Ex vivo comparison of the accuracy of Root ZX II in detecting apical constriction using different meter’s reading

The purpose of this study was to compare the accuracy of Root ZX II to locate the apical constriction with the display meter set at “0.5” and “1’”reading. Seventy single-rooted teeth were soaked in an alginate model and randomly distributed in 2 groups (n = 35). Measurements were taken following canal irrigation with 1% NaOCl.

The length was established using a #20 K-file attached to the holder when the display indicator reached the marks “0.5” (group I) or “1” (group II), after the meter read “Apex.” Then, the file was fixed in position and the teeth removed from the alginate. The apical portion of the root was shaved until the tip of the file could be seen, the distance to the apical constriction verified by means of a stereomicroscope and the measurements compared.

Statistical analysis was performed by using Student t test with the null hypothesis set as 5%. The mean positions of the file tip relative to the apical constriction were —0.23 ± 0.39 mm and —0.42 ± 0.45 for groups I and II, respectively, with no statistical difference (P > .05). The accuracy was 90.5% and 83.78% for the Root ZX II “0.5” and “1” readings, respectively. It was concluded that the meter reading “1” of Root ZX II reduced the risk of working length overestimation. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2009;108:e41-e45)

Precise working length determination is an essential step in root canal treatment. Several clinical studies support that the position of proper canal length preparation and obturation is a significant predictor of successful outcome in endodontics. The cemento-dentinal junction (CDJ), where the pulp tissue changes into periodontal tissue, has been considered the ideal apical limit of the working length. However, the CDJ is not a constant or consistent feature and, therefore, is not an ideal landmark to use clinically. The apical constriction is the narrowest part of the root canal with the smallest diameter of blood supply and preparation to this point results in a small wound site and optimal healing conditions. Therefore, setting the constriction as the apical limit of the working length has been recommended.

Traditionally, periapical radiograph has been the primary method of canal length determination. To overcome its drawbacks, electronic apex locators (EALs) were designed and marketed to objectively and accurately determine the terminus of the root canal. The EAL Root ZX II (J. Morita, Kyoto, Japan), which simultaneously calculates the ratio of 2 impedances in the same canal using 2 different frequencies (8 kHz and 0.4 kHz), became reference in investigations. This device works with the same principle of the original Root ZX, which was tested in various studies. 

Several authors have claimed that Root ZX was accurate when considering the dial “0.5” reading. Nevertheless, measurements of canal length using this mark on the display have also been related to the file tip being located at or just beyond the apical foramen. Clinically, this condition could lead to an overextended preparation, and, consequently, a poor prognosis. These findings raised the question of whether the working length should be established at the point where the EAL indicates the constriction, or some distance coronal to that point. As a result, some authors have suggested withdrawal of 0.5 or 1.0 mm to reduce the frequency of working length overestimation. Thus, the aim of this ex vivo study was to compare the accuracy of Root ZX II to locate the apical constriction using marks “0.5” or “1” on the display, and the percentage of measurements surpassing the apical foramen.

Material and methods

Specimen selection and preparation

The protocol for this experiment was reviewed and approved by the Research Ethics Committee of the Universidade Federal de Uberlândia (protocol no. 267/ 06). A total of 70 extracted, intact, straight, single- rooted human teeth with completely formed roots that had been stored in distilled water containing 10% formalin were used. No information was available regarding the reasons for their extraction. Before the test, the teeth were placed into 5.25% sodium hypochlorite (NaOCl) solution for 2 hours to remove organic residues. The remaining tissues from external root surfaces were removed using a scaler. After rinsing in tap water, standard access preparation was carried out using high-speed diamond round burs (1016HL, Metalúrgica Fava Prod. Hosp. Dent., Pirituba, SP, Brazil) under water coolant. The coronal and middle portions 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 checked using a size 08 K-file.

Subsequently, the roots of the teeth were embedded up to the cemento-enamel junction into freshly mixed alginate (Avagel, Technew Comércio e Indústria Ltda., Rio de Janeiro, RJ, Brazil) and placed in a plastic box. The teeth were randomly distributed in groups I (n = 35) and II (n = 35), corresponding to the marks “0.5” or “1” of the EAL display, respectively.

Electronic working length determination

All measurements were made within 2 hours after model preparation to ensure sufficient alginate humidity. For electronic measurement, the metal lip clip, embedded into the alginate, was stabilized with transparent adhesive tape, and the 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 excess of irrigation solution, with no attempt at drying the canal. A # size 20 K-file was attached to the file holder, slowly inserted into the canal, and apically introduced until the meter read “Apex.” Then, the instrument was retrieved until the flashing bar “0.5” (group I) or “1” (group II) was reached. Measurements were considered suitable if the instrument remained stable for at least 5 seconds. All teeth were measured individually by one operator (M.A.V.) with several years of clinical experience with EALs. Upon completion of electronic measurement, the file was locked in place using a light-curing composite material (Heliomolar, Vivadent, Schaan, Liechtenstein) and its handle was cut using a diamond bur in a high-speed handpiece. The teeth were then removed from the alginate and immersed in tap water.

Macroscopic evaluation

After drying, the apical portion of the root was shaved in the direction of the long axis with a low speed diamond disk, until the file tip could be seen through a very thin layer of dentin. This layer was then carefully removed using a #15 scalpel blade. Subsequently, the distance from the file tip to the apical constriction was measured by means of a stereomicroscope (Mitutoyo, Mitutoyo Corporation, Kanagama, Japan) at ×40 power. Measurements were taken and averaged to the nearest hundredth of a millimeter by 3 evaluators. Positive or negative values were recorded when the tip was detected beyond or short of the apical constriction, respectively.

Data analysis

For each reading, the error in measurement was calculated as the absolute difference, in millimeters, between the file tip and the apical constriction. Accuracy was determined on stable measurements within ±0.5 mm. Student t test was used to compare the results and a significant difference was determined at a 95% confidence level. The analysis was carried out with SPSS version 15 (SPSS Inc., Chicago, IL).

Results

The mean positions of the file tip relative to the apical constriction, as determined by the Root ZX II, were —0.23 ± 0.39 mm and —0.42 ± 0.45 for groups I and II, respectively, with no statistical difference (P >.05). The accuracy was 90.5% and 83.78% for the “0.5” and “1” display readings, respectively. Group I showed percentage of measurements surpassing the apical foramen to be 5.7% (n = 2), whereas in group II no tooth exhibited file tip beyond the foramen.

Discussion

EALs have been considered valuable additions to the clinical endodontic armamentarium. The results of numerous publications have supported this notion demonstrating that EALs can accurately determine the working length in 75.0% to 96.5% of the root canals with mature apices. This seemingly large discrepancy may not only be a result 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 diameter (apical constriction), and others measured from the major diameter (apical fora men). Instead, the best results were obtained with the latest generation devices, such as Root ZX II, that became the benchmark to which other apex locators are compared. According to the manufacturer, the new device Root ZX II works with the same ratio-method as the original Root ZX. Consequently, the results of this research were compared with previous findings.

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. It has been suggested that EALs operate on the principle of electricity rather than the biological properties of the involved tissues. Therefore, models in which extracted teeth are immersed in media with similar electrical resistance to the periodontal tissue can provide valuable information on their function. The materials most often used are alginate, agar, saline, and gelatin.

In the current study, alginate was used as a medium because of its suitable electroconductive property, simulating the periodontal ligament colloidal consistency. This model has been shown to be an effective tool in evaluating EALs and familiarizes the operator with electronic root canal length measurement because of its high degree of stability, low cost, and the simplicity of achievement and preparation. Some authors also reported that this model allowed testing greater number of canals over a shorter period of time than could have been achieved by clinical means. Furthermore, after alginate setting, the roots embedded within are sufficiently strongly held to resist the force exerted by mechanical instruments. It also allows for hiding the roots and making it possible to objectively measure with minimum bias. Yet, the model revealed a disadvantage, which was its inability to fully simulate in vivo conditions. 

It has been suggested that preflaring root canals before using the Root ZX led to an increased device accuracy. Thus, in the present study the canals were carefully preflared with Gates-Glidden burs. In addition, 1% NaOCl was used because it is widely accepted as an irrigation solution during root canal treatment. Its possible influence on electronic reading has been assessed by various authors who observed no interference with the readings; the use of hypochlorite prompted no deterioration of the alginate model, as well. 

To reduce the potential for operator variability, in the current research only one calibrated operator (M.A.V.) carried out the readings, according to the operator’s manual, inasmuch as experience with apex locators has been considered essential for good and consistent results. Yet, the operator ensured the correct use of the EAL avoiding any complications by technical inadequacies or deficient clinical handling. 

Electronic apex locators have traditionally afforded some latitude of acceptable error in locating the apex. As a result, several studies, including the current one, used an error range of ± 0.5 mm to assess their accuracies. Measurements attained within this tolerance are considered highly accurate. Other studies relied on a more lax clinical range of ± 1.0 mm. One reason cited for accepting a ± 1.0 mm margin of error is the wide range seen in the shape of the apical third. Root canals do not always end with an apical constriction, a well-delineated minor or major apical diameter, or an apical foramen within the base of the cemental cone. The absence of such demarcations makes an error tolerance of ± 1.0 mm deemed clinically acceptable. Nevertheless, whatever the apical limit, the measuring device used should be precise and reliable. The precise means being able to locate the chosen limit, and reliable means giving similar readings when used by one or more operators. 

Different ex vivo methods have been used to investigate the accuracy of the Root ZX. In several studies, the actual root length is measured with a caliper and compared with the electronic reading performed by the EAL, without shaving the apical portion of the root. Considering ± 0.5 mm as an acceptable range, the results of published research showed high accuracy, ranging from 92.0% to 97.5%. The manufacturer of the Root ZX II does not claim that the numbers on the display indicate the distance to minor or major constriction in millimeters; rather, they are arbitrary units indicating if the file is moving closer or farther from the constriction. In fact, the instruction manual states that “the bar indicating the apical constriction flashes on and off, which indicates that the tip of the file is in the vicinity of the apical foramen (an average of 0.2 to 0.3 mm past the apical constriction toward the apex).” Although some of these results could not be precise, the “0.5” meter reading on the display of Root ZX II indicates that the tip of the file is at the apical constriction and not at 0.5 mm from the apical foramen as claimed by some authors. 

The location of apical constriction varies considerably from root to root and its relationship to the CDJ is also variable, as the CDJ is highly irregular. So, regarding identification of the apical constriction, a more precise ex vivo method for achieving the accuracy of an EAL is by shaving the apical portion of the root, along the long axis of the tooth, in a plane that was determined to show the best representation of the minor diameter in relation to the file, as carried out in the present study. Besides, if the apical portion is not shaved, the relationship between the file tip and the constriction cannot be established. The accuracy of Root ZX reported in studies using this methodology ranged from 75.0% to 90.7%, in agreement with the present results.

Nevertheless, the high accuracy of Root ZX reported by many authors, on the electronic measurement of root canal using the “0.5” mark, has been also related with some overestimated working length. El Ayouti et al., in evaluating in vitro the ability of Root ZX to avoid instrumentation beyond the apical foramen in premolars, observed that, in 7% of the sample, the electronic measurements had passed the apical foramen. D’Assunção et al., comparing the capacity of the Root ZX II and Mini Apex Locator to prevent overestimated working length, showed that in 2.56% of the canals the file tip was beyond the foramen. Lucena-Martin et al., testing the in vitro accuracy of 3 EALs in vitro, showed that in 5% of the canals the measurements surpassed the apical foramen. These results are in agreement with the present study, in which the file tip was beyond the apical foramen in 5.7% of the canals when using “0.5” mark on the dial.

This fact must be seriously considered because 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 working length could lead to a poor prognosis. Dunlap et al., using the Root ZX, compared in vivo, the canal length to the actual apical constriction in vital and necrotic cases and found 2 measurements in the necrotic group that were 1.5 mm beyond the apical constriction. Welk et al. compared the accuracy of the Root ZX and Endo Analyzer Model 8005, under clinical conditions, and found 6.2% of overestimated working lengths in the Root ZX group. The same way, Wrbas et al. compared the accuracy of 2 electronic EALs in the same teeth, in vivo, and found that the file tip was beyond the major foramen in 8 cases for the Root ZX.

These findings raise a question of whether the working length should be established at the point where the EAL indicates the constriction, or some distance coronally. In this way, some authors have proposed withdrawal the instrument of 0.5 or 1 mm when using the Root ZX with the display meter setting at “0.5” to ensure that the file tip does not protrude beyond the apical constriction, avoiding root canal overpreparation. Thus, in accordance with the results of the present study, to prevent overestimation of the root canal length, the “1” mark should be used instead of “0.5” on the display of Root ZX II.

Conclusion

The application of the Root ZX II did not result in a precise location of the apical constriction in both reading conditions. Despite this, its precision has proven to be acceptable on both display marks. Within the limitations of the study, the accuracy was 90.5% and 83.78% when using the display marks “0.5” or “1,” respectively. The use of meter reading “1” showed no file tip beyond the foramen, reducing the risk of overestimation of the working length. Further clinical research is necessary to confirm these results.

Authors: Marco Aurélio Versiani, Bianca Palma Santana, Cristiane Melo Caram, Elizeu Álvaro Pascon, Cássio José Alves de Souza, João Carlos Gabrielli Biffi, Minas Gerais

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