By Kenneth Koch, DMD, and Dennis Brave, DDS
The past 15 years has seen the acceptance of nickel titanium rotary files to perform endodontic procedures. Their introduction is perhaps one of the great technological advances in the recent history of dental medicine. Excellent though they may be, there is always the possibility of an unpredicted separation. While we may never be able to totally eliminate separation, the key to minimizing separation is to understand what causes it. The following discussion points are tips that we believe will help you to minimize the separation of any rotary file.
1) Understanding torque and cyclic fatigue
The most common factors that can result in file separation are excessive force (torque) and the build-up of cyclic fatigue (metal fatigue). Torque is a result of a twisting motion, similar to twisting a pencil in opposite directions. We actually can see excessive torque as an unwinding of a rotary file. Naturally, when we see this, we immediately discard the instrument. Cyclic fatigue, on the other hand, cannot be seen. It is, in fact, insidious. It is a result of a repetitive bending motion, similar to bending a clothes hanger. Cyclic fatigue accumulates to a point where the next bend will separate the instrument. The only way to monitor cyclic fatigue is to discard an instrument after it has been used in a troublesome canal or tooth.
It has been our experience that rotary files that separate in the apical 1 to 3 mm are usually the result of excessive torque. Files that separate higher up on the shank, such as 5 to 6 mm are the result of cyclic fatigue. Additionally, those separations that are immediate or involve large sizes such as a # 35 / .06 taper file are generally a result of accumulated cyclic fatigue.
2) Landed vs. nonlanded rotary files
The current selections of rotary files are, by design, either landed or nonlanded instruments. Each has its own benefits and limitations when it comes to separation.
Landed rotary files, by definition, rub against the inside of the canal wall and increase lateral resistance. The result is a significant increase in the amount of torque required to spin the file effectively. Nonlanded rotary files, on the other hand, have less torque requirements than landed files and are more efficient in cutting and shaping a preparation. However, rotary files without lands do have a thinner diameter and are somewhat more prone to cyclic fatigue.
So what’s the solution? Every clinician must know the risks and benefits of whatever rotary file system he or she employs. It also depends upon how one uses the instruments. For example, if you perform a minimal number of endodontic procedures and want to use the files multiple times, perhaps you should think about using a thicker landed file run at a slow RPM.
But, if you do a lot of endo, and are into precision-based endodontics (synchronicity) and efficiency, then a nonlanded file is most likely best for your situation. The proof of this is a study recently completed in Japan.
Many academic studies that investigate file separation use a “grab and twist” method. This is when the tip of the instrument is embedded in resin and is twisted until it breaks. The thinner diameter file seemingly always breaks first. This is unrealistic in simulating how a file works in a canal. Factors such as cutting efficiency, torque reduction, load, and flexibility are not taken into account with this methodology.
Recently, a study in Japan (C. Kobayashi, H. Suda, Tokyo Medical and Dental University) produced entirely different results. Their study investigated how many times a canal could be instrumented before the file separates. The study demonstrated that an electropolished, nonlanded file (EndoWave) would prepare four to five times as many canals (before separation) as landed files. This is a study that makes sense because it takes into account the multiple factors that can cause separation.
3) High RPM vs. low RPM
As a result of the introduction of nonlanded files, we are now seeing rotary files run at a higher RPM. Multiple systems call for a speed of 500 to 600 RPM, but will this increase in speed result in increased separation? Not if you know the implications for an increased rate of speed.
A higher RPM will allow the file to work more effectively and will give the clinician greater tactile awareness. This is why we do crown preparations with high-speed turbines. The only limitation in using nonlanded files at a higher RPM is the increase in speed decreases the cycles to failure, meaning you can’t use the files as often. This is really no problem because we should all get into the habit of single-use, especially after molar endodontics. Moreover, when running a nonlanded file at a greater RPM, you have gained a dramatic increase in tactile awareness as a result of the increased speed.
However, if someone is very “frugal” and insists on using the same files over and over, that individual most likely will be best served by using a thicker landed file run at very low RPM, such as 250 RPM. Naturally, this approach will not take advantage of any of the gains in endodontic efficiency that we have witnessed in the past few years.
4) Always keep a file moving
When a developer of a rotary file system receives the initial prototypes, one of the first tests performed is to see how you can break it. All rotary files can be separated if used improperly. One of the easiest ways to break a file is to take it around a curve and hold it in the same place (no vertical movement). Cyclic fatigue rapidly accumulates in the instrument and it will eventually break. This is especially true for greater taper files in large sizes, such as a 35 / .06. Therefore, always keep a rotary file moving as this distributes the stress throughout the length of the instrument. You will find yourself having a lot less “separation incidents.”
5) Torque control engines
Many manufacturers now produce excellent torque control engines and these units can help reduce those separations that are principally caused by excessive torque. This is especially true during the early stages of rotary file experience. However, torque control engines are not a substitute for proper technique. They act simply as a safety cushion in case the operator gets heavy-handed and leans too much on the file. In fact, separation actually increased when torque control engines were initially introduced, simply because they were not being used correctly. When used correctly, torque control engines can be an additional plus, but they do not eliminate all concerns about separation. Remember, there is no such thing as a cyclic fatigue control engine.
Additionally, three things will happen to the experienced rotary user. With experience, you will take rotary files deeper into the canal, be able to use more torque during the procedure, and ultimately use dramatically less total number of instruments. This applies to all rotary instruments.
The easiest and best way to prevent separation of rotary files is to use them one time, then discard. This concept has finally taken hold in the endodontic community. As a result of the dramatic increase in efficiency (and production) the new generation files have created, it makes little sense to use the files multiple times. There is no excuse for having a file separate because you used it too many times. Some clinicians will discard files after one molar case, but will reuse them after anterior and premolar procedures. The rationale behind this is that, as a result of the lack of curvature associated with these teeth, minimal cyclic fatigue is generated during the procedure. This is OK, but not ideal.
The single use of rotary files is wonderful because it dramatically reduces the stress in the office for both the doctor and assistant. If you don’t go the single-use route, you will always have the following questions when you open the sterilized package of files - “How often were they used?” and “How difficult was the case?” This especially applies to multiple partner (associates) offices. Simply put, it’s hard to argue against single-use.
7) Files should be well lubricated
This is kind of a no-brainer - never run a rotary file dry. Always use a lubricant with your rotary file because a file without lubrication requires six to seven times more torque.
Any of the standard dental lubricants work well and are especially helpful at the beginning of the procedure. As you get further into the procedure, the sodium hypochlorite itself will act as a lubricant, not merely as an irrigation agent.
8) Never force a file
Rotary instrumentation is light-handed dentistry. Never force a file; doing so will subject the instrument to excess stress. There are multiple ways to handle the situation where the file no longer progresses easily down the canal. The first option is to go to the next smaller size instrument. This is often the case when instrumenting around curvatures. Another choice would be to recapitulate and repeat the sequence. One can also go to a smaller taper. If the .06 taper is “hung up,” one can always slide over to a .04 taper instrument. Also, remember that a hand file can be used to create a glide path that will facilitate the use of subsequent rotary files.
9) Sensible operatory management
The assistant must monitor the treatment room and cabinets in an orderly fashion. Disorganization in endodontics will drive everyone crazy. It’s like mixing up pin systems! Make certain that there is an order to the presentation and storage of both instruments and materials. Inventory control is paramount to sensible, efficient endodontics.
Also, just as you do in prosthodontics, please use your assistant effectively in endodontics. The assistant needs to visually check each instrument and wipe it off at the end of each sequence of use. Any unwound instruments need to be immediately discarded. Furthermore, the assistant should pass the instrument to the doctor through the beam of the dental unit light. Passing the instrument in such a manner will help prevent separation because any previously missed unwinding of an instrument will deflect light in such a way that it immediately becomes apparent. When utilized properly, the assistant can be a huge help in preventing separation.
10) Difficult anatomy
The most common cause of separation is difficult anatomy that creates excessive torque or a dramatic increase in cyclic fatigue. This is particularly noteworthy in the case where two canals merge into one common canal. It is absolutely critical to take radiographs and fully evaluate them before beginning an endodontic procedure. Clinicians need to trace the ligaments around the tooth as this will provide a good indication of the tooth’s anatomy. In addition to merging canals, the clinician must be alert for short radius curves. Short radius curvatures stress a rotary file far more than banana-shaped canals in maxillary molars.
There is an old expression in endodontics - “Why do we always take the X-ray after we break the instrument?” Keep this in mind and always take a good angled radiograph or digital image before you do any endodontic procedure. It will help keep you out of trouble.
In this article, we have given you 10 tips that should help you reduce the separation of rotary files. While it is true that we can never totally eliminate instrument separation, if you follow these 10 rules, you will significantly reduce the number of separations in your practice.
Furthermore, if we have offered one especially significant tip, it is this - use your rotary files once and discard them. This does not mean one canal, but rather one case (tooth). Do the case, discard the files, and work smart.
Dr. Dennis Brave is a diplomate of the American Board of Endodontics and a member of the College of Diplomates. In endodontic practice for 27 years, he was the senior managing partner of a group specialty practice. Dr. Brave, formerly an associate clinical professor at the University of Pennsylvania, currently holds a staff position at The Johns Hopkins Hospital. Dr. Kenneth Koch is the founder and past director of the program in postdoctoral endodontics at the Harvard School of Dental Medicine. In addition to having maintained a private practice limited to endodontics, he has written numerous articles on endodontics and maintains a faculty position at Harvard. They can be reached at Real World Endo at (866) 793-3636 or through www.realworldendo.com.