Page 18 - The Beauty and Sorrow in Endodontics (WL and C/S)

apical tissue, the circuit is completed, and the apex locator calibrates the measurement to

0 at this location. As the file move back coronally, the machine will then calculate the
distance from the tip of the file to the apex using a set of algorithm, which in reality, is

still just an estimate. Furthermore, some operators may be hesitant to peek the file

through the apex during measurement and stops when the apex locator signals that
0.5mm from the apex is reached; this result is highly inaccurate as the apex locator had

not been calibrated yet. Therefore, Dr. Schilder recommended the use of radiographic

terminus as the reference point for working length for the following benefits: 1. It
eliminates the potential discrepancies in value from elongation / foreshortening of the

films; and 2. No matter which angle the radiographs are taken at, the point at which the
file goes beyond the radiographic terminus will always be the same.

c. Last WL; lWL:

Generally speaking, lWL is only used after cleaning and shaping for verification of

the master GP cone during cone fit try-in. It is a variable number as the value can change
until a desirable cone fit radiograph is obtained. The only rule is that the lWL will always

be shorter than the iWL as canals will straighten up as they are shaped.

The term “lWL” is defined as the WL identified with the largest file in a series used

to clean and shape the apical canal (even if said file is only used for size verification or as

the master apical file). lWL is used to verify the length of the master cone prior to any
cone fit radiographs. Once the master cone reaches the WL with adequate tugback, a plier

is used to mark the outer reference point on the master cone. The cone is then removed

from the canal and placed side by side with the master apical file for comparison (Fig
A22), and if their lengths are reasonably similar, then the cone may now be placed back

into the canal for a cone fit radiograph to be taken. If the lengths marked on the file and
cone are obviously different, then the operator needs to consider the following

possibilities: either 1. the apical cross section diameter is different between the file and
the cone; or 2. the tugback felt was not from the resistance in the apex, but from the cone

binding to the wall in the coronal or middle thirds of the canal.

Since the horizontal section of the canal is never a perfect circle even after cleaning

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