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Accuracy of electronic apex locators in single-rooted teeth during endodontic retreatment with chloroform — an ex vivo study

Educational aims and objectives

This article aims to evaluate the accuracy of two electronic apex locators (EALs) in the presence of chloroform during endodontic retreatment ex vivo.

Expected outcomes

Endodontic Practice US subscribers can answer the CE questions by taking the quiz online at endopracticeus.com to earn 2 hours of CE from reading this article. Correctly answering the questions will demonstrate the reader can:

  • Realize some history of electronic apex locators (EALs) and the limitations of earlier models.
  • Realize some positive aspects of the newer generation of EALs.
  • Recognize the effect of chloroform alone or in combination with gutta percha on the accuracy of electronic apex locators.
  • Realize other factors that may influence measurement accuracy.
  • Recognize why the use of an EAL is important for accurate determination of WL.

Dr. David Keinan and colleagues study the effect of chloroform on the performance of electronic apex locators.

Drs. David Keinan, Aviv Shmuel, Shlomi Ritz, and Iris Slutzky-Goldberg study the effects of the presence of chloroform in the canal during retreatment



The aim of the study was to evaluate the accuracy of the Apex NRGXFR and Apit 11 electronic apex locators (EALs) in the presence of chloroform during endodontic retreatment ex vivo.

Methods and materials

Thirty-five extracted single-rooted teeth were used in this study. Following access cavity preparation, the actual (AL) and electronic (EL-1) canal lengths were measured. The teeth were divided into four groups. Group O (5 teeth) was the negative control. Electronic measurements were performed in empty canals (EL-2). In group A (10 teeth), canals were filled with chloroform, and electronic measurements were performed (EL-2). The remaining teeth were obturated. After 7 days, the root fillings were removed using chloroform and stainless-steel files; these teeth were divided into groups B and C (10 teeth each). In group B, the electronic length during retreatment (EL-2) was measured when patency was achieved. In group C, the EL-2 was measured after chloroform had been allowed to evaporate for 10 minutes. The EL-1 and EL-2 were compared to the AL.


The average EL-1-AL distances using the Apex NRGXFR and Apit 11 were 0.6157 mm  ± 0.1179 mm and 0.5297 mm ± 0.8993 mm, respectively; the EL-1 was significantly shorter than the AL (p < 0.01). The average AL-EL-2 distances using the Apex NRGXFR and Apit 11 were 1.0 ± 0.6152 mm and 0.5667 ± 0.5381 mm, respectively; the EL-2 was significantly shorter than the AL (p < 0.05). In group A, the readings (EL-2) were longer for both EALs than for the EL-1 (p < 0.01). In group B, the EL-2 reading was not significantly longer than the AL using both EALs. In group C, the AL-EL-2 distance was shorter than the AL-EL-1 distance using both EALs (p < 0.05).


The presence of chloroform in the canal during retreatment is associated with inaccurate prolonged EAL measurements.


Determining the apical terminus of a root canal during preparation is an important step in root canal treatment. Thus, electronic apex locators (EALs) are currently used in root canal treatment. The use of EALs is based on the findings of Custer in 1918 and Suzuki in 1942; they reported a constant difference in electrical resistance between the oral mucosa and the root canal.1 The first EALs had several limitations, particularly regarding the presence of fluids inside the root canal.2 These limitations restricted their use, and the necessary working length (WL) was determined based on radiographic estimation of the canal length, combined with obtaining a tactile sensation in the apical constriction using an endodontic file.3 The distance between the radiographic apex and the location of the apical foramen can be diverse,4 and the apical constriction can assume a wide range of locations and histological shapes within the canal;5 thus, prudent clinicians used EALs to determine the WL.6,7 New generations of EALs that are more user-friendly and precise have become available. These devices are not affected by the presence of solutions in the root canal.8,9 EALs are highly accurate, as evidenced by a clinically accepted tolerance of 0.5 mm –1 mm.10 Various factors may influence measurement accuracy, including the pulp status11,12 and type of EAL used.13,14 Notably, the presence of solutions in the canal during root canal retreatment affects the accuracy of measurements using EALs.15-18

Figure 1: Study design

The apical extent of canal filling is an important prognostic factor for root canal treatment, including retreatment.19-21 The use of apex locators during endodontic retreatment has been evaluated.22-24 However, controversy exists regarding their accuracy for WL determination after removal of the root canal filling material.24

In a study regarding the effects of organic solvents on the accuracy of EAL, electronic measurements were conducted after the canal had been filled with chloroform, orange solvent, or eucalyptol. There were no differences among the solvents or devices used.15 In contrast, after the removal of gutta percha, the Root ZX Mini (J. Morita) yielded a reading shorter than the actual WL in the presence of Guttasolv (Septodont), Endosolv R (Septodont), and Resosolv (Acteon).24

The aim of this study was to evaluate the effect of chloroform alone or in combination with gutta percha on the accuracy of two electronic apex locators during endodontic retreatment.

Methods and materials

The protocol was adapted from the method used by Alves, et al.23 Thirty-five extracted intact, straight, single-rooted human teeth with completely formed roots were used. The extracted teeth were obtained from a pool of teeth that had been extracted for reasons irrelevant to the study (generally during periodontal treatment); written informed consent had been provided by all patients prior to extraction. The teeth were stored in 0.9% sterile saline until use. The teeth were serially marked to enable comparison of the results of the two apex locators. The crowns were ground to establish a flat surface, which was used as a reproducible reference point for all measurements.

The access cavity was created using A3 diamond burs (Strauss Co., Israel); Gates-Glidden burs Nos. 1–3 (Dentsply Maillefer, Ballaigues, Switzerland) were used to prepare the coronal third of each canal. During access cavity preparation, the canals were irrigated with saline; the patency of the apical foramen was verified using a No.10 K-File (Dentsply Maillefer). Canal length was established by passive introduction of a 15K-File (Dentsply Maillefer) into the canal until its tip was visible at the apical foramen. This procedure was conducted under 2.6 x magnification using surgical loupes (Orascoptic™). After adjustment of a silicone stop at the incisal surface, the file was removed from the canal, and the distance between the file tip and the rubber stop was measured using an endodontic ruler (Dentsply Maillefer). This measurement was repeated 3 times, and the mean value for each tooth was calculated and recorded as the actual length (AL).

Next, the roots were mounted in alginate (Plastalgin, Septodont) in a plastic container. Each root was passed through a central hole in the top of the container and fixed with acrylic resin to prevent movement during instrumentation. A second, smaller hole was made in the top of the container to stabilize the electrode (lip-clip) and allow it to contact the alginate; this simulated the conductivity of the periodontium. The ex vivo model was adapted and modified from the method of Kaufman, et al.25

Apex locator accuracy

For each tooth, an electronic length measurement was carried out using a No. 15K-file (Dentsply Maillefer) attached to the EAL until the 0.5 marking on the Apex NRGXFR (Medic NRG Ltd., Tel Aviv, Israel) or the middle of the yellow zone for the Apit 11 (Osada, Tokyo, Japan) was reached. This measurement was repeated 3 times for each apex locater; the mean value for each EAL was calculated and recorded as electronic length-1 (EL-1).

Experimental phase

The 35 root canals were serially enlarged up to the apical size of a No. 45K-file (Dentsply Maillefer). The root canals were irrigated with 2.5% sodium hypochlorite for each instrument change; at the end of preparation, the canals were dried using paper points.

  • Group O: 5 teeth (control). These teeth did not receive root filling. The canals were irrigated with sodium hypochlorite (0.9%) prior to electronic length measurement.
  • Group A: 10 teeth (positive control). The root canals were filled with chloroform after preparation.
  • The remaining teeth were filled using lateral compaction of gutta-percha cones and AH Plus Sealer (Dentsply De Trey, Konstanz, Germany). The access cavities were sealed with Coltosol® (Coltene/Whaledent AG, Switzerland); the roots were stored at 37°C and 100% humidity for 7 days. The temporary fillings were removed using an A3 round diamond bur (Strauss Co., Israel), and the teeth were randomly divided into the following two groups:
    • Group B: 10 teeth. Following removal of the filling material from the cervical and middle thirds of the canal using Gates-Glidden drills (No. 1 and No. 2), the access cavities were filled with 0.1 mL of chloroform to soften the gutta percha.
    • Group C: 10 teeth. Following removal of the filling material from the cervical and middle thirds of the canal using Gates-Glidden drills (No.1 and No. 2), the access cavities were filled with 0.1 mL of chloroform to soften the gutta percha. The chloroform was allowed to evaporate for 10 minutes. The working area was kept ventilated.

A No. 15K-file was inserted through the residual root canal filling. When the file reached the “APEX” signal of the EAL Apex NRGXFR (Medic NRG Ltd.) or Apit 11 (Osada), a second electronic length measurement was carried out for each tooth. These measurements were repeated 3 times for each apex locater; the mean value for each EAL was calculated and recorded as electronic length-2 (EL-2) (Figure 1). All measurements were performed by the same operator (A.S.), who was blinded with respect to the true length measurements.

Statistical analysis

Differences among AL, EL-1, and EL-2 were analyzed by Student’s t-test and two-way analysis of variance. Statistical significance was set at p < 0.05.


Differences between AL and EL‑1: After gaining access to the root canal and before preparation, the mean differences between the AL and EL-1 were 0.6157 ± 0.69752 mm (p = 0.00001) and 0.5297 ± 0.89939 mm (p = 0.002) with the Apex NRGXFR and Apit 11, respectively. The EL-1 reading was shorter than the AL reading for both EALs (p > 0.05).

Experimental phase

One of the teeth in group B (chloroform only) was excluded from the study because it contained an obstruction in the root canal that prevented electronic measurement (EL-2). The results are listed in Table 1.

In group O, the average AL-EL-2 distances for Apex NRGXFR and Apit 11 were 1 ± 0.6152 and 0.5667 ± 0.5381 mm, respectively (p < 0.05). In group A (chloroform only), the mean readings were similar to the AL (0.1852 and 0.2778 mm for Apit 11 and Apex NRGXFR, respectively); the difference in AL-EL-2 was smaller but remained significant (p < 0.01). In group B (chloroform and gutta percha), the mean differences between AL and EL-2 were −0.15 and −0.375 for the Apex NRGXFR and Apit 11, respectively (p > 0.05). In group C (gutta percha, after evaporation of chloroform), the EL-2 was shorter than the AL using both EALs (p < 0.05); average differences of 0.4188 and 0.4083 mm were found for the Apex NRGXFR and Apit 11, respectively.


Chloroform is still widely used as an organic solvent for GP removal during retreatment, despite concerns regarding risk of cell cytotoxicity and carcinogenicity.26 The aim of the study was to examine the possible effect of solvents on EAL’s accuracy. Accurate determination of WL is a significant prognostic factor for the outcome of root canal retreatment.19-21 Therefore, the use of EAL during endodontic retreatment is compulsory.22-24 Epidemiological studies have shown that length determination impacts the outcome of endodontic treatment.19-21 The apical extent of canal filling was also a significant prognostic factor for root canal retreatment,19-21 although it was recently reported that overfilling does not influence treatment outcome.27 Furthermore, apical extrusion of debris may increase the risk of flare-ups28 and persistent periapical radiolucency.28-30 Hence, locating the apical constriction is an important step during retreatment.19

The use of apex locators during endodontic retreatment has been suggested.22-24 However, their accuracy in determining the WL after removal of root canal obturation materials is controversial.23,24 Evaluation of the difference between AL and EL-1 enabled assessment of the accuracies of both apex locators. The differences in their readings were not statistically significant. Using both devices, the minor foramen was 0.5 mm–1 mm from the major foramen. These results are consistent with reports of a 0.5 mm–0.8-mm distance between the foramen and the apical constriction, depending on patient age.4,5 It should be emphasized that AL represents the major foramen and differs from the desired WL; thus, the AL was considered to be reached when the tip of the file was visible through the major foramen, which is beyond the apical terminus of the canal. Accordingly, readings shorter than the AL represent measurement locations closer to the apical constriction. Notably, a study of the efficacies of two types of EALs integrated into rotary motors to accurately determine canal length while removing the gutta percha found that the passive insertion of files was more accurate than rotating insertion; however, it led to readings beyond the foramen in both types of EALs.31

The readings in empty canals (group O) demonstrated the position of the apical constriction. The EAL readings in the presence of chloroform alone (group B) or in combination with gutta percha (group C) were longer than the apical constriction and closer to the AL. Therefore, it can be assumed that chloroform disturbs the EAL readings, leading to overinstrumentation and overfilling. This may increase postoperative pain after retreatment.28 These findings are in contrast to a report that the readings of apex locators were not affected by solutions in the root canal, including chloroform, orange solvent, and eucalyptol.15

This issue has been addressed in several studies, although the accepted tolerance was ± 0.5-1 mm,16,22 allowing overestimation of the WL. Concerns have been raised regarding inaccurate measurement of the WL during root canal retreatment.17,18,24 However, some of the reported measurements were performed without dissolving solution.17 Remnant root-filling material reportedly influences the impedance of extracted human roots,32 which may explain the longer EAL readings in group C after evaporation of chloroform. The longer readings in group B are likely attributable to coating of the file with gutta percha, which prevented contact with the canal walls, thereby disrupting EAL readings similar to the inaccurate readings obtained in very wide canals.33 Furthermore, the ADA recommends the use of other solvents during retreatment.34 In view of the results, it may be advisable to use rotary retreatment files without a solvent, as previously suggested,35 which may be safer to the patient and prevent inaccurate readings.

Moreover, the presence of chloroform in the root canal prolonged readings in groups A and B. The electrical conductivity of chloroform is lower than that of most of the other solutions used in endodontic treatment (Table 2). This effect was reversed after chloroform was allowed to evaporate in group C. In comparison, eucalyptol, which is used for dissolution of the root-filling material, has a higher electric conductivity, and the risk of disrupting the EAL reading can be reduced. Further research is needed to compare the effect of solution conductivity on EAL readings.

A comparison of electric conductivity of different materials used during root canal treatment.


Longer measurements in the presence of chloroform were observed for both apex locators. Prudent clinicians should acknowledge the potential influence of chloroform on electrical readings during endodontic retreatment. To prevent overinstrumentation and extrusion of debris to the periapical tissue because of incorrect readings, remnants of chloroform should be allowed to evaporate from the canal space prior to the use of EALs.

“Accuracy comparison of three different electronic apex locators in single-rooted teeth — an in vitro study” discusses determination of the location of working length. Read all about it here: https://endopracticeus.com/accuracy-comparison-of-three-different-electronic-apex-locators-in-single-rooted-teeth-an-in-vitro-study/

Author Info

Drs. David Keinan, Aviv Shmuel, Shlomi Ritz, and Iris Slutzky-Goldberg are affiliated with the Department of Endodontics, faculty of dental medicine, Hebrew University-Hadassah School of Dental Medicine, Jerusalem, Israel 91120.


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