## 4 Discussion  
In the last decades, numerous technologies and improvements in endodontic instrumentation of root canal systems have been developed to achieve proper chemo-mechanical disinfection of root canals, avoiding as many as possible procedural errors and achieving predictable results in the shortest possible clinical time [3]. Recently, the reciprocating motion is gaining in popularity due to its positive clinical experience and scientific results. Undoubtedly, the introduction of single-file reciprocating systematics has facilitated the shaping procedures, simplifying the instrumentation protocols [1,3]. Despite this, the use of a single file for the entire procedure leads to the concentration of cyclic fatigue, flexural and torsional stresses on the single instrument, increasing its wear, despite the mechanical loads being severely reduced thanks to the reciprocating movements [13]. According to this, manufacturers strongly suggest the single use of those systematics, in order to reduce as much as possible their intracanal fracture, as well as the cross-contamination between patients. Nevertheless, this kind of approach leads to the waste of several instruments, increasing the actual cost of root canal treatments without the certainty that it is necessary to avoid instruments' intracanal failure. Regarding this, the wear analysis of reciprocating instruments is crucial to determine the possibility of safely reusing them after a single treatment. To date, there are few data on this topic and there are no data on the evaluation of wear resistance of WOG, R25 Blue, EOF and OR through SEM observation. [PERSON] et al. evaluated the difference in terms of wear resistance between WaveOne and Reciproc after three uses in narrow and straight canals, finding no instrument fractured and no macroscopic signs of plastic deformation, spiral distortion and limited surface defects such as pitting and microcracking [14]. [PERSON] et al. evaluated the wear resistance of WOG and ProTaper Gold (PTG) (Dentsply Maillefer, Ballaigues, Switzerland) after clinical use, stating that the damage of the tip and cutting edge of WOG instruments (which underwent three instrumentation/ sterilization cycles) was significantly more frequent than in PTG. According to the authors, the rationale for those findings is related to the different systematics between the two brands' instruments, with the stress concentrated on a single instrument in WOG [13]. According to the above-mentioned reasons, the aim of this study was to assess and compare the wear resistance of two recently introduced single-file reciprocating instruments, OR and EOF, with other two well-known martensitic single-file reciprocating instruments, R25 Blue and WOG, through SEM observation, by evaluating surface defects before and after the simulated use of the instruments in four canals of resin lower molars. The surface characteristics of NiTi instruments have mainly been evaluated using SEM or atomic force microscopy (AFM) [11]. The first one is considered the most effective and diffused tool for the assessment of wear on instrument surface; however, some limitations have been raised, such as the inability to quantitatively assess the surface characteristics [15]. Despite this, the analysis of the wear defects could be greatly performed through a qualitative method since it consists of the evaluation of the presence or absence of defects. Despite many studies that have demonstrated the relationship between the increase of surface defects and the fracture of NiTi rotary instruments, the assessment of the relationship between the severity of defects and the probability of fracture is still unknown since they propagate slowly through the instruments until a sudden fracture occurs [13]. For those reasons, we believe that the benefits derived from the use of SEM for this aim are much greater than the drawbacks.