A Timeline for Endodontic Innovations: Overcoming Cognitive Dissonance

Within my 50 years as an endodontist here is my perception of the innovations that have been introduced and to varying degrees adopted for the endodontic procedures of instrumentation, irrigation, obturation and restoration. In this post we will confine the discussion to innovations in instrumentation. The memory of my perceptions starts in the third year of my dental education at the University of Pennsylvania around 1968. The protocol for instrumentation was primarily based on the manual use of stainless steel reamers followed by the manual use of K-files.?

The reamers with their predominantly vertically oriented flutes shave dentin away from the canal walls with a combined watch winding motion, today defined either as reciprocation or oscillation coupled to vertical out strokes. The watch winding in stroke was employed to incrementally gain further apical access. Depending upon the resistance encountered doing this procedure, the time needed to reach the apex varied. In all cases, employing reamers means the dentin is being removed via horizontal arcs of motion. The out stroke’s function is simply to bring the captured shavings, engaged pulp tissue and bacteria coronally where they can be removed from the flutes.

The K-reamer was followed by the K-file used in a slightly different way. The purpose of the watch winding motion using files with their predominantly horizontal flute orientation is to have the flutes cut into the canal wall, engaging without yet shaving dentin away. The shaving process only occurs with the vertical pull stroke when the file’s flutes are more or less at right angles to the plane of motion. I don’t think we really understood the mechanics at that time, but following the protocol was the order of the day. One of the downsides we occasionally observed was the impaction of debris apically with a loss of length. This undesirable outcome was far more likely to happen after using the K-files. It rarely occurred with the K-reamers. Today we know that the horizontal flute orientation of the K-files are far more likely to impact debris apically than the vertically oriented flutes of a K-reamer that tend to bypass any debris encountered.

Clinically, the following observations could be made. The reamers always negotiated to the apex with less resistance than the K-files despite the fact that they were used first while the K-files impacted debris apically far more frequently. These observation begged the question why were we using K-files of the same dimensions as the K-reamers. Did we in fact need them? Were they adding some benefit? Clinically, they seemed to be more of an impediment than a help. Despite the school’s protocol many of us abandoned the K-file when going through the instrumentation sequence only to be employed when an instructor came over to watch our procedures. Its deletion had no deleterious consequences during our school years and to date 50 years later that record is intact.

From my perspective it is unfortunate that many schools limit the initial canal preparation to the use of K-files alone. Without a comparsion between the two, the unintentional advantage we had in our endodontic education is missing and the use of K-files as the sole instrument for initial canal preparation is accepted as an absolute. That they impact debris more than K-reamers, that they are less adaptable to curved canals, that their greater number of horizontally oriented flutes encounter significantly greater resistance, that their greater stiffness leads to an increased potential for canal distortions never become comparable when the alternative of equally sized reamers is entirely missing from clinical experience.

For those of us who experienced both, reamers became our primary tool for endodontic instrumentation. The next great innovation that comes to mind is not a new product, but insight into how instruments could be used in a way that minimized canal distortions even in highly curved canals, namely the balanced force technique an innovation in thinking by Dr. James Roane. He used K-files as if they were reamers meaning they shaved or cleaved dentin from the canal walls, not using the pull stroke that is normally associated with files, but rather horizontal motions generally associated with reamers. How did he do that? He employed K-files first with a 90o clockwise arc of motion to first have the predominantly horizontal flutes cut into the canal wall without shaving any dentin away. He followed this with a counterclockwise motion while at the same time applying apical pressure preventing the engaged instrument from rising and disengaging. The result of such manipulation is a cleaving of the engaged dentin from the canal wall. Repetition of this procedure allows the instrument to negotiate to the apex and most surprisingly prevents any distortions that one might expect in curved canals using stiff stainless steel instruments.?

The most notable achievement, non-distortion is explained by Dr. Roane, that short arcs of motion reduce the ability of the tips of the stainless steel files from penetrating the dentin along the length of the canal walls. Upon contact between the tip of the instrument and the canal walls, it is the canal walls that offer greater resistance to deformation than the instruments forcing the instruments to deflect into the path of least resistance, the already existing canal. This was a great insight that never really reached its full fruition simply because the manual implementation of balanced force is slow and hand fatiguing. Most dentists continued to use K-files with the twist and pull motion they learned at school.

It was in an environment dominated by the traditional use of K-files that in 1988 the first manual NiTi instruments were introduced with their far greater flexibility being a highly touted advantage over stainless steel K-files. It was not until 1992 that the first rotary systems became available and promised dentists a reduction in hand fatigue and the time needed for the instrumentation procedures. While the K-files were still necessary for glide path creation, a necessity to reduce the incidence of instrument separation, the implementation of rotary NiTi became an attractive alternative to full manual canal preparations.?

The following years of rotary NiTi innovation center around ways to reduce instrument separation while offering the dentist the convenience of a rotary process including reciprocation, a form of interrupted rotations that reduced the torsional and flexural stresses the instruments were being subject to, the increasing emphasis on the martensitic phase of NiTi making the instruments more flexible and less prone to cyclic fatigue, a series of precautions that the dentists take to minimize stresses including staying centered and crown-down preparations both of which reduce the stresses the instruments are subject to. Given the vulnerability to separation that rotary still bears and the obvious iatrogenic appearance of a separated instrument dentists have had to compromise in their use leading to the documentation of inadequate debridement of oval canals and thin flat isthmus-like spaces.

Given the persistence of separation as an unpredictable outcome, the evolution of rotary NiTi has had to compromise the primary goal of effective three-dimensional debridement. Those employing rotary or reciprocating systems take precautions to minimize functional stresses. They will knowingly not be used vigorously in the buccal and lingual extensions of oval canals. Rather than defining this confined usage as a weakness, the manufacturers state removal of lateral pulp tissue and bacteria in oval canals is the function of irrigation be it sonic, ultrasonic or acoustic. This indeed is the case due to the vulnerability of rotary to separation in such situations, but it should be called for what it is, a weakness that to date the technological improvements in NiTi have yet been able to fully resolve.

It is at this stage of rotary NiTi technology that I find myself once again as I did as student realizing that K-files were more of an impediment than a help, also realizing that the basic weakness of NiTi can be overcome with high frequency short arcs of motion using stainless steel relieved twisted reamers in a handpiece that limits the arc of motion to 30o. The most basic problem, instrument separation, is virtually eliminated. With separation eliminated, none of the precautions rotary requires are necessary. The instrumentation system is engine-driven from the first instrument to the last, for the most part eliminating hand fatigue and speeding up what was the need for manual canal preparation prior to using rotary NiTi.

Cognitive dissonance is likely to result when dentists who have been taught that rotary NiTi represents the most informative and effective way to shape canals not only become familiar with the shortcomings research documents, but are exposed to alternatives that eliminate all the precautions rotary requires, precautions that compromise the goals of complete non- destructive debridement. The resulting dissonance can be enlightening, but it is also likely to lead to denial, annoyance and outright hostility. At its best, it should lead to a more critical appraisal of what is being taught and a more open mind that offers possible solutions to problems that to date have not been adequately resolved.

Regards, Barry