An ex vivo comparison of working length determination by 3 electronic apex locators

Objective. To compare the accuracy of 3 different electronic apex locators (EALs) in establishing the working length in recently extracted teeth.

Study design. Sixty teeth (100 canals) were soaked in an alginate model and electronically measured with 3 EALs (Dentaport ZX, Raypex 5, and Elements Diagnostic Unit and Apex Locator). The real working length was calculated as 1.0 mm short of the real length of the canal. The electronic measurements were taken following the manufacturers’ orientations within ±0.05 mm and ±1.0 mm using a #15 K-file attached to the holder, after canal irrigation with 1% NaOCl. Data were analyzed with Friedman test and Wilcoxon signed rank test, at a significance level of 5%.

Results. Within ±0.5 mm and ±1.0 mm, the accuracy was 39% and 90% (Dentaport ZX), 31% and 82% (Raypex 5), and 37% and 73% (Elements Diagnostic Unit and Apex Locator), respectively, with statistically significant differences between Elements Diagnostic Unit and Apex Locator and the other EALs.

Conclusion. None of the EALs yielded an accuracy of 100%. Within the limitations of the present study. Elements Diagnostic Unit and Apex Locator proved to be less reliable than Dentaport ZX and Raypex 5 in the determination of the real working length. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2009;108:e147-e151)

The determination of the apical limit for instrumentation and obturation is one of the most important steps in root canal treatment, and it has been a challenge in endodontics. Many studies support that performing the root canal treatment at a proper length is a predictor for a successful outcome. For many years, the position of the instrument tip in relation to the radiographic apex has been the guideline to establish the working length (WL). However, because the radiograph provides a 2-dimensional image of a 3-dimensional structure and the foramen commonly does not coincide with the apex, it does not consistently reveal the end point of the root canal system. The idea of defining the WL electronically was introduced in the beginning of the twentieth century. Since then, the electronic apex locator (EAL) started to be developed through generations of different systems.

Dentaport ZX (J. Morita, Kyoto, Japan), a third-generation device, which simultaneously calculates the ratio of 2 impedances in the same canal using 2 different frequencies (8 kHz and 0.4 kHz), works with the same principle as the original Root ZX, which was tested in several studies. The Raypex 5 (VDW, Munich, Germany) and the Elements Diagnostic Unit and Apex Locator (SybronEndo, Anaheim, CA, USA) are claimed to be a fourth-generation of apex locators. Raypex measures the impedance with the same frequencies as Root ZX II; however, according to the manufacturer, the combination of using only 1 frequency at a time and basing measurements on the root mean square values of the signals increases its accuracy and reliability. Elements Diagnostic Unit and Apex Locator does not process the impedance information as a mathematical algorithm, but instead compares the resistance and capacitance measurements with a database to determine the distance to the major foramen, using a composite wave-form of 2 signals (0.5 kHz and 4 kHz). To date, few studies have been conducted to analyze the accuracy of Dentaport ZX, Raypex, or Elements Diagnostic Unit and Apex Locator. 

Therefore, the purpose of the present ex vivo investigation was to compare the accuracy of Dentaport ZX, Raypex 5, and Elements Diagnostic Unit and Apex Locator in establishing the WL in recently extracted teeth.

Material and methods

Sixty teeth (12 lower molars, 10 upper molars, 4 upper premolars, and 34 single-rooted), with a total of 100 root canals, extracted due to periodontal disease or orthodontic reasons, with mature apices, were selected. Tooth suitability was determined by visual inspection using a dental operating microscope (×10 magnification) and radiographs. Immediately after extraction, all teeth were kept in 10% buffered formalin for at least 48 hours and, before testing, they were placed in 5.25% sodium hypochlorite solution (NaOCl) for 2 hours to remove organic residues. The remaining tissues were removed from external root surfaces using a periodontal scaling instrument. Afterward, the teeth were numbered, rinsed in tap water, and decoronated at the cementoenamel junction, providing a flat surface perpendicular to the long axis of the roots. A standard access preparation was performed, the coronal and middle portions of the canals were shaped using #3 and #4 Gates-Glidden burs (Dentsply-Maillefer, Ballaigues, Switzerland), and the remaining pulp tissue was removed with a barbed broach, without any attempted to enlarge the canal. After that, the canals were irrigated with 5 mL of 1% NaOCl, and the patency of the apical foramen was confirmed with a #08 stainless steel K-file (Dentsply-Maillefer, Baillagues, Switzerland).

To establish the actual root canal length, a size 10 K-file (Dentsply-Maillefer), was placed into the root canal until the tip of the file reached the plane of the major foramen. Proper positioning was verified using a stereomicroscope (Wild Makroskop M420; Heerbrugg, Switzerland) at a magnification of ×15. With the file tip at the apical foramen, the silicone stop was adjust to the coronal surface, the file was removed, and the distance from the stop to the file tip was measured with an endodontic ruler to the nearest 0.5 mm. The measurements were repeated 3 times by different operators, and the average of them was taken as the real length (RL). The real working length (RWL) was established 1-mm short of the RL of the canal.

Subsequently, all teeth were randomly assigned to 3 subgroups (n = 20) and their roots were embedded up to the cementoenamel junction in freshly mixed alginate (Hydrogum; Zhemarck, Rovigo, Italy). Within 2 hours after model preparation, all canals were measured, individually, with the 3 EALs by 1 operator who did not know the preliminary measurements.

For electronic measurement, the metal lip clip was embedded into the alginate and stabilized with transparent adhesive tape. Root canals were irrigated with 1% NaOCl using an endodontic syringe (Navy Tip; Ultradent, South Jordan, UT). The pulp chamber was gently dried with air, and sterile cotton pellets were used to dry the tooth surface and eliminate the excess of irrigation solution, with no attempt of drying the canal. Using the Dentaport ZX, a #15 K-file attached to the holder was advanced within the root canal to just beyond the foramen, as indicated by the flashing “APEX” bar and the solid tone. The file was then withdrawnuntil a flashing bar between “APEX” and “1” had been reached. Using the Raypex 5, the same file was advanced to just beyond the foramen (red light), and withdrawn until all flashing green bars had been reached. Using the Elements Diagnostic Unit and Apex Locator, the #15 K-file was advanced into the canal to just beyond the foramen, as indicated by “0.0” on the LCD display. The file was then withdrawn until the reading of the EAL showed a consistent “0.5” with corresponding symbol and audible signal indicating that the root canal constriction had been reached. Measurements were considered to be suitable if the instrument remained stable for at least 5 seconds.

When the EAL exhibited the specified reading, the silicone stop was adjusted to the coronal surface, the file was removed, and the distance from the stop to the file tip was measured with an endodontic ruler to the nearest 0.5 mm. A mean value of 3 measurements was recorded for each canal as the electronic working length (EWL).

For each reading, the error in measurement was calculated as the absolute difference, in millimeters, between EWL and RWL. Positive or negative values were recorded when the tip was detected beyond or short of the RWL, respectively. Accuracy was determined on stable measurements within ±0.5 mm and ±1.0 mm. Data were analyzed by using nonparametric Friedman test followed by post hoc Wilcoxon signed rank test at a significance level of 5%. The analysis was carried out with the statistical package SPSS version 15 (SPSS, Chicago, IL, USA).

Results

The results are shown in Table I and Fig. 1. The mean distance measurements to the foramen (RL) were found to be 0.72 ± 0.09 mm (Dentaport ZX), —0.69 ± 0.11 mm (Raypex 5), and —1.10 ± 0.15 mm (Elements Diagnostic Unit and Apex Locator), while the mean distances to the RWL were 0.28 ± 0.09 mm (Dentaport ZX), 0.31 ± 0.11 mm (Raypex 5), and —0.10 ± 0.15 mm (Elements Diagnostic Unit and Apex Locator).

Within a tolerance of ±0.5 mm or ±1.0 mm, the accuracies were 39% or 90% (Dentaport ZX), 31% or 82% (Raypex 5), and 37% or 73% (Elements Diagnostic Unit and Apex Locator), respectively. Measurements beyond RL and at the foramen were made in 4% and 30% (Dentaport ZX), 10% and 24% (Raypex 5), and 9% and 17% (Elements Diagnostic Unit and Apex Locator) of the samples, respectively (Table I). Statistical analysis showed significant differences between Elements Diagnostic Unit and Apex Locator and the other tested EALs tested (Wilcoxon signed ranks test, P < .05). No differences were observed between Dentaport ZX and Raypex 5, as well as regarding the type of tooth (P > .05).

Discussion

Electronic apex locators have been considered to be valuable additions to the clinical endodontic armamentarium, and the results of numerous publications have supported this notion, demonstrating that these devices can accurately determine the working length in 75%-96.5% of root canals with mature apices. This seemingly large discrepancy may be a result not only of different experimental protocols but also of the inherent difficulty in repeatedly measuring lengths of files from a common reference point. Some authors measured from the minor apical foramen (apical constriction), whereas others measured from the major apical foramen.

The validity of measurements made with in vitro models (i.e., the extent to which they depict the clinical accuracy of EALs) is unknown. However, they do provide a valuable insight into the function of EALs and enable objective examination of a number of variables that are not practical to clinical testing.

Although nowadays technology related to assembling such new generations of EALs that “is not affected by the presence or absence of blood, other discharges, electrolytes, saline, tap water, or hydrogen peroxide” and that has automatic calibration which “delivers extreme accuracy and reliability,” promising “no more overextended or underextended root fillings,” most investigators have demonstrated that the application of the EALs did not result in a 100% precise location of the apical constriction or major foramen. As recently stated, the results of the present study cannot support the results presented in other investigations in which the accuracy of the EALs was >90% within ±0.05 mm. Bernardes et al. found that the accuracy rate of the apex locators at exactly 1 mm short of the apical foramen was 97.5% for Root ZX and 95% for Elements Diagnostic Unit and Apex Locator. Plotino et al., comparing the differences between measurements obtained with 3 EALs at the apical foramen, showed that the percentage of measurements within ±0.5 mm was 97.37% for Root ZX and 94.28% for Elements Diagnostic Unit and Apex Locator. Stavrianos et al. compared in vivo accuracy of Dentaport ZX and Raypex 4 in locating the apical foramen in 80 single-rooted teeth with vital pulp tissue showed that the Dentaport ZX located the apical foramen in 95% of the cases and the Raypex 4 in 92.5%, with no significant statistical difference.

This might be explained as a result of different research parameters and landmark definitions used in different investigations. In the present study, considering the RWL, it was also observed that the frequency of shorter measurements was always higher compared with the longer ones (Table 1). These results are partially supported by different reports that demonstrated that exact measurements within ±1.0 mm could be achieved in 73%-90% of the cases. Ebrahim et al. showed that Dentaport ZX was able to determine the root canal length measurement 81% and 100% of the time within ±0.5 and ±1.0 mm, respectively. ElAyouti et al. demonstrated that the accuracy of Root ZX and Raypex 4 were, respectively, 90% and 74% in locating the apical terminus of root-end resected teeth within a tolerance level of ±1.0 mm. Wrbas et al. compared in vivo accuracy of Root ZX and Raypex 5 in 20 single-rooted teeth and showed that the minor foramen was located within ±0.5 mm in 75% and 80% of the cases with Root ZX and Raypex 5, respectively. Tselnik et al. demonstrated that the accuracy in locating the minor constriction with Root ZX was 75%, 83.3% to ±0.75 mm, and 88.9%, while the accuracy of Elements Diagnostic Unit and Apex Locator was 75%, 88.9%, and 91.7%, with tolerance levels of ±0.5 mm, ±0.75 mm, and ±1 mm, respectively.

As soon as the measurements for each tooth were carried out under the same conditions, the most remarkable results of the present study to be discussed are related to the overextended readings. It was observed that measurements beyond RL and at the foramen were achieved in, 4% and 30% (Dentaport ZX), 10% and 24% (Raypex 5), and 9% and 17% (Elements Diagnostic Unit and Apex Locator) of the samples, respectively (Table I).

Similarly, some authors have reported evidence of overestimated working length using EALs as recommended by the manufacturers. ElAyouti et al., evaluating in vitro the ability of Root ZX to avoid over-instrumentation in premolars, observed that 7% of the electronic measurements had passed the apical foramen. D’Assunção et al., comparing in vitro accuracy of the Root ZX II and the Mini Apex Locator showed that in 2.56% of the canals the file tip was beyond the foramen. Lucena-Martin et al., testing in vitro accuracy of 3 EALs, showed that in 5% of the canals the measurements surpassed the apical foramen.

Considering earlier anatomic studies of root apex, these results must be seriously considered, in clinical conditions, in contrast to in vitro studies, a higher variation of measurements is expected because the favorable circumstances for precise measurements are not available and, in consequence, an overestimated WL and a potential overextended root filling could led to a poor prognosis. These findings raise a question of whether the WL should be established where the EAL indicates the apical constriction or at some distance coronal to that point. Thus, in order to avoid over-preparation, some authors have recommended a withdrawal of the instrument of about 0.5 to 1.0 mm from the electronic measurement.

Conclusion

None of the tested devices yielded an accuracy of 100%. Within the limitations of the study, if a tolerance limit of ±0.5 mm or ±1.0 mm was allowed, the achieved accuracy was, respectively, 39% and 90% (Dentaport ZX), 31% and 82% (Raypex 5), and 37% and 73% (Elements Diagnostic Unit and Apex Locator), re, with significant differences between Elements Diagnostic Unit and Apex Locator and the other tested EALs.

Authors: Elizeu Álvaro Pascon, Massimo Marrelli, Orsola Congi, Rosetta Ciancio, Federica Miceli, Marco Aurélio Versiani

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