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By Barry L. Musikant, DMD

The most productive way to think about endodontic cleaning and shaping is to think about what we have to shape rather than how we shape it. If we understand exactly what the challenges are, then deciding what is the best way to shape a canal should be more an exercise in common sense than rote learning of a technique devoid of the appreciation of the anatomical challenges.

So, lets start with canal anatomy the way it is rather than the simplistic notion that they are routinely conically shaped from stem to stern. As you can see in this axial view of a mandibular central incisor, [youtube height="315" width="560" align="none"]http://www.youtube.com/watch?v=2ux90dseCdY&index=75&list=UUGSe5P1-bcZcTGGxHSiBHBw[/youtube]

the pulp takes on a configuration that is far removed from a conical shape. Please note the concavities on the mesial and distal walls

[youtube height="315" width="560" align="none"]http://www.youtube.com/watch?v=lvR9rHlwPzc&feature=youtu.be&t=26s[/youtube]
. A centralized conical canal preparation would stay centered producing the greatest amount of dentin removal where the walls of the canal are the thinnest to begin with (fig 1) . Like the mandibular central incisor, many roots are highly oval or sheath-like in the bucco-lingual plane with the minor mesio-distal plane being quite narrow.

Given this reality, crown-down shaping using greater tapered rotary instrumentation can often be counter-productive. Why would we want an 06 or 08 greater tapered shape imposed on a canal that may be no wider than 50-60 microns to start with? An instrument with an initial tip size of 25 equals 250 microns and would have difficulty gaining access to the widest part of the canal and an even more difficult time gaining lateral access to the canal’s narrower extensions. Having a greater taper, the instrument rapidly becomes wider removing excessively more tooth structure in the more vulnerable mesio-distal plane. Given the narrow buccal and lingual extensions mechanically the instrument will be confined to the widest part of the canal, creating the inappropriate conical shape while having little ability if any to touch the buccal and lingual extensions.

Some may say that greater tapered shaping does not occur until the glide path to a 20 has been established. Yet, too often, the negotiation of K-files to the apex encounter significant resistance and the problem is somewhat alleviated by creating a more or less straight pathway coronally with the use of a one or more greater tapered rotating NiTi instruments. From a point of view of mechanical advantage, the straight-line access created with greater tapered instruments reduces the resistance of the manual K-files, but the tooth pays the price in terms of a more undermined coronal preparation. The dentist is in a bit of a dilemma. He/she can prepare the glidepath without flaring the coronal access making instrumentation with wider and stiffer K-files all the more difficult. This is not likely to occur, given the need for productivity and the downsides of over preparation are generally not seen for at least several years following the procedure if they are seen at all.

Most of the literature and technique courses today are willing to sacrifice coronal dentin for the sake of expediting the endodontic procedures, reduce the incidence of instrument separation and creating a greater tapered space that can be obturated more predictably. Yet, despite the dearth of literature on alternative methods, safer more effective choices exist.

Rather than initially using K-files with a predominatnly horizontal flute orientation, we can use a series of vertically fluted instruments that incorporate a flat along their working length from size 15 onwards. Vertically fluted instruments have half the number of threads along an equal working length of 16 mm compared to K-files and they are predominantly vertically oriented (fig 2) . Both are used with a watch winding motion with the occasional pull stroke incorporated to bring the instrument out of the canal to wipe away the debris accumulated within the troughs of the flutes. Given the vertical orientation of the flutes along the length of the instrument, the first clockwise motion immediately engages and shaves the dentin away from the canal walls. The K-file, used with the same clockwise stroke, will cut into the dentin engaging it without shaving the engaged portion away until the pull stroke is employed. That is a substantial difference between the K-file and the vertically fluted instrument. Not only does the vertically fluted instrument incur less engagement due to the smaller number of flutes, but that engagement is further reduced with the first clockwise stroke shaving the engaged dentin away immediately. Less engagement translates into less resistance allowing the vertically fluted instrument to negotiate to the apex with far faster and with less effort than a typical K-file.

Starting with a 15, the vertically fluted instruments incorporate a flat along their working length further reducing engagement along length and providing two vertical columns of chisels (where the flat meets the flute) that also shave dentin away with the watch winding motion. Several other advantages accrue to the use of a relieved vertically fluted instrument. The instrument is significantly more flexible than its K-file equivalent. Combining greater flexibility, reduced engagement along length and a superior ability to shave dentin away with the first clockwise stroke, the relieved vertically fluted instruments produce a superior tactile perception compared to the K-file giving the dentist the ability to distinguish between the tip of the instrument hitting a solid wall and being in a tight canal with the former situation producing no tugback and the latter producing immediate tugback. The superior tactile perception allows the incorporation of a cutting tip that pierces tissue rather than impacting it apically the way the non-cutting tip on K-file would do.

Most importantly, the watch winding motion that is entirely appropriate for a relieved vertically fluted instrument can be automated to be run on a 30º reciprocating handpiece that duplicates the hand motion but at a frequency of 3000-4000 cycles per minute. One might think that such frequencies would lead to separated instruments, but while the frequency is quite high, the amplitude of motion is confined to 30º or 1/12 of a circle or 5 minutes on the face of a clock. This amplitude is so small that it virtually eliminates the potential for instrument separation. That in turn leads to greater confidence on the part of the dentist to use these instruments against the narrow walls of highly oval canals without reservations. The high frequency has the added advantage of activating the irrigants throughout the shaping procedure opening the dentinal tubules through the length of the canal and providing a superior seal of obtruation materials.

Please realize that the bulk of the shaping is being done with 02 tapered instruments that preserve coronal dentin and are used with short arcs of motion that not only virtually eliminate fracture of the instrument, but are not associated with the production of dentinal micro-cracks1-9 that arise via the use of rotating NiTi. Once we appreciate the inherent safety of a system employing vertically fluted instruments with a short arc of motion, we can come to the clear understanding that unlike greater tapered NiTi instruments that impose conical shapes independent of the original anatomy of the canal, the 02 tapered stainless steel relieved instruments can address all the walls of a canal with equal vigor removing a more or less uniform layer of dentin resulting in a final shape that mirrors the original shape except for its uniformly larger dimensions. The end result is preservation of dentin in the all important coronal aspects of the canal preparation, minimal chance of dentinal micro-cracks developing and the virtual elimination of instrument separation allowing the dentist to use the instruments several times before replacement not only producing substantial savings, but dramatically reducing procedural stress.

References:
1. Bier C.A.S., Shemesh H., Tanomaru-Filbo M., Wesselink P.R., Wu M.K.
The Ability of Different Nickel-Titanium Rotary Instruments To Induce Dentinal Damage During Canal Preparation
Journal of Endodontics (JOE). 2009; 35(2): 236-238

2. Adorno C.G., Yoshioka T., Suda H.
The Effect of Root Preparation Technique and Instrumentation Length on the Development of Apical Root Cracks
Journal of Endodontics (JOE). 2009; 35(3); 389-392

3. Shemesh H., Bier C.A.S., Wu M.K., Tanomaru-Filho M., Wesselink P.R.
The effects of canal preparation and filling on the incidence of dentinal effects
International Endodontic Journal (IEJ). 2009; 42; 208-213

4. Kim H.C., Lee M.H., Yum J., Versluis A., Lee C.J., Kim B.M.
Potential Relationship between Design of Nickel-Titanium Rotary Instruments and Vertical Root Fracture
Journal of Endodontics (JOE). 2010; 36(7); 1195-1199

5. Burklein S., Tsotsis P., Schafer E.
Incidence of Dentinal Defects after Root Canal Preparation: Reciprocating versus Rotary Instrumentation
Journal of Endodontics (JOE). 2013; 39(4); 501-504

6. Liu R., Kaiwar A., Shemesh H., Wesselink P.R., Hou B., Wu M.K.
Incidence of Apical Root Cracks and Apical Dentinal Detachments after Canal Preparation with Hand and Rotary Files at Different Instrumentation Lengths
Journal of Endodontics (JOE). 2013; 39(1); 129-132

7. M.S., Moraes R.A., Rosa R.A., Moreira C.H.C., So M.V.R., Bier C.A.S.
Vertical Root Fractures and Dentin Defects: Effects of Root Canal Preparation, Filling, and Mechanical Cycling
Journal of Endodontics (JOE). 2012; 38(8); 1135-1139

8. Yoldas O., Yilmaz S., Atakan G., Kuden C., Kasan Z.
Dentinal Microcrack Formation during Root Canal Preparations by Different NiTi Rotary Instruments and the Self-Adjusting File
Journal of Endodontics (JOE). 2012; 38(2); 232-235

9. Adorno C.G., Yoshioka T., Suda H.
The effect of working length and root canal preparation technique on crack development in the apical root canal wall
International Endodontic Journal (IEJ). 2010; 43; 321-327

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