Recently I was asked this question by a reader. The answer is primarily dependent on the brand of rotary nickel titanium (RNT) file used, taper, tip size, how many times it has been used previously and under what conditions. Obviously, clinicians want to obtain the greatest benefit from the given RNT while not having to purchase more RNT than necessary. Strategies for determining the optimal number of uses will be discussed and specific recommendations made.
Every rotation of a RNT is one rotation closer to the fracture of the file. In other terms, there is a finite number of insertions and rotations for any RNT file. At best, clinicians can make empirical estimates of how many insertions and rotations a given file can endure before it should be discarded. The endodontic literature for the most part advocates RNT files as disposable instruments, a position with which I do not agree. While at a theoretical level this may make sense, as it is the safest recommendation, it is not practical due to the cost of the files and their ability to be sterilized.
Complicating matters, how much clinical life a given RNT has left is empirical. A given insertion into one canal might not be equal to another insertion with regard to the time the file is used, the torque applied to it, the type of dentin being cut, the number of rotations, the effect of curvature, etc. In other words, file wear is very subjective, especially before the file shows some obvious defect or deformation.
This said, irrespective of the given file tip size or taper, if the file shows any wear or deformation, it should be discarded immediately. Having a magnifying glass with an enhanced light source built in where the assistant or doctor can carefully evaluate the file for deformation can very quickly pay for itself as the clinician can eliminate the files which need to be thrown away early and save those that have more useful life. One additional useful side benefit to using the surgical operating microscope is that the file can be visualized clinically as it is inserted into the canal and as it exits. If it is kinked or deformed pre- or post-use, the deformation can be observed and the file discarded.
Strategies to maximize the amount of life of a RNT include:
1) utilizing a glide path, i.e. getting the canal to at least a size 15 hand file before a RNT is placed into the canal;
2) minimizing engagement of the RNT to 1-2 mm per insertion into the canal as opposed to engaging significant amounts of the canal wall, i.e. 8-10 mm at a time;
3) use of a light, gentle and touch in insertion of the files, done from the fingers and not from the forearm;
4) copious irrigation during all stages of treatment with recapitulation to make sure that the canal path is open and patent;
5) crown down endodontics where the coronal third is instrumented first, the middle third second and the apical third is prepared last as doing so reduces the amount of torque placed on the files as instrumentation progresses.
A file should be considered for disposable when (aside from the obvious defects mentioned above):
1) the RNT file has been used in a significant curvature and comes out of the canal with any form of curve (even without deformation) imprinted upon it;
2) the particular dentin does not cut easily and insertion, despite the existence of an excellent glide path, is labored. This lack of ideal cutting can be due to dulling of the file or it could be due to the nature of the dentin, in either clinical case, the file should be discarded.
3) If orifice openers are placed into an orifice or curvature that makes an acute bend, it is relatively easy to fracture the tip, especially if the file is inserted with greater than necessary force. Such files are often best used once.
In my empirical opinion, the optimal RNT file on the market now is the K3 RNT from SybronEndo, Orange, CA, USA. The file cuts very well, has an excellent tactile feel, is difficult to fracture if used properly and is more than sufficiently flexible to negotiate even the most challenging curvatures.
Excellent endodontic treatment can be performed with many different files and file designs, this said, the user friendliness of the K3 with regard to the above considerations has an additional advantage, extended useful life. I use K3 on average in approximately 5 molar teeth, this could be 20 or more canals.
Some files might be used more and some less, but overall, this is my experience with K3. While I have extensive clinical experience with a number of other RNT files, this is not a recommendation I can make for all other brands. One reason this is possible is the fact that K3 has uneven flute widths and flute depths and as such has more metal at its tip than files of a U shape cross sectional design. This metal gives the K3 greater resistance to torque failure at its tip, an attribute not shared by all brands.
In addition, in the most general terms, smaller files resist torque less well than their larger counterparts, but they resists flexural failure better. This must be borne in mind when making blanket recommendations about how many times a RNT can be used. .02 tapered files will have less useful life than the larger tapers. .04 and .06 tapered files in the smaller tip sizes 15 and 20 will have less uses than their larger counterparts.
Finally, it could be asked, why not just sort the files by numbers of uses and arbitrarily discard them after a certain predetermined number of canals. With another brand, this precaution might be advisable; with K3, it is simply not necessary. Such limits are wholly arbitrary and for K3 use, simply unnecessary.
I welcome your questions and feedback.
Dr. Mounce lectures globally and is widely published. He is in private practice in endodontics in Portland, Oregon, USA. Amongst other appointments, he is the endodontic consultant for the Belau National Hospital Dental Clinic in the Republic of Palau. Korror, Palau (Micronesia). Dr. Mounce can be contacted at Lineker@aol.com.
