Strip perforations can be easily anticipated and avoided when they are expected if simple steps are taken. They usually occur because of overzealous use of Gates Glidden drills and Peezo reamers used in the middle third of roots, especially lower molar mesial roots and the mesial buccal roots of upper molars.
Any preparation that moves the canal away from its original path through the root is the precursor to a perforation. As a practising endodontist, I have not used Gates Glidden drills or Peezo reamers in many years, even safe ended ones.
A parabolic safe ended tip on a Gates Glidden drill or Peezo might reduce the risk of perforation, but it is the lateral motion of dentin removal that creates the iatrogenic event. If inserted incorrectly, excessive dentin can easily be removed on the thinnest wall of the canal and a perforation result. In my opinion, the risk of perforation with Gates and Peezos is far greater when compared with their rotary nickel titanium (RNT) counterparts.
I use the K3 shapers (SybronEndo, Orange, CA, USA). The shapers are available in .12, .10 and .08 tapers and a fixed 25-tip size. If the canal is open and patent, the shapers can be placed into the orifice initially 2-3 mm (or less) and create a minimal diameter into which irrigants can flow and allow hand exploration of the canal space with small hand K files.
Once the canal is open to at least a #15 hand file, a glide path is created and further enlargement can be made with RNT files. Once the canal path is open and negotiable through a mid-root curvature or highly fluted root, it is essential that the shapers, or any RNT file used in such a root, be done so with intentional placement against the wall of the root which has the greatest bulk and volume of dentin.
In other words, removal of dentin is performed away from the furcation or any thin dentinal wall. When the file is pulled out of the canal it should be removed with upward intention away from the furcal wall.
In addition, if the root is thin, curved and/or highly fluted, the root will require less taper than a root that is relatively straight and has significant width of dentin circumferentially. As a result, most canines can easily accept a .12 shaper with almost no risk of perforation, but the same cannot be said of many lower molar roots. Using .12 shapers in the middle third of lower molar roots will lead to perforations, especially if the clinician is not mindful of the apical insertion of these files into the middle third.
Preventing apical perforation is easier now in that apical preparation can be done with RNT files, whereas if the apical preparation is hand generated, the perforation and ledging and blockage risk is still unacceptably high. Hand K files above a 20 are stiff by nature, and trying to take a lower molar apical preparation to a 30 or 35 by hand (with one exception) is highly likely to transport the canal and possibly lead to perforation.
Pre-operatively, the clinician needs to observe the tooth before starting and not only address where the root is likely to be perforated (or where an iatrogenic event will occur), but also determine the most likely taper and master apical diameter that will be utilised at the completion of treatment.
The clinician should figure out what the estimated working length is. This information can be assembled into a working plan pre-operatively for how the tooth will be subsequently treated. For example, if the tooth is approximately 21mm, the roots will be 12mm of the 21 (on average) and the crown will be about 9mm (on average). This means that the coronal third will be from 9-13mm, the middle third from 13-17mm and the apical third from 17-21mm.
Reading into the anatomy present from the films (highly fluted root, significant mid-root curvature, narrow or calcified canals, etc), if a perforation is expected without evasive action in the middle third, the clinician may only take the orifice to a .08 taper and should spend significant time with hand files getting the canal to a minimal #15 hand file size, first in the middle third and then enlarge the canal with smaller tapers in RNT files.
In the case of K3 files, this means using .02 tapered files first, often in a 15 and 20 tip size variety so as to give the canal some minimal diameter before progressing to larger tip sizes and taper.
Clinically, doing this will require significant irrigation and recapitulation to assure that the more apical portions of the canal are not blocked with debris. Blockage is the precursor to canal transportation and instrument fracture amongst other issues aside from perforation. As the canal shape unfolds and is created, the clinician must be mindful to keep the canal being enlarged in its original position relative to the external walls of the canal (i.e. it is enlarged circumferentially but is not moved disproportionately toward any root wall).
Inherent in these considerations is the need for the minor constriction of the apical foramen to be maintained at its original position and size and be kept open at all times. Treatment that leads to the closure of the minor constriction of the apical foramen because of debris blockage creates a strong potential for the subsequent RNT files or hand files used to deviate from the canal path and, as such, to create an apical perforation.
Finally, the use of viscous EDTA gel like File Eze (Ultradent, South Jordan, UT, USA) can hold the pulp in suspension and prevent such blockage apically that is the precursor to perforation and untoward clinical outcomes. I welcome your questions and feedback.