Rotary NiTi’s Inescapable Limitations Are Overcome Via Innovation

We are all well-versed in what rotary NiTi does. The word rotary as applied to NiTi gave the dentist the engine-driven means to debride and shape curved canals. Prior to that curved canals were essentially done by hand using K-files with short arcs of motion. The coronal portion of curved canals could be straightened by the use of engine-driven peeso and gates glidden reamers, but any curves in the middle and apical portions of the canals would place too much stress upon these inflexible tools and lead to their breakage at the point of greatest curvature, canal distortions or both.

?Rotary’s use of highly flexible NiTi reamers essentially changed the instrumentation of canals from a manual technique to an engine-driven one speeding up the instrumentation process while eliminating a good portion of the procedure that resulted in hand fatigue. The introduction of rotary instrumentation was also welcomed because the reduction in the use of K-files also reduced the incidence of debris impaction resulting in loss of length, a problem intimately associated with the use of K-files.??The concept of an engine-driven rotating instrument debriding the entire length of curved canals was a new concept.?

Prior to the introduction of rotary NiTi, Dr. James Roane demonstrated that K-files could indeed shape the full length of curved canals without distortion. However, the technique he described and demonstrated was a manual one, making it slow and hand-fatiguing. Interestingly, he described the mechanics of what he termed “the balanced force technique” as one where the K-files are utilized with a reaming action, not the pull stroke conventionally employed with K-files. The key fact is that any alternative to engine-driven rotary NiTi remained for the most part manual, an unattractive comparison for dentists that logically wanted greater speed, less hand fatigue and a reduced incidence of blocked canals. Consequently, with the encouragement of extensive marketing, rotary NiTi became a popular technique in canal shaping and debridement.?

When a new technique is broadly adopted both its advantages and disadvantages become more apparent. The touted advantages were the selling points for their broad acceptance. Their utilization made apparent those aspects of rotary NiTi that were not desirable. Without doubt, the biggest disadvantage was the tendency for these instruments to separate when negotiating curved canals, an iatrogenic event that could be detrimental to the success of treatment and be cause for a lawsuit unless the dentist told the patient of the event at the time of treatment. These are not pleasant issues that dentists would wish to deal with. Nevertheless, to date, the increased productivity and reduced hand fatigue introduced by an engine-driven system have outweighed the negatives of these potential events and rotary utilization remains quite popular.

While dentists may continue to use rotary NiTi, they are certainly receptive to all the precautions that constitute the safer usage of these instruments. In that regard, a greater emphasis on glide path creation, perhaps opening the canals to a #15 or #20 K-file rather than the #10 file as was originally recommended by the manufacturers and their advocates. The greater the canal preparation done with the manual use of K-files the less resistance the rotary instruments will encounter; a reduction in instrumentation separation at the expense of increased procedural time and hand fatigue.?

Curved canals may also be oval in cross-section. To reach what are often wider buccal and lingual extensions would require leaning vigorously in these directions. However, buccal and lingual leaning combined with a canal curving to the distal creates an off-centered s-shaped pathway that the instrument is now following, one that imposes significant stresses on the rotating instruments leading to a higher incidence of separation. Staying centered without deviation to the buccal and lingual reduces those stresses. Knowing the safety inherent in a centered approach, most dentists will follow this protocol reducing the incidence of instrument separation, but leaving untouched portions of the canals in what are most likely the buccal and lingual extensions, something well documented in the endodontic literature. The inadequate buccal and lingual canal preparations are undetectable because the periapical view on x-ray??is limited to the mesio-distal dimensions leaving any buccal and lingual inadequacies unobservable.?

Still, the greater flexibility of rotary NiTi promises rapidly produced undistorted shaping. Yes, a bit more canal preparation with K-files may be a necessary precautionary step, and staying centered is another, but the pluses of a fairly rapid engine-driven system still outweigh the increased safety produced by a slow hand fatiguing manual system. And that still holds true when a good portion of the dentists ascribe to the concept of “one and done” confining the use of these rotary instruments to a single procedure. Sure it’s a bit more expensive to continuously buy new instruments, but the increased productivity pays for itself and it reduces the incidence of separation, which is most important to avoid.??

Is the description of the options above accurately describing the total amount of choices dentists have? The short answer is no. Research subsequent to Dr. Roane’s article on the manual application of the balanced force technique reports its effective usage with the use of the M4 30o reciprocating (oscillating) handpiece operating at a frequency of about 3000-4000 cycles per minute and utilized at the very start of instrumentation eliminating practically all hand fatigue while inducing no canal distortions. Furthermore, the short arcs of motion virtually eliminate any chances of instrument separation regardless of canal curvatures in turn give the dentists the confidence to apply them vigorously against all canal walls assuring a far better chance of three-dimensional debridement.

?The comparison today is not between a slow manual hand-fatiguing technique and a rotary system that while fraught with some risk is far more rapid in its utilization. Today, we can compare the rotary system with the risks and precautions that must be taken to one where separation is no longer a threat allowing for their vigorous application against all canal walls be they oval or isthmus-like in the configuration as well as highly curved assuring improved debridement and shaping. With the absence of separation, multiple usage makes sense with great cost savings.?

What dentists might consider of secondary importance, but isn’t, is the fact that the short arcs of motion that guarantee the integrity of the instruments is also in line with Newton’s Third Law of Motion guaranteeing the integrity of the canal walls. Research documents a strong relationship between rotary instrumentation and the production of dentinal defects. When instruments have a tendency to separate in canals, it is consistent that they also produce defects in the canal walls. The opposite is also true. When instruments are not subject to separation they are unlikely to produce dentinal defects.

Further refinements of the mechanized balanced force technique include the use of reamers rather than files, a change that makes particular sense since in the original paper of Dr. Roane he stated the use of K-files with a reamer motion. Using reamers with fewer, but more vertical flutes enhances the shaving of dentin in the clockwise direction allowing for greater penetration into the canal with the 30o arc of motion. In the counterclockwise motion, the dentin that limited the continued progress of the reamer in the clockwise direction is cleaved off and the process is repeated in this manner at a frequency of about 60 cycles per second making it a highly efficient and rapid procedure from the start of the instrumentation process. By then incorporating a flat along the reamer’s working length we further reduce resistance within the canal providing for even more penetration in the clockwise direction. The relieved reamer is: significantly more flexible than a K-file adapting to any curved canals more easily. We now have a system that meets the needs of dentists as well as their insecurities.?

One last point, the same research that documents the ability of the mechanized balanced force technique via the 30o reciprocating handpiece to shape even highly curved canals without distortion has been shown to minimize the amount of debris extruded over the apex. All the concerns associated with rotary NiTi are solved with the use of the relieved reamers in the 30o handpiece.??The only thing left is the skepticism from an audience where it is well-understood that confirmation bias still dictates that what dentists will use is most dependent upon what they have been previously taught. One way to break through that psychological wall is to try the concept out, to see if it meets the claims made for it and how close it comes to overcoming the insecurities that rotary imposes on us.

Regards, Barry