Simple endodontic questions, universal answers

By Richard E. Mounce, DDSRecently I received this question from a reader. The clinician asks an excellent pair of clinically relevant questions. The letter has only been corrected for grammar. Hello Dr. Mounce,

I have used Twisted Files (TF)* for six months and K3* for seven years. Based on my experience, I have some questions:

1. I run my TF at 600 rpm. When .10 or .08 TF is used, I always feel the "screwing-in" force. The force is too strong for me to control my handpiece. The .06 or .04 TF is much smoother. Should I slow down my rpm when I run .08 or .10 TF? I know you run your TF at 900 rpm. Do you encounter that problem?

2. I find TF tends to get distorted easier than K3. Is this the property of a heat-treated NiTi material? Sometimes I run my TF for only a single case and then discard it because of the separation concern, which creates a cost issue for me. I appreciate your experience and suggestion.I do not encounter the issue in question one for the following reasons. Rotational speed is one variable in rotary nickel titanium file (RNT) use. Higher rotational speeds (assuming a constant torque) improve the cutting efficiency of virtually all RNT systems if used appropriately. Specifically, higher rotational speed allows a rapid single insertion that cuts dentin more efficiently. This insertion is a single, continuous, controlled motion that takes two to three seconds and is withdrawn at undue resistance. It is entirely unproductive to pump an RNT file vertically in the canal, especially without determining the true working length. Doing so can lead to debris blockage, extrusion of irrigants through a patent apex, locking an RNT file's tip, and breakage, among other iatrogenic outcomes. All things being equal, lower rotational speeds require the clinician to push harder to achieve the same cutting efficiency as an RNT file rotating at higher speeds. Using TF or any RNT file optimally requires the file flutes to engage the wall for the minimal amount of time at the optimal speed and torque to cut dentin but not risk breakage due to either torsion or cyclic fatigue. The clinician should remember that once the file is inserted to the desired depth in the canal, placing the file back into the canal to the same depth has no clinical benefit and only risks removing excessive tooth structure and locking the file's tip. Once the RNT file reaches the desired apical level, the next file in the sequence should be employed. The clinician reports that TF is “screwing in.” TF is designed to cut dentin and advance down the canal. The problem lies not with the RNT but with the clinician’s tactile control. It is essential to obtain a strong, yet flexible, tactile grasp of the handpiece and only allow the RNT to go where the clinician directs it and not further apically. Said differently, a file that flexibly cuts dentin and advances down the canal with ease such as TF is far preferable to one that must be used with some degree of apical pressure to advance it. RNT use, irrespective of the brand, is certainly easier under the surgical operating microscope (SOM) (Global Surgical, St. Louis, MO). I read the laser markings on my TF files during insertion under the SOM, all the while establishing a strong fulcrum to have precise control over the handpiece and file to prevent it screwing in. With regard to the second question, TF is highly ductile, much more so than an RNT manufactured by a grinding process. Deformation in ground RNT instruments is a direct indication for disposal. Should unwinding occur during the TF case, the tooth can be finished and the instrument discarded. The lighter the touch during insertion, the greater tactile control the clinician will have over TF and the less unwinding will occur. A .08 taper (using the .08/25 TF) can be prepared generally in three to four insertions in the mesial roots of most lower molars with an adequate glide path. Larger master apical diameters can also be prepared with TF (up to a #50 master apical diameter). This information directly relates to the clinician’s concern with cost. Virtually always, three to four insertions of the .08/25 TF prepares a .08 master apical taper with one file in the mesial root of a lower molar. This is certainly economical relative to using five or six files to create the same shape. I welcome your feedback.*SybronEndo (Orange, CA)