June 2, 1994...Butte, Montana
Visualize yourself scrubbing the inside of a tapered beer glass with straight bottle brush.
Each stroke of the brush is straight in and straight out but in a different place against the circumference of the glass.
The root canal is tapered and the only access you have to it is from the top, like the glass.
The instruments are essentially the same as the bottle brush.
The motion is essentially the same.
This motion is known as linear oscillation.
The length of the stroke must be at least as long as the distance between the flutes.
Or to put it in technical language the amplitude of the stroke must be greater than the inter-flute distance.
With Fine-Cut Files those flutes are very close together and so the up and down stroke can be very small.
Almost not a stroke at all but more of a jiggle.
I have often said that it helps if you drink a lot of coffee.
Your hand shakes just the right amount.
With either the STS system or the Fine-Cut Endo Handpiece this stroke is minimal...usually about a millimeter.
Theoretically when a file has been pre-curved, the up and down motion should be a curved up and down motion, but is a little silly talking about an arc of less than a millimeter and so we just say jiggle up and down.
But, because the stroke is so small, it is almost irrelevant to talk about doing anything different in curved canals than in straight canals.
You must understand that most sonic and ultrasonic systems do not and cannot move the file up and down in the canal.
In fact most move the file in a waggling motion, that is, the tip is moved back and forth, not in and out.
If, in fact, the ultrasonic had any cutting action at all it would be dangerous because the waggling would cause a ledge to form at the tip of the file.
The only function of most of these units is to keep the file cutting smoothly like a vibrating sander keeps the sanding block from skipping and gouging.
The cutting must be done by YOU moving the file in and out of the canal.
It is essential for circumferential filing for the file to fit loosely in the canal.
As each circumnavigation of the canal is made, the canal gets slightly larger.
When it is large enough for the next file to fit loosely, the next larger file is used.
And thus through the sizes until either the canal is clean enough or the canal is large enough.
Each file is used in each canal for about 20-30 seconds depending on:
Thus usually a canal can be enlarged with a total time of less than 3-4 minutes.
And since in a molar the proportion of time devoted to changing files is less, the average per canal is slightly less.
Usually it takes about 5-10 minutes to prepare a molar.
With the STS or the Fine-Cut Endo Handpiece the frequency of the stroke can exceed 350 per second(22,000 rpm), this process is materially accelerated.
You should note that it is almost impossible to do circumferential filing with
Since you can only apply as much pressure as the strength and flexibility of the file allow, you must have a positive rake instrument which cuts with the slightest touch.
You cannot apply enough pressure laterally to get a negative rake instrument to cut.
They must be wedged in, such as in PQTP or Balanced Force Filing, which produce lateral force as a function of the force vectors of the downward pressure.
One of the advantages of circumferential filing is that is causes NO LEDGES.
Ledges can only be caused by an end cutting instrument that cuts inward from the crown down to the apex.
Since in circumferential filing the file is passed to depth with no resistance and the cutting is upward, it will in fact erase any ledges it might encounter, not create them.
The end result of a canal prepared with circumferential filing is a canal that is hyperbolic in shape, that is the angle between the walls gets greater as you get further from the apex i.e.
This is the ideal shape for condensing gutta percha and also incidentally for filling with cement...that of a gradually decreasing diameter going toward the apex.
In the electronic web page course Automated Endo, there is a description and an animated GIF that explains this.
In fact it explains two different approaches to filing, named "X"-filing
One of the unique features of circumferential filing is that it maintains the basic shape of the canal.
That is, if the canal is round it keeps it round, just a little larger.
If it is ovoid, ribbon-shaped or kidney shaped the canal ends up bigger but still ovoid, ribbon-shaped or kidney shaped.
This is of absolutely no concern to those of us who fill with cement because the cement always assumes the shape of the canal at the extrusion point of the syringe.
If you are planning on filling with gutta percha and the canal is anything but round, the smaller dimension of the canal may be too small to get your pluggers in and it may be necessary to use smaller pluggers or to over-enlarge the canals to accommodate the larger pluggers to avoid splitting the root.
This problem does not occur in syringe endodontics.
(see also Crown-Down Prepping) (see also orifice opening)
E-mail your comments to drjack@BetterEndo.com