We often limit our awareness of a learning curve to simply that which must be accomplished to master a technique. However, the learning curve itself gives us insight into the intelligence built into the system we are about to learn. When I observe the learning curve for rotary NiTi instruments, I quickly realize that it is one that is bracketed by limitations. One must not use them in canals that merge, bifurcate, recurve or dilacerate. In addition, they are not to be used until a sufficient glide path is created for them. That glide path can be anything from a preparation from a 10 to a 25 k-file. These limitations are imposed for one reason, to prevent breakage of the instrument.
We can initially conclude that we will have the success promised by these far more flexible instruments, but their utility will be restricted by a multitude of cases that fall into dangerous categories. Further insights follow, if we ponder why these instruments used in a rotary handpiece are so vulnerable to breakage. The literature has clearly demonstrated the relationship between increased incidences of breakage and the degree of canal curvature, the abruptness of canal curvature, the thickness and taper of the NiTi instrument, the speed of the instrument and the amount of apical force applied to the instruments when in use. There are two modes of failure. They may break due to excessive torsional stress, excessive cyclic fatigue or, some interactive combination of both. One of the challenges to the use of rotary NiTi instrumentation is not knowing how to quantify excess stress be it torsional or cyclic fatigue. As a result, the manufacturers recommend its single usage. This may be a wise decision on the part of the manufacturers since the literature demonstrates that torsional stress can reduce the resistance to cyclic fatigue and vice versa.
The torsional stress is a product of engagement along length or a locking in of the tip of the instrument apically. To reduce engagement and apical lock, a major part of the learning curve is to master crown-down shaping, a form of shaping that first uses greater tapered and thicker tip sized instruments to open the more coronal portions of the canal before going deeper with lesser tapered and thinner tipped instruments. The purpose is to limit the engagement along length, reducing the torsional stress and reducing the incidence of instrument breakage. This approach has proven successful in reducing the incidence of separation, but shaping of curved canals is often limited to an apical preparation of 20 or 25 with a taper not exceeding more than an 04. Once again, the literature has clearly shown that canal preparations to at least a 30 and preferably 35 are required to adequately irrigate canals to remove tissue chemically that has not been removed mechanically. A preparation of 20 to 25 with a taper of 04 or 06 is insufficient to not only remove the tissue mechanically, but is also insufficient to allow effective irrigation to remove what has been left chemically.
The sophisticates of rotary NiTi have written extensively on tuning and gauging the apical preparations, letting us know that a 20 or 25 may well be sufficient to clean the apical portions of the canal. The rationale is that an instrument of either dimension engaging at the apex tells us that the instrument is touching the walls of the canals and the tissue has been removed. In reality, this approach works only in canals that are round in cross section. A symmetrical instrument can make no distinction between a round and oval canal. The resistance will be defined by the smallest diameter of an oval canal eliminating any possibility that the operator will know that he is in an oval canal with a wider dimension harboring tissue. Using the resistance of the smaller diameter is a rationale for instrumentation to a 20 or 25 which fits nicely into a technique which minimizes the exposure of the instruments to greater degrees of torsional stress or cyclic fatigue that could lead to instrument breakage. This is not conjecture. The literature shows that 1mm from the apex of a mb root the average mesio-distal width is on average 0.22mm while the buccolingual diameter is 0.43mm. A 25 instrument would bind in the mesio-distal dimension telling the dentist that he has shaped sufficiently and the walls are clean. Yet the bucco-lingual dimension of the canal would be left untouched leaving tissue remn ants. Clearly, the information given by a symmetric instrument is insufficient to make accurate apical judgements on what the correct apical preparation dimensions should be.
What works against this rational discussion of the short-com ings of the rotary NiTi techniques is the fact that a 20 or 25 preparation will allow the placement of a radiopaque material to the apex. The radiograph only shows the narrower mesio-distal dimensions of the canal inadvertently confirming visually to the practioner that he has done a thorough job when, in fact, the buccolingual preparation may be insufficient, something he will not know as long as the tooth remains in the mouth.
A detailed understanding of the learning curve in the case of rotary NiTi, demonstrates dichotomies between the need for the safety of the instrument and the tooth’s biologic needs of full debridement and irrigation. Perhaps, the value of a system for cleaning and shaping canals is better measured by how thoroughly, it has the ability to fully cleanse the canal walls, leaving a space that is free of debris, wide enough to allow the irrigants that digest any remaining tissue, remove the smear layer and open the dentinal tubules to do their job. To compromise because of the vulnerability of a system is needlessly limiting our abilities to do the best job possible.
In contrast, to the dichotomies presented with rotary NiTi instrumentation, let us now look at a far less vulnerable one that allows us to shape even highly curved canals to a minimum of 35, 40 1mm back and with an overlayed taper of a 25/06. The learning curve does not require learning where not to use the instruments. They are used in all situations including dilacerated, recurved, merging and bifurcating canals. The simple reason for their universal use is that they are not subject to any significant torsional stress or cyclic fatigue that can lead to breakage. They are used with either a tight watch winding motion, in the 30 reciprocating handpiece or both. There is no great metallurgical breakthrough because none is needed. Rather, the approach we advocate is a synergistic combination of instrument design and an oscillating engine driven handpiece which duplicates the proper hand motion, at a much higher frequency.
With the elimination of breakage as a concern, the learning curve is simply about negotiating canals be they straight forward or highly tortuous. Since the predominant motion used in this technique is horizontal (watch winding and reciprocation) the flutes are oriented more vertically, reducing their number along the working length of the instrument and more at right angles to the motion employed making the cutting tip more effective. What we are describing is an instrument with a reamer rather than a file design. Files make no sense when the motion is mainly horizontal. A watch winding motion with a file leads to a lot of screwing in and screwing out with dentin only being cut on the pull stroke. The more vertically oriented flutes on a reamer make every horizontal oscillating stroke effectively cut dentin. In addition, fewer flutes mean less resistance along length and greater flexibility in the shank. Together these design characteristics endow the reamer with superior tactile perception which gives the dentist the ability to distinguish between a tight canal and a solid wall, a crucial skill needed to prevent distortions or outright perforations.
By putting a flat along the length of the reamer (Fig. 1) we have further enhanced its working properties while in no way making the instrument more vulnerable to breakage. The flat reduces the engagement of the
reamer along length and makes it more flexible. With engagment reduced and flexibility increased two of the three factors that improve tactile perception have improved. Furthermore, the instrument is now asymmetrical in cross section giving the dentist the added ability to differentiate between a round and oval canal (Figs. 2 -5). This ability tied to a set of instruments that are virtually immune to separation, gives the dentist the ability to shape canals to any dimension required whether curved or not.
The learning curve now consists of knowing how to shape canals without impacting debris which is the main cause of canal distortions in the apical third. This is routinely done, by going 0.5mm to 1.0mm beyond the constriction of the canal through a size 25. This leaves the canal patent even as the canal is further shaped to a 35 or 40. Without the buildup of debris, full patency to length is never compromised. It is the debris impacted at the point of constriction that causes instruments to deviate to the outer walls causing the transportations that we want to avoid. Because debris does not build up transportation is avoided.
The other thing to learn is how to negotiate abruptly curved canals. Because the instruments tell us when we are hitting a wall, we now consistently know when to bend the instrument to manually negotiate around the blockage. Relieved reamers make this a far less challenging task than k-files. Once around the blockage the instrument may be reattached to the reciprocating handpiece to both widen the rest of the canal and guide the instrument to full apical length. The flexibility of usage of these reamers used manually and/or in the reciprocating handpiece, used either straight or prebent, allows the dentist to deal with the challenges of any canal configuration he may encounter.
In conclusion, I hope I have emphasized that a careful understanding of the learning curve imposed by every system tells us a good deal about the system itself. One that limits its usage, is also limited in design. One that safely expands ones capabilities is by definition of superior design. The fact that it costs 90% less than the more limiting technically more difficult approaches is also something that is worthy of the consideration of dentists.
We offer a number of different teaching programs, some free others tuition based that give the dentists the full ability to judge the merits of our approach compared to rotary NiTi or any other system on the market. For information on any of these courses please call Ginger Pierro at 1-888-542-6376.
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Dr. Musikant is a member of the American Dental Association, American Association of Endodontists, Academy of General Dentistry, The Dental Society of NY, First District Dental Society, Academy of Oral Medicine, Alpha Omega Dental Fraternity, and the American Society of Dental Aesthetics. He is also a fellow of the American College of Dentistry (FACD).
Oral Health welcomes this original article.
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We often limit our awareness of a learning curve to simply that which must be accomplished to master a technique
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A 25 instrument would bind in the mesio-distal dimension telling the dentist that he has shaped sufficiently and the walls are clean
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The learning curve now consists of knowing how to shape canals without impacting debris which is the main cause of canal distortions in the apical third
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Because the instruments tell us when we are hitting a wall, we now consistently know when to bend the instrument to manually negotiate around the blockage
