By Barry L. Musikant, DMD
In determining the best way to shape canals, we should consider the use of rotating NiTi as one general method of instrumentation with many variations in design and utilization to choose from. To limit the methodology of instrumentation to systems that incorporate either continuous or interrupted rotation needlessly narrows the possibilities and sheds no light on the abilities of other forms of canal instrumentation.
Let’s start off with a discussion of both continuous and interrupted rotating NiTi systems in shaping canals, but let us not limit the discussion to just rotating NiTi systems (Fig 1).
Within the category of rotating NiTi instruments the term “best” may mean different things such as:
1. Least likely to separate during usage
2. Involving the least number of instruments or are most efficient
3. Least likely to produce canal distortions
4. Most conservative in removing coronal tooth structure
5. The ability to be used several times before replacement
6. Less expensive
If we consider each of these desirable characteristics as factors that help us separate the best from the rest, a dose of clinical reality is a necessity. Separation is endemic to any system that rotates with continuous rotation being more vulnerable than interrupted. Rotation, particularly within a curved canals, produces torsional stress and cyclic fatigue within the instruments, the two factors most responsible for instrument breakage. To minimize the adverse effect of these two factors, some manufacturers have taken the following steps:
1. Incorporating crown-down canal preparations that reduce the engagement of any given instrument along length.
2. The development of heat-treated NiTi alloys that are more resistant to breakage
3. The incorporation of asymmetric reciprocation reducing the amplitude of motion compared to continuous rotation
4. Variable tapered instruments that resist apical locking and binding
5. Off-centered instrument design that reduces engagement along length
6. Single use instruments
7. Greater canal preparations using standard K-files prior to the use of rotating NiTi
8. More specific case selection, shaping highly curved canals by hand.
Despite these steps, separation is still a source of concern and leads to increased procedural stress. The search for a truly safe predicatable rotating NiTi system has yet to be achieved.
With the high cost of rotating NiTi, any system that promises the need for fewer instruments is enticing given the additional fact that it promises reduced time requirements. Fewer instruments also imply simpler usage where one size more or less fits all. That these systems are used in asymmetric reciprocation making them less vulnerable to breakage is another attractive feature. There is no question that this approach solves several of the concerns that dentists harbor when using conventional rotating NiTi sequences. What these systems don’t address in many cases is the highly oval sheath-like pulpal anatomy that exists
[youtube height="315" width="315" align="none"]https://www.youtube.com/watch?v=VRHO1IJ7Ilo[/youtube]
These systems, like all rotating NiTi are for the most part limited to preparing conical shapes that mirror the largest instruments used in the rotating NiTi sequence be that one instrument or several. One talks about “brushing” these instruments against the walls of oval canals, but that ability is severely limited given the thin isthmus-like pulpal configurations that often exist in the buccal and lingual extension compared to the dimensions of an 06 or 06 tapered instrument with a 25 tip size. An additional concern that has increasingly entered the dental literature is the impact of rotating NiTi on the integrity of the canal walls. Over 23 studies since 2008 have correlated the use of rotating NiTi to the development of dentinal micro-cracks with greater tapered NiTi instruments a further aggravating factor. Given the concavities often found on the furcal side of roots, highly conical shapes produced by rotating NiTi have the potential to both thin out the coronal dentin in this critical area producing the micro-cracks that are more likely to occur where the dentin is the thinnest (http://rootcanalanatomy.blogspot.com.br/2014/04/publication-joe.html).
The single file NiTi instruments used in asymmetric reciprocation still produce 200 full rotations per minute that can lead to excessive amounts of torsional stress and cyclic fatigue. Interestingly, studies confirm the production of greater degrees of apical dentinal micro-cracks when single file NiTi systems are used. This may be a result of the concentration of stresses produced by a single file system as opposed to such stresses being shared among several instruments.
Most dentists determine the quality of the shaping procedure based on the mesio-distal periapical x-ray. When we see a smoothly tapered conical shape from orifice to apex, we conclude that the shaping is well done. Yet, the periapical x-ray gives us little information on the shape of the canal
[youtube height="315" width="315" align="none"]https://www.youtube.com/watch?v=lvR9rHlwPzc#t=13[/youtube]
in the bucco-lingual plane nor do we have a full appreciation of just how much dentin we have removed coronally in the mesio-distal plane from the concave furcal side of roots because the more bulbous buccal and lingual portions of the root superimpose themselves over the concave anatomy that exists between these more bulbous portions. One can look at the subject of undistorted versus distorted shaping from different points of view. Is a canal distorted if the instruments don’t touch the pulpal extensions of the canal? Is it distorted if the instruments widen the canal several times the original anatomy in the mesio-distal plane? I would say yes to both questions. Distortion means very little if we limit the discussion to small variances that happen in the last mm of the tooth without appreciating both the excessive removal of dentin in the mesio-distal plane and the inadequate removal of dentin in the bucco-lingual plane.
The first three sources of concern are intimately tied to the use of rotating NiTi be it used in continuous or interrupted rotations. The next concern, being most conservative in the preservation of coronal dentin, is addressed best in my opinion by considering the use of non-relieved vertically fluted instruments through a 10 and relieved with a flat along length from 15 onwards. By confining most of the preparation to 02 tapered vertically fluted relieved instruments, we minimize imparting greater tapers in the mesio-distal plane while increasing our ability to shape the long thin pulpal extensions that often exist in the bucco-lingual plane. Rotating instruments must be used with a crown down technique because it is critical to prevent the locking and binding that can occur along length. When 02 tapered relieved vertically fluted instruments are used in 30º reciprocation binding along length is no longer an issue that can lead to instrument separation. With breakage no longer a concern, the dentist can employ these 02 tapered instruments oscillating at 3000-4000 cycles per minute aggressively against all the canal walls, but particularly the buccal and lingual extensions. From a procedural point of view, an 06 tipped 02 tapered vertically fluted instrument may at first be quite snug within a canal that is first being negotiated to the apex manually. If one then attaches the instrument already at the apex to the 30º reciprocating handpiece and works the instrument aggressively against the canal walls for a few seconds, one will create enough space along the entire length that the next instrument in the sequence used from the start in the reciprocating handpiece will negotiate to the apex with minimal resistance. Employing this instrument in the same manner as the 06 then creates enough space for the 10 to drop to the apex with minimal resistance and so on.
The result is a canal that more or less mirrors the original anatomy in larger form without superimposing a highly conical shape that removes excess tooth structure in the mesio-distal plane and inadequate removal of debris in the bucco-lingual plane. Please understand that we are making a basic point. Instruments thinner than the canal, have a great capacity to shape canals far larger than their own size allowing for far more conservative canal preparations than greater tapered NiTi instruments. Furthermore, it should be appreciated that 02 tapered vertically fluted relieved instruments are quite flexible through a 20 preparing even highly curved canals without distortion. As the less flexible instruments beyond a 20 are employed, with their use confined to short arcs of motion, they are still flexible enough to follow the pathway that was created by the previous instruments. In this way, we only use greater tapered NiTi instruments rarely exceeding an 04 taper at the end of the procedure and again confined to a 30ºarc of motion that virtually eliminates breakage.
Once it becomes evident that 30º reciprocation does not lead to instrument separation it is obvious that the instruments can be used several times before replacement. The downside of over usage is a dull instrument not a broken one. Generally, I use most of the instruments at least 6-7 times before replacement. Knowing dullness is the downside gives the dentist the freedom to determine for him/herself just how many usages one will try before replacement. It is now even more obvious that the multiple usage of such instruments are a tremendous cost savings. The recommended instruments are not only significantly less expensive to start with, but their multiple usage will reduce the cost compared to rotary NiTi by as much as 90%.
As you can see, I did not limit the discussion to which rotary NiTi system is the best. That would have confined the discussion to a far less useful function. Rather, I included the criteria one desires when using any system and given those criteria, I believe we are better served by canal preparation techniques that minimize instrument separation, preserve coronal dentin while at the same time effectively working the thin isthmuses that are most often present in the bucco-lingual plane. That these approaches can save money and time while reducing procedural stress is a combination of fortunate events that is worth advocating for.