Clinical Accuracy of Electronic Apex Locator in Measuring Working Length during Root Canal Treatment
AKM Bashar1, Tanzila Rafique2, Ranjit Ghosh2, Md. Kamal Abdullah3, Mahmood Sajedeen2, Gazi Shamim Hassan2

1Dept. of Conservative Dentistry & Endodontics, Bangabandhu Sheikh Mujib Medical University, Bangladesh

2Dept. of Orthodontics, Bangabandhu Sheikh Mujib Medical University, Dhaka, Bangladesh

3Dental Unit, Rajshahi Islami Bank Medical College, Rajshahi, Bangladesh

Principal Contact


Obtaining a correct working length is critical to success of endodontic therapy. The procedure for establishment of working length should be performed using techniques that have been proven to give valuable and accurate results and methods that are practical and efficacious. The development of the electronic apex locator has helped to assess the working length more accurate and predictable. So the objective of this in vivo-ex vivo study was to evaluate the clinical accuracy of electronic apex locator in measuring the working length of the root canal in vivo, and comparing the lengths so measured, to the actual working length, ex vivo and after extraction. Electronic apex locator C-Root-I VI (Foshan COXO Medical Instrument Co. China) was used to measure the working length in 100 root canals (one palatal canal and one buccal canal in fifty maxillary 1st premolar) in vivo before extraction, that were scheduled for orthodontic treatment in twenty-five patients. Teeth were then extracted and apical constrictions were identified by careful preparation of the apical 4 mm of all the roots. Actual working lengths were determined by adjusting an endodontic file in the root canal upto the constriction from the coronal reference point. Electronic working lengths obtained in vivo were then compared for coincidence with the actual lengths thus measured after extraction (ex vivo). The data were statistically analysed by a paired Student 't′ test and Pearson correlation-coefficient test. In 14 canals out of 100 electronic working lengths (EWL) truly coincided with actual working length (AWL). Eighty two canals out of 100, EWLs were ≤0.5 mm short in measurement than AWLs but the difference with AWL was statistically insignificant (p >0.01). Only in 4 canals (4%) EWLs were either >0.5mm short or more than AWL, thereby fail to meet the criteria of acceptable range of coincidence (≤0.5 mm from the apical constriction). On reliability analysis, all (100%) electronic working length significantly correlate with the actual working length (r=0.971). Within a clinically acceptable range of ≤0.5 mm, C Root I apex locator device showed a high degree of success (96%) in determination of working length during root canal treatment.

Key words: Electronic apex locator (EAL), working length (WL), Root canal treatment, vivo-ex vivo


The features of proper root canal treatment procedures include the complete removal of infected pulp tissues, thorough canal cleaning, shaping, disinfection and three-dimensional filling.1 To achieve this objective, the preparation terminus (working length) must be detected accurately and must be maintained during the process.2The apical constriction (AC) is recommended as the ideal end-point for the instrumentation and filling of the root canal system.3Available techniques to determine WL are: periodontal sensitivity, tactile sense, radiographic method and electronic methodbut till now none of these are completely perfect.4

Radiography is the most commonly used diagnostic aid in endodontics, as described by ingle.5However, accuracy in determining working length is difficult to achieve in this technique because radiography can, at best, give an estimate of histological structure (apical constriction) and although clinically desirable, averages used to define the apical constriction from the radiographic apex could lead to over or under filling.6 Moreover, the variables in techniques, angulation and exposure distort this image and lead to error due to laterally situated foramina.7 In addition, there is radiation hazard both to the patients and the dental personnel.6 Furthermore, a radiograph provides a two dimensional image of a three dimensional structure which lacks of a real representation.7Dense bone and anatomical structures,the superimposition of the zygomatic arch has been shown to interfere radiographically and can make the radiographic visualization of root canal files unfeasible by obscuring the apex.8

The tactile perception being the oldest, because of the simplicity of the technique and its virtual effectiveness are factors that motivate a few clinicians in endodontic practice to still follow this technique. But this technique obviously depends on the sensitivity and experience of the operator,generally inaccurate in root canals with immature apex, excessive curvature and if the canal is constricted throughout its length.6

In general, the methods currently available for root canal length measurement, neither the manual nor the radiologic approaches allow precise localization of apical narrowing.2

The new generation of apex locator provides the operator with a digital read out, graphic illustration and an audible signal. It has been claimed that it can measure pulp space lengths accurately even in wet canalin presence of biological phenomena such as vital tissue, or conductive fluid like NaOCl, Nacl, EDTA solution, etc.9-11

In vitro testing in dry and in presence of different electro conductive fluid, accuracy of apex locator revealed as low as 83%upto 100%.12,13 But the clinical efficacy of EALs regarding its clinical accuracy is yet a matter of dispute. Since in vitro testing could give accuracy results higher than those obtainable in clinical practice.So,the present study was conducted to evaluate whether and how much the Electronic Apex Locator is clinically accurate in measuring the working length of root canal.

Materials and Methods

The in vivo- ex vivo study was carried out in the Department of Orthodontia and Department of Conservative Dentistry &Endodontics, Bangabandhu Sheikh Mujib Medical University, Dhaka, Bangladesh. Methodology followed in this study was based on previous studies.6,14,15 Human maxillary 1st premolar teeth with mature apices that were scheduled for extraction for orthodontic purpose was included as a study specimen. The teeth having metallic restorations, open apex or blunderbuss canal, root fractures, calcification in the pulp chamber or root canal, radiographically invisible canals, internal or external resorption as determined both clinically and radiographically or previously root canal treated teeth were excluded from the study.6,7,15 Finally, the study was conducted on a total of one hundred root canals15 (Fifty maxillary 1st premolar teeth with one palatal and one buccal canal in each) those were used as study sample consecutively both for electronic working length determination in vivo then for actual working length measurement after extraction (ex vivo).

Procedure: After obtaining informed consent, access cavity prepared and pulp extirpated with a barbed broach. A univocal reference point was determined on the occlusal surface. The canals were irrigated using 1% sodium hypochlorite solution and finally flushed copiously with distilled water. The irrigant solution overflowed to the pulp chamber was soaked with small cotton ball.

Electronic working length determination: The 'lip clip/attachment′ of the digital apex locator instrument (C-Root-I VI digital apex locator, Foshan COXO Medical Instrument Co. China) was attached to the patient′s lower lip. A no. 15 K-file was taken and the unit′s cable was clipped to its metal shank. The digital apex locator was turned on and the file was inserted slowly into the palatal canal.

During insertion of the file deep into the canal, when the apex locator gave the signal for the apical constriction (the flashing bar diagram reached at the level '0′ digital display), the rubber stop was adjusted on the file shaft in such a way that it touched the reference point. The instrument was carefully withdrawn and the distance from the tip of the instrument to the rubber stop was measured using a graduated scale; the value was noted and registered as Electronic Working Length (EWL). Now electronic working length in buccal canal was measured similarly. Same protocol was followed for all the 100 root canals.

Actual working length determination: The teeth were extracted and the apical 4 mm of the root canal and the apical canal constriction were exposed by careful sectioning the root apices in a longitudinal direction; at first with a diamond bur, then the remaining thin layer of dentin with a probe. The apical constriction and the major foramen were identified under microscope at 16x magnification.14 An endodontic file was carefully introduced into the canal upto the apical constriction, the rubber stop was adjusted so that it touched the reference point and was withdrawn. Distance from the apical constriction upto the coronal reference point was measured. All the obtained readings in 100 canals by this method were registered as Actual Working Length (AWL). Now the difference between the AWLs and EWLs were calculated. Levels of coincidence were assigned according to predetermined evaluation criteria (table I). Paired Student 't′ test was done to compare between AWLs and EWLs on different level of coincidence. Finally, different levels of coincidence as attained by EWLs were co-related with AWLs utilising Pearson correlation-coefficient test. A 'p′ value <0.0001 was considered significant.


The results showed that in 14 canals out of 100 (14%) Electronic working length (EWL) as measured clinically with C Root I Electronic apex locator truly coincided with the actual working length (AWL) (figure 1). Eighty two canals out of 100 (82%) EWL were ≤0.5 mm short in measurement than AWL (figure 2) but the difference with AWL was statistically insignificant (p >0.1). Only in 4 canals (4%) EWLs were either >0.5 mm short or more than 0.5mm AWL, thereby fail to meet the criteria of acceptable range of coincidence (≤0.5 mm from the apical constriction) but were within 1 mm from the apical constriction. So, all together (true coincidence and within clinically acceptable range) a higher percentage of EWLs (14%+82%=96%) under clinical conditions were either accurate or marginally short of AWL within a clinically acceptable range of ≤0.5mm.

Table I: The level of coincidence determined for the study

level of coincidence



Exact coincidence: Zero difference between the value obtained by electronic method and the value obtained using the Awl method.


Acceptable coincidence: 0.5 mm or less than 0.5 mm decrease in measurement (≤0.5 mm) when compared with that obtained using AWL method


Non-acceptable coincidence: more than 0.5 mm (>0.5 mm) short of the actual working length or more than the AWL

On reliability analysis, all (100%) EWL significantly correlates with the Actual Working Length (AWL) r= 0.971]. So, it could be assumed that Electronic Apex Locator like C Root I can measure working length of root canal clinically with a high degree of accuracy (96%).

The various levels of coincidence obtained by the electronic Apex locator (C Root I) during clinical measurement of the root canals and reliability analysis at various levels vis-a-vis actual working length are tabulated (Table II-V).

Table II: Interpretation of coincidence (n=100)









Acceptable (>0.5 mm short in measurement than AWL)



-0.50 to -0.20

Nonacceptable (>0.5 mm short or more than AWL)



-1.50 to +0.50




-1.50 to +0.50

Table III: Comparison between AWL and EWL (n=100)


AWL (mm)

EWL (mm)

p value








Paired Student′s 't′ test, *** = Significant AWL = Actual working length; EWL = Electronic working length

Table IV: Comparison between AWL and EWL based on coincidence


AWL (mm)

EWL (mm)

p value

Exact (n=14) Mean±SD








Nonacceptable (n=4) Mean±SD




Table V: Reliability analysis (AWL vs EWL)



p value




Acceptable (n=82)



Nonacceptable (n=4)



Pearson correlation coefficient test ns = Not significant, *** = Significant

Figure 1: Electronic working length(EWL) truly coincided with the Actual working length(AWL)

Figure 2: Elcetronic working length (EWL) were ≤0.5 mm short in measurement than Actual working length (AWL)


Findings of this study revealed that the electronic working lengths were only 14% coincident with actual working length. Using different brand apex locator and different evaluation criteria, other former in vivo studies resulted higher success rates than the present study.6,7The reason forgetting a higher coincidence rate was probably due to difference in the process of determining the actual working length. Those resulted higher success rates than the present study in consideration of actual coincidence, often used the major foramen as a reference point. But in this study, it was explored that the minor apical foramina (apical constriction). However, the overall higher success rate (96%) in the present study may be due to using brand of apex locator, method used to compare the device and parameters used for comparison.

Hoer and Attin when determining the accuracy of apex locator set the target intervals 'apical constriction to major foramen′.14 Finally, the position of the file tip in relation to target interval was recorded. If the file tip hit the target interval, the measurement was recorded as 'success′, if not, it was recorded as 'non-success′. Thus, 82.4% file tips were found within the target interval 'apical constriction to major foramen′. In the present study, a total of 14% EWLs were either truly coincided with the target goal (just at the apical constriction) and 82% EWLs were within 0.5 mm from the apical constriction. If we would consider a target interval of 0.5 mm from the apical constriction a total of 96% could attained accuracy. Previous studies demonstrated that electronic apex locators (EALs) can determine canal length within 0.5 mm from the apical constriction in 84 -100% of canals.16-19

Though EWL in 4 canals out of 100, found to be within non acceptance range as per set criteria for the study; But if it is considered the previous study2 where the target intervals were ±1 mm around the apical constriction or around the major foramen, even these values can be acceptable.20,21

Finally, overall reliability analysis for 100 canals showed that EWLs had a significant correlation co-efficient (r=0.971); while considering individual group, EWL in 82 canals being within acceptable range (±0.5), was also statistically co-relate with AWL (r= 0.991); having similarity with the study of Shanmugaraj et al, where an interclass co-relation co-efficient value was 0.99 between EWL and AWL.7


The results of the present study confirm that C Root I apex locator device showed a high degree of success (96%) to identify the apical constriction i,e, in determination of working length during root canal treatment within a clinically acceptable range of ≤0.5 mm. It was tried to keep the canals lightly moistened, but it was crucial. During the process of clinical experiment, errors may occur, as in other types of apex locators, from patient′s excessive hydration or dehydrations.

Based on previous study, it was inspected and evaluated apex and apical constriction at 16x magnification, but it is true that microscopic evaluation (SEM) of the apex and apical length should have been more accurate scientifically.17


  1. Ding J, Gutmann JL, Fan B, Lu Y, Chen H. Investigation of apex locators and related morphological factors. J Endod. 2010; 36:1399-03.
  2. Nekoofar MH, Ghandi MM, Hayes SJ, Dummer PMH. The fundamental operating principles of electronic root canal length measurement devices. International Endodontic Journal. 2006; 39: 595-09.
  3. Kuştarci A, Arslan D, Altunbaş D. Dent Res J (Isfahan). 2014 Sep-Oct; 11: 568-73.
  4. Marcos-Arenel JL, Caplan DJ. Oral Surg Oral Med Oral Pathol Oral RadiolEndod 2009;108: e101-e105
  5. Loyd AL, Ingle JI. Electronic apex locator. In: Ingle Ji, Backland LK, Baumgartner JC editor. Endodontics.6th ed. Hamilton: BC Decker Inc; 2008. P. 848.
  6. Mandik J, Shah N, Pawar K, Gupta P, Singh S, Shaik SA. An In Vivo Evaluation Of Different Methods of Working Length Determination. J Contemp Dent Prac 2013; 14 :644-48
  7. Shanmugaraj M, Nivedha R, Mathan R, Balagopal S. Evaluation of working length determination methods: An in vivo / ex vivo study. Indian J Dent Res 2007; 18:60-62.
  8. Jarad FD, Albadri S, Gamble C, Burnside G, Fox K, Ashley JR, Peers G, Preston AJ. British Dental Journal 2011; 211: 595-98
  9. Wrbas K T, Ziegler A A, Altenburger M J, Schirrmeister J F. In vivo comparison of working length determination with two electronic apex locators. Int Endod J 2006; 40: 133-38
  10. Gordon MPJ, Chandler NP. Electronic apex locators. International Endodontic Journal, 2004; 37: 425-37.
  11. Kang JA, Kim SK. Accuracies of seven different apex locators under various conditions. Oral Surg Oral Med Oral Pathol Oral RadiolEndod. 2008 Oct;106: e57-62.
  12. Czerw RJ, Fulkerson MS, Donnelly JC, Walmann JO. In vitro evaluation of the accuracy of several electronic apex locators. Journal of Endodontics. 1995; 21:572-75.
  13. Meares WA, Steiman HR. The influence of sodium hypochlorite irrigation on the accuracy of the Root ZX electronic apex locator. Journal of Endodontics. 2002; 28: 595-98.
  14. Hoer D, Attin T. The accuracy of electronic working length determination. International Endodontic Journal. 2004; 37: 125-31.
  15. Paludo L, Souza SL, Só MVR, Rosa RA, Vier-Pelisser FV, Duarte MAH. An in vivo radiographic evaluation of the accuracy of Apex and iPex electronic Apex locators. Braz Dent J. 2012:23;54-58.
  16. Vieyra JP, Acosta J, Mondaca JM. Comparison of working length determination with radiographs and two electronic apex locators. Int Endod J. 2010; 43:16-20.
  17. Vieyra JP, Acosta J. Comparison of working length determination with radiographs and four electronic apex locators. Int Endod J. 2011; 44:510-18
  18. Kim E, Marmo M, Lee CY, Oh NS, Kim IK. An in vivo comparison of working length determination by only Root-ZX apex locator versus combining Root-ZX apex locator with radiographs using a new impression technique. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2008;105: e79-83.
  19. Plotino G, Grande NM, Brigante L, Lesti B, Somma F. Ex vivo accuracy of three electronic apex locators: Root ZX, Elements Diagnostic Unit and Apex Locator and ProPex. Int Endod J. 2006; 39:408-14.
  20. Vasconcelos BC, Macedo do Vale T, Menezes AST, Pinheiro-Junior EC, Vivacqua-Gomes N, Bernardes RA, Duarte AH. An ex vivo comparison of root canal length determination by three electronic apex locators at positions short of the apical foramen. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2010; 110: e57-61
  21. Versiani MA, Santana BP, Caram CM, Pascon EA, de Souza CJA, Biffi JCG. Ex vivo comparison of the accuracy of Root ZX II in detecting apical constriction using different meter's reading. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2009; 108: e41-e45.