Correct length determination of a root canal system can influence the outcome of endodontic treatment. ¹ It is widely accepted that the apical constriction is the point that root canal instrumentation and filling should be terminated, where the pulp ends and the periodontal ligament begins. ² Radiographic and electronic methods are currently used to determine the correct root canal length. Due to the limitations of radiography, ³ , ⁴ the use of electronic devices has increased in recent years. Although apex locators effectively and reproducibly locate the apical foramen, ⁵ , ⁶ some factors such as endodontic solutions which used during preparation, ⁷ , ⁸ , ⁹ apical constriction diameter, ¹⁰ root canal preenlargement, ¹¹ retreatment procedures, ¹² and intracanal medicaments ⁷ can influence the accuracy of readings. Due to the complexity of the root canal system, none of exciting cleaning and shaping methods can completely remove the bacteria. Therefore, intracanal medicaments used in multiple visits are used to reduce the pathogenic species associated with pulp necrosis. ¹³

Calcium hydroxide (Ca[OH] ₂) is an intracanal medicament, which has been widely used, especially in cases with necrotic pulp. Ca(OH) ₂must be removed before obturation because its presence on root canal walls adversely affects the penetration of sealers into the dentinal tubules. ¹⁴ , ¹⁵ Many techniques and irrigants were introduced for Ca(OH) ₂removal. ¹⁶ , ¹⁷ To remove the Ca(OH) ₂from the root canal, instrumentation by master apical file and simultaneously irrigation by different endodontic irrigants are suggested. There are various techniques to remove Ca(OH) ₂from the root canal. This usually accomplished through several irrigant rinses in conjunction with hand or rotary instrumentation or ultrasonics. Most common irrigants have been used included saline, sodium hypochlorite (NaOCl), ethylenediaminetetraacetic acid (EDTA), and combinations thereof. ¹⁸ NaOCl is the most common used irrigant in endodontics, and the most frequently described method to remove Ca(OH) ₂from the root canal is combination use of NaOCl and EDTA. ¹⁹ However, it was shown that instrumentation and irrigation cannot completely clean the root canal walls, ²⁰ and it has been reported that in this method, Ca(OH) ₂residues were remained at the apical portion of the canal. ²¹

Raypex 6 is the last member of Raypex series electronic apex locators (EALs). The clinical performance of Raypex 4 and 5 was found to be successful. ²² , ²³ Moscoso et al. in an in vitro study showed that there were no statistically significant differences between Dentaport ZX and Raypex 6 under in vivo clinical condition. ²⁴

Uzunoglu et al. in an in vitro study showed that the residues of Ca(OH) ₂adversely affected the accuracy of Root ZX. ⁷ To the best of our knowledge, there is no information to evaluate the influence of Ca(OH) ₂residues on Raypex 6 EAL. Thus, the aim of this study was to evaluate and compare the effect of Ca(OH) remnants after irrigation by different irrigants on the accuracy of Raypex 6 and Root ZX EAL.

Eighty mandibular single-rooted premolars were selected for this in vitro study. Calculus and soft-tissue debris were removed from the teeth and then stored in 1% thymol solution for disinfection. To minimize the variables, the crowns of the teeth were decoronated at the level of cementoenamel junction with a diamond disc to standardize the length of roots to 15–17 mm. The root canals patency was checked using a K-file size 10 (Dentsply Maillefer, Switzerland), and the working lengths (WLs) were determined using size 10–20 K-files depending on the apical foramen sizes. A K-file was inserted into the canal and the tip of the file was checked until it became visible through the foramen under microscope (Dunwell Tech, USA) at ×12 magnification. An actual WL (AWL) was measured 0.5 mm short of the apical foramen, and a single operator prepared all root canals. The coronal one-third of the canals were flared with size 2–3 Gates-Glidden burs (Jota AG co., Switzerland) and remaining root canals were shaped using protaper rotary instruments (Dentsply Maillefer, Switzerland) at a speed of 300 rpm up to size F3 (size 30). Between each instrument, copious irrigation by sodium hypochlorite (NaOCl) 5.25% solution (Poura, Iran) with a syringe and a 27-gauge needle 2-mm shorter than WL was done. Then, all the root canals were finally irrigated with 5 ml 17% EDTA (Master-Dent, Dentonics Inc., USA) and 5 ml distilled water. The teeth were dried by paper point (Meta Biomed, South Korea) and filled with injectable Ca(OH) ₂paste (SURE-Paste, Sure Dent Corp., Korea), and the access cavities were temporarily sealed with Cavit (Cavisol, Golchai Co., Iran). Mesiodistal and buccolingual radiographs were taken to confirm complete root canals filling. An alginate model was used for testing apex locators to mimic the clinical situation. Four plastic boxes were filled with alginate (Tropicalgine, Zhermack, Italy) and the teeth were randomly distributed in them. Two boxes were designed for experimental groups according to the apex locator and irrigants (n = 30), and two boxes were prepared for control groups (n = 10). The pool of alginate was wrapped in wet gazes and stored at 37°C and 100% humidity for 7 days. The apex locators used in this study were: Root ZX (J Morita Corp., Tokyo, Japan) and Raypex 6 (VDW, Munich, Germany).

Ca(OH) ₂removal was performed by two different irrigants: normal saline (Darou Pakhsh, Iran) and NaOCl 5.25% followed by EDTA. To avoid bias, during Ca(OH) ₂removal, apical foramens of all the teeth were filed until size 30. The teeth in the positive control group were not filled with Ca(OH) ₂paste, and in the negative control group, the Ca(OH) ₂was not removed from the root canals. Then, WLs were measured by two EALs when the root canals were filled with irrigants such that the apical third of the tooth was fulfilled with the irrigants. The metal lip clip was placed into the alginate, and the silicon stop was adjusted to the nearest available anatomical landmark of the teeth. The measuring file was moved into the canal until the display level the APEX point displayed on the monitor for Root ZX, then operator passed the file over the APEX point and the termination point was the first line in the red area. With the Raypex 6, the termination point was determined and the red bar began flashing according to the manufacturer's instruction. Then, the K-file was fixed as the WL and withdrawn from the root canal, and the distance from the reference point to the file tip was measured, and then, 0.5 mm was subtracted from every measurement to achieve the situation of apical constriction. Each measurement was repeated three times by a static endodontic ruler, and the mean average electronic WL (EWL) was recorded. Differences from AWL were calculated (AWL–EWL). Furthermore, the accuracy of apex locators was calculated as the percentage of acceptable measurements (tolerance limit of ±0.5 and ±1 mm). One-way ANOVA with post hoc paired t-test was used to analyze the data (P < 0.05). Chi-square tests were used to compare percentages (P < 0.05).

The difference between the mean EAL and the AWL was calculated for teeth in the presence of different irrigants in tolerance limit of ±0.5 and ±1 mm. The calculated absolute difference values are reported in Table 1. There was no significant difference between the accuracy of two EALs and irrigants (P > 0.05) Table 1.{Table 1}

The control group with no Ca(OH) ₂removal was significantly different from experimental groups (P < 0.05), and the control group with no Ca(OH) ₂placement was not significantly different from experimental groups (P > 0.05). The calculated apex locators' accuracy in the presence of two irrigants is shown in Table 2.{Table 2}

In this study, the accuracy of two different EALs was evaluated after Ca(OH) ₂removal by different irrigants.

The use of Ca(OH) ₂paste between therapeutic sessions is suggested due to its excellent antimicrobial efficacy. The selected method for Ca(OH) ₂removal was supported by Lambrianidis who suggested instrumentation by master apical file with copious irrigation. ¹⁹ , ²⁵ Previous studies showed that none of the different removal techniques and irrigants can completely clean Ca(OH) ₂from the root canal walls and considerable amounts of medicament remnants were remained, ⁷ , ¹⁸ , ²⁵ and these remnants of Ca(OH) ₂can adversely affect the outcome of treatment. Holland et al. reported that the residual Ca(OH) ₂in dentinal tubules causes the decreasing tubular permeability and endangers the outcome of the treatment. ²⁶

NaOCl is primarily used as an endodontic irrigant and has great efficacy in disinfecting root canals. ²⁷ , ²⁸

Van der Sluis et al. reported that NaOCl is an effective irrigant in removal of Ca(OH) ₂paste, especially in the apical root canal and when used during passive ultrasonic irrigation. ²⁹ Adversely, Salgado et al. showed that the worst results of Ca(OH) ₂removal were in the NaOCl irrigation group. ³⁰ Removal of Ca(OH) ₂from the root canal with copious irrigation of NaOCl and EDTA is the most frequently described method. ¹⁹ Hence, the irrigation with EDTA after NaOCl was used in this study. Although it is likely that EDTA may chelate Ca(OH) ₂remnants and the removal of this paste became easier by following NaOCl irrigation, other studies could not confirm that these combinations of irrigants can remove Ca(OH) ₂totally and still found extensive Ca(OH) ₂remnants. ¹⁹ , ³¹

There are several studies about the influence of different irrigants on the accuracy of EALs. According to the manufacturer, the accuracy of new generation EALs is not adversely affected from irrigation. Some studies showed that there was no significant correlation between the accuracy of EALs and irrigation solution and the irrigation do not affect the accuracy of the electronic readings. ³² , ³³ , ³⁴ Similarly, in this study for each EAL, regardless of the irrigants, the accuracy of readings was not significantly different in both ±0.5 mm and ±1 mm tolerance. Adversely, Kang and Kim reported that NaOCl gave rise the readings to significant differences among tested EALs include Root ZX. Furthermore, they concluded that when saline was used as an irrigant, the mean discrepancies for Root ZX were >0.05 mm. ⁹

Raypex EAL is a widely used EAL and Raypex 6 is a recently developed EAL. Stöber et al., evaluated the clinical performance of Raypex 5, found that it is 75% accurate to ±0.5 mm and 100% to ±1 mm. ³⁵ Furthermore, in an in vivo study, Somma et al. reported that the accuracy of Raypex 5 and Dentaport ZX is not significantly different and both of the EALs have correct measurements. ²²

Some studies which compared different EALs to Root ZX reported that accuracy of the Root ZX is high. ³⁶ , ³⁷ In accordance with the previous studies, ³⁶ , ³⁸ , ³⁹ based on the results of this study, the Root ZX apex locator revealed high success rate in the presence of both irrigants Table 2, and similarly, the success rate of Raypex 6 is high and not significantly different from that of Root ZX.

In this study, an ex vivo alginate model was selected for investigating EALs. This media has similar electrical resistance to the periodontium. The advantages of this ex vivo model were the simplicity, ease of use, and the ability to have strict control over the experimental conditions tested. The disadvantages of laboratory models are the inability to simulate conditions in vivo. ⁵ Ex vivo studies use electroconductive materials such as alginate to simulate the clinical situation. Sometimes, the media can leak through the apical foramen and may cause premature readings. Furthermore, it has been shown that some ex vivo experimental models report greater accuracy for apex locators than clinical conditions. ⁴⁰ Therefore, the results of laboratory studies could not be directly extrapolated to the clinical situation, and the authors of the present study suggest further in vivo studies to better evaluate these EALs after Ca(OH) ₂paste removal.

The present study revealed that no statistical significant differences were observed between Raypex 6 and Root ZX EALs after Ca(OH) ₂paste removal with normal saline and NaOCl irrigation.

Acknowledgment

The authors would like to thank the Vice-Chancellor of the Shiraz University of Medical Science for supporting this research (Grant#13958).

This manuscript is based on the thesis by D. Zaeri. The authors also thank Dr. Vossoughi from the Center for Research Improvement of the School Of Dentistry for the statistical analysis.

Financial support and sponsorship

Nil.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.