Written by Richard Mounce, DDS Tuesday, 30 November 2004 19:00
Starting with the end in mind usually gives the clinician a much better chance of arriving at a desired end point. Having given many hands-on endodontic courses, it is my observation that many of the challenges clinicians face in creating excellence in endodontic cleansing and shaping result from misconceptions about what the desired final prepared canal shape and obturation goals are. Said differently, if clinicians in their minds’ eye understand exactly what should be created, then they will have a far better chance of achieving it with greater efficiency.
For example, if one knows where the pulp chamber lies under the clinical crown, then creating straight-line access is infinitely easier than finding a pulp exposure and trying to feel one’s way through the crown to unroof the pulp chamber. Such access without a clear mental picture of the desired outcome leads to an access that often is too small and leaves significant undercuts in the line angles leading into the canals. A byproduct of this access deficiency is that the files, which are entered into the canals, do so at an angle, which creates an unnecessary bend and predisposes them to fracture, torsional stress, and cyclic fatigue, and reduces visibility and capacity for irrigation.
Regarding instrumentation, it is vital that the following key principles are observed regardless of the rotary or hand instrumentation system used to achieve these objectives:
|Figure 1. Ideal canal shape that follows the biologic objectives of root canal treatment.|
(1) The canal must remain in its original position.
(2) The apical foramen must remain at its original location and size.
(3) The canal must resemble a tapered funnel and be kept as small as is practical.
(4) The preparation should resemble a tapering funnel (Figure 1).
Traditionally, there have been 2 methods for achieving these objectives, one of which is “step back” (SB) instrumentation. This method emphasizes getting to the apex first, creating an apical stop at some arbitrary level of the canal, and creating canal shape above this point subsequently in the procedure. The second method is known as “crown down” (CD). CD instrumentation emphasizes cleansing and shaping the coronal third first, the middle third second, and apical third last. Marshall and Pappin first advocated the CD instrumentation technique.1 While to the author’s knowledge, no body of literature proves conclusively that CD instrumentation achieves a statistically significant higher level of endodontic success, intuitively the benefits of a CD approach are myriad. CD instrumentation is consistent with the following:
(1) enhanced volumes of irrigation earlier in the cleaning and shaping process;
(2) better tactile sense over files (both rotary and hand);
(3)enhanced debris re-moval at all phases of the process;
(4) fewer instrument fractures;
(5) less iatrogenic possibility; and
(6)more rapid instrumentation.
|Figure 2. RealSeal (SybronEndo).||Figure 3. Bonded obturation at 1,000x. Resin bonded into the tubules in a monoblock of soft-core resin contiguous from the tubules to the core resin canal filling. (Image courtesy of Dr. Martin Trope.)|
Because the CD sequence allows more irrigation to reach deeper levels of the canal system rapidly, irrigation is certainly enhanced. In addition, restrictive dentin is removed high in the canal early in the sequence, creating greater tactile sense on the instrument for the clinician. In addition, because the file is bound less when compared to a SB procedure, the instrument encounters less torsional resistance against the canal wall, reducing fracture potential. Due to the greater tactile control, not only will the file have a greater tendency to resist breakage, but ledging and transportation are reduced as well. The cumulative effect of all of these factors is an enhanced speed of instrumentation, which when used in combination with adequate irrigation and removal of the smear layer, can create optimal prepared canal shapes as well as the potential to bond the obturation with Resilon (Resilon Research) and RealSeal (SybronEndo,2-4 Figures 2 and 3).
|Figures 4a and 4b. Dentin mud and its iatrogenic potential to cause transportation of the canal.|
In a SB procedure, the potential exists to have shaping files in the apical third without first determining a true working length (TWL). This is the accumulation of debris that cannot be easily removed (and can become dentin mud) and leads to ledging, possible perforation, and transportation of the apex amongst other iatrogenic outcomes including instrument fracture (Figures 4a and 4b).
|Figure 5. The LightSpeed rotary file system (LightSpeed Technologies).|
The CD technique can be accomplished with any rotary nickel titanium file system. I use the K3 system (SybronEndo) for its handling characteristics, efficient cutting ability, and fracture resistance. That said, the sequence that follows can be adapted to almost any other rotary system. In addition, it is possible, regardless of the rotary system used, to employ the LightSpeed rotary file system (LightSpeed Tech-nologies) to achieve larger apical diameters for potentially greater apical irrigation and apical cleaning (Figure 5).
Use of the following sequence makes several as-sumptions. This would be appropriate for an “average” tooth, if average exists. This tooth would be easily negotiable with a 10K file to the radiographic apex, not be severely curved in any dimension, not be severely calcified, nor have severe limitations with regard to access or visualization of canals, amongst other potential issues. Any of these clinical challenges will require a modification of techniques, and while they may incorporate much of the CD model listed below, a “nonaverage” tooth may require somewhat different steps and a different sequence of use. This said, the following suggested protocol would cover a wide variety of commonly seen clinical cases.
CROWN DOWN TECHNIQUE
The coronal third is prepared first with K3 Shapers. The Shapers can be used as the master apical file and canal-shaping files. More generally, they function as orifice openers. These Shapers come in 3 tapers—0.12, 0.10, and 0.08—and have a fixed 25 tip size. While the Shapers come in various lengths, the 21-mm Shapers have an especially useful length. In a large canal, a larger tapered 0.12 Shaper can be used first, followed by a 0.10, and then a 0.08 Shaper. In a smaller root, the 0.08 can be used first, followed by the 0.12, and then the 0.10. If the canal does not readily accept a given Shaper, choose a smaller one and see if it can easily be entered into the canal.
The Shapers can be run at a higher rpm than many other files (900 to 1,500 rpm) without torque control, if desired, once adequate experience is acquired. This sequence is CD, in that each successive Shaper creates more room for the subsequent file, which reaches deeper into the canal. For the average tooth, these 3 files in succession will often shape at least the coronal two thirds of a root. For those clinicians who are still hand filing, use of the Shapers (or a diminishing taper via such orifice openers of any brand) in this CD fashion will save a tremendous amount of time in that only the apical third remains to be shaped after their use. Shapers are a dramatic improvement over Gates Glidden drills in their ability to create canal shapes rapidly and effectively and minimize the risk of strip perforation.
Irrigation is frequent and copious. In a vital tooth, viscous EDTA gel such as RC Prep (Premier Dental Products) is placed into the pulp chamber to emulsify the pulp and remove it, with sodium hypochlorite delivery passively applied with a side-venting needle.
After glide path creation, a 0.02 K3 size 15 canal-shaping file can be inserted to true working length passively. The 0.02 15 K3 helps refine and enhance the glide path for subsequent apical canal-shaping files. As mentioned, irrigation and recapitulation are ideally completed after inserting every file above to achieve and maintain canal patency during all stages of treatment. After the 0.02 15 K3 is taken to true working length, a 0.06 K3 35 is taken to resistance, followed by a 0.06 30, 25, 20, etc. Once the clinician has created the desired apical preparation, the instrumentation sequence is completed.
|Figure 6. An instrumentation sequence that employs crown down instrumentation.||Figure 7. A menu of potential master apical file sizes that can be achieved using LightSpeed files.|
Apical preparation is also completed in a crown down sequence (Figure 6). Regard-less of whether a 0.06 taper or 0.04 taper is used, K3 canal-shaping files are used in a crown down fashion from larger to smaller tip sizes to create the final master apical diameter preparation.
|Figures 8a and 8b. Clinical cases performed using the technique described.|
Evaluating the initial radiograph and, as mentioned initially, having a clear objective that guides the hand of the clinician can pay handsome dividends for all involved as the end point of treatment. This can often be decided upon well before the treatment commences (Figures 8a and 8b).
1. Marshall FJ, Pappin J. A Crown-Down Pressureless Preparation Root Canal Enlargement Technique [manual]. Portland, Ore: Oregon Health Sciences University; 1980.
2. Shipper G, Orstavik D, Teixeira FB, et al. An evaluation of microbial leakage in roots filled with a thermoplastic synthetic polymer-based root canal filling material (Resilon). J Endod. 2004;30:342-347.
3. Teixeira FB, Teixeira EC, Thompson JY, et al. Fracture resistance of treated roots using a new filling material. Paper presented at: IADR/AADR/CADR 82nd General Session; March 10-13, 2004; Honolulu, Hawaii.
4. Shipper G, Teixeira FB, Arnold RR, et al. Periapical inflammation after coronal microbial inoculation of dog roots filled with gutta-percha or Resilon. J Endod. In press.
The author would like to thank Dr. Gary Carr, EIE 2, PERF, The Digital Office Program for Endodontists, and Dr. Arnaldo Castellucci for the images in Figures 1 and 4.
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