Comparison of accuracy of two electronic apex locators in the presence of various irrigants: An in vitro study

Abstract

Aim:

This study was designed to compare the accuracy of Root ZX and SybronEndo Mini, electronic apex locators (EALs), in the presence of various irrigants.

Materials and Methods:

Sixty extracted, single-rooted human teeth were decoronated and the root canals coronally flared. The actual length (AL) was assessed visually and teeth mounted in the gelatin model. The electronic length (EL) measurements were recorded with both EALs in the presence of 0.9% saline; 1% sodium hypochlorite (NaOCl); 2% chlorhexidine digluconate (CHX), and 17% EDTA solution, at “0.5” reading on display. The differences between the EL and AL were compared.

Results:

The accuracy of EL measurement of Root ZX and Sybron Mini within±0.5 mm of AL was consistently high in the presence of NaOCl and found to be least with EDTA.

Conclusion:

EL measurements were shorter with 1% NaOCl, whereas longer with 2% CHX for both the devices. Sybron Mini was more accurate with 1% NaOCl and 2% CHX than Root ZX.

INTRODUCTION

The determination of an accurate working length is a critical step in endodontic therapy.[1] Establishing the working length at the apical constriction is considered ideal for endodontic treatment.[2] The apical constriction (minor apical diameter) is the narrowest apical portion of the root canal with a variety of morphological variations that make its identification unpredictable.[1,3] The radiographic assessment technique is sensitive in both its exposure and interpretation.[4]

Electronic devices for assessing the root canal length have gained popularity and eliminate many of the problems associated with radiographic measurements.[5] Based on Suzuki's discovery that electrical resistances between the periodontal ligament and oral mucosa registered constant values of 6.5 kΩ, Sunada in 1962 developed the first electronic apex locator (EAL).[6] Since then, different generations of EALs have been developed.[4]

Whilst the simplest devices measure resistance, other devices measure impedance using either high frequency, two frequencies, or multiple frequencies. In addition, some systems use low frequency oscillation and/or a voltage gradient method to detect the canal terminus.[3]

The Root ZX (J. Morita Mfg Corp., Kyoto, Japan), a dual frequency device, based on the “ratio method”, has been investigated extensively as regards its accuracy and its efficacy in the presence of various irrigants.[7,8] SybronEndo Mini Apex Locator (SybronEndo, Sybron Dental, Glendora, CA, USA) is multifrequency based and is also claimed to be accurate in the presence of various intracanal conditions, but has not been investigated much.

Thus, the purpose of this in vitro study was to compare the time tested Root ZX with the newly introduced SybronEndo Mini Apex Locator in the presence of various intracanal irrigants.

MATERIALS AND METHODS

Sixty extracted, straight, single-rooted permanent human teeth with mature apices were selected for this study. Teeth were stored in a 0.2% sodium azide solution until use. Residual soft tissue on the root surface was removed by soaking the teeth in 5% sodium hypochlorite (Nice chemicals, India) for 3 h. The type I canal configuration was confirmed by using digital radiograph (Gendex, Dentsply) in mesiodistal and labiolingual planes. Teeth with resorption, curvatures, open apices, or radiographically invisible canals were excluded.

The teeth were decoronated at the level of cementoenamel junction with a diamond disc to allow access to the root canal and to provide a stable reference for all measurements. The coronal portion of each canal was preflared using sequential Gates Glidden drills #4, #3, and #2 (Mani Inc., Japan), irrigated with saline and pulp extirpated with a barbed broach (Spirocolorinox, Dentsply).

Teeth were numbered 1–60 and the actual length (AL) was determined by introducing a size 10 or 15 k-file (Mani Inc., Japan) into the canal until its tip emerged through the major apical foramen at ×10 magnification under a stereomicroscope (WILD M2Z, Heerbrugg, Switzerland). The long axis of the tooth was placed perpendicular to the line of sight and the tip of the file was positioned tangential to the major apical foramen.[9,10] After carefully adjusting the silicone stopper to the reference point, the file was withdrawn from the root canal, and the distance between the file tip and silicone stopper was measured with a digital caliper (Mitutoyo Co., Japan) to the nearest 0.5 mm; 0.5 mm was subtracted from this length and recorded as AL.

To simulate the periodontium, this study used the in vitro model as designed by Donnelly.[11] A polystyrene specimen bottle (40 ml) was filled with warmed gelatin solution and refrigerated for 2 h to allow gelatin to set. The apical two-third of the root was embedded in gelatin, and the tooth was stabilized to the lid of a container with auto-polymerizing resin as described by Higa et al.[10] The lip electrode was also placed in gelatin through another opening in the lid [Figure 1].

Figure 1.

Experimental set-up used in the study

The irrigants tested were: 0.9% saline (Denis Chem., India), 1% sodium hypochlorite (NaOCl) (Nice chemicals Pvt. Ltd., India), 2% chlorhexidine digluconate (CHX) (Dentochlor, Ammdent, India), and 17% disodium edetate solution (EDTA) (Canalarge, Ammdent, India). The irrigant to be tested was introduced into the canal with a 23-guage needle.

Two EALs tested in this experiment were: Root ZX (J. Morita Mfg Corp., Kyoto, Japan) and SybronEndo Mini Apex Locator (Sybron Endo, Sybron Dental, Glendora, CA, USA). Both EALs were used according to manufacturer's instructions.[12,13] Depending on the size of the canal, #15 or #20 K-file (Mani Inc., Japan), was attached to the file holder and introduced into the canal.

For Root ZX, the meter's 0.5 mm reading was set between the “APEX” and “1” (factory setting) as indicated by a flashing bar and was used for electronic measurements. For each one of the devices, the file was gently inserted into the root canal until the “APEX” signal was displayed. The file was then gently retracted until the display showed a flashing image of the root canal and a flashing bar between APEX and 1 (0.5 reading) for Root ZX and “0.5” blue LED was “on” for Sybron Mini. The silicone stopper on the file was carefully adjusted to a reference point, and the file was withdrawn to measure the distance between the silicone stopper and the file tip to the nearest 0.5 mm. This was recorded as the electronically measured canal length (EL).

The canal length was assessed twice by two operators individually for each of the EAL and individual irrigants, amounting to four readings per tooth. To prevent cross-contamination: (a) fresh gelatin was used for the individual irrigant, and (b) the root canals were irrigated with ethanol and dried with paper points. The results obtained (in millimeters) were recorded. The difference between the median of electronically measured length (EL) and the AL were calculated for each tooth for all irrigants, and AL±0.5 mm was used to evaluate the accuracy of the two EALs.[14] The relative conductivity and the pH of irrigants were also determined using a calibrated conductivity meter (Conductivity meter 304, Systronics Ltd, India) and a digital pH meter (Digital pH meter 335, Systronics Ltd, India).

A paired t-test was employed to statistically analyze the significance of mean difference between EL and AL. One-way ANOVA was employed to assess the difference among various irrigants in their estimations of the canal length. Significance was set at P<0.05. Inter operator agreement was calculated through the kappa co-efficient range from 0.7 to 0.88 which are highly significant for all the four irrigants for both the apex locators. Hence there is high reliability between two operators. The analysis was performed with Statistical and Presentational System Software (SPSS 16.0, SPSS Inc, Chicago, IL).

RESULTS

The mean and standard deviation of actual length (AL) and electronic length (EL) measurements obtained by Root ZX and Sybron mini in the presence of various irrigants are shown in Table 1. The mean differences between the electronic and the AL were 0.002 mm and –0.21 mm for Root ZX and Sybron Mini, respectively. ANOVA showed a highly significant difference between the two EALs at P=0.000. The effect of irrigant on the canal length determinations measured with Root ZX and Sybron Mini is as shown in Table 2.

Table 1.

Mean (SD) of AL and EL measurements with Root ZX and Sybron mini in the presence of various irrigants

Table 2.

Mean difference between the actual length and the electronically determined length by Root ZX and Sybron Mini in the presence of various irrigants in the root canal (mm)

The measurements of Root ZX in the presence of saline (P=0.209) and 1% NaOCl (P=0.443) were closer to the AL and with no significant difference between them, while significant differences were observed with 2% CHX (P=0.001) and 17% EDTA (P=0.005). Sybron Mini, in the presence of saline, 1% NaOCl and 17% EDTA, gave measurements which were shorter than the AL and the results were significant (P=0.000), whereas, 2% CHX was more accurate (P=0.542) and without significance.

Although statistically significant differences existed between the irrigants, the majority of the readings were within the acceptable range of ±0.5 mm for both EALs. The overall accuracy of measurements within ±0.5 mm of AL by Root ZX and Sybron Mini was 88.3% and 87.5%, respectively. The distribution of measurements of Root ZX and Sybron Mini with various irrigants is presented in Table 3.

Table 3.

Distance between AL and EL (AL-EL) with various irrigants for Root ZX and Sybron Mini

The results of conductivity testing of the irrigants ranked from most to least conductive were: 1% NaOCl>17% EDTA>0.9% Saline>2% CHX, and pH from high to low: 1% NaOCl>17% EDTA>0.9% saline>2% CHX.

DISCUSSION

The first-generation EALs were resistance-based and the second-generation EALs were impedance-based apex locators.[1] The main shortcomings of these EALs included poor accuracy in the presence of fluids and pulp tissue, and the need for calibration.[5] The frequency-based third-generation EALs have more powerful microprocessors and are able to process mathematical quotient and algorithm calculations required to give accurate readings.[4]

Root ZX (J. Morita Mfg Corp., Kyoto, Japan) is a third-generation EAL that uses dual frequency and comparative impedance principle is based on the “ratio method” for measuring canal length. This method simultaneously measures the impedance values at two frequencies (8 and 0.4 kHz) and calculates a quotient of impedances. This quotient is expressed as a position of the file in the canal.[15] Root ZX requires no calibration, and can be used when the canal is filled with a strong electrolyte.[1]

Multifrequency based EALs have been developed to further increase the accuracy of EALs.[3] The SybronEndo's Mini Apex Locator uses a sophisticated, multifrequency measurement system to calculate the distance from the tip of the file to the foramen by measuring changes in impedance between two electrodes. According to the manufacturer, an all-digital signal and an 80% shorter cable than other EALs add up to increased signal integrity, easy operation, and consistently reliable measurements. The manufacturer does not specify any other technical characteristics.[13]

The fourth-generation apex locators do not process the impedance information as a mathematical algorithm, but instead they take the resistance and capacitance measurements separately and compare them with a database to determine the distance to the apex of the root canal.[4]

However, there is still a concern as to whether high electroconductive irrigants such as saline, anesthetic solution, and sodium hypochlorite can affect the of these new-generation EALs performance.[5]

The gelatin model was used in this in vitro study to simulate the periodontium, has the advantage of simplicity, ease of the use and the ability to have a strict control over the experimental conditions tested.[16] It is generally accepted that apical constriction is on average located 0.5–1.00 mm short of the apex.[3] This study employed the “0.5” reading (apical constriction) on the display of Root ZX.[12,14] In addition, the “0.5” reading on the display of Sybron Mini was suggested as apical constriction and used as an apical endpoint for all the length measurements. The manufacturer of Sybron Mini does not correlate any histologic landmark with the readings.[13]

To evaluate the accuracy of EALs, the ±0.5 mm range from AL was chosen, which is considered clinically acceptable and highly accurate.[14]

In vivo studies have shown the accuracy of Root ZX to be varying from 82.3% to 96.2% within ±0.5 mm.[14,17] The result of our study for Root ZX is in general agreement with a previously reported in vitro study.[7] The overall accuracy of Sybron Mini in this study with various irrigants was 87.5%; however, with NaOCl the accuracy was 93.3% comparable to a previous study of 97.5% accuracy with NaOCl as irrigant.[18] The difference in electroconductivity of various irrigants could be the reason for lowered overall accuracy of Sybron Mini. The literature review revealed that there are no studies evaluating the accuracy of Sybron Mini in the presence of various irrigants.

The use of irrigating solutions is an important aspect of endodontic treatment. The irrigants investigated were: 1% NaOCl, with tissue solvent and antibacterial activity; 2% CHX with antibacterial activity even against Enterococcus faecalis; 17% EDTA, a chelator which facilitates canal preparation and removes the smear layer;[19] and normal saline with only flushing action served as a control. 1% NaOCl was used in this study because in vitro studies found that, 1% NaOCl dissolved pulp tissue entirely in the course of an endodontic treatment session[19] and studies indicate that the accuracy of EALs is not significantly influenced by different concentrations of NaOCl.[20]

In the presence of saline, the accuracy of Root ZX and Sybron Mini within ±0.5 mm was 95%, which was in agreement with a previous study,[21] and 88.3%, respectively.

Wrbas et al. in an in vivo study with 1% NaOCl found the accuracy of Root ZX within ±0.5 mm to be 75%,[22] while Weiger et al. in an in vitro study found this to be 95.7% within ±1 mm.[23] de Camargo et al. found the accuracy of Sybron Mini with 1% NaOCl to be 97.5% (–1 ± 0.5 mm from total length, meter reading “1”),[18] while D’Assunção et al. with 2.5% NaOCl found this to be 100% within ±0.5 mm (meter reading “Apex”).[24] The results of our study with 1% NaOCl with Root ZX and Sybron Mini within ±0.5 mm were 90% and 93.3%, respectively.

With 2% CHX in the canals the accuracy of Root ZX and Sybron Mini was 86.7% and 93.3%, respectively, and with 17% EDTA solution this was 81.6% and 75%, respectively. Jenkins et al. found the accuracy of Root ZX was not influenced by 0.12% CHX or 17% EDTA.[7] Kaufman et al. found measurements with 17% EDTA with Root ZX were similar to dry canals or filled with 3% NaOCl or normal saline.[8] There were no studies to compare the accuracy of Sybron Mini with 0.9% saline, 2% CHX, and 17% EDTA.

The 1% NaOCl was most electroconductive (66 mS) and had a highly alkaline pH (11.72), followed by 17% EDTA (40 mS, pH 7.01) and accordingly the measurements with these solutions were short of AL, with both EALs. The 2% CHX had poor electroconductivity (1 mS, pH 6.5) and measurements were longer with both Root ZX (P=0.001) and Sybron Mini (P=0.542). The electroconductivity findings are in agreement with the findings of a previous study.[25]

The better performance of Sybron Mini with 1% NaOCl and 2% CHX could possibly be due to its multifrequency mechanism and shorter transmission line.

The major limitation of this in vitro study is the length measurements recorded to the nearest 0.5 mm, which is from a statistical point of view gross; since an error tolerance of ±0.5 mm was used to assess the accuracy of the EALs. Better statistical results could have been achieved if the measurements were made to the nearest 0.01 mm. The authors have used 0.5 mm accuracy for the length measurements since this was easy to transfer and reproduce clinically.

The results obtained in this in vitro study cannot be applied to the clinical situation, but can provide an objective assessment of a number of variables that are not practical to test clinically. At best the EALs should be used as an adjunct, and not as the only method to determine the canal length in endodontic therapy. Clinicians should exercise caution when interpreting the canal measurements using EALs with different irrigants.

CONCLUSION

Under the conditions of this in vitro study,

• Root ZX was more accurate compared to SybronEndo Mini Apex Locator.

• With 1% NaOCl as irrigant, there was a tendency toward shorter measurement, whereas longer measurements were recorded with 2% CHX for both the devices.

• SybronEndo Mini was more accurate with 1% NaOCl and 2% CHX than Root ZX.

Further, in vivo studies with different irrigants are needed to better evaluate the accuracy of SybronEndo Mini Apex Locator.