Retrospective Thoughts on Endodontic Instrumentation

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OVER THE COURSE OF a nice peaceful Christmas day, I’ve had time to put things into perspective. Much research has been posted supporting the use of rotary NiTi and non-linear reciprocating NiTi in the creation of non-distorted shaping. Without question, it works. The same is true for K-files. They work too. Yet, the question is not whether or not something works. If that were the question, we would still be using K-files made of carbon steel the way endo was done more than 100 years ago simply because it worked.

Obviously, we incorporate new ways of doing procedures looking for the advantages of improved results—or at a minimum equal results attained more safely, efficiently, in the most economical way possible. This is where there have been strong differences of opinion. Rotary NiTi carries with it an increased chance of breakage. It has been written about for decades, and the major manufacturers are bringing out new products with the promise of reduced rates of breakage. The subject of breakage is challenged in the literature by studies showing that it is an infrequent event. Yet, for anyone who experiences separation clinically, literature that says it is a rare event is not quite convincing enough. As a result, it remains an issue. To say that it is not an issue is to ignore the experiences of dentists who are concerned about this trait.

The defense of rotating NiTi includes not only downplaying the effects of instrument separation, but also emphasizing studies that show that the alternative use of reamers used both manually and in the thirty-degree reciprocating handpiece cause a greater degree of distortion. These papers are challenged by studies that refute these effects, showing superior centering results for the reamers. There are no studies that show one technique superior to another in success outcome studies. When it comes to rotating NiTi, there are so many systems to choose from that their sheer numbers detract from the realization that other existing systems significantly different from rotating NiTi virtually eliminate breakage as a concern. One can get so bogged down in deciding which rotating NiTi system to use that one is not aware that the problem of breakage has already been solved.

If we make a list of the other problems that still confront us they would include:

  • excessive removal of coronal dentin
  • the problem of dentinal micro-fractures
  • poor preparation of sheath-like highly oval canals
  • in the bucco-lingual plane
  • blockage in the creation of the glide path
  • Common sense tells us that excessive coronal removal of dentin has to be associated with greater tapered instruments. A peeso can remove excess coronal dentin, too, but having cutting blades on a stiff shank, it can be selectively used against the outer wall of a root, straightening coronal access and leaving a greater thickness of dentin on the inner wall, adjacent to the furcation, something crucial to preventing strip perforations and the increased chances of dentinal micro-cracks in thin sections of the root.

    From the perspective of rotating NiTi advocates, the production of dentinal micro-cracks is best ignored. Saying that they are of no consequence may be a bit premature, knowing that the longer a tooth that has undergone root canal therapy exists in the mouth the greater the incidence of vertical fracture. Micro-cracks coalesce and propagate over time. It would be the rare dentist who over time has not seen endodontic failure due to vertical fractures. Is it a direct result of rotating NiTi? Rotating NiTi is probably one of many factors, but in no way can the production of micro-cracks be ignored as irrelevant. We know that vertical fractures occur, and we have to seriously consider all the factors that may be involved. It is hard to ignore 14 studies that have pointed out their production with rotating NiTi being the prime cause.

    I have heard that rotating NiTi can be used in a brushing motion, meaning that it can be adapted to highly oval canals. Yet, I would think that anyone who uses rotating NiTi knows that the final preparation nearly always presents itself as a round hole at the level of the orifice. With this observation, it is unrealistic to think that canal preparations further apically are now going to be anything else other than a tapered round preparation along the entire length of the canal. In short, highly oval canals result in canal preparations that are inadequate in the bucco-lingual plane. To remove pulpal tissue from these thinner bucco-lingual sections, our best bet is to use thin 02 tapered stainless steel reamers with short arcs of motion. The short arcs prevent breakage. Knowing that the instruments will not break, the dentist can apply pressure against all the walls. If the instrument is used in a thirty-degree reciprocating handpiece, this pressure can be applied to the lateral walls at a frequency of 3000–4000 cycles per minute, enough action to widen the lateral canal spaces, and also fast enough to activate the irrigants that are present in the canals.

    A major concern in creating the glide path—particularly in tight, tortuous canals—is the maintenance of depth. We don’t want to lose length due to blockages. If blockage occurs, attempting to regain that length often results in distortions in the apical third of the root. There has been major contention over the issue of whether or not reamers work better than K-files. Once again, most dentists have never been exposed to the use of reamers, so they have nothing to compare to the use of K-files. Yet, if we correlate the differences of design between a reamer and a file, it defies logic to not understand the reasons why reamers work better. While this has been covered many times before, it never hurts to reinforce basic mechanical principles.

    1. Reamers have fewer and more-vertically-oriented flutes than K-files. Implications: Reamers engage dentin less than K-files, and the vertically-oriented flutes immediately shave dentin away with the first clockwise stroke rather than further embedding themselves into dentin with the first clockwise stroke.
    2. The fewer flutes and the flat along the working length of relieved reamers make them more flexible than corresponding sized K-files. Implication: Reamers negotiate curved canals more easily than stiffer K-files.
    3. Relieved reamers’ greater flexibility, shaving ability, and reduced engagement along length provide the dentist with a superior tactile perception. Implication: The dentist is able to differentiate between a solid wall and a tight canal, which in turn prevents distorting curved canals.
    4. The reamer has a cutting tip. The file does not. The implication: The cutting tip can pierce pulpal tissue and not impact it apically.
    5. The asymmetric cross section of a relieved reamer can distinguish between a round and oval canal. The flat along length tends to line up with the thin narrow sheaths that most often exist in the bucco-lingual plane. K-files don’t provide this advantage.
    6. The vertical flutes along the length of a reamer tend to bypass dentinal debris when negotiating apically. The horizontal flutes on a file tend to impact debris apically when directed apically.

    All these issues can be discussed at length. I think that there is a strong case to be made for the use of reamers, both unrelieved and relieved, in the complete shaping of canals. While not everyone may agree, it is worthy of an objective discussion in which many of us can relate our clinical experiences.

    It really does come down to clinical experience. By broadening the base of that experience, we have a better chance to understand the implications of design as it relates to function. Research gives us useful information, but it is the individual’s experience that is most relevant to what is used.

    January - March 2014

    By broadening the base of clinical experience, we have a better chance to understand the implications of design as it relates to function. Research gives us useful information, but it is the individual’s experience that is most relevant to what is used.