Barry Musikant

  ANSWERED a question on the Endo-Mail message board the other day concerning flare-ups, and it got me to thinking once again about the ability of instruments to negotiate and widen canals with the least likelihood of pushing debris over the apex, and, given that some apical debris is inevitable, how to render that debris as non-virulent as possible.
    Many articles have been written about rotary NiTi’s being less likely to push debris over the apex than stainless steel instruments are. I don’t believe that this is where the discussion should start, simply because rotary NiTi is recommended only after the glidepath has been created with stainless steel instruments, K-files being the most frequently mentioned. Rather, I think we will benefit from an analysis of K-reamers, both unrelieved and relieved, compared to K-files. I justify the start of discussion at this point because either of these two instruments is the first one placed into the canals, and it is a reality that the canal is most often shaped to a least a 20 prior to the use of rotary NiTi. If debris is going to be pushed over the apex and in so doing create a flare-up, it will happen at this stage of canal preparation.
    All instruments can be used in rotation, in a watch-winding motion, or with a push-pull stroke. These motions may be generated manually or by a motor-driven handpiece. The design of the instruments and the motion used will have an impact on the amount of debris generated and extruded apically. In a comparison of K-files and K-reamers, both relieved and unrelieved, we find the following differences.

1. Along their 16 mm of working length, files have twice the number of flutes that reamers have.
2. The inclination of the flutes of a file is about twice as horizontally oriented as those of a reamer, a natural consequence of the increased number of flutes along the shank of a file.
3. The shank of a reamer is more flexible than that of a K-file because the reduced number of flutes means that the reamer has been twisted less. In the case of relieved reamers, the shank is even more flexible because it is thinner in cross section.
4. The extra space created by the relieved shank provides a place for the debris generated during negotiation, and according to recent research that space appears to be critical in giving the relieved reamers less resistance and increased ability to cut more dentin than their unrelieved counterparts.
   

    These four differences between K-files and K-reamers produce far different experiences when each instrument is asked to negotiate a canal. The differences are more obvious the more difficult the anatomy of the canal.
    The K-reamers, and particularly the relieved K-reamers, engage the canal walls significantly less than K-files do. Less engagement immediately translates into less resistance to apical negotiation. It is a given that both K-reamers and K-files are predominantly used with a watch-winding motion, with the occasional vertical pull stroke used to remove the instrument to wipe off the debris. For watch-winding to be effective in cutting the dentin along the length of the canal walls, the blades on the shank should be more or less at right angles to the plane of motion. This orientation is the case for K-reamers, both relieved and unrelieved, but is not the case for K-files. Consequently, when the watch-winding motion is used with K-files it is highly inefficient. The operator must count on the pull stroke to plane the walls. The horizontal flutes on a K-file are actually far more efficient at cutting dentin with a pull stroke. The problem here is that the pull stroke tends to work selectively against the outer wall. As a result, repeated vertical strokes with K-files transport the apical canal shape in the direction of the outer wall. In addition, a pull stroke is often accompanied by a push stroke. Here, the horizontally oriented flutes of the K-file are quite efficient at impacting debris apically as well as driving it over the apex.
    Figure 1 shows a photo of a K-file and a relieved K-reamer (SafeSiders®) demontrating the differences in design between the two and the effect of that design on the shaping ability of each.
    The design of the relieved reamers, coupled with their use in the 30º reciprocating handpiece operating at 3000–5000 cpm, makes them so efficient and effective at shaping canals in an undistorted fashion that they should be the initial instruments used in canal shaping as well as the instruments used for complete shaping, precluding the need for rotary NiTi. To elaborate a bit on this last point, rotary NiTi really was a partial answer to the limitations of K-files. It made sense to find a tool superior to K-files, given their rapidly increasing stiffness, their exaggerated engagement with the dentinal walls that leads to rapidly increasing resistance to apical negotiation, their poor cutting potential when used with a watch-winding stroke, their excessive potential to distort the apical ends of canals, and their unwanted ability to impact debris apically. Rotary NiTi short-circuited many of these problems because it limited the use of K-files to a 20 or 25 preparation. However, rotary NiTi instruments put us on guard against their use in many situations lest we break them in the canal. The consequences of this concern are canal preparations that may be somewhat greater in taper than the K-files initially produced but are rarely wider in tip size. In short, we limit the use of rotary NiTi in curved canals to conservative preparations because of separation anxiety.
    Under these circumstances, conservative preparations make sense, but they often do not meet the biological needs of the tooth. The literature clearly states that minimum preparation to a 30 and preferably a 35 is required for adequate irrigation as well as the activation of those irrigants. Rotary NiTi instrumentation stays centered. Given the diverse nature of canal morphology, staying centered is a distinct disadvantage for complete debridement. We want the instruments to have the ability to work the walls of the canals aggressively. Many of those canal walls have extensions that are fin-like. Leaving those spaces untouched makes removing the tissue chemically far more challenging when the irrigants are placed. If the fins themselves were opened to a minimum of 30, removing any remaining debris would be accomplished with far more predictability. We know that the irrigants, particularly when activated, can digest the tissue, the only requirement being preparation to at least a 30, but the 30 must be the minimum dimension for all the walls, not just a centered round preparation in the presence of an oval canal with narrow buccal and lingual extensions as depicted in Figure 2.
    Relieved reamers meet the requirements for widening all canal walls safely and efficiently when used manually with a tight watch-winding stroke or in the 30º reciprocating handpiece oscillating between 3000 and 5000 cycles per minute. Thirty-degree arcs of motion effectively eliminate torsional stress and cyclic fatigue, the two factors most responsible for instrument separation, allowing the instruments to be directed laterally against all the walls of a canal in a strong deliberate fashion, opening narrow grooves that may be harboring necrotic tissue into wider spaces that allow the irrigants to digest the small amount of remaining tissue that was not removed mechanically. Not only is there less tissue to digest, but the volume of irrigant that does the digesting is increased.
    We developed these safer, more effective, and far less expensive ways to do endodontics primarily for our own practice, but over the years, despite the presence of giant rotary NiTi manufacturers and their monopolization of the marketplace, the SafeSiders approach has been accepted not only by more and more dentists, but also by more and more endodontists. In recent years, I have been given the chance to speak at more and more universities in front of their graduate student bodies to discuss and demonstrate the advantages of our approach. More often than not, these opportunities arise because one or more members of the faculty personally use the systems and want their student body exposed to these approaches. This is not as good as getting an invitation from the chairman of the endodontic department, but, the world being the way it is, we recognize these invitations as progress and part of a continuum of steps leading to much wider recognition of a more logical and benign approach for effective endodontics.