The Philosophy Supporting Various Endodontic Instrumentation Techniques

Philosophy may be to fancy a term to use. Perhaps, simply using the word “reasons” describing the rational for the various techniques would do as well and not be as pretentious. When we discuss the stated reasons, we should look out for inconsistencies on the one hand that may weaken the reasoning and on the other hand the consistencies that add strength to the reasoning process. With this in mind lets look at the reasoning supporting different methods of endodontic instrumentation.?

Hand instrumentation?with the auxiliary help of peesos and gates glidden stainless steel rotary reamers to straighten the coronal portions of curved canals:

This technique would generally be defined as the original and traditional method of shaping canals prior to the introduction of rotary NiTi. As in anything original, there was little to compare it to and the techniques employed seemed to reflect the goals of instrumentation as we understood them at the time. Despite, the many studies describing the complex state of a good deal of pulpal anatomy, the goal was to create an undistorted conical shape in the canals to more or less match a gutta percha point and any gap between the wall and the point would be filled with a sealer and the introduction of additional gutta percha points into any spaces following lateral and vertical condensation.?

?The initial instrumentation was pretty much done with K-files. While the watch winding motion generally employed is truly more suitable to a K-reamer, the early instruments were fabricated from carbon steel, a brittle material that was subject to separation if it encountered excessive torsional stresses. Knowing that K-files used minimal rotation to engage a small amount of dentin and then employed a pull stroke to shave away the engaged dentin made them safer to use from the point of view of keeping the instruments intact than the K-reamer that was only effective with greater degrees of rotation. In this manner, the watch winding motion was kept to a short arc followed by the pull stroke that did not lead to stresses that might separate the instrument.?

By the time stainless steel was introduced with its far greater resistance to torsional stress the concept of using K-files was pretty well accepted as conventional wisdom and has remained so to this day. Such is a good part of the human mentality that we resist a challenge to traditions. The great variation in results from this technique is based on the complexity of the cases and those performing the tasks. Where curved canals are present, it takes a great deal of skill to negotiate and widen them without incurring distortions, blockages, ledges, transportations and outright perforations. These problems become all the more obvious when these types of cases are aggressively approached manually with excessive apical forces applied, ignoring tactile perception of what the tip of the instrument is encountering and excessive degrees of rotation. Many of these distortions are obvious in periapical x-rays and these??represented mechanical defects became the subject of a host of research sudies.?

Interestingly, the bulk of the articles regarding different forms of canal distortion did not make their way into the dental literature until after the introduction of rotary NiTi. The greater distortions produced by the manual use of K-files had been present since their use many decades before the introduction of rotary NiTi, but they only became a subject of increasing interest with the advent of the more flexible NiTi The short comings of the manual use of K-files became interesting when their limitations became a selling point for rotary NiTi. What had been an acceptable technique for endodontic instrumentation was now seen as old fashion, from an obvious early era before we were privileged to employ the advanced technology of rotary NiTi.

Rotary NiTi?with an initial glide path created using the traditional techniques of K-files manually:?

?The superior flexibility of rotary NiTi compared to K-files made a strong impression on those doing the most endodontic procedures, namely the endodontists. This might not have been case, if NiTi had been limited to the use of handfiles, used similarly to the manual use of K-files, but that would not have had nearly the market impact if they could be used in continuous rotation providing the endodontists with??quicker means to shape canals while incurring a much smaller amount of hand fatigue. That truly was a market changer with an appeal that went way beyond the endodontist in attracting converts.

?For many, switching to rotary was evidently worth the far higher costs associated with this technique. Hand fatigue like fatigue in general tends to reduce quality and increase errors and now the dentists had the means to avoid a good deal of it. As noted earlier, the first research papers were an endless stream of studies that noted the reduction in distortions, time requirements and generally superior shaping when rotary NiTi was substituted for the manual use of K-files.

?The weaknesses of manual instrumentation with K-files are hand fatigue, greater distortions in curved canals and at times excessive time requirements. As it turned out, rotary NiTi has its weaknesses, a tendency to separate as canals become more curved, an inability to cleanse oval canals thoroughly because of separation concerns, the tendency to impact debris bucco-lingually, a result of greater tapered rotary instrumentation removing excess dentin mesio-distally and the production of dentinal micro-cracks, a reflection of the unpredictable tendency for rotary NiTi to separate and consistent with Newton’s Third Law of Motion that states that two interactive bodies will have an equal and opposite effect on each other.?

Unlike the stagnancy of the manual use of K-files that showed little to no adaptability in the face of obvious shortcomings, rotary NiTi manufacturers. taking note of separations, the high expense of these instrument and their destructive impact on the remaining dentin came up with solutions that while not eliminating these problems reduced their impact. Among these solutions was the introduction of reciprocating or interrupted rotations that reduced the torsional stresses and cyclic fatigue the continuously rotating instruments were exposed to. While not consistent in their findings, a good deal of research stated that interrupting the rotations reduced the incidence of instrument breakage and from that it could be inferred that the incidence of dentinal defects would also be reduced.

A second approach was and is today a popular transition to far more conservative canal preparations eliminating or significantly reducing the need for crown-down preparations, using fewer instruments in total and subjecting the thinner instruments to far less torsional stress and cyclic fatigue resulting in lower procedural costs, reduced incidence of separations and reduced preparation time. While conservative preparations have become increasingly adopted, the procedures are not designed to debride and cleanse canals in three-dimensions. This would not be a problem, if the majority of canals were conical in shape where all the walls of the canals are contacted simultaneously and fully scraped clean of pulp tissue and bacteria.?

Since the introduction of rotary NiTi, far superior techniques have been developed that give us the tools to fully appreciate the three dimensional configuration of pulpal anatomy. The micro CT scan and the CBCT produce images of pulpal configurations in three-dimensions confirming with crystal clarity that the majority of pulpal anatomy is not conical in shape. Rather it is composed of tight isthmuses, narrow buccal and lingual extensions, a wide variety of partially calcified states,?c-shaped, flattened sections and a wide variety of cul de sacs. Under these conditions, the more conservative preparations are likely leaving a greater portion of pulp tissue and bacteria than the earlier rotary preparations that needlessly sacrificed more dentin, but also removed more pulp tissue. On balance, perhaps the conservative preparations offer more benefits, but at best it is a compromise where rotary advocates state that any pulp tissue and bacteria untouched mechanically should be removed by activation of the irrigants, something the research says helps, but is not a complete answer.

?In the same way rotary was at least a partial answer to the shortcomings of manual endodontics, 30o oscillations of stainless steel relieved twisted reamers both unrelieved and relieved provides solutions to the shortcomings of rotary NiTi.

?30o oscillation?at a frequency of 3000-4000 cycles per minute:

The single most important advantage engine-driven 30o oscillation of stainless steel relieved twisted reamers brings is the virtual elimination of instrument separation, the subject of an endless number of research papers regarding such iatrogenic events when using rotary NiTi. The reason separations are virtually eliminated is simply because a 30o arc of motion, 1/12 of a full rotation, does not generate anywhere near enough torsional stress or cyclic fatigue to overcome the elastic limit of stainless steel. With that basic advantage, several other advantages are derived. These instruments can be used from the beginning in the oscillating handpiece to prepare the canals. At no point, in the vast majority of cases is it necessary to use these instruments manually. It should be noted here that the relieved K-reamers have manual handles that fit into the oscillating handpiece and can be used manually if so desired, Fully mechanized instrumentation eliminates all hand fatigue. With no chance of separation of even the thinnest instruments they are used aggressively in the buccal and lingual planes with strong lateral movement even when curves are present anywhere along the length of the canals they are in. The result is a form of internal routing that touches and debrides all the canal walls (save those that are so thin that they are impenetrable). Less obvious, but still of importance that consistent with Newton’s Third Law of Motion, an instrument under function that is virtually immune to breakage will also have far less impact on the canal walls reducing if not eliminating completely dentinal micro-cracks.

From the manual use of K-files to rotary to oscillation we are observing an evolution in instrumentation that is becoming safer and more thorough for the teeth being treated and the instruments providing the treatment. Given the fact that evolution is a gradual affair, we have always seen a blending of techniques as transitions occur.??In this light, it should be understood that 30o oscillation has the added virtue of being a safety valve for the safe usage of rotary NiTi, that anywhere a dentist senses the rotary instrument he/she is employing is being subjected to excessive stresses that could lead to separation, oscillation provides the means to eliminate that stress. Simply widen the canal slightly larger than its present preparation with the oscillating reamer of choice following that with the rotary reamer that was previously being used. The slightly larger preparation provided safely with oscillation has now made it safe to take the rotary instrument to the apex without encountering a level of resistance that raised concerns.

I am looking forward to the day we can safely restart our group workshops so dentists can experience the advantages of the approach I have been advocating over the years. Nothing beats hands-on experience and hopefully we will begin again in the next few months. In the meantime, I am offering free 1-on-1 workshops in my office. If you are interested and capable of being in NYC, to arrange a workshop simply call 212-582-8161 to set up a date.?

Regards, Barry?