Contested issue #3: Greater Tapered Rotary Instrumentation

For many dentists rotary instrumentation is not a contested issue. It is an engine-driven system that speeds up the shaping process, reduces hand fatigue and imposes a shape to the canal that has been described as faithfully recreating the original canal anatomy in larger form. We have been taught that the results, observed in periapical x-rays, are not only superior to the previous outcomes achieved with 02 tapered K-files used in a manual watch winding motion, but are attained with less effort. Our endodontic educational system has reinforced these interpretations. One or another rotary endodontic system has been adopted by many of the dental schools. The AAE provides huge exposure to the companies producing these systems and the majority of articles written about endodontic instrumentation are about rotary systems. There is little serious discussion about alternatives and the advantages that alternatives might offer. For that to happen, the dialogue would need a critical review of the impact of rotary NiTi instrumentation to date.

A critical review in turn would challenge a good deal of the marketing effort that has gone into selling rotary instrumentation. It would challenge the education that the students are receiving in their respective dental schools and it would question the emphasis and access the AAE and the major dental magazines give to the sponsors of these systems. Let’s examine the issues that would be brought up by a critical review of the impact of rotary NiTi instrumentation .

1. Rotary NiTi essentially produces conical shaping with varying tapers. Is this a good thing? As long as the taper is a relatively close approximation of the original canal taper, shaping of the canal in this manner is effective without deleterious impact on the tooth’s residual strength. With the exception of maxillary anteriors, the original shape of the canals are not conical. They are exceptionally thin in the mesio-distal plane with a taper no greater than 02, a taper that mirrors the external anatomy of the tooth mesio-distally. In all these other situations, the imposition of greater tapered rotary instrumentation is by definition removing excess tooth structure defined as the removal of dentin that does not improve the conditions for success. Conical shaping only makes sense when the original pulpal anatomy was conical and then only to the degree that it removes a uniform layer from all the walls of the canal.
2. Not only are canals thin mesio-distally they are often quite wide bucco-lingually. Their original shape is far more oval with thin isthmuses extending buccally and lingually. In creating conical shapes, rotary NiTi is needlessly sacrificing tooth structure on the mesial and distal walls of the canal, but is leaving significant amounts of pulp tissue buccally and lingually.
3. The tendency to impose conical shaping is reinforced by a concern for instrument separation, an iatrogenic event that dentists want to avoid. They do this by staying centered reinforcing the concept of conical shaping despite the highly oval isthmus-like anatomy of the pulpal spaces that are being cleansed and shaped.
4. To further avoid instrument separation, straight-line access and crown-down preparations are recommended, two steps that remove even more dentin in the coronal third of the root.

The concept of greater tapered rotary instrumentation is first plagued by excessive removal of dentin mesio-distally, an inherent outcome when greater tapered instruments are used that is then accentuated by the need to use these instruments in a way that minimizes their potential for breakage that then leads to the secondary cause of excessive removal of dentin in the mesio-distal plane while inadequately debriding the canal in the bucco-lingual plane. Perhaps, rotary NiTi would work more effectively if the tapers of the instruments were reduced. Certainly, less tooth structure would be removed in the mesio-distal plane. However, used in rotation whether interrupted or continuous, the instruments are still subject to unpredictable amounts of torsional stress and cyclic fatigue that can lead to instrument separation. The dentist will continue to use them in a centered position avoiding the buccal and lingual extensions where significant pulp tissue may be lying. They will still seek straight-line access adding to the amount of tooth structure that is being needlessly removed.

The disparity between actual pulpal anatomy and the shapes imposed by greater or even lesser tapered rotary instruments discourage three-dimensional instrumentation. Any comparison between the shapes imposed and the highly oval anatomy that originally existed clearly demonstrates a conceptual deficit that should not exist let alone encouraging greater implementation. Furthermore, a large and growing body of evidence strongly links greater tapered instruments and rotary instrumentation as two factors that lead to the production of dentinal micro-cracks, defects that can coalesce and propagate into full-blown vertical fractures.

Those advocating the use of greater tapered rotary instrumentation recognize the negative impact that critical thinking has on its continued use. The advocates can argue that interrupted rotations produce reduced rates of micro-cracks. This is not borne out in the literature. They can argue for the use of a reduced number of instruments. This approach only concentrates the stresses resulting from instrumentation into fewer instruments, an approach that is likely to further increase the incidence of instrument separation and the number of defects. They can argue for reduced tapers, but that decreases their effectiveness in the bucco-lingual plane. The literature leaves little room for escape for those advocating rotary NiTi. The last avenue of escape is the denial of the bulk of the literature.

Such denial takes three different forms.
1. Much of the studies on the production of dentinal defects induced by rotary NiTi be it continuous or interrupted are on extracted teeth that are then sectioned at 3mm, 6mm and 9mm from the apex for microscopic examination. Research denial derives from the possibility that extracted teeth and the sectioning process is inducing defects that would not exist in teeth that are still in the mouth and surrounded by a periodontal ligament. These studies demonstrate a far higher incidence of dentinal defects after instrumentation. The controls and those done with short amplitudes of hand instrumentation do not display such defects. By stating that extracted teeth subject to sectioning do not reflect reality, the advocates of rotary NiTi are stating that the results documented to date are anomalies and should be ignored.
2. To partially answer those doubting these studies, researchers have documented dentinal micro-cracks after rotary instrumentation using micro-ct scans, a technique that does not involve sectioning. While they reinforce the sectional studies one could still say that extracted teeth are the cause of defects that would not exist in reality.
3. Other studies using finite element analysis, a process that is computer derived reproduce the same results attained in both the sectional studies and the micro-ct scans. Those advocating rotary NiTi cannot blame the results on extracted teeth. They must also deal with the fact that the majority of studies mutually confirm these results.

Yet, the desire to retain the momentum of a long, intense and effective marketing strategy that has propelled rotary NiTi instrumentation as the paradigm improvement it claims to be is ever present. In this vein, one cadaver study on bodies over 82 years old displayed the presence of dentinal defects without any endodontic procedures being done concluding that dentinal defects exist whether endodontic procedures are done or not. That is one interpretation. Another would be that dentinal defects present before endodontics are likely to coalesce and propagate during and after endodontic procedures. Their preexistence is imo no excuse to not be cautious about the use of greater tapered rotary instrumentation.

Finally, another study has stated that dentinal defects are a product of instrumentation regardless of the system used. In short, whether the instrumentation system employs short arcs of non-rotary motion, continuous arcs of full motion or interrupted arcs of full motion the results are the same and the system employed should be based on other factors. The advocates rationalize that because all systems are sinners why not stay with what you are using, likely a rotary one. This research suffers from its obvious denial of Newton’s Third Law of Motion, that interactive bodies have an equal and opposite effect on each other. Those familiar with rotary NiTi know the instruments are subject to unpredictable separation. Those using 30º reciprocation far less so. The short arcs of motion that reduces the stresses imposed by the canal walls on the reciprocating instrument in turn subjects the canal walls to equally reduced stresses giving a scientific reason why so many studies confirm minimal dentinal damage when the shaping instruments are limited to short arcs of motion.

There is good reason why greater tapered rotary motion is a contested issue. Other than commercial interests, there is no good reason why this issue has not been brought front and center in endodontic educational circles. Commercial interests influence ducational centers impeding and disrupting true education.  They should actively be encouraged to drop these relationships. Commercial interests will never stop trying to influence content. It is up to honest educational centers to remove all impediments to that honesty