It is generally accepted that root

It is generally accepted that root canal treatment
procedures should be confined within the root canal
system. To achieve this objective the canal terminus
must be detected accurately during canal preparation
and precise control of working length during the process
must be maintained. Several techniques have been used
for determining the apical canal terminus including
electronic methods. However, the fundamental electronic
operating principles and classification of the
electronic devices used in this method are often
unknown and a matter of controversy. The basic
assumption with all electronic length measuring devices
is that human tissues have certain characteristics that
can be modelled by a combination of electrical components.
Therefore, by measuring the electrical properties
of the model, such as resistance and impedance, it
should be possible to detect the canal terminus. The root
canal system is surrounded by dentine and cementum
that are insulators to electrical current. At the minor
apical foramen, however, there is a small hole in which
conductive materials within the canal space (tissue,
fluid) are electrically connected to the periodontal
ligament that is itself a conductor of electric current.
Thus, dentine, along with tissue and fluid inside the
canal, forms a resistor, the value of which depends on
their dimensions, and their inherent resistivity. When
an endodontic file penetrates inside the canal and
approaches the minor apical foramen, the resistance
between the endodontic file and the foramen decreases,
because the effective length of the resistive material
(dentine, tissue, fluid) decreases. As well as resistive
properties, the structure of the tooth root has capacitive
characteristics. Therefore, various electronic methods
have been developed that use a variety of other
principles to detect the canal terminus. Whilst the
simplest devices measure resistance, other devices
measure impedance using either high frequency, two
frequencies, or multiple frequencies. In addition, some
systems use low frequency oscillation and/or a voltage
gradient method to detect the canal terminus. The aim
of this review was to clarify the fundamental operating
principles of the different types of electronic systems that
claim to measure canal length.