###### Abstract  
The aim of this study is to assess the surface alterations of four reciprocating instruments before and after the shaping of four resin-simulated root canals. The following four different reciprocating instruments are selected: 10 Reciproc Blue (RB25), 10 WaveOne Gold (WOG), 10 EdgeOne Fire (EOF) and 10 recently introduced instruments OneRECI (OR), for a total of 40 new instruments. Before root canal shaping, each instrument is mounted on a stub in a standardized position and observed using a scanning electron microscope (SEM) to detect any surface alteration, such as microfractures, metal defects, deformations, blunt and disruption of cutting edges, debris, pitting and tip flattening. Micrographs are acquired at the level of the tip, 4 mm, 8 mm and 12 mm from it. After that, each instrument is used in four simulated resin root canals. SEM observation was repeated after the simulated clinical use to assess the wear resistance of the instruments. Surface alterations are registered before and after instrumentation and statistical analysis is performed using a Chi-Square test to verify homogeneity of defects distribution and GLM to evaluate the differences of RMS at baseline and after use for both groups (\(\alpha\) level 0.05). Before simulated clinical uses, no alterations are found except for three cases of EOF with metal strips in correspondence to the tip, disruption of the cutting edge of WOG and oily spotting on two different OR. After simulated clinical use, EOF shows a statistically significant difference in terms of spiral distortion and flattening of the cutting edges. The OR shows the highest presence of debris despite the ultrasonic cleaning procedures. No instrument fractures are observed. EOF should be discarded after four clinical uses and carefully inspected after each insertion into root canals. Moreover, tough attention should be paid during disinfection and cleaning procedures after instrumentation considering the copious debris detected in each instrument, particularly in OR. Keywords:endodontics; nickel-titanium rotary instruments; reciprocation; root canal treatment; scanning electron microscopy; wear analysis +
Footnote †: journal:reciprocating cycles according to the difference between the angle in the cutting verse and the angle in the noncutting one [1; 3]. The benefits arising from this kinematic have been thoroughly investigated, concluding that the reciprocating motion is able to increase both cyclic fatigue and torsional resistance. Regarding the first one, it has been demonstrated that reciprocating movements increase the cyclic fatigue resistance in comparison to continuous rotation. This is due to the fact that the alternating rotation leads to a distribution of the tension-compression strain cycles acting on the region of the instrument engaged in the point of the maximum curvature of the root canal, reducing the concentration of stresses [4]. Moreover, since the instruments used in reciprocation are subjected to less torsional stress since they continuously engage and disengage dentine, reducing the incidence of torsional fracture by taper-lock [5; 6]. Despite this, over the years, several limitations have been highlighted. Firstly, reciprocating instruments have been criticized for the possibility of extruding debris and bacteria in the periapical space in comparison to continuous rotary instrumentation [3; 7; 8]. Despite this, to date, the literature includes conflicting results, so further studies are needed [3]. Nevertheless, during the use of single-file reciprocating instruments, it is suggested to recapitulate and irrigate after each insertion into the canal system in order to avoid any blockage and extrusion of debris [9]. Another point that has been raised is the less cutting efficiency of the instrument used with a reciprocating motion. Nevertheless, even in this case, the data are conflicting and a unanimous consensus was not reached [3].