- Reaming with Rotating Instruments
Any time an instrument is rotated in the confines of a canal and one portion of it gets stuck and the rotation continues torsion is created.
- Reaming by Hand
The advantage of hand reaming is that it is easier to determine when the torsion begins.
- Push Quarter-Turn Pull Filing
PQTP filing is nothing other than intermittent reaming coupled with a slight amount of augering at the end and thus is subject to the same torsional stresses as in hand or engine reaming.
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From the Design of the File
In many cases files will break even when no rotating is apparent. This stems from the fact that when blades engage and the file is pulled on, the force vectors create torsion in the blade. And since in most fixed helical angle files the helical angle at the tip is larger than at the shank, the greatest amount of torque is on the weakest part of the file.
Fine Cut files have almost twice as many cutting blades per millimeter and not incidentally have the lowest helical angle both at the tip and at the shank of any file available today.
When gravity exerts linear force on a skier and he is on a hill, the foce vectors cause horizontal force and the skier moves forward. Sitting on a flat surface, there is no horizontal vector.
Wire has tremendous tensile strength as witnessed by bridges handing on cables which are essentially strands of wire. Wire has very poor torsional strength.
Helical Angles and Torsion
The higher the angle the greater the torsion...in spades.
Any time axial load is applied to a file, it is divided into axial load and torsional load. The percentage that is torsional is a function of the tangent of an angle not the actual angle.
Tangents range from zero to infinity as the angle goes from zero to 90 degrees.
So if you double the helical angle of a file you more than double the torque component.
Osciallating only produces intermittant torsion and permits the file to come unstuck periodically and thus minimize the likelihood of fracture.