When I write about the winding path endodontic shaping procedures that have taken over for the last 35 years, I am sure that I sound like an old curmudgeon bragging he walked 5 miles to school every day through the snow without Uggs. Back in the day, we actually did root canal therapy (RCT) without handpiece-driven files! Nowadays, when I look at the amazing panoply of instruments available, I get a bit confused by the remarkable list of files introduced in just the last 2 years, never forgetting that this is a quality problem, compared to the days of stainless steel K-files.
So how do we sort through an embarrassment of file design riches? Do we just keep on using what we have become comfortable with, or do we continue seeking the next advance? How do we assess and compare all the new files to determine whether another inflection point has arrived with enough promise in these late-generation instruments that it would behoove us to choose again?
My answer is that we must first consider the anatomy of root canal systems. Why? Because endo anatomy is the only aspect of endodontics that has not, and will not, change. Also, it’s because that anatomy is attached to our patients, whom are our first responsibility. The second order priority is to understand the functional characteristics of the range of available files when they operate in the full range of anatomic possibility. The third order priority is to be certain we understand our procedural objectives when using these instruments, because the right instrument used toward a flawed objective will not get us where we want to go.
SCHILDER’S SHAPING LEGACY
The first endodontic literature I read (as a student) was Dr. Herbert Schilder’s Shaping Root Canals.1 This text was recommended by my first mentor in endodontics, Dr. Alan Gluskin, and it blew my mind. At the time, the Standardized Apical Stop Preparation was de’ rigeur, but Schilder’s concepts made much more sense to me, even as a dental student, when considering the natural anatomy of root canal systems.
Schilder’s primary shaping principles were to keep the terminus as small as was practical and to maintain the original path of the canal (no apical ripping allowed) as a continuously tapering preparation is cut with files to the terminus of the canal (Figure 1). When Schilder wrote this paper,1 shaping was done with hand-driven stainless steel K-files and engine-driven Gates Glidden burs (GGBs). Although it was the only shaping method that made sense to me at the time, his technique was complicated and confusing, requiring 12 to 18 instruments and about 20 to 50 separate steps to cut shape in a single canal.
I was able to master serial step-back shaping after working through about 150 to 200 cases in grad school. However, as I began teaching the art of procedural endodontics, I was struck by how difficult it was to transfer this step-back shaping method to others, leading me to the epiphany that it was only complicated because we were using relatively non-tapered files (K-files have .02 tapers) to indirectly cut the tapered shape in root canals. The alternative? Why not make these instruments with the various tapers desired at the completion of shaping?
The first part of this vision quest took 3 years to get variably tapered prototypes made and, after using them, I realized why we had never used files of greater taper than .02 before. In stainless steel, files with .04, .06, and .08 tapers are as stiff as boards, and they break before shaping a single curved canal. Oops! Then Derek Heath, owner of Quality Dental Products, the independent file manufacturer that cut the first 3 prototypes, asked me if I wanted to try nickel titanium (Ni-Ti), an exotic metal alloy that had revolutionized orthodontics, for the next round. Of course, I said yes! The first Ni-Ti 20-.06 variably tapered prototype was a huge improvement as it was dramatically more flexible than comparable files made out of stainless steel. In addition, it was tougher; however, it took further work before we were able to create what became Hand GT, or Greater Taper Files (DENTSPLY Tulsa Dental Specialties [DTDS]).
|Figure 1. An illustration of Schilder’s shaping
objective—a continuously tapering preparation—with the narrowest diameter located at the apical foramen and the widest diameter at the orifice level.
|Figure 2. The 1994 US Patent, Claim 1 illustration showing maximum flute diameter limitations of what became Greater Taper (GT) Hand Files (DTDS), and later, GT and GTX Rotary Files. This design feature, when applied to a series of files having the same tip diameters but differing tapers, results in a shortening of cutting flute length as file tapers increase.|
|Figure 3. The 1994 US Patent, Claim 10 illustration showing file design with apically accelerating flute angles.||Figure 4. CT reconstruction of shaped canals
comparing results from stainless steel K-files and Gates Glidden Burs versus GT Hand Files. Note the more regular, more conservative shape cut with a single 20-.06 GT Hand File.
About that time, Dr. Ben Johnson (the inventor of Thermafil and founder of Tulsa Dental Products [TDP]) was directed to me by Derek, as he had asked if Derek could make files with different tapers. After being told by Derek that I arrived at the variably tapered file party first, he asked if I would be interested in working with him on GT Files. I responded positively, and it was game on.
The US Patent Office denied my claim for variably tapered endo files, but granted the claims2 that made variably tapered files work safely and effectively—namely Claim 1 (Figure 2) for maximum flute diameter limitations at the shank end of the file and Claim 10 for apically accelerating flute angles (Figure 3). The first time I used a Hand GT File made to these specifications in an epoxy curved-canal practice block (1991), I distinctly remember sitting in a beach chair in my backyard in a t-shirt, shorts, and flip-flops cutting a perfect 20-.06 canal shape with just a single instrument—the first ever single-file shape in endodontics!
Up until this time, my files were designed to be used by hand as I was prejudiced by the poor result seen with SS Sargenti handpiece-driven files. However, Drs. John McSpadden and Ben Johnson were then experimenting with using Ni-Ti to make handpiece-driven files (Quantec Files by McSpadden and .04 Taper Rotaries by Johnson), and, they had it right! Why do hand work if we can do it better with a slow-speed handpiece? So GT Files were redesigned to be rotary files by using the same radial flute lands used by McSpadden and Johnson (Derek was making all of the above), but with the same maximum flute diameter limitations and apically accelerating flute angles that defined Hand GT File geometry (Figure 3).
TDP’s sales manager at the time, Steve Cornelius, saw the value of this first-time, system-based rotary file set—one uniquely designed so the gutta-percha points, paper points, pluggers, obturators, posts, etc, automatically fit in the canal shapes cut by these files—and made GT Rotary Files TDP’s flagship product. The rotary file revolution was on. Five years later, TDP was sold to DENTSPLY for more than $150 million by completion of the deal.
The introduction of Ni-Ti rotary files to endodontics ushered in one of the greatest productivity improvements ever seen in dentistry. Instead of spending 20 to 40 minutes of clinical time to perfectly shape a root canal, we were now doing it in 5 to 10 minutes with 3 to 4 instruments. Dentists loved the procedural simplification, patients loved the shortened RCT time (contrary to Bill Murray’s character in “Little Shop of Horrors,” nobody wants a long, slow RCT), and of course dental companies were overjoyed to make and sell these new instruments for $8 when their stainless steel K-files only brought $1.50. Everybody was happy.
As to the clinical outcomes using GT Files, the first instrumentation research study3 ever done using CBCT imaging showed they delivered dramatically superior shaping results, both in terms of fidelity to the original canal path, but also in the highly reproducible canal shapes that even first-time dental students were able to cut with them, especially compared to shaping outcomes cut with stainless steel K-files and GGBs (Figure 4).
Deconstructing Rotary Failures
With their hyper-flexibility, radiused tips, and landed cutting flute geometries, as well as their uncanny ability to capture and remove virtually all cut dentin debris, this first generation of Ni-Ti rotary files eliminated apical ripping, mid-root transportation, and apical blockage with dentin debris. For those users who eliminated GGBs from their shaping protocol, they even eliminated coronal over-enlargement, which was a nearly ubiquitous problem at the time. Literally, the only remaining challenge faced by dentists using rotary shaping files was instrument separation; an event that seldom causes a lesser prognosis, but one that is feared more by dentists than apical laceration (a far more devastating iatrogenic event). This is because when dentists break files, they are immediately accountable, as they must inform patients to avoid negligence claims.
Breakage of narrow rotary files (.02 and .04 tapers) is usually caused by torsional or twisting forces. Bind the tip of a narrow file tight enough in the apical half of a canal and the shank end will simply twist off and separate from the tip portion. Wider files (.06 and up) come apart most often due to the metallurgic rending force called cyclic fatigue. Cyclic fatigue occurs when metals are bent and re-bent, imparting compressive forces into the molecular structure on the inside of the curvature and, simultaneously, stretching forces on the outside of the curvature. Bend a coat hanger back and forth 12 to 15 times, and it will come apart the same way.
During our early days at the barriers of the Ni-Ti revolution, one of the most troublesome and undecipherable breakage incidents occurred occasionally when brand new files were being used in apparently uncurved, uncalcified canals. The answer to that conundrum came when educators realized that straight-line access was much more important when using rotary files than it ever was for hand-driven instruments. Leave a bad access path that flexes the wider shank-end of rotary files, rotate them 5 times a second (300 rpm), and in a very short time you have cyclic fatigue failure, or, as we say, “short file syndrome.” Poorly aligned access cavities have a much less damaging effect on hand files, as not even a caffeine-addicted dentist can rotate a hand file faster than 70 rpm. After we figured out the access thing, the new-file thing, and the crown-down thing, rotary shaping was providing quality outcomes and manageable separation frequency.
Most revolutions go astray after the original objective has been met, and it was a bit like that after the initial change ushered in by Ni-Ti. In the first part of a revolution, the unworkable is replaced by a better alternative, and if it is successful, there is the 10x improvement that Peter Drucker mentions in “Innovation and Entrepreneurship.” However, the potential released from the first big bang empowers every competitor, be they political or technological, to join and attempt to subvert the revolution to their own interests.
This unfolded about 3 years into our rotary experience, when the non-landed instruments arrived.
Chasing Speed to Avoid Breakage
Of course, file breakage continued then as it does to this day and, like then, having a file come apart in a canal always makes for a bad day, unless you are Dr. Yoshi Terauchi with an amazing expertise in removing broken files.4 I am convinced that this fear is more related to the immediate, unescapable accountability of the devil’s bargain related to file separation and legal liability; you cannot be sued for breaking the instrument in a root canal, but you must raise your hand when the penalty occurs and inform your patient of the event before he or she leaves the office that day. Ironically, apical laceration has caused far more RCT failures than instrument breakage ever caused.
When the first non-landed, speed-cutting rotary files—EndoSequence (Brasseler USA) and ProTaper (DTDS)—came to market, they became overwhelmingly popular for several reasons. First, because they required little or no crown-down routine to cut to length in small curved canals—a significant simplification that removes the need to judge when it is time to go to the next smaller file as length is approached. Cut the S1, S2, F1, F2, and F3 to length. Done.
With landed-flute files (Figure 5), the less efficient blades required cutting large to small files closer and closer to the end of the root canal, moving on to a smaller instrument to cut deeper in the canal when the larger file balks. The challenge of crown-down shaping routines is the decision point of when to pull the working instrument and replace it with a smaller file. Miss the inflection point and the risk of file separation goes way up, an experience we have all had with these instruments.
With these much sharper, non-landed files, each file usually cuts to length, especially when used at 600 rpm (see “directions for use” with EndoSequence Files). This single fact explains much of the popularity of this non-landed generation of rotary files, because when files always cut to length rapidly, the perception is that the risk of file breakage is much lower, as most of the breakage experiences dentists have occur when the file balks and the dentist keeps it spinning in the canal for an extended period of time in the hopes that persistence will get the file to cut to length. Add the increase in apical pressure that clinicians unconsciously apply as the file slows its apical progress, and it’s easy to understand a file coming apart.
|Figure 5. Cross sections of landed (left) and
non-landed (right) file flute geometry and a CT reconstruction showing the difference in shaping outcomes despite both files having a 30-.06 external geometry.
|Figure 6. CT analysis of shaping outcomes of landed versus non-landed files when used around apical curvatures in small molar canals. These 30-.06 shapes were cut in side-by-side canals in mesial roots of lower molars. Note the apical transportation of the canals shaped with non-landed files (right) and the fidelity of the shape (left) cut with landed files. The shaping result on the right is the typical predecessor to overfilling.|
|Figure 7. Single-canal premolar treated
without cutting any dentin with shaping files. After confirming that the canal had ideal
natural shape, it was soaked with sodium hypochlorite for 40 minutes and filled.
So when the salesperson says, “Here doctor, try these new files out in this plastic block,” and each of them cut to length, there is a misplaced sense of relief on the part of the clinician—misplaced because these sharper fluted instruments are prone to rip the ends of small curved canals after they get to a large enough tip diameter to become less flexible. The classic experience with ProTaper Files was that everything was fine during use of the S1, S2, and F1 files, but then the apical transportation began with the F2 and F3 files (tip diameters of 0.25 mm and 0.3 mm) (Figure 6). This occurred so often that endodontists who continued using ProTaper files left the file set after the F1 was used, switching to landed files for tip sizes larger than No. 20.
Why no hue and cry about this significant problem? First off, because most clinicians have no idea as to why they overfilled. In fact, most over-extended fills are caused by poor apical shaping results—either incompletely shaped or, the worst, completely ripped. In addition, because there is no immediate accountability (these chickens come home to roost one or 2 years after the RCT has been done), apical transportation is feared much less than file separation. If an apical third is destroyed, but nobody saw it, did it really happen at all?
Hybrid File Sets: Efficiency Versus Safety
The answer? Understand that it is not just file geometry, but also tip size that causes these problems, and put together a hybrid technique, as many endodontists currently do in their practices. A Ni-Ti file with a tip diameter equal or greater than 0.4 mm is essentially as stiff as an SS file, so first use a razor blade-like file with a small tip diameter and then switch to a landed blade when the terminus is larger than 0.2 mm.
My current routine is to do initial entry and cut to length (if it can safely be done) with a 15-.06 Vortex Blue File (DTDS). This single file in the Vortex series is awesome because it is triangular, providing 57% more chip space between cutting flutes than a file with a square cross-section, so it can be cut to length without being concerned about apical blockage (the chip space collects the pulp tissue encountered). It has apically accelerating flute angles, so it doesn’t thread into canals, and its tips tend not to break off when bound. Furthermore, they are heat-treated to have limited shape memory, so they tend to untwist rather than break when bound in a canal; these are very important functional characteristics when using a rotary file with such a small tip diameter.
The tiny tip diameter is the key to this file’s remarkable function when used as the first instrument through the root canal in an RCT procedure. If we had an infinitely flexible file material, we could put razor-blade cutting flutes on all sizes and never worry about transportation because transportation requires sharpness plus stiffness of blade. As it is, I cut a 15-.06 VB file to length in most canals, after which I gauge with Ni-Ti K-files to measure the terminal diameter of the canal, and then cut a GTX File (DTDS) of appropriate tip size to length to finish. Two files total, and if the canal gauges at a No. 40 terminal diameter, I don’t have to worry about damaging the end of the root canal because I finish with a landed file.
So, do you need a $40 shaping file to be successful and profitable? Not really. The WaveOne File (DTDS) was marketed as a single-use file to rationalize a very high price per file, which is ironic as I have been cutting single file shapes since 1996, using files that cost $9. To this day I shape at least half of all the medium and large root canals I treat with a single 30-.08 GTX File. The other half are finished with a 40-.08. Two files ($9 each), and eminently reusable as they accumulate little or no cyclic fatigue in these less curved canals. When you use 2 files in 4 canals, the cost (2 for $18) is divided by 4, for a shaping cost per canal of $4.50. This is much better than $40 per canal, and, furthermore, you do not need a new handpiece either.
The Best Shaping Is No Shaping
The primary objective of shaping procedures is to cut, when necessary, shape in a root canal that will allow ideal irrigation and obturation. That’s it. Contrary to popular opinion, we don’t clean canals with our shaping files; we clean canals with our irrigating routines.5 When you really get this, you understand that it is not necessary to shape canals that already have ideally tapered shapes—an eventuality found most often in younger patients, as their teeth have undergone less trauma or have not had the time to calcify when pupally challenged. You will also understand that it is nonsensical to enlarge the apical terminus of any root canal, thus removing the most common cause of apical derangement—non-landed rotary files with large tip diameters erroneously taken to length.
My treatment plan when I find myself treating a single canal premolar on a 17-year-old is to broach the pulp, establish patency, determine length with an apex locator, gauge to see if the canal has adequate shape, and when that is the case, I cut no dentin and prepare the canal for obturation solely with my irrigating routine (Figure 7). This might sound a bit radical, but if you think about it, using shaping files actually creates the sludge we call smear layer, a collection of debris that must be etched off canal walls before we use sodium hypochlorite. Skip the shaping when there is already adequate shape in a young canal and this dirt will never be deposited on the canal wall.
There will be increasingly efficacious irrigation methods coming to market in the near future, and ultimately, as irrigation becomes more thorough and faster, our need for shape in the root canals we are treating will lessen. However, between the challenge of coronally occluded canals and the need for some semblance of a known shape to effectively obturate—even if photon-initiated photoacoustic streaming and/or Sonendo come to be successfully marketed and implemented—we will be shaping root canals into the foreseeable future.
HOW TO BEST CUT SHAPE IN A ROOT CANAL IN 2015?
Take your choice—there is a long list of methods and tools that can do the trick now. Just be certain that: (1) the terminus is respected as file tip sizes get larger, (2) shaping should require no more than 4 files total, and (3) most medium and large canals should be shaped with just one or two $9 rotary files. That’s today, not sometime in the future.
My best forward-looking guess about how we will be shaping root canals in the future is that shaping will end up where it started before Schilder, with apical preparations that are a bit larger than we cut now, and with much, much less coronal enlargement. Shaping objectives for our apical preparations will get larger because it will aid irrigation (especially negative pressure irrigation) in this most complex territory of root canals and also because we can cut larger shapes with landed-flute files without consequence where we could not before.
In the ‘70s, when Schilder wrote his treatise on shaping root canals, he needed continuously tapering shapes because only stainless steel files were available and that amount of coronal enlargement was required to allow rigid stainless steel files to be maneuvered around even severe apical curvatures. With the advent of Ni-Ti files and landed blade geometry for file tip sizes greater than 0.2 mm, the whole game changed and we suddenly needed less coronal shape to accomplish our apical purpose.
- Schilder H. Cleaning and shaping the root canal. Dent Clin North Am. 1974;18:269-296.
- Buchanan LS, inventor. Endodontic treatment system. US patent 5,897,316. Filed April 28, 1994, Issued April 27, 1999.
- Gluskin AH, Brown DC, Buchanan LS. A reconstructed computerized tomographic comparison of Ni-Ti rotary GT files versus traditional instruments in canals shaped by novice operators. Int Endod J. 2001;34:476-484.
- Terauchi Y, O’Leary L, Yoshioka T, et al. Comparison of the time required to create secondary fracture of separated file fragments by using ultrasonic vibration under various canal conditions. J Endod. 2013;39:1300-1305.
- Lussi A, Nussbächer U, Grosrey J. A novel noninstrumentated technique for cleansing the root canal system. J Endod. 1993;19:549-553.
Dr. Buchanan is a Diplomate of the American Board of Endodontics, a Fellow of the National and International Colleges of Dentists, and part-time faculty at University of California, Los Angeles’ and University of Southern California’s graduate endodontic programs. He is the founder of Dental Education Laboratories, a hands-on teaching center in Santa Barbara, where he also maintains a practice limited to conventional/microsurgical endodontic therapy and implant surgery. He can be reached via the websites www.delendo.com or www.dentalcadre.com.
Disclosure: Dr. Buchanan discloses that he has a financial interest in Care Credit, Obtura Spartan, SybronEndo, and DENTSPLY.