Defining Areas Where The Mechanics of Endodontics Can Be Improved

?In past posts I have dwelt on the greater efficiency reamers have over files in negotiating canals. The resistance encountered in negotiating canals is a measure of the amount of contact the instruments have with the canal walls and the ease with which they reduce that engagement by shaving the dentin away that is producing the engagement. By employing reamers, instruments with half the number of flutes along a working length equal to that of a file, we immediately reduce the amount of contact and consequent resistance. The flutes on the reamer are twice as vertically oriented as those on a file and will shave dentin away from the canal walls with the first clockwise motion of the instrument.??The more horizontally oriented flutes on a file will first penetrate a depth of dentin without removing any of it and only shave the engaged dentin when a vertical pull stroke is employed. The files can carry out this function, but it takes the implementation of a second stroke, making it less efficient and increasing the possibilities of impacting dentinal debris when the motion of the file is directed apically.

The innate superiority of reamers over files can be further enhanced by placing a flat along the working length of the reamers further reducing their engagement while increasing their efficiency of shaving dentin away from the canal walls with the first clockwise stroke,??a result of the two vertical blades produced with the incorporation of the flat. Nothing is more basic than the initial instruments we choose to use in gaining patency to the apex and establishing a safe glide path for further shaping. From my perspective, the continued use of K-files for these initial tasks is counterproductive, a maintenance of the status quo that makes our procedures more challenging and less predictable..

?As stated in previous posts, rotary endodontics, propelled by the use of highly flexible NiTi instruments was greeted with enthusiasm because it reduced the amount of hand fatigue required for full canal shaping. Hand instrumentation in many situations was not needed beyond a canal preparation to a 10/02 or 15/02 leaving the rest to non-hand fatiguing greater tapered rotary. Dentists like the greater tapered space provided by these instruments that allowed for more impressive and consistent obturations of the canals.?

The introduction of NiTi based rotary instrumentation would have unquestionably become the standard save for the fact that instrument separation, a result of excessive torsional stress and cyclic fatigue in curved canals became a recurrent problem reported extensively in the dental literature and noted as a concern by the majority of dentists using these systems. That is not to say, that the pleasing results it did produce and the reduction in hand fatigue still count heavily in its favor with dentists continuing to employ these systems albeit with a good deal of caution.??

A partial resolution to instrument separation came about with the evolution of rotary into a reciprocating system where continuous rotations are interrupted with a smaller counterclockwise arc of motion reducing without eliminating the torsional stresses and cyclic fatigue associated with uninterrupted rotations. At the same time, greater tapered preparations were being critically reviewed in the dental literature with greater tapered rotary preparations being associated with the production of dentinal defects that over time can coalesce and propagate into full blown vertical fractures. This insight has led to the introduction of significantly more conservative preparation, preserving tooth structure making the teeth less prone to vertical fracture, reducing the incidence of instrument separation and lowering the number of instruments needed for full shaping, thus lowering the cost of the procedures to the dentists. That is presently where NiTi rotary stands today. And the question is can we do better and by better I mean more thorough cleansing, the preservation of more tooth structure, two goals that at first glance appear to be incompatible, the complete elimination of instrument separation and hand fatigue.

That’s where the 30o oscillating handpiece comes in. The horizontal motion generated by the handpiece oscillating at 3000-4000 cycles per minute is entirely compatible with reamers both unrelieved and relieved. The high frequency of oscillations shaves the dentin away from the canal walls in small increments reducing the torsional stresses that the instrument encounters while the short arc of motion eliminates cyclic fatigue as a source of separation. In short, it is virtually impossible for stainless steel twisted reamers non-relieved or relieved to separate. While rotary instrumentation removed the potential for hand fatigue after the creation of the glide path, the reamers used in 30o oscillation eliminate hand fatigue throughout the entire cleansing and shaping process. That is not to say that there are occasions in extremely calcified or curved canals to employ the relieved reamers manually, but this is definitely the exception.

Never underestimate the advantages from using a system that is virtually assured of not separating. Where rotary must stay pretty much centered in the canal, the oscillating system gives the dentist the freedom to vigorously work the reamers against all the canal walls and to do so with the thinnest reamers available (06/02). These lesser-tapered instruments will remove less tooth structure mesio-distally where the majority of canals have their narrowest dimensions and will extend their cleansing abilities bucco-lingually where the canal lengths are often extended. Where rotary removes excess dentin mesio-distally only to impact some of it bucco-lingually disrupting the cleansing potential of the irrigants, high frequency oscillations with thin reamers penetrate these narrower extensions, removing the smaller amount of debris that might be there and then activate the irrigants to greater effectiveness due to the high oscillating frequency of the handpiece. These benefits need not be limited to the creation of the glide path. With relieved reamers sized to a 40/02 they are more than large enough to accommodate adequate cleansing where the pulp tissue has greater dimensions. They can be used alone to entirely shape the canal or used in conjunction with rotary, to be implemented wherever the dentist determines the rotary instruments are encountering too much resistance. The end result is the preservation of more tooth structure, greater debridement in the bucco-lingual plane, far lower procedural costs since the instruments are only replaced when they become dull and not out of fear of separation.

Three dimensional shaping is often thought of as being attained by chemical means, the irrigants and, indeed, a number of innovations have been introduced to enhance the irrigants effects, namely sonic, ultrasonic and laser induced acoustic activation. To date, the literature has not associated any of these methods with higher success rates. That does not mean they may not be beneficial, only that the benefits have yet to be shown to produce higher rates of success. The approach I have the most confidence in is the use of our thinnest reamers, the ones most likely to penetrate the deepest into thin isthmuses and narrow buccal and lingual extensions activating the irrigants at sources potentially closer to biofilms and vestigial tissue that may be ensconced in these narrow spaces. I can do this because I am assured that the instruments will not break. But because I have that assurance the concept of “internal routing” takes on real meaning and is the basis effective instrumentation in three dimensions.

?One final point regarding stainless steel relieved reamers reduced potential to produce distortions when enlarging curved canals. If stainless steel were used in rotation in curved canals, rotation beyond a 20/02 reamer would likely produce transportation to the outer walls as has been pointed out in various research papers. This understanding of the use of stainless steel in rotary to induce distortions in curved canals has been accepted as a generalized truth about stainless steel and not the alternative ways they may be used. Confined to short arcs of motion, not full rotations, we have demonstrated that 30o oscillating relieved reamers stay true to the original canal anatomy apically even when vigorously applied to all the canal walls coronally. In effect, we are staying true coronally and the application against all the canal walls is simply mirroring the original anatomy. The stainless steel relieved twisted reamers confined to short but rapid arcs of motion are going to follow the pathway of least resistance, the original canal anatomy, a fact that we have demonstrated in the many posts where I placed x-rays of obturated highly curved canals without any indications of distortions.

A detailed analysis of the mechanics involved in endodontics shows us that there is a whole gradation of results that can be produced depending on what mechanics we employ. Simpler more predictable answers are hiding in plain site and once discovered can go a long way in simplifying our endodontic life.?

Regards, Barry