Fifth generation (dual frequency ratio type) EALs measure the resistance and capacitance separately, and there can be different combinations of values of capacitance and resistance that provide the same impedance and thus the same foraminal reading. Therefore, this generation of EAL solved the problem of fourth generation EALs, which must be operated in relatively dry or partially dried canals. The effects of various irrigants, such as saline, hydrogen peroxide, sodium hypochlorite solution, and ethylenediaminetetraacetic acid (EDTA) solution, on fifth generation EAL performance have been investigated. Numerous studies indicate that endodontic measurement can be performed in the presence of any conductive fluid, but the type of irrigant solution may affect the accuracy of the EAL. The most tested irrigants are sodium hypochlorite and chlorhexidine solutions. Erdemir et al. showed no significant difference in file tip position between 2.5% NaOCl and 0.2% chlorhexidine gluconate using a Tri Auto ZX [
21]. There were no differences in the accuracy of endometric measurements in the presence of sodium hypochlorite and CHX in the Prasad [
42] and Gomes [
43] studies. Different results were obtained by Özsezer et al. in an in vivo study; they evaluated the performance of Propex after extirpation and in the presence of different irrigation solutions, including 2.5% NaOCl and 0.2% chlorhexidine gluconate. The results of their study showed that among the irrigation solution groups, the chlorhexidine gluconate group was closest to the actual length [
8]. Shin et al. noted that measurements made in the presence of CHX are most consistent [
32]. This finding was confirmed in this study. The distance to the apex was smallest when CHX substances were used rather than when NaOCl was used (Table
2., Table
4). Therefore, the first null hypothesis was rejected. This result could be a product of the different electrical conductivities of the solutions used. Electrical conductivity is the ability of different types of matter to conduct an electric current. The electrical conductivity of a material is defined as the ratio of the current per unit cross-sectional area to the electric field producing the current [
44]. Electrical conductivity is an intrinsic property of a substance that is dependent not on the amount or shape but on the temperature and chemical composition of the substance. WL measurements tended to be slightly shorter in solutions of higher electrical conductivity, such as NaOCl solutions [
37].
The literature review revealed that in accurate EAL measurements, usually 0.1, 0.2, 0.8% [
4,
8,
21,
45] CHX solutions were tested. However, more recent studies have evaluated 2% CHX solution and 2% CHX gel. Most authors agree that the CHX concentration for root canal treatment should be 2% [
46‐
48]. Chlorhexidine is a bis-biguanide that has broad-spectrum antimicrobial activity and is active against both gram-positive and gram-negative microbes. Only 2% CHX has a sufficiently high kill rate against the common endodontic pathogen
E. faecalis. In Dammaschke’s research, 2% CHX-G was as effective as camphorated-and-mentholated chlorophenol (ChKM) against
E. faecalis [
49]. Thus, when choosing a root canal medicament, the established better biocompatibility of CHX compared with that of ChKM should be taken into consideration [
49]. CHX-G, also used as an intracanal medicament in other Dammaschke studies, showed good periapical regeneration, suggesting that the gel may be an alternative to calcium hydroxide root canal dressing [
50]. It was shown that CHX can be retained in root canal dentin at effective antimicrobial concentrations for up to 12 weeks [
30]. Due to the excellent antibacterial properties, especially against
E. faecalis, and the long-lasting bactericidal effect observed in this study, a 2% concentration of chlorhexidine in solution and gel form was used.
Many studies assessed the accuracy of endometric measurements performed with different equipment. In Jung et al. [
4], an in vitro study tested seven different apex locators in the presence of different solutions, including 0.1% chlorhexidine and 5.25% NaOCl. The researchers obtained no statistically significant differences between the tested EALs (Apex Finder 7005, Apit, Bingo-1020, e-Magic Finder, ProPex, Root ZX, and SmarPex.). Oliveira et al. [
51] tested five EALs: the Root ZX II, Raypex 6, Apex ID, Propex II, and Propex Pixi. The researchers revealed no difference in accuracy between the evaluated devices. Gurel et al. [
15] also tested a Raypex 6 and two other new generations of apex locators Raypex 5, iPex and iPex II. The accuracy of the WL measurement was similar for all devices. Khandewal et al. [
14] conducted comparative evaluation of accuracy of Raypex5 and Apex NRG XFR EALs with conventional radiography in ex vivo study. They obtained the same accuracy in determining the WL for both EALs when compared with radiography Saatchi et al. [
52] tested Raypex 5 and Root ZX in the presence of blood and also found no significant differences. Al-Hadlaq [
53] evaluated the accuracy of the Root ZX and two compact apex locators, the Root ZX mini and Mini Apex Locator, in the presence of different endodontic solutions, including 2% CHX and NaOCl at 5.25, 2.625, and 1% concentrations. The function of all three devices was similar and was not affected by the type of solution. The lack of differences in the accuracy of measurements made with two different apex locators was also confirmed in this study. Between the Apexdal and Raypex 5, no statistically significant differences were observed for all intracanal media tested. Thus, the second null hypothesis was confirmed. All devices tested in the different solutions provided a high percentage of correct measurements. Generally, measurement accuracy is 83% [
4], 90% [
54] or higher [
55]. In this study, good results were achieved only when canals were filled with CHX solution or CHX gel. Measurement accuracy was 79.3 and 86.2%, respectively, for the devices. Measurements that were made in canals irrigated with 2% NaOCl were correct only in 53.2 and 48.2% of cases for Raypex 5 and ApexDal EAL, respectively. Some studies differ in establishing the reference point from which measurement accuracy is determined. Some researchers measure the minor diameter (apical constriction), whereas others measure the major diameter or apical foramen [
56]. It is worth noting that EALs are highly accurate in determining the location of the minor constriction, but the mean distance from the file tip to the minor constriction always has a positive value [
56,
57]. This means that EALs mostly overestimate the WL. In this research, the file tip was located in a more apical positive relative to the apical constriction. Such a position of the file tip resulted from the fact that measurements were made until the “apex” was marked by the devices. In this research, correct measurements were taken, and the tip of the file was ±0.5 mm within the apical foramen. For both apex locators, the distance to the apex was smallest when CHX solution and CHX gel were used instead of NaOCl. Ebrahim et al. [
45] examined the effects of 0.5% NaOCl, 2.5% NaOCl and 0.8% chlorhexidine on the accuracy of a Dentaport ZX in enlarged canals. They found that the Dentaport ZX was accurate and not affected by the presence of both NaOCl concentrations when small and large files were used but was accurate in the presence of CHX only when large files were used. In another in vitro study, Jung et al. [
4] tested seven different apex locators in the presence of different solutions, including 0.1% chlorhexidine and 5.25% NaOCl. There were no statistically significant differences between the EALs (Apex Finder 7005, Apit, Bingo-1020, e-Magic Finder, ProPex, Root ZX, and SmarPex.). The same conclusions are presented in this study. Between the ApexDal and Raypex 5, no statistically significant differences were observed for all intracanal media tested. However, the studies by Shin noted that the measurements taken in the presence of CHX were the most consistent [
32]. This means that the CHX solution resulted in the least variability in the performance of the devices. This study confirms this observation. There was also no significant difference between CHX-S and CHX-G, but there were differences when CHX-S or CHX-G were compared with 2% NaOCl. Jenkis et al. [
37] tested a Root ZX in the presence of 2% lignocaine, 5.25% NaOCl, RC Prep, EDTA and 3% H
2O
2. The researchers did not find an influence of the irrigant solution on the performance of the device, but Root ZX worked least precisely in the presence of 5.25% NaOCl. In this study, in addition to 2% NaOCl, different solutions were tested than those in the Jenkis study, but both EALs achieved the poorest results in the presence of NaOCl. Kaufman et al. [
58] tested a Root ZX and Bingo 1020. They found significant differences based on the canal conditions; measurements were closer to the actual length in the presence of EDTA and saline than in dried or Xylol-filled canals. Their results also indicated that electric measurements can be safely performed in the presence of CHX, because these results were similar to those obtained in the presence of NaOCl.