Written by Richard Mounce, DDS Tuesday, 28 February 2006 19:00
Recently I did a series of donated endodontic demonstration cases. The teeth were challenging because I was in a developing country without my usual instruments or a surgical microscope. The local dentist had saved the teeth and patients for my arrival due their difficulty level and risk factors. After the treatment, I was asked how I had chosen the sequence for using the various nonmatching files (of many brands and types present that day). Because I was relying on a set of principles to accomplish the treatment, and not a dependence on a particular set of instruments or a cookbook, I was able to utilize the materials at my disposal optimally.
This paper discusses the tangible steps needed to enhance the predictability, safety, and efficacy of canal preparation irrespective of the given instrumentation system being used.
A bewildering array of endodontic shaping instruments (materials, tapers, tip sizes, etc) are available worldwide in the commercial marketplace. For the general practitioner and specialist alike, choosing and mastering one of these systems (or blending several together) can be a daunting task. What is most relevant, especially from a general dental perspective, is to provide principles that are universal and that all endodontic clinicians can utilize irrespective of the particular system employed.
As a starting point, it has value to think of shaping canals from an “out-of-the-box” viewpoint. Specifically, the canal is already present within the tooth, and in purely mechanical terms, it represents a tube or space that ideally must be enlarged, but not moved in its spatial relationship from the root that contains it. Much of the debate surrounding shaping of root canal systems revolves around the decision as to how much enlarging is required. Ideally, again in mechanical terms, the clinician would want to make a small hole (the tube) slightly larger without moving the canal in any dimension relative to where it existed before. In addition, the clinician wants to enlarge the tube (canal) to a degree that will accomplish 2 key objectives: (1) allow adequate irrigation so as to digest the contents of the pulp space, and (2) facilitate the placement of a 3-dimensional obturation material into the canal space from the canal orifice to the minor constriction of the apical foramen.
How can clinicians take a pile of endodontic instruments they are unfamiliar with and create an ideal shape? Simply stated, by observing Principle-Centered Endodontics.
The principles are simple, and if followed, they will provide the platform for excellent results. If not followed, the clinician proceeds at his or her own risk. By analogy, if one is scuba diving in caves in Florida, certain rules will keep the diver alive, but violation of these rules will put the diver’s life and those of the dive team members at serious risk. By following the rules, the odds are overwhelming that the dive will be safe; violate the rules, and morbidity ensues (and quickly).
In the endodontic arena, the results of rule violation are not so catastrophic. But an outcome that is not ideal for the tooth and the value that it represents for the patient is a compromise that risks removal and loss of the tooth, which similarly becomes a catastrophic event, albeit on a much smaller scale. In addition to the principles that will follow, it is essential that the clinician observe the following time-honored guidelines for the final prepared canal shape irrespective of the particular system to be utilized:
•Maintain the canal in its original position.
•Maintain the apical foramen in its original position.
•Maintain the apical foramen at its original size.
•Keep the apical foramen as small as is practical.
•Facilitate obturation yet keep the canal as small as is practical.
•Preparation should resemble a tapering funnel.
To facilitate these objectives, the following principles are suggested:
•Never place any instrument into a canal unless the access is ideal (straight line, all caries and overhanging tooth structure have been removed, etc).
•Utilize ideal visualization: at a bare minimum loupes, and ideally the surgical operating microscope.
•Always utilize a rubber dam.
•Never put a rotary Ni-Ti (RNT) file in a canal that has not first been explored by a hand file and/or will accept it passively without any undue force.
•Never try to instrument a canal with a rotary Ni-Ti file that has not first been instrumented with a hand file to at least a 10 or 15 K-file, ie, had a glide path created in the canal first.
•Never instrument in a dry canal; copious irrigation is essential. The author uses a combination of sodium hypochlorite 5.25%, 2% chlorhexidine (Vista Dental Products), and SmearClear (Sybron-Endo). Each of these products has a slightly different rationale for its use. Sodium hypochlorite is used for tissue digestion and disinfection, chlorhexidine is used for disinfection only, and SmearClear is used to remove the smear layer to promote open tubules for bonded obturation with a material such as RealSeal (Sybron-Endo).
•Utilize RNT in a crown-down fashion, irrespective of the brand. In the broadest context, after negotiation and glide path creation with hand files, this means using RNT from largest to smallest. Largest to smallest is a broad term, but in the most general sense, this means from larger taper to smaller taper and larger tip sizes to smaller. In addition, if one wishes to mix tapers utilizing crown-down instrumentation, it is possible to do so simply by utilizing a sequence as detailed below. (I use K3 files [SybronEndo] for their cutting ability, fracture resistance, and tactile feel, amongst other positive attributes. They will be referenced, although the clinician can utilize these principles with any brand of files.)
DIMINISHING TIP SIZES
If using diminishing tip sizes (assuming that the canal has been negotiated and a glide path created to the true working length), a common sequence for a larger canal might be as follows:
(1) K3 shapers 0.12 taper (K3 shapers are the equivalent of orifice openers and can be used as canal shaping files as well.)
(2) K3 shapers 0.10 taper
(3) K3 shapers 0.08 taper
(4) K3 0.06 taper 40 tip size
(5) K3 0.06 taper 35 tip size
(6) K3 0.06 taper 30 tip size
(7) K3 0.06 taper 25 tip size
(8) K3 0.06 taper 20 tip size (As the sequence progresses, the files will move apically toward the estimated working length. In a smaller and more curved canal, a fixed 0.04 tapered sequence of instruments with similar tip sizes can be used.)
The value in using diminishing tip sizes is that each successive smaller tip size will advance farther apically than its successor. Critics of this method feel that because the taper is not changing, the next smaller tip-sized instrument is more fully engaged than the last (because of the constant taper), and as such risks torque-related failure to an essentially over-engaged instrument. While the point is well taken, it is my empirical opinion that if the amount of force placed on the instrument is minimized and the area of engagement of the tip is minimal (1 to 2 mm or no more than 4 mm), the chances of separation are remote, assuming that all the other principles are observed.
VARIABLE TAPER SEQUENCE
Alternatively, the clinician can vary the taper as the instruments move down the canal. The shapers can be used first as recommended previously, followed by these steps:
(1) K3 0.06 taper 40 tip size
(2) K3 0.04 taper 35 tip size
(3) K3 0.06 taper 30 tip size
(4) K3 0.04 taper 30 tip size
(5) K3 0.06 taper 25 tip size
(6) K3 0.04 taper 20 tip size
Advocates of this method believe that varying the taper diminishes the chance for torque-related failure, as each successive instrument has less chance of being fully engaged. Having utilized both methods, I do not find one method entirely superior to the other. The important point is that regardless of the system utilized, the taper should diminish or the tip size should ideally diminish between instruments to create a crown-down instrumentation rather than a step-back preparation. Step-back preparation, if utilized exclusively, means that the clinician is trying to reach the apex as rapidly as possible and instrument the canal from the apex up. In other words, instrumentation proceeds from the bottom to the top, as opposed to the top to the bottom (crown-down). While a comprehensive discussion of the subject is beyond the scope of this paper, to instrument canals in a purely step-back manner means that the clinician will be working through a significant distance of canal while trying to enlarge the desired apical area first. In other words, the chance for torque-related failure is increased, as the instrument has a greater chance to bind, and additionally, diminished quantities of irrigation can reach the apical third. Conversely, crown-down instrumentation allows greater volumes of irrigation to reach the apical third more quickly and allows the clinician to work in the apical third without having the restrictive dentin present that will likely bind the instrument.
Several important points need to be mentioned. The above sequences are for illustration purposes only. Roots don’t read textbooks, and no two canals in my hands are ever instrumented exactly the same. In addition, what is relevant is that the general flow of instrumentation is observed (larger tapers and larger tip sizes to smaller). The suggested sequences are for “average” molars, if such a tooth actually exists. The K3 files come in G Packs and VTVT Packs (amongst others) that are sequenced to facilitate the above sequences. G Packs have a fixed tip size and diminishing tapers, and the VTVT Packs have variable tapers and tip sizes that diminish, aside from shaper files that act as orifice openers.
Figures 1 to 4. Cases prepared in the manner described in the paper using K3 rotary Ni-Ti files and RealSeal bonded obturation (SybronEndo).
As an aside, it bears mentioning that some canals are already open enough to be obturated with little if any enlargement. Enlarging canal systems to the extent they need to be, and no further, can only serve to diminish the number of iatrogenic events. Specifically, prior to ever placing a file in the canal, the clinician should have some general idea to what extent the canal will need to be instrumented. Canals that are wide open, easily negotiable, etc, will require less mechanical instrumentation, but they will certainly require needed irrigation delivered optimally and for the correct amount of time. Said differently, some canals will require relatively little instrumentation and others will require a significant amount of time to negotiate, create a glide path, instrument, irrigate, etc. While it is beyond the scope of this paper, there is very strong evidence in the endodontic literature that the creation of larger master apical file sizes, if created appropriately and without iatrogenic sequelae, provide cleaner canal systems into which an obturation material can be expressed. Intuitively this makes sense; a canal instrumented to a larger master apical file size will allow greater volumes of irrigant as well as removal of debris than one instrumented to a smaller diameter. To dismiss the creation of larger apical diameters as unnecessary is to argue that a smaller apical diameter is equivalent to a larger one; this is not only counter-intuitive, but with all due respect, amounts to an “it works for me” rationale. The protocols given above, viewed in light of the desire to create larger “biologic” master apical diameters, might be considered a “basic” preparation or a preparation that sets the stage to then perform the true shaping, which actually cleans the canal. This is a debate that will continue (Figures 1 to 4).
Disclosure: The author receives an honorarium from SybronEndo for certain lectures.
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