This PhD thesis is focused on the study of the fatigue of the Ni-Ti endodontic instruments that are used during the shaping phase of a root canal treatment. The studies carried out have analyzed different aspects of the topic. In the first work was evaluated the influence of the type of movement on the fatigue of the instrument. In particular, 5 different types of endodontic instruments were tested in vitro, they were associated with 2 types of movement: continuous rotation and optimum torque reverse. The results obtained show that the use of the instruments tested in association with the OTR movement causes less fatigue compared to the association with the continuous rotation movement. In the second research work the same survey was carried out concerning the use of the two different rotational movements applied to two types of instrument that are used as single-file, also in this case the results show an advantage in terms of resistance to cyclic fatigue when the reciprocating mode of the OTR movement is selected. The third work deals with analyzing how the cutting efficiency varies and how the surface of the root canal instruments is modified according to the number of canals treated. Resin blocks were used to simulate the root canals, in order to standardize the working conditions of the root canal instruments. A type of reciprocating single-file instrument was used: the Reciproc R25. Each instrument has been used to shape 6 artificial canals. Each instrument has been analyzed, after each instrumentation, using SEM for the evaluation of superficial alterations. At the same time each resin block is photographed and processed with AutoCAD for the calculation of the lumen area, in order to quantify the amount of resin taken away from the instrument during the shaping phase. The results obtained show that the cutting efficiency for the instruments tested is progressively reduced with their use, the statistically significant difference, in terms of cutting efficiency, is shown between the instruments used to instrumentate 1 and 2 canals and the instruments used to instrument 5 canals. The fourth work is about the influence that the type of access cavity performed exerts on the fatigue of the instrument during the shaping phase. The Reciproc R25 blue was chosen in combination with a conservative access and a truss access. The scientific literature supports the use of conservative access cavities, because this type of access offers an adequate compromise between the preservation of the tooth substance and the possibility of correctly tackling the subsequent phases of the endodontic treatment. There is no information about the truss configuration, which however some clinicians use. In the aforesaid work it has been shown that the truss access cavity determines a greater fatigue of the reciprocating instruments during their use. On the basis of the work carried out, it is possible to state that the technology available applied to endodontic instruments allows today the clinician good chances of success as regards the correct execution of an endodontic treatment. The variables to be considered are many and it is necessary to continue the development of each aspect in order to have an ever greater control over the procedures that will be carried out. It is also evident that, given the wide range of endodontic instruments available, it is fundamental for the operator to know the characteristics of each to choose the appropriate instrument based on the case to be treated.
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