## 4 Discussion  
It is well known that a correct shaping is fundamental for the endodontic success. Especially in case of complex anatomies, a less invasive approach leads to a better prognosis [24; 25]. Instrument alloy and other geometrical variables influence the shaping behavior of the endodontic files [1; 2; 4; 5; 6; 9; 10; 12]. The super-elasticity of modern NiTi instruments reduces the occurrence of canal aberrations and allows for a more centered root canal preparation [9; 10; 26]. Canal centering ability is essential to provide a less invasive shaping, without removing exceeding radicular structure [27; 28]. The instrument pressure patterns against the canal walls is a predictive indicator of the centering ability during shaping [4]. The invention of new endodontic instruments mostly occurs by industrial drawings and derived prototypes, which must pass severe laboratory tests before commercialization: this is a time-consuming and expensive process. FEA was firstly introduced in endodontics for the static analysis of the forces applied to the NiTi files [15; 16; 17; 18; 21]. Afterwards, a dynamic FEA was proposed for the visualization of the instrument mechanical stresses during shaping [17; 18; 19; 29; 30; 31; 32]. In all these studies, the differences found between the instruments were due to the geometry of the instruments and to the alloy stiffness: especially, it was demonstrated how the most rigid instruments caused the highest stress on the apical portion of the canal, confirming that these two instrument characteristics influence their mechanical behavior [4; 19; 26; 33; 34]. Since instrument pressures and their cutting capability may determine canal aberration and it is of utmost relevance to analyze these factors during the project management of endodontic instruments. The discretization of the instruments and the root canals in which instruments rotate allows for a proper dynamic structural analysis during canal instrumentation. Recently, FEA has been introduced as a reliable virtual testing approach to predict the mechanical behavior of NiTi files through a reverse engineering approach [20]. The reliability of FEA for failure prediction was demonstrated by a strict correlation between virtual simulation results and in vitro laboratory tests in terms of the number of cycles to failure and of the localization of breaking points of PTN instruments [20]. Therefore, FEA could be considered as an effective aid for the dynamic analysis and prediction of the mechanical properties of the endodontic instrument [20]. However, nowadays, the shaping ability of the newly developed mechanical NiTi instruments is evaluated only after commercialization by using micro-CT studies [35; 36]. Therefore, the virtual prediction of the instrument shaping ability through a dynamic FEA could significantly ease the design of even more performing instruments [20]. The possibility of a virtual modification of the instrument parameters during an early design phase could optimize the instrument behavior, avoiding the creation of several instruments' prototypes. In the present study, the instruments and the shaped endodontic resin blocks were virtually replicated to perform a dynamic finite element analysis for the evaluation of the pressures generated during the canal instrumentation. Two widely used NiTi endodontic instruments with equal size and taper (#25, 0.06) at tip level but different design, alloy and sections were tested in standardized simulated root canals. The resin blocks were virtually reproduced after their shaping with the two different instrumentation systems, in order to obtain a dedicated enlargement of the root canals for each tested shaping instrument before applying FEA [37; 38; 36]. The pressures variation (_Var_ value) against the Top and Down canal surfaces was systematically lower for PTN X2 than for BR 3 for each different zone of the root canal. Consequently, PTN X2 instrument seemed to remain more centered in the root canal during shaping.