Allan Deutsch

Figure 1: An ISO #25 stainless steel instrument, showing the position of the flat. 

What the Flat Does

THIS FLAT does several good things: it makes the instrument easier to use, prevents the accumulation of dentin debris, reduces stress on the instruments, and increases the flexibility of the instruments.

1. Because the flat reduces the amount of the instrument’s cutting surface in contact with the canal wall, less dentin is engaged during each cutting stroke.  This decrease in the engaged area results in a slightly less efficient instrument but one that is easier to use in the canal because it is not fully engaging the canal wall circumferentially.  You may have to turn the instrument a few extra turns to completely instrument the canal, but turning it within the canal is much easier now because the entire instrument is no longer cutting the root.  You will notice much less hand fatigue. 
2. The flat gives the dentin debris that is generated during cutting someplace to go.  The debris accumulates in the space between the flat and the canal wall. Therefore, the debris does not wedge between the instrument and the canal wall making the instrument more difficult to turn in the canal.  As the instrument is turned, the debris falls into the space created by the flat, and you do not have to work against the accumulated debris.  Because less dentin is being cut at any one time and the debris has someplace to go, you will find that the clinical effect is that it feels about 25 percent easier to instrument the canal using the SafeSiders. 
3. Because less of the instrument is cutting at any one time, less stress is placed on the instruments.  Lowering the stress lowers the chance of instrument breakage, and consequently the instruments last longer.  They do not have to be replaced as often as conventional instruments. 
4. The flat is not cut deeply into the core of the metal of the instrument, so it increases the flexibility of the instruments without sacrificing strength.  The flat removes some metal from the length of the cutting edge to the tip, resulting in more flexibility, but the durability and strength of the instrument are maintained. 

FIGURES 2 AND 3 show the configuration of the two NiTi instruments in the series.  The Orange 30/.04 is an ISO standardized #30 at the apex with a .04 taper up the shaft.  The flat can easily be seen to extend the entire length of the cutting surface to the tip.  The other NiTi instrument is the Brown 25/.08.  It is an ISO standardized #25 at the apex with a .08 taper up the shaft.  This is the last instrument in the sequence; once the canal is prepared with this instrument it is ready to be obturated with a medium gutta-percha point.  Once again we can see the flat extending the entire length of the cutting edge of the shaft.

Figure 2: Note the flat extending the entire length of the cutting edge of the NiTi 30/.04 instrument.

Figure 3: Note the flat extending the entire length of the cutting edge of the NiTi 25/.08 instrument.

ALL THE INSTRUMENTS in the sequence are meant to be used with a circular cutting stroke. An up-and-down filing stroke is not the way to go!  Use a “wrist-watch-winding motion” while applying slight apical pressure.  During the clockwise winding motion, a point will come when you feel resistance from the flutes of the instrument cutting into the dentin.  At that point, go to a counterclockwise motion.  When you now rotate the instrument counterclockwise, you will cleave off the dentin that was engaged between the flutes, and the instrument usually will move about 0.5 mm closer to the apex. 
    Remove the instrument from the canal, clean off any debris, and inspect the instrument for any deformation.  If the instrument looks fine, repeat the cutting procedure until the instrument goes to the desired length in the canal.  When the instrument is spinning freely (in a clockwise direction) and does not engage the dentin, it is time to move to the next size instrument in the sequence. 
    Always cut wet!  It will make instrumentation much easier.  Keep the canal flooded with irrigant at all times.  When the canal is dry, you will find that binding, ledging,  and other bad things occur far too easily. 
    Remember, there is a learning curve to all new techniques.  After completing four or five anterior cases, you will be surprised at how much easier endo will have become.