REVIEW ARTICLE

https://doi.org/10.5005/jp-journals-10015-2563
World Journal of Dentistry
Volume 16 | Issue 1 | Year 2025

A Systematic Review and Meta-analysis of the Precision of Electronic Apex Locators vs Three-dimensional Imaging in Root Canal Length Determination

Rutuja S Rajput¹ https://orcid.org/0009-0000-6199-9354, Anuradha B Patil² https://orcid.org/0000-0001-9007-2611, Margasahayam V Sumanthini³ https://orcid.org/0000-0002-2313-7021, Divya S Naik⁴ https://orcid.org/0000-0001-5987-6486

^1–4Department of Conservative Dentistry and Endodontics, MGM Dental College and Hospital, Mumbai, Maharashtra, India

Corresponding Author: Anuradha Patil, Department of Conservative Dentistry and Endodontics, MGM Dental College and Hospital, Mumbai, Maharashtra, India, Phone: +91 9819875730, e-mail: anuradhapatil32@gmail.com

Received: 09 December 2024; Accepted: 27 January 2025; Published on: 13 March 2025

ABSTRACT

Aim: To compare the accuracy and reliability of an electronic apex locator (EAL) and a three-dimensional imaging technique in determining the working length (WL).

Materials and methods: An electronic search was conducted for scholarly articles written in English or translated into English, listed in PubMed, Scopus, ProQuest, Google Scholar, and SciELO, for relevant literature. A total of 1,452 potentially relevant journal articles were identified. The search strategy yielded 887 articles. Out of these, 817 were excluded after reading the title and abstract, and 18 were selected for full-text reading. Of these, 2 were excluded based on exclusion criteria. Finally, 16 articles were included for the final search. The data subsequently extracted from the 16 included studies were recorded and analyzed. A scoring system was used to evaluate the risk of bias in the included studies. Due to the heterogeneity of studies, a meta-analysis was not performed in all studies.

Results: The meta-analysis was performed on seven studies that qualified with the required data outcome that could be analyzed quantitatively. The meta-analysis of seven studies assessing the standardized mean difference for WL between cone beam computed tomography (CBCT) and the apex locator was carried out as a subgroup analysis using a random-effects model according to the type of teeth. The standardized mean difference for WL did not show any significant difference between the two groups.

Conclusion: The accuracy and reliability of newer generations of apex locators, when compared with three-dimensional imaging techniques such as CBCT and micro-CT, show reliable results for WL determination. Micro-CT is a novel imaging modality effectively adapted for the 3D visualization and analysis of the internal anatomy of the tooth at high spatial resolution. EALs are a superior choice for determining WL in endodontic treatment, and radiographs should be utilized alongside EALs for optimal results.

Clinical significance: Preexisting CBCT scans may be appropriate for WL determination, but acquiring a new CBCT for endodontic treatment is inadvisable due to cost and the as low as reasonably achievable (ALARA) principle.

Keywords: Cone beam computed tomography, Electronic apex locator, Micro-computed tomography, Permanent dentition, Working length determination

How to cite this article: Rajput RS, Patil AB, Sumanthini MV, et al. A Systematic Review and Meta-analysis of the Precision of Electronic Apex Locators vs Three-dimensional Imaging in Root Canal Length Determination. World J Dent 2025;16(1):80–91.

Source of support: Nil

Conflict of interest: None

INTRODUCTION

In order to achieve successful endodontic treatment, it is imperative to thoroughly remove infected pulp, necrotic debris, and microorganisms from the root canal space. A well-executed root canal obturation is essential for promoting the healing of existing periapical lesions and preventing the formation of new ones.¹ Accurate assessment of the working length (WL) is crucial to avoid potential complications such as excessive instrumentation, perforation, and underfilling.²

The most common methods for determining the WL are radiographic imaging and the use of an apex locator. However, the radiographic method has disadvantages such as increased radiation exposure, time consumption, and potential errors due to observer bias and superimpositions caused by surrounding anatomical structures.³^,⁴

Electronic apex locators (EAL) are popular for determining root canal length and overcoming issues with radiographic measurements. Current models measure impedance differences between frequencies or electrical impedance ratios.⁵^,⁷ The accuracy of electronic measuring devices, particularly EALs, has been widely studied.⁸^,¹⁰ Factors such as the conductivity of intracanal liquids, immature apex formation, periapical lesions, canal patency, metallic restorations, apical constriction size, cardiac pacemakers, and the file size used can all affect the accuracy of EAL.¹¹^,¹²

Despite its questionable usefulness in WL estimation, the apex locator, when used in combination with radiographs, has proven to be a useful adjunct to radiographs in WL determination.

Cone-beam computed tomography (CBCT) and micro-computed tomography (micro-CT) are advanced imaging systems that produce undistorted three-dimensional images.¹³ These modalities are widely accepted for evaluating and visualizing the complex morphology of individual teeth in three dimensions.¹⁴ CBCT facilitates precise diagnosis, effective treatment planning, and reliable canal instrumentation in clinical scenarios. However, micro-CT is limited to in vitro and ex vivo studies only.

The determination of WL is a critical step in root canal treatment, as its accuracy directly influences treatment success. Traditional methods, like two-dimensional radiographs, are subject to limitations such as potential distortion and reliance on the operator’s skill. Emerging technologies, including EALs and three-dimensional (3D) imaging techniques such as micro-CT, have shown potential to enhance accuracy in WL determination. However, there remains limited consensus on their comparative effectiveness.

To address this gap, it is necessary to conduct a thorough evaluation comparing the effectiveness of EALs and 3D imaging in determining WL. The purpose of this systematic review and meta-analysis is to aggregate findings from various studies to more accurately assess the accuracy of EALs and 3D imaging techniques in determining root canal WL. This review aims to provide insights into the effectiveness and practical value of these methods in clinical practice for teeth undergoing root canal treatment.

MATERIALS AND METHODS

Research Questions

The study was conducted at MGM Dental College and Hospital, Kamothe, New Mumbai. A research question was formulated as far as possible, adhering to the PICOS (population, intervention, control, and outcomes) format. This aimed to evaluate the accuracy and reliability of an EAL and a three-dimensional imaging technique in determining the WL.

”Is there any difference in the accuracy of EALs (I) when compared to three-dimensional radiographic imaging techniques (C) in determining the root canal WL (O) of human permanent teeth undergoing endodontic treatment (P)?”

Protocol and Registration

The systematic review protocol was registered at the International Prospective Register of Systematic Reviews (PROSPERO–CRD42023448624) and performed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analysis–Diagnostic Test Accuracy (PRISMA-DTA) checklist (Salameh et al., 2020).¹⁵^,²¹

Sources, Search, and Study Selection

The electronic search was conducted across various databases, including PubMed, Scopus, ProQuest, Google Scholar, and SciELO, to find relevant literature published between 2012 and November 2023, with the search period spanning until May 2024. An adequate search strategy was used in each database with the following terms: human permanent teeth OR electronic apex locators AND three-dimensional radiographic imaging technique CBCT OR micro-CT OR accuracy of working length OR randomized and nonrandomized clinical studies OR clinical trials OR ex vivo studies Table 1.

Studies without a proper study design, inadequate data, untranslatable scripts, and those dated earlier than 2012 were excluded.

Literature Screening and Data Extraction

Two authors (AP and RR) independently screened the relevant titles and abstracts of all studies to identify potentially eligible studies. Based on the inclusion and exclusion criteria, two authors (AP and DN) independently reviewed the full texts of the studies for eligibility. Notably, the value of Cohen’s kappa (κ) for study selection was determined to be 0.9 after the full-text reading, indicating considerable agreement in the selection process. The references of all eligible studies were also examined. Disagreements were resolved through discussion with a third author (SMV). The complete search strategy with the yields is presented in Table 2. Furthermore, the reference lists of the included studies and previous reviews were meticulously searched to identify potential articles for inclusion.

**Table 1:** Concept table
	Concept 1 Population	Concept 2 Intervention I1	Concept 3 Comparison 1	Comparison 2	Concept 4 Outcome	Concept 5 Study design
Key concept	Human permanent teeth	Electronic measurement of the working length	CBCT	Micro CT	Accuracy of the EALs for the working length determination	Clinical study
Free text terms		Electronic apex locator	CBCT Cone beam computed tomography	Micro CT		Ex vivo study Clinical study Clinical trial
Controlled vocabulary terms/subject terms (MESH terms, entry terms)	Permanent Dentition Dentition, Secondary Secondary Dentition Dentition, Adult Adult Dentition		Computed Tomography, Cone-Beam Cone Beam Computed Tomography CT Scan, Cone-Beam CT Scan, Cone Beam CT Scans, Cone-Beam Cone-Beam CT scan Cone-Beam CT scans Scan, Cone-Beam CT Scans, Cone-Beam CT Tomography, Cone-Beam Computed Tomography, Cone Beam Computed Tomography, Volume Computed Computed Tomography, Volume Volume Computed Tomography Volumetric CT CT, Volumetric Volumetric Computed Tomography Computed Tomography, Volumetric Tomography, Volumetric Computed CAT Scan, Cone-Beam CAT Scan, Cone Beam CAT Scans, Cone-Beam Cone-Beam CAT Scan Cone-Beam CAT Scans Scan, Cone-Beam CAT Scans, Cone-Beam CAT Cone-Beam Computer-Assisted Tomography Computer-Assisted Tomography, Cone-Beam Cone Beam Computer Assisted Tomography Tomography, Cone-Beam Computer-Assisted Cone-Beam Computerized Tomography Computerized Tomography, Cone-Beam Cone Beam Computerized Tomography Tomography, Cone-Beam Computerized Cone-Beam CT CT, Cone-Beam Cone Beam CT Volume CT CT, Volume	Microtomography, X-Ray X Ray Microtomography MicroCT MicroCTs X-Ray Micro-CAT Scans Micro-CAT Scan, X-Ray Micro-CAT Scans, X-Ray Scan, X-Ray Micro-CAT Scans, X-Ray Micro-CAT X Ray Micro CAT Scans X-Ray Micro-CAT Scan X-Ray Micro-Computed Tomography Micro-Computed Tomography, X-Ray Tomography, X-Ray Micro-Computed X Ray Micro Computed Tomography Xray MicroCT MicroCT, Xray MicroCTs, Xray Xray MicroCTs X-Ray Micro-CT Scans Micro-CT Scan, X-Ray Micro-CT Scans, X-Ray Scan, X-Ray Micro-CT Scans, X-Ray Micro-CT X Ray Micro CT Scans X-Ray Micro-CT Scan X-Ray Microcomputed Tomography Microcomputed Tomography, X-Ray Tomography, X-Ray Microcomputed X Ray Microcomputed Tomography X-ray MicroCT MicroCT, X-ray MicroCTs, X-ray X ray MicroCT X-ray MicroCTs Xray Micro-CT Micro-CT, Xray Micro-CTs, Xray Xray Micro CT Xray Micro-CTs Microcomputed Tomography Tomography, Microcomputed X-Ray Micro-CT Micro-CT, X-Ray Micro-CTs, X-Ray X Ray Micro CT X-Ray Micro-CTs	Accuracies, Dimensional Measurement Accuracy, Dimensional Measurement Dimensional Measurement Accuracies Measurement Accuracies, Dimensional Measurement Accuracy, Dimensional	Ex vivo study Clinical study Clinical trial In vivo studies

Inclusion Criteria

Articles describing studies performed on fully developed human permanent teeth.
Clinical trials, in vivo studies, randomized controlled studies, and nonrandomized control trials are included.
Quasi-experimental studies, cohort studies, clinical retrospective studies, and ex vivo studies are included.

Exclusion Criteria

Observational study designs, case reports, case series, cross-sectional studies, reviews, in vitro studies, and animal studies were excluded.
Studies not fully available in the database were excluded.
Articles reporting only abstracts were also excluded.
Studies in foreign languages without an English translation were excluded.
Additionally, studies reporting only a single intervention were excluded.

Data Collection

The data were subsequently extracted from the 16 included studies and recorded in respective Excel data extraction sheets. The relevant items were summarized as follows: Study ID, author, year of publication, study design; Sample characteristics (number of patients, number of teeth, type of teeth, number of canals assessed); Intervention characteristics (generation of apex locator); Comparator characteristics (type of three-dimensional imaging, i.e., micro-CT/CBCT); Number of examiner agreements; Statistical analysis; Protocol for reliability measurements of EAL; Validation method; Method of accuracy measurement; Method of reliability measurement; Conclusion based on the extracted data.

Two independent observers (AP, RR) collected the data. In case of disagreement, a consensus was reached through discussion with a third author (SMV). Interrater reliability tests were undertaken, with a kappa value above 0.81.

**Table 2:** Search strategy
Search found	PICO	Search strategy input query	No. of items
#1	P	(((((((((((((patients) OR (mature permanent teeth)) OR (mature permanent tooth)) OR (mature apex)) OR (mature apices)) OR (permanent molar)) OR (permanent premolar)) OR (permanent canine)) OR (permanent incisor)) OR (incisor)) OR (bicuspid)) OR (cuspid)) OR (molar)) OR (dentition permanent)	89,16,895
#2	Ia	((((((((((((((((((((((((((((((((((((((((((((((Microtomography, X-Ray) OR (X-Ray Microtomography)) OR (MicroCT)) OR (MicroCTs)) OR (X-Ray Micro-CAT Scans)) OR (Micro-CAT Scan, X-Ray)) OR (Micro-CAT Scans, X-Ray)) OR (Scan, X-Ray Micro-CAT)) OR (Scans, X-Ray Micro-CAT)) OR (X Ray Micro CAT Scans)) OR (X-Ray Micro-CAT Scan)) OR (X-Ray Micro-Computed Tomography)) OR (Micro-Computed Tomography, X-Ray)) OR (Tomography, X-Ray Micro-Computed)) OR (X Ray Micro Computed Tomography)) OR (Xray MicroCT)) OR (MicroCT, Xray)) OR (MicroCTs, Xray)) OR (Xray MicroCTs)) OR (X-Ray Micro-CT Scans)) OR (Micro-CT Scan, X-Ray)) OR (Micro-CT Scans, X-Ray)) OR (Scan, X-Ray Micro-CT)) OR (Scans, X-Ray Micro-CT)) OR (X Ray Micro CT Scans)) OR (X-Ray Micro-CT Scan)) OR (X-Ray Microcomputed Tomography)) OR (Microcomputed Tomography, X-Ray)) OR (Tomography, X-Ray)) OR (X Ray Microcomputed Tomography)) OR (X-ray MicroCT)) OR (MicroCT, X-ray)) OR (MicroCTs, X-ray)) OR (X ray MicroCT)) OR (X-ray MicroCTs)) OR (X-ray MicroCTs)) OR (Micro-CT, Xray)) OR (Micro-CTs, Xray)) OR (Xray Micro CT)) OR (Xray Micro-CTs)) OR (Microcomputed Tomography)) OR (Tomography, Microcomputed)) OR (X-Ray Micro-CT)) OR (Micro-CT, X-Ray)) OR (Micro-CTs, X-Ray)) OR (X Ray Micro CT)) OR (X-Ray Micro-CTs)	5,13,824
#3	Ib	((((((((((((((((((((((((((((((((((((((Computed Tomography) OR (Cone Beam Computed Tomography)) OR (CT Scan, Cone-Beam)) OR (CT Scan, Cone Beam)) OR (CT Scans, Cone-Beam)) OR (Cone-Beam CT Scan)) OR (Cone-Beam CT Scans)) OR (Scan, Cone-Beam CT)) OR (Scans, Cone-Beam CT)) OR (Tomography, Cone-Beam Computed)) OR (Tomography, Cone Beam Computed)) OR (Tomography, Cone Beam Computed)) OR (Computed Tomography, Volume)) OR (Volume Computed Tomography)) OR (Volumetric CT)) OR (CT, Volumetric)) OR (Volumetric Computed Tomography)) OR (Computed Tomography, Volumetric)) OR (Tomography, Volumetric Computed)) OR (CAT Scan, Cone-Beam)) OR (CAT Scan, Cone Beam)) OR (CAT Scans, Cone-Beam)) OR (Cone-Beam CAT Scan)) OR (Cone-Beam CAT Scans)) OR (Scan, Cone-Beam CAT)) OR (Scans, Cone-Beam CAT)) OR (Cone-Beam Computer-Assisted Tomography)) OR (Computer-Assisted Tomography, Cone-Beam)) OR (Cone Beam Computer Assisted Tomography)) OR (Tomography, Cone-Beam Computer-Assisted)) OR (Cone-Beam Computerized Tomography)) OR (Computerized Tomography, Cone-Beam)) OR (Cone Beam Computerized Tomography)) OR (Tomography, Cone-Beam Computerized)) OR (Cone-Beam CT)) OR (CT, Cone-Beam)) OR (Cone Beam CT)) OR (Volume CT)) OR (CT, Volume)	7,27,771
#4	C	(Electronic apex locator) OR (apex locator)	4,792
#5	O	((accuracy) OR (clinical accuracy)) OR (precision)	10,72,063
#6		((working length) OR (working length determination)) OR (endodontic working length)	65,967
#7		(Apical foramen) OR (Tooth apex)	5,865
#8		#1 AND #2	2,88,731
#9		#1 AND #3	4,29,152
#10		#1 AND #4	2,335
#11		#1 AND #5	3,56,992
#12		#1 AND #6	19,413
#13		#1 AND #7	3,867
#14		#2 AND #5	46,554
#15		#3 AND #5	64,017
#16		#4 AND #5	652
#17		#6 AND #5	4,357
#18		#7 AND #5	626
#19		((#1) AND (#2)) OR (#3)	7,30,776
#20		((#1) AND (#2)) AND (#3)	2,85,726
#21		((((((#1) AND (#2)) OR (#3)) AND (#4)) AND (#5)) AND (#6)) AND (#7)	20
#22		((((((#1) AND (#2)) AND (#3)) AND (#4)) AND (#5)) AND (#6)) AND (#7)	7

Quality Assessment

Evaluating the quality and risk of bias in individual studies is critical to ensuring the accuracy and reliability of the findings in a systematic review. For this review, we applied a quality assessment methodology based on an approach previously outlined by Shivkumar et al.,²² with modifications to suit the objectives of this study Table 3.

The assessment process involved the use of a comprehensive checklist, which included the following aspects:

Selection criteria for teeth.
The number of canals evaluated.
The setting in which the study was conducted.
The number of examiners involved.
Reported reliability of the tests used.
Validation methods applied.
Criteria for validation.
Reliability of the validation process.

Each study was assigned a score based on these factors, with a total score ranging from 0 to 20. This score was then converted to a percentage by multiplying the score by 5. Based on this percentage, studies were categorized as follows:

Low risk of bias: Studies with a score above 50% were considered to be of very high quality and had a low risk of bias.
Average risk of bias: Studies with scores between 40 and 50% were categorized as having an average risk of bias.
High risk of bias: Studies scoring below 40% were classified as having a high risk of bias.

The findings of this quality assessment are summarized in Table 4. Two authors (AP and DN) independently assessed these criteria and gave a judgment of “high risk,” “average risk,” or “low risk.” The interreader agreement in the process of quality assessment was evaluated using the kappa statistic. According to Landis and Koch’s classification, the interreader agreement is considered almost perfect if the kappa (κ) value falls within the range of 0.81–1.00, substantial for κ values of 0.61–0.80, moderate for κ values of 0.41–0.60, fair for κ values of 0.21–0.40, and poor for κ values below 0. The reliability between the two reviewers was high, as indicated by a kappa coefficient exceeding 0.89.

Qualitative and Quantitative Syntheses

Depending upon the characteristics of the selected studies and the various data items retrievable from these studies, qualitative and quantitative syntheses of the collated data were performed.

RESULTS AND ANALYSIS

Search Details

A total of 1,452 articles were obtained through the electronic searches, and 1,374 were exported into the Zotero Desktop software. After removing duplicates, 1,374 articles were screened for titles and abstracts, providing a total of 18 studies for full-text reading, out of which 16 articles were included for the systematic review. The PRISMA 2020 Flow Diagram (Fig. 1) shows the reasons for the exclusion of studies after full-text reading.

**Table 3:** Scoring criteria for assessing the risk of bias of included studies
Elements of internal validity	Points	Criteria
Tooth selection	3	Both posterior and anterior teeth
	2	Only anterior or only posterior teeth
	1	Selected anterior or posterior teeth
	0	Single tooth type
Study setting	2	In vivo
	1	Ex vivo
	0	In vitro
No. of canal assessed	3	120 or more
	2	60–119
	1	30–60
	0	<29
Number of observers	2	2–3
Number of observers	1	1
Test reliability report	2	Inter- and intra-evaluator reliability reported
	1	Either intra- or intra-evaluator reliability reported
	0	No evaluator reliability reported
Validation method	2	Three-dimensional radiographic imaging technique with EAL with other methods
	1	Three-dimensional radiographic imaging technique with EAL
	0	Only three-dimensional radiographic imaging technique or single EAL
Validation criteria	1	Criteria explicitly stated
Validation criteria	0	Criteria not explicitly stated
Validation reliability	1	Intra- and inter-evaluator reliability reported
Validation reliability	0	No reliability reported

**Table 4:** Assessment of risk of bias
Author and year	Study setting	No. of canals assessed	Tooth selection	No. of examiner	Test reliability reported	Validation method	Validation criteria	Validation reliability	Total score	Risk of bias
Cinar 2020³⁴	Ex vivo	25	16 incisors and 9 premolars	1	No evaluator reliability reported	WL measured with 3rd and 5th generation EAL and postoperative micro-CT images	Criteria explained	No reliability reported	35	High
Cury et al. 2021²³	Ex vivo	90	47 incisors, 28 canines, 15 premolar	1	No evaluator reliability reported	WL measured with 3rd and 4th generation EAL and postoperative micro-CT images	Criteria explained	No reliability reported	45	Average
De-Deus et al. 2023²⁴	Ex vivo	23	5 incisors, 4 canine, 6 premolar, 8 molars	1	Blinding of the evaluator reported	WL measured with 3rd and 5th generation EAL and postoperative micro-CT images	Criteria explained	No reliability reported	40	Average
De-Deus et al. 2022³⁵	Ex vivo	11	9 anterior, 2 posteriors	1	No evaluator reliability reported	WL measured with 3rd generation EAL and postoperative micro-CT images	Criteria explained	No reliability reported	35	High
Üstün et al. 2016²⁸	Ex vivo	73	67 incisors and canines, 6 premolars	1	Intraoperative reliability reported	WL measured with Preoperative CBCT images and 5th and 6th generation EAL	Criteria explained	Reliability reported	55	Low
de Morais et al. 2016²⁹	In vivo	30	13 incisors, 14 lateral incisor, 3 canines	1	No evaluator reliability reported	WL measured with periapical radiograph,3rd generation EAL and postoperative CBCT images	Criteria explained	No reliability reported	50	Low
Hussien 2023²⁵	In vivo	47	12 upper central incisors, 10 upper lateral incisors, 13 upper canines, and 12 lower canines	1	No evaluator reliability reported	WL measured with preoperative CBCT images and EAL postextraction actual WL	Criteria explained	No reliability reported	45	Average
Janner et al. 2011³⁰	In vivo pilot study	10	6 incisors, 2 canine, 1 premolar	1	Inter- and intra-evaluator reliability reported	WL measured with preoperative CBCT images and 3rd generation EAL	Criteria explained	Reliability reported	55	Low
Jeger et al. 2012³¹	In vivo	40	32 incisors, 8 canines	1	Inter- and intra-evaluator reliability reported	WL measured with preoperative CBCT images and 3rd generation EAL	Criteria explained	Reliability reported	55	Low
Pietrzycka et al. 2021²	In vivo retrospective study	58	13 incisors, 5 canine, 30 premolars	1	Inter- and intra-evaluator reliability reported	WL measured with preoperative CBCT images and 6th generation EAL	Criteria explained	Reliability reported	65	Low
Obeid et al. 2018²⁶	In vivo	178	Maxillary molar	1	No reliability reported	WL measured with 3rd generation EAL the preoperative CBCT images and actual WL using microscope	Criteria explained	No reliability reported	50	Average
Kamalraj et al. 2021³²	Ex vivo	30	Mandibular premolar	2	Inter- and intra-evaluator reliability reported	WL measured using the tactile method, radiography, 5th generation EAL and postoperative CBCT images and actual WL using loups	Criteria explained	Reliability reported	55	Low
Rabi 2021³⁷	In vivo	20	Mandibular premolar	1	No reliability reported	WL measured using RVG, 3rd generation EAL, postextraction CBCT images and actual WL by stereomicroscope	Criteria explained	No reliability reported	35	High
Svetlozarova and Papancheva 2022³³	In vivo	130	45 incisors, 24 canines, 31 premolars	1	Intra-evaluator reliability reported	WL measured with preoperative CBCT images and 3rd generation EAL	Criteria explained	Reliability reported	65	Low
Sherwood and Piasecki 2021²⁷	In vivo	24	21 maxillary central incisors and 3 maxillary lateral incisors	1	Blinding of the evaluator reported	WL measured with preoperative CBCT images and 3rd and 4th generation EAL and RVGs	Criteria explained	Reliability reported	45	Average
Eslinger 2012³⁶	In vivo	40	15 maxillary 1st molar, 21 mandibular 1st molar, 2 maxillary premolar, 1 maxillary central incisor, 1 mandibular central incisor	–	No reported	WL measured with preoperative CBVT images, EAL, RVGs	Criteria explained	No reliability reported	30	High

Fig. 1: PRISMA flow diagram of study selection process

Qualitative Assessment

Out of the sixteen studies included, five studies²³^,²⁷ showed an average risk of bias, while another seven studies²^,²⁸^,³³ showed a low risk of bias. Four studies³⁴^,³⁷ exhibited a high risk of bias. The high risk of bias is attributed to the lack of reported inter/intra-evaluator reliability and small sample sizes.

Notably, the studies by De-Deus et al. (2023)²⁴ and Sherwood and Piasecki (2021)²⁷ reported blinding of the evaluator, while the studies by Üstün et al. (2016)²⁸ and Svetlozarov and Papancheva (2022)³³ reported intra-evaluator reliability. Additionally, four studies reported inter/intra-evaluator reliability, which was validated using correlation methods.²^,³⁰^,³²

In the 2012 study conducted by Eslinger,³⁶ the sample size was appropriate, but there was no evaluation of inter/intra-evaluator reliability, so it was rated as a high risk of bias. Some studies had small sample sizes, but they reported inter/intra-evaluator and validation reliability, so these studies were considered to have an average risk of bias.²³^,²⁷ Four studies³⁴^,³⁷ show a high risk of bias, the reason being small sample size, no test reliability reported, and lack of validation reliability reported. All included studies explained their validation methods and criteria. Inter/intra-evaluator reliability was validated through correlation coefficients, with a few studies²⁷^,²⁸^,³⁰^,³² using Pearson correlation coefficients. The study by Svetlozarova and Papancheva³³ used the degree of correlation to measure the values of two different groups.

Quantitative Synthesis

Review Manager (RevMan) 5.4 was used for statistical analysis. The combined results were expressed as standardized mean and standard deviation for the continuous data at 95% confidence intervals (CIs) and P50% and p ≤ 0.10. For I² > 50%, the random-effects model was applied. Also, statistical significance was set at p-value (two-tailed).

A total of 7 studies²^,²³^,³⁷ met the inclusion criteria for quantitative analysis. The meta-analysis required the use of standard deviation and mean difference as outcome parameters; however, these were absent in nine studies,²^,²³^,²⁵^,²⁸^,²⁹^,³¹^,³⁴^,³⁵ leading to their exclusion from the analysis. Additionally, some studies utilized the median as an outcome parameter, which was not suitable and contributed to heterogeneity.

During the quantitative synthesis process, two studies by Janner et al.³⁰ and Eslinger³⁶ presented individual patients’ data instead of pooled values. Hence, mean and standard deviation values were calculated and considered for meta-analysis. In the study by Obeid et al.,²⁶ mean and standard deviation for 3 separate roots against two different types of apex locators were presented as separate values; therefore, each group was analyzed separately as six different studies. In the study by Sherwood and Piasecki,²⁷ five different types of apex locators were compared with CBCT; therefore, each group was analyzed as five different studies.

Subsequently, two meta-analyses, including one subgroup analysis for different types of teeth, were performed for assessing the WL using the CBCT as compared to apex locators.

The meta-analysis was performed on seven studies²⁶^,²⁷^,³⁰^,³²^,³³^,³⁶^,³⁷ that qualified with the required data outcome that could be analyzed quantitatively. The meta-analysis of seven studies assessing the standardized mean difference for WL between CBCT and the apex locator was carried out as a subgroup analysis using the random-effects model according to the type of teeth (Fig. 2).

Fig. 2: Forest plot of seven studies assessing the standardized mean difference for WL between CBCT and apex locator for all types of teeth

For incisors, the standardized mean difference in WL between both groups did not show a significant difference (SMD, 0.17, 95% CI = –0.05–0.38, p = 0.13, I² = 0%). For canines, only a single study was eligible, and the standardized mean difference in WL between both groups did not show a statistically significant difference (SMD, –0.09, 95% CI = –0.66–0.48, p = 0.76).

Similarly, for premolars and molars, the standardized mean difference in WL between both groups did not show a statistically significant difference (SMD, 0.02, 95% CI = –0.29–0.33, p = 0.90, I² = 0%) and (SMD, –0.01, 95% CI = –0.15–0.14, p = 0.92, I² = 0%), respectively.

For unspecified types of teeth, the standardized mean difference in WL between both groups did not show a significant difference (SMD, 0.13, 95% CI = –0.26–0.53, p = 0.51, I² = 0%).

When the total analysis using the random-effects model irrespective of the type of teeth was considered for a total sample size of 675 and 681 samples for CBCT and apex locator groups, respectively, the standardized mean difference in WL between both groups did not show a statistically significant difference (SMD, 0.05, 95% CI = –0.06–0.15, p = 0.41, I² = 0%).

Further, the meta-analysis was performed on two studies that qualified with the required data outcome that could be analyzed quantitatively. The results of the overall comparison have been depicted as a forest plot. The forest plot of two studies³⁰^,³⁶ (Fig. 3) assessing the standardized mean difference in WL for mandibular premolars between the CBCT and apex locator group was carried out using the random-effects model. The standardized mean difference for WL did not show a statistically significant difference between the two groups (SMD, 0.05, 95% CI = –0.34–0.45, p = 0.79, I² = 0%).

Fig. 3: Forest plot of two studies assessing the standardized mean difference in WL for mandibular premolars between CBCT and apex locator groups

DISCUSSION

The present study undertook an exhaustive literature search using various electronic search engines and manual searches to identify studies dealing with the precision of EALs in WL measurement compared to three-dimensional imaging techniques in human permanent teeth undergoing root canal treatment. To overcome heterogeneity of data, strict inclusion and exclusion criteria were applied to the studies. Sixteen studies were identified as eligible for the systematic review.

Ideally, the canal preparation and obturation should terminate at the cementodentinal junction or the apical constriction. The minor constriction, major constriction, and cementodentinal junction are three distinct landmarks on the terminal part of a tooth root. In 39 cases, the cementodentinal junction does not coincide with the point 1 mm short from the radiographic apex. The distance between the apical minor constriction and major constriction ranged between 0.4 and 1.2 mm.³⁸ Since the pulp canal does not always exist at the anatomic apex, greater consideration should be given to obtaining a sound knowledge of different techniques of WL determination.³²

The most commonly used methods for determining the WL are related to radiographic imaging or the use of an apex locator. Film-based radiography has the advantage of being an easy tool, but it has some drawbacks, such as increased radiation exposure and longer duration of processing.³⁸ Digital radiography is a cutting-edge imaging technology that allows for image enhancement, reduced radiation exposure, and shorter turnaround times. However, technical sensitivity and thereby accuracy remain still controversial.

In general, the radiographic method has a greater potential for errors because of observer bias and is prone to superimpositions by surrounding anatomical structures and distortions caused by converting a three-dimensional object to a two-dimensional representation.³⁹ Therefore, it is expected that electronic measurements that determine the WL will be achieved at this level. In terms of accuracy, ease of use, and patient comfort, using an EAL to determine endodontic WL removes many of the drawbacks of the radiographic process.¹⁵

The functionality of this equipment is based on the fact that the electrical conductivity of the tissues surrounding the root apex is greater than the conductivity inside the root canal system, which is either dry or filled with nonconductive fluid.¹⁶ The Root ZX EAL (J. Morita Co., Kyoto, Japan) has undergone comprehensive in vivo and ex vivo testing and has established itself as the gold standard against which new devices are measured.³⁹ According to the study conducted by Cinar,³⁴ the accuracy of RayPex 5, Root ZX mini, and Propex Pixi EALs are not affected by the existence of blood-pulp tissue or NaOCl. Apex locators, when used in combination with radiographs, have proven to be a useful adjunct to radiographs in WL determination due to decreased chances of error compared to when radiographs are used alone.¹⁴

Among the different generations of EALs, the first and second generations are obsolete and no longer manufactured or used in modern dentistry. With the advancement of science and technology in dentistry, new generations of EALs, the third, fourth, fifth, and sixth generations (modified fifth generation), with higher accuracy in measuring WL, have been developed.¹⁸ A systematic review and meta-analysis conducted by Nasiri and Wrbas (2021)¹⁹ showed no significant difference in the determination of WL among the four (third, fourth, fifth, and sixth) generations of apex locators, and it concluded that all generations can be equally useful and accurate in determining WL. A WL determination that makes endodontic treatment more accurate and less stressful for clinicians by reducing operating time and increasing patient satisfaction has been noted.¹⁴

Another three-dimensional imaging technique is micro-CT, which is a nondestructive method for evaluating anatomical landmarks in tooth samples. With the aid of micro-CT technology, it is possible to determine the narrowest apical part of the entire root canal.⁴⁰ With MicroCT scans, it was even possible to visualize mean apical constriction diameters of 0.296 mm.⁴¹ In addition, the use of a double-scan protocol provided the possibility of evaluating the real position of instruments without interference from artifacts created by the metal alloy during the scanning procedure.

The assessment of the risk of bias based on in vivo/ex vivo studies was conducted based on an analysis previously recorded by Shivkumar et al.²² This analysis was modified for the present systematic review. The checklist includes the scoring based on tooth selection, number of canals assessed, study setting, number of examiners, test reliability reported, validation method, validation criteria, and validation reliability.

Of the sixteen studies reviewed, five²³^,²⁷ showed an average risk of bias, while seven²^,²⁸^,³³ had a low risk and four³⁴^,³⁷ showed a high risk of bias. The high risk was primarily due to the lack of reported inter/intra-evaluator reliability and small sample sizes.

De-Deus et al. (2023)²⁴ and Sherwood and Piasecki (2021)²⁷ reported blinding of evaluators, while Üstün et al. (2016)²⁸ and Svetlozarov and Papancheva (2022)³³ documented intra-evaluator reliability. Four studies validated inter/intra-evaluator reliability using correlation methods.²^,³⁰^,³²

Eslinger (2012)³⁶ had an appropriate sample size, but the absence of inter/intra-evaluator reliability resulted in a high risk of bias. Conversely, studies with small sample sizes that reported reliability had an average risk of bias.²³^,²⁷ All studies explained their validation methods, with many using the Pearson correlation coefficient²⁷^,²⁸^,³⁰^,³² and Svetlozarova and Papancheva³³ employing a degree of correlation to measure values between groups.

A total of seven studies²⁶^,²⁷^,³⁰^,³²^,³³^,³⁶^,³⁷ met the criteria for quantitative analysis. During the process, two studies³⁰^,³⁶ provided individual patient data, leading to calculated mean and standard deviation values for meta-analysis. One study²⁶ analyzed three separate roots for two types of apex locators, treated as six distinct studies, while another compared five apex locators with CBCT, resulting in five additional analyses.²⁷

The meta-analysis evaluated the standardized mean difference in WL between CBCT and apex locators, with a total sample size of 675 for CBCT and 681 for apex locators. No statistically significant difference was found for mandibular premolars. A separate meta-analysis of two studies on this topic also confirmed that there was no significant difference between CBCT and apex locators for WL.

The results of the present study showed that CBCT scans can be used as an alternative method for ascertaining the WL. If a patient has a preexisting CBCT scan, the dental practitioner should take advantage of this technique as an additional, reliable method for the determination of WL. Therefore, if a preoperative CBCT image is available, to conform with the rule of “as low as reasonably achievable” (ALARA), an additional preoperative X-ray should not be taken to avoid excessive radiographic exposure. EAL measurements can provide inconsistent results in teeth with metallic restorations because of electrical short-circuiting. In such cases, CBCT images could be helpful in obtaining a more precise evaluation of the WL and could increase the success rates for endodontic treatment.

It is well established in the scientific literature that the use of EALs is the most effective method for determining WL in permanent teeth. The rationale for this study was to determine the current level of evidence on the accuracy of EALs compared with three-dimensional imaging techniques. A prior systematic study was conducted, in which WL obtained using CBCT and EAL was compared, and its accuracy and reliability were evaluated.⁴² Another systematic review and meta-analysis⁴³ was conducted, this time comparing WL acquired from preoperative CBCT images to EAL. However, significant shortcomings of this study were addressed in the current systematic review and meta-analysis to improve the quality of evidence.

Thus, this review has important strengths to highlight: (1) the inclusion of only in vivo and ex vivo studies. Systematic reviews carried out only with in vivo studies tend to generate results that are more in line with the number of limitations that come with traditional radiographic images and EALs for WL determination, which have been overcome in three-dimensional examinations. CBCT scans have been popular in endodontics for both diagnosis and planning treatment. Different reports stated and analyzed to consider the CBCT as one of the reliable methods in clinical practice, thus being able to contribute to the implementation of evidence-based practices in dentistry; (2) the methodological analysis of the included studies was assessed by an analysis previously recorded by Shivkumar et al.; (3) a wide data collection approach, through multiple databases, including the gray literature, without the use of filters; (4) meta-analysis of data: grouping data from different studies increases the sample size, generating results with greater statistical power; (5) this is the first systematic review that includes micro-CT for measuring the WL. Several studies have been performed in which WL obtained from different apex locators was assessed through micro-CT images. However, the results of these studies have not been as meta-analysis so far.

There are some limitations of this study, as only studies published in the English language were included. Also, a limitation of the study was that research and publication related to micro-CT studies are limited, leading to restriction in our systematic review. Thus, future comparative studies between apex locators and micro-CT/CBCT should use standardized methodology and consider different clinical conditions, such as the presence of multirooted teeth (molars), teeth with periapical lesions, and anatomical variations, to devise clinical techniques for routine endodontic practice.

CONCLUSION

EALs are essential for determining WL in endodontic treatment and should be used alongside radiographs to enhance accuracy and reduce complications like over-instrumentation and postoperative pain. If available, pre-existing CBCT scans can serve as a reliable alternative for WL measurement, aligning with the ALARA principle to minimize radiation exposure. While three-dimensional imaging can improve accuracy, it should complement rather than replace EALs. In summary, combining EALs with radiographs is the gold standard for accurate WL determination in root canal treatment, contributing to higher success rates without increasing patients’ radiation exposure.

Clinical Significance

It’s important to accurately determine the WL in root canal treatment to avoid complications such as over-instrumentation, perforation, and overfilling, which can lead to postoperative pain. Studies have shown that preoperative cone beam computed tomography (CBCT) can help identify the apical foramen (AF), and specialized software can measure its distance from a reference point. This measurement can be compared with EAL for accuracy.

Additionally, micro-CT studies have been conducted to evaluate WL using different apex locators. A systematic review is underway to provide a more precise estimate of the accuracy of WL established with EAL and three-dimensional imaging techniques and their usefulness in root canal treatment.

Supporting information: PRISMA checklist.

ORCID

Rutuja S Rajput https://orcid.org/0009-0000-6199-9354

Anuradha B Patil https://orcid.org/0000-0001-9007-2611

Margasahayam V Sumanthini https://orcid.org/0000-0002-2313-7021

Divya S Naik https://orcid.org/0000-0001-5987-6486

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