Further Endodontic Insights Insight #2: The plusses and minuses of rotary NiTi endodontics:
We all know that the most touted of all NiTi instrumentation is their greater flexibility compared to comparably sized stainless steel counterparts. In tapers greater than 02, stainless steel instruments are too stiff to negotiate even the mildest of curves. NiTi, on the other hand, retains significant flexibility in their more apical portions even when the tapers are as great as an 08. Along with their flexibility, NiTi, particularly in the more recent heat-treated products, has an increased resistance to separation when rotated in a curved canal. Stainless steel instruments adapt poorly to curved canals during rotation with an increased risk of separation.
Under rotating conditions, rotary NiTi will always prove superior to rotating stainless steel in terms of reduced distortions and a far lower incidence of separations even when NiTi is employed in tapers greater than 02. As long as the comparison is one measuring the impact of rotation on the canal shape and the integrity of the instruments doing the shaping, rotary is always the correct choice. However, limited to rotation whether continuous or interrupted, NiTi, while superior to rotating stainless steel is not without the well-documented concern of instrument separation. Yes, rotary stainless steel will separate more than rotary NiTi, but that doesn’t preclude our desire to employ instruments that are virtually immune to breakage.
In the quest to minimize NiTi rotary separations, it is recommended to use these instruments in a crown-down fashion to minimize engagement along length at any given time and increase straight-line access to reduce the torsional stress that rotation imposes on instruments and in so doing reduce the incidence of separation that can result. Crown-down preparations also reduce the number of rotations that an instrument needs to reach the apex. In a curved canals fewer rotations leads to a reduced amount of cyclic fatigue, the other factor responsible for instrument separation. Staying centered is another way to reduce the stresses imposed on a rotating NiTi instrument. The final way to reduce the stresses a rotary NiTi instrument might encounter is to simply prepare the canal far more conservatively.
From the perspective of the instrument, the techniques mentioned above all reduce the amount of torsional stress and cyclic fatigue that the rotating NiTi instruments are exposed to and reduce the incidence of separation. From the perspective of the teeth being treated, a potential price is being paid. It can be the price of weakened tooth structure as a result of straight-line access and crown-down preparations. It can be the production of dentinal micro-cracks that have been intimately associated with the rotary preparation of canals with NiTi. It can be the inadequate removal of pulp tissue and associated bacteria as a result of centered preparations that fail to cleanse thin isthmuses or narrow buccal and lingual extensions or simply the result of overly conservative preparations that while looking good in the mesio-distal plane inadequately cleanse the canals in the bucco-lingual dimension.
Seen in the light of two different perspectives, I appreciate rotary NiTi’s ability to produce undistorted apical preparations creating a space that at least mesio-distally conforms to gutta percha of similar dimensions. I also appreciate the speed at which I can prepare canals when little resistance to apical negotiation exists. This last point is one that I have become increasingly aware of. The resistance encountered in negotiating to the apex changes from tooth to tooth depending upon the canal’s length, curvature, patency and state of dentinal sclerosis. Even the ability to insert the instrument into the initial canal orifice is often dependent upon the patient’s ability to open wide enough. Think of a mesio-buccal canal of a maxillary second molar in an older patient who cannot open wide to start with. Unintentionally, inserting a rotating instrument into an orifice at an acute angle can immediately lead to instrument separation.
At this stage of the game, I use rotary instrumentation. Admittedly, I prefer the system we developed, the Safesiders HF incorporating a helical flat that reduces engagement along length thereby subjecting the instruments to reduced stresses as they negotiate apically; but even with our system, I am always aware of the potential for excessive resistance, something defined as much by my gut as my tactile perception. It is a response to both my gut and tactile sense that literally drove me to think of endodontic instrumentation as an integrated system rather than one first using a manual or oscillating system followed purely by rotation either continuous or interrupted.
By integration, I mean that any time I feel what I consider a dangerous amount of resistance, I have a virtually separation-free way of shaping the canals to the point where rotation can be done with complete security. That system can be employed either manually or in a 30o oscillating handpiece. Either way, full rotations are completely eliminated and canals can be safely widened sufficiently to then employ rotary NiTi with complete confidence. These oscillating relieved reamers are employed first routinely to open the canal to a minimum of a 15/02 preparation and then depending upon the resistance that rotary encounters on a case by case basis as needed. My threshold for their use has steadily lowered over time simply because of the guaranteed safety they offer any time they are inserted into the instrumentation sequence.
The insights into the rational use of rotary NiTi center around the fact that given the vulnerability of rotation as an efficient, but somewhat unpredictable outcome, means beyond the modified usage of rotary was still necessary. The short arcs of motion provided by relieved stainless steel reamers represent the means I now employ. Adaptable to all situations, the confidence in knowing that we employ techniques that virtually eliminate instrument breakage has helped our endodontic practice and taken an edge off the typical stresses we face. In today’s environment and for the foreseeable future, the reduction in human stress is a goal worth striving for.
Perhaps, in my next post, I’ll talk about the benefits of single point (for the most part) obturation and why imo it beats any thermoplastic technique.
Regards, Barry