Page 27 - The Beauty and Sorrow in Endodontics (WL and C/S)

developed for dentists to enlarge and shape the irregular coronal and middle thirds first

before exploring the apical segment.
Benefits to Crown-Down Concept
1. Once the obstructions from the coronal and middle thirds are removed, cleaning and
shaping of the canal becomes much easier;
2. With the coronal two-thirds enlarged, irrigants can penetrate more easily into the

apical portion and disinfect it;
3. Once the coronal and middle thirds have been enlarged, the operator’s tactile sensation
becomes sharper when exploring the canal terminus with a file;

4. It is easy to shove debris out the canal terminus if you push the file to the RT without
opening the coronal two-thirds first. If the coronal and middle thirds have been
cleaned first, less toxic material will be pushed out the apex even with small files
peeking out later. This reduces the risk of post-operative pain.

Determine the Ideal Apical Taper

Generally, as long as there is adequate root thickness, a final apical taper of 0.06 or

more (up to 0.10) is recommended. This is in line with the traditional step-back technique.
For example, if the first file that binds to the apex is an ISO #10 file that reaches the
working length at 20mm, it was recommended that the apex be enlarged to three sizes
larger than the initial file (in this case, ISO #25 at 20mm). From this point on, each
subsequent larger file will have a working length that is 1mm shorter than (thus ending

1mm coronal to) the previous file (eg. #30 19mm, #35 18mm, #40 17mm), this, by
definition, results in a taper of 0.05. Then, after every set of three files used, the irregular
“steps” on the canal wall will then be smoothened, resulting is a slightly larger taper

closer to 0.06. If, however, each larger file is used every 0.5mm coronal to the previous
file instead (eg. #30 19.5mm, #35 19mm, #40 18.5mm), then the canal would have > 0.10
taper once shaping is completed.

The Theory Behind the Envelope of Motion

1. It is the action of simultaneous rotation (360° turn) and translation (coronal
withdrawal movement) of the file.

2. The files are pre-curved for more efficient contact to various point on the canal wall.
The simultaneous rotation and translation motions also creates a vortex of the

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