Examining the Endodontic Terrain
OVER TIME, I believe we have taken the effort to digest the impact of rotating NiTi and its effect on shaping canals. One feature of rotating NiTi that stands out in my mind is its clear separation between the creation of the glide path and the subsequent use of NiTi independent of the system used. To my knowledge, every NiTi system, be it used with continuous or interrupted rotations, does not address the creation of the glide path, leaving the dentist on his own in establishing a pathway that is best suited for the subsequent use of NiTi.
I don’t consider this a casual oversight. The glide path is the most crucial part of canal instrumentation. Its importance has been born out by the trends that are occurring in endodontics. Traditionally, K-files were used to produce canal preparations to 30 or 35, an apical dimension that was compatible with effective irrigation according to a multitude of research studies. In fact, K-files have unfortunate design characteristics that increase the chances of impacting debris apically when shaping curved canals, leading to loss of length and various canal distortions, including transportation to the outside wall and frank perforation. Such events demanded an improvement in the tools and led to the eventual introduction of NiTi, a significantly more flexible metal that has the potential, within limits, to shape canals in rotation without distortion. The primary goal in introducing NiTi was to expedite a reduction in the use of K-files without eliminating their use altogether.
At first, it was stated that the glide path need only be shaped to a 10, at which point the crown-down rotary use of NiTi would over and quickly prepare canals to an equivalent apical preparation with the added advantage of imparting a greater taper to the canal along length. As it turned out, the minimal use of K-files often did not define the glide path sufficiently and for that reason and a host of others those experiencing the use of rotating NiTi had to deal with a significantly increased incidence of instrument separation. As a response to such stress-producing events, articles started emphasizing the creation of a glide path to a minimum of 20 while at the same time stating that the final apical canal preparation need not exceed a 25. The goal changed from optimizing the cleanliness of the canal and its ability to be well irrigated to maintaining the structural integrity of the instruments.
The NiTi systems today have clearly been retrogressive in their apical preparations, particularly in canals of increasing curvature. What is fascinating is the consistency with which all rotating NiTi systems still insist that K-files are the instrument of choice in glide path creation. These poorly functioning instruments by definition were the main incentive to seek out improved methods of canal shaping and when the various NITi innovations resulted in instrument breakage, they were simply assigned less work with the default use of K-files again on the increase.
It is no wonder that there are no studies showing an improvement in success rates over the past several decades. We are presently in a closed-loop system where what obviously does not work well is again compensating for the shortcomings of innovations that do not live up to their promises and in many cases add to procedural stress.
A way out of this dilemma is substituting K-reamers unrelieved through a 10 and relieved with a flat along the working length thereafter. Substituting reamers for files makes the creation of the glide path far less challenging for the following reasons:
1. K-reamers have half the number of flutes. As a result they will engage the canal walls far less. Less engagement means less resistance.
2. The flute orientation on a K-reamer is twice as vertical as their orientation on a K-file. When the instrument is used with a watch-winding motion, the flutes will immediately shave dentin away from the canal walls with the first clockwise stroke, further reducing the engagement along length and making it still easier to negotiate the instruments apically. Compare these mechanics with those of a K-file, where the predominantly horizontal flutes will immediately engage the dentinal walls along length without shaving any dentin away until the pull stroke is employed.
3. With such minimal engagement along length, the dentist has a superior tactile perception of what the tip of the instrument is encountering. Reamers with minimal contact along length give the dentist the ability to differentiate between encountering a solid wall and being in a tight canal. A K-file cannot make such a distinction, increasing the chances of canal distortion in the apical third.
Ultimately, what we have eliminated is the continual battle between the inefficiencies of K-files and the vulnerability of rotating NiTi, resulting at best in under preparation apically and excessive preparation coronally as instruments of greater taper widen the canal in what are often roots that are thin mesio-distally. This excessive coronal preparation also includes underpreparation coronally in the bucco-lingual plane.
Knowing that instruments will not break when used with a tight watch-winding motion or in a thirty-degree reciprocating handpiece, the dentist is comfortable in working the shaving blades against all the walls of the canals. Procedural stress is minimized for the same reason, and from a practical point of view, all the instruments are used at least 6 or 7 times before replacement without fear of breakage, saving significant amounts of money.
July - September 2013
We are presently in a closed-loop system where what obviously does not work well is again compensating for the shortcomings of innovations that do not live up to their promises and in many cases add to procedural stress.