The review was reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for ScRs (PRISMA-ScR) Checklist [Supplementary Table 1]. A review protocol was created and registered on the Open Science Framework (OSF) Registries (osf.io/9sjdg) at https://doi.org/10.17605/OSF.IO/VCEH4.

A review question was developed using the population, concept, and context (PCC) framework. This PCC framework was developed to review the factors influencing the accuracy of EALs. What factors can affect the accuracy of EALs (concept) when determining the root canal terminus of human permanent teeth (population) in experimental studies (context)?

Inclusion criteria: Experimental studies that investigated the factors affecting the accuracy of EALs and studies that used multiple-frequency EALs in permanent teeth. Exclusion criteria: Studies in which length determination was done during rotary instrumentation, studies in which the reference length was determined by observing the file through the apex without any magnification, studies that only compared the accuracy of different EALs without investigating an influential factor, and studies that compared the accuracy of different radiographic modalities with EALs. Studies meeting the inclusion criteria were marked as “Eligible” and subjected to data extraction.

Three online databases (PubMed/Medline, Scopus, and Web of Science) were searched in April 2023 with a restriction to the English language. Two reviewers conducted the search using the following six keywords: “Apex finder,” “Apex locator,” “Electronic apex locator,” “Electronic root canal length measurement device,” “Electronic foramen locator,” and “Electronic working length measurement devices.” The search strategy for each database is available in Table 1. Forward and backward reference searches were conducted. Gray literature was searched in the first 100 hits of Google Scholar and ProQuest Dissertation and Theses. The search results were exported to EndNote, and duplicate publications were identified and removed.

The articles were independently reviewed based on their titles and abstracts by two reviewers. In case of a disagreement, a final consensus was reached after a discussion with a third reviewer. The full texts of the possibly relevant studies were accessed. Furthermore, two reviewers independently extracted relevant data using a standard data collection form. The extracted information included the name of article, the first author’s name, the year of publication, the influencing factors, and the conclusion. Any disagreements were resolved through discussion with a third reviewer.

Instead of using the more common “generation” classification, which lacks scientific basis and is more appropriate for marketing concerns, it is suggested to classify EALs based on their electrical working mechanism. Early EALs like Root Canal Meter and Dentometer (Dahlin Electromedicine, Copenhagen, Denmark) used electrical resistance to estimate the position of the MAF. Once the electrical resistance between the file tip and the oral mucosa reached 6.5 kΩ, the device announced the point as MAF.^[14] The accuracy of these devices decreased when the root canal was not dry, and the patients felt a “shock sensation” because of the direct electrical current used in these devices.^[15]

Later EALs started using the electrical impedance of the root canal system to determine the MAF. Endocater (Yamaura Seisokushu, Tokyo, Japan), Endo Analyzer (Analytic/Endo, Orange, CA, USA), and Sono-Explorer (Hayashi Dental Supply, Tokyo, Japan) are the early examples^[14] that used a single frequency alternating electrical current, had to be calibrated for each tooth, and were also inaccurate when electrolytes such as sodium hypochlorite (NaOCl) were present in the root canal.^[16]

Later devices used multiple alternating electrical current frequencies, either simultaneously or to compensate for the presence of different contents in the root canal. Some of them use the “ratio” of the impedance at these frequencies to determine the position of MAF.^[17] RootZX (J. Morita Corp., Tokyo, Japan) is an example that calculates the ratio using 8-kHz and 400-Hz alternating electrical currents “simultaneously.” This ratio is 0.67 at the MiAF (or AC) and nearly 1 at the MAF.^[14] Some devices (Raypex 4 and 5 [VDW, Munich, Germany]) use two or more frequencies “in order” or use the “difference” of the impedance values at two frequencies instead of the “ratio.”^[14] Recently, neural networks, which are computer algorithms that try to mimic the human brain’s data processing mechanism, have been used for WL estimation, and they may be promising for better accuracy in different root canal conditions.^[18]

The process through which EALs use the impedance values and locate MAF can vary significantly from one device to another, affecting the inherent and clinical accuracy of EALs.^[13]

Due to limited information from manufacturers about the working mechanism of some EALs, classification can be challenging. In general, most modern EALs deliver clinically acceptable results in ideal clinical scenarios.^[19-24] Nonetheless, one EAL may outperform another in more complex clinical situations, despite being from the same “generation.” Therefore, it is recommended that manufacturers disclose the exact electrical working mechanism and clinicians have a basic understanding of the electrical concepts used in EAL to gain better insight into its behavior in various clinical situations.^[13]

In 2017, Oliveira et al. compared four methods for the application of EALs: (1) insertion of the file to “1.0” reading; (2) insertion to the “apex” reading and then withdrawing to “1.0;” (3) insertion to the “apex” reading; and (4) insertion to beyond the “apex” reading and then withdrawing to the “apex.” The third method provided the best accuracy.^[25] EALs’ ability to locate the MAF with the best accuracy was also mentioned in another study.^[26] However, the higher accuracy of method 3 (insertion of the file to the “apex” reading) than method 4 (insertion to beyond the “apex” reading and then withdrawing to the “apex.” reading) was attributed to the fact that the file size chosen for measurements in this study was fitted to the canal size. Taking a file beyond the MAF enlarges the MAF, and the file is no longer as fit as before in the MAF, affecting the electrical characteristics of the circuit formed by the file, the tooth, and the PDL. Hence, it was suggested that there is no benefit to taking the file beyond the MAF and retracting it for the measurement.^[25] The accuracy of the 0.5 reading with passing the MAF, then returning to the 0.5 without passing the foramen, and only reaching the 0.5 point, was also investigated, and both were equally accurate.^[27]

Two studies have evaluated the influence of root canal curvature on the accuracy of EALs.^[7,28] An investigation showed that root canal curvature (9°–58°) did not affect the accuracy of EALs.^[7] However, another study found that Root ZX EAL was less accurate when measuring canals with >20° of curvature.^[28] The former study used the “0.5” reading of the RootZX and compared it with the actual length of the canal minus 0.5 mm, whereas the latter used the “0.0” reading and compared it to the position of the MAF. It was suggested that the effect of root canal curvature on file contact with dentinal walls might explain the subtle differences observed in the accuracy of EALs. Therefore, it is advisable to reconfirm the WL in curved canals during the preparation steps.

Saatchi et al., in 2015, concluded that RootZX is more accurate in shorter teeth, and there is a tendency toward underestimation of length in longer teeth.^[29]

In 2011, Mancini et al. demonstrated that EALs were more accurate in premolars than in molars and anterior teeth. This difference may be attributed to the wider characteristics of the MAF in molar and anterior teeth, as well as the lateral positioning of MAF on the root surface in those teeth.^[30]

El Hachem et al., in 2019, showed that the proximity of the maxillary sinus to a root end can also affect the length estimation accuracy. In a cone-beam computed tomography, RootZX was found to overestimate the length of palatal roots in contact with the sinus. As an explanation, it is suggested that proximity to the sinus may change the impedance characteristics of the root canal system.^[31]

In teeth with lateral MAF anatomy, the accuracy of the “Apex” reading on the RootZX was affected,^[32] and more overextended readings were observed. However, the accuracy of the “0.5” reading was not affected.^[32,33] The presence of lateral canals also reduced the accuracy of RootZX but not to a clinically significant level.^[34]

The accuracy of EALs decreases with wider MAF sizes.^[35-37] However, EALs are still “clinically” useful in larger sizes despite the statistically reduced accuracy.^[38,39] The accuracy of RootZX with MAF sizes below 0.6 mm was independent of the file size; at 0.7–0.8 mm, the accuracy improved when the file matched the MAF size closely, and in MAF sizes >0.9 mm, the accuracy reduced regardless of the file size.^[40] Propex Pixi (Dentsply Maillefer, Ballaigues, Switzerland) was more accurate in MAF sizes <0.6 mm compared to larger sizes, independent of the file size.^[41] In an investigation, the AC area of root canals was gradually widened to 1.02 mm using a sequence of hand files to assess the ability of RootZX to detect the narrowest area of the canal. No significant difference in the measured length was noted during the enlargement up to 0.6 mm. However, a significant difference was observed in the accuracy of the EAL in a 1-mm wide AC. These findings show that the accuracy of RootZX is affected by the AC size and also the size and fit of the file in the apical area.^[42] The accuracy of RootZX with the MAF sizes below 0.6 mm was independent of the file size; with MAF sizes of 0.7–0.8 mm, the accuracy improved when the file matched the MAF size closely, and in MAF sizes >0.9 mm, the accuracy decreased regardless of the file size.^[40] Moreover, the accuracy of RootZX (J. Morita Corp., Tokyo, Japan) started to decrease when the file size reached 0.7 mm, and for RootZX MINI (J. Morita Corp., Tokyo, Japan), this occurred at 0.55 mm, which was attributed to thinner root canal walls in canals with larger diameters, which can affect the measured impedance.^[43]

iPex (NSK, Tochigi, Japan) and Propex II showed decreased accuracy as the foramen diameter increased from 0.27 mm to 0.47 mm and 0.72 mm.^[35] In addition, the accuracy of RootZX and Raypex 6 (VDW, Munich, Germany) decreased when the MAF size was >0.57 mm.^[36]

Propex Pixi was more accurate in MAF sizes <0.6 mm, regardless of the file size.^[41] This factor should be considered carefully when the canal is filled with blood rather than NaOCl using RootZX.^[44] In addition to the size of the MAF, the readings get more accurate as the file fits more tightly in the apical regions.^[45-48]

The fit of the file in the apical regions may affect the accuracy of EALs;^[33,34] selecting a file size that closely matches the apical diameter of the canal may improve the accuracy.

Based on these findings, the results of EALs should be interpreted cautiously in wide MAF sizes, but with the correct fit of the measuring file, readings can be clinically acceptable.

Ten studies have shown that the alloy of the file does not affect the accuracy of EALs.^[45-54] However, one study found that RootZX was more accurate with NiTi files, whereas RootZX MINI was more accurate with stainless steel files.^[55] In contrast, another study found that RootZX was more accurate with SS files; the lower accuracy with NiTi files was attributed to the higher flexibility of NiTi instruments or their electrical characteristics.^[56] Elements Diagnostic Unit (Sybron Endo, Sybron Dental, Anaheim, CA, USA) and ProPex Pixi were unaffected by the file alloy used.^[55] Moreover, the Self-adjusting File (ReDent Nova, Raanana, Israel) has also produced accurate results for electronic length estimation in canals with wide apical diameters.^[57]

Lack of foraminal patency reduced the accuracy of Raypex 6 and RootZX, which was more pronounced in the latter.^[58] Apex Finder (Endo Analyzer 8001; Analytic Technology, Redmond, WA, USA) was also more stable than RootZX when the canal was not patent.^[59] Apex ID was the most accurate in the absence of foraminal patency, followed by Propex II (Dentsply Maillefer, Ballaigues, Switzerland) and RootZX.^[60] RootZX and D10 (Parkell Electronic Division, NY, USA) were more accurate in wide AFs than Apex NRG (Kibbutz Afikim, Israel) and Apit 7 (Osada, Tokyo, Japan).^[61] This might be due to their different operating mechanisms. It has been claimed that inconsistent readings can be associated with the lack of foraminal patency.^[62]

Nine studies have found that preflaring can increase the accuracy and consistency of EALs.^[52,63-70] However, no significant difference was observed with iPex^[71] and RootZX,^[72] irrespective of whether the canal was pre-enlarged or not. Preflaring was found to have a more significant effect on the predictability of the readings compared to using larger file sizes for length determination.^[63] It appears that EALs are accurate irrespective of whether the canal is preflared or not. Preflaring of canals before electronic length measurement is advocated by most studies.

Apical periodontitis might cause root resorption, which can change the root canal morphology, which EALs rely on to determine the apical foramen.^[73] Most evidence shows that apical periodontitis does not affect the accuracy of EALs;^[8,74-78] also, most studies have found EALs to be accurate in apically resorbed teeth.^[79-82] However, lower accuracy of length determination in teeth with apical periodontitis has also been noted.^[83,84] A study that used RootZX on freshly extracted human teeth with attached apical lesions showed a negative effect, causing longer readings. This tendency to more frequently locate a point beyond the AC in teeth with apical periodontitis was also reported in another study.^[8] The altered apical morphology of the root secondary to the resorption accompanying the apical periodontitis may cause this wrong determination.^[85] RootZX and Raypex 6 were more accurate in teeth with simulated apical resorption compared to iPex.^[86] Therefore, it can be generally concluded that EALs can be used in teeth with apical periodontitis with clinically acceptable accuracy.

In 2011, da Silva et al. concluded that an EAL is accurate in canals with internal resorption without perforations.^[79]

EALs can reliably locate perforations,^[87-95] and one study found that EALs are more reliable than radiographs in detecting perforations.^[96] EALs were found to more accurately detect perforations in dry conditions^[93] and also with CHX as the irrigant compared to NaOCl.^[92,97] Normal saline, SmearOff, and EDTA (an EDTA-based solution with added CHX) have also produced more accurate results than NaOCl for the detection of perforations.^[98,99] The same experiment has also been carried out with MTAD, Qmix, or NaCl in the root canal. The perforation was most accurately located with NaCl, and the least accurate results were measured using MTAD.^[100] Since EALs can detect perforations, clinicians should be aware of this interference when measuring length in root canals with a perforation.

Studies are controversial regarding the effect of irrigants and solvents on the root canal system. Some studies have found that the accuracy of EALs is not negatively affected by the presence of irrigants or solvents.^[9,101-112] Some authors believe that the most accurate results can be achieved with NaOCl since it is conductive and decreases the impedance of the root canal system.^{[45,101,113,114]} However, in an investigation, the most accurate results were achieved in a dry canal.^[115] One study stated that the readings were statistically different from the actual length with RootZX in the presence of NaOCl; however, they were all within an acceptable clinical range.^[116] No association was found between the concentration of NaOCl and the accuracy of EALs.^[114]

An experiment showed that RootZX was accurate in the presence of EDTA and NaOCl using smaller and larger files. However, higher accuracy was achieved with larger files when using saline or CHX.^[117] The accuracy of RootZX was not adversely affected by the presence of NaOCl or EDTA in pre-enlarged canals, whether small or large files were used. However, when CHX or RC-Prep was used, larger files produced more accurate results, and smaller file sizes caused long readings.^[118] It can be assumed that in low conductive conditions in the canal, the “fit” of the file in the apical regions becomes more critical. Using RootZX, short readings with NaOCl and long readings with CHX and EDTA were observed.^[119,120] ProPex had more accurate results with CHX compared to NaOCl in another study.^[121] Raypex 5 is more accurate with CHX (whether in solution or gel form) instead of NaOCl.^[122] The interaction of the internal mechanisms of EALs and the electrical characteristics of the root canal contents might produce these conflicting results. Interestingly, it has been found that the irrigant itself can act as an active electrode in a wide-diameter canal. Connecting the file holder of an EAL to an irrigation needle and monitoring the displayed number on the EAL screen during irrigation can verify that the irrigants have reached the apical area.^[123]

While the 0.9% saline did affect the accuracy, 2.5% NaOCl, 3% H₂O₂, 0.2% chlorhexidine, 17% EDTA, and the anesthetic solution had no impact on the accuracy of the integrated EAL with the Tri Auto ZX (J. Morita Corp., Tokyo, Japan) handpiece.^[124]

EndoPilot (Schlumbohm, Brokstedt, Germany) and iPex both showed zero readings regardless of the distance of the file from the canal terminus in the presence of isopropyl alcohol, indicating that isopropyl alcohol acts as a complete isolator. Inaccurate readings were also observed with hydrogen peroxide and citric acid.^[50] However, concerning hydrogen peroxide, one study reported accurate readings with RootZX.^[101]

A study showed that the accuracy of EALs was not affected when chloroform, orange solvent, or eucalyptol was present in the root canal.^[125] However, another study showed that using Guttasolv or Resosolv solvents could decrease the accuracy of RootZX.^[126]

It has been demonstrated that isolating the part of the file that passes through the access cavity, regardless of the presence and type of the restoration, can improve the stability and consistency of the EAL reading as it provides an additional means of isolation.^[127]

RootZX is designed to work in wet canals with high conductivity; therefore, unstable readings can result in dry canals; the reverse is true for EALs of earlier generations, which give unstable readings in wet canals.^[128]

The remaining calcium hydroxide (CH) in the root canal resulted in decreased accuracy of RootZX, proportional to the surface area of the residual CH, as observed through an operating microscope.^[129,130] Therefore, it is suggested to use NaOCl or EDTA in combination with hand filing using the master apical file to achieve more accurate results with EALs following the use of CH.^[129] However, it has also been observed that CH removal using EDTA in combination with either NaOCl or normal saline solution results in the same accuracy of length measurement in EALs.^[131]

The effect of blood as a root canal content on the accuracy of EALs was also investigated, and it was reported that blood did not negatively affect the accuracy of RootZX, Raypex 5, RootZX MINI, and Propex Pixi.^[109,132]

Another study evaluated the effect of the presence of dentinal debris and canal instrumentation on the accuracy of Sonoexplorer EAL. Before instrumentation, 63% of the readings were long, whereas after instrumentation, 70% of the readings were short. Therefore, removing dentinal debris using recapitulation was necessary for accurate length measurements.^[133]

In conclusion, the electrical conductivity of root canal contents seems to marginally affect the readings of EALs; however, in most studies, the reported readings are still within a clinically acceptable range; therefore, EALs can be safely used clinically in the presence of various contents.

Pulp condition had no significant impact on the accuracy of EALs.^{[109,134-140]} However, AFA (Analytic Endodontics, Orange, CA) EAL was more accurate in vital cases compared to necrotic ones.^[141] Furthermore, extirpating the pulp tissue improved the accuracy of EALs.^[142] In conclusion, EALs can be accurately used clinically regardless of the vitality of the pulp.

Orthodontic brackets bonded to the tooth surface have been demonstrated to affect the accuracy of electronic WL measurement. A short-circuiting mechanism between the brackets on the tooth surface and the EAL lip clip might be responsible for this interference. A minimum distance of 3 cm between the bracket on the tooth being measured and the lip clip has been suggested.^[143]

EALs can be accurately used in retreatments after removing the root-filling material.^[61,144-149] Using RootZX in specimens obturated with a carrier-based system led to an overestimation of root canal length.^[150] Situations in which EALs fail to provide any data are more common in retreatment cases.^[144] RootZX was not able to detect the MiAF (0.5-mm mark) accurately but was accurate in the “apex” mark in retreatment cases,^[151] which can be explained by the fact that residual root filling material increased the electrical impedance of the canal.^[152] However, regardless of the presence of residual root-filling material, the most distinct decrease in impedance of the root canal occurred at the MAF,^[153] which was less affected than the anatomy of the MiAF that could have been disrupted in the primary treatment or retreatment process. Overall, EALs can be used with clinically accepted accuracy in retreatments. It is advisable to rely more on the measurement that indicates the MAF in each EAL rather than any point short of that in retreatments.

Some EAL manufacturers state that the apex locator is intended for use in an electromagnetic environment with controlled radio frequency disturbances. Therefore, several studies were conducted in this regard.^[154-165] It has been reported that cell phones do not interfere with EALs’ internal mechanisms.^[154-157] However, one study showed the direct contact of cell phones with the enclosure of RootZX MINI while an active call might cause instability of the EAL reading at the “Apex” mark.^[158] An active call is unlikely to happen in clinical scenarios. In another study using RootZX, the mean error of length estimation increased when a cell phone, ADSL modem, cordless phone, or an MP4 player was within 50 cm of the RootZX. In this study, the cell phone was ringing but not in an active call, which is more likely since clinicians or patients usually do not answer their phones during the procedure.^[159] While clinicians should be aware of this potential interaction, available information shows that patients can safely keep their cell phones “on” in the dental operatory without affecting the accuracy of EALs. Evidence has shown that EALs can be safely used in clinical scenarios for patients with cardiac pacemakers.^[160-165]

The limitations of the present study should be noted. The critical appraisal of individual sources was not done due to the high number of included studies. Since the optimal WL can be determined through experimental studies by visualizing the endodontic file tip or by histological section, no clinical studies were identified. For future studies, the application of artificial intelligence in EAL could be examined.

Nil.

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