Regardless of what system is used to shape canals, the goal is to render a canal devoid of as much tissue as possible, producing a shape that can be obturated as close to 100% as possible. Knowing that tissue in canals can be in a variety of invaginations that prevent its physical removal, we also want to create a shape that allows the efficient introduction of chemical irrigants that will digest physically untouched tissue not removed thru instrumentation and kill any remaining bacteria that are present in that tissue and the dentinal tubules.
The goal is very clear. The way to attain that goal is subject to much discussion. Much of what we want to accomplish comes under the category of what not to do. Included in this list is:
* Minimizing canal distortion along the entire length of the canal.
* Preventing instrument separation in the canal.
* Neither over-instrumenting nor under-instrumenting.
If we prevent the above three procedural error and thoroughly debride the canals, our results will produce successful results on a predictable basis.
Let us examine a few different approaches to instrumentation and obturation to see their potential for meeting our goals.
The use of traditional files used in a stepback fashion often produced canal shapes that inadequately prepared the middle third of the canal where many of them open wide bucco-lingually. The use of k-files often bind excessively in the apical third impacting debris and impeding complete negotiation to the apex.
The blockage of canals in the apical third often leads to overzealous pressure being applied to these instruments in an effort to gain apical length. Depending on the degree of curvature in these canals, excessive apical pressure and rotation can lead to apical distortion to the outer wall of the canal or worse, a complete lateral perforation of the root (Fig. 1).
Apical distortion may produce a shape where the most apical preparation is wider than the shaping coronal to it, producing a blunderbuss, which is difficult to obturate effectively (Fig. 2).
Given the increasing resistance to k-files in curved canals as the files become wider, many canals are under-instrumented simply because preparing the canals to a wider diameter is too difficult and if attempted leads to greater distortions. The consequences of minimal shaping leads to inadequate irrigation leaving more tissue in the canal that can support bacterial growth while preventing subsequent effective obturation.
Rotary NiTi instrumentation is essentially a crown-down technique which widens the more coronal aspects of the canal before going more apically with thinner and less tapered instruments. Crown-down limits the amount of engagement that any one instrument encounters as it negotiates apically. The NiTi instruments are about three times more flexible than stainless allowing a thicker instrument to negotiate apically without the same potential for distortion.
Rotary NiTi requires two conditions to be met before they can be safely used. First, the access opening must allow for straight line access to approximately half way down the root. If this condition is not met, the potential for the instrument to separate at the coronal end of the flutes increases significantly. Two, the entire distance to the apex must have an unencumbered glide path most often created with the hard to use k-files.
A sufficient glide path is one where a minimum of a No. 20 file will slide to the apex of the canal with a push motion. If this glide path is not established, the potential for separation in the apical third of the instrument is also increased (Fig. 3).
Short of achieving these two goals, instrumentation must be done by alternative means. This may include the use of peeso and gates glidden reamers, the use of hand filing in the apical third and a host of other alternatives from Hedstroms to ultrasonics.
While separated instruments can be reduced thru the use of conservative alternatives, it is their vulnerability under a host of different circumstances, that makes the dentist most cautious when they are used. The consequences of this reality include rapid replacement of these expensive instruments and a learning curve that truly expands on when not to use them in the first place.
Balanced against the anxiety produced by unpredictable separation is the fact that when they perform well, they produce shapes that are far superior to anything that traditional techniques routinely produced.
The next logical step is to produce a system that is free of separations, yet produces shapes that clean the canals at least as well as rotary NiTi. To meet that standard, a set of instruments must be used that negotiate to the apex with far less resistance than the k-files. Relieved reamers accomplish this goal creating a glide path thru a 20. There is far less resistance to apical negotiation because there are fewer flutes to engage the walls of the dentin, the flutes are more efficient at removing dentin from the walls of the canal because they are more vertically oriented and the relief along the length of the shank produces two columns of chisels that cut in both the clockwise and counterclockwise direction (Fig. 4).
These relieved hand reamers may be used either manually or in a 30 degree reciprocating handpiece taking full advantage of the efficient clockwise-counterclockwise cutting potential of the two columns of chisels plus the vertically oriented flutes.
Motion limited to a 30 degree arc (Fig. 5) virtually eliminates torsional stress and cyclic fatigue the two most important factors in the separation of rotary NiTi instruments.
Using these relieved reamers in the reciprocating handpiece, allows the rapid creation of a glidepath with an apical preparation of 0.20mm. A glidepath of this dimension is more than enough to guide a tapered peeso down the canal. The purpose of the tapered reamer is primarily to straighten the coronal curve at the expense of the outer wall, thereby leaving the tooth structure on the furcal side of the canal intact. Ideally, the peeso should get within 6mm of the apex which is roughly half way down the root. The tapered peeso is thinner at the tip than a conventional No. 2 peeso and consequently goes to length with less resistances and less ability to bottle.
Once the coronal curve is straightened all that is usually left even in a complicated molar is an apical curve. Because of the peeso, the subsequent relieved reamers used either manually or in the reciprocating handpiece need only negotiate one curve, not two making their apical progress that much more rapid and their potential for apical distortion far less.
The canals are most often instrumented to a 35 to the apex and 40 1mm short of the apex.
Now that 95% of the canal has been shaped with tough inexpensive stainless steel instruments, the remaining 5% is shaped with two NiTi relieved reamers with its use limited to the motion of the reciprocating handpiece.
As a result, they are highly unlikely to separate even if firm, but fleeting apical pressure is applied. The net result is a minimal preparation of 35 at the apex with a modified 08mm/mm taper, a shape that is most efficiently irrigated and obturated with epoxy-resin cement and either a medium or a 30/06 gutta percha point.
This entire sequence represents a complete alternative to rotary NiTi. The tapered peeso is a more efficient instrument than any rotary NiTi instrument because it can follow the natural architecture of the coronal wall of the canal space in both a mesio-distal and bucco-lingual direction. Rotary’s need to stay centered rather than working the walls selectively limits their ability to clean asymmetrically shaped canals.
The more ubiquitous work of the tapered peesos allow for better irrigation than that created by rotary NiTi. The fact that instruments are far less likely to separate even in curved canals, allows for a greater degree of instrum
entation which cleans the canals better and produces a space that is again more efficiently irrigated.
With minimal exposure to torsional stress and cyclic fatigue, all these relieved reamers may be used several times without concern for separation. In fact, they are replaced simply because they become dull after several uses and if abused by overuse they simply will cut less efficiently without an increased incidence of separation. As a result the dentist using this type of system will spend less than 1/10 the cost of rotary NiTi which are recommended by the manufacturers to be used once and discarded.
Even in unusually curved canals these tough inexpensive instruments may be pre-curved and placed into canals manually, still 3 to 4mm shorter than the apex, negotiated around the curve and then reattached to the reciprocating handpiece.
Pre-curved instruments cause no distortion when placed into the curved canals even when used in the reciprocating handpiece because a 30 degree envelope of motion is too constricted to scribe a sufficient arc beyond the confines of the canal the instrument is already in. This is certainly not the case for rotary NiTi where the last thing one can do is pre-bend an instrument.
Rotary NiTi is a tremendous improvement over traditional techniques and deserves a place as a tool that increased the effectivness of endodontics. However, it has been replaced by alternative systems, that have none of rotary NiTi’s vulnerabilities while producing results that are at least as good as rotary NiTi.
Given the reduced costs of these alternative systems, their simplified learning curve and their far superior resistance to separation, one can easily envision the day where rotary NiTi, today’s paradigm technique, will no longer be relevant.
Dr.’s Musikant and Deutsch are fellows of the American College of Dentistry (FACD). Dr. Musikant is the president, Dr. Deutsch is the executive vice-president as well as co-directors of Research at Essential Dental Systems.