File Design and Why You Should Care

	File Design and Why You Should Care Most dentists tell me they have never looked at an endo file under magnification. The main reason seems to be that even if they did, they would not know what they were looking for. That is, they would not be able to relate what they are seeing to what effect it has on the use and usefulness of the file. It's sort of like looking for a tool to cut a tree branch and picking up a hammer. Sure it would work, eventually, but a saw would work better. There are differences between one file and another as profound as the difference between a saw and a hammer. I will attempt to explain these differences here. And to do that I will attempt to 'dissect' an endodontic file for you. Take it apart so to speak and look at the individual parts.
	Interflute distance - the physical distance from one flute to another Simple enough? Not so. The assumption here is that there is only one set of flutes on a file. Some files have one, two or even three sets of flutes intertwined, sort of a double or triple helix, to coin a phrase. This is very obvious on a file where the intertwined flutes are of different design as in the K-Flex file on the left in the photo. The importance of double or triple flutes comes in when using the interflute distance to calculate the helical angle. It is in fact possible to calculate the helical angle of a file by looking at a photo of it.
	The helical angle is in effect the angle of a right triangle whose height is the interflute distance and whose base is the circumference of a cylinder around which it is wrapped. Thus by... ...measuring the interflute distance and the diameter and ...deriving the circumference by multiplying by pi, ...dividing one by the other, and ...looking up the ratio on a table of tangents ...we can determine to a high degree of accuracy, the helical angle of a file by simply looking at it's picture.
	Helical angle - the angle between a tangent laid on a blade and a right angle to the central axis of the file. The implications of the helical angle affect how you may use a file and whether it breaks under a particular kind of use. If a file is rotated and the helical angle is steep, it probably will not dig in and thus probably not break. Therefore a low helical angle is not only not good but definitely bad when the file is rotated because will dig in very quickly, i.e. with not much of a rotation and will twist itself off very quickly.
	A problem enters when there are multiple intertwined helixes (helices??). In these cases it is necessary to skip every other flute when measuring the height of the triangle. The implication of a double helical design should now be apparent. That is all other things being equal, if there are two helixes where the interflute distance is the same, then the helical angle must be greater, as compared to a file with a single helix. If you have an interflute distance of one unit.
	And then compare to what you would have it you were to attempt to put one flute of double the interflute distance in the same area
	and then to put two of double the interflute distance overlapping so that the flute frequency is the same as if the interflute distance was smaller

	you can see that in order to have the same flute frequency with a double flute you would have to increase the helical angle. The point is that it can be confusing to determine the actual interflute distance when there is a double or even triple helix design. If a file is going to be used for circumferential filing, it is desirable to have the lowest helical angle as possible because this produces less torque when the blade engages dentin. If the file is going to be used for probing is is essential. The only way a low helical angle can be accomplished is to have a single fluted design.
	Rake Angle - the angle between the cutting edge of a file and a perpendicular to the surface being cut. Negative Rake Positive Rake
	Included Tip Angle - the angle between two surfaces at the tip If this angle is too steep the tip tends to dig in If this angle is too flat, the edges of the file tend to gouge in and not negotiate the curve. If the outside surface of the blade rides the outside of the curve the file will slide around the curve.
	The Depth of the Flute - the distance between the cutting edge and the bottom of the flute. The deeper the cut of the flute, the weaker the metal is when stressed. If the interflute distance is great then the depth must be greater. If the interflute distance is small then the depth can be shallower and thus more metal in the core of the file.
	The Taper - the angle between two lines laid against a row of flutes and a center line of the file. If one file of the same taper follows another, then all surfaces of the file contact simultaneously but at a lesser depth. If on the other hand, a larger taper is used, the file will touch only at the entrance of the prep. Then as the file is worked apically more and more of the file will touch and only at the bottom will the tapers completely match. Until that point, the tip of the file is not stressed. This is not, however a strategy for deepening a prep, only for widening it. It minimizes the cutting at the depth and thus the canal assumes a wider and wider taper.
	The Material it is made From - i.e. the flexibility or strength of the metal.
	The blade length is therefore the hypotenuse of a right triangle, whose height is the interflute distance and whose base is the circumference of the file at the shank end of the flute. The only reason the blade length might be important is that it is one of the factors in determining the helical angle based on a photograph or a microscope view of a file. Not only is the helical angle of a file critical to the likelihood that it will twist off in use but it is also very easy to determine. Simply measure the interflute distance, keeping in mind that some files have a double helix and therefore you'd have to measure from on flute to the second next flute. These measurements, since they are relative measurements can just as well be done on a photograph of a file as on the file itself.
	The Interflute distance will be the height of our right angle triangle. Next measure the diameter which of course multiplied by PI becomes the circumference of a file and in our process here is the base of our right triangle. Then to determine the helical angle, that is the angle between the hypotenuse( flute ) and the circumference (file diameter times pi) Divide the interflute distance by the circumference and looking up the angle on a table of tangents, available in any trig or machinists handbook. The steeper the angle the greater the torque when a file gets stuck and is pulled out. This is proportional to the tangent of the angle, not the angle itself.