# In Vitro Accuracy of Two Different Electronic Apex Locators Under Various Conditions
## 1 Introduction  
Determining the correct working length (WL) of a root canal is essential and an important factor for successful endodontic treatment. Underestimation of the WL can lead to the insufficient debridement of the root canal, while overestimation may result in damage to the periapical tissues, which will delay or prevent healing [1]. Although the apical constriction may vary widely in shape, it is generally defined as the narrowest portion of the canal that has the smallest diameter blood supply. It creates the smallest wound site and provides optimal healing conditions [2]. According to histological studies, the apical constriction is often at some distance (0-2 mm) from the radiographic apex [3]. Another study showed distances of 0.5 to 0.75 mm from the anatomical to apical constriction, with great variations between the anterior and posterior teeth [4]. Therefore, locating the exact terminus of the canal at the apical constriction is an important clinical factor [5].  
The development of electronic apex locators (EALs) stemmed from the findings of [PERSON] in the year 1942 [6]. It was established that the electrical resistance between the periodontal ligament and the oral mucosa of dogs in vivo was a constant value. Thefirst-generation EALs were resistance-based, the second-generation EALs were impedance-based, and the third-generation EALs were frequency-based [7]. They had many limitations, so mostly fourth and fifth-generation devices are employed currently. It is typical for fourth-generation devices to measure and compare the complex electrical characteristics of the root canal using two or more frequencies of electrical impulses [8]. Nowadays, there is also a sixth generation of devices, which represent a modified version of the fifth generation [9].  
Conductive solutions used as canal irrigants inside the root canal have been reported to affect the accuracy of EALs in vivo, in vitro and ex vivo [8; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34]. It has also been reported that different irrigants and methods affect the accuracy of EALs under in vitro conditions [16; 18; 19; 20; 21; 22; 23; 24; 26; 27; 28; 29; 30; 31; 32; 33; 34], but some studies showed no impact on the performance of apex locators [24; 25; 26; 27; 28].  
Sodium hypochlorite (NaOCI) is used in many situations at concentrations ranging from 0.5% to 6% during endodontic treatment because of its antibacterial properties and ability to rapidly dissolve remains of pulpal tissue. Also, chlorhexidine (CHX) is used as an antimicrobial agent (exhibit bacteriostatic or bactericidal) in the form of a solution or gel. Ethylenediaminetetraacetic acid (EDTA) is a chelating agent used to remove mineral debris and the smear layer in endodontic treatment. The varying accuracy of different available electronic apex locators under various canal irrigation solutions is problematic for clinicians. Moreover, a comparative evaluation of different apex locators under similar simulated clinical conditions is deficient in the literature, as well as research on the influence of different root canal irrigants. There are many commercial root canal irrigants with different chemical compositions on the market today. It is possible that they have a greater or lesser impact on the accuracy of EALs. However, a limited number of studies have evaluated the influence of different root canal irrigants on the accuracy of different EALs.  
The aim of this study was to determine the impact of seven different root canal irrigation preparations used in clinical practice on the accuracy of two different EAL devices. The null hypotheses tested were as follows: (i) the accuracy of both EALs would not depend on the type of root canal irrigation agent used and (ii) there is no difference in the accuracy of measurement between the two EALs.