An overview of electronic apex locators: part 2 R. Ali, *1 N. C. Okechukwu, 2 P. Brunton 3 and B. Nattress 4 VERIFIABLE CPD PAPER are based on a simple model. They apply a small direct current to the tooth under investigation of known voltage. The resist- ance at each level of the RCS can be cal- culated using these two variables using Ohm’s Law. At the periodontal ligament space (PDLS), the resistance of the circuit will equal 6.5 kΩ and the RBEAL are pro- grammed to detect this value (Fig. 1). Although these devices were accu- rate under dry conditions, their accuracy decreased when electrolytes, pulp tissue, inlammatory exudate or excessive haem- orrhage were associated with the RCS. 4,5 As soon as the ile tip touched an electro- lyte, the direct current (DC) voltage would polarise the tissue, complete the circuit and incorrectly register that the PDLS had been reached. The devices also ignored the capacitance component of the circuit. Furthermore, the use of a DC would often cause an electric shock sensation to be felt by the patient which is clearly disad- vantageous. 6 The fact that these devises were less reliable than using radiographs to determine RCS length 7 led practitioners to stop using them. IMPEDANCE BASED (FIRST GEN- ERATION) APEX LOCATORS (IBAL) To overcome the aforementioned prob- lems, the next set of EALs were based on the impedance of the circuit set up within INTRODUCTION In Part 1 of this series, readers were intro- duced to the micro-anatomical features of the apical terminus and the ability of a tooth to function as a capacitor. In the second part of this series, readers will be introduced to: (a) the different types of electronic apex locator (EAL); (b) their modes of action; (c) their relative accu- racies and (d) methods to optimise their success in clinical practice. RESISTANCE BASED (FIRST GENER- ATION) APEX LOCATORS (RBEALS) Sunada 1 carried on earlier work by Custer 2 and Suzuki 3 and determined that the elec- trical resistance between an endodontic instrument at the apical foramen and an electrode attached to oral mucous mem- brane was approximately 6.5 kW. RBEALs (such as the Root Canal Meter (Onuki Medical Co., Tokyo) and Dentometer (Dahlin Electromedicine, Copenhagen) A number of electronic apex locators are available for use during endodontic treatment. The use of third and fourth generation electronic apex locators (EAL) are recommended to help clinicians determine the apical limit of the root canal system (RCS). The presence of different irrigating media in the RCS does not impact signiicantly on the performance of third/fourth generation apex locators. The devices are most accurate at determining the apical limit when the attached en- dodontic ile contacts the periodontal ligament space and the visual analogue displays ‘Apex’ or ‘0.’ Given the accuracies of modern generation EALs, the clinician should be able to consistently identify the apical limit of the tooth under treatment. Their use in conjunction with appropriate radiographs and the clinician’s knowledge of average RCS lengths and anatomy will maximise the successful outcome of any orthograde endodontic treatment. the RCS. This would in theory be more accurate than the solely resistive devices. However, the impedance (and therefore capacitance) of the RCS was dependent on many variables and would vary between different RCSs. Consequently, the biggest disadvantage for IBALs was the need for 1 Specialist Registrar in Restorative Dentistry, 3 Profes- sor/Honorary Consultant in Restorative Dentistry, 4 Senior Lecturer/Honorary Consultant in Restorative Dentistry, Leeds Dental Institute, Clarendon Way, Leeds, LS2 9LU; 2 Senior House Of ice in Oral & Maxillofacial Surgery, York Hospital, Wigginton Road, York, YO31 8HE *Correspondence to: Dr Rahat Ali Email: rahat224@hotmail.com Refereed Paper Accepted 10 October 2012 DOI: 10.1038/sj.bdj.2013.215 © British Dental Journal 2013; 214: 227-231 • Describes the different generations of electronic apex locators, how they function and their relative accuracies. • Describes how the use of third and fourth generation electronic apex locators are recommended to help clinicians determine the apical limit of the root canal system. • Describes clinical tips to help optimise the use of an electronic apex locator whilst carrying out orthograde root canal therapy. IN BRIEF PRACTICE Fig. 1 Schematic representation of an endodontic instrument, the root canal system and the periodontium functioning as a capacitor. (From Nekoofar et al. 14 ). The resistance of the system is 6.5 kW when a ile touches the PDLS at the apical foramen. Different irrigants in the RCS will have different diaelectric constants (ε) (Part 1 of this series, Fig. 4) EALs need to take this feature into account to avoid generating inaccurate readings BRITISH DENTAL JOURNAL VOLUME 214 NO. 5 MAR 9 2013 227 © 2013 Macmillan Publishers Limited. All rights reserved.