Fig. 1

Radiographic apex, apical foramen and apical constriction. The position of each area can be seen in this image.

Root canal wall that is the dentin will stop at the level of apical constriction (CDJ) and this is where should we stop our preparation

Fig. 2

There are several techniques have been used for determining the apical constriction. Traditionally, the determination will be used by taking radiographs. But nowadays the procedure has become slightly complicated because the apical constriction is a histological landmark that difficult to identified radiographically, and variables in technique, angulations and exposure distort this image leading to errors and multiple exposures which can be lead us in working length determination. 

From this Xray, we might think that the working length is a little bit under. But we need to remember that this is a 2D image.

Fig. 3

From the other side we can see that instead of under length this length is already over for few mm. 

Fig. 4

The apical foramen is not always the same with radiographic apex

Fig. 5

The apical foramen is not always the same with radiographic apex

Fig. 6

Nowadays, the use of an electronic method to determine working length using an apex locator have proven to be powerful and useful adjunct. The development of the electronic apex locator has helped make the assessment of working length more accurate and predictable.

Apex locator is an electronic device used to determine the position of the apical foramen and its location and thus determine the length of the root canal space. Precise name of electronic apex locator (EAL) is Electronic Root Canal Length Measurement Device (ERCLMD) or Electronic Canal Length Measuring Devices. An electronic apex locator has been shown in Fig.6. The work of an Apex Locator is determined by comparing the electrical impedence of periodontal membrane with that of the oral mucosa. An electronic apex locator cord has two ends. One end is called 'lip hook' which is placed in contact with the oral mucosa of the patient. Other end is termed as 'file holder' which is a probe attached to the endodontic instrument in use. The attached file is slowly inserted into the root canal up to the estimated working length. As the file touches the soft tissues of the periodontal membrane, the electrical resistance gauges for both oral mucosa and periodontal membrane would have similar readings and the length of the canal can be determined here.

The development of apex locator has evolved since its first generation to now the 6th generation of apex locator. From resistance type in 1st generation, impedance type in 2nd generation the frequency type in 3rd generation, nowadays the latest generation of electronic apex locators are using multi frequency to be more precise and accurate.

The picture shows an Electronic Apex Locator with Multi frequency. The multi frequency apex locator eliminating necessity of drying the canal and improve precision and accuracy in reading the working length.It provides with a digital read out, graphic illustration and an audible signal.

How we use electronic apex locator clinically

Fig. 7

Other function of Apex Locator are: 

  • To detect root perforations to clinically acceptable limits
  • Determine the location of root and floor perforations
  • To detect horizontal fractures
  • Recognize any connection between the root canal & periodontal membrane such as root fracture, cracks & internal or external resorption. 

This apex locator (wispex) from Fanta dental is a multi frequency apex locator, auto calibration and adjustable reference position for different cases.

Fig. 8

Case of symptomatic apical periodontitis, lesion was evident on both mesial and distal root. Underfilled root canals more likely is the cause of this endodontic failure

Fig. 9

After root canal retreatment, working length can be reached, shaped, cleaned and packed. 

Fig. 10

2 years follow up showed healing lesion


It is undoubtful that higher survival rates were recorded for teeth with root canal fillings of the correct working length. No individual technique is truly satisfactory in determining endodontic working length. But modern electronic apex locators can determine this position with accuracies of greater than 90%. The correct use of an electronic apex locators are user friendly, less time consuming and reliable in most of the clinical situations yet safer compare with the radiographic. However, knowledge of apical anatomy still important for us to understand. Although apex locators can be used as an individual technique to determine working length, but the use of radiographs can not be replace completely, it will serve as an effective adjuvant. 


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