## 4 Discussion  
The loss of preload is one of the most frequent complications in implant-supported prosthodontics, belonging to the subcategory of mechanical complications [27; 28; 29]. It always precedes the screw loosening, that has an incidence of 5.3% after one year of loading and between 5.8% and 12.7% after 5 years [16; 17; 29; 30; 31]. According to the literature the loss of preload is influenced by several factors [17; 27; 28], including the type of material used for the fabrication of the abutment and the connecting screw, the fixture geometry connection, the type of prosthesis, the shape of the abutment screw head and the implant screwing method [18; 19; 20; 24; 32; 33; 34; 35]. In the present study all these factors, except the metal alloy composition of the screw and the abutment of the two joint systems, were controlled. In particular, in the MUA system used, both the abutment and connecting screw are composed of grade 4 commercially pure titanium while the OT-Bridge system has a titanium nitride coating the low-profile attachment with an anodized titanium anchoring screw. This could be the reason why only in the MUA system a difference was found between initial torque value and the torque value registered after 10 min in absence of loading. Probably the high Young's modulus and the low elastic recovery of grade 4 commercially pure titanium produce an important friction during the mating threads [18; 19], resulting in a loss of preload. We can thus hypothesize that the alloy composition of OT-Bridge system prevents this situation. In literature, some studies concluded that the material of the connecting screw plays a great role in the loss of preload, evidencing that Gold-Tite screws better adapt the mating counterpart of the implant bore, thanks to the ductility and malleability properties of this material [18; 19; 36; 37]. An interesting finding of this study was that the two connection systems did not show statistically significant differences regarding the loss of preload following the simulation of 400,000 loadings on distal cantilever, that correspond approximately to 1-year of cyclic loading [24; 25; 26]. The application of the occlusal forces on the distal areas, perpendicular to the occlusal plane and parallel to the long axis of the anterior implants, in light of the considerable mechanical stress, produces a statistically significant difference between the connecting screws on anterior and posterior implants only in Model A (MUA system). The absence of this evidence in OT-Bridge system could be due to the presence of the acetal ring, which provides a snap retention opposed to the upward traction force. These results suggest that MUA and OT-Bridge are reliable prosthetic anchoring systems even in unfavorable conditions like the application of occlusal forces on a 7-mm distal cantilever. However, an important difference for the biomechanical comparison of these two systems is the presence of the acetal ring on the OT-Bridge system that stabilizes the prosthetic structure, in case of screw loosening. In other studies, it would be important to assess the behavior of this acetal ring during all the masticatory functions and evaluate the significance of the \"snap\" retention on the extra-grade abutment of the prosthesis, once the correct tightening of the screw has been lost. The limits of this work are principally due to the in -vitro nature of the study. The two joint systems were assessed in only one important stress condition, by performing compressive cycles on 7-mm distal cantilever. Other loading conditions were not investigated but occur in the patient's mouth. As the effects of saliva and food on the longevity of MUA system are well-known in literature, little or nothing is present for OT-Bridge system. However, OT-equator is also used for overdenture rehabilitation and is proved to be effective in the mouth of the patient [14; 38]. Also the temperature may influence the preload loss and in this study we do not evaluate this.