nickel - titanium ( niti ) instruments are very popular and provide many advantages during 
 root canal treatment . however , niti instruments have a high risk of separation due to 
 torsional or flexural fatigue ( cyclic fatigue ) . fractures due to cyclic fatigue occur because of metal fatigue 
 and are more prevalent in curved root canals .
the instrument rotates freely at the 
 curvature without binding , so that the tensile and compressive stresses are concentrated 
 at the point of maximum flexure . resistance to fracture prior to an actual fracture is 
 determined by the number of cycles in which an instrument operates under a specific 
 loading condition . when the tip or 
 some part of the file binds in the root canal during rotation , torsional failure 
 occurs .
chemomechanical preparation , cleaning procedures , chemical disinfection , and 
 sterilization cause the corrosion of endodontic instruments .
sodium hypochlorite ( naocl ) is the most commonly used 
 irrigant in endodontic treatment , and 
 its active clo- ions have the potential to increase corrosion .
these are the areas where stress gets concentrated , forming cracks and increasing the probability of 
 fatigue failure .
the corrosive effect of naocl on endodontic niti 
 instruments has been studied previously only in a static mode and for long contact 
 times , which did not simulate actual clinical conditions . the resistance to cyclic fatigue of niti rotary instruments
can be increased via 
 improvements in the manufacturing process or by the use of new alloys with superior 
 mechanical properties .
twisted files ( sybronendo , orange , ca , u.s.a . ) , which were 
 recently introduced , are reported to have better physical and mechanical properties than 
 traditional niti instruments .
they are produced with a special form of thermomechanical 
 processing , which yields a new , heat - treated niti instrument that contains a martensitic 
 phase stable superelastic alloy under clinical conditions .
it has been 
 reported that the additional heating of niti instruments might increase their 
 flexibilityand 
 microhardness .
there have been many studies on the cyclic fatigue resistance of different niti rotary 
 files having different designs or compositions .
there are also several reports related to the effect of 
 autoclave sterilization , for either improving or degrading the surface , mechanical , and 
 cutting properties of various niti rotary systems .
however , 
 there is no information on the influence of both naocl and autoclave sterilization on 
 the cyclic fatigue resistance of niti rotary instruments .
therefore , the aim of this 
 study was to assess the effect naocl solution and autoclave sterilization has on the 
 cyclic fatigue resistance of three traditional ( profile , flexmaster , and mtwo ) niti 
 instruments and one heat - treated ( twisted files ) niti instrument in conditions similar 
 to those used in clinical practice .
four niti rotary instruments - profile ( dentsply maillefer , ballaigues , switzerland ) , 
 flexmaster ( vdw , gmbh , munich , germany ) , twisted files ( sybronendo , orange , ca , u.s.a . ) , 
 and mtwo ( vdw , gmbh , munich , germany ) - were selected for the fatigue resistance test ; 
 all the instruments were of the same size ( # 25 , 0.06 taper ) .
each group included 40 
 unused instruments that were divided into four subgroups ( n=10 ) to evaluate different 
 protocols . in group 1 ,
the instruments were dynamically immersed in 2.5% naocl for 5 min at room 
 temperature . in group 2 ,
the instruments were immersed in naocl , under the same 
 conditions as in group 1 , and then subjected to one cycle of autoclave sterilization 
 ( statim 5000s , scican , leutkirch i m allgu , germany ) at a temperature of 135c for 32.5 
 min ( 17.5 min of sterilization and 15 min of drying ) . in group 3 ,
the instruments were 
 subjected to five cycles of the same test protocols as in group 2 . in the control group , 

the instruments were not immersed in naocl and were not sterilized . for the dynamic immersion , the endodontic instruments were attached to an endodontic 
 motor and rotated freely at a constant speed ( 300 rpm for profile , flexmaster , and mtwo ; 
 500 rpm for twisted files ) in a small glass container with an amount of the naocl 
 solution that was sufficient to be in contact with the instruments but not the 
 shaft .
the files were rinsed 
 with distilled water to neutralize the effect of the naocl and then dried .
a total of 160 instruments , 40 from each subgroup , were tested for dynamic cyclic 
 fatigue resistance .
artificial canals were constructed initially by providing a copper 
 duplicate of each instrument to obtain accurately - sized canals .
the copper duplicate was 
 milled by increasing the original size of the instrument by 0.1 mm , using a computer 
 numeric control machining bench ( ajan cnc , izmir , turkey ) , according to the curvature 
 properties ( 60 angle , 2.5 mm radius , curvature beginning 2 mm from the tip of the file ) 
 that were chosen for this study .
it was then constructed in a stainless steel block 
 using the negative mold , using a die - sinking electrical - discharge machining process 
 ( hartford m 65 series , she hong industrial co. ltd . ,
the depth of the 
 artificial canal was machined to the maximum diameter of the instrument plus 0.2 mm , 
 thus allowing the instrument to rotate freely .
the artificial canal was also covered 
 with tempered glass to prevent the instruments from slipping out and to allow 
 observation of the instruments .
the dental hand - piece was fixed to permit precise 
 placement of each instrument inside the artificial canal and to guarantee the same 
 three - dimensional positions of the instruments ( figures 
 1a , b ) .
all the instruments except the 
 twisted files were rotated at a constant speed of 300 rpm using a 16:1 contra angle and 
 powered by a torque - controlled electric motor ( x - smart , dentsply maillefer , ballaigues , 
 switzerland ) with back - and - forth axial movements until fracture occurred .
the twisted 
 files were rotated at a constant speed of 500 rpm , as per the 
 manufacturer 's recommendation .
the amplitude of the axial movements was 3 mm , with about 
 2 s for every displacement .
high - flow synthetic oil ( dme , gloucester , uk ) was poured 
 into the simulated canal after each file change to reduce the friction between the 
 instrument and the metal canal walls .
a ) cyclic fatigue testing device and the used canal type b ) trajectory of the size 
 25/.06 instrument and curvature properties of artificial root canal the time - to - fracture was recorded visually with a 1/100-s chronometer .
the number of 
 cycles to failure ( ncf ) was determined by converting the time required to fracture the 
 instrument into decimal units and then multiplying the time by the number of rotations 
 per minute . the ncf values ( n=40 )
obtained from four subgroups with 
 different protocols were averaged to calculate the mean ncf value for each instrument 
 ( profile , flexmaster , twisted files and mtwo ) .
bartlett 's test showed that the data did not provide preconditions of homogeneity of 
 variance ( p<0.05 ) and were not normally distributed according to anderson - darling 
 test ( p<0.05 ) .
thus , the significances among the tested instruments were 
 statistically analyzed with kruskall - wallis test ( p=0.05 ) .
bartlett 's test showed that the data did not provide preconditions of homogeneity of 
 variance ( p<0.05 ) and were not normally distributed according to anderson - darling 
 test ( p<0.05 ) .
thus , the significances among the tested instruments were 
 statistically analyzed with kruskall - wallis test ( p=0.05 ) .
kruskall - wallis test indicated significant differences among the tested instruments in 
 terms of ncf , p=0.000 .
the mean ncf of mtwo ( 556.75 ) was higher than that of the twisted 
 file , flexmaster and profile , 483.1 , 376.12 , 365.25 , respectively ( table 1 ) .
the mean ( sd ) number of cycles to failure ( ncf ) * and fragment lengths in mm ( sd ) of tested niti 
 instruments sd = standard deviation ; different letters in the same column indicate statistically significant 
 difference kruskal - wallis test showed that naocl immersion and autoclave sterilization have no 
 effect on the ncf values of the tested instruments ( p>.05 ) , when the effects of naocl 
 immersion and autoclave sterilization on each instrument were searched ( table 2 ) .
mean ( sd ) of the number of cycles to failure ( ncf ) and fragment lengths in mm ( sd ) 
 of tested niti instruments according to the groups sd = standard deviation there were no statistically significant differences among the tested instruments in 
 terms of the length of fractured segments , ( p>.05 ) ( table 1 ) .
the mean lengths of the separated segments as measured from the 
 instruments ' tips were 2.7 , 2.6 , 2.5 and , 2.5 mm for mtwo , flexmaster , twisted file and 
 profile , respectively ( table 1 ) .
in the literature , there is still a debate regarding the impact of torsional stress and 
 metal fatigue on the fracturing of niti rotary instruments .
a number of studies have 
 stated that fatigue is the predominant mechanism in material failure . on the other hand ,
the fatigue life of an instrument is determined by the radius and angle of curvature , 
 which are associated with the diameter of the instrument in the region of maximum 
 bending .
thus , as the diameter 
 of the instrument increases and the curvature radius of the canal decreases , the fatigue 
 resistance of the rotary instruments decreases .
other factors related to cyclic fatigue are the design and cross 
 section of the instrument , the working speed , torque and 
 sterilization procedures and clinical usage .
to simulate clinical conditions , the test protocol chosen for this study included 
 dynamic immersion in 2.5% naocl , a maximum of five cycles of autoclave sterilization , 
 and dynamic cyclic fatigue resistance testing . to remain within a realistic time - frame 
 of clinical practice and considering the mean life of the control instruments ( between 1 
 min , 51.36 s for mtwo ; and 1 min , 13.06 s for profile ) , 5 min was selected as the 
 contact time of the solution with the instrument .
the shaft of the instrument was not 
 immersed in the irrigation solution , as is the case in root canal instrumentation 
 procedures .
a dynamic model that 
 incorporates cyclic axial movement not only provides better simulation of the clinical 
 environment but also lengthens the lifespan of the rotary files , because the compressive 
 and tensile stresses are distributed along the instruments .
however , the 
 ncf values obtained in the present study were lower than those obtained in other studies 
 that used a dynamic model .
those studies used an artificial canal 
 that was not constructed based on the specific dimensions of each file group to ensure a 
 suitable file trajectory .
therefore , the canal design they used may have affected the 
 fatigue life and resulted in a wide variation in the fatigue life of the instruments 
 tested , as was claimed by plotino , et al .
the number of times that rotary instruments can be reused is suggested to vary between 1 
 and 10 , depending on the canal conditions and the type of instrument 
 usage .
reuse for five 
 times was simulated by five cycles of naocl immersion and autoclave sterilization . in a recent study , pedull , et al . 

( 2011 ) reported that static or dynamic immersion in 5% naocl did not significantly 
 reduce the cyclic fatigue resistance of twisted files , mtwo , or revo s su .
if the corrosion is not in the maximum 
 stress area , then , the instrument 's resistance to cyclic fatigue probably can not be 
 reduced . in another study , 
 profile and
race instruments showed a reduction in fatigue resistance after immersion in 
 a heated naocl solution for 1 h. in the present study , the results showed no significant difference for the instruments 
 made of traditional niti alloy ( profile , flexmaster and mtwo ) before and after naocl 
 immersion and autoclave sterilization .
the findings of other studies that investigated 
 autoclave sterilization effects on niti instruments were in agreement with the present 
 results .
( 2006 ) reported that the ncf of profile 
 rotary instruments ( 25 , 0.06 ) increased by 16% after five autoclave cycles in a 
 temperature range 122 - 128c .
furthermore , it has been demonstrated that the five cycles 
 of autoclave sterilization increased the microhardness of the niti endodontic 
 instruments .
naocl immersion and autoclave sterilization also had no significant influence on the 
 cyclic fatigue life of the heat - treated instruments ( twisted files ) . in contrary , 

( 2011 ) found that 
 repeated autoclaving significantly reduced the mean ncf of 25/.06 twisted files . as they 
 explained it , 135c is too low a temperature to produce a heat treatment effect , because 
 it is known that such changes require a temperature of at least 170c , and that the 
 maximum fatigue resistance can be obtained at 430 - 440c .
furthermore , multiple autoclave cycles have been 
 reported to increase the depth of niti file surface irregularities , causing fatigue 
 propagation . on the other hand , plotino , et al .
( 2012 ) suggested that autoclave 
 sterilization might improve the mechanical properties of instruments ( k3 xf protypes , 
 sybronendo ) that are manufactured using a type of heat treatment .
the quality of 
 manufacturing processes and different thermal treatments influence the performance of 
 niti rotary instruments because niti is an alloy that is very sensitive to both the 
 thermal and mechanical ( machining ) stresses that can occur during the manufacturing 
 process .
a better arrangement 
 of the crystal structure and changes in the percentage of phases of the alloy obtained 
 via thermal treatment provide improvements in superelastic and plastic 
 behaviors . in the production 
 process of twisted files ,
r - phase heat treatment technology modifies the crystalline 
 structure , making it finer than that of traditionally processed materials , and it 
 maximizes the flexibility and resistance to breakage without affecting the 
 superelasticity .
( 2012 ) reported that 
 this new manufacturing technology enhanced the mechanical properties of instruments , the 
 results of the present study did not confirm this finding .
the traditional niti 
 instrument , mtwo , showed better cyclic fatigue resistance than the other traditional 
 instruments , profile and flexmaster , and the heat - treated instrument , twisted files . 

twisted files showed better cyclic fatigue resistance than profile and flexmaster , as 
 reported previously .
2010 ) demonstrated that 
 twisted files have fewer surface defects ( microfractures ) because of twisting , and a 
 smaller cross - sectional area than profile instruments , characteristics that might result 
 in their better fatigue resistance .
however , scanning electron micrography ( sem ) 
 micrographs revealed wrinkle - like grooves because of twisting on the flute faces of 
 twisted files , which may increase their level of stress during instrumentation .
all tested instruments fractured at the point of the maximum flexure within the curved 
 part of the artificial canal where the stress concentrates . as expected , the lengths of 
 the fractured segments of the instruments were not influenced by the naocl immersion and 
 autoclave sterilization .
the comparison within the conditions of this study showed that cyclic fatigue resistance 
 of the tested niti instruments can not be adversely affected by naocl immersion and 
 autoclave sterilization .
production process ( twistedfiles ) or design ( twisted files , 
 flexmaster , mtwo and profile ) of the instruments can influence their cyclic fatigue 
 resistance
. however , further studies investigating the different factors which can 
 affect the instruments cyclic fatigue resistance , fracture modes and different apparatus 
 design with more similar features to root dentine are necessary .
 niti instruments have a high risk of separation due to torsional or flexural fatigue 
 ( cyclic fatigue ) . 
 chemomechanical preparation , cleaning procedures , chemical 
 disinfection and sterilization cause the corrosion of endodontic instruments that may 
 weaken the fracture resistance of the instruments . 
 objectiveto assess the effects of naocl immersion and autoclave sterilization on the cyclic 
 fatigue resistance of profile , flexmaster , mtwo and twistedfiles niti instruments 
 ( tip size 25 , 0.06 taper , n=160).material and methodsthe instruments ( n=10 for each subgroup ) were dynamically immersed in naocl ; 
 immersed in naocl and sterilized in one autoclave cycle ; 5 cycles immersed in 
 naocl and sterilized in autoclave and not immersed in naocl and not sterilized 
 ( control group ) . 
 dynamic cyclic fatigue resistance was tested . 
 the number of 
 cycles to failure ( ncf ) were statistically analyzed ( p<0.05).resultskruskall - wallis test indicated significant differences among the tested 
 instruments in terms of ncf ( p=0.000 ) . 
 the mean ncf of mtwo ( 556.75 ) was higher 
 than that of the twisted file , flexmaster and profile , 483.1 , 376.12 , 365.25 , 
 respectively . 
 naocl immersion and autoclave sterilization have no effect on the 
 ncf values of the tested instruments ( p>.05).conclusionscyclic fatigue resistance of the tested niti instruments can not be adversely 
 affected by naocl immersion and autoclave sterilization . production process 
 ( twistedfiles ) or design ( twisted files , flexmaster , mtwo and profile ) of the 
 instruments can influence their cyclic fatigue resistance . 
INTRODUCTION
MATERIAL AND METHODS
Statistical analysis 
RESULTS
DISCUSSION
CONCLUSION