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Comparison of working length determination during root canal treatment

CE Publish Date: June 24, 2017
CEU (Continuing Education Unit): 2 Credits

Educational aims and objectives

This clinical article aims to evaluate in vivo the accuracy of two electronic apex locators —
Root ZX® and CanalPro™ — for determining the working length (WL) as compared to radiographs.

Expected outcomes

Endodontic Practice US subscribers can answer the CE questions to earn 2 hours of CE from reading this article. Take the quiz by clicking here. Correctly answering the questions will demonstrate the reader can:

  • Realize that electronic apex locators were more accurate compared to radiographs in determining working length (WL), greatly reducing the risk of instrumenting and filling beyond the apical foramen.
  • See some history behind the electronic determination of WL.
  • Realize the challenges of accurate determination of WL.
  • Identify some advantages for electronic apex locators.
  • Recognize some drawbacks to the use of electronic devices to determine WL.

Drs. Jorge Paredes Vieyra, Mario Ignacio Manriquez Quintana, Francisco Javier Jiménez Enríquez, and Fabian Ocampo Acosta examine the working length accuracy of two electronic apex locators compared with radiographs

Accurate determination of working length (WL) during root canal treatment is a challenge. Anatomically, the apical constriction (AC), also called the minor apical diameter or minor diameter (Kuttler, 1955), is a logical location for working length since it regularly coincides with the narrowest diameter of the root canal (American Association of Endodontists, 2003). However, locating the AC clinically is problematic.

Dummer, et al., 1984, concluded that it is difficult to detect the minor foramen clinically with certainty because of its location and topography. The cementodentinal junction (CDJ) has also been suggested as the position for WL because it represents the transition between pulpal and periodontal tissue (Grove, 1931). The site of the CDJ is widely accepted as being 0.50 mm to 0.75 mm coronal to the apical foramen (Ricucci, Langeland, 1998) but, as with the AC, the exact location of the CDJ is difficult to identify clinically. In general, the CDJ is considered to be co-located with the minor foramen (Stein, Corcoran, Zillich, 1990); however, this is not always the case (Dummer, McGinn, Rees, 1984).

WL is defined as “the distance from a coronal reference point to the point at which canal preparation and filling should terminate” (American Association of Endodontists, 2003). Radiographic determination of WL has limitations such as distortion, shortening, and elongation; interpretation variability; and lack of three-dimensional representation. Even when a paralleling technique is used, elongation of images has been found to be approximately 5% (Van de Voorde, Bjordahl, 1969).

A WL 1 mm short of the radiographic apex may result in over- or under-instrumentation because of the variability in distance between the terminus of the root canal (minor foramen) and the radiographic apex (Gutiérrez, Aguayo, 1995). Thus, this often used “rule” is not predictable or reliable.

Custer (1918) was the first to determine WL electronically. Suzuki (1942) investigated the electrical resistance properties of oral tissues and developed the first electronic apex locator. The device was resistance-based and measured the resistance between two electrodes to determine the location of an instrument in the canal. Later devices were impedance-based (Nekoofar, et al., 2006) and used multiple frequencies. More recently, resistance- and capacitance-based devices emerged that measure resistance and capacitance, directly and independently.

The Root ZX® (J. Morita USA, Inc.) uses the “ratio method” to locate the minor foramen (Kobayashi, Suda, 1994) by the simultaneous measurement of impedance using two frequencies. The Root ZX claims to work in the presence of electrolytes and non-electrolytes and requires no calibration (Kobayashi, 1995).

CanalPro™ (Coltene-Whaledent) is a modern apex locator that uses multiple frequencies. (Unlike conventional apex locators, two measuring frequencies are alternated, not mixed, eliminating noise and the need for signal filtering.) Signal intensity is used to calculate the file tip position, making the measurement immune to electro-magnetic interference in an attempt to eliminate the influence of canal conditions.

In addition to improving WL accuracy (Nekoofar, et al., 2006), electronic apex locators (EALs) address concerns about radiation, as they have the potential to reduce the number of radiographs taken during root canal treatment (Pagavino, Pace, Baccetti, 1998).

The purpose of this study was to evaluate in vivo the accuracy of two EALs (Root ZX and CanalPro) for determining WL as compared to radiographs. The null hypothesis is that these apex locators provide identical results related to working length determination in vivo.

Materials and methods

Root ZX and CanalPro apex locators (ALs) with equal working frequencies (0.4kHz and 8kHz) were used. One hundred and eighty patients, aged 28 to 75 years, contributed to the study. One hundred and sixty teeth (493 canals) with fully formed apices (confirmed by radiographic evaluation before treatment) and apical periodontitis were used (Table 1). Pulps in 31 teeth were non-vital; the rest of the teeth gave positive responses to hot and cold tests and were scheduled for extraction for periodontal or prosthodontic reasons. Approval by the institutional review and ethical board before the start of the study was obtained, and written consent was given from each patient.

After local anesthesia, rubber dam isolation and access cavity preparation were prepared in such a way that straight line access to the root canals was provided, and undercuts were avoided. After the identification of the root canals, the canals were flared coronally with size 1 and 2 Orifice Shapers (DENTSPLY Tulsa Dental Specialties) using 2.5% sodium hypochlorite for irrigation. The final rinse was aspirated, but no attempt was made to dry the canals.

The AC of each tooth was located with two EALs. The working lengths were then determined by a single operator. The Root ZX and the CanalPro were used according to the manufacturers’ instructions. The lip clip was attached to the patient’s lip, and a size 15 file was coupled to the electrode of the apex locators.

The minor foramen was located with the Root ZX by advancing a size 15 stainless steel K-file in the canal until the locator indicated that the minor foramen had been reached, according to the manufacturer’s instructions. The LCD showed a flashing bar between “Apex” and “1” and a flashing tooth. The silicone stop on the file was positioned at the reference point. The instrument was fixed within a removable light-curing composite pattern (ceram.x®, DENTSPLY). This was the insertion length. Then the composite pattern was removed from the tooth.

The procedure was repeated in the same tooth with another instrument using the CanalPro. In each case, the composite pattern was repositioned exactly in the respective tooth. The AC was located with the CanalPro by advancing the same size 15 K-file in the canal until the locator indicated that the minor foramen had been reached, per the manufacturer’s instructions. The stop was positioned at the reference point and the insertion length measured. The sequence of testing alternated between the two locators.

According to CanalPro’s user’s guide, the apical zone is divided into 11 segments graduated from 1.0 to 0 (Apex) as visual information of file progression. When the apex is reached (read bar at the mark “0” and reading “Apex”), a solid tone is emitted. To determine the working length for shaping, it is recommended to subtract 0.5 mm from the apical length.

The minor foramen was located radiographically by advancing the size 15 K-file until its tip was 1.0 mm from the radiographic apex (determined from a pretreatment parallel technique radiograph). A radiograph was exposed, and if the file tip was seen not to be 1.0 mm from the radiographic apex, the file was repositioned and another radiograph taken to ensure that it was. The distance from the stop to the tip was the insertion length. The file was then reinserted to the insertion length (1 mm from the radiographic apex) and cemented in place with Fuji II™ LC dual-cure glass ionomer cement (GC Corp). The file handle was sectioned with a high-speed bur, and the tooth was extracted without disturbing the file, placed in 6% sodium hypochlorite for 15 minutes to remove remaining tissue from the root surface, and stored in a 0.2% thymol solution. All of the clinical procedures were conducted by the principal investigator.

After the tooth was removed from the solution, and with the file still in place, the apical 5 mm of the root was ground parallel to the long axis of the canal with a fine diamond bur and abrasive discs. When the file became visible, additional dentin was removed under 20x magnification (OPMI® Pico microscope, Carl Zeiss) until the file tip, the canal terminus, and the foramen were in focus.

The first digital image was taken and stored in Adobe Photoshop 5.5, and the distance of the file tip to the minor foramen was measured. This distance was recorded as being -1.0 mm from the minor foramen; -0.5 mm from the minor foramen; at the minor foramen; +0.5 mm from the minor foramen or +1.0 mm from the minor foramen. A minus symbol (-) indicated a file short of the minor foramen; a plus symbol (+) indicated it was long.

The second image was made with the repositioned composite pattern for the measurement with the Root ZX, and the third image was made with the composite pattern for the working length determination with CanalPro.

Once the actual length to the minor foramen was measured visually, the distance from the minor foramen determined by the two EALs was also completed (-1.0 mm from the minor foramen; -0.5 mm from the minor foramen, etc.) by comparing their insertion lengths to the actual length (distance to the AC) (Tables 2-4). The measurements obtained by the two EALs and radiographs relative to the actual location of the minor foramen were compared using a paired samples t-test, X2 test, and a repeated measure Anova evaluation was conducted at the 0.05 level of significance.


For anterior teeth, the Root ZX, CanalPro, and radiographs located the minor foramen 83%, 70%, and 22% of the time, respectively.

For premolar teeth, the Root ZX, CanalPro, and radiographs located the minor foramen 75%, 64%, and 28% of the time, respectively. For molar teeth, the Root ZX, CanalPro, and radiographs located the minor foramen 63%, 51%, and 14% of the time, respectively. There was no statistically significant difference between the two EALs, but there was a difference when the EALs and radiographs were compared (Tables 2-4).

For anterior premolar and molar teeth, none of the measurements were 1.0 mm short of the minor foramen. For anterior and premolar teeth, none of the measurements were 0.5 mm short of the minor foramen, but for molar teeth 1%, 7%, and 8% of the measurements using the Root ZX, CanalPro, and radiographs, respectively, were short.

For anterior teeth, the Root ZX, CanalPro, and radiographs were 0.5 mm long of the minor foramen 17%, 30%, and 52% roots, respectively. For premolar teeth, the Root ZX, CanalPro, and radiographs were 0.5 mm long of the minor foramen 25%, 85%, and 53% roots, respectively, and for molar teeth, it was 34%, 36%, and 40%, respectively.

No EAL measurements were 1.0 mm long of the minor foramen for anterior, premolar, and molar teeth; but for radiographs it was 26% for anterior teeth, 18% for premolar teeth, and 31% for molar teeth. There was no statistically significant difference between the two EALs, but there was a significant difference (p = 0.05) when the EALs and radiographs were compared.


The present in vivo study was performed to evaluate the accuracy and predictability of two EALs for determining WL as compared to radiographs.

The use of electronic devices to determine WL has gained in popularity. When using them, an important consideration is being aware of the possible sources of error such as metallic restorations, salivary contamination, dehydration, etc. However, as shown in this and other studies, the accuracy of EALs is superior to radiographs (Van de Voorde, Bjorndahl, 1969; Pratten, McDonald, 1996; Venturi, Breschi, 2007).

Accurate determination of the WL is a critical step for the success of endodontic treatment. Radiography is the most commonly used technique for WL determination in clinical practice. However, it has some drawbacks. Therefore, taking into consideration the limitations of conventional radiography, the present study was designed to evaluate the accuracy of WL measurements obtained with two EALs and radiograph.

One of the reasons a radiographically determined WL lacks accuracy is that it is based on the radiographic apex rather than the canal terminus — the minor foramen. WL is obtained with a radiograph by positioning the tip of a file a certain distance (usually 1.0 mm) from the radiographic apex. Nonetheless, WL should be based on the position of the minor foramen rather than the apex because the foramen frequently is not at the apex (Wrbas, et al., 2007).

In this study, radiographs correctly located the minor foramen 22% of the time, whereas for the Root ZX and CanalPro, it was 83% and 70% of the time, respectively. Both EALs were within ±0.5 mm from the minor foramen 85% of the time, whereas radiographs were within ±0.5 mm of 22% of cases.  An in vivo study by Shabahang, et al., 1996, reported that the Root ZX was within 0.5 mm from the minor foramen 96% of the time, a value similar to the present findings.

In general, this study also agrees with others (Usun, et al., 2007) that EALs are more accurate than radiographs and greatly reduce the risk of instrumenting and filling short or beyond the canal terminus. Since the minor foramen varies in location and anatomy (sharply defined, parallel, or missing), caution should be used to avoid overestimating working length (Nekoofar, et al., 2006). According to Gutierrez and Aguayo (1995), over-instrumentation of the root canal must be a common and un-
detected occurrence.

An instrument passing through a necrotic pulp and through the foramen most likely carries bacteria and toxins into the apical tissues (Siqueira, et al., 2002; Siqueira, Barnett, 2004). An indication by an EAL of reaching the minor foramen or foramen is very helpful in avoiding mishaps.

Indeed, this study showed that WL obtained with radiographs was 1.0 mm long of the AC 34% of the time but 0% for the two EALs. This high incidence of error is clinically important because a WL of 1.0 mm long would result in canals being instrumented outside the foramen.

Using an EAL as an aid to endodontic therapy could also help to reduce radiation dose required for WL determination, thereby reducing the radiation hazard to the patient. Under the ex vivo conditions of this study, it can be concluded that there was no significant difference between the accuracy of the two EALs in determining the WL when compared with conventional radiography.


Under clinical conditions, the EALs identified the minor foramen with a high degree of accuracy. EALs were more accurate compared to radiographs with the potential to greatly reduce the risk of instrumenting and filling beyond the apical foramen.

Author Info

Dr. Jorge Paredes Vieyra is an endodontist and professor of endodontics and pulp therapy at the School of Dentistry, Universidad Autónoma de Baja California, Campus Tijuana, Tijuana, Baja California, Mexico.

Dr. Mario Ignacio Manriquez Quintana is a professor of oral rehabilitation at the School of Dentistry, Universidad Autónoma de Baja California, Campus Tijuana, Tijuana, Baja California, Mexico.

Dr. Francisco Javier Jiménez Enríquez is a professor of oral surgery and periapical surgery at the School of Dentistry, Universidad Autónoma de Baja California, Campus Tijuana, Tijuana, Baja California, Mexico.

Dr. Fabian Ocampo Acosta is a histopathologist and professor at Universidad Autónoma de Baja California, Campus Tijuana, Tijuana, Mexico.


The authors thank Professor Dr. Michael Hülsmann (Göttingen, Germany) for his valuable assistance in reviewing this manuscript.

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