Common Steps to Safe and Efficient Canal Preparation

By Richard Mounce, DDS (Comfort@MounceEndo.com) – Private endodontic practitioner, Portland, Oregon, USA.
Author of a comprehensive DVD on cleansing, shaping and packing the root canal system for the general practitioner.

There are universal steps to preparation of canal spaces that ideally can and should be observed irrespective of the particular type of rotary nickel titanium files used for canal preparation. These steps can create more ideal canal shapes and do so in a manner that can diminish the incidence of iatrogenic problems. While there are an infinite variation in the methods and materials that might be employed in instru- mentation, the steps described below have empirically proved highly productive for the author.

1) Negotiation: Once straight line access has been made, it is essential to negotiate the canal to determine if the canal path is clear and can be instrumented easily. Negotiation ide- ally takes place with small (6-8) K files that are used with a gentle and passive insertion to determine the curvature, length, calcification, and diameter of the canal. Such files are precurved by hand and placed into the canal with the curva- ture inserted in the expected direction of canal curvature. If a shape is imprinted on to the file upon its removal (it comes out of the canal with a distinct curvature), this is a clear indi- cator of the curvature of the canal and gives useful informa- tion to the clinician as to the degree and direction of canal curvature and guides the future use of rotary nickel titanium instruments in the given canal.

It is noteworthy, for certain canal thirds (coronal and middle thirds), it may not be necessary to first negotiate with hand files first. While relatively infrequent, such negotiation with small hand files would usually not be necessary in a canine or other radiographically visible canal space that is wide open and straight, especially in the coronal and middle canal third.

It is recommended that the clinician always explore the apical third by hand first before ever placing rotary nickel titanium files into this most delicate canal space.

2) Achieve patency: Patency is an extension of negotiation. Patency refers to the ability to pass a small K file (6-10) through the apical foramen approximately 1-2 mm. While a comprehensive discussion of canal patency is beyond the scope of this paper, suffice it to say that in the author’s empirical opinion, it has great value to clear the entire canal path from the canal orifice to the minor constriction of the apical foramen. To not clear the canal path through the apical fora- men risks, the creation of “dentin mud” at the apex that can create an unnecessary opportunity for iatrogenic outcomes. Such dentin mud can create an obstacle which might not later be bypassed and or could cause, amongst other undesirable outcomes, instrument fracture and canal transportation.

3) Glide path creation: A glide path is a hand created “clearing” or minor enlargement of the canal up to approximately a size 15 K file. The Endodontic literature is unanimous that the creation of a glide path is essential for reduction in the frequency of separation for rotary nickel titanium files. In the context of the steps detailed here, once having negotiated the entire canal and assured that the canal is patent through the apex (usually accom- plished with a #8-10 K file) the clinician should then instrument the canal slightly larger to a #15 or #20 hand file to the true working length prior to employing rotary nickel titanium files.

4) Rotary instrumentation: Simply put, there is no sub- stitute for the efficiency and creation of excellence possible with rotary instruments relative to hand files and Gates Glidden drills. If used with a gentle touch, minimizing engagement of the rotary files against the canal walls to 1-2 mm per insertion amongst other strategies, fracture can be reduced if not eliminated entirely. The author uses the K3 System from SybronEndo (Orange, CA, USA) for its flex- ibility, cutting efficiency, fracture resistance and excellent tactile sense. The files come in various tapers and tip sizes and represent a complete line of instruments universally applicable to virtually any anatomy a clinician may be chal- lenged with. For example, aside from more commonplace anatomies, with K3, the clinician can create larger master apical diameters if desired (i.e. larger final apical preparations) than those traditionally created. K3 is available in .04 and .06 tapers to a 60-tip size and when clinically appro- priate, it is possible to create larger final master apical file sized preparations.

Alternatively, it must be recognized by the clinician that many of the other rotary systems avail- able to clinicians have a much smaller “largest” master apical diameter made possible by the limitations of the file system. More specifically, if a given file system has, as its largest tip size, a 30 or 35, the clinician is limited in what can be achieved with regard to the largest master apical diameter. Such limitations do not exist with the K3. Irrespective of the rotary system used, the author recommends that, all things being equal, case dependent, rotary nickel titanium files be used from largest tip size and largest taper to smaller to ensure a crown down instrumentation and minimize file breakage.

5) Irrigation: While a comprehensive discussion of irriga- tion is beyond the scope of this paper, the greater the vol- ume, more efficient the delivery, the more bactericidal the irrigant, the better. The author uses 5.25% sodium hypochlorite, 2% chlorhexidine (Vista Dental, Racine, WI, USA) as bactericidal irrigants. An average molar case will require approximately 75-150 cc per tooth. Irrigation is always carried out with a close ended and side venting nee- dle. It is possible to heat the irrigants and utilize ultrasonic activation in their activity if desired. These two irrigants are never mixed in the canal due to the precipitate that forms. They are cleared with SmearClear (SybronEndo) that is also used to facilitate bonded obturation through removal of the smear layer after instrumentation.

6) Recapitulation: After irrigation, placement of a K file again through the apex approximately 1 mm can assure the clinician that the canal path is still open and that no block- ages of dentin have been created which might create the aforementioned negative outcomes.

7) Repeat sequence: Irrespective of the canal third, the cli- nician can employ the series of steps mentioned above and as one progresses down the canal and the series mentioned above can be repeated as necessary. These steps provide for optimal safety in reducing instrument fracture and improve possibilities for canal cleanliness.