The Factors That Lead To Safer Endodontics
When we say safer Endodontics, we are really talking about 2 separate, but related factors, the safety of the instruments performing the endodontic tasks and the impact of the instruments on the teeth being treated. The mutual safety of both the tooth and the instruments is primarily the responsibility of the instruments. No doubt that a skilled practitioner can enhance the safety of both, but the whole purpose of technology is to maximize the inherent safety of the instrument as well as the recipient of the actions those instruments produce, namely the tooth.
Measuring the safety of the instruments is fairly straightforward. We don’t want them to break even under the most severe anatomical challenges. The safety of the tooth is a bit more complicated. First, the safety of the tooth, as measured by the longevity in which it can function healthily is jeopardized if the instrument separates in a canal. Two, a tooth can be weakened by gaining straight-line access and crown-down preparations, steps employed to reduce the incidence of instrument separation, but at the same time making the tooth more vulnerable to vertical fracture at some point down the road. Three, to reduce the torsional stresses and cyclic fatigue associated with shaping curved canals, the safety of the instrument is increased as their preparations become more conservative. In oval canals the conservative approach can only aggravate the inadequate cleansing that has already been documented in numerous studies.
What is being described above is a dichotomy, a conflict between the inherent need to satisfy the safety requirements of the instruments and the optimum functions with which we want these systems to perform. Removing excess tooth structure for the safety of the instrument weakens the tooth. Inadequate three-dimensional debridement leaves pulp tissue and bacteria that reduce success. Yet, if we don’t employ these steps in the utilization of rotary instruments the instruments are more vulnerable to breakage. It is a self-perpetuating sequence, a self-contained loop that cannot be eliminated as long as instrumentation is based on rotary mechanics.
Rotary advocates discount this dichotomy stating that the success rate using rotary NiTi is high proving their utility and the negative research is really not relevant. And even if it were relevant at the earlier stages of rotary development the most recent generation of rotary NiTi instruments make the incidence of instrument separation far less likely. That along with the more conservative preparations in vogue today assure a stronger tooth, but one that is likely to be even less derided where oval anatomy and thin isthmuses are present.
Since its introduction rotary NiTi can certainly claim that where canal anatomy appears to be fairly simple, that its utilization produces faster results and that the obturation as observed in mesio-distal periapical x-rays is consistently uniform compared to the traditional manual use of K-files. What it cannot claim is a higher success rate. No study has yet to be published that states that rotary NiTi produces higher long-term success rates. Today, the best statistics put endodontic success at about 90% dropping to about 80% after 5 years. What that means simply is that there is room for improvement.
If one looks at the marketing of rotary NiTi today, it appears to be centered around two points of competition, resistance to separation and the fewest number of instruments required for the completion of the procedure. By employing one of these latest forms of rotary NiTi technology the implication is that one will be doing a superior job resulting in higher success rates while saving time and making the dentist more productive. The implication of higher success rates, however, is not substantiated in the literature.?
As I have stated in previous posts, what is needed to break out of this self-imposed conundrum is some common sense thinking. If rotary motion makes the instruments vulnerable to breakage, are there alternatives to rotary other than the time-consuming hand fatiguing manual watch-winding use of K-files? We know that the 30o M4 handpiece oscillating at 3000-4000 cycles per minute represents one such alternative. What impact does its use have on the instruments? It confines the motion to 1/12 of a full rotation that reduces the torsional stresses and cyclic fatigue to the point where they cannot exceed the elastic limit of the stainless steel files resulting in the complete elimination of instrument separation even where the most challenging anatomy is present.?
With the absence of separation a fact, all the precautions that were imposed to reduce those separations are no longer required. Vigorous lateral instrument movement in three dimensions rather than light centered shaping is the new norm. That lateral movement is enhanced by a handpiece oscillating at approximately 60 cycles per second using stainless steel relieved twisted reamers that in addition to vigorously physically contacting the canal walls also activate the irrigants that are constantly present in the canals. One might note that I transitioned from K-files to relieved reamers. Given the long history of their use why change to reamers and then go further and modify them with a flat along their working length??
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I’ve posted other papers in the past that noted the advantages that are associated with K-reamers in general and the flat-sided SafeSiders in particular. Just recently I came across a paper from 1982 published in the JOE titled, Flute Design of Endodontic Instruments: its influence on cutting efficiency. In a comparison between reamers and files, the authors concluded the following: “the reamer group tested was significantly more efficient than the group of all files tested. The reamer group from any one brand was significantly more efficient than the group of files from the same brand or any other brand.”
These results corroborated the experiences I first had as a student in 1968 where we were taught to use a reamer first followed by the same sized file. It became immediately apparent that the file was simply not necessary and employed it only when the instructor came around to observe our techniques. Today we use reamers modified with a flat reducing their resistance within a canal while making them more flexible and adaptable in curved canals. They also shave dentin away via a horizontal motion rather than the pull stroke associated with files. When a reamer is introduced into a canal, its predominantly vertical flutes bypass any debris in the canal unlike the predominantly horizontal flutes on a file that tend to impact encountered debris apically often resulting in a loss of length.
For me the most satisfying aspect regarding 30o oscillations as opposed to full rotations either continuous or interrupted is the insight provided by Dr. James Roane and his paper on the balanced force technique, a way of using stainless steel instruments that prevents canal distortions. For years prior to appreciating Dr. Roane’s balanced force technique I had been using the relieved stainless steel reamers in curved canals and was always pleased to see how they negotiated curved canals without any apparent signs of distortion. Only after reading his paper on balanced force, did I appreciate exactly why distortions were not occurring despite using stiffer stainless steel instruments. After all, it is obvious that if stainless steel reamers were used with full rotations in curved canals distortions to the outer walls would routinely occur. Why not when using them in 30o arcs of motion??
As Dr. Roane pointed out, short arcs of motion generate much less stress against the canal walls. Indeed as the arc of motion reduces, the resistance of the canal wall to that of the instruments increases. With short arcs of motion the resistance of the canal wall exceeds that of the stainless steel instruments. Consequently as they contact each other in negotiating to the apex, it is the instrument that gives being deflected into the path of least resistance, namely the patency of the canal. Now the balanced force technique employed a 90o clockwise arc of motion followed by about a 270o counterclockwise motion with apical pressure applied, arcs of motion that prevented any distortions. The 30o oscillating handpiece reduces even those arcs of motion significantly further reducing the potential for distortion while dramatically speeding up the debridement process from a handpiece operating at a frequency of 3000-4000 cycles per minute. Short arcs coupled to high frequency gives the dentist the ability to shape canals rapidly, debride them in three dimensions with minimal potential for distortion.
These advantages represent the solutions to the rotary dichotomy. Furthermore, they reduce the potential for the production of dentinal defects, a result associated with rotary instrumentation. I simply look at dentinal defects as the other side of the coin represented by separated instruments. A rotary instrument breaks in the canal due either to excessive torsional stress or cyclic fatigue imposed on the instruments by its contact with the canal walls. Newton’s Third Law of Motion stating that two interactive bodies have an equal and opposite effect on each other, tells us why an instrumentation system that can result in separated instruments also has the potential to produce dentinal defects. It is a result of the Third Law of Motion. 30o arcs of motion do not lead to instrument separation. Consequently, it is far less likely produce dentinal defects. Perhaps, this is the reason that the manual use of K-files are associated with the least amount of dentinal defects. They are generally used with a watch winding motion that by definition uses short arcs of motion.
I find the discussion of instrumentation has different levels of understanding. I personally enjoy delving into this subject, enjoy the views of others and hope the readers of these posts feel they have gained some measure of greater insight.
Regards, Barry
Doktora- ?stanbul üniversitesi
2 年Hi Dr. Musikant. Thank you for visiting my page.