricate RBFPDPs. However, the clinicians should evaluate, with attention, the static and dynamic contacts of the clinical case before use of lithium disilicate RBFDP. Whereas, PMMA-based material with ceramic filler RBFPDPs represents a cheap and aesthetic solution for a temporary prosthesis for patients waiting for implant treatments. Therefore, the choice of a correct material, according to therapeutic needs, is crucial for the survival of RBFPDPs.