Rotary and Oscillating Mindsets Compared

 Rotary endodontics utilizes NiTi reamer-like instruments to shave dentin away from the canal walls using repetitive full rotations coupled to varying degrees of apical pressure dependent upon the amount of resistance these instruments encounter as they negotiate to the apex. Oscillating endodontics utilizes stainless steel twisted reamers unrelieved with a flat through a 10/02 and relieved with a flat along their working length from 15/02 onwards through a 40/02 if necessary. The motion is confined to a 30o arc, but is applied to the canal walls with a frequency between 3000-4000 cycles per minute or about 60 cycles per second. Between these two poles is a modified form of rotation, reciprocation, that interrupts the rotations with a short counterclockwise arc reducing the stresses that the instruments encounter as they engage the canal walls. Looked at in this light, 30o oscillations limit their arc of motion to 1/12 of a full rotation virtually eliminating torsional stress and cyclic fatigue to the point where it is virtually impossible for these instruments to separate.

The most obvious advantage of shorter arcs of motion is its direct relationship to the reduction in the stresses the instrument will encounter. This is pretty well recognized by the fact that the glide path essential for the use of rotary instrumentation is generally created with the manual use of K-files or better yet K-reamers that are confined to a watch winding motion that is simply another name for a form of oscillation, consisting of a series of short arcs of a forward and backward motion. Those advocating rotary endodontics believe that full rotations while not as safely used, are far more efficient than the short arcs of motion generated in oscillation and therefore, merit their use despite the higher incidence of separation. Let’s examine that belief.

A continuous or interrupted rotary motion will shave dentin away from the canal walls as the instruments rotate within the canals. 30o oscillation is not without movement. It will travel 30o of the 360o that define a full rotation. That 30o arc of rotation travels circumferentially shaving dentin away from the canal walls wherever the flutes touch the canal walls. We know this is true because upon removal of the instrument the flutes are loaded with dentinal debris and the fact that the next instrument inserted into the canal is a lot looser than it would be if a previous instrument were not first used. 

Now for each cycle of 30o only a small portion of dentin is shaved away from the walls, far less than is removed with one full rotary cycle. The small increments removed per cycle reduce the stresses that are typically produced in rotation. The compensating factor is the high frequency of the cycles that in any given amount of time will remove at least as much dentin from the canal walls as a rotary system. To state the compensating feature of 30o oscillations it is relevant to compare its 3000-4000 cycles per minute to the typical 300-400 revolutions produced in rotary or the 200 full rotations produced in reciprocation. The 30o oscillating handpiece generates cycles of movement at a frequency approximately 10 times that of rotary and 20 times that of reciprocation or interrupted rotations. 

Advocates of rotary attain their sense of these instruments after the creation of the glide path. They most likely have had experiences of hand fatigue in creating that glide path using K-files manually and possibly a loss of length. Creating the glide path using stainless steel relieved and unrelieved twisted reamers in the 30o oscillating handpiece would be a completely different experience. Hand fatigue is eliminated and the time required for glide path creation is dramatically reduced. Having not experienced the oscillating approach with the prescribed reamers, they cannot imagine a further extended use of stainless steel instruments beyond the creation of the glide path.

As I demonstrated in a previous post, stainless steel relieved twisted reamers confined to short arcs of motion stay centered within the canal. One might think that stiffer stainless steel reamers will automatically start to distort a curved canal to the outer wall. Consider this, however. A glide path has already been well defined by a series of reamers both unrelieved and relieved through a 20/02. The contention of rotary advocates is that the next instrument a 25/02 will start the distortion process in a curved canal. To do that the instrument confined to a 30o arc of motion will start to selectively remove dentin from the outer wall. This will not happen with short arcs of motion because the path of least resistance is the channel that has already been established by the previous instruments. This type of centered shaping has been demonstrated with the balanced force technique that is quite similar to the mechanics involved when employing 30o oscillations. 

In like manner, the last stainless steel relieved twisted reamer has more clearly defined a pathway that each subsequent stainless steel reamer will follow as long as the arcs of motion are confined to 30o. While I am making a case that rotary can be completely replaced by oscillating stainless steel relieved reamers, rotary NiTi still has its conveniences. It creates a space in the mesio-distal plane that is often greater than an 02 taper and can be sized in that dimension to a gutta percha point of the same dimensions simplifying the obturation process. At least in the mesio-distal plane rotary preparations can produce smooth walls that give the final x-rays a pleasing appearance. I would add here that the final irrigation of the canal walls should be done with a smaller oscillating instrument that will remove the smear layer that develops when rotary instruments are used. Removing the smear layer allows the sealer to penetrate the dentinal tubules more effectively during obturation.

From my perspective, I typically employ the oscillating reamers through a 20/02 and then use our rotary system consisting of three instruments, a 20/02, 30/02 and a 30/04. We designed the rotary system with a helical flat to reduce engagement of the instrument as it negotiates the length of the canal and many times this relieved rotary system gains that length with minimal resistance. However, there are also many times that I encounter what I consider excess resistance in advancing these instruments apically. I have not had experience in our instruments separating either in testing or clinically, but I am completely aware that in the presence of what I consider excess resistance, I always have the option to intervene with the oscillating system to widen the canals sufficiently that the subsequent rotary instruments now engage far less resistance allowing me to negotiate to the apex with no trepidations. And then there are the situations where the canals require preparations wider than a 30/04. Again confined to the 30o arc of motion I can easily widen the canals to any dimension that I consider consistent with complete removal of the pulp tissue and associated bacteria. That preparation can be as great as an 80/02 in unusually wide situations.

That’s pretty much the story that I tell from about 50 years of endodontic experiences and about 40 years of product development. It can be told in many different ways, but it always comes back to the basics. How do we deal most safely with excessive resistance from the perspective of the tooth, the final arbiter of what we want to accomplish. We may protect the instruments by incorporating certain techniques, but if it in any way compromises the quality of treatment of the tooth we are not optimizing our treatment. Learning is a long arduous process, but also quite exciting as we gain greater clarity.

 Regards, Barry