the success of endodontic treatment is directly correlated to an efficient biomechanical preparation aiming to clean and disinfect the root canal system .
the use of mechanized nickel - titanium ( niti ) files for root canal shaping is associated with well - centered preparations , reduced operator fatigue and less operatory time , but the file separation is still a major concern in clinical practice [ 1 - 5 ] .
reciprocating motions have been recently introduced as an alternative way of shaping root canals with niti , aiming at reducing instrumentation stress .
the basic concept is that when the two reciprocating angles are different , the resulting motion is a non - continuous rotation .
the partial rotation in one direction engages the niti instrument inside the canal , allowing cutting , debris removal and apical progression .
the partial rotation in the opposite sense ( usually with a significantly lower angle ) promotes its disengagement and releases the stress .
the safety is also increased by using cutting angles smaller than the elastic or fracture limit ( deflection angle ) of the instrument . whereas continuous rotation generates the accumulation of metal fatigue ,
the reciprocating motions promote alternating stress and release , therefore reducing both torsional and flexural stresses .
several studies clearly showed the increase in mechanical resistance of niti instruments used in reciprocation when compared to their use in continuous rotation [ 1 - 5 ] .
the twisted file adaptive ( tfa ) system ( kerr endodontics , glendora , orange , ca , usa ) consist of a sequence of three niti instruments that are meant to be used with a specific reciprocating motion , given by the elements motor ( kerr endodontics , glendora , orange , ca , usa ) .
the tfa small / medium ( sm ) shaping sequence consists of three niti instruments with the following tip sizes and tapers : sm1 ( 20.04 ) , sm2 ( 25.06 ) and sm3 ( 35.04 ) . according to the manufacturer ,
the tfa motion uses a range of reciprocating angles that automatically vary according to the detected intracanal stress ( from 600/0 up to 370/50 ) , aiming at optimizing efficacy and prevention of failures .
this kinematic was reported to significantly increase in vitro instrument s resistance to fracture compared to the rotary motion , to reduce screwing - in effect   and to reduce the amount of apically extruded debris .
the tfa system also resulted in fewer dentinal cracks and higher cutting efficiency when compared to other reciprocating instruments . however , no data are reported about clinical resistance to breakage of tfa instruments .
previous studies that investigated different brands of rotary niti instruments after clinical use suggested that the occurrence of defects is a multifactorial problem and can be affected by the number of uses , the experience of the operator , method of use , instrumentation technique , anatomical complexity of the root canals , design , diameter and taper of the instruments and manufacturing process [ 11 - 18 ] . since different motions result in different instrumentation stresses , deformation and fracture rate
may also be affected . however , to the best of our knowledge there is no information available about the incidence of defects in tfa instruments after repeated clinical use . therefore , the aim of the present study was to investigate the incidence of deformation and fracture of twisted file adaptive nickel - titanium instruments after repeated clinical use and to identify and check whether the three instruments within the sm sequence showed similar or different visible signs of metal fatigue .
a total of 120 tfa sm packs ( 360 niti instruments ) were collected after clinical use over 15 months . prior
to use , all instruments were observed under stereomicroscope at original magnification x20 ( karl kaps gmbh , asslar , germany ) to discard those with deformations or defects .
each set of tfa sm was assigned to prepare three molar cases ( 8 to 10 canals ) by three endodontists ( gg , gm , ddn ) .
the case selection excluded teeth presenting immature apex , root fractures , previous endodontic treatment , extensive root resorption , c - shaped canals and dilacerated roots .
after coronal access , a mechanical glide path was created , by using the 0.02 stainless steel ( ss ) manual k - files ( which are included in the sm package ) attached to an m4 reciprocating 30 - 30 hand - piece ( m4 program available in the elements motor ) .
the ss files iso sizes 8 , 10 and 15 were used with the m4 , until the 15 k - file could be inserted into the canal without pre - bending or pressure .
after the establishment of glide path , the sequence of three niti tfa instruments were used according to manufacturer s instruction , running with the non - editable tfa program available in the elements motor .
all the instruments reached working length , progressing in a series of steps of 1 - 2 mm .
after each step , the instrument was retrieved from canal and cleaned to limit packing of debris inside flutes .
following manufacturer s instructions , if any sign of distortion or fracture was detected , the instrument was identified , discarded and replaced for a new one ; being that the substitute instruments were not included in the data . irrigation with 5% sodium hypochlorite   was performed during all the shaping procedures . after completing the preparation of each molar case , the set of instruments was sterilized by autoclave and also identified to be used up to a maximum of 10 canals .
the length of each collected instrument was measured by using a digital caliper to a precision of 0.01 mm , this aimed to determine if occurred any fracture that was not previously clinically detected . in the same way
, they were visually inspected for plastic deformation under a stereomicroscope at original magnification x20 ( karl kaps gmbh , asslar , germany ) .
the surfaces of fractured instruments were also viewed under a scanning electron microscope ( sem ) to evaluate the fracture mode .
data was statistically analysed using the spss 17.0 software ( spss incorporated , chicago , il , usa ) .
chi - squared test was used to assess differences in frequency of defects among sm2 instruments and operators and to compare frequencies of fractures and unwinding .
deformation of files after the use in the first molar was observed in 4 kits ( 8 instruments ) , and these files were no longer used to prepare more cases .
the remaining 116 kits of tfa sm instruments were used to prepare a minimum of 8 canals , up to a maximum of 10 for each set of instruments .
the incidence of instrument deformation ( figure 1 ) was 22.2% , which was not evenly distributed within the three instruments of the sequence ( table 1 ) .
the defect rate of sm2 instruments was statistically higher than the other two ( p < 0.001 ) .
of the 80 deformed instruments , 71 ( 88.75% ) were elongated and 9 ( 11.25% ) presented shortening of the flutes .
there was no difference among the three operators regarding the percentage of incidence of defects ( chi - square test p > 0.05 ) .
regarding the number of uses , in 53.75% of the cases the distortion was noted in the preparation of the third molar but with no statistically significant difference ( table 2 ) . 

example of plastic deformation ( unwinding of flutes ) of twisted file adaptive sm2 and sm3 . 
 

number and percentage of defects on twisted file adaptive ( tfa ) instruments 
 
 different superscript letters indicate statistically significant difference within column . 
 

n = number of tfa instruments used ; n = number of deformations or fractures observed ; sm = small / medium . 
 

incidence of defects on twisted file adaptive instruments according to the number of uses 
 
 different superscript letters indicate statistically significant difference within row . 
 

n = total number of deformations and fractures observed ; n = number of deformations and fractures observed after each case . 
 
there was a difference in the site of deformation among instruments : while most of sm2 deformed at middle part , the sm1 and sm3 deformed close to the tip . during the clinical procedures , three instruments fractured ( two sm2 and one sm3 ) .
the overall fracture rate was 0.8% , significantly lower than the incidence of unwiding ( p < 0.0001 ) .
sem fractographic analysis of the broken instruments showed signs of both torsional and flexural fatigue ( figure 2 ) . 

fractographic analysis of twisted file adaptive broken instruments showed signs of flexural fatigue ( indicated by the arrows ) and in some specimens also torsional fatigue ( indicated by the circle ) . 
 

previous studies have shown that the clinical fracture incidence of niti instruments varied from 0.3% to 23% among different clinics worldwide [ 11 - 16,18,20,21 ] .
the overall fracture incidence of tfa recorded in the present study ( 0.8% ) is close to the lowest values of the reported range .
this low fracture rate could be related to different factors , including the motion , the design of the instruments and its manufacturing process [ 1 - 3,7 ] .
.   showed that the twisting process of tf instruments resulted in an increased resistance to breakage compared to instruments produced by grinding .
it is known that intracanal file breakage is more likely to occur in severe , abrupt curvatures or s - shaped canals .
the fact that file separation occurs mainly in molars , when compared to other tooth types , have been correlated not only to the anatomical complexity but also to its clinical accessibility . however , clinically it is not possible to standardize the anatomy of the teeth or the difficulty of the cases ( e.g. the canal diameter , the curvature angles , dentin hardness ) .
most of previous clinical studies that reported low fracture rates of different brands of instruments , have included all dental groups .
nevertheless , in the present study it was set a level of difficulty by including only molar cases .
these tooth type was selected because tfa sm sequence is recommended for small and/or curved canals , which are more likely found in molars .
even tough , the incidence of fracture in the present study was similar to that reported in studies that included also anterior teeth and premolars .
the deformations rate of niti files reported in the literature vary from 0.3 to 28% , which commonly is slightly lower or similar to the fracture rate [ 11 - 14,16 - 21 ] . in the present investigation ,
the incidence of defects was significantly higher ( 22.2% ) than the fractures rate ( 0.8% ) .
it has been suggested that the design of the instrument play a major role in determining resistance to intracanal stress .
however , for the tfa instruments , the manufacturing process might also explain the greater occurrence of deformations .
the combination of heat treatment and twisting process leads the tfa instruments to present a clinical behaviour different from the majority of grounded niti rotary instruments .
likewise stainless steel files , when subjected to excessive intracanal stress , they tend to show a greater amount of plastic deformation ( unwinding of flutes ) before breakage ( figure 1 ) .
this propensity for unwinding flutes can be considered as a safety feature   that allows clinicians to visualize metal fatigue and avoid overuse .
the plastic deformation of the files in the present investigation could be detected clinically in the majority of cases ( 96.25% ) . in four cases ,
it was observed plastic deformation during the first use of the tfa set , probably related to the complex anatomy of the teeth . by showing clinical evidence of metal fatigue
, these instruments allow the clinician to discard the deformed instruments on a rationale base . the majority of grounded niti rotary instruments usually undergo to unexpected fracture without any previous signs of metal fatigue or unwinding of flutes .
the present study also confirmed previous ones showing that instruments within a sequence are subjected to different intracanal stresses , due to different dimensions and different blade engagement . among the three tfa instruments , the first and third one ( sm1 and sm3 ) presented lesser deformation / fracture rates . although unwinding was observed in 15% of sm1 , none of the 120 instruments fractured .
this finding is probably related not only to the instrument smaller size , but also to the previous establishment of an adequate glide path , up to a number 15 k - file .
similarly , the sm3 is subjected to stress only in the last 4 mm , which explains their low deformation rate and only one fractured file .
this is correlated to the dimensions , because this file increased both taper and size preparation , being more subjected to intracanal stress .
these results showed sm2 to be the most critical instruments inside this sequence , thus requiring a more delicate and cautious approach , to minimize blade engagement and tip lock .
clinical studies showed that among the protaper sequence , the s1 instrument corresponded to 62% of all separated files   while for k3 instruments , the defect rates were significantly higher for the tapers 0.06 and 0.04 .
accordingly , the present findings highlight that to provide useful information , studies about fracture and deformation rates of niti instruments should be more detailed and analyse their behaviour inside a sequence .
fractographic analysis of tfa sm2 and sm3 broken instruments showed signs of both torsional and flexural fatigue ( figure 2 ) .
this fact might be also related the influence of the kinematics ( reciprocation ) on the response to mechanical stresses .
this assumption is corroborated by a recent study that evaluated the incidence of defects after clinical use of waveone and found that these reciprocating instruments fractured by shear stress .
the evaluation of the other two commercially available reciprocating instruments also showed a very low incidence of fracture and deformation after clinical use , respectively 0.47% and 0.35% for reciproc , and 0.5% and 9.6% for waveone .
however , it is important to notice this low fracture rate of reciproc and waveone are also related to the fact that , according to manufaturers instructions , both are single - use system while tfa is intended for multiple uses .
the repeated use of niti files ( more than 8 times ) , have been reported to increase significantly the separation rate . in the present study despite the repeated clinical use , only 3 instruments fractured ( 0.8% ) which are probably correlated to the combination of tfa instruments with the adaptive motion .
therefore , it was concluded that twisted file adaptive instruments presented a very low fracture rate and a high incidence of plastic deformation after repeated clinical use . since the small / medium 2 instruments presented significantly more deformation , the clinical relevance of the present study lies in demonstrating that instruments within a sequence might be differently subjected to intracanal stress and clinicians must understand the different behaviour and perform instrumentation accordingly .
the authors declare that there are no financial or other conflicts of interest related to this publication .
 abstractobjectivesthe aim of the present study was to investigate the incidence of deformation and fracture of twisted file adaptive nickel - titanium instruments after repeated clinical use and to identify and check whether the three instruments within the small / medium sequence showed similar or different visible signs of metal fatigue.material and methodsone - hundred twenty twisted file adaptive ( tfa ) packs were collected after clinically used to prepare three molars and were inspected for deformations and fracture.resultsthe overall incidence of deformation was 22.2% , which was not evenly distributed within the instruments : 15% for small / medium ( sm)1 ( n = 18 ) , 38.33% for sm2 
 ( n = 46 ) and 13.33% for the sm3 instruments ( n = 16 ) . 
 the defect rate of sm2 instruments was statistically higher than the other two ( p < 0.001 ) . 
 the fracture rate was 0.83% ( n = 3 ) , being two sm2 instruments and one sm3.conclusionsit was observed a very low defect rate after clinical use of twisted file adaptive rotary instruments . 
 the untwisting of flutes was significantly more frequent than fracture , which might act as prevention for breakage . 
 the results highlight the fact that clinicians should be aware that instruments within a sequence might be differently subjected to intracanal stress . 
INTRODUCTION
MATERIAL AND METHODS
RESULTS
DISCUSSION
CONCLUSIONS
ACKNOWLEDGMENTS AND DISCLOSURE STATEMENTS