Endodontic K-Files: Invaluable Endangered Species or Ready for the Smithsonian?

Do hand-powered K-files have a place in a rotary nickel-titanium (RNT) endodontic world? Will the Smithsonian some day display a K-file? This is possible, but not for the foreseeable future. The author is skeptical of claims that an entire preparation with rotary is now possible or desirable. Said differently, RNT files blended with K-files create the potential for excellent canal shaping in modern endodontics. Neither file is exclusively the solution to all cleansing and shaping needs or individually efficient to the exclusion of the other.

To eliminate K-files from the endodontic armamentarium, RNT instruments would need to meet the following requirements that at this point they are not able to provide:

(1) RNT would need to be able to explore, track, and resist fracture due to torsional failure and cyclic fatigue irrespective of the diameter and/or multiplanar curvature of the given canal. RNT would have to cut efficiently and hold dentin chips while cutting, and ideally it would have to be used multiple times with identical cutting efficiency.

(2) RNT would also have to be able to hit a canal wall and not wind-up on itself. In addition, it would have to hit a flat wall and remain rotating indefinitely without either transportation or failure due to cyclic fatigue (similar to what happens when a piece of metal is bent repeatedly at the same location and snaps).

(3) RNT would have to be able to track a canal at the orifice irrespective of its size or how acute the angle of the orifice is to the pulpal floor.

(4) RNT would have to be able to break through canal blockages consisting of dentin mud without risk of fracture

It is obvious that no single file design fulfills these objectives at this time. That said, if used astutely K-files can have a significant impact on prevention of RNT separation, glide path creation, exploration of the apical third, achievement and maintenance of apical patency, and exploration and access to the orifice (especially one that is severely constricted). They can also go a very long way toward overcoming the present limitations of RNT files (listed above). In short, these vital functions are at this point more effectively fulfilled by hand K-files.

Stainless steel K-files have attributes not possessed by RNT:

(1) far better tactile sensation

(2) less separation potential

(3) K-files, once removed from a canal,  often have the impression of the canal curvature imprinted on them, which should guide the clinician as to the curvatures present in the given canal.

(4) K-files make excellent exploratory files with regard to determining curvature and achieving and maintaining patency.

(5) K-files can bypass obstructions with far greater ease than their RNT counterparts. In fact, bypassing foreign body obstructions with RNT is not advised. For example, trying to bypass a separated RNT file with another RNT is hazardous and ill-advised.

(6) The stiffness of hand stainless steel files is valuable in canal exploration and breaking through blockages, especially dentin mud and calcifications. In fact, the flexibility of nickel-titanium hand files offers no advantage in the author’s opinion over stainless steel K-files. Stainless steel K-files, especially in the small sizes, are so flexible (especially 6, 8, 10, and even to a large degree 15 K-files) that nickel-titanium does not offer any particular advantage. As expected, stainless steel K-files are far less expensive that their nickel-titanium counterparts.

(7) It is important to mention that a stainless steel file has the inherent ability to tell the clinician in a tactile sense of the various curvatures and canal eccentricities, if the tactile messages are heeded. Said differently, if used correctly, a stainless steel K-file can (in most cases) and will tell the clinician a great deal about the given anatomy, including a clue about the working length as well as what the subsequent master apical rotary file will be.

All K-files should be precurved before they are placed into canals. To do so will allow the given file to traverse canals efficiently as well as more easily bypass ledges, break through blockages, and sensitize the clinician’s tactile sense to anatomical complexities that are not radiographically visible. In addition, when viewed through a surgical microscope in cases of especially difficult accesses and/or severe calcification (especially in access through crowns in nonvital cases), small K-files are invaluable. To place a RNT file into such a canal without first traversing it with a K-file is highly problematic, risking fracture irrespective of the manufacturer.

Figure 1. Glide path and apical patency achieved clinically.

(8) K-files are small enough that they can traverse virtually any negotiable canal, especially in the small sizes (6 to 10). At present (to the author’s knowledge) the smallest RNT file commercially available is a 0.02-tapered 15-tip size file (such as the K3 RNT, SybronEndo). While this is a valuable file, it is not small enough or designed to negotiate initially or explore canals without prior K-file exploration (Figure 1).

CLINICAL USE OF K-FILES

 

Initial exploration of any given canal third with a K-file is essential prior to using RNT in that given third. Using a RNT file in the middle or apical third without exploration and glide path creation with K-files and establishment of apical patency creates significant risk for a torsional locking that could separate the file, especially at its tip. File separation creates a significant retreatment challenge. Even if it can be removed, its removal will require otherwise unneeded removal of tooth structure to facilitate its reacquisition. In addition, if for various technical reasons the fragment must be left, its presence certainly creates the potential for uncleaned and unfilled canal space alongside and beyond  the fragment, depending on the stage of the cleansing and shaping process in which the separation occurred.

A discussion of K-file importance would be incomplete without a discussion of which K-files are actually required in addition to how these files should be used. The author uses 21-mm and 25-mm 6, 8, 10, 15, and 20 K-files. In the author’s inherent and arbitrary opinion, there is little, if any, indication for K-files of a larger tip size than a 20, except for their use in apical gauging (determining the diameter of the apical foramen after preparation) and perhaps electronic apex location. In fact, a 20 hand file would be used very sparingly in actual canal instrumentation. All canal shaping beyond a 15 K-file can be performed with RNT in a manner that will be described shortly. RNT instruments are more than capable of enlarging canal spaces that have been opened to a 15 K-file. Beyond the 15 K-file size, for greatest efficiency, all instrumentation should proceed with RNT use.

Blending hand and RNT use in this manner brings out the best in both sets of instruments and can help prevent the worst iatrogenic possibilities inherent in each system. For example, trying to use a 30 K-file in the apical third is unnecessary and can easily be the genesis of ledges, apical transportation, and tearing of the foramen, etc. RNT files will more easily track canals in the apical third, with less risk of transportation, than larger K-files (for example a 30 K-file, if an adequate glide path has been created), and their speed and efficiency are unmatched by their hand-powered counterparts.

In contrast to RNT, a 30 K-file is a relatively stiff instrument and difficult to handle without forcing it apically in many canals. Its 0.02 taper will create little inherent shape in the canal above the tip. Operator fatigue as well as the aforementioned iatrogenic possibilities also weigh against protracted use of hand K-files for an entire preparation when an alternative such as RNT is available. It is noteworthy that even a 15- and 20-tip size rotary instrument such as the K3 (SybronEndo) is too large to explore canals apically without first being explored and negotiated by hand files. These files are very effective at enhancing and refining a glide path that has been negotiated and prepared to a 10 to 15 K-file first. In essence, the glide path created by hand can be blended with subsequent RNT instrumentation by the 0.02 K3 15- and 20-tip sizes, and can set the stage for a very smooth crowning down with subsequent files such as the K3 in diminishing tip sizes (for example 0.06 40 followed by a 0.06 35 and 0.06 30, etc).

Figures 2a to 2c. Completed cases with a blend of hand and rotary nickel-titanium instrumentation as described in the article. 2b.
2c.

As an aside, there is little if any indication for Hedstrom files in routine endodontics, given the natural synergy that exists between K-files and RNT. The potential for breakage in addition to transportation of the foramen is too great. This said, the use of Hedstroms for removal of gutta-percha, as an adjunct to separated file and post removal, and removal of failed carrier-based products can be invaluable. However, as a cleaning and shaping instrument its indication is dubious in light of the existing K-file and RNT synergy (Figures 2a to 2c).

CONCLUSION

 

It is entirely possible (and likely) that in the future a RNT system will come into being that might make K-files obsolete and a candidate for display at the Smithsonian. Such a system would possess the qualities that were mentioned previously. Until that time, the tried-and-true hand-powered 0.02 K-file will remain an essential item in the endodontic armamentarium.



Dr. Mounce is in private endodontic practice in Portland, Ore. He is the author of a 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 Webcast pay-per-view. For more information, e-mail This e-mail address is being protected from spambots. You need JavaScript enabled to view it . Dr. Mounce can be reached at (503) 222-2111, This e-mail address is being protected from spambots. You need JavaScript enabled to view it , or MounceEndo.com.

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