Describing internal routing as a different method of instrumentation and a paradigm shift, especially in light of the introduction of greater tapered rotary NiTi instrumentation about 25 years ago, might sound a bit presumptuous. This is particularly the case if you are a fan of rotary NiTi. Yet, the very reason it’s both a paradigm shift and an improvement is because of the previous introduction of greater tapered rotary NiTi. Please allow me to explain.
Rotary NiTi sprung from endodontic grounds that were dominated by 02 tapered stainless steel K-files, with the additional use of some Peeso and Gates Glidden reamers to entirely shape the canals. All K-files were used manually, often producing operator hand fatigue and impacting debris apically, leading (at times) to a loss of length, followed by distortions such as ledging and transportations to the outer walls of curved canals. The Peeso and Gates Glidden reamers widened the coronal third of the canal to straighten the access to these canals so that the thicker stainless steel K-files could more easily negotiate to the apex and, in addition, a tapered gutta-percha point would more consistently go to length.
Engine-Driven, Greater Tapered Preparations
Much was made of step-back preparations because it was generally accepted that wider preparations would be more easily irrigated and better obturated. In short, traditional endodontic instrumentation took a lot of work to be done in a way that would produce greater tapered shapes. Along came greater tapered rotary endodontics, with instruments able to impart a greater tapered shape without having to do step-back, made possible only because of NiTi’s greater flexibility. Thus, the greater tapered shape is a product of crown-down preparations. Lo and behold, rotary NiTi introduced a technique of instrumentation that produced the greater tapered shape with the instruments powered by a motor, doing away with a good deal of hand fatigue and leaving a space that was more readily accessible to both room temperature and thermoplastic obturation techniques. From the perspective of the dentist, these were attractive improvements. Who can argue with reduced operator hand fatigue, procedures often done more rapidly, and a result that coincided with what were, and are, considered by many to be superior canal preparations? So, rotary NiTi became the basis of a dominant form of cleansing and shaping and has been anointed by its advocates as a paradigm improvement over the traditional use of K-files.
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| Figure 1. Removing excess tooth structure in the mesiodistal plane is counterproductive, especially considering the fact that the canals are typically barely visible on a periapical radiograph. |
Figure 2. By eliminating rotation, whether continuous or interrupted, and substituting 30° oscillations, we have virtually eliminated instrument separation. If you do not break instruments in the first place, you need not learn all the steps required to avoid the problem. |
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| Figure 3. These illustrated microCT scans clearly demonstrate the amount of pulp tissue that may remain after mesiodistal preparations with greater tapered rotary instrumentation. On a mesiodistal radiograph, the results may look quite good because this same radiograph does not give us information on the buccolingual state of debridement, something the microCT scan clearly demonstrates is inadequate. |
Mandatory Compensation for Unintended Consequences
With the use of greater tapered rotary shaping, more root structure is removed in the mesiodistal plane (Figure 1), resulting in teeth that are less resistant to vertical fractures.1 A myriad of studies observed the introduction of dentinal micro-cracks resulting from rotary NiTi instrumentation used in either a continuous or an interrupted motion.2-6 From the perspective of the remaining tooth structure, the root has been weakened quantitatively and qualitatively. From the perspective of the rotating instrument, we know from personal experience (Figure 2) and numerous articles that these instruments are subject to separation, leaving hard-to-retrieve sections within the canal systems that we are trying to shape and cleanse.7 At a minimum, the blockage of access to the apex of the canals does not enhance endodontic success. Knowing that rotary NiTi instruments are subject to breakage, we are told to stay centered, with minimum brushing to the outside walls. This is done with the increased possibility of leaving significant amounts of pulp tissue in the thin buccal and lingual sections of oval canals (Figure 3). Finally, it is strongly suggested that these instruments be used once and discarded, with the clinician’s understanding that such rapid replacement will reduce, if not eliminate, the chances of instrument separation.
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| Figure 4. The same instrument that was used manually can now be inserted into the EndoExpress (Essential Dental Systems) handpiece that oscillates at 3,000 to 4,000 cycles per minute for rapid and thorough debridement and irrigation activation. | Figure 5. The ability of the SafeSiders (Essential Dental Systems) to internally route all the walls of a canal produces a final shape that reflects the original canal anatomy in larger form. The alternative to greater tapered rotary shaping needlessly removes excess dentin while reducing the ability to remove the pulp tissue in the buccal and lingual extensions of oval canals. |
With the passage of time, we can now appreciate that the introduction of greater tapered rotary NiTi instrumentation as a mixed bag. On the one hand, it creates a space that can be better irrigated and can lead to what appears in a 2-D periapical radiograph to be excellent obturation of the canals. On the other hand, the instruments are vulnerable to breakage,8 taking away excess tooth structure mesiodistally and removing inadequate pulp tissue, most often buccolingually, while having to be replaced after shaping a single tooth.
In the author’s opinion, the enumerated inadequacies of greater tapered rotary NiTi do not currently constitute a paradigm shift of improvement in endodontic techniques. If far more pulp tissue can be removed while, at the same time, preserving more dentin using the instruments in a way that virtually eliminates instrument separation and, consequently, provides the ability to use them several times before replacement, we would now have a genuine base for a paradigm shift of improvement, saving time and money and being more secure in the process. The question is: Does a system exist that can deliver on these goals?
This question brings us back to the subject of this article: internal routing. This concept is a simple one. Use instruments that fit within a preexisting space (the canals) as closely as possible. In most situations, this is not possible given the thin mesiodistal width of many canals. However, it is entirely possible in the broader buccolingual dimensions of the canals that are often present. Under these circumstances, we do the best that we can by first utilizing the thinnest instrument made: the 06 tipped, 02 tapered, twisted stainless steel reamer. It may be somewhat wider than the mesiodistal dimensions of the canal, but it will certainly be thinner than the broader buccolingual dimensions of an oval canal.
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| Figure 6. The flat surface on the SafeSiders provides for a thinner cross-sectional area that is more flexible, reduces engagement along length, creates a space for debris, acts as a paddle to further activate the irrigants, and creates 2 vertical columns of chisels that shave dentin away in both clockwise and counterclockwise motions. |
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| Figure 7. Note the large common buccolingual space that the MB and ML canals share in the coronal two-thirds of the canal length. The SafeSiders were able to clean this common isthmus space because of their ability to internally route the pulpal anatomy. |
The 06 tipped, 02 tapered stainless steel reamer is manually negotiated through the length of an irrigated canal. Take note of the length when the apex is reached. After reaching the apex, the instrument is withdrawn from the canal and placed into a 30° handpiece (Figure 4) that oscillates at 3,000 to 4,000 cycles per minute and, most often, can now be easily negotiated to the apex. Unlike rotary instrumentation, where it is a necessity to stay centered with minimum deviation, the 30° oscillating instruments are vigorously applied to all of the canal walls, particularly in the broader buccal and lingual extensions. Thin stainless steel, while still highly flexible, has enough body to remove the pulp tissue and a small amount of dentin wherever the oscillating reamer is applied to the canal walls. It is the ability to apply pressure without immediate deviation of the instrument, and the removal of concern regarding separation, that allows us to employ this technique (Figure 5). It cannot be done with rotary NiTi that, in thin dimensions, has minimum body and would deviate from the canal wall almost immediately, thus preventing it from shaving any dentin away. The ability of the instruments to be rigorously applied to all of the canal walls produces a shape that mirrors the original canal anatomy in a larger form. This is what we mean by internal routing.
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| Figure 8. These canals were quite thin to begin with and did not call for a greater mesiodistal taper, and yet all the canals were vigorously worked in the buccolingual plane because separation is not a concern with which we have to contend. |
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| Figure 9. These canals were slightly larger and called for a minimally wider preparation in the mesiodistal plane, but this still preserved far more tooth structure using 02 tapered internal routing than would have resulted from greater tapered preparations. |
The Proof is in the Pudding
One might question whether an instrument confined to such a short arc of motion can effectively remove sufficient dentin. In fact, we know that these instruments that are used in the 30° oscillating handpiece are effective because we routinely skip sizes. After negotiating a canal that offered significant resistance to a 06 tipped, 02 tapered reamer, I routinely go right to the 10 tipped, 02 tapered reamer, skipping the 08 tipped, 02 tapered reamer. Typically, the instrument I skip to is looser in the canal than the previous instrument that I had used, demonstrating the previous instrument’s ability to shave away dentin from the full length of the canal walls. From there, I frequently skip to the 20 tipped, 02 tapered reamer, and, quite often with one more instrument, I complete the instrumentation with a 30 tipped, 02 tapered relieved reamer (Figure 6). Then I prepare the canal for obturation with, in most situations, a fine gutta-percha point combined with an epoxy resin sealer to obturate those lateral spaces that are not occupied by the gutta-percha point. I have not employed the crown-down technique used by greater tapered rotary systems that, from the start, sacrifice excess amounts of dentin for the safety of the instrument while, at times, leaving pulpal remnants in the buccal and lingual dimensions.
Short Arches of Motion: Safer for the Teeth and Shaping Instrument
What is the difference between rotating instruments, whether used in continuous or interrupted motion, and those employed in a 30° oscillating handpiece? Rotation can produce excess amounts of torsional stress and cyclic fatigue, which are the main causes of instrument separation. Thirty-degree oscillations, representing one-twelfth of a full rotation, produce minimal amounts of torsional stress and cyclic fatigue. Furthermore, twisted stainless steel reamers do not have their flutes ground-in—a manufacturing process that produces defects within the core of the instruments that make them more vulnerable to breakage. Twisted instrument fabrication is a process that produces flutes without weakening the reamer’s core structure. By combining instruments with their flutes ground-in with full rotations, whether interrupted or not, the potential for separation increases.
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| Figure 10. In this long-rooted molar, the canals are close to disappearing in the apical third. However, the SafeSiders were able to adapt to the canal shape reflecting the wider coronal preparations compared to the far thinner preparations in the apical third. The key is the instruments’ adaptation to the canals rather than the tooth adapting to the shaping instruments. |
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| Figure 11. This case demonstrated the SafeSiders’ ability to cleanse complex anatomy without concern for instrument separation that, in turn, allowed for negotiation of the canal preparation to the apex without any visible signs of canal distortion. |
What really constitutes a paradigm improvement? If you think about it, the application of greater tapered rotary instruments (instruments far wider than the canals in the mesiodistal plane) is forcing the tooth to adapt to the shape of the instruments, regardless of the original canal anatomy. The thin tapered reamers, confined to the 30º arc of motion generated by the handpiece, adapt to the canal anatomy, removing a thin layer of dentin circumferentially from all canal walls, while targeting the pulp tissue wherever it is ensconced. The result is a more conservative canal preparation that preserves dentin while more effectively removing all the tissue in the canals, regardless of the canals’ configurations.
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| Figure 12. An example of typical curved canals shaped by the SafeSiders and obturated using a single room temperature gutta-percha point in combination with EZ-Fill epoxy resin cement (Essential Dental Systems). |
Preserving the Quality and Quantity of the Remaining Dentin
Greater tapered rotary preparations are not what we want. Teeth do not get stronger by removing more tooth structure than necessary. Without rotary NiTi’s ability to deviate from the centered canal position, pulp tissue is not more effectively removed. According to Newton’s third law of motion, when 2 bodies interact, they create equal and opposite forces on each other. Following this law, we know the impact of the canal walls on the integrity of the instrument can be obviously compromised by its separation within a canal. What we may not be aware of, but should be, is the impact of the rotating instruments on the canal wall. The studies that included examination of the production of dentinal micro-cracks from rotating instruments confirm the dual nature of this law, and, given the fact that dentin has fatigue properties similar to metal, this point should be driven home. From the opposite perspective, the fact that oscillating, stainless steel, twisted, relieved reamers are virtually invulnerable to breakage tells us that their likelihood of causing dentinal defects would be inconsistent with Newton’s third law.
CLOSING COMMENTS
Internal routing is a concept that makes sense. It makes the tooth the center of attention, as it should be! It also respects the structure that is being shaped. Relieved stainless steel reamers, in combination with a 30° oscillating handpiece, conservatively and more completely shape the canal the way it should be shaped, leaving a stronger tooth. This is my opinion of what constitutes a paradigm shift to improved endodontics. The fact that it is a far less expensive approach than rotary NiTi should also tell us that progress need not always be more technologically sophisticated and expensive. Figures 7 to 12 clinically demonstrate the results for the technique described herein.
References
- Ossareh A, Rosentritt M, Kishen A. Biomechanical studies on the effect of iatrogenic dentin removal on vertical root fractures. J Conserv Dent. 2018;21:290-296.
- Ashwinkumar V, Krithikadatta J, Surendran S, et al. Effect of reciprocating file motion on microcrack formation in root canals: an SEM study. Int Endod J. 2014;47:622-627.
- Barreto MS, Moraes Rdo A, Rosa RA, et al. Vertical root fractures and dentin defects: effects of root canal preparation, filling, and mechanical cycling. J Endod. 2012;38:1135-1139.
- Liu R, Kaiwar A, Shemesh H, et al. Incidence of apical root cracks and apical dentinal detachments after canal preparation with hand and rotary files at different instrumentation lengths. J Endod. 2013;39:129-132.
- Bier CA, Shemesh H, Tanomaru-Filho M, et al. The ability of different nickel-titanium rotary instruments to induce dentinal damage during canal preparation. J Endod. 2009;35:236-238.
- Yoldas O, Yilmaz S, Atakan G, et al. Dentinal microcrack formation during root canal preparations by different NiTi rotary instruments and the self-adjusting file. J Endod. 2012;38:232-235.
- Kazemi RB, Stenman E, Spångberg LS. A comparison of stainless steel and nickel-titanium H-type instruments of identical design: torsional and bending tests. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2000;90:500-506.
- Sattapan B, Nervo GJ, Palamara JE, et al. Defects in rotary nickel-titanium files after clinical use. J Endod. 2000;26:161-165.
Dr. Musikant has lectured worldwide in more than 150 locations and has coauthored more than 300 dental articles published in major dental journals. As a partner in a New York City endodontic practice, his 40-plus years of clinical experience have crafted him into one of the top authorities in endodontics. He’s currently the course director of endodontics at the Touro College of Dental Medicine at New York Medical College in Valhalla, NY. He can be reached at (888) 542-6376, via email at info@essentialseminars.org, or via the website essentialseminars.org.
Disclosure: Dr. Musikant is president of Essential Dental Systems.
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