it is a generally accepted standard that root canal treatment procedures should be confined to within the root canal system . the cementodentinal junction ( cdj ) is the ideal end point for a root canal preparation . clinically , we can not identify this histological structure and the extension of the cementum into the root canal is variable [ 2 , 3 ] . generally , the apical constriction ( ac ) , which usually does not coincide with the cdj , is a more clinically dependable and practical demarcation to use as a terminus for the canal preparation . moreover , in those cases with an ac , the distance between the ac and anatomic apex varies . as a consequence , detection of the canal terminus by traditional methods , such as radiography , still remains a challenge . the use of new generations of electronic apex locators ( eals ) has gained increasing acceptance in the recent years because of their high precision in determining the working length [ 710 ] . however , using these devices , the operator faces some limitations . as some teeth do not have a clear and reproducible reference point , alteration in the primary measured distance during instrumentation seems likely . besides , because of parallax errors when placing the rubber stop on the crown , the accurate adjusting and repositioning of a series of files is very difficult if possible [ 1113 ] . using rotary nickel titanium instruments to determine the working length lead to more acute parallax errors due to a wider shaft than the primary hand file . in addition , accuracy of file positioning and the working length is affected by the changes in the insertion path as canal preparation progresses . more recently , devices have been introduced that consist of an eal combined with a rotary handpiece . these ealhs have the capability to stop or reverse the rotation of the nickel - titanium files as the estimated end point of the root canal is reached . thus , they could eliminate the problem of maintaining working length with multiple files and the need for a clear reference point on the tooth . as sufficient evidence suggests that accurate working length determination can profoundly affect the success of root canal treatment , further studies to evaluate the reliability of these devices are worth pursuing.in the current study , we used an in vitro model to compare the motor - driven mode of two newly developed ealhs . each tooth was carefully examined under a loupe ( magnification 3 ) to detect the presence of external cracks , apical resorption or wide - open apices that might alter the accuracy of the working length measurements . endodontic access cavities were prepared by using high - speed diamond bur ( dentsply , maillefer , ballaigues , switzerland ) . preflaring of the mesiobuccal ( mb ) canals was performed with # 2 and # 3 gates - glidden burs ( mani , japan ) [ 7 ] . subsequently , apical patency was confirmed by inserting a size 10 k - file ( mani , japan ) until the tip was just visible at the apical foramen using 3 magnifications . the specimens were then randomly divided into two groups of 18 and were placed into individual alginate settings used to simulate the periodontium . using a blunt 27-gauge needle , then , ewls were determined by vdw gold ( vdw , munich , germany ) and dentaport zx ( j. morita corp . , tokyo , japan ) in groups 1 and 2 , respectively . a size 10 mtwo .04 taper niti rotary file ( vdw , munich , germany ) was mounted in the handpieces . the automatic stop function of the devices was preset at line 0.5 on the panel . after the rotary instrument was introduced into the canal and reached the predetermined level , it automatically stopped . subsequently , the rubber stop was adjusted to the coronal reference point and the distance from the base of the rubber stop to the file tip was measured with a digital caliper to the nearest 0.01 mm . analysis of variance ( anova , 1-way ) and tukey pairwise multiple comparison intervals ( 0.05 ) were used to compare the accuracy of the two electronic devices . for vdw gold , 61.1% of the measurements were within  0.5 mm and 88.9% were within  1 mm of the awl . 88.9% of dentaport zx measurements were within  0.5 mm and 94.4% were within  1 mm of the awl . for both of the devices , there was no significant difference between awl and ewl ( p=0.466 for dentaport zx and p=0.283 for vdw gold ) . no significant difference was found between the accuracy of the two devices in determining the wl ( p=0.8 ) . the current in - vitro study was aimed to compare the accuracy of two ealhs ( dentaport zx and vdw gold ) using a nickel - titanium rotary file . unlike the similar previous studies in which single rooted teeth had been used , mb canals of the mandibular molars were selected for this study . using the motor - controlled mode of the ealhs if the rotating file reaches any obstruction such as packed debris , the ability of the apex locatorto accurately function will be compromised . canal blockage is possible by packing of the dentin debris as the motor progressively stops . in addition , binding of the file in the irregularities of the canal will cause it to be automatically reversed or stopped . mb canal of the mandibular molars is greatly narrower , more irregular and complex compared with the single rooted teeth . under the conditions of the present study , although dentaport zx recorded more measurements in acceptable range compared with vdw gold , there are no significant differences between the accuracy of the devices . although literature search failed to reveal any study that evaluated the accuracy of the vdw gold regarding dentaport zx , the results of the current study were in agreement with previous studies showing that dentaport zx and tri - auto - zx ( a previous version of dentaport zx ) are safe and reliable . they compared motor - operated root zx ii , apex nrg xfr , and mini apex locator using 0.04 tapered niti rotary profiles sized 4020 in a crown - down manner . once the first rotary file reached the ac , the rotary motor was automatically stopped.they showed that the devices were able to locate the ac within 0.5 mm only 50% or less of the times.this discrepancy might be attributed to the following reasons . the most important factor could be the different condition of their study as it was clinical . another possible explanation might be the technique of filling . using a series of niti files driven by ealhs , the canal may be partially plugged by debris , leading to modification of the electrical conductivity of the root canal and cross - section of the file [ 20 , 21 ] . for this reason , some authors recommend recapitulation with a small diameter hand file ( 14 ) . under conditions of the current study , as the strictest clinical tolerance (  0.5 mm ) was applied , the accuracy of the motor - driven mode of dentaport zx ( 88.9% ) was in the range of reported accuracy for its manual mode in previous studies [ 22 , 23 ] . therefore , in comparison to the results of previous studies , it might be inferred that even if we consider the narrowest acceptable range , the rotational mode of dentaport zx is reliable similar to the manual mode . on the other hand , although the accuracy of vdw gold in  0.5 mm ( 61.1% ) was less than the reported average for manual mode of new generated eals , considering the clinical range of  1.0 mm to the foramen , the results of this study were in agreement with those obtained by other investigators for the newest eals ( 12 , 24 ) . in an ex vivo study conducted by alves et al . , it was mentioned that if the motor - driven mode of tri auto zx was set at 1 , differences in working length measurements of manual and rotational mode were not significant . as cited in previous studies , because of the lack of a well - delineated limit for canal terminus , an error tolerance of  1.0 mm is deemed clinically acceptable . although in this study both of the devices detected the apical end point within an acceptable range , it should be noted that many practitioners believe that a distance from the file tip to the major foramen of 0.5 mm results in over instrumentation and incorrect working length [ 14 , 18 , 24 ] . for this reason , that concluded if the apical line of the tri auto zx was preset at 0.5 , it would prevent over preparation of the canal , carneiroet al . found tri - auto - zx to be more accurate at measuring root canal length when it is preset at 1 rather than the 0.5 value . it has been reported that a slight screwing effect of the rotary file might be a cause for longer measurements as observed with ealhs [ 26 , 29 ] . this factor could be an explanation for the relatively deep penetration of a number of files inside the root canals reported in the present study . we concluded that the motor - driven mode of these devices appeared to be clinically safe , as previous studies have shown . under the condition of the current study , it may be concluded that using motor - driven mode of apex - locator- controlled handpieces such as vdw gold and dentaport zx offered an accurate method for endodontic working length determination .
objectives : the aim of this in - vitro study was to compare the accuracy of two new electronic apex - locator controlled handpieces ( ealhs ) using the first rotary mtwo file while rotating in the canal.materials and methods : a total of 36 extracted mandibular molar teeth were selected . 
 the lengths of the mesio - buccal canals to the major foramina were measured directly using a size 10 k - file introduced in the canal until the tip was visible under a loupe and then 0.5 mm was subtracted from the recorded lengths as the actual working lengths ( awls ) . 
 subsequently , the teeth were randomly assigned to two groups and embedded in an alginate model . 
 auto - stop function of the ealhs was preset at  0.5  mark and then electrical working lengths ( ewls ) in groups 1 and 2 were recorded by vdw gold and dentaport zx , respectively . 
 analysis of variance ( anova , 1-way ) and tukey pairwise multiple comparison intervals ( 0.05 ) were used to compare the accuracy of the two electronic devices.results:for vdw gold , 61.1% of the measurements were within 0.5 mm and 88.9% were within 1 mm of the awl . for dentaport zx , 
 88.9% of the measurements were within 0.5 mm and 94.4% were within 1 mm of the awl . 
 there was no significant difference between awls and ewls ( p=0.466 for dentaport zx and p=0.283 for vdw gold ) and between the accuracy of the two devices in determining the ewl ( p=0.8).conclusion : both dentaport zx and vdw gold were suitable for determining working length using a rotary file . to avoid over instrumenting the canal , 
 we recommend setting the devices to automatically stop or reverse the rotary file at 1 mm level .