The effects of file size, sodium hypochlorite and blood on the accuracy of Root ZX apex locator in enlarged root canals: an in vitro study

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1 SCIENTIFIC ARTICLE Australian Dental Journal 2006;51:(2): The effects of file size, sodium hypochlorite and blood on the accuracy of Root ZX apex locator in enlarged root canals: an in vitro study AK Ebrahim,* T Yoshioka, C Kobayashi, H Suda Abstract Background: The initial electronic apex locator (EAL) length measurement is generally established with a small-sized file. It is not known whether file size would be interfering with the reading accuracy of the EAL. This study aimed to evaluate the effect of file size on the accuracy of Root ZX apex locator using an agar model when sodium hypochlorite solution or blood was present during electronic measurements in enlarged root canals. Methods: A total of 36 extracted lower premolars were used. In stage 1, the canals were instrumented using size K-files with a size 40 K-file as the master apical file (MAF). The teeth were then divided randomly into two groups of 18 teeth each. In group A, the teeth were mounted in one per cent agar and irrigated with six per cent sodium hypochlorite solution (NaOCl), while in group B the teeth were mounted in agar and irrigated with human blood. In stage 2, the canals were enlarged using a size 60 K-file as the MAF. In stages 1 and 2, the apical portions of the canals were instrumented using the step-back sequence (up to a size 80 K-file). In stage 3, the canals were enlarged using a size 80 K-file as the MAF. In each stage, the length was measured with a Root ZX until the meter value reached APEX using small and large size files. Results: Three-way ANOVA and Bonferroni test showed that file size, stage of preparation and type of irrigant all had a significant influence on the measurement error (P<0.0001), with all the interactions between these three factors being significant (P<0.0001). *Postgraduate Endodontic Student, Pulp Biology and Endodontics, Department of Restorative Sciences, Graduate School, Tokyo Medical and Dental Assistant Professor, Pulp Biology and Endodontics, Department of Associate Professor, Pulp Biology and Endodontics, Department of Professor, Pulp Biology and Endodontics, Department of Conclusions: As the diameter of the root canal increased, the measured length with the smaller size files became shorter. A file of a size close to the prepared canal diameter should be used for root length measurement in the presence of blood. In the presence of NaOCl, the Root ZX was highly accurate even when the file was much smaller than the diameter of the canal. The agar model was effective and suitable for testing EALs in vitro. Key words: Agar, blood, electronic apex locator, file size, root length determination, root canal preparation, sodium hypochlorite. Abbreviations and acronyms: EAL = electronic apex locator; MAF = master apical file; NaOCl = sodium hypochlorite solution. (Accepted for publication 7 July 2005.) INTRODUCTION Successful root canal treatment is partly dependent upon the correct assessment of the working length. 1,2 It is imperative that this procedure be confined to the canal in order to prevent irritation of the periapical tissues and possible overextension of the root filling. 3 Anatomically, the canal is believed to terminate at the apical constriction, or minor foramen. 4 Therefore, locating the exact terminus of the canal at the apical constriction is an important clinical step. 5 Electronic apex locators (EALs) have been used clinically for more than 40 years as an aid to determine the file position in the canal. These devices, when attached to a file, are able to detect the point at which the file leaves the tooth and enters the periodontium. Using radiography followed by subsequent tooth extraction and sectioning, Stein and Corcoran found that the radiographically-established working length did not actually coincide with the true apical vertex. 6 EALs obviate this problem because their readings are not related to the apical vertex but rather to the apical foramen. Sunada demonstrated that the electrical resistance between the periodontal ligament and oral mucosa had a constant value that could be measured. 7 Australian Dental Journal 2006;51:2. 153

2 However, this value was influenced by electrolytes in the canal during measurement. Thus, a frequencydependent apex locator has been introduced. 8 Finally, Kobayashi and Suda developed an apex locater (Root ZX, J. Morita Co., Tokyo, Japan), 9 which simultaneously calculates the ratio of two impedances in the same canal using two different electric current frequencies to determine the canal length, even in the presence of electrolytes or vital pulp tissue in the root canal. As shown in in vitro and clinical evaluations, 13 EALs give accurate readings in about per cent of canals. The measurements appear to be less accurate when the apical foramen is immature or otherwise large. 11,12 Clinically, the initial EAL length measurement is generally established with a small-sized instrument that can negotiate the canal to its terminus. However, it has not been clarified whether the accuracy of the instrument would be affected by the use of a small-sized instrument in enlarged canals with irrigants. This question may be particularly relevant to situations where the working length is verified by EAL after the completion of canal preparation. The aims of this in vitro study were: (i) to evaluate the accuracy of Root ZX apex locator measurements in enlarged root canals with small size files and files that match the actual canal diameter; and (ii) to observe effects of the agar model when sodium hypochlorite solution (NaOCl) or human blood was present in the canal during electronic measurements. MATERIALS AND METHODS A total of 36 extracted, straight, single-rooted human lower premolars with complete root formation and stored in distilled water containing 10 per cent formalin were used. No information was available regarding the reasons for their extraction. Dental digital X-ray images were taken in both buccolingual and mesiodistal directions to evaluate the root canal anatomy. The crowns of the teeth were removed with a low speed diamond saw (ISOMET, Buehler; Model # , Lake Bluff, Illinois, USA) to standardize the root length to 14.50mm and to allow access to the root canal and establish a level surface to serve as a stable and unequivocal reference for all measurements. The actual canal length was determined by introducing a size 10 K-file (Zipperer, Munich, Germany) in the canal until the tip of the file became visible at the major apical foramen under a digital microscope (VH-8000; Keyence, Osaka, Japan) at x10 magnification. A rubber stop was then carefully adjusted to the reference level and the distance between the rubber stop and the file tip was measured to the nearest 0.01mm with a digital caliper (Sankin, Mitutoyo Co., Kanagawa, Japan) and recorded. All canals were checked with a size 10 or 15 K-file to ensure the initial canal size before canal enlargement. Gates Glidden drills (size 1 4, Mani, Tochigi, Japan) were then used to prepare the coronal Fig 1. Experimental setup. (a) K-file. (b) Self-curing resin. (c) Bottle cap. (d) Specimen bottle. (e) Tooth. (f) Agar. (g) Stainless steel screw. portion of the canal, while the middle and apical portions of the canal were prepared using size K-files with six per cent NaOCl for irrigation. All canals were prepared to the APEX reading length. The canal preparation was done in three stages. In stage 1, a size 40 K-file was used as the master apical file (MAF) and it was confirmed that the larger size files (>40 K-file) did not reach the apex. The apical portion of the canal was then instrumented using the step-back sequence by decreasing the working length of larger files by 0.5mm. The canal was irrigated with 2ml of NaOCl using an endodontic syringe with a 27-gauge needle (Nissho, Osaka, Japan) in an up-down motion. The teeth were then randomly divided into two groups of 18 teeth each. Each tooth was fixed to the lid of a polystyrene specimen bottle (20ml, Iuchi, Osaka, Japan) with self-curing resin. A stainless steel rod, screwed into the body of the specimen bottle, was used as a neutral electrode. The specimen bottles were then filled with one per cent concentration of heated agar (Bacto-Agar; Difco Laboratories, Detroit, Michigan, USA). The caps were immediately placed over the specimen bottles and the model assemblies were refrigerated for two hours to allow the agar to set as has been described previously. 10,12 Before electronic canal measurements, the canals were irrigated with NaOCl in group A, while in group B, the canals were filled with human blood containing EDTA anticoagulant. Each tooth and an apex locator, Root ZX were subsequently connected together in the experimental set-up as shown in Fig 1. Size K-files attached to the file holder were inserted into the root canal until the meter value reached APEX on the Root ZX. A rubber stop was then carefully adjusted to the reference level and the distance between the rubber stop and the file tip was measured with a caliper. In stage 2, the teeth were removed from the specimen bottles and the canals were enlarged using a size 60 K-file as the MAF. The apical portion of the canal was 154 Australian Dental Journal 2006;51:2.

3 Table 1. Mean and standard deviation of actual length and initial canal length measurements (in millimetres) obtained before canal enlargement for both groups A and B Group A Group B AL IL* AL IL* 14.48± ± ± ±0.054 AL: actual length, measured with a size 10 K-file. IL: initial length, measured with a size 10 K-file. *Measured with Root ZX. Mean error (mm) then instrumented using the step-back sequence by decreasing the length of the larger files by 0.5mm. New agar was placed into the specimen bottles and the caps were immediately placed over the specimen bottles. The canals were then irrigated with NaOCl in group A or filled with blood in group B. The tooth and the Root ZX were then connected as described above and the canal length was measured using size K-files. In stage 3, the teeth were removed again from the specimen bottles and the canals were prepared using a size 80 K-file as the MAF. New agar was then placed into the specimen bottles and the caps were immediately placed over the specimen bottles. The canals were irrigated again with NaOCl in group A or with blood in group B. As described in stages 1 and 2, the tooth and the Root ZX were connected and the canal length was measured using size K-files. At each stage, the electronic measurement was taken three times for each file and the average value was calculated. For each average reading, the error in measurement was calculated as the absolute difference between the electronically measured canal lengths and the actual canal lengths. Three-way ANOVA was conducted to investigate the influence of file size, stage of preparation and the type of irrigant on the measurement error. Multiple comparisons were performed with Bonferroni test. The analyses were carried out with SPSS version 11.5 (SPSS Inc., Chicago, Illinois, USA). To compensate for the influence of differences in the actual canal lengths on the measurement errors, the actual canal length was set as a covariant in the statistical analysis. RESULTS Table 1 shows the mean and standard deviation of actual length and initial canal length measurements obtained before canal enlargement for both groups A and B. Three-way ANOVA and Bonferroni test showed that file size, stage of preparation and type of irrigant all had a significant influence on the measurement error (P<0.0001) with all the interactions between these three factors being significant (P<0.0001) (Figs 2, 3 and Table 2). At stage 3, the measurement error showed the largest absolute difference in both groups A (0.19mm) and B (1.11mm) when a size 10 K-file was used. At all stages in both groups A and B, the measurement error was less than 0.03mm when the MAFs were used. Fig 2. Mean error and standard deviation of absolute differences from actual canal lengths for group A. (Statistical analyses are described in Table 2.) Mean error (mm) Fig 3. Mean error and standard deviation of absolute differences from actual canal lengths for group B. (Statistical analyses are described in Table 2.) DISCUSSION This study aimed to evaluate the effect of file size on the accuracy of Root ZX apex locator and to observe effects of the agar model when NaOCl and blood were used in the canal during electronic measurements in enlarged root canals. One per cent concentration of agar was selected as a medium to simulate the normal periodontium. This agar model has been shown to be an effective tool for evaluating EALs and familiarizing the operator with electronic root canal length measurements. 10,12 In group B, the canals were filled with human blood to simulate bleeding in the root canal. EALs are frequently used with a small size endodontic file. However, the effect of file size relative to canal diameter on the measurement has not been clarified. In this study, the canals were enlarged in three stages and the lengths were measured using small size files and those matching the canal diameter to determine whether any discrepancy existed in the presence of NaOCl or blood. In the present study, the apical portion of the canal was enlarged and the apical constriction was destroyed, although the conical shape of the canal was still maintained. Group A showed statistically significant better scores than group B. In the presence of NaOCl, the Root ZX was accurate and the length measurements obtained with small and large size files were comparable. The results of group A Australian Dental Journal 2006;51:2. 155

4 Table 2. The file distances in millimetres (mean±standard deviation) from the reference point to the tip of the file in groups A and B (stages 1, 2 and 3) Stage 1 Stage 2 Stage 3 File size Group A Group B Group A Group B Group A Group B # ± 0.03* ± 0.06* ± 0.02* ± 0.11* ± 0.02* ± 0.02* # ± 0.02* ± 0.07* ± 0.03* ± 0.07* ± 0.01* ± 0.12* # ± 0.02* ± 0.07* ± 0.02* ± 0.09* ± 0.02* ± 0.06* # ± 0.02* ± 0.06* ± 0.02* ± 0.12* ± 0.02* ± 0.08* # ± ± ± 0.01* ± 0.09* ± 0.02* ± 0.09* # ± ± ± 0.01* ± 0.08* ± 0.02* ± 0.06* # ± ± ± 0.01* ± 0.07* ± 0.01* ± 0.04* # ± 0.02* ± 0.05* ± 0.01* ± 0.04* # ± 0.01* ± 0.03* ± 0.02* ± 0.08* # ± ± ± ± 0.03* # ± ± ± ± 0.02* # ± ± 0.01 # ± ± 0.02 *Statistically significant (P<0.0001) compared to actual length. confirmed those of Nguyen et al., who found that the Root ZX was accurate even when the file was much smaller than the diameter of the canal. 14 Many studies have used a ±0.5mm error range to assess the accuracy of the EAL. 15,16 Measurements attained within this tolerance are considered highly accurate. Other studies rely on a more lax clinical range of ±1.0mm to the foramen. 17 One reason cited for accepting a ±1.0mm margin of error is the wide range seen in the shape of the apical zone. 18 The results obtained in group B with the smaller size files may not be clinically acceptable because the measurement error showed the largest absolute difference value (1.11mm) when a size 10 K-file was used. Blood may affect some of the variables in electronic root length determination. In group A, the canal was irrigated with NaOCl, which is highly electro-conductive and infiltrates into dentinal tubules, 19,20 resulting in reduction of electrical impedance of the root canal wall. Electro-conductive solutions may allow better electrical contact with the apical tissues. 20 The measured canal length is adversely affected by different circumstances such as the size of the file, 21 and the diameter of the apical foramen. 22 McDonald recommended the use of files with sizes comparable with the root canal diameter, 23 claiming that this would result in more accurate readings. Huang demonstrated that the moisture content of the root canal was another factor influencing the accuracy of electronic root canal measuring, 24 although when the foramen was sufficiently small, it did not disturb the accuracy of electronic root canal measurement. The present study and other previous studies appear to indicate that, even in fully controlled in vitro study conditions, 11,12 there is some inconsistency in the EAL measurements. Because of this potential inconsistency, EALs should not be used to replace the routine radiographic confirmation of the canal length in endodontic therapy. CONCLUSIONS As the diameter of the root canal increased, the measured length with the smaller size files became shorter. This suggests that the size of the root canal diameter should be estimated and a snug-fitting file should be chosen for root canal length measurement in the presence of blood, and possibly serum or pus. In the presence of NaOCl, the Root ZX was highly accurate even when the file was much smaller than the diameter of the canal. The agar model was found to be effective and more suitable for testing EALs in vitro. Further clinical studies are needed to evaluate EALs. ACKNOWLEDGEMENT We express special thanks to Dr. Pallegama Ranjith for his kind advice in the statistical part of this research. REFERENCES 1. Seltzer S, Bender IB, Turkenkopf S. Factors affecting successful repair after root canal therapy. J Am Dent Assoc 1963;67: Bramante CM, Berbert A. A critical evaluation of some methods of determining tooth length. 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5 13. McDonald NJ, Hovland EJ. An evaluation of the Apex Locator Endocator. J Endod 1990;16: Nguyen HQ, Kaufman AY, Komorowski RC, Friedman S. Electronic length measurement using small and large files in enlarged canals. Int Endod J 1996;29: Dunlap CA, Remeikis NA, BeGole EA, Rauschenberger CR. An in vivo evaluation of an electronic apex locator that uses the ratio method in vital and necrotic canals. J Endod 1998;24: Pagavino G, Pace R, Baccetti T. A SEM study of in vivo accuracy of the Root ZX electronic apex locator. J Endod 1998;24: Keller ME, Brown CE, Newton CW. A clinical evaluation of the Endocater an electronic apex locator. J Endod 1991;17: Shabahang S, Goon WW, Gluskin AH. An in vivo evaluation of Root ZX electronic apex locator. J Endod 1996;22: Yamashita K. Variable factors in the impedance method of determining the working length of a root canal. Jpn J Conserv Dent 1981;24: Pilot TF, Pitts DL. Determination of impedance changes at varying frequencies in relation to root canal file position and irrigant. J Endod 1997;23: Waki H. A study of the electronic method for measuring the root canal length AC impedance of reamer tip in electrolyte. Jpn J Conserv Dent 1981;24: Baker GJ, Lankelma P, Wesselink PR, Thoden van Velzen SK. Electronic determination of root canal length. J Endod 1980;6: McDonald NJ. The electronic determination of working length. Dent Clin North Am 1992;36: Huang L. An experimental study of the principle of electronic root canal measurement. J Endod 1987;13: Address for correspondence/reprints: Dr Aqeel Khalil Ebrahim Pulp Biology and Endodontics Department of Restorative Sciences Graduate School Tokyo Medical and Dental University Yushima, Bunkyo-ku Tokyo Japan aqeel_endo@hotmail.com Australian Dental Journal 2006;51:2. 157