Appreciating the Function of 30º Oscillating Reamers

I’ve written a number of posts regarding the benefits of 30º oscillating reamers instrumenting canals compared to the traditional use of the K-files. I think it would enhance a greater appreciation of those benefits if we go into a bit more detail in exactly how the reamers function when used with high frequency short oscillations  compared to K-files. But, first let’s consider the use of K-files that are still dominant to this day. 

The most rational explanation I can think of why K-files rather than K-reamers are traditionally used to shape canals derives from their originally being manufactured from carbon steel, a material that is quite brittle and tends to separate if it encounters excessive torsional stress and/or cyclic fatigue. To minimize the stresses from these two sources, the instruments are most safely used with short rotational arcs combined with a pull motion. Confined to these actions where the vertical pull motion is dominant, a K-file is a more functional design at shaving dentin away from the canal walls. As long as carbon steel was the only choice for instrument fabrication the rational for a vertical file type of motion rather than a horizontal reamer motion made sense.

With the introduction of stainless steel, a metal that is far less brittle, a horizontal reamer action was now possible, but by the time of its introduction the use of the K-file had become entrenched with little effort to evaluate the advantages that may have accrued from a reamer design. That is where we sit today. The K-file is still the dominant design despite their being made from stainless steel, a metal that could beneficially be adapted to the safe utilization of reamers. Given the unquestioned allegiance to the K-file design, the instrumentation of curved canals were shown to be vulnerable to various canal distortions including transportation, zipping, ledging and canal blockages.  

Recognizing these inherent shortcoming, Dr. James Roane in 1985 published a paper on using K-files with what he described as a balanced force technique comprising a 90º clockwise horizontal motion that embedded the threads of the K-file into the canal walls followed by approximately a 270º horizontal counterclockwise motion with enough apical pressure applied to prevent the instrument from rising. Used with this technique, the counterclockwise horizontal motion cleaved off the dentin in what he described as a reamer type of action. In effect, he used a K-file in such a way that it mimicked the action of a K-reamer. The major insight of the balanced force technique was its ability to use increasingly stiffer stainless steel instruments in such a way that during its advancement through the length of the canal, it would stay true to the original canal anatomy. 

This fidelity is based on the fact that used with the short arcs of motion utilized with this technique, both the tip of the instrument and the dentin contacting the instrument at that point are both subject to an equal amount of pressure. What Dr. Roane discovered was that confined to short arcs of motion the resistance of the canal wall to penetration by the instrument was greater than the resistance of the instrument. In short, upon contact, it was the tip of the instrument that gave being forced into the path of least resistance, namely the canal. Repetition of this clockwise/counterclockwise motion with applied apical pressure gives the instruments the ability to stay within the confines of the canals even when highly curved. The key is the short arc of motion.

Balanced force was introduced as a manual technique. Prior to the introduction of rotary NiTi, this was simply a way to avoid canal distortions. The fact that it was time intensive and led to hand fatigue was to be expected given that the state of the art at the time included very little in the way engine-driven devices. The introduction of rotary NiTi instrumentation addressed two problems, hand fatigue and excessive time requirements. Engine-driven rotation of NiTi instruments (all designed as reamers) significantly reduced the amount of instrumentation that had to be done manually and being engine-driven the time requirements were much reduced in most cases. As well all know the most obvious downside of rotary instrumentation are separated instruments, a defect in these systems that led me to seek ways of canal preparation that would eliminate that iatrogenic potential.

The answer to this concern turned out to be the utilization of stainless steel twisted reamers unrelieved through a 10/02 and relieved from a 15/02 through a 40/02  powered by a 30º handpiece oscillating at 3000-4000 cycles per minute. Now I have covered the basic mechanics of why this approach works safely and efficiently in previous posts, but let’s get into the details of exactly why this approach works.

Please remember that in describing balanced force, Dr. Roane described it as a reaming action, namely shaving dentin away via a horizontal arc of motion. The oscillating reamer provides precisely that, rapidly generated short arcs (30º) of motion. The potential for canal distortion is directly related to the arc of motion. The shorter the arc of motion the less the potential for canal distortion. Where the manual balanced force technique employed 90º clockwise motions coupled to 270º counterclockwise motions , the oscillating handpiece limits both the clockwise and counterclockwise motion to 30º. Like balanced force, apical pressure is being continuously applied with the dentin engaged in the clockwise motion shaved away from the canal walls in the counterclockwise motion. Now admittedly, the shorter arcs of motion are removing less dentin per cycle than that occurring with the original balanced force technique, but that is more than compensated for by the much higher frequency (3000-4000 cycles per minute) that engine-driven oscillating handpiece generates.

Let’s discuss a few other advantages. The unrelieved K-reamers through a 10/02 are more flexible than K-files simply because they are less work hardened. From 15/02 onwards they incorporate a flat along their working length that makes them more flexible than unrelieved K-reamers that even in their unrelieved state would be more flexible than K-files. The increased flexibility means they are even more adaptive to the canal walls. When the tip of a reamer contacts the canal wall, the reamer will deviate into the open canal space more easily than the K-file as exemplified in the balanced force technique. Combined with a shorter arc of motion this enhances the effectiveness of following the canal anatomy without introducing distortions. Being a mechanized technique that eliminates hand fatigue and confined to arcs of motion that are so small that they virtually eliminate instrument separation, we have the means to vigorously contact all the walls of a canal be it round or highly oval including thin isthmuses and narrow buccal and lingual extensions. The irrigants used throughout the instrumentation process are sonically activated by the handpiece that is oscillating at 3000-4000 cycles per minute.

One  last bit of insight: It is the clockwise motion that slightely embeds the reamer’s flutes into the canal wall. It is the counterclockwise motion that shaves the engaged dentin from the canal walls. Interestingly, the counterclockwise motion that does the dentin shaving, is also the motion that prohibits deeper engagement. That is the point in time where the canal dentin is deflecting the instrument into the path of least resistance, namely the canal. The fact that the same counterclockwise motion is responsible dentin removal and keeping the instruments within the center of the canal makes this system exceedingly safe and accurate. As seen in this video, the tip of the instrument even when applied to a solid wall will not penetrate the dentin as long as the motion is limited to short arcs. While the tip of the instrument makes a point contact, please note the 30º arc of motion that is generated in the more coronal portions of the instrument, exactly what is desirable when shaping canals along length.

I’m teaching these techniques now on a 1-on-1 basis in my office and will be expanding the program once we get over this Covid  environment. In the meantime for those who can visit me in Manhattan the number to make a workshop appointment is 212-582-8161.

 Regards, Barry