Interview with Dr. John T. McSpadden on Instrument Design and Technique Protocol (Part 2)

By Dr. Barry H. Korzen

One of the greatest concerns that clinicians have is that they not have a procedural accident during canal instrumentation. What measures would you suggest to the clinician in order to reduce the possibility of breaking an instrument?


Other than skill, the most valuable commodity that a dentist has is time. If the dentist did not have to be concerned with file breakage, root canal instrumentation could be accomplished with no concern for time. My observation has been that the greatest, and I might say needless, time spent during canal instrumentation has been for the prevention of a file breaking or for removal of a broken file.
The most important objectives during instrumentation is to maximize efficiency and to minimize risks. By definition, achieving these objectives result in less time spent but require what I term as ”know time.” We all know the essential physics employed during instrumentation but few of us tend to keep physics “tuned in” during instrumentation. Some important concepts and their ramifications are as follows:

  1. Fatigue of a file increases with the square of the file’s diameter. When the diameter is 2 times as great as another diameter, breakage may be 4 times as likely. Consequently, coronal curvatures may be more of a threat than apical curvatures.
  2. Fatigue of the file increases with the degree of curvature and the number of rotations. Consequently, if one curvature is 2 times as great as another, only half the number of rotations may be required to fatigue the file.
  3. In a straight canal the ability of a file to withstand torsion increases with the file’s diameter. Consequently, even in the straight canal you don’t want to apply excessive apical pressure with a small tip size.
  4. The torque required to rotate the file varies directly with the surface area. The ramifications of this fact is mostly underestimated. For instance, if a canal is instrumented to a size 25/.06 and the preparation is extended apically only 2 mm with a 25/.04 file, 7 mm of the file becomes engaged.

In summation, be careful not to apply excessive apical pressure, don’t stay too long in one position, change file tapers often and don’t go too far apical with one file in order to engage a minimal length of the file.

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