The Extra Baggage (Pros and Cons) of Rotary Instrumentation

?An interesting phenomenon occurred about 25 years ago in endodontics with the introduction of rotary NiTi instrumentation. Rather than emphasizing canal shaping with stainless steel (typically K-files), the new NiTi metal instruments, all designed as reamers after the first product, a file design, proved to perform poorly in shaving dentin away from the canal walls. Nevertheless, the unique property of NiTi, its great flexibility was welcomed as a major advance in the mechanics of endodontics. Indeed, an instrument with much greater flexibility than stainless steel was found to have the ability to shape even significantly curved canals via repetitive rotations without producing transportation of the canals to the outer walls. Even greater tapered instruments, with their greatest flexibility at its most apical ends were able to negotiate canals with little stress imparted to the outer walls preparing spaces that were easily obturated by gutta percha points of similar dimensions. The greater tapered preparations made thermoplastic obturation more predicable by reducing the potential for extrusion beyond the apex. 25 years later, the unique properties of rotary NiTi can be assessed in determining their overall impact on endodontic instrumentation.?

Perhaps, the use of NiTi in rotation has had the greatest clinical impact. The flexibility of NiTi is no guarantee that the instruments in rotation are potentially subject to excessive amounts of torsional stress as they negotiate thru narrow spaces and cyclic fatigue when negotiating curved canals, both of which, if excessive lead to instrument separation. Separation turned out not to be a trivial concern. Many research articles have been published regarding the factors that lead to excessive torsional stress and cyclic fatigue followed by the precautionary steps needed to mitigate those factors. Once it was clearly understood that separation was a distinct possibility, rotary NiTi lost the luster of perfection and was better utilized knowing it was a mixed blessing requiring an appreciation for the precautionary steps needed for its safer usage.?

The standard technique for rotary NiTi is a crown-down preparation reducing the engagement of any given instrument with the canal walls. The less the engagement, the less the resistance reducing the chances of excessive torsional stress and shaping curved canals in a graduated manner also helps to reduce cyclic fatigue. The disadvantages of crown-down preparations are two-fold. First, they potentially remove far more tooth structure than is necessary for canal debridement and cleansing. Two, these instruments are expensive with the manufacturers generally suggesting single usage before replacement. Rotary NiTi instrumentation in its original form, weaken teeth and increased the procedural costs to the dentists dramatically.??

There was no getting away from these observations both of which could impact sales and would leave an opening for competitive modifications of the technique that would address these two concerns. The innovation of heat-treated NiTi made the instruments less vulnerable to separation, consequently reducing the need for crown-down preparations thereby reducing the number of instruments required to shape a canal. The result was more conservative preparations, leaving a stronger tooth less vulnerable to vertical fracture. The reduced number of instruments significantly cut the procedural costs. Rotary NiTi was evolving, resolving the issues of excessively weakened teeth and high costs with the added advantage of reducing the dentinal micro-cracks that research studies correlated to greater tapered preparations.

While the introduction of heat-treated NiTi instruments were the impetus behind more conservative preparations, these lesser-tapered highly flexible instruments deviate from the canal walls if and when there is a space that they can deflect into. That has minimal impact in the mesio-distal dimension where the canals are generally narrower than the instrument penetrating that space. There is no room for deviation in that plane. Bucco-lingually, the canals are often wider than the inserted instrument allowing the instruments to deviate into the adjacent space??limiting their ability to shave dentin away from the canal walls they were applied to. It is almost like rotary NiTi in its evolutionary progression is digesting itself. They are used far more conservatively today than when first introduced. Their very flexibility is a detriment when adjacent spaces are present. Rotary NiTi has been shown to leave anywhere from 35-50% of the pulp tissue where complex anatomy is present, something that is not improved upon with more conservative preparations utilizing significantly more flexible instruments.

Of significance is the reality that the gap between the glide path created with stainless steel (preferably relieved reamers) and the final shaping with rotary NiTi has often narrowed to the point of insignificance. An oscillating stainless steel relieved reamer in a handpiece generating 3000-4000 cycles per minute can routinely shape a glide path to a 20/02 preparation, but also retain the ability to widen the glide path preparation to a 40/02 if and when the canal curvatures are so severe that there would be hesitation in utilizing even the heat-treated NiTi instruments in rotation. We might still call the utilization of oscillating instruments the glide path stage, but in reality the space has been enlarged adequately to define the preparation as complete especially given the oscillating reamers’ ability to be vigorously applied??to all the canal walls even in their thinnest configurations without the deviation of the instruments into adjacent spaces as occurs with heat-treated rotary NiTi.

Oscillating stainless steel instruments effectively close the gap between glide path creation and full canal preparations. From my perspective, rotary NiTi has one remaining value. It can shape a canal at least in the mesio-distal dimension that closely matches a corresponding gutta percha point making its accurate placement highly predictable. For that reason alone, dentists may utilize rotary NiTi for the final shaping. However, even here we should not forget that rotary is also associated with the creation of a smear layer that has been shown to be effectively removed using 17% EDTA activated by a smaller reamer oscillating at 3000-4000 cycles per minute. I routinely make that my final step whether the final shaping is done with oscillation alone or in conjunction with rotary.

From a historical perspective, rotary NiTi came with baggage. As rotary NiTi evolved that baggage changed, but it never completely disappeared. To a large extent rotary NiTi has consumed itself, but not before exposing all of us to a history of complications that existed only because its advantages were widely touted while its downsides took much more time to percolate into our consciousness. I suppose this is a path that many products take when they are backed by powerful marketing agents. We can all learn a lesson from the rotary NiTi saga. There is some value here, but not what was touted all those years ago when it was first introduced.?

Regards, Barry