March 1, 2004
by Richard Mounce, DDS
Excellence in cleaning and shaping in endodontics stems from adherence to sound principles, including:
1) pre-operative assessment of the tooth for canal numbers, location, length and curvatures at all levels calcification, anomalous anatomy, access difficulties, etc.;
2) adequate visualization, ideally with a surgical microscope;
3) creation of straight line access to canals;
4) crown down instrumentation (instrumenting the apex of the tooth last);
5) adequate irrigation;
6) creation of a tapered funnel with narrowing apical cross sectional diameters;
7) maintenance of the apical foramen at its original size and location;
8) maintenance of apical patency throughout the process (Figs. 1-3);
9) elimination of the smear layer with EDTA as a final rinse before obturation.
Crown down instrumentation is desirable and has several important implications for the apical third. Crown down instrumentation allows a greater volume of irrigant to penetrate the apical third because restrictive dentin coronally has been removed allowing enhanced access for the irrigating needle. A lack of restrictive dentin allows greater tactile control over files (hand or rotary) as there is less chance the file will bind coronally.
Excellent instrumentation in the apical third is facilitated by first scouting with K files, next creating a hand generated “glide path”, refining this hand glide path with rotary files and performing final hand or rotary instrumentation to complete the preparation.
Scouting with small K files (6-10) is undertaken after coronal and middle third shaping with the intent to “learn the path” of the canal or “know” the canal. This discovery in the apical third would include evaluating the canal for curvatures, calcifications, size, bifurcations, trifurcations, foramen diameter, etc. If a K file comes out of the apical third in a “pig tail” (with a curvature imprinted onto the file) the shape imprinted can be thought of as an impression of the apical third and gives strong clues as to canal anatomy. Understanding and appreciating the canals curvatures and nuances can go a long way toward the operator “owning” the result and being able to produce the best possible shape. Scouting with K files involves pre curving the file with an Endo Bender plier (SybronEndo, Orange, CA) and the file can be directed into the canal at the same angle as the canal’s curvature. Initial scouting with a 6 K file is often desirable as in many roots; this is the first file that will fully negotiate the apical foramen to achieve patency. Throughout the entire process of instrumentation including scouting, it is vital that apical patency be maintained and that irrigation is adequate. In an average molar, scouting may require between 2-6 packs of K files especially if the canal has any significant apical curvature.
Once scouting is complete, a “glide path” is created initially with hand files and refined with specialized rotary files to be described. A glide path helps prevent torque failure and cyclic fatigue. Initially, when rotary files were introduced there was no recommendation for glide path creation. Subsequently, instrument fracture became a significant issue until glide path creation became known as an adjunct to safe rotary use. A glide path is now recommended by virtually all manufacturers of rotary nickel titanium files. Without a glide path, rotary files can easily screw themselves into canals by engaging more dentin than ideal and separate. A common example of this occurs when the apical 3-4 mm of the file becomes locked in a canal and the coronal shaft of the file remains spinning. In addition, if the file tip should run into a canal wall perpendicular to the path of file advancement the file will very quickly twist upon itself and fail. Scouting the canal and creating a glide path minimizes these risks. The glide path assures the operator that the tip of the file will not become locked as it moves apically and that the canal is free and clear of significant debris and blockage, could lead to iatrogenic events.
Once the canal is scouted to the apex with a 6, it is generally easy to do the same with an 8 and 10 K file. Once the 10 K file reaches the estimated pre-operative working length, the true working length (TWL) can be verified and the glide path created by hand. The hand glide path is finished when a pre curved 15 K file spins freely at TWL.
Refinement of the hand created glide path is easily accomplished with the .02 15 and .02 20 K3 rotary nickel titanium files (SybronEndo, Orange, CA) used successively. The .02 15 and .02 20 act as a “bridge” or “transition” between the hand glide path and subsequent rotary files to create efficiencies with regard to irrigation and decreased file breakage. In essence, these transition files speed the process by more uniformly removing dentin within the canal as compared to the hand glide path and subsequent rotary files have a smoother track to the apex. In essence, whatever irregularities were present with the hand glide path are generally removed to create a track, which is easily followed by the larger tapered K3 files or the rotary file of choice.
I advocate the K3 because it has an asymmetrical cross sectional design and a variable flute pitch which is significant in that the file does not tend to screw itself into the canal as is common with symmetrical U shaped files (Fig. 4). The clinical correlation to this attribute is that the K3 possesses an excellent sense of tactile control, which minimizes the chance of apical perforation. Also, the K3 is robust in that it is difficult to fracture. It should be used in a single up and down motion. “Pecking” with this file is not recommended. The .02 K3’s are a single use file. The .04 and .06 files above a 25 tip size can be used more than once.
The .02 15 and 20 K3’s are small enough with regard to taper and tip size that they flow easily down even tortuous canals after the hand glide path has been created. The K3 and especially the .02 15 and 20 have the added benefit that they will spring out of a straight alignment if they have been employed beyond there elastic memory and should be discarded. Use of the K3 .02 15 and 20’s in the manner recommended facilitate apical debris removal and further enlarge the apical third prior to final instrumentation to allow the greatest penetration of irrigants.
After the glide path is refined, the final apical sequence of instrumentation is indicated.
Specifically, in an average molar, the following sequence (using the K3 as an example) will efficiently shape the apical third:
– .06 taper K3 40 tip size to resistance
– .06 taper K3 35 tip size to resistance
– .06 taper K3 30 tip size to resistance
– .06 taper K3 25 tip size to resistance
– .06 taper K3 20 tip size to resistance
– .06 taper K3 25 tip size usually will reach TWL with the above sequence (Fig. 5).
In summary, glide path creation is essential for prevention of rotary file separation and most effective rotary use. Blending a hand created glide path with the .02 tapered 15 and 20 tip sized K3 to facilitate subsequent rotary use can be an important adjunct in this endeavor.
Dr. Mounce has no commercial interest in any of the products discussed in this article. Dr. Mounce is in private Endodontic practice in Portland, OR. Dr. Mounce would like to thank Dr. Gary Carr, The Digital Office Program for Endodontists, EIE 2, PERF and Dr. Arnaldo Castellucci for images 1-3.
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