The Mechanics of Endodontic Instrumentation That Have Made My Life Easier

The longer the arc of rotation the more aggressive the instrument is as it negotiates to the apex. The more quickly it can advance and the less time it takes to complete the instrumentation process. In addition to greater speed, the longer the arc of motion, the greater the stresses that are endured by both the instruments and the canal walls in contact with the instruments. Furthermore, the greater the degree of curvatures through which the instruments negotiate the greater the flexural stresses encountered by the instruments and their equal degree of stresses imparted to the canal walls.

The way to mitigate the stresses is to limit the debridement of the canals to short arcs of rotation, minimizing both torsional stresses particularly in the apical third as well as the flexural stresses encountered in canals of increasing curvature. The major advantage of short arc kinetics is the virtual elimination of instrument separation when employing stainless steel relieved twisted reamers. However, of equal importance is the fact that the shorter the arcs of motion the less the potential for canal distortions including ledging and outer wall transportation. Confined to short arcs of motion 02 tapered stainless steel reamers are less resistant to deformation than the canal walls. When the two contact each other as they inevitably do during instrumentation it is the instrument that deforms rather than the canal wall. That deformation deflects the instrument into the path of least resistance, the patency of the canals.?

The shorter the arc of motion, the greater the gap between the resistance of the canal walls and the resistance of the instruments with that gap favoring the canal walls. Employing relieved twisted stainless steel reamers, instruments significantly more flexible than traditional K-files, further enhances the dominance of the canal walls over the instruments in terms of resistance to deformation. This basic principle was first described by Dr. James Roane in his paper describing the mechanics behind the balanced force technique. The use of the more flexible relieved reamers and the shorter arcs generated by the 30o oscillating handpiece further reduce the potential for canal distortions compared to the use of K-files and the greater arcs of motion employed in the original balanced force paper.?

The 30o arcs of motion are generated by an oscillating handpiece at a frequency of 3000-4000 cycles per minute in contrast to the manual use of K-files first described by Dr. Roane. The traditional balanced force technique is employed manually making it slow and hand fatiguing. Those two limitations are overcome with the 30o oscillating handpiece. At approximately 3000-4000 cycles per minute or about 60 cycles per second does the oscillating handpiece mimic the much slower manual mechanics of the balanced force technique??

While the speed of the handpiece is far greater than that generated manually, the mechanics of each cycle are similar to the balanced force technique. The 30o clockwise stroke first shaves a small amount of dentin away from the canal walls followed by a slight apical advancement of the preparation as the flutes engage the canal walls. The 30o counterclockwise motion combined with the apical pressure that is being consistently applied cleaves off the small amount of engaged dentin resulting from the clockwise rotation directing those shavings into the depths of the flutes. While each 30o cycle results in only a small amount of dentin being removed and only a small amount of further apical progress, the process is a rapid one because of the high frequency of the oscillations. The upstroke is simply a means to carry the dentinal shavings coronally where immersed in the irrigants they are suctioned away and those now clogging the flutes are wiped away.

There is little potential to impact dentin apically when the instruments are directed apically because the predominantly vertical orientation of the flutes along the length of the relieved reamers are designed to bypass any debris present in the canals. This particular property has been shown in a video that prepared a highly oval canal configuration in three dimensions with a small amount of apically extruded debris. Considering the fact that unlike rotary instrumentation of oval canals where to avoid instrument separation the preparations are limited to central shaping with documented inadequate cleansing of the buccal and lingual extensions of oval canals, the oscillating reamers are vigorously applied to all the canal walls assuring?improved three-dimensional cleansing, substantiated by cross-sectional studies shown in previous posts as well as the video that has also been shown in previous posts. With far more vigorous three-dimensional shaping it should not be unexpected to see some debris extruded apically. Research studies have concluded that no instrumentation technique devised to date completely eliminates the extrusion of debris. That being the case, the minimum we should be seeking is as thorough a capability as possible to employ vigorous three-dimensional cleansing of the canals even as the pulpal anatomy becomes more complex. One could make the case that it is better to push debris through the apex where it will be in contact with a viable blood supply than leave it within the confines of the canals. Admittedly, neither is desirable, but it is a subject worth reflecting on.

Another advantage of employing a high frequency oscillating instrumentation system is its secondary ability to activate the irrigants killing bacteria and dissolving pulp tissue remnants before they are extruded apically. Rendering what is extruded apically less toxic is a potential benefit that should not be overlooked. Now here is the kicker. Do all the potential advantages ascribed to 30o oscillations of relieved twisted stainless steel reamers improve success rates? To date no technique can legitimately claim a higher success rate. So, one can conclude that given this dearth of data what is employed should be based on other factors. Before we jump to this simplistic conclusion, the fact that any and all systems appear to produce similar outcomes does not relieve us of the responsibility of doing any given technique correctly. In that regard finding all the canals, cleansing them to the apex, leaving as little pulp tissue and bacteria as possible, sacrificing as little dentin as possible consistent with effective debridement are necessities.Given these necessities that must be part of all instrumentation systems, the following factors might be incentives to use one system over another.

• Elimination of instrument separation thereby reducing procedural stress
• The elimination of the need for rapid replacement of the instruments
• The ability to shorten the procedure making the dentist more productive
• The elimination or at least the reduction in hand fatigue
• The overall cost of the systems including the handpieces that are a part of many systems

From a personal choice having been exposed to the contemporary systems to date, I found the above list to be the motivation behind the development and use of the 30o oscillating relieved twisted stainless steel reamers. Their use fulfills the above objectives. And while I cannot argue with the research that states that all systems done properly lead to similar success rates, I find it a lot easier to do things properly when the mechanics involved are purposely oriented to non-breakage of the instruments that directly leads to their more vigorous usage against all canals walls that from a common sense perspective should result in improved debridement. With instrument separation no longer an issue, multiple usage becomes the norm reducing costs. Employed at the earliest stages of canal preparation they eliminate hand fatigue from the beginning to the end of the procedure. These are all attractive properties to me that have made my life a good deal less stressful and perhaps one of the reasons, after 50 years as an endodontist I am still going strong.

Regards, Barry