Having lectured throughout the country over the past 25 years, I can remember statements made by some attendees that took on the air of certitudes such as, "The gold cast post is the 'Cadillac' of posts," something that if it even stood the test of time would prefer not to any longer be analogous to a car from a manufacturer that may not be in business much longer. Other certitudes include, “Only use K-files. Reamers will distort the canal. Never go over the apex with anything greater than a 10 file. The gutta percha must be adapted as close as possible to the walls along the length of the canal. Thermoplastic obturation is the only way to fill lateral canals.”
The above notions all had some validity at some point in time. However, the criteria upon which these convictions were based has changed over the years to such a degree that the validity of the certitude is no longer necessarily true. Let’s take an example to see what I mean.
Constriction still an issue?
Does the statement, “Never go over the apex with anything greater than a 10 file”still have merit? It was originally invoked because the constriction, which we will define here as the apex, was the natural stop for a gutta percha point. If it was widened to a larger diameter the point would go through the constriction. Yes, the point could be cut back and even produce tugback, but given the 02 taper of the point, the somewhat greater taper of the canal even in the narrower mesio-distal diameter and the plasticity of the gutta percha point when subjected to vertical and lateral forces, it would be exceedingly easy to overcome the small amount of vertical resistance offered by the 02 taper in the point itself and produce a fill that extended beyond the apex. In the world of 02 tapers, this was reason enough not to widen the constriction.
Of course, problems arose when the 02 tapered point hit the ledge created at the constriction. It would often buckle upon itself creating a final fill where the canal looked well filled and adapted to the canal walls along the entire length minus the last couple of mm where the point crinkled after hitting the purposely-made ledge. The best that could be done would be to flood the canal with cement, covering up the voids that were produced by the crinkled point or just flooding the canal and placing the point with no extra vertical force applied, eliminating the subsequent buckling that would have been sure to follow. And that is how it stood for a very long time until greater tapered shaping came along.
Changing the rules of canal shaping
Figure 1 – Illustration of the greater taper wedge shape versus the parallel .02 taper. |
Greater tapered instrumentation changed the rules of shaping. Instead of having an 02 taper, the canals could now routinely be shaped to 04, 06 or greater tapered shapes (Fig 1). Gutta percha points with tapers close or equal to the tapers created in the canals were placed. Having a greater tapered point wedge into a locked position offered far greater resistance to apical overfill than an 02 tapered preparation and gutta percha point. With 02 tapered shapes the creation of a ledge to prevent overfills was necessary. With greater tapered shaping, the need for a ledge to prevent overfills was no longer needed. The point kept its place in the canal because it was wedged in along the entire length of the canal.
Once ledges were no longer needed, there was no longer a reason to limit the dimensions of an instrument going over the constriction to a 10. In fact, opening up the constriction now prevented the buildup of dentinal debris that often occurred with the old ledge making system. Debris buildup is always associated with loss of length. The attempt to regain the lost length often causes deviation of the instrument to the outer wall causing distortions and transportation in the apical third of the canal.
Evolving concepts trump old beliefs
So what we see is an evolution in concepts where the incorporation of new ideas does away with the need for old concepts. With the advent of greater tapered shaping and greater tapered gutta percha points, the creation of a ledge was no longer necessary and widening beyond the apical constriction to prevent debris buildup became a practical step in distortion-free canal shaping. The lesson to be learned from this example is that holding onto old beliefs when things change around us can make us retain lessons that are no longer relevant in the world we presently operate in.
NiTi Issues
Figure 2 – Unlike Rotary NiTi which has “Shape Memory”, this photograph shows the ability to be prebent a relieved reamer to conform to the shape of a canal. |
Another principle widely held today is that rotary NiTi is the only way to shape curved canals free of distortion. None of us would disagree that NiTi instruments are far more flexible than stainless steel and if used in the same manner the NiTi would always create less distortion if used in identically curved canals. Yet, stainless steel instruments may be designed as relieved reamers which while still stiffer than equivalently sized NiTi instruments, have the ability to be pre-bent to conform to the shape of a canal while (Fig 2) NiTi must adapt from the straight position with a shape memory that is always makes it attempt to snap back to the straight position.
Figure 3 – Photograph showing the more vertically oriented flutes of a relieved k-reamer versus the horizontal flutes on a K-file. |
The relieved reamer is more flexible than the K-file. It engages far less along length consequently encountering less resistance than a K-file as it negotiates to the apex. The more vertically oriented flutes of a relieved k-reamer cut the dentin more effectively than the horizontal flutes on a K-file (Fig 3) when the motion is rotation or reciprocation.
Figure 4 – Illustration of the tight watch-winding stroke of the instrument in a 30° reciprocating handpiece |
A combination of events has caused this principle to lose its original validity. The substitutions of relieved reamers for files have introduced instruments that when used with a tight watch-winding stroke or in a 30° reciprocating handpiece (Fig 4) produce no more distortion than that of a rotary NiTi instrument. Yet, this is only the beginning of the advantages that these relieved reamers have. Unlike rotary NiTi they are highly resistant to breakage and can be used in all situations no matter how abrupt the curve of a canal is, whether they merge, bifurcate, re-curve or dilacerate and they can be used several times without any increase in the incidence of separation.
Sometimes new paradigms work together. We talked about two examples, namely the ability to now widen the constriction beyond a 10 and the use of relieved K-reamers both manually and in the reciprocating handpiece. The new paradigm allows these relieved reamers to open up the canal to a 20 or 25 0.5 mm to 1.0 mm beyond the constriction preventing the buildup of debris that occurred when such overextensions was not permissible. The greater shaping of the canal constriction prevents the loss of length and the distortions that would have likely occurred in the attempt to regain that length. Two concepts work because they are both predicated on the safe creation greater tapered shaping.
Replacing rotation with reciprocation
By substituting reciprocation for rotation, all the relieved instruments can be used without fear of separation due to torsional stress or cyclic fatigue. In fact, we also incorporate the use of relieved tapered NiTi instruments which are just as safe to use as long as their usage is confined to the short arc of rotation produced by the 30° reciprocating handpiece.
No more K-files?
Once we understand the interplay of these two concepts we can now eliminate the use of K-files which always limited us in the quest for distortion free shaping. Having produced greater tapered shapes with a distortion-free system used in a reciprocating handpiece that has also eliminated the impaction of apical debris, we are no longer required to obturate the canal with total dependency on gutta percha being intimately adapted to the canal walls along its entire length.
Rather, the wider space can be flooded with epoxy resin cement along its entire length without driving cement over the apex using a simple device called the bidirectional spiral. A gutta percha point that conforms to the space is then coated liberally with cement and placed into the canal. The point has a taper between an 05 and 06 which makes it function much like a spreader driving the excess cement first laterally and then letting it escape coronally. The result is a non-interrupted cement interface between the walls of the canal and the gutta percha point.1-2
Furthermore, the low viscosity of the cement combined with the lateral forces from the tapered gutta percha point are sufficient to fill a plethora of lateral canals. Examples below illustrate this point clearly. The qualities of epoxy resin cement that make for a stable interface include no shrinkage upon polymerization, bonding to both dentin and the gutta percha point 3-4, actual expansion of the cement and gutta percha as they warm from room to body temperature, antibacterial 5-6 and highly resistant to hydrolytic degradation.
Conclusion
What we see is that old truths have a way of morphing into something less than ideal when we consider what is going on around those old ideas. New techniques and insights interact with each other bringing forth better approaches to endodontic treatment by making them safer, more effective efficient and affordable
As for the gold cast post, I’d make a good case that prefabricated posts with creative designs can surpass the goals of high retention and minimum stress better than cast gold, but that is the subject of a different paper. And comparing it to a Cadillac ain’t what it use to be.
Reference
- Gordon MP, Love RM, Chandler NP. An evaluation of .06 tapered gutta-percha cones for filling of .06 taper prepared curved root canals. Int Endod J. 2005;38:87-96.
- Investigating gutta percha and sealer distribution. Deutsch AS, Cohen BI, Musikant BL. Endodon Practice 2003;6:23-6.
- Teixeira CS, Alfredo E, Thomé LH, Gariba-Silva R, Silva-Sousa YT, Sousa-Neto MD. Adhesion of an endodontic sealer to dentin and gutta-percha: shear and push-out bond strength measurements and SEM analysis. J Appl Oral Sci. 2009;17:129-35.
- A. Jainaen, J.E.A. Palamara & H.H. Messer. Push-out bond strengths of the dentine-sealer interface with and without a main cone. Intl Endod J 2007;40:882-90.
- Heling I, Chandler NP. The antimicrobial effect within dentinal tubules of four root canal sealers. J Endod. 1996;22:257-9.
- al-Khatib ZZ, Baum RH, Morse DR, Yesilsoy C, Bhambhani S, Furst ML. The antimicrobial effect of various endodontic sealers. Oral Surg Oral Med Oral Pathol. 1990;70:784-90.