Claims of strength for endo files: Hype or Fact

Creating the most useful file requires balancing properties of flexibility, resistance to torsion stresses, cutting ability, and debris removal.

Creating the most useful file requires careful balancing of properties.

Is there a Relationship between Cyclic Fatigue and Strength?

The short answer is no. Metal fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic or regular repetitious loadings such as a file rotating around a curvature during which each side of the file is alternately undergoing tension and compression forces. Strength, on the other hand, can best be explained as a file’s ability to resist or withstand an applied load without being deformed from its original shape. If a file is heat treated to increase its ability to resist fatigue yet it unwinds more easily, its strength is actually reduced and the claim for having more strength is erroneous. Further defined, the yield strength of a file is the stress at which the file begins to show plastic deformation. The ultimate strength of a file is the maximum stress and deformation a file can endure without breaking.

A more useful term for an endodontic file might be the measure of its toughness.  In order to be tough, a material must be both strong and ductile. For example, brittle materials (like ceramics) that are strong but with limited ductility are not tough. Conversely, very ductile materials with low strengths are also not tough. To be tough, a material should withstand both high stresses and high strains. Generally speaking, strength indicates how much force the material can support, while toughness indicates how much energy a material can absorb before breaking. The key consideration for designing the most useful endodontic file is balance.

The objective of NanoEndo is to maximize efficiency and minimize risks. That requires balancing properties of flexibility, resistance to torsion stresses, cutting ability and debris removal. You might think of these properties as members of a team; not one can stand alone, no matter how superior, without the support of the others and expect to be effective.

Does the quality of manufacturing make much difference for endo files?

SEM courtesy Nova Southeastern University College of Dental Medicine

Before different types of files are studied, it should be stressed that the quality of manufacturing is the most basic consideration for determining the success or failure of endodontic files independent of their composition or design. Less than ideal manufacturing quality controls result in the formation of micro-cracks and defects along the surface of a file. Cracks can propagate to failure at a stress level lower than the stress ordinarily encountered during instrumentation and other defects can cause stress concentration points that lead to file failure and jeopardize endodontic success. It should be noted; considerably less force is required to propagate a crack than is required to form it. If the defects are in a position of high stress, failure can occur quickly. The area of highest stress is along the blade or leading edge. The formation of micro-cracks shown on the file’s cutting edge indicated by the arrow is an area for potential failure.

First Reported Broken One Endo File

 

NanoEndo appreciates feedback from our customers and is committed to excellence in manufacturing and customer service.

NanoEndo appreciates feedback from our customers and is committed to excellence in manufacturing and customer service.

We received our first report of a broken One Endo file. The file was broken at its maximum diameter, a position that is most resistant to torsion failure. On the other hand, this location is most susceptible to fatigue failure due to its greatest diameter that also has a stress concentration point where the grinding wheel leaves an angle as it is lifted from the shaft at the end of its grinding cycle. Since torsion failure would be very unlikely to occur at maximum diameter, our first thought was that the breakage was due to a bending force being applied because of insufficient mouth opening access. The operator assured us, however, no bending forces were applied. That leaves us only to speculate. The other choices are a void or, more likely, an inclusion in the raw wire stock, both being rare occurrences. The only other contributing factor to consider would be a “burn” spot from grinding, also a rarity. At this point the cause remains undetermined. We appreciate the clinician taking the time an effort to keep us informed. NanoEndo is committed to using and conveying information to enhance the quality of our instruments and to expand knowledge and understanding for instrumentation.

How to Interpret Graphs from NanoEndo’s Endo File Evaluator

View the test at http://nanoendo.com/evaluator-v2.php

View the test at http://nanoendo.com/evaluator-v2.php

What do the graphs we display provide to us as beneficial information that can be used to enhance our instrumentation expertise? Generally speaking, as long as the plotted coordinates remain within the grid boundaries, the file is suitable to be used with the parameters specified in a canal with identical dimensions and properties if the file did not distort, break or excessively screw- in. That may sound like too many limitations to consider, but the trial run in practice blocks offer surprisingly relevant information clinically.

As an example, let’s take a look at the graph above for the Edge file 20/06. First we note that the tip distorted, although positive coordinates (points above base line zero) do not seem alarmingly high. We only have to consider positive measurements for torque because this is this is the only stress for torsion that will cause distortion. Negative pressure, on the other hand, indicates the screwing-in factor. Negative readings exceeding -2lbs should be a matter of concern (note the reading at 11:46:06 where the measurement is more negative than -2). Remember in this situation the device automatically reverses after 1mm of advancement. Manual reflexes might not be as quick since screwing-in forces are more insidious. Note also, the spike in negative pressure was accompanied with a spike in torsion and both occurred early in the cycle. It is at this point that the tip probably distorted.

What does this information tell us if we are to use this file? It means that we should be careful to limit the progress of advancement into the canal with very short pecking motions and to exercise caution with each insertion to counteract any sudden screwing-in forces. In addition, we should check for distortion frequently.

Steps of Endodontic Instrumentation Technique

Many endodontists have spent a great deal of time deciding on the file sequence that serves them best and would prefer not to have to learn a new technique. For those, we would recommend no technique change since the One Endo file was designed to give better results with any technique simply by using One Endo files approximating the sizes of the files of your technique.

On the other hand, we have had requests for specific technique recommendations of how we would recommend the One Endo file should be used. Practitioners should keep in mind the purpose of any recommendation is to provide a means for maximizing efficiency for enlarging the canal space while minimizing instrument failure—not to advocate a particular canal size or taper.The final canal dimensions should be adjusted to conform to the judgment of the operator and the requirements of the obturation technique used.

1 – No Change:

The One Endo file was designed to be used without changing the operator’s preferred technique. The fact is, if the One Endo file performs better than other files of the same approximate size, they can be used exactly in the same way  and achieve better results.

2 – Same Technique with added Parameters:

This step requires no technique change but does make the following recommendations during the procedure:

•Advance into the canal using no more pressure than was required to advance the first 1mm. If more pressure is required, change files.
•Engage no more than 6mm of a file if it is engaged in a curvature (the exception would be a size 20-.02 or smaller). If engagement is greater than 6mm, change files.
•Advance a file into the canal with no more than 1mm increments with insertion/withdrawal motions.
•Advancement into the canal should be able to occur at a rate of approximately 0.5mm/s without increasing the pressure of insertion.
•Withdrawing at a rate greater than the rate of insertion improves debris removal.
•Follow the use of one file with a file having a different taper.

Even though more files may be used in following these parameters, the total cumulative time can be less when compared to other techniques. Each step of instrumentation can occur quickly without repetitive non-productive attempts.

3 – Technique Change:

We recommend a technique change to incorporate the findings of our research to maximize efficiency while avoiding probable risks. We refer to this technique as the zone technique in which we divide the canal into two zones, the coronal zone or the portion of the canal coronal to a curvature, and the apical zone or the portion of the canal apical to a curvature.The zone technique was designed with two objectives for minimizing file stress: One, the canal diameter should be large enough coronal to a curvature to prevent any stress due to engagement in that portion of the canal when any file is being used apical to the curvature. Two, the file diameter is not too large to rotate safely in a curvature; the file diameter is sufficiently small in a curvature to avoid excessive cyclic stress.

ZoneExamples

The first step is to determine if there is a curvature of any significance and how far the curvature is from the apex. Withdrawing the file used to establish the working length, and passively re-inserting will indicate a curvature if it meets any resistance short of the working length since the canal is now larger than the file. The canal portion short of the resistance defines the coronal zone and the portion beyond the resistance defines the apical zone. The length of the canal to the curvature, the coronal zone, is measured and recorded with the same importance as determining the working length. The working length minus the coronal zone length provides the distance the curvature is from the apex, the apical zone length.

The second step is to determine the distance each of the files having different sizes and tapers can safely be advanced around the curvatures and which size file will need to be used in the coronal zone to prevent any subsequent file from binding in the apical zone. At the point of curvature in the, the file diameter should be no greater than .60mm for a .02 taper, .55mm for .04 taper, .50mm for .06 taper, and .35mm for a .08 taper. (This consideration is the result of testing for 45-degree curvatures having 8 mm radii and applies only to these dimensions for rotary NiTi files. File diameters should be smaller for more severe curvatures and can be adjusted larger for less severe ones.) By using the parameters suggested above for diameter limitations, we can calculate if the diameter of a selected file would exceed our limitations.

Example: If the point of curvature is 6mm from the working length and we are considering using a .25/.06 file. The parameters above state that a .06 tapered file should not have a diameter greater than .50mm at the point of curvature and that diameter for a .25/.06 file is at 4mm from its tip. Therefore, in this situation the .25/.06 should only be advanced 4mm into the apical zone and if it is carried to that point, the terminus of the coronal zone should previously be prepared to a size .50mm. Following this procedure at first may seem complicated but it can be learned quickly. The result is a very expeditious and relatively worry free of threat of breakage.