Barry Musikant

NE SIGN OF MATURITY is the understanding that we have to take the good and the bad, that nothing is black and white, and that personal growth develops as we understand these subtleties. A good example of balance is the incorporation of rotary NiTi into one’s practice. These instruments are significantly more flexible than stainless steel K-files and are engine-driven. Compared to the sole use of K-files, the use of rotary NiTi gives the dentist the increased benefit of shaping curved canals to about a 25/06 without distortion to the outer wall. Being engine-driven, rotary NiTi instruments also reduce the hand fatigue associated with the manual use of K-files. Against these advantages, the dentist learns to take precautions against the instruments’ increased tendency to separate. In taking these precautions, canal preparations have been changed from step-back to crown-down to reduce the amount of engagement of these instruments along their length. The manufacturers also recommend single usage to further reduce the incidence of breakage. The net result is a canal shaped with minimal distortion, in a timely fashion. These results are more quickly achieved and with less chance of distortion than results achieved with K-files alone.
    One could say that the above example represents what experience brings to the successful implementation of new technology: a balance between the advantages and disadvantages of NiTi metallurgy, taking precautionary steps that mitigate NiTi’s shortcomings while taking advantage of NiTi’s greater flexibility to shape canals in an undistorted way. Having used rotary NiTi for a substantial period of time, I would agree that if K-files were the only alternative for thorough canal shaping, we would indeed need to employ NiTi with all the noted precautions. However, this is distinctly not the case. While maturity and balance in our approach are desirable professional traits, these qualities can be employed in the use of systems that don’t require such a fine balancing act.
    For example, employing relieved reamers—either manually with a tight watch-winding stroke or in a 30-degree reciprocating handpiece—does away with the main disadvantage of rotary NiTi, the tendency to separate. Using relieved reamers doesn’t just reduce this possibility. It virtually eliminates separation. The reason is not based on new metallurgy. Rather, the technique minimizes the two factors that cause separation, torsional stress and cyclic fatigue. Minimizing their impact means that the metal is never subjected to stresses that lead to breakage. Stainless steel’s present-day metallurgical characteristics are more than adequate for the instruments’ safe repetitive use in short arcs of motion. By changing the flute design from that of a file to that of a reamer and then creating a flat along its entire working length, the instrument is designed to have the following advantages over a K-file:

1. It is far less engaging along its length, reducing the amount of resistance as the instrument negotiates to the apex.
2. Its more vertically oriented flutes cut the dentin more effectively when used with the horizontally employed watch-winding motion.
3. Its greater flexibility due to the incorporation of the flat along its length results in less work-hardening from the incorporation of fewer flutes and varying degrees of heat treatment that produce an increasing amount of dead softness or adaptability to curved canals.

These design features lead to secondary advantages, including:

1. An increased tactile perception of what the tip of the instrument is encountering, giving the dentist the ability to differentiate between a solid impediment and a tight canal. The former will have no immediate tugback while the latter has tugback from the start.
2. The ability to incorporate a cutting tip that pierces tissue rather than impacts it. Reciprocation eliminates the chances that a cutting tip will produce a perforation.
3. The ability to distinguish between round and oval canals, an advantage when one is deciding just how wide the preparation of a canal should be.
4. A flat that allows the instrument to orient itself in the most efficient pathway when negotiating tight canals to the apex.

    The major advantage of 30-degree reciprocation over rotation is the elimination of breakage. However, there are some beneficial secondary implications:

1. A paper by Venkateshbabu et al published in the Australian Endodontic Journal demonstrates that the tips of the instruments stay centered within the confines of the canal, producing less distortion than rotary NiTi systems produce.
2. On the pull stroke, the instruments can be worked aggressively against all the canal walls, removing debris in a circumferential manner without the need to enlarge the entire canal to the width of the major diameter of an oval canal.
3. As has been pointed out by the Laurent paper, the back-and-forth “churning” action of the reciprocating handpiece creates a motion that encourages the removal of the smear layer more effectively than rotation.

    Relieved reamers confined to a tight arc of motion represent an alternative to both K-files and rotary NiTi. With separation eliminated as a cause of concern, the learning curve is substantially easier. The dentists can concentrate on the challenges that the canals pose without worrying about the impact of the canal anatomy on the integrity of the instruments they are using to shape the canals. If a system is correctly designed, canal negotiation should not require special steps taken to reduce the possibility of instrument separation. The rules that must be followed should all be directed at negotiating the canals, not the prevention of broken instruments.
    From a practical point of view, the same approach that prevents breaking instruments also allows using them several times before replacement. In fact, overuse leads to dullness, not breakage, further reducing the anxiety that rotary NiTi instruments engender with their use. The natural consequence of using relieved reamers with a short arc of motion is drastically reduced costs. The metal composition and the system of use is less expensive to begin with. Combined with an average usage of 6–7 teeth, the costs compared to rotary NiTi are approximately 90 to 95 percent less expensive.
    In summary, we always want to use a sophisticated mindset when we are using any endodontic system. A system that is inherently more stable, removes the smear layer more effectively, and maintains the integrity of the original canal shape has many more advantages when weighed against its disadvantages. At a meeting some time ago, I was asked to enumerate the disadvantages of relieved reamers in the reciprocating handpiece. Without being arrogant, I said that the questioner was asking the wrong person because as the inventor of the system and the primary beneficiary of it I had done my best to design out all the obvious disadvantages. That doesn’t mean that the system cannot still be improved, but that it places the emphasis on increasing the advantages while the disadvantages have already been eliminated.
    For those who are intrigued by this article’s claims, I have been giving free two-to-three hour one-on-one workshops for more than twenty years to help expose dentists to more rational endodontic techniques and tuition-based two-day workshops that include more than ten hours of hands-on experience. If you are interested in the free workshops, please call 212-582-8161. If you are interested in the tuition-based two-day courses, please call 201-487-9090.