Richard Mounce, DDS
Recently, I heard an interesting tale of misplaced frugality. A clinician was working in an office where the owner had collected all of the rotary nickel titanium (RNT) files which had separated. He kept them for future use in a box labeled "Shorties." Apparently, when a situation called for it, this doctor would ask his assistant to go "get a shorty," and then he would use these broken rotary files again. These files were reused in an obvious effort to save a few dollars in purchasing files.
When I give lectures, one of the most frequent questions I am asked is how many times a given RNT file can be used in order to keep costs down.
How many times can a RNT endodontic file be used and how do you know when to discard it? How many times a file can be used is a function of which particular file system the clinician is using. I am a strong advocate of the K3 rotary-nickel titanium file system by SybronEndo for its cutting ability, excellent tactile feel, and fracture resistance. In general, the guidelines that follow for how many times a K3 can be used do apply to other file systems, but certainly caution is advised.
RNT files were always intended to be disposable. There is no proven literature-based standard which states how long any given brand of file should be used before discarding. Trying to get every last rotation out of a RNT file is a misplaced economy. File fracture very quickly takes the profit out of the case, especially if it has to be referred for removal.
When a file exhibits deformation of any kind, it must be discarded. To use the file further is to risk instant fracture without warning. Engagement of RNT files should be minimal (1-2 mm per insertion), with a gentle and passive touch, bearing in mind that curvature, calcification, quality and volume of irrigation, apical patency, and prior exploration of the canal with K files - as well as glide-path creation to a 15 K file - are among many factors affecting fracture risk. The greater the curvature of the canal, previous number of insertions, pressure applied to the file by the clinician, torque placed on the file by its movement in the canal, and number of rotations, the more important it becomes to discard the file.
In addition, the rotational speed used, the torque setting of the electric motor, as well as the amount of hand instrumentation which has preceded the RNT file, all make a significant difference. In short, the more times a file is used, the less useful life it has. It also is possible that a given file may possess manufacturing defects before it is ever utilized. Obviously, this, too, can affect the number of advised uses. Utilizing a file with a preconceived expectation of uses is a recipe for separation. Using a separated file (our "Shorty" referenced in the first paragraph) is ill-advised due to microcracks which likely have formed in the metal, among other material considerations.
All the factors I have discussed can have a significant impact on how many times a file can be used. They are much more important than a manufacturer's recommended arbitrary number of uses. With these cautions in mind - and exceptions do exist - I use the K3 above a 20-tip size (independent of taper) in 12 to 24 canals. Below a 25 tip size - i.e., a 15 and 20 K3 (independent of taper), I use the files only once to three times before discarding them.
"Get Shorty" was a movie, not a strategy to improve endodontic profitability. If we could stop and ask any clinician who has just separated a file if he or she wishes the file had been discarded a minute before it snapped, it's a safe bet the answer would be "yes!" Consider this food for thought the next time you have the urge to push a rotary file beyond its useful life. I welcome your questions and feedback.
Dr. Richard Mounce is in private endodontic practice in Portland, Ore. Dr. Mounce is the author of a comprehensive DVD on cleansing, shaping, and packing the root canal system for the general practitioner. The material is also available as audio CDs and as a Web cast pay-per-view. He lectures worldwide and is a widely published author. For more information, contact Dr. Mounce via email at comfort@MounceEndo.com. Visit his Web site at www.MounceEndo.com.