Endodontic Research and Rational Conclusions

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AS AN ENDODONTIST who was clearly not comfortable with rotary NiTi, I looked for alternative means to shape canals that were safer, more effective, and decidedly less expensive. I wanted these alternatives primarily for myself, but strongly believed that if they were achievable, they would help a large number of dentists who felt the way I did. The alternative approach that evolved substituted K-reamers for the traditional use of K-files. These K-reamers were relieved (meaning that they had a flat along their working length) from number 11 upward and unrelieved through number 10. This substitution reduced the instruments’ engagement with the canal walls along their length, increased their flexibility, gave them superior ability to remove dentin more efficiently when a watch-winding motion was employed, and allowed more efficient shaping of canals. The short-watch winding arc of motion that is so effective when employed manually could easily be adapted to a reciprocating handpiece that itself is limited to a 30-degree arc of motion. Reduced engagement, increased flexibility, and more effective dentin removal—all resulting from the fact that the cutting blades are more or less at right angles to the plane of motion—combined to give the dentist a much finer tactile perception of what the tip of the instrument was encountering. Increased tactile perception gave dentists the final tool they needed to shape even highly and abruptly curved canals because significantly increased resistance or outright stoppage of the instrument immediately told them that they were either encountering a solid impediment or negotiating a significant curve. In either case, the instrument signaled to the dentist that he was to remove it from the canal, prebend it, and negotiate manually around the blockage or severe curve. Leaving the instrument in the canal at the newly acquired depth, the dentist then has the option of continuing manually to the apex or reattaching the head of the instrument to the reciprocating handpiece for rapid negotiation to the apex. Limited the instrument to a short arc of motion when used either manually or in the reciprocating handpiece virtually eliminates the torsional stress and cyclic fatigue that leads to the separation of instruments used in rotation. With separation worries eliminated, the dentist can have the confidence to work these instruments against all the walls of oval canals with vigor, removing a more or less uniform layer of dentin from all the circumferential walls of the canal.

The above is a description of the advantages of this approach to canal shaping. When I first started lecturing on this alternative approach I met strong headwinds. Massive advertising by the major manufacturers of rotary NiTi had convinced many dentists that their approach was the new paradigm and what I was advocating was old technology that had been obviously left to the dustbins of history. This was not quite accurate since I was talking about the creative use of reamers rather than K-files, instruments that rotary NiTi still depends on for the creation of the glide path.

I found that over time, the reaction to my lectures became less confrontational. It became apparent to those using rotary NiTi that there were issues associated with their use, the most obvious one being separation, a euphemistic word for unpredictable breakage. This unfortunate side effect of the use of rotary NiTi gave me room to discuss our alternative approach to an audience that had an increasingly open mind.

Published research in the two major endodontic academic journals (JOE and IEJ) has been our unintended ally in support of our alternative approach. For years these journals have published research documenting the various factors that lead to instrument separation, and they have offered suggestions to minimize this deleterious effect. In the meantime, the rotary NiTi industry came up with their own suggestions to reduce breakage, namely to use instruments once and then discard them, a suggestion that was widely accepted. The result was a reduction in separation and vastly more profit for the companies producing these vulnerable instruments. Their genius was in convincing dentists to invest so much more money in a system that was vulnerable from the start. One-time use reduced breakage, but by no means eliminated it.

That is where things might have stood were it not for the continuing research that documented clear visual evidence that canals most often have an oval, sometimes highly oval, cross-sectional anatomy that cannot be cleansed effectively with rotary NiTi. In effect, rotary NiTi makes conically shaped preparations regardless of whether the canal is round or oval, leaving major portions of the canal untouched and their original debris intact. This effect is further exacerbated by the dentists who use these instruments more conservatively (meaning lesser tip size and tapered instruments) to make even the centered preparation less encompassing. Ultimately, the dentist wants to use a system that is effective and robust to accomplish the task of cleansing the canals and creating a space that can be well irrigated. Rotary NiTi does not accomplish either of these tasks. What it does produce is a well-centered preparation that looks good on a mesio-distal x-ray; that image implies a well-done job without its necessarily being so. The fact that there is no published study that has shown an increased rate of success with rotary NiTi systems is an indication that the claimed beneficial effects are more apparent than real.

Further research has yet more to offer. Over the past three years, these same journals have published a number of articles that clearly show a high incidence of microfractures produced along the length of canal walls when rotary NiTi systems are employed. The most recent was published in the February issue of the JOE. These are not rare events; they occur in anywhere between 25 percent and 60 percent of cases, depending upon the system used. If we put together what we know about today’s rotary NiTi systems we see singular negative events that become even worse as they interact. For example, to reduce breakage, less canal shaping is performed; that leaves even more debris in the canals. Take the other approach and shape the canals to greater tip size and taper and an increased rate of separation is possible and the walls constituting the minor diameter of many canals will be excessively thinned out. Coupled to the fact that rotary induces microfractures, it is not a wild assumption that an increased rate of microfractures will occur where the dentinal walls are the thinnest.

One might think that the introduction of single-file systems used in reciprocation would perhaps solve many of the problems encountered with rotary NiTi. This is not necessarily the case. Again it has been documented in several studies including micro CT scans that the single-file system inadequately shapes canal anatomy, leaving large portions untouched. Furthermore, the reciprocating NiTi systems still include 200 full rotations per minute. The incorporation of a 30-degree counter rotation will reduce torsional stress to some degree, but the 150-degree clockwise rotation still leads to a 120-degree advance with every full cycle. Used at 600 full cycles per minute, the handpiece generates the 200 full rotations referred to above that still lead to cyclic fatigue and instrument separation. Of even more significance is the fact that 200 full rotations per minute lead to the same rotational action that produced the microcfractures with full rotary systems. What the companies are selling the dentists is reduced costs, lower incidence of breakage, and more rapid procedures. What the dentists are buying is less effective cleansing while still retaining the negative possibilities of continuing instrument separation (admittedly less likely) and the continued generation of dentinal microfractures.

It is perhaps because all these factors have come to light that our approach, which first sounded somewhat quaint and old-fashioned, has acquired a more solid foundation in the long run, producing cleaner canals while minimizing the potential for distortion, virtually eliminating instrument separation, and—perhaps most importantly—eliminating the potential for microfractures along length. The fact that our approach supports multiple use of the instruments, reducing the cost by 90 percent compared to rotary NiTi systems, is what I consider a secondary benefit, but one that we should still be aware of.

July - September 2012

Ultimately, the dentist wants to use a system that is effective and robust to accomplish the task of cleansing the canals and creating a space that can be well irrigated.