The Counter-productivity of Rotary NiTi Instrumentation

I keep returning to this subject because of the built-in problems produced by greater tapered shaping that also goes under the name of rotary NiTi instrumentation. We can look at rotary NiTi from two perspectives. In their initial introduction and for a long time following, rotary NiTi has been intimately associated with greater tapered preparations. With a growing awareness from the dental literature and clinical experience of the causes of weakened teeth, rotary NiTi has most recently emphasized instrumentation with reduced tapers. The difference between the two is the preservation of dentin in the mesio-distal plane, a comforting result given the association of greater tapered preparations with teeth more prone to vertical fracture.

If we follow the evolution of rotary NiTi to its present state, we are often dealing with preparations that are no greater than a 30/02 or somewhat more aggressively a 30/04. Compared to the greater tapered preparations of the past, a minimal amount of work has to be done beyond the glide path preparations with minimally tapered NiTi instruments. To this day, most manufacturers still strongly insist that glide path creation is the function of K-files, shaping the canals to at least a 20 prior to the introduction of rotary NiTi. It has been stated time and time again that the glide path is an absolute necessity prior to the use of rotary NiTi.

If one thinks about it, a reasonable question is why the glide path must be shaped with stainless steel instruments. Why not just use rotary NiTi instruments with the same configuration and dimensions of stainless steel? The answer lies in the reason NiTi was adapted to greater tapered instruments, those beyond an 02 taper. NiTi is far more flexible than stainless steel. Instruments that are flexible in greater tapered design are excessively flexible in 02 tapers. In canal spaces larger than the instrument, generally found in the bucco-lingual plane, any attempt to use a NiTi instrument of 02 taper or less against a wall would result in its immediate deflection away from the wall with minimum potential to remove any dentin or the tissue ensconced in any isthmus-like anatomy that might be present. This is graphically illustrated in a micro-ct scan produced by researchers Versiani et al in Brazil.

https://www.youtube.com/watch?v=XSjxAVJvWCg

Please note the tissue left in those areas of the canals that are associated with thinner spaces in the mesio-distal plane. The instruments’ inability to negotiate into these thin spaces results from being too flexible causing immediate deflection as soon as the rotating instrument encounters resistance. It will negotiate the center of the canal where minimal resistance exists, but leave untouched a large portion of the canal space and the tissue residing within, namely the tissue in the broad bucco-lingual dimension.

We can see a dichotomy here. We want thinner instruments to have the ability to negotiate into these flatter, but broader spaces to remove a minimum of dentin and as much pulp tissue as possible, but we find that these thinner NiTi instruments lack the body to effectively shave dentin away from the canal walls where highly oval isthmus-like anatomy is present. The result is a central widening of the canal with inadequate removal of pulpal tissue. For this reason, glide path creation, a step that creates a space that subsequent rotary NiTi follows must be the first step and that step must be done with stainless steel.

That glide path creation is done with stainless steel is something that most dentists accept. However, stainless steel is only half the answer for glide path instrumentation. Of equal importance is how these stainless steel instruments are designed and used. The concept of using stainless steel instruments with a file design is wrong. It is incompatible with a watch-winding stroke. It shaves dentin away only when the pull stroke is employed after the predominantly horizontal flutes are rotated into the dentin engaging it minimally. The same flutes that are efficient at shaving the engaged dentin away on the pull stroke are also unfortunately efficient at impacting dentinal debris apically when the files are reintroduced into the canal. Most dentists have experienced this phenomenon recognized by a loss of length when shaping the canals to a 15 or 20. It is the horizontal flute orientation of the files that is most responsible for this event.

Change the flute design from one that is predominantly horizontal to one that is vertical and the mechanics undergo a favorable change. The watch winding motion now not only engages the dentin, but simultaneously shaves it away. The principle is the same one employed when shaving one’s face. To shave our face the blades (flutes) must be more or less at right angles to the plane of motion. Now the watch-motion is removing dentin along the length of the canal walls. When what is now designed as a reamer is removed from the canal to remove debris from the shank, will be far less likely to impact debris apically when it is reintroduced into the canal. The potential to lose length is drastically reduced.

By incorporating a flat along its entire working length, the reamer becomes more flexible, less engaging along length thereby encountering reduced resistance to the apex. The relieved state of the 02 tapered stainless steel reamer still has significantly more body than an equivalently sized NiTi instrument giving it the ability to shave dentin away and remove the associated pulp tissue in three dimensions. In short, the stainless steel relieved reamer gives the dentist a balanced combination of body and flexibility that prevents canal distortions while incorporating three-dimensional debridement.

By using these relieved stainless steel reamers in a 30o handpiece oscillating, not rotating, at 3000-4000 cycles per minute we are simulating manual use of these instruments at a much higher frequency shortening the time requirements for the procedure and virtually eliminating hand fatigue from the procedure’s inception. It must be noted that 30o arcs of motion virtually eliminate torsional stress and cyclic fatigue, the two factors most responsible for instrument separation. Typically, canals that offer resistance to a 15/02 instrument need not be opened any wider than a 30/02. However, the 30/02 preparation will have been prepared vigorously shaped in the bucco-lingual plane effectively debriding the canal.

Insights provided by this approach include:

1. Preservation of dentin in the mesio-distal plane

2. Removal of pulp tissue more completely in the bucco-lingual plane

3. Activation of the irrigants used because they are exposed to 3000-4000

   oscillations per minute.

4.The virtual elimination of separated instruments

5. Minimal hand fatigue in creating a glide path that does not require a secondary

   step of greater tapered preparations

6. Multiple usage of the instruments with great cost savings

The technique represents a simplified approach that enhances cleansing and is consistent with reducing the incidence of vertical fracture, strip perforation, canal transportation, and ledge formation.

Compare the previous video to this one employing the relieved stainless steel reamers.

https://www.youtube.com/watch?v=1gQVeWveLIg

Note the preservation of tooth structure with the complete removal of pulp tissue. Most dramatically at the end of the video observe the thin mesio-distal instrumentation that was necessary to avoid strip perforation in the furcal side of the distal root where concavities on the external surface of the root often are present. Thin mesio-distal preparations are often necessary and the approach offered here delivers on this need consistently.

For us to progress in endodontics, we must have a clear appreciation of our priorities, why they are priorities and how they can best be implemented. That is not an easy process in the presence of ubiquitous advertising, but it is necessary to constructively advance.

Regards, Barry