A critical assessment of endodontic armamentarium
Nickel-titanium endodontic instrumentation systems are being introduced at a rapid pace. Some of the new products include NiTi instruments that are heat-treated and twisted rather than ground, making them more resistant to breakage. Other systems are beginning to incorporate a form of reciprocation that turns continuous rotations into interrupted rotations, reducing the incidence of breakage because of excessive torsional stress and cyclic fatigue.
They are also being introduced as single-instrument systems, although in practical terms a glide path must still be created in all but the easiest cases.
The fact that so many new products are being introduced is ample proof that the prior products are meeting with increased resistance and breakage, along with high cost and excessive time requirements. The other factor is competition from other companies offering systems that minimize or eliminate these concerns.
A basic question we want to ask ourselves: Is the introduction of these new products an effective answer to the problems encountered with the previous products? From a marketing point of view, the introduction of a single-file system used with asymmetric reciprocation does indeed reduce the incidence of instrument breakage. It also reduces costs compared with techniques that use a number of instruments and reduce the time requirements. However, great care must be taken with that single instrument, since it alone is going to introduce a greater tapered shape along the entire length of the root.
So yes, one can conclude that these instruments are more effective in meeting the perceived needs of the dentist. Yet lost in the discussion is the fact that for safety and effectiveness, a glide path must still be created prior to their use. It is at this crucial stage of canal shaping that we are relegated to using the old workhorse of K-files that have been around for more than 100 years. If by some means we can improve the quality of glide path creation, then we are overcoming the greatest obstacle to full canal shaping. Any new NiTi innovation means little if blockages, distortions and loss of length occur at the stage of glide path creation. Therefore, let us discuss ways that improve canal shaping at its earliest and most critical stages.
To first appreciate the improvements that can be made in this area, we must understand why the traditional K-file is poorly designed for the function it is asked to perform. A K-file is composed of approximately 30 horizontally oriented flutes along 16 mm of working length. They are used in one of three ways — watch-winding, a short twist and pull, and balanced force. Watch-winding is a horizontal form of motion. When the orientation of the blades at the edges of the flutes are in the same plane as the plane of motion, they will cut into the dentin without removing it.
When 30 blades along length are embedding themselves into the dentinal walls, a high level of engagement exists between the instrument and the walls of the canal. The greater the engagement, the greater the resistance the instrument encounters as it negotiates apically. This engagement and resistance is overcome only when the instrument is pulled in the coronal direction.
It takes a vertical motion for the horizontal blades aligned along the length of the instrument to shave dentin away, freeing up the instrument for further apical movement. The greater the engagement, the greater the coronally directed force required to overcome that resistance. It is for this reason that some dentists use K-files with a short twist and pull. The short horizontal motion reduces the engagement of the dentin and less coronal force is required when pulling the instrument in the coronal direction.
The problem with both these techniques is that the horizontally oriented flutes along length have a strong tendency to impact debris apically when the instrument is directed apically. This is especially so when negotiating curved canals. In both cases, the problem of apical blockage and subsequent distortion is aggravated by the high level of engagement with the canal wall. The greater the engagement, the poorer the tactile perception of what the tip of the instrument is encountering. With so much engagement along length, the dentist may not be aware that the tip of the instrument is encountering a solid wall and a further twist-and-pull action can lead to ledging and ultimately a lateral perforation of the canal.
Given these problems, the K-files are used at times with a technique called balanced force, developed by Dr. James Roane. In this technique, the dentist advances the K-file with a clockwise stroke until significant resistance is encountered. He then applies apically directed pressure as the instrument is rotated counterclockwise. The apically applied pressure prevents the instrument from unscrewing itself from the embedded dentin with the counterclockwise stroke cleaving off the engaged dentin in the process. This technique has the added benefit of keeping the instrument tip centered in the canal. It is also less likely to impact debris apically because the flutes are cleansed after every counterclockwise stroke. The negatives of this technique are the time it takes to do properly and the chance of still creating a ledge in abruptly curved canals because the excessive engagement along length obscures what is happening at the tip of the instrument (Fig. 1).
As we can see, the K-file is not optimally designed for creation of the glide path despite the fact that it is an instrument used almost universally for this task. Now let’s substitute for the K-file a K-reamer in two configurations, unrelieved and relieved, with a flat along its entire working length. The K-reamer, both unrelieved and relieved, is composed of approximately 16 flutes along the same 16 mm of working length as the K-file. Unlike the K-file, the flutes along the length of the instrument are predominantly vertically oriented.
If used with the same watch-winding stroke as the K-file rather than the threads embedding themselves into the dentin, the first clockwise stroke immediately shaves dentin away from the canal walls. Rather than increasing resistance as the instrument travels apically, the walls are constantly being widened, resulting in less engagement and consequently less resistance. Also consider the fact that having 16 rather than 30 flutes to begin with reduces the initial engagement of the instrument with the canal wall, further reducing resistance.
The major difference between a K-file and a K-reamer is the latter immediately removes dentin rather than first embedding into it, thus enhancing the tactile perception of an instrument that is already superior to a K-file because of the initial reduced engagement. Now consider the impact of a flat along the working length further reducing engagement and resistance, and it becomes obvious why this is a superior instrument for the creation of the glide path (Fig. 2).
Using instruments that are not optimally designed for the initial glide path creation has not been critically assessed in the past because of the promises and expectations made for the various NiTi systems. Yet, if we incorporate the improvements that K-reamers, both unrelieved and relieved, offer, it also allows us to reassess the advantages of NiTi and question whether or not an extension of the same instruments used for glide path creation can be extended to the entire shaping process.
The K-files, both relieved and unrelieved, may be used either manually with a watch-winding motion or in a 30-degree reciprocating handpiece. Please note that unlike the reciprocating NiTi systems, 30-degree reciprocation does not generate full cycles of motion, preventing excessive torsional stress and cyclic fatigue associated with full arcs of motion. By keeping the amplitude of motion to a minimum, the stresses generated to both the instruments and the teeth are also kept to a minimum. This last point has implications beyond mere academic curiosity, given nine studies published during the past four years describing the production of dentinal micro-fractures associated with full rotations and greater tapered instruments.
The limitations of rotary NiTi are easy to observe. They are now recommended for single usage, underlining their vulnerability to breakage (Fig. 3). Glide path creation often takes the apical preparation up to a 20 or 25, with the subsequent use of NiTi creating a greater taper along length with minimal work being done apically. These goals have been imposed after the use of K-files. Imagine the increased ease of creating the glide path with the use of K-reamers, both unrelieved and relieved, and the ability to extend these preparations into complete shaping using instruments that encounter so much less resistance. They are virtually invulnerable to breakage and are consequently used several times before replacement, dramatically reducing costs in the process.
Perhaps the last bastion of defense for rotating NiTi is the claim that a single file can produce a 25/08 preparation. With the increasing concern for dentin conservation, the 25/08 preparation should really be no greater than a 25/06. Using the relieved reamers and one Gates Glidden No. 2 reamer, we routinely create a space that is prepared to a 30 at the apex with a 25/06 overlayed taper. That is the additional use of three instruments beyond those required for the glide path to produce an apical preparation to 30, which research shows is the minimum required for effective irrigation. In addition, the instruments can be used multiple times and, given their resistance to breakage, can be worked against all the walls of oval canals, increasing the chances of three-dimensional cleansing. It should also be noted that the use of these three instruments will often take less time to prepare the canal to greater apical dimensions than the single instrument recommended for complete canal shaping as the manufacturers define it.
Perhaps most importantly, when I talk about glide path and the creation of a minimal canal preparation to a 30 with an overlayed 06 taper, I am talking about a single-system approach that encompasses all aspects of canal preparation. Not one rotary system exists that describes in detail how the glide path should be created. This critical part of the process is left to the dentist to figure out on his own, to determine what constitutes a suitable glide path and how to achieve it. Furthermore, many canals require far greater preparations with special attention directed to highly oval canals. Rotary NiTi does not address these issues in a disciplined fashion. The use of K-reamers, both unrelieved and relieved, used both manually and in a reciprocating handpiece, addresses the full array of endodontic challenges that may arise and gives the dentist the means to address all of them with a system that is safe, efficient and economical.
Note: This article was published in Endo Tribune U.S. Edition, Vol. 8 No. 6, July/August 2013 issue.