K files above a 20-tip size are relatively stiff, inflexible, and encourage ledge formation. If placed beyond the minor constriction of the apical foramen, their use can result in apical perforation and transportation. In fairness, the aforementioned effects can be mitigated to some extent by use of the Roane technique. This technique employs, in part, a counterclockwise motion for hand K file use. While the Roane technique (whose description is beyond the scope of this column) has value in some applications, a simpler method of canal instrumentation for the clinician would include rotary preparation in the apical third. In addition, blending various K files to create a smooth, tapering preparation is far more challenging than what is achieved by an RNT file of either a fixed or progressive taper. While stainless steel files will separate at a lower rate than their nickel titanium counterparts, RNT files - if used safely and appropriately - are far more efficient, ergonomic, and create ideal shapes with less fatigue. It is important to emphasize the need to negotiate and explore all canals with hand files prior to using rotary files, and to create a glide path with at least a 15 K file prior to using rotary files - especially in the apical third. It is also important to recall that some extremely curved and calcified canals might require a significant amount of hand filing to allow RNT to be inserted passively. Once a canal, irrespective of curvature, is prepared by hand to a 15 K file, rotary nickel titanium files can be used to finalize the preparation. This assumes that the appropriate taper and tip size instrument is employed, the correct amount of pressure is applied to the file, and the motion and touch of its use are correct.
The blending of hand files into preparation schemes is relatively simple. Usually the clinician will make sure that the canal is patent and negotiable (either that it is clearly visible on the radiograph, or it can be determined by hand). After canal negotiation, orifice openers are used to shape the coronal and middle thirds of canals, and the apical third is negotiated by hand. A glide path is created, and the apical third is contoured with canal shaping files. I use the K3 rotary system by SybronEndo (Orange, Calif.). The sequence for the K3 rotary system which I utilize in my practice is:
- Canal negotiation to a 10 K file
- K3 shaper files (orifice openers) used .12, .10, and .08 successively. This usually will take a clinician to the junction of the apical and middle thirds in any average case. The shapers have a fixed 25-tip size.
- Apical third canal negotiation and patency is achieved up to a 10 file, TWL is determined, and a glide path is created to a 15 K file by hand.
- Canal size dependent - .06 taper for larger canals and .04 for smaller - the apical third is shaped .06 35-20 K3 to the TWL, and the desired master apical file size is created.
In essence, the rotary files we have now do the “heavy lifting” in cutting efficiency. This makes use of hand K files at sizes above a 20 unnecessary for the vast majority of cases. Hand K files below a 20 might be thought of as negotiating tools and glide path creators - not the actual instruments that provide the final shape to the canal. This suggested technique will vary for any given anatomy. Utilizing a crown down approach - which instruments the coronal third first, then the middle third, and finally the apical third - removes restrictive dentin coronally first, and allows more volume of irrigation to reach more deeply into canal systems. The approach also provides greater tactile sense to the clinician. With the myriad of benefits of rotary nickel titanium files, it is not necessary - in the vast majority of cases - to risk the iatrogenic possibilities inherent in utilizing K files that are larger than a 20-tip size for apical preparation. 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 e-mail at comfort@MounceEndo.com. Visit his Web site at www.MounceEndo.com.