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Canal anatomy as it relates to effective instrumentation and obturation

An illustration showing how a round instrument cannot adequately cleanse an irregularly shaped canal. (DTI/Photo Barry Musikant)
Dr. Barry Lee Musikant, USA

Dr. Barry Lee Musikant, USA

Wed. 20 April 2011

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Recently, there has been a discussion of the shaping techniques that are most consistent with effective cleansing of the canal spaces both in length and cross section. While cleansing length has been the main focus of earlier discussions, there is an increasing awareness of the importance of cross-sectional cleansing.

It is somewhat understandable why cleanliness and shaping have been limited to the mesio-distal length of canals given the fact that this is the only view seen when clinical radiographs are taken.1

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For years, we have considered the standard of equating a highly radiopaque appearance throughout the mesio-distal length of the canal as evidence of a well-shaped and well-obturated canal. The introduction of CBCT radiographs now give us the ability to attain an accurate and easily read picture of the canal anatomy in three dimensions, clearly demonstrating that in an abundance of situations the bucco-lingual diameter hardly reflects the same mildly tapered mesio-distal anatomy.

In short, most canals, at least along major portions of their length, are not round in cross-section. They may be highly oval with many asymmetric tissue outpouchings, fins, sheets and cul de sacs (Fig. 1).

With this more realistic understanding of canal anatomy gradually making its way into the literature, a more critical review of the shaping claims made by manufacturers of rotary NiTi is warranted.2

We single out rotary NiTi because staying centered is a requirement for their safe usage. Yet centered usage, as has been shown by an increasing number of studies, prevents adequate cleansing of the wider extensions of oval canals, those walls that constitute the boundaries of the major diameters of the canals. Because rotary NiTi is limited more or less to the preparation of round holes, the only way to achieve cleanliness when using them is to make wider round holes. If a canal starts off oval then, by shaping the minor diameter to the width of the major diameter, all the walls will be cleansed.

Unfortunately, one can easily see that this cleansing strategy increases the likelihood that the walls constituting the minor diameter can easily be weakened significantly, undermining the resistance of the root to vertical fracture. Turning oval canals into wider round canals is a strategy because it is the only way rotary NiTi instruments can address the problem of asymmetry (Fig. 2).

It is unpredictable when this strategy will produce more problems than it solves. To know when to apply this strategy would require knowledge of the cross-sectional width of the roots as well as the canals within them, requiring each patient to routinely have a CBCT scan.

At best, we would know when not to employ this strategy while still needing a technique that would clean the canals adequately without weakening the walls of the minor diameter. Logically, if we can devise a technique that allows us to clean canals without removing excessive tooth structure, that technique could then be used all the time.

Recently, a new tool has been introduced that makes an effort at cleansing the canal spaces without removing excessive tooth structure from the minor diameters.

The self-adjusting file (SAF) is a hollow tube mesh that is used in a rapid up-and-down stroke that cleanses the walls. Being made of NiTi, it is highly flexible both in length and cross-section. It adapts to the walls of the canal and, in so doing, removes a thin layer of dentin along length, be it in the major or minor diameter.

If the canal’s original diameter is oval, it remains oval at the end of the shaping procedure. This sounds like an ideal solution to the alternative approach: enlarging the minor diameter to that of the major diameter, which can excessively weaken the root.

The shortcoming of the SAF is related to its overall diameter. If at any given level the total circumferential area is less than that of the canal space, then the sides of the instrument will not touch the walls of the canal unless the instrument is aggressively pressed against those walls. This becomes increasingly challenging in the apical third where the SAF is often much thinner than the cross-sectional anatomy of the canal.

Furthermore, the hollow mesh design of the NiTi tubing is so flexible that it cuts very little dentin. The very flexibility that allows it to conform to the canal walls reduces the efficiency with which it removes dentin from the walls of the canal, necessitating approximately five minutes for the cleansing procedure. Despite the amount of time dedicated to the cleansing process, a recent article has shown that the apical third is no better than one-third cleansed.

An alternative way to clean canals that minimizes the amount of cross-sectional distortion (turning oval cross sections into round ones) is via the use of relieved reamers in a 30-degree reciprocating handpiece.3

The reamers are made of stainless steel and have enough body to be effectively placed against all the walls of the canals in a circumferential fashion. The vertically oriented blades cut the dentin as the reamers oscillate thru 30-degree arcs of motion at the rate of 3,000-4,000 cycles/minute.

Cleansing the canals with these instruments accomplishes four goals:

1) The reamers are virtually immune to breakage.

2) Consequently, they can be used six to eight times before replacement, dramatically reducing costs.

3) By using them on the pull stroke in a circumferential manner, the original shape of the canal while being enlarged retains the original canal anatomy.

4) The instruments have enough body in them to effectively remove upward of 200 microns of dentin from all the walls. This is critical because bacteria have been shown to penetrate 200 microns or more when the canals are infected. This is not the case with the SAF and is not recommended for the safe usage of rotary NiTi.

The stainless-steel relieved reamers are far more flexible than K-files and are used routinely through size 35. The tight arc of motion keeps the canals centered in the canal during the manual down stroke and allows them to be directed against the circumferential walls during the upstroke.

The up-and-down strokes provide both centered control at the tip of the instrument while the instruments are able to remove dentin from all the walls. This is the exact action that is needed to adapt to the shapes of the canals that exist, widening and cleansing them without distortion in the process.

At the end of the shaping procedure, those advocating a round hole as a final preparation may state that it is now impossible to produce a decent seal during obturation. This would be true if the fill were dependent on gutta-percha either laterally condensed or thermoplastically adapted to the canal shape.4-6

The former situation cannot be remedied by gutta-percha in the cold state. In the heated state, whatever adaptation that occurs will almost immediately be compromised as the thermoplastically adapted material cools and shrinks between 4–5 percent. The most predictable way to fill oval canals is via a combination of gutta-percha and cement.

However, the cement must flood the entire canal space prior to the placement of the pre-fitted gutta-percha point. The only way to accomplish this goal without concern for driving the cement over the apex is the application of the sealer with a bi-directional spiral (Fig. 3) specially made to flood the canal while preventing its extrusion beyond the walls of the canal.

Once the canals are flooded, the placement of the well-coated master point drives the excess cement laterally, filling the nooks and crannies along the walls of the canals before the excess escapes coronally.

In canals that are highly oval, the creation of lateral space using a spreader with no more force than the weight of one’s hand, allows for the placement of one or more additional well-coated points. This technique allows for the thorough obturation of oval spaces with a combination of cement and gutta-percha.

The purpose of the gutta-percha is to act as a carrier and driver of the cement and then a spacer that can be removed at a later date, if a post-hole is needed or the canals have to be retreated.

Integral to the success of this approach is the use of epoxy resin cement as the sealer.

The properties of epoxy resin (see examples in Figs. 4-6) include:

1) Its low viscosity allows for excellent flow and adaptation to the canal walls.

2) The ability to bond chemically and physically to both the dentinal walls and the gutta-percha.

3) Dimensional stability as it polymerizes.

4) High resistance to hydrolytic degradation.

5) Its placement at room temperature and its subsequent mild expansion as it warms to body temperature.

In summary, we see the shortcomings of rotary NiTi and the SAF. These shortcomings are well-documented in dental literature. We offer an alternative method of shaping followed by an equally necessary technique of filling asymmetrical spaces.

The former is necessary to ensure well-cleansed canals. The latter is necessary to produce a good seal without the potential of undermining the minor diameter of oval-shaped roots in the misdirected quest to make all oval canals round in cross-section.

About the author

Barry Lee Musikant, DMD, has co-authored more than 300 articles in dentistry in various international dental journals. As a partner in the largest endodontic practice in Manhattan, Musikant’s 35-plus years of practice experience have established him as one of the top authorities in endodontics.

Editorial note: This article was originally published in Endo Tribune Vol. 6 No. 4, April 2011. A complete list of references is available from the publisher.

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