Issue 28

A. Brotzu et alii, Frattura ed Integrità Strutturale, 28 (2014) 19-31; DOI: 10.3221/IGF-ESIS.28.03 30 near the sectionwhere the greater curvature is observed and it is usually observed after instruments seizing events.  The samples tested with both types of lubricant, even if sometimes they show failure from unwinding, tend to be subject to sudden breakage always in the section where the curvature is greater. The failures are anticipated by the formation of cracks perpendicular to the instrument axis, which develop on the cutting edges, usually starting from defects (micro cavities) present in the material from its outset. Longitudinal cracks develop from the early cycles of operation starting from thesemicro cavities, but they never seem to be the cause of the failures.  The analysis of fractures shows a dimple structure, whichoccupies almost the entire surface, indicating a breakage due to plastic type overload. Near cutting edges small areas inwhich fatigue striations are evident with high rates of crack propagationwere found.  The operating mode of these instruments develops a fatigue variable loads which is characterized by a frequency imposed by the operator (in this work is about 2.3 Hz) and a load ratio R, considering the particular shape of the  of Nitinol alloys, approximately near -1. FEM analysis shows that stress applied theoretically to the rotary instruments is still within the load plateau inwhich the stress-induced austenite-martensite transformation takes place. However, it is nearing completion of this transformation so that any overhead that is associated, for example, by the seizure of the tip into the canal or from themeeting of narrowing or odd corners of the canal itself brings thematerial to work fully in the martensitic state. At this point any other material deformation is due to "traditional" plastic sliding, related to dislocationmovement in themartensitic lattice and therefore it is an irreversible deformations. This is the cause of the phenomena of unwinding or localized accumulations of plastic deformation that can lead to sudden failure. In any case, even in the absence of hypothetical overloads, the imposed cyclic deformation is very high.  Comparison with bibliographic data shows that, at these levels of stress, Nitinol alloys have a fatigue life, whatever their structure, around 10 3 cycles. Given the small size and the complex design of the instrument, the same stresses (expressed in terms of K), which act at the apex of cracks nucleated from defects always present in these materials, are very high and near to the critical values of failure. Any slight overload causes that K Max , acting at the apex of the crack, exceeds the value of K IC and then leads to the sudden rupture of the instrument. The final break always occurs withplasticmechanisms as highlightedby the extensive networkof dimples that characterizes the fracture surfaces.  The tests performed show that the use of a lubricant leads to an increase of rotating instrument operating life: this is related to a more effective removal of machining debris from the root canal and the reduction of both torsional and axial loads applied.  From the comparison between the two lubricants, the aqueous gel, showing a higher lubricant capacity, seems to guarantee a longer life of the rotating tool with respect to that based on the EDTA emulsion. This is probably due to a better thermal stability, which keeps constant its rheometric characteristics, even if the temperature rising due to friction between instrument and root canal walls. It also has a higher heat capacity which allows aminor temperature variation during the operation. The emulsion-based on EDTA, at low temperatures, has a very high viscosity, which surelywill result in lower lubricating effect of the gel. This viscosity is greatly reduced as the temperature increases up to become lower than that of the Aqueous gel at temperatures slightly higher to body temperature. This behaviour is probably due to its decomposition procured by a combined effect of temperature and stresses. This last consideration refers to the adopted testing procedure. Test carried out employing root canal simulacra realized with plastic or metallic materials can hardly simulate what really happens inside the root canal during the surgical treatment because perspex or metal chemical, mechanical and physical properties are completely different from those of the dentine. Furthermore, working the Nitinol instruments at the limits of their characteristics, the dexterity of the operator, playing a key role, must be considered. R EFERENCES [1] Pruett, J.P., Clement, D.J., Carnes, D.L. Jr., Cyclic fatigue testing of nickel-titanium endodontic instruments, J Endod., 23 (1997) 77- 85. [2] Shen, Y., Peng, B., Cheung, G.S., Factors associatedwith the removal of fracturedNi-Ti instruments from root canal systems, Oral SurgOralMedOral PatholOral Radiol Endod, , 98 (2004) 605–610. [3] Sattapan, B., Nervo, G.J., Palamara, J.E., Messer, H.H., Defects in rotary nickel-titanium files after clinical use, J Endod, 26 (2000) 161-165. [4] Guettier, P., Safe and quality endodontic practice with ProTaper, In: Wei Shy, ed. Contemporary endodontics, Hong Kong: DentsplyAsia, (2002) 21-24.