Barry Musikant

HE MORE I LECTURE on alternative methods of endodontic instrumentation compared to rotary NiTi, the more I realize that rotary NiTi is not the issue. The real nemesis for dentists is their continued use of K-files. Rotary NiTi reduces, but does not eliminate, the supposed need for K-files, the instrument of choice for creating the glide path. If one analyzes the design of K-files and the way they are used, one quickly realizes that their design does not support their function. The problem is that very few teachers in most of our schools have ever bothered to make the analysis that is the first step in understanding the relationship between form and function. So let’s give it a try right now.
    The main difference between a K-file and a K-reamer is the angle of the flutes along the length of the shaft. The working length of both is 16 mm. The flutes on a K-file are about twice as horizontally oriented as the flutes on a K-reamer. Since this is the case, the number of flutes on a K-file will be about twice the number of flutes that are present on a K-reamer. What I want you to appreciate is that this single difference, the orientation of the flutes, makes for a profound difference in function between the two instruments. The K-file, having twice as many flutes along its length, will engage the canal walls about twice as much as the K-reamer. Twice the engagement leads to twice the resistance to apical negotiation; that difference in resistance can become quite appreciable as the curvature of the canals increases. The more twists (flutes) an instrument has along its length, the stiffer the shaft will be. Greater stiffness leads to greater shape memory, a property characterized by the shaft’s snapping back to its original straight configuration.
    Whether one is using a K-file or a K-reamer, the typical hand motion is one of watch-winding, a horizontal back-and-forth motion combined with the occasional up-and-down stroke. Combining a predominantly horizontal motion with a flute design that is significantly horizontal is the height of inefficiency. The initial clockwise stroke does not cut. Rather it engages the dentin by threading (screwing) into it. The cut does not occur until the instrument is pulled up—when the engaged flutes can cut dentin—or apical pressure is applied to the engaged instrument as it is then rotated in the counterclockwise direction with the engaged dentin now being cleaved off. This latter technique is known as balanced force. The first technique requires two motions to have one effective cutting motion. The second technique requires three movements to produce dentin removal: the clockwise engagement, the apical application of pressure, and the counterclockwise motion. In both cases, the instrument is removed with a straight pulling motion in which the more-horizontally-oriented flutes exert their greatest cutting ability. These instruments have a higher degree of stiffness and consequent shape memory and tend to selectively shape to the outside wall, producing apical transportations and blockages as the canals being negotiated become more curved.
    Greater engagement produces increased resistance to apical negotiation. The shaft is stiffer, and the horizontally oriented flutes cut less efficiently. The result is poor tactile perception: dentists working with K-files and K-reamers cannot clearly tell what the tip of the instrument is engaging. As they advance from an .06 to an .08 to a .10 K-file, all that they really know is the length that they are determined to maintain. If the pathway to the apex of a curved canal becomes blocked in the apical third, they may detect that they have lost 1 or 2 mm of length. The typical response is to apply more apical pressure with a twist-and-pull motion until the lost length is regained. Unfortunately, the lost length is too often regained at the expense of apical transportation to the outside wall. At other times, the canal is blocked and the instrumentation is not only transported to the outside wall but also remains short of the apex, potentially leaving infected debris in the original canal anatomy.
    Despite these problems associated with K-files, these instruments are still the ones most prominently used. There are some who use the more flexible version of K-files, and while greater flexibility is a plus, these alternative instruments still suffer from a file flute design that excessively engages without cutting efficiently. It is the inadequacy of fit between form and function that has led to the use of rotary NiTi. Rotary NiTi reduces the use of K-files (and other files) without eliminating their use. The damage that K-files can produce in curved canals will occur while the glide path is being created. A glide path that is already distorting the original canal shape will not be corrected by the subsequent use of rotary NiTi.
    When one considers the separation anxiety that was introduced with rotary NiTi, a tool that was meant to overcome the problems of K-files without introducing its own problems, one has to conclude that it is high time for a more rational approach to endodontic shaping than has been apparent to date. As I have written before, reamers—because they have fewer flutes that are more vertically oriented and because they have more flexible shafts—will negotiate the canals with less engagement, gaining access to the apex with reduced resistance. As soon as the reamer is placed into the canal and rotated clockwise the more-vertically-oriented blades on the flutes cut. This is the way it should be.
    The combination of less engagement, greater flexibility, and vertically-oriented flutes that cut more efficiently produces a superior tactile perception for the dentist. Most significantly, the dentist can differentiate between hitting a solid wall and being in a tight canal. The former produces no tugback, the latter immediate tugback. With this ability to differentiate, when no tugback is present the dentist removes the instrument from the canal, places a 45-degree bend in the last millimeter or two of the instrument, and manually attempts to negotiate around the apparent blockage. You wouldn’t do this critical step if you were not aware that the tip of the instrument was indeed hitting a solid wall.
    The K-reamer is significantly better at accomplishing the task of shaping canals than K-files are. However, the K-reamer can be dramatically improved by placing a flat along its entire working length. The results of this design change include a further reduction in engagement along the instrument’s length, greater flexibility, and the creation of two vertical columns of chisels that cut in both the clockwise and counterclockwise directions. The flat has the added advantage of creating an asymmetrical instrument that can also differentiate between a round and oval canal, another critical observation that leads to recognition and better instrumentation of canals that have oval shapes. While these instruments are effective when used manually, they may also be used in a 30-degree reciprocating handpiece oscillating at approximately 3000–5000 cycles per minute. Even at these higher engine-driven speeds, the reciprocating handpiece is merely mimicking manual motion, employing an arc of motion that prevents excessive torsional and flexural stress, the two factors that are responsible for separated instruments when employing rotary NiTi.
    Reciprocation represents a high-frequency (number of cycles per minute) low-amplitude (arc of motion) way to shape canals that is quite efficient. While the amount of dentin removed per stroke is very little, the frequency of these strokes is so high that—from the dentist’s perspective—in most cases the instrument is almost sliding through the canal. This approach to instrumentation is not limited to thin instruments. There is no need to switch to rotary NiTi as the instruments become thicker. The relieved K-reamers, known as SafeSiders®, remain centered in the canal. The pull stroke, used to remove the instruments, does not selectively work against the outer wall because the more vertical flute design—which works so well with horizontal motion, whether manual or reciprocating-engine-driven—is a poor remover of dentin when a vertical motion is applied, and that is a good thing. To maintain patency, the dentist simply extends the length of instruments 0.5 mm beyond the constriction through a .25 reamer, preventing the buildup of any impacting debris. Without a buildup of debris at this potential bottleneck, the subsequent instruments will not lose length, eliminating the main cause of canal transportation.
    As the instruments become larger, the relieved design of the reamers helps maintain a higher degree of flexibility that reduces the potential for distortion. The dentist will always recognize if and when a wall is being hit and can then prebend any relieved reamer to get around the blockage, preventing the ledging and apical transportation that might otherwise occur.
    What I have described is a more detailed picture of exactly why the approach we advocate works so efficiently. From the dentist’s vantage point, this is a system that is virtually free of breakage, a system in which each instrument can be used six to seven times with dullness, not fracture, being the downside of overuse, and—most significantly in today’s economy—a reduction in cost of about 90 percent compared with rotary NiTi on a per-use basis. Having just commented on costs, I would add that even if times were flush with money, I see no point in using far more expensive systems that bring greater insecurity without producing superior results.
    For anyone interested in experiencing relieved reamers in natural teeth, I offer free 2–3 hour one-on-one workshops in my office. To participate in a workshop, simply call me at (212) 582-8161 to set up a mutually agreeable date.