Instrumentation of root canal systems ideally involves a repeating series of steps regardless of whether one is in the coronal, middle, or apical third. These steps include a cycle of negotiation (or at least assurance that the canal is negotiable given the particular anatomy), creation of patency and a glide path with hand files, rotary nickel titanium file use, irrigation, and recapitulation. The sequence repeats itself, regardless of the given location in the canal, as one advances in the order of steps just mentioned.
As a first step, the negotiation is most often essential, particularly in the apical third. Small K files (6-10s), used for negotiation, should be precurved and placed into the given canal third to explore the canal for curvature, calcification, diameter, ease of insertion, length, patency, etc. Negotiation might be thought of as the process of exploration and discovery to determine the individual canal anatomy noted previously. Despite this, hand file negotiation in a wide-open canal might not be needed as an initial first step, especially the coronal third of a large root. But roots vary in negotiability. Some roots, including those in the coronal third, might be very challenging - even with the benefit of a surgical operating microscope and ultrasonic tips for visualization and location.
Hand files used for negotiation are powered by gentle finger pressure and never forced. The file is inserted passively. If the canal will accept the file easily, insertion is continued. If the canal resists insertion, a smaller file is used to gently attempt advancement. In all canals, especially in narrow, constricted ones, the presence of a solution such as sodium hypochlorite or chlorhexidine is essential. It is vital to replenish irrigant often and copiously.This removes fragments of pulp and dentin debris that are generated from the initial exploration of the canal at any level. The worst-case scenario is to have the initial negotiation advance debris apically, and create “dentin mud” that blocks the end of the root from further instrumentation. Such blockage, if not bypassed and removed, can result in a significant amount of uncleaned and unfilled space within the root canal system.
To neglect the exploration and negotiation of a canal, as required, is to risk misadventure. One of the risk factors for iatrogenic events of all types is the creation of ledges and canal transportations that can result from the advancement of hand and rotary files apically too fast and with too much force. Said differently, to not treat these narrow and delicate root canal systems with the respect they deserve, through the needed cycle of negotiation and glide path creation, can be the precursor to a larger and more challenging iatrogenic event that can compromise the possibilities for success.
Once patency has been achieved following negotiation of the entire root (a 10 or possibly 15 K file, for example, will traverse the entire length of the root to the TWL and spin freely), there is great value in using a smaller, tapered rotary file to refine, enhance, and “smooth out” the canal space. This facilitates the insertion of subsequent rotary files. Such a file would be the .02 tapered K3 rotary file by SybronEndo (Orange, Calif.). Placed passively after hand file negotiation and glide path creation to the TWL, the .02 tapered K3, especially in the 15 and 20 tip sizes, can save significant time and provide greater predictability later in the rotary phase of the instrumentation process. These .02 tapered instruments are robust, flexible, and resist fracture. They can be used confidently. If used appropriately, they will make subsequent rotaries slide down the canal with much more fluidity.
In summary, canal exploration of all thirds - especially the apical third - is done gently and passively with precurved hand K files. After the estimated and/or true working length has been determined, the canal has been negotiated and a glide path has been put in place, .02 tapered rotary files - such as the K3 - can be used to enhance and refine the canal. This makes subsequent rotary instrumentation more predictable. I suggest practicing with extracted teeth to achieve proficiency with such rotary files. I think they can create higher efficiency, provide greater safety, and create the best possible clinical results.
As always, I welcome your questions and feedback on this topic.
Dr. Richard Mounce is in private endodontic practice in Portland, Ore. Dr. Mounce is the author of a comprehensive DVD on cleansing, shaping, and packing the root canal system for the general practitioner. The material also is available as audio CDs and a Web cast pay-per-view. He lectures worldwide and is a widely published author. For more information, contact Dr. Mounce via e-mail at comfort@MounceEndo.com. Visit his Web site at www.MounceEndo.com.