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Rapisarda E., Tripi T. R., Bonaccorso A.
Background. Technological progress in the endodontic field has allowed the creation of innovative nickel-titanium endodontic instruments, made with a superelastic alloy with a reduced modulus of elasticity. The elastic properties of the NiTi alloy can undergo significant variations in connection with mechanical movements and cyclic fatigue induced on the instruments caused by their repeated usage, especially in canals which are anatomically difficult to shape. The state of tension of the instrument determines a deformation of the geometry of the files and, therefore, a modification in their properties. Similarly to mechanical stress, thermal treatments can influence the properties of the alloy. The aim of the study is to subject the NiTi alloy to repeated thermomechanical cycles, examining the influence of the mechanical and thermal stress on the temperature at the beginning and end of the martensite transformation, on the increase of recovery of deformation during the cooling phase and on the change in the internal structure of the material.
Methods. The instruments (30 ProFile Maillefer) were subdivided into 5 groups: Group I, composed of 6 new ProFiles submitted to 14 sterilization cycles in an autoclave for 30 minutes each; Group II, formed by 6 unused ProFiles subjected to one sterilization cycle in an autoclave; Group III made up of 6 new ProFiles used as controls; Group IV made up of 6 ProFiles submitted to clinical use on endodontic simulators in plexiglas for a total time of 240 seconds; Group V consisting of 6 ProFiles used on endodontic simulators for a total time of 480 seconds. All the above mentioned samples were tested using a thermomechanic dynamic analyzer (DMTA) produced by Rheometric Scientific.
Results. The files from Group III (control) show an elastic deformation curve typical of nickel-titanium instruments. The samples from Groups IV and V subjected to sequential use on endodontic simulators evidenced modifications in their elastic behaviour compared to the control instruments. The operative use caused modifications in the original properties of the alloy, with a reduction in elasticity proportional to the number of uses. The instruments that underwent 14 cycles of sterilization in the autoclave (I group) present dynamic analysis curves which are different compared to the new unsterilized ProFiles. After 14 cycles there is a modification in the response of the instruments to thermodynamic stress.
Conclusions. Both the samples undergoing repeated mechanical stress on Plexiglas endodontic simulators and those sterilized in the autoclave at thermodynamic analysis show significant modification in modulus of elasticity. The different taper does not influence the experimental results, there not being any significant differences found in the elastic behaviour between ProFiles.