instrumentation with nickel - titanium ( niti ) instruments can result in some complications such as root canal transportation , perforations and vertical root fracture . shaping procedures can damage the root dentin , resulting in dentinal cracks which can develop into vertical root fractures . the most susceptible teeth to fracture are those with a narrow distomesial dimension compared with the linguobuccal diameter as in mesial roots of lower molars . with the newly introduced single - file niti systems reciproc ( vdw , munich , germany ) and waveone ( dentsply maillefer , ballaigues , switzerland ) it is possible to shape canals with only 1 instrument , thereby requiring less time than rotary full - sequence systems . these files are produced with a special niti alloy ( m - wire ) subjected to a special thermal treatment process to increase the flexibility of the instrument . waveone and reciproc files are used in a reciprocating motion , in order to reduce stress on the instrument and to minimize fracture of cyclic fatigue . another new generation of niti instrumentation system has been introduced called the twisted files adaptive ( tfa ) ( sybronendo , orange , ca ) . the tfa are produced by transforming the niti wire from the austenitic crystalline structure to the super - elastic crystalline r - phase structure by a method of heating and cooling . the tfa is used in a combination of both , reciprocating and continuous rotation . according to the load of pressure exerted on the file , the tfa instrument can change from continuous rotation to reciprocation mode with clockwise and counterclockwise angles varying from 0 to 600 up to 37050. the quantity of dentine structure remnant is correlated to the strength of a root filled tooth . dentine removal during root canal shaping was found to have an incidence on the formation of incomplete cracks . moreover , it might be speculated that when using only one instrument , more stress will be generated during instrumentation , increasing the frequency of dentinal defects . therefore , the aim of the present study was designed to evaluate the frequency of dentinal microcracks observed after root canal preparation with reciprocating ( reciproc and waveone ) and  adaptive motion the null hypothesis was that there would be no significant differences in crack formation among groups . ninety freshly extracted mandibular first and second molars with completely separated canals and mature apices were chosen . only teeth with a curvature of the mesial root < 25 were selected . canals were controlled for apical patency with a size 15 k - file ( vdw ) . the coronal portions and distal roots of the teeth were removed by using a an isomet low speed saw ( isomet ; buhler ltd . , lake bluff , ny ) under water cooling , leaving mesial roots with approximately 18 mm in length to prevent the introduction of confounding variables . all roots were observed with under a  25 magnification stereomicroscope ( expert dn ; mller optronic , erfurt , germany ) to exclude cracks . as a result samples were randomly assigned to each group ( n = 30 ) according to the system used for root canal shaping . all root canals were instrumented with the r25 ( 25/0.08 ) reciproc single - file according to the instructions of the manufacturer . r25 was introduced with about amplitude of 3 mm with a slow in- and - out pecking motion and reciproc vdw silver motor ( vdw gmbh ) . after 3 pecking motions , the canal was irrigated with naocl ( 5.25% , 3 ml ) . root canal patency was verified with a size k15 file to the working length ( wl ) . waveone taper 0.08 and size 25 reciprocating file was introduced into the root canals in a slow in- and - out pecking motion with about amplitude of 3 mm according to the manufacturer 's instructions . the waveone instrument was mounted on the vdw silver reciprocating motor using the manufacturer configuration setup . root canals were irrigated with naocl ( 5.25% , 3 ml ) after 3 pecking motions . instrument sequence was tfa 25 taper 8 followed by tfa 25 taper 6 and 25 taper 4 to the wl . after reaching wl with 25 taper 4 , tfa 25 taper 6 followed by tfa 25 taper 8 were taken to wl . between instruments , after instrumentation , all root canals were irrigated with naocl ( 5.25% , 3 ml ) . irrigation was performed using an endo - eze ( ultradent , south jordan , usa ) irrigator tip mounted on a disposable 3 ml syringe . when preparation was achieved ethylenediaminetetraacetic acid ( 15% , 1 ml ) ( wizard , rehber kimya san . , istanbul , turkey ) was applied for 1-min and canals were irrigated again with naocl ( 3 ml ) . each instrument was changed after 3 canals . the final apical preparation resulting was standardized to size 25 , taper 8% for all groups . one operator performed all root canal preparations , and the assessments of the cross sections were performed by 2 examiners who were blinded in respect to all experimental groups . after preparation , all the roots were sectioned horizontally at 3 ( apical third ) , 9 ( middle third ) and 15 ( cervical third ) mm from the apex with a 0.1 mm low - speed saw ( leitz , wetzlar , germany ) under water - cooling . to avoid any artifact by dehydration , all slices were observed under a digital stereomicroscope ( expert dn ) at  25 magnification by using a cold light source and pictures were taken . in cases of discrepancy in the observations of the 2 examiners , fracture , incomplete cracks , and craze lines , as well as the relative and absolute length of the defects , were recorded by using the imagej software ( national institutes of health , public domain ) . the incidences of the different defects were analyzed by using the chi - square test and the fisher exact test at a significance level of p < 0.05 . no defect is root dentin without any lines or cracks on the external or the internal surface of root [ figure 1 ] . cross section at the 3-mm without any dentinal defects an incomplete crack is a line extending from the canal wall into the dentin without reaching the outer surface . a complete crack is a line extending from the root canal wall to the outer surface of the root [ figure 2 ] . cross section at the 3-mm level showing a complete crack craze lines are all other lines that did not reach any root surface or extend from the external surface into the dentin but did not attain canal wall . all root canals were instrumented with the r25 ( 25/0.08 ) reciproc single - file according to the instructions of the manufacturer . r25 was introduced with about amplitude of 3 mm with a slow in- and - out pecking motion and reciproc vdw silver motor ( vdw gmbh ) . after 3 pecking motions , the canal was irrigated with naocl ( 5.25% , 3 ml ) . root canal patency was verified with a size k15 file to the working length ( wl ) . waveone taper 0.08 and size 25 reciprocating file was introduced into the root canals in a slow in- and - out pecking motion with about amplitude of 3 mm according to the manufacturer 's instructions . the waveone instrument was mounted on the vdw silver reciprocating motor using the manufacturer configuration setup . root canals were irrigated with naocl ( 5.25% , 3 ml ) after 3 pecking motions . instrument sequence was tfa 25 taper 8 followed by tfa 25 taper 6 and 25 taper 4 to the wl . after reaching wl with 25 taper 4 , tfa 25 taper 6 followed by tfa 25 taper 8 were taken to wl . between instruments , after instrumentation , all root canals were irrigated with naocl ( 5.25% , 3 ml ) . irrigation was performed using an endo - eze ( ultradent , south jordan , usa ) irrigator tip mounted on a disposable 3 ml syringe . when preparation was achieved ethylenediaminetetraacetic acid ( 15% , 1 ml ) ( wizard , rehber kimya san . , istanbul , turkey ) was applied for 1-min and canals were irrigated again with naocl ( 3 ml ) . each instrument was changed after 3 canals . the final apical preparation resulting was standardized to size 25 , taper 8% for all groups . one operator performed all root canal preparations , and the assessments of the cross sections were performed by 2 examiners who were blinded in respect to all experimental groups . after preparation , all the roots were sectioned horizontally at 3 ( apical third ) , 9 ( middle third ) and 15 ( cervical third ) mm from the apex with a 0.1 mm low - speed saw ( leitz , wetzlar , germany ) under water - cooling . to avoid any artifact by dehydration , all slices were observed under a digital stereomicroscope ( expert dn ) at  25 magnification by using a cold light source and pictures were taken . in cases of discrepancy in the observations of the 2 examiners , fracture , incomplete cracks , and craze lines , as well as the relative and absolute length of the defects , were recorded by using the imagej software ( national institutes of health , public domain ) . the incidences of the different defects were analyzed by using the chi - square test and the fisher exact test at a significance level of p < 0.05 . no defect is root dentin without any lines or cracks on the external or the internal surface of root [ figure 1 ] . cross section at the 3-mm without any dentinal defects an incomplete crack is a line extending from the canal wall into the dentin without reaching the outer surface . a complete crack is a line extending from the root canal wall to the outer surface of the root [ figure 2 ] . cross section at the 3-mm level showing a complete crack craze lines are all other lines that did not reach any root surface or extend from the external surface into the dentin but did not attain canal wall . number of complete cracks in the different cross - section slices ( n=30 in each group ) and percentage of sections in which defects were present number of incomplete cracks in the different cross - section slices ( n=30 in each group ) and percentage of sections in which defects were present overall , reciproc produced significantly more complete cracks compared with waveone and tfa ( p = 032 ) . there was no significant difference between tfa and waveone ( p > 0.05 ) . regarding the middle and coronal third sections ( 15 and 18 mm ) , no significant difference was found between the 3 files systems ( p > 0.05 ) . only in the apical third section ( 3 mm ) reciproc and waveone produced significantly more incomplete cracks than tfa ( p = 0.004 ) . in the middle third section ( 9 mm ) , only the reciproc system produced significantly more incomplete cracks ( p = 0.032 ) . in the coronal third sections , no significant difference was found between the 3 file systems ( p > 0.05 ) . craze lines were detected in the reciproc group in the middle third with a significant difference ( p = 0.003 ) [ table 3 ] . number of craze lines in the different cross - section slices ( n=30 in each group ) and percentage of sections in which defects were present vertical root fracture is not an instant phenomenon but rather a result of crack propagation . the main goal of the present investigation was to study the effect of two different kinematics using single - file and multiple file systems . this study revealed the incidence of dentinal defects was higher with single - file reciprocating instruments . in the apical part of the canals , reciprocating files produced significantly more incomplete cracks compared with the rotary reciprocating adaptive motion ( p < 0.05 ) . the differences between the instruments tested regarding the incidence of dentinal defects may be due to the preparation technique and the cross - sectional design of instruments . the reciprocal motion seems to enhance debris transportation towards the apex and may increase torsional forces . it should be evaluated in further studies whether these increased torsional forces are associated with an increased risk of creating dentinal defects . the higher frequency may be correlated to the cross - sectional design of the instrument . in fact , the reciproc r25 has an s - shaped geometry with a double cutting edge while waveone has a modified triangular cross section with radial lands at the tip and a convex triangular cross section in the middle and coronal portion of the instrument . the waveone cross section results in lower cutting efficiency and less chip space . in the apical third and in the whole root canal , tfa produced the least dentin defects the file uses continuous rotation when it is exposed to a minimal or no applied load and uses reciprocal motion when it engages dentin and load is applied . the adaptive motion combined with the high flexibility and the multi - file system seems to decrease torsional forces resulting in less dentine defects . no definitive conclusion can be made regarding the clinical implication of these dentinal defects on long - term follow - up . these dentinal defects can become high - stress concentration areas which may propagate to the root canal surface when an external force is applied . currently , there is an evident lack of correlation between the results obtained in this type of studies and the clinical situation . despite efforts to reproduce the clinical conditions , it is impossible to eliminate possible influence of external factors such as storage of the specimens . the sectioning method used in the study allowed evaluation of the impact of root canal treatment procedures on dentin root by direct inspection of the root . however , other methods have been described such as stress distribution measures , observations of the presence of cracks in tooth sections , and resistance of the root canal treated tooth to fracture . the method in the present study differed from that approach because no external forces were applied , and the influence of root canal preparation on the root canal walls and the adjacent dentin was observed directly . in addition , resistance to fracture does only provide information on vertical root fracture , but the occurrence of dentinal defects can not be detected . under the study limitations , there was a significant difference between the groups in the formation of dentinal microcracks . further studies are required to assess the advantage of single - file reciprocating root canal instrumentation concerning its impact on the incidence of root microcracks .
objective : this study aimed to compare the frequency of dentinal microcracks after root canal shaping with 2 reciprocating ( reciproc and waveone ) and 1 combined continuous reciprocating motion twisted files adaptive ( tfa ) rotary system.materials and methods : ninety molars were chosen and divided into three groups of 30 each . 
 root canal preparation was achieved by using reciproc r25 , primary waveone and tfa systems . 
 all the roots were horizontally sectioned at 15 , 9 , and 3 mm from the apex . 
 the slices were then viewed each under a microscope at  25 magnification to determine the presence of cracks . 
 the absence / presence of cracks was recorded , and the data were analyzed with a chi - square test . 
 the significance level was set at p < 0.05.results:instrumentation with reciproc produced significantly more complete cracks than waveone and tfa ( p = 0.032 ) . 
 the tfa system produced significantly less cracks then the reciproc and waveone systems apically ( p = 0.004).conclusions : within the limits of this study , the tfa system caused less cracks then the full reciprocating system ( reciproc and waveone ) . 
 single - file reciprocating files produced significantly more incomplete dentinal cracks than full - sequence adaptive rotary motion .