March 1, 2006
by Rich Mounce, DDS
To answer the question that was asked (Feb/06 pg. 8) by a reader, no, hand K files are not meant to be inserted into canals that cannot support the diameter of the given file. Said differently, the hand file will have to be slightly smaller than the canal space into which they are inserted and as such even if the canal is microscopically bigger than the file, the file can find a space into which it can easily be placed and its rotation can engage dentin within the flutes and begin to clear a path in the canal. If the file were bigger than the canal, it would have to be forced into place which increases the chance of breakage.
The files are precurved to more easily negotiate the given space. It must be remembered that all canals are curved to some extent and a straight file will not traverse the curvature nearly as easily as one that is curved.
With regard to using small K files to bypass blockages, in my empirical opinion, small K files do have a better chance to pierce through some types of apical debris rather than compact the debris, but whether debris is pierced or compacted would be a function of the given diameter of the canal and the level of existing blockage. Some very narrow, curved, and calcified canals are very much at risk of being made impassable if the canal is entered too rapidly with either hand files or RNT files that are too large for the given space. In essence, such forcing of a file apically that is too large for a given canal can easily push pulp debris beyond the file tip and contribute to blockage.
While it may seem a contradiction given the context of this column to state so, it is possible to use small .02 tapered RNT files as negotiators in the correct clinical cases. I use the .02 tapered K3 RNT files (SybronEndo, Orange, CA, USA) for canals that are large enough to easily accept them, even as the first file down the canal. Such a case might be one where the canal is small enough that a larger diameter file such as a .06-tapered K3 is too large to be easily inserted any significant distance, but as above, a .02-tapered file slides easily. Use of .02 tapered RNT K3 files used in this manner should be practiced extensively first in extracted teeth and rotational speed must be carefully controlled so as to diminish the chance for the file to be subjected to cyclic fatigue and/or torsional failure. An ideal rotational speed would be about 350. Cyclic fatigue is the phenomenon that occurs when a file is subjected to tension on one side of the file and compression on the other and the file separates, much like a paper clip would when it is bent repeatedly at the same spot. Cyclic fatigue can occur in a similar manner when a RNT is placed through a curvature in a root canal system. Once any rotary file is used, especially where any significant debris is produced, recapitulation and irrigation is then recommended to keep the canal path open to the greatest degree possible and avoid iatrogenic events prior to the next RNT file insertion.
In summary, copious irrigation, precurvature of hand files, gentle insertion and touch, creation of a glide path and recapitulation after RNT use, can all go far toward creating efficiency and safety in canal instrumentation.
Dr. Richard Mounce is in private endodontic practice in Portland, OR, USA. Dr. Mounce is the author of a comprehensive DVD on cleansing, shaping and packing the root canal system for the general practitioner.
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