## 1 Introduction  
In the last few decades, the screw-retained prosthesis on implants has become increasingly common for the rehabilitation of the totally edentulous patients [1, 2, 3]. Multiple solutions for the rehabilitation of upper and lower jaw were proposed and accepted in literature, basing on the entity of bone atrophy, the prequantization of maxillary sinus, the quantity of bone available and the position of inferior alveolar nerve and mental fororima [4, 5, 6, 7]. The All-On-Four technique for the rehabilitation of completely edentulous mandibles, initially proposed by [PERSON] et al. [8] is nowadays used by several clinicians [9, 10, 11] in order to reach the correct masticatory and speech functions, as well as an optimal prosthetic support, and to re-establish an acceptable situation for the patient [12]. In these type of implant-supported rehabilitation, intermediate components between implant fixtures and prosthetic framework are used in order to correct implant misalignment and to achieve a passive fit of the framework. For this technique, one of the most used anchoring systems is the Multi-Unit-Abutment (MUA), which consists of straight or angulated components of different heights that move the implant internal connection to a conical external connection. In this way, a passive prosthetic fit is allowedeven in case of implant disparallelism. Furthermore, the occlusal stress is moved from the implant screw to the multi-unit abutment screw, that is smaller than the first one and may be the weak point in case of prosthetic complications.  
An alternative to MUA system, recently introduced on dental market, is represented by OT-Bridge(r) system (Rhein 83, Bologna, Italy). The OT-Bridge system is composed of a low profile attachment for overdenture (OT-Equator), a sub-equatorial component represented by an interchangeable undercut acetal ring (Seeger ring) and a cylindrical titanium abutment with a cavity at the retentive extremity designed for insertion of the acetal ring. In this way, the Seeger ring provides a secure and functioning elastic retention system of the abutment, against the possibility of prosthesis unscrewing. The most frequent mechanical complications in implant supported prosthesis are represented by the chipping of the prosthesis, the fracture of the abutment and the abutment/implant screw fracture and loosening [13; 14; 15; 16]. This last complication is always preceded by the reduction of its preload [17; 18; 19; 20], defined as the axial force that, during the tightening of the screw, is generated between the threads of the screw and the internal part of the implant in the long axis direction. Screw loosening is influenced by several factors such as screw geometry, material properties particularly stiffness, surface texture and condition of mating surfaces, degree of lubrication, rate of tightening, integrity of joint, prosthetic misfit etc. [17]. The metal alloy composition of the screw and the abutment is an important factor that defines the entity of the friction during the mating threads and then influence the stability of the joint system in terms of absorption of preload [18; 19]. Also, the type of abutment (machined or cast), and the surface coating or the implant connection systems could play crucial roles relating to this phenomenon.  
At the best of our knowledge, no studies of comparison for prosthodontic complications between MUA and OT-Bridge are today present in literature. The aim of this in vitro study was to compare the loss of preload in MUA and OT-Bridge connection systems using all-on-four prosthetic model, with and without cyclic loading on 7-mm distal cantilever.