Evolutionary Endodontics: Two Pathways, Different Outcomes
Dr. Musikant is a graduate of the University of Pennsylvania School of Dental Medicine. He is a member of the American Academy of Endodontists, the Academy of General Dentistry, and the American Society of Dental Aesthetics, and is a member of the board of the First Madison Group. In addition, he is the President and Co-Director of Dental Research at Essential Dental Systems. He has lectured throughout the world and is the author or co-author of more than 185 published articles.
- For dentists to understand the impact of the mechanics they are imparting to the root when they instrument canals, thereby giving them the ability to observe and differentiate between various modes of canal shaping.
Evolution applies to both life and innovation. In endodontics, productive innovation leads to a better quality of life, at least orally speaking. Evolution is all about competition, with the winners being those that best adapt to a changing environment. In the practice of endodontics, we have competing systems that at times cross paths, but start from different premises. By examining the different premises, we have the ability to judge what works most safely and efficiently for the long-term preservation of the tooth, and in so doing understand better where market forces impact the direction endodontic systems should be taking.
Let’s start with the premises. If we examine rotating NiTi systems, we know that their most obvious vulnerability is one of separation. Not that it happens often, but the unpredictability arising from separation imparts procedural stress and conditions us to employ steps that protect the instrument at the expense of the remaining tooth structure. The instrument was initially meant to shape canals via engine-driven systems that would reduce hand fatigue, impart greater tapered shaping that allows for better irrigation and produce a shape that is designed to offer apical resistance when thermoplastic techniques are employed. When NiTi systems were introduced, there was little in the literature that correlated greater tapered rotating shaping with reduced resistance to vertical fracture. As a result, straight-line access became a priority, reducing the degree of cyclic fatigue the instruments would be exposed to, in turn reducing the incidence of instrument breakage. With no evidence that greater tapered shaping imparted weakness to the roots, greater tapered shaping appeared to be all gain without any loss.
We now know that is not the case. The literature has clearly documented greater tapered shaping, aggravated by straight-line access weakening the resistance of roots to vertical fracture. Furthermore, the action of an instrument rotating within a canal has also been documented to produce dentinal micro-cracks that propagate when active methods of obturation such as lateral and vertical condensation and a host of thermoplastic techniques are employed. Given this new information, the basics of greater tapered NiTi preparations are being fundamentally challenged. From an evolutionary point of view, their continued success must be based on adaptation.
Some of that has occurred. The introduction of asymmetric reciprocation (interrupted rotation) has reduced the incidence of instrument separation while not eliminating it. Newer NiTi instruments are also less vulnerable to cyclic fatigue, although resistance to torsional stresses has not increased. These innovations reduce the vulnerability of the instruments to breakage, but do nothing in reducing the detrimental impact on the dentinal walls that are still subject to the production of micro-cracks. In fact, an evolution has taken place in reducing the number of instruments required to shape the canal. From a marketing point of view, this is an attractive feature decreasing the dentists’ expense. However, fewer instruments concentrate the stresses that were previously shared by a larger number of instruments that imparted canal shaping in a more gradual manner. The research has not documented a decrease in dentinal defects produced by these innovations. On the contrary, some studies document an increase in dentinal defects produced by systems employing fewer instruments. Another innovation is the greater emphasis on rotating NiTi instruments limited to a taper of 06. What is touted as NiTi’s greatest asset, a high degree of flexibility, is also its greatest limitation, however. An 02 tapered NiTi instrument with a tip size of 13 or 15 is so flexible that it flexes away from any canal wall with minimal pressure applied. With little adaptability for leaning into the walls of the canals, the instrument will only shave dentin away where the dimensions of the canal are smaller than that of the instrument itself. In short, flexible metallurgy and poor resistance to torsional stress and cyclic fatigue allows NiTi systems at best to create conical shapes that will reflect the instrument’s dimensions but are unlikely to reflect the dimensions of the canals, which most often will be highly oval and sheath-like in the bucco-lingual dimensions.
From an evolutionary standpoint, NiTi has locked itself into a space that it is hard to escape from. Designed with small tapers, the instruments are too flexible to effectively shape the canals laterally. With increasing tapers they are more vulnerable to torsional stress and cyclic fatigue, necessarily resulting in the removal of dentin that would be better preserved. The less the taper, the more difficult it is to use thermoplastic obturation techniques effectively and the more stress that will be imparted to the roots when vertical and lateral condensation pressures are applied. That is where rotating NiTi more or less stands today and the implications it has on the typical oburation techniques that are subsequently used.
Let’s now go down another evolutionary pathway. One that completely eliminates the use of NiTi in rotation and minimizes its use for the most part to an 04 tapered instrument used in 30º reciprocation. Before we get to that step, however, the premise of this alternate approach is to preserve dentin while effectively cleansing canals that are often highly oval and sheath-like in both the mesio-distal and bucco-lingual planes. For those familiar with rotating NiTi, the goals of shaping the canal in both the mesio-distal and bucco-lingual planes without removing dentin excessively in the thinner mesio-distal plane do not sound doable. One reaches this conclusion because those using NiTi are familiar with the need to keep these rotating instruments centered, knowing attempts at deviation from this pathway can lead to an increased incidence of instrument breakage.
What is true for NiTi is not true for stainless steel 02 tapered reamers, unrelieved through a 10 and relieved with a flat along the working length form 15 onwards. Thinly tapered NiTi instruments are too flexible to effectively shave a wall whose dimensions are greater than the NiTi instrument. Stainless steel, while highly flexible in thin dimensions, has enough body to work against the canal wall, effectively shaving away dentin on a canal surface that is greater than its own dimensions. This point is critical in understanding the difference between the two. When we negotiate an 06 tipped 02 tapered stainless steel reamer to the apex with enough flexibility to negotiate the most curved of canals, we do not serve the instrument’s total purpose. Once at the apex, the instrument is vigorously worked against what are often the broader buccal and lingual walls. This task is made far more effective by using them in a 30º reciprocating handpiece oscillating at 3000-4000 cycles per minute. Stainless steel’s reamer design adds to its flexibility and increases its ability to adapt to the canal walls as it records curves while passing through them. With a high frequency of motion coupled to the short amplitude of motion produced by the reciprocating handpiece, the thinnest 02 tapered stainless steel reamers create a space for the subsequent instruments that allows them to reach the apex with minimal resistance while maintaining the original anatomy in an increasingly larger form. Each instrument in turn widens the canal beyond its own dimensions, defining the original pathway more clearly and allowing the next marginally stiffer instrument to faithfully follow the increasingly defined established pathway. This would not be possible were instruments of greater taper used or if the motion were greater than that generated by the 30º reciprocating handpiece.
The results of this approach are entirely different from those produced by greater tapered instruments. Research has documented that instruments used in short arcs of motion do not produce dentinal defects, whether used manually or in a handpiece generating a high frequency of oscillations. An 02 tapered stainless steel reamer design removes minimal dentinal along the thinner mesio-distal length of the canal while having the ability to shape the broader buccco-lingual extensions that are often present. Virtually immune to breakage, such instruments require neither straight-line access nor crown-down preparations, thus preserving dentin that can only make the tooth stronger.
A beneficial side effect of the use of reamers in a 30º reciprocating handpiece is that it allows them to be used multiple times with the downside of over-usage only being a dull instrument, not a broken one. Overhead is thereby reduced dramatically. I typically use most of these instruments 5-6 times before replacement.
We have discussed two evolutionary pathways. First, rotating NiTi created shapes that removed excess dentin. At the time of their introduction, there were no prohibitions against this practice and such preparations fitted neatly into a technique that also made the instruments less vulnerable to separation. We today know that such preparations weaken the teeth. Thus attempts are being made to reduce the amount of dentin removed, and hence the introduction of 02 tapered NiTi instruments for initial glide path creation that minimize their ability to cleanse the broader bucco-lingual extensions. Users of the NiTi systems remain committed to rotation (be it continuous or interrupted), a technique that is deeply implicated in the production of dentinal defects.
Secondly, by contrast, emphasizing 02 tapered stainless steel reamers enables us to preserve tooth structure in the mesio-distal plane even as we are now capable of extending the preparations into the thin buccal and lingual extensions of highly oval sheath-like anatomy using a handpiece that generates a 30º arc of motion at 3000-4000 oscillations per minute, without the unwanted production of micro-cracks that has been associated with NiTi usage. From an evolutionary point of view, considering the impact of each system on the quality and quantity of remaining dentin, there is little question that short arcs of motion and 02 tapered stainless steel reamers, both unrelieved and relieved, give the tooth a better chance of survival.ADA Recognition Statement
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