Do K-files Function the Way We Want? If Not What Is the Answer

So many dentists use K-files that I thought it would be insightful to write a post on the way they function in contrast to reamers. First of all, files are designed with a predominantly horizontal flute orientation with approximately 30 flutes included within their 16 mm of working length. If used manually, they work most effectively if rotated clockwise anywhere from 45o to 90o to engage and slightly embed the flutes within the dentinal walls. Once embedded, the K-file is drawn coronally, a direction at more or less right angles to the flute orientation giving the file the ability to now shave the engaged dentin away from the canal walls. Please note that to shave dentin away (and that’s what we want to do) the cutting edge of the flutes must be at right angles to the plane of motion.

A technique used to advance K-files apically without distortion is called balanced force. It requires a 45o to 90o clockwise rotation to first engage the dentinal walls. Once engaged, apical pressure is applied to the K-file while at the same time rotating the file counterclockwise. By preventing the instrument from unscrewing, the counterclockwise motion fractures the engaged dentin from the canal walls while keeping the file centered preventing any canal transportation that might occur in curved canals with full rotations. When performing this procedure one can clearly hear the crack of the dentin as it fractures away from the canal walls. 

From the point of view of centered shaping free of canal distortions, using K-files with a balanced force technique works as claimed. One of the issues that has come to light since the introduction of greater tapered rotary endodontics is their production of dentinal micro-cracks, defects in the dentin that can coalesce and propagate into full-blown vertical fractures. Knowing that the production of dentinal defects is a potential outcome of endodontic instrumentation causes concern when literally hearing and feeling dentin fracturing off the canal walls when the balanced force technique is employed. It is not difficult for me to imagine dentinal defects extending deeper into the surrounding dentin when the dentin being removed is fractured off the canal walls rather than being lightly shaved away.

A predominantly horizontal flute orientation encourages the impaction of debris apically. While it is true that K-files shave dentin away on the pull stroke, they have an equal potential to impact debris on the push stroke that occurs when that file or the next file in the sequence is introduced into the canal. Good technique states that excellent irrigation should keep the canal debris-free during the instrumentation process, but we all know that while desirable we really have no way of knowing if indeed the canals are debris-free. Most likely, they are not.  

Let’s also remember that canals are most often not conical in cross-section. Our goal is to remove the pulp tissue in three-dimensions requiring the vigorous application of the files against the buccal and lingual extensions of oval canals as well as the thin isthmuses that may be present in figure eight pulpal configurations. The same pull stroke is required to shave dentin away from these walls compounding the potential to impact debris upon reinsertion of the file. The highly fluted file is stiffer than a comparably sized reamer encountering significantly more resistance to apical negotiation the greater the curvature of the canal. As stated above, distortions can be minimized by using the balanced force technique , but that is likely to be associated with the production of dentinal defects and the minimal ability to reach the pulp tissue in any buccal and lingual extensions that may be present. 

By eliminating balanced force, we are left with the manual twist and pull motion to advance the files apically. We are less likely to cause dentinal defects, but are now more prone to transportation in the apical third. The high number of flutes along the file’s length means greater engagement and resistance within the canals requiring more force applied to advance it. The greater the force needed, the poorer the tactile perception of what the tip of the instrument is encountering giving us a reduced ability to differentiate between a tight canal and a solid wall. If what we believe is a tight canal is really a solid wall we are on our way to apical canal distortion that can significantly reduce our success rate. All this is compounded by the need for application of the files against all the canal walls in a uniform circumferential manner to attain three-dimensional cleansing.

Anyone who has read my past posts knows I am an advocate of reamers and relieved reamers from a 15/02. I purposely emphasized what I consider the disadvantages of K-files despite their broad usage. The replacement of K-files by regular reamers through a 10/02 followed by a vertically relieved 15/02, used in a 30o oscillating handpiece, are all that are required to create a glide path space that is fully cleansed in three dimensions prior to the use of a rotary system that has been designed with a helical flat that gives high assurance that they will remain intact as the canals are opened to a maximum of 30/04. Used with the technique that I describe as “internal routing” dentin is preserved while the pulp tissue is removed not only centrally but in its buccal and lingual extensions and quite often in the very thin isthmuses that separate mb and ml canals.

Reamers resolve all the problems associated with K-files and relieved reamers be they vertically or helically relieved are a further improvement that reduces instrumentation time, increases safety to the instruments and the teeth and provides for more thorough procedures. 

Happy to answer any questions that might arise.

Regards, Barry