What is the Single Most Important Method to Prevent Fracture of Rotary Nickel Titanium Instruments?

by Rich Mounce, DDS

Minimizing flute engagement of a given rotary nickel titanium (RNT) against the canal wall while cutting is the single most effective technique to prevent breakage of rotary files. While this statement, taken by itself, only represents one aspect of a much larger story, it is true. The sequence of files used, the brand of file, the pressure used, rotational speed, the taper, tip size, presence of a glide path created with hand files, straight line access in addition to other factors also have an impact on potential file breakage. Ideally, the engagement of a rotary file against the walls of a canal should be 1-2mm for each insertion and certainly no more than 4-6mm. Engaging more than this amount of dentin can lead to over-engagement of the file with a resulting binding of the file in the canal and possible torsional failure of the instrument.

Torsional failure occurs when a file is subjected to a locking or binding force in rotation which is greater than the inherent strength of the instrument. Minimizing engagement diminishes the torque required to power the file. Minimal engagement ultimately requires more file insertions to remove the same amount of dentin to create an ideal shape. In clinical reality, the amount of additional time needed for these added insertions is nominal. Minimal engagement reduces the amount of dentin that is removed from the canal wall on each insertion and which must be subsequently removed from the canal.

Over-engagement of the file leads to greater production of dentin chips, greater torsion required to power the file and allow it to actively cut as well as diminishes the operator’s tactile control of the file. Placing greater flute surface area into contact also will enhance the phenomenon of cyclic fatigue, which is akin to bending a piece of metal at the same place until it breaks because it is subjected to tension on one side of the bend and compression on the other. A file over engaged especially beyond the area of greatest curvature in the canal is susceptible to both of these forces (torsional failure and cyclic fatigue) at the same time and conversely, one that is not overengaged is far less susceptible to these combined forces.

In practical terms, minimizing engagement requires a dedicated intentionality and focus in rotary file use so as to not force the file to go to a predetermined length and only allow the file to advance 1-2mm beyond the level that the operator perceives tactilely that resistance is being met. While a discussion of file selection is beyond the answer to this question, in light of having used every rotary file system in North America, I am a strong advocate of the K3 rotary nickel titanium file (SybronEndo, Orange, CA) due to its strong fracture resistance, asymmetrical design which resists “screwing in” the canal, and delicate tactile sense. I welcome your feedback and questions.

Dr. Mounce is in private endodontic practice in Portland, OR.