Home' News Bulletin : ADA News Bulletin April 2016 Contents PRACTICE
The determination of working length (WL) is probably the most
important step in root canal treatment. Failure to determine the
correct working length may result in over-filling or under-filling and
has the potential to significantly increase the failure probability of
root canal treatment.1 Apart from that, a wrong WL can influence the
short term effectiveness of treatment, so WL determination should
be done in a very early phase of systematic endodontic treatment.
Even if there is a considerable controversy regarding the exact
termination point for root canal therapy, the most popular concept
is the apical constriction (AC), which is a narrowing in diameter
short of the apical foramen. In some studies less than 50% of roots
seem to have a true AC,2 which may be more a conceptual approach.
In teeth without an AC, a point closely short of the foramen is
recommended as the apical reference point. A precise WL is not
possible without a stable and reproducible coronal reference point.
Traditionally, the working length is determined using a radiological
approach developed by CE Kells in 1899 and perfected by J Ingle
in 1957. This method uses a reference instrument of known length
to mathematically eliminate projection errors and to calculate
the working length in relation to the radiological apex as the only
identifiable landmark in the radiograph. The calculated distance is
typically reduced by around 1 mm, as morphological studies have
shown this is the mean distance from root apex to AC.2
Additional techniques like a true parallel projection combined with
highly radiolucent reference instruments (e.g. silver points) without
the need of stoppers (Fig 1) can make this method very precise.
Gauging of reference instruments with loupes or microscope
together with modern digital X-ray technique and a computerised
implementation of Ingle´s algorithm (Fig 1) can produce a
measurement precision below 0.5 mm.
Another benefit is the radiograph itself, which is a perfect proof of
procedural accuracy and provides information regarding anatomical
details of the case.
Unfortunately, the radiograph does not show the location of the
foramen, which can be located 0-3 mm short of the root apex.2 In
some cases, the use of mean distances may result in severe over
Other traditional methods like tactile sensation or determination of
fluids using paper points are not precise enough as single methods
of length determination, but may contribute to a final clinical
synopsis of different methods in difficult cases.
ELECTRONIC APEX LOCATORS
Determining working length
Fig 1: Traditional radiographic measurement of working length. One
canal was analysed using Ingle´s algorithm implemented in the modern
radiographic imaging software Digora for Windows 2.7 [Soredex, Tuusala,
Finnland]. Silver cones without stoppers have been cut to precisely sit at the
coronal reference points. The entrance cavity was temporarily closed using
silicone material to allow for a precise parallel projection with film holder and
without instrument handles or a rubber dam clamp. The resulting measure of
22.17 mm (probably with false precision) was rounded to a 22.25 mm.
ELECTRONIC APEX LOCATORS
Electronic apex locators (EAL) are based on measurements of an
alternating current impedance between an electrode placed in the
root canal (typically an endodontic file) and the general pool of body
liquid presented through an electrode clip contacting the oral mucosa
('lip clip'). The change of impedance between the file and the lip clip
presents an 'S'-shaped curve as the file approaches the apical foramen,
especially when this measurement is done using different measurement
frequencies processed using special mathematical algorithms as in
modern generation EALs. This makes it possible to calibrate the device
to the position of the apical foramen. Most modern generation EALs are
also able to identify the position of the AC.
It should be noted, that the naming of EALs is incorrect, as they do
not detect the position of the apex, but the position of the foramen
and/or the position of the AC. To use customary clinical terminology,
the name 'EAL' will be kept in this text as a technical term.
36 | ADA NEWS BULLETIN | APRIL 2016
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