The most important feature of rotary file use should be the minimization of engagement of the file at any given time against canal walls. Minimal engagement diminishes the chances of instrument separation due to torque failure. Minimal engagement also promotes removal of canal debris and enhanced cutting efficiency as well as optimal tactile control of the file by the clinician. Dr. John McSpadden from Chattanooga, Tenn., has been the source of much of my information about minimizing engagement, and I am grateful for his teaching.
Rotary file insertion should be passive, gentle, and the file should flow easily down the canal. The file should never be forced. The clinician should be in tactile control of the file, not the other way around, during insertion or withdrawal. Symmetrical U-shaped cross-sectional rotary file designs with fixed tapers are problematic in that they tend to act like a screw and suck down into the canal upon insertion. This diminishes operator control. The K3 rotary file system (SybronEndo, Orange, Calif.) is asymmetrical in cross-section, and does not suffer from this limitation. Ideally, the file should be engaged by only 1 to 2 mm of canal wall at any given time (certainly no more than 4 to 6 mm). Then the file should be withdrawn and reinserted. Knowing when to withdraw a rotary file comes with practice. Suffice it to say that once a rotary file encounters resistance, the operator can advance the file another millimeter or two if the canal will accept it easily. Then the file can be withdrawn, the flutes cleaned and reinserted, or the clinician can move to the next file as required by the clinical case.
For some file brands, relying upon factory preset values of torque and rotational speed - which are programmed into some electric motors, give an audible warning sound, and have an auto-reverse feature when torque limits are approached - is fraught with potential problems. These types of motors may give the operator a false sense of security. I have seen a number of cases of file separation referred to me that were the result of a clinician who was depending upon audible warning sounds or an auto-reverse that never occurred. Relying on auto-reverse encourages a file to be pushed to its torque limits, and risks overengagement. This increases the chances of torsional failure. Relying on auto-reverse also deemphasizes how the file can and should be treated. In other words, these “bells and whistles” do not emphasize the ideal amount of pressure that should be placed upon the file, the rotational speed, torque setting, and/or file engagement. Spending extra money to buy motors that have rotational speed and torque values built into them for specific file models, in my empirical opinion, is money poorly spent. In contrast, the clinician who is aware of the amount of engagement of the file against canal walls puts total control of the file into the operator’s hand. If the file is fully bound at its tip, it is very possible that the operator will not hear a warning since the entire file may not have encountered enough torque resistance to cause the file to auto-reverse. A file bound at the tip in this manner is highly susceptible to fracture.
I run my rotary files at full torque and do not rely upon auto-reverse features, even though virtually all motors available commercially come with auto-reverse. One practical and economical model that possesses torque control as well as auto-reverse without the unnecessary factory presets is the TCM Endo III (SybronEndo).
As an aside, the rotational speeds recommended by a number of manufacturers may actually prevent ideal efficiency from many rotary file brands and systems. Most manufacturers recommend their files be run at 250 to 500 rpm, with 350 being the general number. Factory presets for a given file may actually run the given file too slowly for its optimal use. For example, in the coronal half of a large fully patent canal without curvature, there is little reason to run a rotary nickel titanium file at 350 rpm. With the K3 shapers (orifice openers), which can be used as orifice openers because of their robust sense to tactile control and cutting ability, I confidently rotate these files at approximately 1,000 rpm for the clinical situation described. A future column will discuss optimal rotational speeds in greater detail.
As always, I welcome your feedback and questions.
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 a Web cast pay-per-view. He lectures worldwide and is a widely published author. For more information, contact Dr. Mounce via e-mail at comfort@MounceEndo.com. Visit his Web site at www.MounceEndo.com.