In a word, yes! K files are indeed essential to pre-curve prior to use. First off, canal systems are never straight, no matter how the radiograph may appear. No line in nature is straight and canals contain a dimension of curvature from buccal to lingual as well as mesial to distal. Said differently, if a canal appears radiographically straight in the mesial to distal dimension, it is a very good bet that the canal is curved buccal to lingual. Files that are precurved tend to traverse curvatures present in all dimensions more easily than straight files. Often times, especially in narrow and curved canals placement of a straight file will only allow the file to traverse the canal a limited linear distance. Once a curvature (to be described) is put into the file, it can much more easily travel down the same canal that previously could not be negotatiated.
For acute curvatures, placement of a very small J bend (3-4mm) at the end of a K file can often make the difference between negotiation and, in the worse case scenario, the creation of a ledge. The reason that a J bend negotiates the canal better than a straight file is that often canals make very acute directional changes in which forcing a straight file (if not easily bypassed) may only serve to begin the creation of a ledge or blockage. This underscores the importance of the clinician using very small K files (6-10s) copiously especially in the negotiation of the apical third of root canal systems. An ideal instrument to place such an apical J bend is the EndoBender pliers (SybronEndo, Orange, CA). While curvatures on files can be placed by hand or with cotton pliers, none of these other methods can duplicate the exacting smooth bend of the EndoBender. I have an EndoBender on each of my endodontic trays.
Bypassing ledges and negotiating blockages is also made far simpler with the bend that an EndoBender can impart into the apical 3-4mm of the file. Placing the bend onto the file followed by a series of gentle insertions into a canal that is resisting negotiation can often yield a soft spot in which the file will make headway and allow negotiation to take place. It is important in such clinical entities to use gentle pressure, copious irritation, sharp new files, and never advance to the next larger K file until the smaller ones can predictably negotiate through and beyond the given obstruction and spin freely. While not all ledges and obstructions can be bypassed, many can. Diligent work with K files that have small bends placed in their tips can often create patency if used as described. Conversely, to use larger files to try to “push through” the obstruction often only serves to worsen the ledge or pack the debris further into the root. Unless used judiciously and with special techniques, taking large K files to length, especially in curved canals is a recipe for ledging and blockage.
And finally, it has value to think of K files, especially small ones, as canal negotiators and patency files and for creation of a glide path. It is my arbitrary and empirical opinion that any canal shaping that is done above a size 15 K file can be done more safely and efficiently with a rotary nickel titanium file, especially if used in a correct order and with ideal principles to prevent separation and create efficiency. I welcome your questions and feedback.
Dr. Richard Mounce is in private endodontic practice in Portland, OR, USA. Dr. Mounce is the author of a comprehensive DVD on cleansing, shaping and packing the root canal system for the general practitioner.