A Retrospective: Comparing Rotary NiTi and Relieved Reamers

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IT HAS BEEN more than twenty years since rotary NiTi was introduced as an advanced endodontic instrument to minimize if not eliminate the use of K-files, whose limitations include their being excessively stiff, rebounding too easily to the straight position, engaging excessively along their length, and providing a poor sense of tactile perception, which in the course of canal-shaping would too often lead to transport­ed and blocked canals, or inadequate canal shaping, including insufficient flaring that would lead in turn to poor debridement and canal distortions (Figure 1).

NiTi is inherently more flexible than stainless steel, particularly when the stainless steel instruments have a file design like that pictured in Figure 2. Please note that a relieved reamer designed with a flat along its entire working length has a thinner cross section as well as fewer and more vertically oriented flutes. This design makes the relieved reamer far more flexible than a comparably sized K-file. The fewer number of flutes on a reamer engage the canal walls less, resulting in reduced resistance as the reamer negotiates apically. Corroboration of the effectiveness of a reamer design over a file design is readily observed simply by noting that the NiTi instruments are designed with a similar reamer design minus the flat.

Since the NiTi instruments were essentially an answer to the poorly functioning K-files, it is reasonable to ask whether or not NiTi instruments work better than relieved reamers in the many versatile ways that they can be used. After all, their initial reason for being is their superiority compared to the continued use of K-files. To justify their continued use, they should show the same superior benefits over relieved stainless steel reamers. In fact, if both NiTi and stainless steel reamers were used in rotation, NiTi would still exhibit superior flexibility due to its greater flexibility for comparably sized instruments. However, it is not written in stone that instruments must be used in full-arc rotations. If, alternatively, we use stainless steel relieved reamers with a short arc of motion at a high frequency of oscillation, the reamers will stay centered on the downstroke, can be used on any wall of an oval canal on the upstroke, and will produce excellent tactile perception if and when an obstruction is encountered giving the dentist more than enough notice to remove the instrument from the canal, prebend it, manually negotiate around the blockage, and then reattach it to the handpiece once at the newly gained depth.

Rotary NiTi still counts on achieving this unobstructed glidepath with the poorly designed K-files (Reference 1). Only after this pathway is established do the manufacturers suggest using rotary NiTi instruments. One of the realities of endodontics that helps the sale and enthusiasm for rotary NiTi is the fact that indeed many canals are fairly straight and wide and can be machined rather quickly with very few NiTi instruments, providing a far more efficient procedure than most dentists were able to perform when confined to K-files. Hence their relative enthusiasm for NiTi. If one has not ventured beyond rotary NiTi, one would not be aware that the same procedural brevity in these simple cases is more than achievable with relieved reamers used in the 30-degree reciprocating handpiece.

So would we say that stainless steel relieved reamers are as good as rotary NiTi? No, I would not say that. I would say that they are far superior for the following reasons:

1. As most of us know, rotary NiTi is subject to torsional stress and cyclic fatigue (Reference 2), and most of the dentists who are quite proficient at using rotary NiTi instruments safely, meaning without separations, have learned to use them skillfully. That skill includes shaping the final canal to a conservative preparation in both tip size and taper, and in highly curved canals employing a good deal of recapitulation as is warranted with a crown-down technique.

The reality of the instruments’ vulnerability in highly curved canals has led to techniques that completely avoid their use, depending instead—when confronted with highly curved canals in the apical third—upon the old K-files that they were originally developed to replace. The dentists and endodontists who used these systems are well aware of their vulnerability and have developed techniques to minimize their exposure to forces that could lead to separation.

Another way to say it is that as the knowledge of their limitations increases they are used with greater selectivity, which is a polite way of saying less. Of course, the manufacturers either knowingly or unknowingly confirm this conservative usage by saying that they are single-use instruments to be replaced after one procedure. Relieved reamers used either with a tight watch-winding stroke or confined to a tight arc of motion in the 30-degree reciprocating handpiece (Figure 3) are virtually immune to the torsional stress and cyclic fatigue that afflicts rotary NiTi. As a result, the only challenge facing the dentist when encountering a highly curved canal is the ability to negotiate its length without having to worry about separation. Relieved reamers allow shaping highly curved canals without separation anxiety because they function far better than K-files and are not subject to breakage the way rotary NiTi is.

2. Rotary NiTi’s inherent properties of flexibility cause the instruments to rebound to the straight position. The greater the curvature of the canal, the wider the tip size of the instrument; the larger the taper of the instrument, the greater the potential for the rebound effect to selectively work to the outside wall of a curved canal, introducing unintended distortions. For this reason and to avoid breakage, these instruments are once again used conservatively, a method that may be beneficial for the life of the instrument, but not necessarily compatible with the total cleansing of an oval canal where the wider diameter can often be three to four times greater than the narrow diameter. Now one might think, knowing that stainless steel is not nearly as flexible as NiTi, that the problems described for NiTi would be worse for stainless steel even when the instruments are designed as relieved reamers. While it is true that stainless steel is less flexible than NiTi, stainless steel has the advantage of recording the curvatures of a canal and in cases of severe curves it can easily be prebent to conform to the canal shape. There is little to no rebound. A precurved relieved reamer adapted to the shape of the canal and confined to a narrow arc of motion will not distort the canal to the outside wall because once adapted to the canal shape the instrument retains the bend without then selectively shaping to the outside wall.

3. The relieved stainless steel reamers, not requiring preshaping with other instruments the way rotary NiTi does, negotiate to the apex with minimal resistance due to the relieved design. They cut at least as well because they have the same vertical flute design incorporated into NiTi instruments, unlike the poorly designed K-files, and they are far more flexible than K-files, resulting in a superior tactile perception compared to both K-files and NiTi instruments, giving the dentist the crucial ability to differentiate between a solid wall and a tight canal, in turn telling him when and when not to remove the instrument from the canal, prebend the instrument and manually negotiate around the impediment (Reference 3).

4. The asymmetric design of the relieved reamers gives the dentist the ability to distinguish between a round canal and an oval canal (Figure 4), something no rotary instrument can accomplish. This is a crucial ability in determining just how aggressively a dentist should work the walls of an oval canal as well as which walls to work.

5. We should also take note of the evolution of rotary NiTi. Some of the major companies are now adopting a hybrid form of reciprocation using what is touted as a single-instrument system. In reality, the glide path must still be created to predictably use this single instrument. However, even with this being the case, most canals are not opened beyond a 25/08, a dimension the research studies considered inadequate for proper debridement and irrigation of the canals. Furthermore, this hybrid system still undergoes 200 revolutions per minute, inducing the cyclic fatigue that was one of the plagues of the original rotary systems. Yes, the manufacturers are cutting down on the number of instruments used, but they are doing so by clearly limiting the degree of instrumentation that is consistent with predictably cleansed canals. They had to reduce the number because the expense of using rotary NiTi has truly been getting out of hand, but to do so by inadequately preparing the canal spaces is poor judgment on the part of the manufacturers who are supposed to be working with the universities to create superior products.

It is interesting for us as innovators of relieved reamers confined to a tight arc of motion either manually or with the 30-degree reciprocating handpiece, to see a convergence in the principles we have been advocating for over a decade. The relieved reamers are designed to cleanse canals thoroughly whether the canals are round or oval. We have removed the fear of separation as a factor that must be considered when shaping canals. For examples of instrumentation completed using a relieved reamer in a reciprocating handpiece, see Figures 5–7. Dentists using relieved reamers as suggested need only concern themselves with negotiating through challenging anatomy and not with the impact that canal anatomy may have on the integrity of the instrument being used to perform the task.

For those who may not be familiar with our safe and innovative approach to thorough undistorted shaping, please contact our Manhattan endodontic practice for a free one-on-one workshop to experience exactly what this article describes. The office number is (212) 582-8161. You will be amazed and delighted.

References

  1. Patiño PV, Biedma BM, Liébana CR, Cantatore G, Bahillo JG. The influence of a manual glide path on the separation rate of NiTi rotary instruments. J Endod 2005: 31:114-6
  2. Li UM, Lee BS, Shih CT, Lan WH, Lin CP. Cyclic fatigue of endodontic nickel titanium rotary Instruments: static and dynamic tests. J Endod. 2002 Jun;28(6):448-51.
  3. Wan J, Rasimick BJ, Musikant BL, Deutsch AS. Cutting efficiency of 3 different instrument designs used in reciprocation. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2010 May;109(5):e82-5
  4. July - September 2011

Figure 1
FIGURE 1: A K-file. Note the high number of flutes that are more horizontal in nature, thereby contributing to poor design under function.
Figure 2
FIGURE 2: Showing the vertical flutes and flat of a relieved reamer.
Figure 3
FIGURE 3: A relieved reamer in a reciprocating handpiece.
Figure 4
FIGURE 3: A relieved reamer in a reciprocating handpiece.
Figure 5 Figure 6 Figure 7
FIGURES 5–7: Radiographs illustrating the clinically excellent results obtained by using relieved reamers in a reciprocating handpiece.