Electronic Apex Locators

By Gordon M, Chandler N

Date: 07/2004
Journal: JOE

Importance of working length:

•The proper point to which a root canal should be filled is the junction between the dentin and cementum and the pulp should be severed to this point where it bounds to the Periodontal membrane. (Grove 1930)

•Root canal treatment procedures aim to make use of this potential barrier between contents of the canal and apical tissues. (Schilder 1967)

•Most favorable histological conditions were when instrumentation and obturation remained short of the apical constriction. Extruded GP and sealer always caused severe inflammatory reaction despite the absence of pain. (Ricucci and Langeland 1998 – in vivo histological study) •Epidemiological studies have reported that the best prognosis is when root canal filling lies within 2 mm from radiographic apex. (Sjogren et al 1990)

Anatomy of the apical foramen:

•The anatomy of the apical foramen changes with age.

•The three distinct aspects that should be appreciated are as shown in Fig 1b (1-3)

•The foramen of the main root is offset from the tooth apex by up to 3 mm in 50-98% of the roots. (Dummer et al 1984)

•Foramen to anatomic apex offset mean in young groups was 0.48 mm and 0.60 for older groups (Kuttler 1955)

•Distance is 0.3 mm in anterior teeth and 0.43 in posterior teeth. (Green 1956)

•Greater in posterior and older teeth than in anterior and younger teeth. •Foramen to apical constriction distance mean is 0.5 mm in younger group and 0.8 mm in older group. (Kuttler 1955, Dummer at al 1984, Stein & Corocoran 1990)

•Pathological resorption was not reported to change the measurments. •Types of Apical Constriction Fig 2

Traditional methods of establishing working length:

1.Using anatomical averages

2.Tactile sensation

3.Moisture on paper point 4.Radiography

Limitations of traditional methods:

Tactile sensation:

Seidberg et al (1975) found that even among experienced clinicians only 60% were able to locats the constriction using tactile sensation

•Preflaring increased the ability to determine the constriction by tactile sensation up to 75% of the time. (Stabholz et al 1995)


•In radiographic method, dependence relies on radiographic apex. Apical foramen does not always coincide with the apex and if it  terminates at a lateral aspect or buccolingual aspect it cannot be detected on radiographs. •Overlapping of skeletal structures (zygomatic arch)

Histroy of electronic apex locators:

•First investigated by Custer in 1918, revisited by Suzuki in 1942

•Registered consistent values in electrical resistance between an instrument in a root canal and an electrode on the oral mucous membrane and speculated that this would measure the canal length

•(Czerw et al 1995 – in vivo study) used EAL to determine working length, cemented the file at the detected length, extracted the teeth then located the file under magnification.

•When extraction was not possible, radiographs were used to evaluate working length, and this introduced the problems that associate with radiographic method.

•4 generations of EAL present in the market

Other uses of EAL:

•Detecting cracks and perforations

•Detected simulated horizontal fractures but unreliable with simulated VRF

Problems associated with use of apex locators:

•Inaccurate readings due to presence of vital tissues, blood, exudate, metallic fillings, instrument in other canals

•Immature apices tend to give short measurements as the file does not touch apical dentin

Cardiac pacemakers: EAL could interfere with cardiac pacemakers, should not be used with such patients. However, Garofalo tried 5 third-generation devices and concluded that all except one did not interfere with cardiac pace-maker. Consultation with cardiologist is still advised before using.


•EAL reduced over-estimation of working length

•It is not advised to use EAL alone without RG confirmation

•EAL are over 90% accurate