Assessing The Pluses and Minuses of Rotary Endodontics

Let’s talk about the positives first. Oftentimes, rotary instrumentation is quicker, certainly quicker than the manual use of K-files, but at times quicker than engine-generated 30o oscillations. While the glide path created for rotary is generally accomplished manually, in fairly straight wide canals the initial preparation rarely needs to extend beyond a 10/02 K-file, something accomplished quite quickly followed by rotary instrumentation that may not require more than a single instrument to prepare the canal adequately for obturation.?

?For this approach to be effective in accomplishing the prime requirement of instrumentation, namely thorough cleansing and debridement in three dimensions, requires canals that are round in cross-section and conical throughout their length. Those canal configurations are routinely present in maxillary centrals and laterals, but often not present in all other teeth in the mouth. It is best to assume that the canal being instrument is not round in cross-section and is best addressed by leaning the rotating instruments against all the canal walls, something dentists are reluctant to do as the canals become more curved, long, and narrow. That is why the advantages of faster procedures employing rotary are present under the limited conditions of simple straight and fairly wide canal anatomy.

Another significant advantage derives from rotary NiTi’s greater flexibility compared to the manual use of K-files resulting in a better adaptation to canal curvatures with less potential for distortions such as transportation to the outer walls. Stiffer K-files that remove dentin upon the pull stroke tend to shave more dentin away from the outer wall of a curved canal resulting in this type of distortion. While the adaptability derived from NiTi’s greater flexibility is a plus, its improved effectiveness is balanced by the increasing risk of separation as the canals become more curved. It is a tradeoff that works most of the time in the dentist’s favor but sometimes does not lead to procedural unpredictability.

Two particular advantages that I like with rotary NiTi is their ability to smooth and size the walls of the canals particularly in the mesio-distal plane for the appropriate gutta percha point. The smooth walls produce a more pleasing image when viewed on x-ray and the sizing provides a simple way to determine which gutta percha point should be used during obturation. The advantages rotary offers, faster procedural time, reduced distortions, smoother walls and proper sizing are most safely accomplished where canal anatomy is uncomplicated. In the case of increasingly curved canals, the advantages of non-distortion must be weighed against the greater possibility of instrument separation.?

The above pretty much describes and delineates the advantages that I believe rotary offers the dentist. Now let's talk about the negatives associated with rotary NiTi instrumentation, negatives that have been thoroughly documented in the literature through multiple studies as well as the feedback from dentists regarding their clinical experiences.

Without a doubt, instrument separation is the biggest concern. As the canals become more curved, long, and calcified they are more resistant to apically negotiating to the apex. That resistance translates into increasing torsional stress and cyclic fatigue and if and when a combination of these two forms of stresses exceeds the elastic limit of the NiTi instruments, separation will result. The rotary industry as well as their advocates would like us to believe that it is a rare event and while some literature supports those claims, other studies point to separation as an event that occurs with far greater frequency. From having taught a multitude of dentists in safer endodontic techniques, separation is their prime concern and the reason they seek out alternative methods to shape and cleanse canals.?

I recently saw a comment on LinkedIn stating that if one does not separate an instrument, one is simply not doing endodontics. Nothing could be further from the truth and to state that opinion as some immutable law is misleading, counterproductive and an obstacle in the way of keeping an open mind and examining all possibilities that exist to resolve the concerns regarding instrument separation. To reduce the incidence of separations, various articles include a list of precautionary steps that adversely impact the tooth being treated.

Staying centered is a precaution that limits the amount of stress that the rotary instruments are subject to. In a canal round in cross-section a greater a tapered instrument will contact all the walls mirroring the original round anatomy in larger form even when used in the centered position. In oval canals, particularly ones that are also curved or those canals with flat anatomies such as isthmuses connecting a mesio-buccal and mesio-lingual, centered positioning limits the stresses the instruments are exposed to, but leaves untouched tissue and bacteria in thinner buccal and lingual extensions as well as the tissue and bacteria present in the thin flat configurations of isthmuses between the two main canals. In balancing the need for thorough debridement against the increased possibility of instrument separation, the avoidance of separation generally wins. You are unlikely to see pulpal remnants on an x-ray. A separated instrument is generally all too obvious and undeniable evidence that an iatrogenic event has occurred.?

Another precaution is to prepare the canals employing a crown-down technique, a method of instrumentation that prevents the entire working length of the instrument from having contact with the canal walls at any one time. By first widening the canal coronally to a greater taper and following up with lesser tapered instruments in sequence, the full length of the canal is cleansed, at least mesio-distally in oval canals, with a reduction in torsional stress and cyclic fatigue for each particular instrument involved. The instruments have a reduced chance of separation due to a technique that sacrifices excessive amounts of dentin for the safety of the instruments. Please remember that most canals are their thinnest mesio-distally. That thinness is a reflection of the external mesio-distal dimensions of the root. To excessively widen canal preparations using a crown-down sequence that are their thinnest in the very dimension where the most tooth structure is being removed weakens the roots and makes them more likely to fracture under function.

In the same way that we cannot see on x-ray tissue and bacteria that are remaining in the buccal and lingual extensions of oval canals, we cannot see the dentinal micro-cracks that are produced via rotary NiTi as documented in innumerable studies. Some advocates state that rotary instrumentation is not a causative factor in their presence, that they were there previously or are a by-product of a flawed research protocol. What can be least discounted, however, is Newton’s Third Law of Motion stating that two interactive bodies will have an equal and opposite effect on each other, the two factors here being the instruments and the canal walls.

We know beyond doubt that rotary instrumentation can lead to instrument separation. The instruments separate due to the action of the canal walls on the rotating instruments. In curved, long and calcified cases the stresses applied to the rotating instruments via the canal walls increases significantly making separation more likely in these more complex situations. Newton’s Third Law states that the impact of the canal walls on the rotary instruments will be equaled by the impact of the instruments on the canal walls. The fatigue properties of dentin are similar to that of NiTi (and stainless steel for that matter). Those who deny the impact of the instruments on the canal walls are saying that Newton’s Third Law as it relates to endodontic instrumentation is a oneway street. They accept the fact that the resistance of the canal walls can lead to instrument separation, but the other does not exist. That position is untenable.

A final precaution is one that the rotary manufacturers love. It is suggested by many advocates and manufacturers that the instruments should be discarded after a single use, paraphrased as “one and done”. A policy such as that reflects on the recognized vulnerability of rotary endodontics. That so many dentists accept this as a viable solution amounts to a marketing coup. The solution to the problem of unpredictable separations is not the abandonment of the systems. Rather, it is their rapid replacement after minimal use, a precaution that increases the costs of endodontic instrumentation immensely. Rotary NiTi as the dominant instrumentation system imo substitutes instruments purchased at premium prices and then with wisdom suggests their rapid replacement. Granted, rotary gained favor by comparing their engine-driven usage against the manual use of K-files a traditional technique that took a great deal of time and often incurred a large amount of hand fatigue. That traditional system is not the sole comparison that can be made today.

This post is long enough today, but to review the benefits of 30o oscillation using stainless steel twisted reamers one only need to read some of my previous posts.

Regards, Barry