There is a general consensus that root canal procedures
should be limited within the confines of the root canal,
with the logical end-point for preparation and filling
being the narrowest part of the canal. It is not possible
to predictably detect the position of the apical constriction
clinically, indeed, the constriction is not uniformly
present, or may be irregular. Equally, it is not logical to
base the end-point of root canal procedures on an
arbitrary distance from the radiographic apex as the
position of the apical foramen is not related to the
‘apex’ of the root.
Electronic root canal length measuring devices offer
a means of locating the most appropriate end-point for
root canal procedures, albeit indirectly. The principle
behind most ERCLMDs is that human tissues have
certain characteristics that can be modelled by means
of a combination of electrical components. Then, by
measuring the electrical properties of the model (e.g.
resistance, impedance) it should be possible to detect
the canal terminus.
Thus, most modern ERCLMDs are capable of recording
the point where the tissues of the periodontal
ligament begin outside the root canal, and hence from
this a formula can be applied to ensure that preparation
is confined within the canal. Most reports suggest that
0.5 mm should be subtracted from the length of the file
at the point when the device suggests that the file tip is
in contact with the PDL (zero reading). This does not
mean that the constriction is located; rather it means
that the instrument is within the canal and close to the
PDL. It is not appropriate to rely on any device reading
0.5 mm short of the foramen as this will often be
inaccurate. The use of ‘generation X’ to describe and
classify these devices is unhelpful, unscientific and
perhaps best suited to marketing issues.