the use of nickel - titanium ( niti ) rotary instruments for root canal system preparation has increased due to their undeniably favorable qualities ; however , unexpected fracture is an important disadvantage of these instruments [ 13 ] . controversy remains regarding the contribution of torsional fracture , fatigue fracture and the combination of both to the separation of niti rotary instruments . some have implicated fatigue fracture to be a main reason for the separation of endodontic files in the clinical setting [ 2,5 , 6 ] . fatigue fracture occurs due to repeated compressive and tensile stresses accumulated at the point of maximum flexure of an instrument rotating in a curved canal without the instrument being bind to the root canal . it should also be noted that controversy exists regarding the effect of un - retrieved separated files in root canals on the outcome of endodontic treatments . considering that the retrieval of separated instruments from the root canal system is very difficult and in some cases impossible and that un - retrieved fractured files may have an adverse effect on the outcome of endodontic treatments , overcoming or reducing the risk of this problem is of high clinical significance . neoniti ( neolix , chtres - la - fort , france ) is a newly introduced niti rotary system with a non - homogeneous rectangular cross section and multiple taper in a single instrument ; it consists of one c1 and three a1 ( with # 20 , # 25 and # 40 tip sizes ) files . the taper in the a1 # 25 file is 0.08 from d0 up to d5 ; whereas from d5 to d16 the taper is 0.04 . it is manufactured using a newly developed wirecut electrical discharge machining ( wedm ) process . the manufacturer claims that this manufacturing process is highly precise down to the micron , oil - free and clean and stress is limited to the metal surface during this process . furthermore , it produces a rough surface , resulting in abrasive properties that enhance the speed of root canal preparation . moreover , the manufacturer claims that these files undergo appropriate heat treatment that results in high flexibility and shape memory of this system . protaper next ( dentsply , tulsa dental specialties , tulsa , usa ) is another newly marketed rotary file system made of m - wire niti with a rectangular cross - sectional design . twisted file ( sybronendo , orange , ca , usa ) is manufactured by twisting a niti wire with a triangular cross - section and r - phase crystalline structures created by heating and cooling . after completion of twisting , the file is heated and cooled again to maintain its new shape while converting back into its austenite crystalline structure . mtwo ( vdw gmbh , munich , germany ) and race ( fkg dentaire , la - chaux - de - fonds , switzerland ) rotary instruments are two routinely used systems for root canal cleaning and shaping . s cross - section with two cutting edges resembling that of the s - file . race instruments have a triangular or square - shaped cross - section and alternating cutting edges . considering that no research has been done regarding the mechanical properties of neoniti rotary files , the aim of this study was to compare the cyclic fatigue resistance of neoniti a1 # 25 , race 25/.06 , mtwo 25/.06 , twisted file 25/.06 and protaper next x2 file . artificial grooves simulating three canals measuring 1.5 mm in width , 20 mm in length , and 2.5 in depth with a u - shaped cross - section were machined into 316l stainless steel blocks by computer - assisted milling and hardened with polished chrome plating . three types of trajectories were designed according to the following parameters : group a : a straight cervical segment measuring 5.29 mm with an arc length of 9.42 mm , a curvature radius of 6 mm , a straight apical segment measuring 5.29 mm , and the arc in the middle portion of the canal ; group b : a straight segment measuring 7.44 mm with an arc length of 12.56 mm , a radius of 6 mm , and the arc in the apical portion of the canal ; group c : a straight cervical segment measuring 10.58 mm with an arc length of 9.42 mm , a curvature radius of 6 mm , and the arc located in the apical portion of the canal . a 4-mm - thick glass was screwed in front of the simulated canals to prevent the instrument from slipping out ( fig . 1 ) . comparison between trajectories a and c was done to evaluate the influence of the arc location ( i.e. in the apical vs. the middle portion of the canal ) on the ncf of rotary instruments . comparison between trajectories b and c was done to evaluate the influence of the arc length ( i.e. 9.42 mm or 12.56 mm ) on the ncf of rotary instruments . five types of rotary files with a similar tip size were used and evaluated in this study : 
 neoniti a1 # 25race 25.06mtwo 25.06twisted file 25.06protaper next x2 file protaper next x2 file three types of trajectories designed for this study . b. trajectory with curvature located in the apical third . c. trajectory with curvature located in the apical third with smaller arc length . the hand - piece of an endodontic electromotor ( x - smart ; dentsply maillefer , ballaigues , switzerland ) with a reduction ratio of 1:20 was fixed above the block so cyclic fatigue could be assessed in a static mode ( fig . 2 ) . after putting a glass slab on the block and fixing the hand - piece to the block with the file in it , the instruments ( n=10 from each subgroup ) freely rotated in a the rotational speed and torque suggested for all evaluated rotary instruments by their manufacturers were similar except for that of twisted file . to standardize the study , twisted file placed in the trajectory of group a ( right image ) liquid paraffin ( kimiagar toos , mashhad , iran ) was used as lubricant during the file rotation and it was applied on the canal walls by a micro - brush . the instruments were used until fracture occurred and the time to fracture was recorded in seconds by two methods : ( a ) direct visualization with a 2.5 loupe ( heine hr binocular loupes , heine optotechnik , herrsching , germany ) and listening ( b ) playing captured videos by corel video studio prox2 software ( corel corp . , ottawa , canada ) . the ncf was calculated using the following formula : ncf = time ( seconds ) to failure  rotational speed/60 . the exact angle and radius of the curvature in which each file was constrained in , were calculated from the snap shots taken from the files while rotating in the trajectories as measured by pruett et al . . fractured tips were evaluated using a stereomicroscope ( model la - szm45-b1 , nanjing sunny optical instrument co. , ltd . , nanjing , china ) at 7 magnification and am423x dinoeye digital eyepiece and rll64 led light source ( led model ) . images were saved in jpeg format by dinocapture 2.0 software and the length of the fractured file tips was measured . one - way anova and tukey s hsd test were used for inter- and intra - group pairwise comparisons of ncf , separated fragment length , angle and radius of curvature ; the significance was determined at the 95% confidence level . the overall mean time of fracture calculated visually and on the captured film was 82.0291.09 seconds and 80.5289.53 seconds , respectively . the difference seen between the two different methods of calculation was not statistically significant ( p=0.059 ) . the mean , minimum and maximum values of ncf for each group are shown in table 1 . according to the results of the current study in all curve types neoniti showed the highest and race showed the lowest ncf values ( p<0.05 ) , indicating the highest and lowest fatigue resistance , respectively . in group a , a significant difference was seen in the ncf among the evaluated files ( p<0.001 ) . 3 ) : the mean  standard deviation of ( a ) number of cycles to fracture ( b ) fragment length ( c ) angle of curvature and ( d ) radius of curvature of experimental subgroups . the mean  standard deviation ( sd ) , minimum and maximum ncf values of each subgroup . groups with statistically significant differences are shown with similar symbols ( alphabets or numbers ) . neoniti and twisted file showed a significant difference when compared with each other ( p<0.001 ) . furthermore , both neoniti and twisted file showed significant differences when compared to other subgroups ( p<0.001 ) . in groups b and c , the ncf values from the lowest to the highest were as follows ( fig . 3 ) : race < protaper < twisted file < mtwo < neoniti . in groups b and c , a significant difference was seen in the ncf among the evaluated files ( p<0.001 ) . race , protaper and twisted file showed no statistically significant differences ( p>0.3 ) ; twisted file and mtwo had no statistically significant difference either ( p>0.4 ) ; whereas , neoniti had a statistically significant difference with all other groups ( p<0.001 ) . the mean length of separated fragment of instruments in each subgroup is shown in table 2 . overall , the fragment length of separated instrument in group a was significantly longer than that in group b ; furthermore , these values in group b were significantly longer than those in group c. in group a , no statistically significant difference was seen ( p>0.1 ) . the twisted file separated fragments were the longest while the race separated fragments were the shortest . in groups b and c , significant differences were seen in the separated fragment length of different groups ( p<0.001 ) . b , the longest separated fragment length was seen in race ; whereas , in group c , the longest separated fragment length was seen in protaper . in group b , protaper , mtwo and twisted file had no significant difference with all other file types ( p>0.2 ) ; whereas , race had a statistically significant difference with neoniti ( p=0.003 ) . in group c , neoniti had a statistically significant difference when compared to all other file types ( p<0.001 ) ; whereas , twisted file and mtwo had no statistically significant difference ( p=0.4 ) . the mean angle and radius of curvature of each subgroup are shown in table 2 . overall , the highest and the lowest curvature angles were seen in race and neoniti , respectively . regarding the radius of curvature , neoniti and protaper showed the lowest and mtwo showed the highest angle of curvature . in groups b and c , the angle and radius of curvature also showed significant differences ( p<0.001 ) . the highest and the lowest angle of curvature in group b belonged to protaper and neoniti , respectively ; whereas , in group c the highest and the lowest values were seen in race and neoniti , respectively ( fig . 3 , table 2 ) . in group a , neoniti showed the lowest radius of curvature ; whereas in groups b and c it showed the highest values ( fig . the overall mean time of fracture calculated visually and on the captured film was 82.0291.09 seconds and 80.5289.53 seconds , respectively . the difference seen between the two different methods of calculation was not statistically significant ( p=0.059 ) . the mean , minimum and maximum values of ncf for each group are shown in table 1 . according to the results of the current study in all curve types neoniti showed the highest and race showed the lowest ncf values ( p<0.05 ) , indicating the highest and lowest fatigue resistance , respectively . in group a , a significant difference was seen in the ncf among the evaluated files ( p<0.001 ) . 3 ) : the mean  standard deviation of ( a ) number of cycles to fracture ( b ) fragment length ( c ) angle of curvature and ( d ) radius of curvature of experimental subgroups . the mean  standard deviation ( sd ) , minimum and maximum ncf values of each subgroup . groups with statistically significant differences are shown with similar symbols ( alphabets or numbers ) . neoniti and twisted file showed a significant difference when compared with each other ( p<0.001 ) . furthermore , both neoniti and twisted file showed significant differences when compared to other subgroups ( p<0.001 ) . in groups b and c , the ncf values from the lowest to the highest were as follows ( fig . 3 ) : race < protaper < twisted file < mtwo < neoniti . in groups b and c , a significant difference was seen in the ncf among the evaluated files ( p<0.001 ) . race , protaper and twisted file showed no statistically significant differences ( p>0.3 ) ; twisted file and mtwo had no statistically significant difference either ( p>0.4 ) ; whereas , neoniti had a statistically significant difference with all other groups ( p<0.001 ) . the mean length of separated fragment of instruments in each subgroup is shown in table 2 . overall , the fragment length of separated instrument in group a was significantly longer than that in group b ; furthermore , these values in group b were significantly longer than those in group c. in group a , no statistically significant difference was seen ( p>0.1 ) . the twisted file separated fragments were the longest while the race separated fragments were the shortest . in groups b and c , significant differences were seen in the separated fragment length of different groups ( p<0.001 ) . in group b , the longest separated fragment length was seen in race ; whereas , in group c , the longest separated fragment length was seen in protaper . in group b , protaper , mtwo and twisted file had no significant difference with all other file types ( p>0.2 ) ; whereas , race had a statistically significant difference with neoniti ( p=0.003 ) . in group c , neoniti had a statistically significant difference when compared to all other file types ( p<0.001 ) ; whereas , twisted file and mtwo had no statistically significant difference ( p=0.4 ) . the mean angle and radius of curvature of each subgroup are shown in table 2 . overall , the highest and the lowest curvature angles were seen in race and neoniti , respectively . regarding the radius of curvature , neoniti and protaper showed the lowest and mtwo showed the highest angle of curvature . in groups b and c , the angle and radius of curvature also showed significant differences ( p<0.001 ) . the highest and the lowest angle of curvature in group b belonged to protaper and neoniti , respectively ; whereas , in group c the highest and the lowest values were seen in race and neoniti , respectively ( fig . 3 , table 2 ) . in group a , neoniti showed the lowest radius of curvature ; whereas in groups b and c it showed the highest values ( fig . different types of rotary files exhibit differences in resistance to fatigue failure due to differences in various determinants such as their manufacturing process , structural characteristics and geometric designs , surface texture , the type of curve they are introduced into   and the method used for fatigue failure calculation . according to the results of this study , no significant difference was seen between the fracture time estimated by direct visualization and watching a captured video clip . when reviewing the captured video clips , the time of fracture can be determined more precisely and easily due to lack of eye strain . fatigue failure can be evaluated in two different modes of  static  and  dynamic  . in the dynamic mode , the hand - piece is promoted in a back - and - forth axial motion while the instrument is driven in a full rotational movement inside the trajectory . this model is more similar to clinical conditions in which rotary files are used ; but to standardize the amplitude and speed of the axial movement of the hand - piece , a device must be designed . it should be noted that the fatigue resistance of rotary files may be higher when used in the dynamic mode due to the axial movement , which prevents stress concentration in the same area allowing stress distribution along a larger portion of the instrument . conducting another research that compares the fatigue failure of these rotary instruments in the dynamic mode is suggested . three ways for improving the longevity of endodontic files have been suggested which include the following : ( i ) thermal treatments before machining ; ( ii ) choosing machining conditions adapted to the niti alloy and ( iii ) electro - polishing . this study compared the cyclic fatigue resistance of different types of niti rotary files with different manufacturing techniques and different improvement strategies . twisted files are formed by twisting a triangular blank in combination with heat treatment and special surface conditioning , which conserves the natural grain structure . race files are developed by a grinding process , which is followed by electro - polishing . the mean ncf of all rotary files was lower in group a compared to groups b and c implying that the resistance to fatigue failure is lower in files used in root canals exhibiting a more coronal curvature than apically placed curvatures . in more coronally located curvatures , the portion of the instrument situated in the critical stress concentration point has a greater diameter and undergoes higher levels of metal fatigue   opposed to when the arc is located in the apical portion . also these values were higher in group b in comparison to group c indicating that the shorter the length of the arc of curvature the higher the resistance of the file used in that trajectory to fatigue failure . controversy remains regarding the effect of electro - polishing on the fatigue failure of rotary files . some studies have shown that electro - polishing improves the instrument s working properties such as resistance to failure by producing a smooth and homogeneous protective surface oxide layer with less defects and residual surface stress ; whereas , herold et al . inferred that electro - polishing did not prevent the development of microfractures and barbosa et al .   concluded that electrochemical polishing had no influence on the resistance to fracture of the rotary instruments tested . according to the findings of our study race had the lowest ncf values in comparison to all other experimental rotary files in all canal types evaluated . mtwo files are expected to show lower fatigue resistance than files undergone electro - polishing , such as race , due to the crack - like surface features created subsequent to their machining manufacturing process . on the contrary , the mtwo group exhibited higher ncf values than race , although the difference was not statistically significant . in addition , while m - wire alloys are considered to be more flexible and more resistant to fatigue than conventional niti alloys , mtwo showed no statistically significant difference when compared to protaper next . gambarini el al ,   also concluded that instruments made of m - wire alloy did not show higher resistance to fatigue when compared to instruments produced by the traditional niti . twisted files showed significantly higher ncf values than race , protaper next and mtwo in curvatures located in the middle third of the trajectory ; whereas , in more apically located curvatures no significant difference was seen between the aforementioned files . this may be due to the unique manufacturing process and heat treatment of this file . further research regarding the physical and mechanical properties of this newly marketed file is required . it should be noted that there is a slight inconsistency between the shape of the files and that of the trajectory they are rotating in . this was seen when evaluating the angle and radius of curvature that each rotary file formed in the trajectories . it is unclear whether the slight differences seen in the angle and radius of curvature between rotary file types have a clinically significant effect on the resistance to fatigue failure of the evaluated files . for instance , while neoniti files used in the group a trajectory had a similar angle of curvature and shorter radius of curvature compared to other files in group a , this file had the highest ncf values . it should be noted that lower fatigue resistance is expected in shorter radius of curvatures . in group c , although race files showed the highest angle of curvature , these files exhibited the lowest ncf values . furthermore , neoniti files in this group exhibited the highest resistance to fatigue even though they had the lowest angle of curvature . further research is required regarding the clinical impact of slight differences in the angle and radius of curvature on the resistance to fatigue failure of endodontic rotary files . finally it should be noted that in clinical conditions , rotary files are used in a dynamic mode , endure torsional stresses and cyclic fatigue at the same time and are bind to the root canal walls ; therefore , their fatigue resistance may be different from the results of this study . the fatigue resistance of the evaluated rotary files was lower in more coronally located curvatures . neoniti exhibited the highest and race exhibited the lowest fatigue resistance compared to other evaluated files .