The physical properties of nickel titanium endodontic files that are heat treated for enhancing flexibility differ so widely that definite correlations are difficult to determine. This is particularly true when there are no controls for heat treated files having the same design dimensions. D&S Dental, LLC provided identical files having different degrees of heat treatment for testing and determining correlations. One group of files had no heat treatment (Group A), one group had medium heat treatment (Group B) and one group had greatest heat treatment (Group C). Each group was tested for rotation until failure (suspended at 3mm from tip), peak torque at failure, and force required for 5mm deflection on a 60 degree inclined plane to measure tip flexibility. Group A, no treatment, values were used as base lines for comparisons.The results are as follows:
|Degrees Rotation to Failure
|Peak Torque at Failure
|Force on 60 Degree Incline
Although there is an approximate direct correlation for peak torque at failure and force for 60 degree incline flexibility, the degree of rotation to failure appears to be an aberration of expectation. Within the scope of this limited research, we can conclude that heat treatment applied to enhance flexibility will accompany a decrease in resistance in torsional stress.
In short, progressive flexibility means the flexibility increases as the need for flexibility increases. This is in contrast to the all or nothing approach of other brands. Flexibility of a file can be increased by the following means:
- Increase the depth of grooves. However, increasing the depth of flutes can make the instrument more vulnerable to torsional failure, particularly if applied to all file sizes.
- Increase the number of spirals. Yet, increasing the number of spirals has two consequences: the file has a greater tendency to screw-in, and debris has a greater distance to travel causing more abrasion and torsional stress.
- Apply heat treatment. However, heat treatment usually means decreasing the file’s resistance to torsional stress. Some manufacturers claim that their heat treatment process actually enhances or has no effect on the file’s resistance to torsion stress. These claims are usually justified by measuring only the number of turns of rotation a file can be twisted before failure without consideration for peak torsion (greatest amount) the file can resist.
For virtually all cases, increasing flexibility decreases a file’s resistance to torsional failure. On the other hand, once the file has adequate resistance to torsional failure due to size, greater cross-sectional mass or flute design, the file’s flexibility can be increased by any or all of the means listed above progressively to enhance the file’s performance. The objective of the One Endo file series is to balance the need for flexibility with the need for torsional resistance. You will notice that One Endo files size 35 and larger are heat treated. The exception is size 35 .04/.06 in which its flexibility is increased by increasing flute depth. File size 55 .04/06 is heat treated to a greater degree to provide the needed flexibility for this large size.