Normally when circumferential filing is done it is thought that the canal essentially ends up conical.
And that is essentially true in theory.
Theory, however, fails to explain how some canals end up with a trumpet or hyperbolic shape.
What really happens is that varying amounts of pressure are applied by the operator at various points on the file and due to varying stiffnesses along the shaft of the file, varying amounts of dentin are cut.
It is easy to see that in a conical file, the greatest amount of pressure is applied by the thicker, less flexible part of the file, i.e. coronally.
And less pressure is applied apically.
Thus results in the trumpet or hyperbolic shape.
Normally the operator moves the file in an apical-coronal direction and applies lateral pressure.
As the coronal part gets larger the pressure bends the file and the hyperbolic shaping is exaggerated.
When the operator switches to a larger file, this tendency is less pronounced and the canal walls get straighter and the canal more nearly conical in shape.
In effect, the center of rotation of the file on the z-axis is the tip of the file. Thus in cross section, the extremes of the file position laterally form a "V" shape.
In theory, with circumferential filing, a canal can be enlarged to any size with any file that will fit initially by simply continuing the filing until the canal is the desired size.
The reason for using a progression of file sizes is because larger files cut more per stroke than do smaller files. This in turn increases efficiency and saves effort.
But there is another, less obvious reason for progressing through the sizes: larger files tend to produce a more conical canal.
Furthermore. smaller files tend to drop into the same vertical grooves and thus deepen them.
The result is a canal that in cross section is amoeba shaped.
Now that we are using the Fine-Cut Endo Handpiece system, to move the file up and down over 300 or more strokes per SECOND, there is little reason from an efficiency standpoint to go to the larger files, thus accentuating the amoeba shape and the hyperbolic shape.
Thus we have devised a simple strategy for eliminating the hyperbolic shape.
The answer is simply to make the middle point of the file, the z-axis point of rotation.
This, in cross-section, makes an hourglass or an "X" shape.
Thus this motion has been dubbed, for want of a better term,
"X-Filing" (Pun intended)
This causes the file to cut more at the tip than at the shank and enlarges the canal more at the apical end.
Very frequently, if this is not done, the next file seems not to fit because the apical portion has not been sufficiently enlarged.
The recommended technique is to move the file in a "V" motion, i.e. "V-filing" for about 20 seconds and then "X-Filing" for another 20 seconds or so.
When a hand file of the desired size fits to the depth filed to, then the canal is sufficiently enlarged and we are done with circumferential filing.
