- Most roots are wider in the bucco-lingual plane than the mesio-distal plane. This asymmetry is reflected in the pulpal anatomy. Pulpal tissue is most often quite thin mesio-distally, but often quite broad bucco-lingually. Yet, despite this fact, canal preparations are widened broadly in the mesio-distal plane where the roots and the pulp tissue are thin and inadequately instrumented in the bucco-lingual plane where both the roots and the pulp tissue are wider. In short, creating a canal preparation that is round in cross-section is counterproductive when the canals are highly oval and isthmus-like to begin with. The mesio-distal preparation needlessly weakens the tooth and the bucco-lingual preparation leaves tissue in the canal’s extensions.
- The creation of greatly tapered conical preparations via engine-driven rotations have been shown in repeated studies to predictably produce dentinal micro-cracks. The greatest susceptibility for micro-cracks is where the dentin is the thinnest, namely on the furcal sides of roots where significant concavities are routinely present. The greater the taper of the instrument the more significant the amount of tooth removed in the mesio-distal plane. For example, a 40/02 instrument to the apex will create a mesio-distal preparation of 0.64 mm 12 mm coronally. A 30/04 instrument will create a mesio-distal preparation to 0.78 mm 12 mm coronally, 0.14 mm wider, a 22% increase over the 40/02 preparation despite the fact that the 40/02 preparation is creating a wider space apically. We should take into account the fact that the external shape of roots generally have a far greater taper bucco-lingually than their mesio-distal counterpart.
- Logically given the above facts we would want to create an oval preparation mirroring the original anatomy. However, that is rarely done because rotary NiTi is increasingly susceptible to breakage the more it is worked laterally. The safest way to use them from the instruments perspective, is to stay centered and use light pressure in a pecking motion no more than 3 mm in amplitude. Unfortunately, it is the perspective of the tooth that ultimately counts and that perspective is too often sacrificed to prevent instrument separation.
- We produce beautifully tapered mesio-distal preparations recorded on x-ray and convince ourselves that the mesio-distal beauty is confirmation of a job done well in three-dimensions despite the lack of evidence to reach that conclusion. I am providing a series of videos gathered from researchers on pulpal anatomy that clearly depict pulpal anatomy as a far more complex entity than that simplistically described by a conical shape. The discrepancy between real anatomy and the shapes imposed by rotary NiTi continuous or interrupted leads to weakened teeth and less than ideal cleansing. To ignore this discrepancy does not make the problem go away, but it does discourage critical thinking that is necessary for progress to be made in any field.
We cannot make progress in the mechanics of endodontics until we realize the ideal goal of removing all tissue and bacteria with the smallest sacrifice of dentin. We are not there yet. The incorporation of greater tapered instrumentation ignores those goals and the companies manufacturing these systems have employed effective marketing at all levels of education starting with the universities to advance their product lines and in so doing have imo led to retrogression in the endodontic treatment of teeth.
Interestingly, at the last AAE meeting, Dr. Robert Roda, immediate past president of the AAE brought up the observation of an increased incidence of vertical fractures over the past 20 years, an issue many in the audience concurred with. While he did not ascribe causes, other than people living longer, something that has not dramatically changed in well over 20 years, the one fundamental shift in endodontic techniques in the last 20 years is the implementation of greater tapered instruments used in engine-driven rotation. We know greater tapers are obviously associated with an increased sacrifice of tooth structure. We know the rotation of greater tapered instrumentation is associated with the production of dentinal defects that can coalesce and propagate into full-blown vertical root fractures.
One is not stretching credulity by concluding less remaining tooth structure encourages the incidence of crack formation, something confirmed in the dental literature. What is left unstated because of the intimate relationship between present day academics and corporate largesse is the fact that documented evidence strongly suggests we are employing systems that are weakening the teeth, that we are entranced with the beauty of greater tapered fills while ignoring all the evidence that we are compromising the tooth’s integrity.
Until we retreat from a concept that puts the safety of the instrument ahead of the integrity of the remaining tooth structure, we will remain in a state of rationalization where denial and misdirection take the place of critical thinking. The goal should be maximum removal of tissue and bacteria with the least loss of tooth structure necessary to accomplish that goal. As stated above, when an 06 tipped 02 tapered instrument encounters mesio-distal resistance within the first 3 mm of penetration, it makes no sense from the perspective of the tooth to ultimately open the canals coronally to a width often exceeding 1 mm, a manmade width, 10 times wider than the original width that led to resistance. If we think about it, we accept these wide preparations because they protect the instruments. They are doing nothing for the tooth. Our problem is that we are more concerned about a separated instrument than we will ever be about dentinal micro-cracks. We see the former immediately and it is embarrassing to say the least. The latter is just a bit of research that has no immediate clinical consequences.
Once we understand where weakness to both the instruments and the tooth come from, we have a rational basis for devising techniques that prevent the removal of excess tooth structure and the separation of instruments that propel us to use techniques that are weakening the tooth. Some of these steps include:
1. Employ short amplitudes of motion that virtually eliminate the torsional stress and cyclic fatigue that leads to instrument breakage.
2. For the most part don’t exceed an 02 taper. Again, consider an apical preparation with a 40/02. 12 mm coronally, the mesio-distal preparation will be .64 mm. Compare this preparation to that made using a 30/04 instrument. Coronally, the mesio-distal preparation will be .78 mm, a .14 mm difference and 22% wider where greater mesio-distal width removes excess tooth structure needlessly. By using 02 tapered instruments we can preserve coronal dentin while having the ability to increase the apical preparation if we find that necessary.
3. Attaching 02 tapered reamers modified with a flat along their working length to further reduce resistance along length and using them in an engine-driven handpiece oscillating at 3000-4000 cycles per minute provides for rapid canal preparation from start to finish without hand fatigue. To date, those recommending rotary NiTi still claim the need to prepare a glide path manually, a portion of the shaping process that can take longer than the entire instrumentation process when using a 30º reciprocating handpiece.
To make genuine progress, we must value the perspective of the tooth. For the most part, that has not been the case through the entire evolution of rotary NiTi whether used in a continuous or interrupted manner. To progress beyond this point, we need techniques that encourage us to be conservative when necessary (mesio-distally) and aggressive at other times (shaping bucco-lingually). For sure the final shape will rarely be conical and that is a step in the right direction.