What do dentists want from their rotary shaping instruments? Back in 1996, when rotary nickel titanium shaping files were introduced to the marketplace,1-4 the primary question that dentists grappled with was, “Should we dare put a handpiece-driven file into a root canal?” Today, the decisions are different. The question dentists ask themselves is not whether to use them, but which set of instruments, out of the multitude available, is right for them. With every new company entering the market with a NiTi rotary file claiming that theirs is the next generation of design, when it may be just the latest to arrive in this intensely competitive field, it has become increasingly difficult to separate the marketing from the reality of using them in patients’ teeth.
So, how do we figure this out? I would say the answer lies in looking at what we most want from our shaping instruments and comparing the offerings to those standards. A marketing research firm polled dentists and found out that the things they want from their files are, in this order: 1. resistance to breakage, 2. fidelity to the original canal path, and 3. efficiency. I was gratified to note that the first 2 are both related to safety, as it should be. However, as simple as the concept of safety seems, the multivariate and often oppositional functional characteristics of file geometry make for a serious analytical challenge.
Examples of these countervailing features are abundant. While larger core diameters enhance a file’s resistance to torsional stresses in straight canals, they radically increase their susceptibility to cyclic fatigue in curved canals. While sharper cutting flutes decrease friction and the resultant torsional stresses accumulated in a file during cutting, they significantly increase the amount of transportation from the original canal path in curved canals.
Beyond the difficulties of understanding factors that come to bear in safety objectives, it is no less obscure to determine how these geometric variants affect efficiency. Do we look only at the cutting efficiency of a file blade during the use of a single file in a single cutting cycle, or do we consider the number of instruments and steps necessary to complete the root canal preparation? Do we compare files relative to how long they can cut in an apical direction before their flute spaces are jammed with debris?
THE IMPORTANCE OF SHAPING OBJECTIVES
Often lost in this cacophony of information about rotary shaping instruments is the issue of shaping objectives. Without considering the preparation result desired it is difficult to choose an instrumentation system, as it would be difficult to choose a mode of transportation without knowing where you wanted to go. Most clinicians have only asked themselves whether they want to use hand files vs. handpiece-driven files, with the intention of creating the same outmoded root canal preparations they were taught in dental school some time ago. I am of the opinion that it is a bigger step forward to stay with hand files but to cut more predictable shapes than it is to use rotary files to cut shapes with a greater chance of poor short- or long-term outcomes. So, let’s first consider our options in this department. In that light, the consideration of shaping objectives can be divided into coronal, middle, and apical categories.
Traditionally, root canal preparations have been relatively large coronally and apically with varying, often anemic, amounts of shape between those regions. Large coronal shapes became the rage when Schilder and others5,6 noted, correctly at the time, that greater amounts of coronal enlargement allowed for better results in apical regions. But that was only true for the relatively stiff stainless steel files and relatively large irrigating needles we had at that time. The other reason that clinicians to this day cut big shapes in these regions is to facilitate the filling of root canals with lateral condensation of cold gutta-percha.
The problem is that over-enlargement of this part of the canal unnecessarily weakens root structure, and in the case of molar canals, increases the chance of strip perforation. When I talk to prosthodontists about their fears of using endodontically-treated teeth as critical abutments, it revolves primarily around structural integrity issues (assuming that the apical region has been thoroughly treated).
When I asked Carl Reider, a retired but renowned prosthodontist from Newport Beach, California, what he most wanted from his endodontist, he said he would prefer it if the endodontist could just suck the dying pulp out of the tooth without removing any dentin. He made that paradigm-shifting statement to me back in 1990. Fast-forward to 2006, when a young prosthodontist stated to a room of endodontic graduate students that he would rather do full-mouth reconstructions supported only by implant fixtures rather than endodontically-treated teeth (they were emotionally distraught), and I hear that same fear about losing important abutments due to structural failure. It is, therefore, clear to me that controlling the amount of coronal enlargement is vital to long-term endodontic success, as well as to the long-term success of endodontics as a credible treatment in dentistry (Figure 1).
What about the shaping objectives in the middle third of the canal? The problems associated with a paucity of shape in this region are seldom appreciated. Undershaping the middle third of root canals is largely responsible for irrigation and conefit problems, and when carrier-based obturation is used, it will readily strip off gutta-percha from the carrier and allow it to arrive at the end of the preparation without gutta-percha around it—all of these being set-ups for failure. Fortunately, the advent of variably-tapered shaping files ended the commonly under-shaped outcomes we saw when push-pull filing with .02 tapered k-files, so virtually all rotary shaping instruments of .06 or greater taper will solve this problem.
Apically, the traditional shaping objective has been the “stop prep,” an intentional ledgeform short of the apical foramen. While the stop prep will work well when done perfectly, it is prone to failure as it is unforgiving of length determination errors. When length is erroneously determined short, the stop prep, created by cutting larger and larger files to working length, is a ledge that will confound efforts to bypass it when attempting to fill to an acceptable length in the canal. When length is erroneously determined long, or when a curved canal is shortened by straightening during shaping, the large and relatively stiff instruments that are taken beyond the root canal terminus will inevitably rip the end of a canal—a set-up for overfilling.7
In fact, the volume of literature correlating overfills with a higher incidence of failure8-11 have largely misunderstood coincidence for etiology, as most of the endodontic failures studied were overfills that happened in canals prepared with the stop preparation. As an apical stop is really an intentional ledge, any overfill occurring with this shaping objective is by definition indicative of a shaping failure, not a filling failure. When the apical foramina of canals are ripped open it is virtually impossible to adequately seal them with conefit techniques. This is the only possible explanation for those failures when the biocompatibility of our filling materials is considered.12-14
If we cut a tapered shape (with files that have safe tip geometry) that is short of ideal length, it is easy to just cut the preparation farther into the canal when the error is detected before filling. If we cut a tapered shape (with files that have landed flutes) too long, the tapered resistance form still exists and only requires that the fitted point be cut back to correct length before obturation—a recovery that takes less than 20 seconds.
For my money, I want the apical preparation that is most forgiving, one that is tapered. In addition, when clinicians choose this apical shaping objective they get a “twofer.” Not only is it more predictable with variably-tapered shaping files, it requires fewer instruments to achieve that result.
HOW DO WE CREATE THE SHAPES WE WANT SAFELY?
Figure 1. Endodontically-treated maxillary molar with vertical root fracture in mesio-buccal root. While the endodontic therapy was quite good in the apical regions (note the apical accuracy of filling and the significant lateral canal in the mesio-buccal root), the unnecessary over-enlargement of the coronal regions of the canals resulted in a loss of structural integrity and the splitting of the MB root within 5 years of treatment.
Figure 2. Micro-CT view of a curved mesial root of a mandibular molar shaped with size 30-.06 landed and nonlanded rotary files. The canal shape on the left, significantly more conservative in size, was shaped with a landed-blade GT File. The canal shape on the right was shaped with a nonlanded file having the same size tip and taper.
Figure 3. Micro-CT reconstruction of curved canals shaped in a mesial root of a mandibular molar, comparing outcomes in the apical third with rotary files having radiused vs. aggressive tip geometry. Note the canal on the right showing severe transportation (aggressive tip) and the canal on the left staying true to the original canal path as the canal terminates (radiused tip).
What characteristics should we look for to meet our shaping objectives and ultimately answer the 3 primary needs dentists cited in the aforementioned market survey? With the shaping objectives listed above it becomes clear that we need a file set that provides limited coronal enlargement and a tapered shape at least in the apical half or two thirds of the canal, preferably without serial step-back techniques required. Not an easy wish list to fulfill, given the fact that most files with tapers greater than .08 mm/mm have dangerously large shankend flute diameters.
While there are many files on the market that provide .06 tapers, which are adequate for small-root canals, tapers larger than that are typically desired for medium-root and large-root canals. The concept of maximum flute diameter (MFD) limitations is unique to the GT File (DENTSPLY Tulsa Dental) family of instruments, and it not only controls coronal enlargement but also allows preparation tapers greater than .06 to be created without unwanted cutting in those regions. While there are instruments that have somewhat limited flute diameters, no other file set limits the MFD to 1 mm throughout the series as seen in the GT designs.
When I hear a dentist tell me that he or she uses GT Files but still likes to use Gates Glidden Burs at the start or end of the preparation, I know that they are still filling canals with lateral condensation because GT Files cut shapes that are just slightly larger than GT Gutta Percha Points, and there is not much room at the orifice level to place a spreader between the canal wall and the point. In the 21st century, with thermoplastic obturation techniques available that take less time than well done cold lateral condensation, I would say that dentists need to pick a different filling technique if the one they are using requires dangerous over-instrumentation. While the 1 mm coronal enlargement created by GT Files may be smaller than many dentists are used to working through, Continuous Wave Obturation, carrier-based obturation, warm lateral condensation, and even single cone fills (although I don’t recommend this) are better than needlessly weakening root structure.
To the endodontists reading this article, be aware that over-cut coronal shapes are part of the reason prosthodontists are increasingly choosing implant-supported prostheses over endodontically-treated abutments. All nonlanded cutting flutes cut shapes significantly larger than the external geometry of the file itself, as seen in Figure 2. Anticurvature shaping routines do nothing to minimize the shaping size; they just reduce the chances for strip perforating narrow curved roots. The best method to retain maximal root strength and totally eliminate the possibility of exiting the root is to definitively control coronal enlargement, one of the most distinguishing features of both the GT as well as the new GT Series X File lines (DENTSPLY Tulsa Dental)
Moving to the other end of file geometry, file tip designs must be safe-ended. Every 5 years a dental company introduces a new file with aggressive or semi-aggressive tip geometry. While the initial sensation of effortless apical progress is seductive, micro-CT research and clinical experiences do not bear out manufacturers’ claims of safety (Figure 3). The real story is told when the manufacturer’s directions for use (DFU) is read and the technique recommended mentions never taking the files beyond the apical foramen, and that the file tips should never be used for more than one second at full length. Underappreciated in this regard is the dangerous combination of aggressive tip geometries and nonlanded cutting flutes in curved canals. When a nonlanded instrument is used in curved canals, mid-root curvatures are inevitably straightened, thereby shortening canal lengths, resulting in inadvertent placement of aggressive file tips beyond the apical foramen, an invitation to apical ripping. Of course, all aggressive tip, nonlanded shaping files can be used with excellent outcomes if the user is experienced and adept, however, the work-arounds necessary to avoid apical damage often requires additional files and additional procedural steps to circumvent the inherent dangers.
Next on the list of file features are blade edge and core geometries. While the file tip and shank features are quite obvious in their effect during shaping procedures, the functional characteristics of cross-sectional file geometry are less obvious. If we look at the 4 leading files on the market in cross-sectional view there are 3 primary things to observe: cutting blade edge, core size, and conversely, the size of the chip space between the flutes.
Each of these features has countervailing advantages and disadvantages—the trick is to optimize them. Cutting blade edges cannot be too sharp, or significant transportation of curved canal paths surely result. Yet, if they are too inefficient and take too long to cut, cyclic fatigue will rapidly accumulate when these instruments are held around canal curves. Core diameters that are larger add strength to the instrument when it is used in a straight canal (improved torsional strength), but become quite dangerous in curved canals (decreased resistance to cyclic fatigue). Larger core diameters combined with nonlanded cutting blades are the most dangerous, as the stiffness activates the cutting blades to transport and they are more prone to cyclic fatigue failure.
Furthermore, if the core diameter is smaller, the chip space becomes more capable of gathering cut dentin debris before the file stalls in its apical cutting progress. Small chip space means that the file needs to be removed and cleaned more often before it can cut farther in an apical direction. Also, as the core diameter becomes smaller, the inherent flexibility of the instrument increases—a very good thing in curved canals.
What is needed is a small core diameter and the resultant large chip space, and a blade edge that is effective without being too aggressive—a difficult set of parameters.
GT SERIES X FILE FEATURES: M-WIRE, VARIABLE-WIDTH LANDS, GREATER CHIP SPACE, AND FLEXIBILITY
Figure 4. GT Series X File. Note the Maximum Shank Diameter at 1 mm, the radiused tip, the consistent, more open blade angle, and the variable width lands. At the tip and shank ends the land widths are half the size of the lands in the middle region of the flutes, allowing rapid cutting without transportation.
Figures 5A and 5B. Micro-CT reconstructions comparing Standard GT and GT Series X File function relative to transportation of curved canals. In this severely curved mesial root of a mandibular molar, the shapes in the adjacent canals shaped with the different file sets are virtually identical and show exceptional fidelity to the original canal paths.
GT Series X Files have the same radiused tip geometry, the same limited MFD, and they are still landed instruments but with a significant improvement—the land widths vary along the length of the file (Figure 4). Experience gained through the 12 years of GT File manufacture and use has revealed that the width of radial lands is critical. Too large and they don’t cut fast enough (a set-up for failure due to cyclic fatigue). Too small and they begin to transport curved canals like nonlanded files. The key finding during the design and prototype development process was that we need different degrees of sharpness at different regions of a landed file. Because transportation is a function of blade sharpness and the rigidity of the instrument at a given position along the file, testing showed that the tip flutes, in the most flexible part of the file, could be safely narrowed to gain cutting efficiency without transportation in the highly curved apical regions of canals (Figures 5a and 5b).
Furthermore, at the shank end of the file the lands could also be thinned without danger, in spite of the stiffness of that part of the file, because the shank end cuts through the straightest region of roots. With these efficiencies at hand it became apparent that the degree of stiffness in the middle section of the file, coupled with the significant amount of curvature encountered in the middle third of root canals, dictated maintenance of the original land width to prevent straightening of mid-root canal curves. The outcome of this optimization was an at least 2X increase in cutting speed as well as less taper-lock during apical progress.
The final blade change is that the blade angles have been opened to a consistent 30° along the length of GT Series X Files, thereby nearly doubling the chip space between flutes. This increases the flexibility of these files and significantly extends the length of each cutting cycle. Where standard GT Files cut for about 4 to 6 seconds before clogging up, GT Series X Files will cut continuously for 10 to 12 seconds before they need to be removed and cleaned.
A serendipitous result of these blade improvements is that rather than using a 20-.10, then a 20-.08, then a 20-.06 GTX File to cut initial shape in small curved canals, a single 20-.06 GTX File, used in 2 cutting cycles, will often create the initial shape in these canals. Obviously, this should be considered a gift when it happens, rather than an expectation—or breakage could result from pushing this file beyond its limits in tortuous canal morphology—but the 20-.06 GTX File will accomplish this amazing feat more often than not. When it balks at cutting to length, a 20-.04 GTX File is cut to length, after which the 20-.06 will usually make it to length as well.
Figure 6. GT Series X File Family. This reduced file set, with its .2 mm, .3 mm, and .4 mm tip sizes will shape all canals but those with huge apical diameter. Standard GT .12 Accessory Files would be used if tip diameters are needed in .5 mm, .7 mm, and .9 mm sizes.
Figure 7. Clinical case shaped with GT Series X Files. The mesio-buccal canals were shaped with a 20-.06 and a 30-.06 GTX File, the disto-buccal canal required a 20-.04 and a 20-.06, and the palatal canal was shaped with a 20-.06 and a 40-.08. Shaping time (length of time files were cutting) for this case was 1.5 minutes and required just 4 GT Series X instruments. Each of the canals was negotiated to a No. 15 k-file size prior to the use of rotary GT Series X Files with no other hand files used to cut dentin.
The final 2 changes from the standard GT line are a latch-grip handle that is shortened from 13 to 11 mm, and a reduced file set from 15 to 8 instruments in the set (Figure 6). The shorter handle is a no-brainer when considering the small interocclusal distance between posterior teeth, but the reduced file set needs explanation.
The short explanation for the reduced file set is 2-fold: (1) a common cause of rotary file breakage is the erroneous selection of a taper size that is inappropriate for the canal curvature being shaped, and (2) the consistently ideal preparations created by landed-rotary instruments require less tapered resistance form to achieve apical accuracy of obturation. The longer, more complete explanation will be included in a future article on GTX shaping techniques. Virtually any canal, short of those with open apices, can be ideally shaped with this 8-instrument file set, al-though for bigger apical sizes and greater coronal enlargement clinicians can bring a standard GT File, for instance the 40-.10 or the .12 GT Accessory Files with tip diameters of 0.5, 0.7, and 0.9 mm.
WILL THESE EFFICIENCY IMPROVEMENTS USHER IN THE NEW AGE OF ENDODONTIC EXPEDIENCY?
Nothing could be farther from the truth. While I have never seen a patient in my 27 years of practice who has asked for a long, slow root canal, I have also never had a patient tell me that it’s OK to shortcut the procedural necessities. Providing nearly 100% success in endodontic procedures requires endless patience during certain parts of the procedure, for example when looking for calcified canals or when negotiating difficult molar canals to their terminal points. Certainly, that is the case when doing conventional retreatment of botched cases. So it is imperative that we improve the efficiency of any part of the procedure so that we can, for instance, spend the time needed to adequately irrigate root canals after they are shaped. When I have endured the negotiation phase of a tortuous set of root canals I am thankful that I can then shape the canals in just a couple of minutes (Figure 7).
- Walia HM, Brantley WA, Gerstein H. An initial investigation of the bending and torsional properties of Nitinol root canal files. J Endod. 1988;14:346-351.
- Tepel J, Schafer E, Hoppe W. Properties of endodontic hand instruments used in rotary motion. Part 3. Resistance to bending and fracture. J Endod. 1997;23:141-145.
- Tepel J, Schafer E, Hoppe W. Properties of endodontic hand instruments used in rotary motion. Part 1. Cutting efficiency. J Endod. 1995;21:418-421.
- Schafer E, Tepel J, Hoppe W. Properties of endodontic hand instruments used in rotary motion. Part 2. Instrumentation of curved canals. J Endod. 1995;21:493-497.
- Schilder H. Cleaning and shaping the root canal. Dent Clin North Am. 1974;18:269-296.
- Ram Z. Effectiveness of root canal irrigation. Oral Surg Oral Med Oral Pathol. 1977;44:306-312.
- Weine FS, Kelly RF, Lio PJ. The effect of preparation procedures on original canal shape and on apical foramen shape. J Endod. 1975;1:255-262.
- Seltzer S, Bender IB, Turkenkopf S. Factors affecting successful repair after root canal therapy. J Am Dent Assoc. 1963;67:651-662.
- Sjogren U, Hagglund B, Sundqvist G, et al. Factors affecting the long-term results of endodontic treatment. J Endod. 1990;16:498-504.
- Strindberg LZ. The dependence of the results of pulp therapy on certain factors. An analytical study based on radiographic and clinical follow-up examinations. Acta Odontol Scand. 1956;14(suppl 21):1-175.
- Swartz DB, Skidmore AE, Griffin JA Jr. Twenty years of endodontic success and failure. J Endod. 1983;9:198-202.
- Costa GE, Johnson JD, Hamilton RG. Cross-reactivity studies of gutta-percha, gutta-balata, and natural rubber latex (Hevea brasiliensis). J Endod. 2001;27:584-587.
- Orstavik D. Materials used for root canal obturation: technical, biological and clinical testing. Endodontic Topics. 2005;12:25-38.
- Tavares T, Soares IJ, Silveira NL. Reaction of rat subcutaneous tissue to implants of gutta-percha for endodontic use. Endod Dent Traumatol. 1994;10:174-178.
- Shen Y, Cheung GS, Bian Z, et al. Comparison of defects in ProFile and ProTaper systems after clinical use. J Endod. 2006;32:61-65.
Dr. Buchanan, a diplomate of the American Board of Endodontics, is an assistant clinical professor at the postgraduate endodontic programs at USC and UCLA. He has a practice limited to endodontics and implant surgery in Santa Barbara, California and is the founder of Dental Education Laboratories, a hands-on training center serving general dentists and endodontists in upgrading their skills in new endodontic and implant technology. He can be reached at (805) 899-4529 or email@example.com, or visit endobuchanan.com.
Disclosure: Dr. Buchanan consults for and holds patents to the GT System of instruments manufactured and sold by DENTSPLY. He also holds patents with the System-B Heat source and is the inventor of Continuous Wave of Condensation Technique.