A More Profound Understanding of our Endodontic Options

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A More Profound Understanding of our Endodontic Options

Postby BarryMusikant on Sat May 30, 2015 9:01 am

When it comes to the proper way to endodontically negotiate and shape canals there is no end of opinion and controversy. This controversy did not exist 25 years ago when the typical armamentarium for shaping canals was limited to K-files used in conjunction with peeso and gates glidden reamers. With the advent of greater tapered rotary NiTi and the many systems it has spawned, more choices are available and what one thinks works best is generally the result of training, marketing and personal experience.

One interesting phenomenon of rotary NiTi is the need to divide the shaping sequence into two distinct phases, the creation of the glide path generally followed by the conical shaping of the canal with rotary NiTi used in a crown-down way. An 02 taper is created with the K-files generally to a 20 establishing a pathway defined enough for the greater tapered rotary systems to follow. The glide path shaped with a series of 02 tapered files is the key to shaping the entire canal length to completion. If one creates a ledge or impacts debris apically at the early stage of canal shaping, optimum shaping is compromised.

The advantage of dividing the canal preparation into two distinct phases allows us to analyze the advantages and disadvantages of both components individually. Let’s start with the K-file, probably the most universal instrument used in endodontics to date. Does the instrument function so well that it optimizes the creation of the glide path? Those who have blocked canals and lost length during their use might disagree, but few would suggest an alternative approach given the standardization of their use over several generations of dentists.

I am suggesting that K-files be abandoned and relieved reamers be adopted. Most dentists are not aware that reamers exist and they are designed significantly differently from K-files.

  


Both have working lengths of 16 mm. Within these 16 mm the K-file has 30 flutes and the relieved reamer has 16 making the orientation of the reamers’ flutes twice as vertically oriented. Both are used with a watch winding motion. However, the reamer that can be used manually is adapted for use in a 30º reciprocating handpiece oscillating at 3000-4000 cycles per minute. Does this difference in design and its ability to be used engine-driven bestow any advantages? Several advantages result from a relieved reamer design and usage including:
1. Less engagement with the canal wall a result of 16 flutes contacting the canal walls rather than 30.
2. Immediate shaving away of dentin as the vertically oriented flutes horizontally oscillate along the length of the canal walls. Compare this action to that of the K-file where predominantly horizontal flutes cut into and engage the dentin without removing it until the pull stroke is employed.
3. With less engagement to start and the immediate shaving away from dentin from the canal walls with the first clockwise stroke, the relieved reamer encounters far less resistance as it negotiates to length.
4. When one has reduced resistance, the tactile perception increases letting the dentist know if and when the tip of the instrument is hitting a solid wall or is ensconced in a tight canal. If it is the former then the instrument must be removed and prebent in an attempt to gain entry into a patent pathway. If it is the latter, the dentist knows to proceed as he/she is doing. That is a significant difference between the two and important in the initial phase of canal instrumentation.
5. A superior tactile perception due to reduced engagement along length and short arc of motion generated either manually or in the 30º reciprocating handpiece allows the instruments’ design to incorporate a cutting tip that can pierce any tissue apical to it rather than impacting it, a likely outcome when employing the non-cutting tip of a typical K-file.
6. The incorporation of fewer flutes and a relieved flat along length makes the shank of the instruments more flexible giving them an enhanced ability to negotiate curved canals.
7. A 30º reciprocating handpiece oscillating at 3000-4000 cycles per minute accomplishes the twin goals of rapid  and virtually separation-free shaping eliminating the hand fatigue associated with K-files.
8. This in turn allows the instruments to be used several times before replacement. The downside of over usage is dullness not breakage and the cost savings resulting are dramatic.
9. With breakage not an issue, these rapidly oscillating instruments can be applied against all the walls of the canal in a uniform manner retaining the original canal shape in larger form in contrast to the large conical shapes that are imposed when rotary NiTi is employed.
10. When uniform shaping is the outcome, a uniform thickness of dentin external to the canal remains maintaining more of the tooth’s original structural integrity.

The advantages of relieved reamers over K-files are so obvious that one might be tempted to simply do away with K-files and employ relieved reamers followed by the crown-down use of rotary NiTi. There is no question that this will improve the endodontic prospects for anyone making this switch. However, we should now address the features of rotary NiTi whether it is used in continuous or interrupted rotation. These features, good and bad are innate to rotary NiTi.
1. Rotating NiTi eliminates the hand fatigue encountered during the manual creation of the glide path when using K-files.
2. On the other hand, rotating instruments, continuous and interrupted produce cyclic fatigue and torsional stresses in the metal that lead to breakage.
3. Their flexibility  maintains a larger version of the original canal anatomy in the mesio-distal dimension as seen on x-ray.
4. By creating straight-line access and crown-down preparations excessive dentin is removed coronally
5. To reduce breakage, the instruments are used with a light centered pecking motion not conducive to removing tissue that will be present in any buccal and lingual extensions.
6. Subject to both torsional stress and cyclic fatigue, most of the rotary NiTi systems are recommended for single usage imposing high cost when employed.
7. A large and growing body of research that is documenting the production of dentinal micro-cracks associated with greater tapered preparations and large amplitudes of motion, the decrease in resistance to vertical fracture and the increased amount of leakage when greater tapered preparations are obturated.


There is little question that we have been trained to uncritically accept greater tapered conical shaping as an improvement over what was accomplished by K-files alone. The far wider coronal preparations were not considered until recently a detriment to the long-term survivability of the root. Today, we know that pulp tissue is most often thin in the mesio-distal plane and wide in the bucco-lingual plane accurately reflecting the external dimensions of the root. Rotary NiTi takes away excessive amounts of dentin where the root is the thinnest to begin with and inadequate tissue where the canal is the widest. The recent development of systems that require fewer instruments, but still produce overly tapered preparations does not resolve this fundamental flaws in the use of greater tapered instruments compounded by their use in rotation.

In my own mind, I believe that rotary NiTi is reaching the endpoint in development. In the latest iteration of rotating NiTi, the shank of the instrument incorporates an s-shaped curve that prevents the instrument from touching all the walls of the canal, reducing contact and the resistance encountered on the way to the apex. I thought about this design and understood the need for reduced engagement along length. They are attempting to duplicate the reduced resistance that relieved reamers have by incorporating a flat along working length. Where a relieved reamer will always make less contact compared to an unrelieved instrument, an instrument with an s-shaped curve designed into it can only have reduced contact if it’s dimensions are smaller than the canal within which it is being placed. Going into a canal of smaller dimension will cause the instrument to adapt to its shape straightening out the s-shaped configuration in the process. The tapered preparation will still be no less than the greatest tapered NiTi instrument used. Driven by continuous rotation, they are subject to separation and the induction of dentinal defects. I see little that supports such innovations as having substance.

Let’s put a bit of common sense to work and make the following observations:
1. When the arc of motion is small, neither defects in the roots or instruments are produced. The teeth and the tools remain intact. The instruments can be used several times before replacement and the canals can be better cleansed because their use can be extended far more aggressively buccally and lingually.
2. Short amplitudes of motion allow for their use in high frequency handpieces making the process of instrumentation a rapid one free of hand fatigue.
3. The result is going to be far lower costs and a major reduction in procedural stress.

Please note that the use of instruments in a 30º reciprocating handpiece is predominantly the domain of 02 tapered stainless steel relieved reamers with the last a relieved NiTi instrument generally no greater than a 30 at the tip with an 04 taper. Oscillating at 3000-4000 cycles per minute they are safely used against the buccal and lingual extensions protecting the dentin in the mesio-distal plane at the same time.

The goal is to cleanse the canal in both the mesio-distal and bucco-lingual plane sacrificing the least amount of tooth structure in the process. The use of thin highly flexible stainless steel relieved reamers confined to short arcs of motion accomplish this goal with far greater efficiency and safety than those employing greater tapered systems whether used in continuous or interrupted rotation. Greater tapered rotary instruments cannot escape their legacy of dentinal damage as documented in an overwhelming number of studies. To ignore this data is a disservice to ourselves and our patients.

Regards, Barry
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