A Sliding Scale of Rotary NiTi’sEffectiveness: Risk Versus Reward

Certainly the security of knowing the instruments will remain intact is inversely related to the degree and number of canal curvature. There is far less chance to separate an instrument when the canals are straight and don’t bifurcate or merge. They are also more effective cleansing conically shaped canals. As the canals become more oval or contain isthmuses and other forms of flattened pulpal anatomy, the rotary instruments generally confined to centered shaping particularly in canals of increasing curvature become less effective in three-dimensional cleansing.??

These deficits are generally not seen on periapical x-rays. The mesio-distal image provides little information on any instrumentation inadequacies in what is often the wider bucco-lingual plane and what is not seen is most often not considered a problem. From a clinical perspective, centered shaping even leaving voids of cleansing and obturation in the bucco-lingual plane does not appear to be a major source of failure supporting the dentists’ decision that what appears to be adequate instrumentation in the mesio-distal plane is indeed sufficient, except for those occasions when it is not.?

The separation of an instrument is stark evidence of an iatrogenic event while an undetectable area of inadequate cleansing and obturation is barely a concern. From the human perspective, it is completely understandable why a dentist when faced with instrumenting highly curved oval canals would use these instruments in the most conservative fashion, intent on minimizing the potential for instrument separation, rather than risking separation by leaning laterally into the buccal and lingual extensions of oval canals. Rotary’s vulnerability to separation has set the standards for their utilization, with increasingly conservative preparations both in size and lateral displacement being employed as the pulpal anatomy becomes more complex.

The constricted movement of rotary instrumentation leaving pulpal remnants in the buccal and lingual extensions as well as thin isthmuses is claimed to be compensated for by activating irrigants either sonically or ultrasonically that then digest any tissue remaining and killing any bacteria that had not been mechanically removed. Yet, numerous research studies have shown residual tissue and bacteria remaining in these extensions even after activated irrigation is applied. The answer to these deficits is not to simply live with compromised results and saying this is the best that can be done bottom line. Rather, these inadequacies should clearly define exactly what we want to accomplish and then use the available means to accomplish it.?

As I have written in many previous posts, we must employ a system that can be vigorously applied in all planes without any concern regarding instrument separation. Only by eliminating separation as a cause of concern completely can we then have the confidence to apply whatever these instruments are in a way forceful enough to eliminate the pulpal tissue and bacteria wherever they may be residing. The more pulp tissue and bacterial biofilms are removed from all the canal walls, the greater the chance of total three dimensional obturation via a combination of one or more gutta percha points and a cement interface that is not only present between the canal walls and the gutta percha but is flowable enough to penetrate the more thinly prepared isthmuses and buccal and lingual extensions.

For those reading my previous posts, the obvious answer is a combination of stainless steel twisted reamers unrelieved through a 10/02 and relieved with a flat along their entire working length through a 40/02 all used in a 30o oscillating handpiece running at a frequency of??3000-4000 cycles per minute or about 60 cycles per minute. The 30o arc of motion prevents the instruments from ever being exposed to stresses that exceed their elastic limit. Consequently, the torsional stresses and cyclic fatigue that unpredictably plague rotary systems don’t apply when the arc of motion is limited to 30o.?

The oscillating system is further enhanced by using instruments that gain their flutes by means of twisting rather than having them ground in, a process that again unpredictably produces defects in the core that can then propagate into full fractures under function. By using twisted stainless steel reamers we eliminate the potential for these defects. By employing a reamer design we are using instruments with half the number of flutes along length compared to a K-file and a flute orientation that is twice as vertically oriented, a design that reduces engagement and resistance along length providing a means to reach the apex more quickly with a reduced potential to impact debris apically compared to K-files.

Most notably, employing these instruments with a short arc of motion has been documented in the literature to prevent canal distortions even as they negotiate the length of highly curved canals. I have also shown numerous examples of highly curved canals that I have completed to demonstrate this crucial point. The key to non-distorted shaping is the short arc of motion. The fact that the instruments from 15/02 onwards incorporate a flat along their working length certainly aids in making the instruments more flexible, but it is the short arc of motion that minimizes any chances of distortion. A relieved stainless steel 02 tapered reamer would still produce distortions in curved canals if used with full rotations.?

Rotary NiTi ,used in full rotations, even if some systems use it in reciprocation (interrupted full rotations) as flexible as they are does not entirely compensate for the distorting potential when used in repetitive 360o motions as shown in this short video. Here we see a NiTi instrument shaping a curved canal from stem to stern resulting in a noticeable blunderbuss created at the apical end of the preparation. The gutta percha point fits snuggly, but still leaves a void at the apical end of the preparation. This result certainly does not mean such a case would fail, but it does not correspond to accepted endodontic principles. What this short video does illustrate is the unpredictable power that full rotations not only have on the integrity of the instruments, but on the undesirable removal of dentin from the outer walls of curved canals. As an aside, a blunderbuss leaves open the greater potential to extrude debris and irrigant over the apex.

(Full video: https://youtu.be/SxOXsJs_a_Q)

There is a bit of irony in the fact that the major rotary companies have introduced an endless array of products that compete amongst themselves on the basis of being less prone to separation. Non-distorted shaping, the original reason NiTi was introduced was a given, automatically accepted as being far superior than stainless steel in that regard. As it turns out NiTi in rotation still has a greater potential to distort canals than stainless steel relieved twisted reamers in 30o oscillation with the added precaution that in curved canals they are going to be used in the centered position to avoid separation while the oscillating reamers virtually immune to breakage will routinely be used vigorously against all the canal walls.

I don’t mean to offend anyone using rotary instrumentation. I use it myself, admittedly in a very limited way and only when I know the canals have been prepared sufficiently for me to be totally sure it will remain intact as it negotiates to the apex. There are enough critical studies on all aspects of rotary usage to realize at a minimum that this technology represents at best a mixed bag. I simply choose to use it in a limited way to smooth walls and create a final space that correlates to a corresponding gutta percha point if only in the mesio-distal plane. The rest of the canal preparation is most safely and thoroughly done with the oscillating reamers.

Regards, Barry