## 4 Discussion  
The aim of this study was to assess the static and fatigue resistance of Lava Ultimate crowns bonded directly to a titanium abutment (group 1) or bonded to a custom zirconia framework and then cemented to a standardized titanium abutment (group 2).  
The composite crown directly bonded to a titanium abutment failed first in the static test compared to the multilayer solutions, and in the fatigue test, it systematically failed before ending all the cycles' load, thus suggesting that the customized zirconia framework (size and shape) significantly influences both the fatigue and static resistance.  
In group 1, the static load results showed a mean fracture resistance 200 N lower than in group 2. Moreover, the type of fracture observed in group 1 was always complete and left the titanium abutment completely exposed.  
Instead, in group 2, the fracture has occurred with complete debonding of just one portion of the crown at the point of load application. A large part of the composite crown, however, remained perfectly bonded to the zirconia abutment.  
The differing response shown in the dynamic testing between the two sets of samples can be explained by the geometric morphology of the two abutment designs. In group 1, the applied forces are distributed fully on the abutment and discharged vertically on the implant, delaying the creation and spread of fractures due to the shape of the zirconia tailored abutment, which is free of sharp edges.  
In group 2, the forces applied to the occlusal surface are concentrated on the titanium abutment roof, acting as a wedge against the internal composite crown. The load forces are not discharged in a uniform manner on the titanium abutment, resulting, as discussed in [22], in mechanical conditions (i.e., sharp edges/fillets) that allow the propagation of cracks, which increase during load cycles, until sample failure. Therefore, the shape of the titanium abutment determines premature failure of the samples rather than the material itself.  
The values in the static test recorded in our study are higher than those reported in a similar test, which investigated prosthetic restorations of premolars. In this study, the fracture resistance of monolithic lithium disilicate (1900 N) and metal ceramic [23] (1800 N) and zirconia veneered with porcelain was evaluated [24] (1700 N).  
From the point of view of the dynamic load, one study found that the breaking value for a lithium disilicate premolar prosthesis was 1200 N to 800 K cycles [25]. Another paper [26] am average value of failure of 1280 N at 160 K cycles in 70% of some of a CAD-CAM ceramic material (paradigm C), and tested with a protocol similar to that used in this study. In the same article, the author tested identical samples manufactured in a CAD-CAM composite material (MZ 100), detecting a 100% survival rate at 1400 N for 180 K cycles.  
It is obvious that these values are strongly influenced by the morphology of the sample. In fact, incisors tested in another study [27] reached values of 240 (paradigm C) or 280 N (MZ 100) at about 120 K cycles in both prosthetic solutions. Another study [2] compared CAD-CAM crowns made with ceramic material (MKII) and composite (MZ 100) on extracted molars using a loading protocol similar to that used in previous studies and measured a fatigue fracture resistance of 1147 N and survival of 0% in the first group and a survival of 73% of the samples in the second group.  
An unsolved problem is the use of coupling agent, including both its type and resin-implant connection, and this is worthy of a deeper analysis to provide a more comprehensive understanding of the mechanical relations of the implants in the mouth during their lifecycle.  
In vitro study is important to achieving mechanical quality of materials, but it is clear that only long-term clinical trials will provide evidence of in vivo success of materials and production techniques.