So much has been written about the benefits of rotary Ni-Ti for so long and from so many different sources that it is understandable that many dentists consider this way of doing endodontics the standard. Yet a closer examination shows that this system, in all its many forms, has deficiencies and inconsistencies that limit its use and require many precautions to be employed safely. Perhaps the greatest inconsistency is the reliance on traditional instruments to create a glide path so they can be used safely in the first place.
To this day, advocates of rotary Ni-Ti state that a glide path sized generally to at least a 20 be created with K-files. The K-file came of age in a day so long ago that endodontic instruments were made of carbon steel, a metal so brittle that using it in a rotary fashion would have led to immediate separation. With this limitation it made sense to design instruments that were used with a push-pull stroke, namely files. When stainless steel was introduced as a metal for endodontic instruments about 60 years ago, the teachers and manufacturers of the day said it was then safe to rotate these formerly brittle instruments because they would no longer break. No consideration was given to the fact that dentists were now being asked to use these push-pull designed instruments in a rotary fashion.
Figure 1. Horizontal flutes of an endodontic file.
Figure 2. Wood screw with horizontal flutes.
The push-pull action of a file requires the more horizontal flute (Figure 1) orientation to pull cleaved dentin from the canal. Once these horizontal flutes are employed with a rotary or reciprocating motion, their efficiency dramatically drops because they are removing minimal dentin while engaging the tooth far more. Greater engagement is not necessarily associated with greater efficiency. For example, the threads on a screw are quite horizontal (Figure 2). They are sharp enough to engage the dentin maximally, but at the same time remove practically none of the material into which they are being embedded. They are doing what a screw is designed to do. Yet the design of a file, as compared to a reamer, is much more screw-like. In short, what the dentists of the day were taught, and are still being taught, is counterproductive when trying to efficiently remove dentin along the walls of the canal.
A simple comparison of the features of a K-file compared to those of a K-reamer should further amplify what is stated above. A typical K-file is fabricated from a square wire, which includes 24 flutes along its 16 mm of working length. Because the original wire is square, each flute has 4 contact points, making a total of 96 contact points from the 24 flutes incorporated into the file design. By comparison, a K-reamer is fabricated from a triangular wire, which includes 16 flutes along its 16 mm of working length. Because the original wire is triangular, each flute has 3 points of contact, making a total of 48 contact points from the 16 flutes incorporated into the reamer design.
|Figure 3. Comparison of file versus reamer.|
There is a direct correlation between the amount of engagement and the resistance an instrument will encounter as it negotiates apically.1 Obviously, on the basis of the number of contact points alone, a reamer with one-half the contact points of a file will encounter one-half the resistance, making it far easier to negotiate apically. Yet as obvious as this fact is, it is really not the most important difference between a K-file and a K-reamer. With 16 flutes evenly spaced along the 16 mm of working length, the orientation of the flutes on a reamer are significantly more vertically oriented than the 24 flutes that must be accommodated in the same 16 mm of working length on a K-file (Figure 3). While horizontal flutes were justified on a file using a push-pull stroke, they make no sense when the motion is rotational or reciprocal. The flutes on a K-reamer are compatible with the rotary or reciprocating motion with which these instruments are used.
Figure 4. Flat on a reamer.
Figure 5. Motion with minimal amplitude but very high frequency.
The above discussion is not academic in nature. Anyone comparing the ease of apical negotiation through a curved, tight canal must observe far less resistance using a K-reamer versus a K-file. Given the advantages of more vertically oriented flutes, fewer of them, and half the contact points, other factors enhance the shaping capabilities of K-reamers. Having fewer flutes, they are less work-hardened, making them more flexible than files. The overall reduction in engagement and the greater ease of cleaving dentin augments the tactile apical perception these instruments give to the dentist. This increased tactual capability allows the instruments to be designed with a cutting tip, unlike the noncutting tip of a K-file. A cutting tip allows piercing and penetration of tissue and debris far more efficiently than the noncutting tip that tends to impact debris, resulting in loss of length, most particularly in tight, curved canals.
What is most fascinating in making the comparison between K-files and K-reamers is the realization that the K-reamer can be altered to make it even more efficient. We realize that reamers work better than K-files because they engage less while cleaving peripheral dentin more efficiently. By placing a flat on the entire working length of a K-reamer we create a relieved reamer that has only 32 points of contact, or one-third that found on a K-file (Figure 4). In addition, the placement of a flat along the entire working length also creates 2 columns of chisels. Where the flat meets the flute, a cutting edge is formed. When used in the recommended 30-degree reciprocating handpiece, one column of chisels cleaves in the clockwise direction while the other cleaves in the counterclockwise direction. The 30-degree reciprocating handpiece produces a motion with minimal amplitude (only one twelfth of a circle or 5 minutes on the face of a clock) but very high frequency (Figure 5). Because the instrument's degree of rotation is limited to 30 degrees, torsional stress and cyclic fatigue, the 2 factors most responsible for instrument separation in rotary Ni-Ti systems, are virtually eliminated. As a result, reciprocating speeds of up to 2,000 cycles/minute are well-tolerated with little danger of instrument separation.
One of the great attributes associated with rotary Ni-Ti is its superelasticity, which allows it to bend with far more flexibility than stainless steel. Yet a careful comparison of the properties of Ni-Ti and stainless steel point out pluses and minuses for both. Along with the superelasticity of Ni-Ti comes a high degree of shape memory, meaning that the metal wants to spring back to its original straight position. While these instruments can be prebent with difficulty, bending weakens the instruments considerably; it would never be used in a rotary system after it was bent. The advantages of Ni-Ti's superelasticity diminish as the canals become more curved and the Ni-Ti instrument employed becomes thicker because of tip size, taper, or both. It is also a fact that as the canals become more curved and the instruments used increase in tip size and taper, they increasingly become vulnerable to separation.2,3 It is ironic that these instruments designed to shape curved canals more easily without distortion must be used with ever increasing caution as they encounter the very canals they were designed to shape.
Figure 6. Placing relieved reamer in reciprocating handpiece.
On the other hand, a relieved reamer (Figure 6) can be prebent to take any curve that the canal may present, have that curve negotiated manually without distortion, and then be connected to a reciprocating handpiece for the final drive to the apex. The envelope of motion is so constricted with the use of the 30-degree reciprocating handpiece that attaching the prebent relieved reamers to the handpiece results in no significant distortion. Dr. Herb Schilder discussed the benefits of a constricted envelope of motion many years ago.4 The reciprocating handpiece is consistent with that minimal envelope of motion. The reciprocating motion is also consistent with the balanced force technique. Given the instrument's high frequency-low amplitude motion, the dentin is milled off in an even fashion rather than being cleaved in larger amounts, making the removal of dentin less harsh on both the instruments and the tooth being shaped.
What is being described is an alternative method of shaping canals with increased efficiency from the start of the glide path. We use a unique, modified tapered peeso that straightens the coronal curve of all teeth at the safe expense of the outer wall, easily gains depth to within 6 mm of the apex, and allows for the continuous use of relieved reamers in the reciprocating handpiece in a most efficient manner. On average, from the moment the canal measurement is made it takes approximately 3 to 5 minutes to shape the canal up to the point where it is ready for obturation.
|Figures 7a to 7f. Six typical cases performed with relieved reamers in a reciprocating handpiece.|
Because separation is no longer an issue, dentists will feel comfortable in tackling more and more challenging cases (Figures 7a to 7f). The learning curve is a forgiving one because unlike rotary Ni-Ti, we are never asking more of the instruments than they can deliver. This is rotary Ni-Ti's biggest drawback. Yet, in the opinion of the authors, rotary and Ni-Ti should not be used in the same sentence. The fact that some dentists can use them in what they describe as a "free and easy manner" is a testament to their cautious nature and conservative clinical judgment.
|Figures 8a to 9b. Clinical cases that exemplify the straightforward techniques used to shape and obturate.|
We use some Ni-Ti to create apical tapers without distortion, but all the instruments are used in the reciprocating handpiece, which not only prevents separation but also allows them to be used several times without any fear of separation. In fact, the only downside to using these instruments too many times is one of dullness. As a result, we have an organizer that includes a counter that records the number of times the instruments are used before they are discarded.
Two clinical cases will exemplify the straightforward techniques we use to shape and obturate (Figures 8a to 9b).
In the first case involving a lower second molar requiring endodontic treatment, traditional access was made with a No. 4 high-speed surgical round bur. Once the access was extended to the borders of the pulp chamber, warm 6% NaOCl was used to dissolve any of the remaining tissue, allowing us to have better visualization of the canals and any further tooth structure that should be removed for better access to the canals. The use of an explorer confirmed that we had at least coronal patency. We then placed an 08 K-reamer into each canal and attached an apex locator to determine the length of the canal to the apex. Gaining full depth to the apices presented no difficulties. We used the following sequence for full shaping, using hand reamers in the reciprocating handpiece:
- Nos. 10, 15, and 20 SafeSider (Essential Dental Systems) reamers to the apex
- No. 2 peeso to within 6 mm of the apex in all the canals
- No. 25, 30, and 35 SafeSiders to the apex
- No. 40 1-mm short of the apex
- No. 2 Gates Glidden to within 3 mm of the apex
- No. 30/04 and 25/08 Ni-Ti SafeSiders to the apex.
We constantly irrigated the canals with 6% NaOCl and lubricated each instrument with RC-Prep (Premier Dental). After the final shaping was done, we irrigated the canal with 17% EDTA and a final rinse with 2% chlorhexidine, which we left in the canals while we fitted the points.
After the points (mediums from DENTSPLY Maillefer) were fitted with significant tugback, we dried the canals and placed EZ-Fill epoxy resin cement (Essential Dental Systems) into the canals using the bidirectional spiral, which completely floods the canals while preventing excess from spilling over the apices. We then liberally coated the points with the same epoxy resin, seared off the excess gutta-percha, removed the excess cement, cleansed the pulp chamber with a cotton ball moistened in alcohol, and then etched and bonded a composite filling within the access preparation.
We used virtually the same technique for the second case with equally good results. Of course, the Safe-Siders can be used with re-treatment cases as well. In fact, a significant number of the endodontic procedures we do today are retreatments. As with any technique, greater experience leads to greater facility with the technique. The one feature that stands out with the SafeSiders is the undeniable fact that separation is not part of the learning experience, and as a result dentists will be far more willing to take on increasingly challenging cases.
This article makes the case that rotary Ni-Ti should not be compared with traditional techniques. There is no question that rotary Ni-Ti outperforms these antiquated systems. Rather, the comparison should be between this new alternative way to accomplishing endodontic treatment and rotary Ni-Ti. As so many former rotary Ni-Ti users who now use this alternative method of instrumentation have found out, all the advantages of rotary Ni-Ti are retained, while the disadvantages of both traditional techniques and rotary Ni-Ti are virtually eliminated. When one considers the fact that the cost of doing endodontics drops by about 90% on a peruse basis when this alternative system is employed, without sacrificing quality, the motivation to grow acquainted with this system becomes obvious.F
1. Musikant BL, Cohen BI, Deutsch AS. Comparison instrumentation time of conventional reamers and files versus a new, noninterrupted, flat-sided design. J Endod. 2004;30:107-109.
2. Pruett JP, Clement DJ, Carnes DL Jr. Cyclic fatigue testing of nickel-titanium endodontic instruments. J Endod. 1997;23:77-85.
3. Li UM, Lee BS, Shih CT, et al. Cyclic fatigue of endodontic nickel titanium rotary instruments: static and dynamic tests. J Endod. 2002;28:448-451.
4. Schilder H. Cleaning and shaping the root canal. Dent Clin North Am. 1974;18:269-296.
Disclosure: The authors hold 18 patents for co-inventing endodontic products for Essential Dental Sys-tems, a company they co-founded.