Issue 28

A. Brotzu et alii, Frattura ed Integrità Strutturale, 28 (2014) 19-31; DOI: 10.3221/IGF-ESIS.28.03 20 I NTRODUCTION n the practice of endodontics the introduction of Ni-Ti rotary instruments has reformed root canal treatment by reducing time required to finish the preparation andminimizing procedural errors associatedwith stainless steel hand instrumentation. The use of Ni-Ti alloys for those applications comes from their important properties such as shape memory and superelasticity phenomena, good corrosion resistance and high compatibility with biological tissues. The stainless steel instruments, employed in the surgical treatment of root canal, are used as files inserting the instruments in the channel and applying to it amanually axialmovement. Therefore, the operatingmethods give to the instrument almost exclusively tensile stress. These tools are very thin and with little taper in order to reach the root apex. The channels machined with these tools are characterized by surfaces that are not perfect and so the risk of failure in final sealing is highly recurrent. The use of rotary instruments with greater taper allows to get perfectly conical channels, easily sealable, thus reducing the risk of recurrence. Obviously strains are high because the instrument works in a curved channel, thus it is necessary to use strong materials able not only to work dentin but also to get significant deformations. Those deformations have to be fully recovered once instrument is pulled out from the channel. Therefore, superelasticNitinol® alloys are particularly suitable for such use, being able to withstand deformations exceeding 5%, and then completely retrieving. Despite their increasing popularity, a major concern with the use of Ni-Ti rotary instruments is the possibility of unexpected failure in use [1, 2]. The breakingmode ofNi-Ti rotary instrumentsmay be classified into flexural fatigue and torsional (shear) fracture according to their appearance after breaking [3]. Torsional failure is usually accompanied with macroscopic distortion or unwinding of the flutes adjacent to the fractured end, whereas flexural fatigue often presents as an unexpected fracture without unwinding defects. All manufacturers stated that the only predictable way to prevent flexural fatigue is to discard the instrument regularly after a certain number of uses. The cycle number for instrument re- using depends on the type of tooth that was treated, with the greatest number in anterior teeth and the lowest inmolars [4]. In the biomedical field these materials are often subject to cyclical loads that can lead to fatigue failure, and that the particular mechanical behavior of these alloys is derived not from the usual dislocationmovement but from the dysplastic transformation austenite-martensite. That transformation allows thematerial to accumulate large reversible deformations; many researchers, in recent years, have tried to correlate this property with the experimental evidence of studies performed both by fatigue standard tests carried out directly on artifacts for simulating the operating conditions ofNitinol instruments asmuch as possible. The fatigue life of endodontic instruments, whatever their structure and the stress levels at which they are subjected, is somewhat limited (around 1000 cycles, [5]). Tests directly performed on the drills into channel simulacra show that the fatigue life for these instruments depends on several factors, amongwhich is: the surface defective state, that usually characterizes them, and instrument geometry. However, the fatigue life of the tool in these cases is lower than the theoretical one and does not exceed 500 cycles [6-8]. The initial step of crack formation of these tools is the initial step of trigger as the level of stress applied is such that once the fatigue crack develops, the stress expressed in terms of ΔK applied is so high and close to the levels of rupture of Nitinol alloys [1, 9] that the instruments rapidly fail. It is usually assumed that lubrication during root canal preparation would lower mechanical stress on rotary root canal instruments and therefore prevent instrument failure [10]. However, few data occur in the literature to confirm or refute that opinion. A lubricant may play a physical effect by moving debris away from the rotating instrument. Furthermore, chemical additives could operate on the root canal dentin to facilitate instrumentation, as example Calcium-chelating lubricant, by dissolving inorganic dentin components, softens the root canal wall, whereas sodium hypochlorite (NaOCl) attacks the organic dentin matrix [11]. Thus, both NaOCl and Calcium chelators, like EDTA, can lower root dentin microhardness [12]. The impact of lubricant parameters on simulated root canal instrumentation has been investigated and the performed tests allow to demonstrate that an aqueous lubricant ismore beneficial than a gel-type counterpart [13]. The potential difficulty in removing instrument fragments from root canal and a perceived adverse prognostic effect of this procedural complication, together with short time duration of those rotary instruments are the main reason to study the instrument fracture mechanism and how it may be prevented rather than treated. The present research completes a previous preliminary study [14], with the target to determine the incidence and the mode of instrument failure of Ni-Ti rotary systems with and without aqueous lubricant. Furthermore a deepened FEM analysis has been performed on the rotary instruments inorder to verify the applied stress. I