Tapered Preparations are Not Necessarily Conical Preparation

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Tapered Preparations are Not Necessarily Conical Preparation

Postby BarryMusikant on Sun May 31, 2015 8:09 am

From a practical point of view, our endodontic shaping protocol includes steps to reduce the chances of instrument breakage. Included in this protocol is the need for straight-line access and highly conical shaping, a result of crown-down preparations. It should be understood that the imposition of greater tapered shaping is done to increase the preservation of the instruments. While infected dentin is unquestionably removed in the process, a good deal of dentin is needlessly sacrificed to minimize the stresses that would be applied to these rotating NiTi instruments if fully engaged. The rational for such preparations has mainly been one of endodontic esthetics where a significantly tapered preparation from stem to stern has been uncritically defined as a paradigm improvement and certainly the creation of a beautiful curved preparation gently widening as it travels coronally is pleasing to the eye. What has been completely deemphasized until recently is the loss of dentin in the mesio-distal plane that is a consistent component of greater tapered crown-down shaping. Today research has confirmed that greater tapered shaping reduces the resistance to vertical fracture based simply on that greater loss of dentin as well as the production of dentinal micro-cracks, a result of instruments rotating within the canal. Together greater tapered shaping and rotation are two factors documented in the research that directly weaken the roots.

These conclusions make sense. We have known for years that rotary NiTi instruments are vulnerable to breakage as a result of the pressure applied to these instruments by the surrounding dentinal walls. The observation of a separated instrument from these stresses is obvious. What is not obvious is the damage these instruments have on the integrity of the canal walls. That is where a large number of studies are now directing our attention, to the unseen damage that is routinely occurring as a result of the utilization of these approaches.

Bringing these issues up can result in a state of cognizant dissonance. We learned early on that a basic principle of canal shaping is a continuous taper. Without it, we are unlikely to create a good seal with our gutta percha and cement. It takes a tapered preparation for the cement to flow well and to apply thermoplastic obturation techniques efficiently. Having learned this principle early on, any criticism of greater tapered preparations might appear to be weakening the principle of continuous tapers.  In fact, one must differentiate between continuous tapers and conical shapes. While it is true that conical shaping creates equal tapers in two planes (mesio-distal and bucco-lingual) the principle of continuous taper does not state that the tapers in both planes must be equal. It states that they must be present, but not necessarily the same. Insisting on equal tapers leads automatically to the removal of excess dentin in what is most often the thinner and far less tapered mesio-distal plane. It also leads to an inadequate taper in the far wider (but still thin) bucco-lingual plane, a result that leads to inadequate tissue removal in this plane as documented in several research studies.

We often describe pulpal inclusions being in a sheath-like configuration. Sheath-like describes a configuration where the pulp tissue is generally very thin in the mesio-distal plane, but quite wide in the bucco-lingual plane accurately reflecting the external shapes of the roots. The imposition of a conical shape in these situations can only lead to inadequate debridement and the weakening of the root in the mesio-distal plane and the potential for strip perforation where the furcal sides of roots often have concave anatomy.

If we look at the mesio-distal anatomy of pulpal tissue as seen in a typical periapical x-ray we first notice just how thin the roots are in this plane even at their widest coronal extensions.

  


These roots may be so thin that attempting to negotiate an 06 tipped 02 tapered stainless steel reamer into them is met with significant resistance within the first few mm of entry. This resistance tells us that these canals are extremely thin at their widest. We can also see that from orifice to apex, these pulpal spaces are almost parallel with minimal taper along length. Such pulpal configurations do not call for greater tapered preparations. They are inappropriate in what is most often the thin mesio-distal plane. On the other hand, sheath-like tissue, wide in the bucco-lingual plane often requires a preparation with a far greater taper to effectively remove the tissue in this plane. What we are describing is a canal preparation that far more accurately reflects the original anatomy in larger form. By elongating the canal preparation in the bucco-lingual plane we will remove more tooth structure, but it will not be excessive because the root is far wider in this plane. At the same time, the minimum taper in the mesio-distal plane will preserve tooth structure where the roots are thin to begin with.

The imposition of conical shaping is a simplistic approach to canal shaping that ignores the oval shealth-like pulpal configurations that are often present. It’s popularity stems from its relative ease of application in many canals that are wide enough mesio-distally to allow unimpeded progress to the apex. The x-ray limited to viewing the mesio-distal preparation does not show excessive removal of dentin due to the more bulbous anatomy of the buccal and lingual roots superimposed over the centered concave anatomy that exists between them and further reinforced by former concepts that depict such preparations as superior to lesser tapered preparations. Nevertheless, as increasing amounts of research document, roots are being weakened and subject to increasing chances of vertical fracture.

A more constructive approach would include an emphasis on lesser-tapered instruments used with short, but rapidly applied arcs of motion. By using 02 tapered stainless steel vertically fluted relieved reamers in a 30º reciprocating handpiece oscillating at 3000-4000 cycles per minute we create conditions where it is virtually impossible to separate an instrument. Once the dentist is convinced that the instruments will remain intact despite the degree of curvature and the resistance encountered, he/she will feel liberated to work these instruments vigorously in the wider buccal and lingual plane. 06 and 08 tipped 02 tapered stainless steel reamers will encounter minimal resistance mesially and distally as they extend their preparations buccally and lingually. The result is a preparation that will remain minimally tapered in the mesio-distal plane, yet extend itself to engage and remove the tissue ensconced in the thin buccal and lingual extensions.

With separation no longer an issue, the steps incorporated to reduce separation are no longer critical.  Straight-line access and crown-down preparations are no longer required. Indeed, greater tapered conical shaping, the major reason for NiTi metallurgy, can no longer be considered optimally beneficial for the integrity of the tooth. The vast majority of canal preparations are better served with the predominant use of relieved stainless steel reamers confined to a short but rapid arc of motion defining a glide path that each subsequently larger tipped 02 tapered reamer will faithfully follow.

The principles make sense and are supported directly and indirectly from the research. If one looks at the cases posted in this discussion we see how conservative preparations can be in the mesio-distal plane while extending the debridement more effectively in the bucco-lingual plane, something that is not readily apparent on x-ray, but is now an easily attainable goal for any dentist aware of its need.

  

  

  

Please note that in the last case posted above that what you are looking at is an off-angled x-ray that is displaying the sheath-like anatomy in the bucco-lingual plane. The last thin we would want to do is to create a wide tapered preparation in the mesio-distal plane that would significantly undermine the strength of the root.

Regards, Barry
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