The term “Working Length” (WL) is defined in the Glossary of Endodontic Terms as “the distance from a coronal reference point to the point at which canal preparation and obturation should terminate.”
Errors in determinon of WL may result in length being too long leading to apical perforation, overfilling or overextension and increased post-operative pain with prolonged healing period and a lower success rate. A WL too short of the apical constriction can lead to incomplete cleaning and under filling causing persistent discomfort, and continued periradicular infection.
Accurate determination of root canal length is critical for successful endodontic treatment. The debridement, shaping, and obturation of root canal system cannot be accomplished properly unless the working length (WL) is correctly determined. Underestimation of the WL can lead to insufficient debridement of the root canal; whereas, overestimation may result in damage to the periapical tissues, which will delay or prevent healing. Optimal healing condition with minimal contact between the obturation material and the apical tissue is achieved when root canal treatment terminates at the apical constriction.
Traditional methods for establishing WL have been the use of anatomical averages and knowledge of anatomy, tactile sensation, moisture on a paper point, and radiography. The most popular method of WL determination has been the use of radiographs. There are, however, a number of limitations associated with it; The increasing concern about the radiation exposure led to the development of the electronic apex locator (EAL).
The principles of EAL can be explained by Ohm’s law.9 Ohm’s law is expressed as voltage/current = resistance. Rather than DC, because alternating current (AC) is typically used in electrical measurements and expressed in sine wave form, Ohm’s law is changed to voltage/current = impedance in AC. Observation that the ratio between two electrical impedances decreases as the file tip approaches the apical foramen led to the development of the ratio method for WL determination.