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Endodontics for the Recent Graduate: Instrumentation by Kenneth Koch, DMD, and Ali Nasseh, DDS, MMSc

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Endodontics for the Recent Graduate Instrumentation

by Kenneth Koch, DMD, and Ali Nasseh, DDS, MMSc

Endodontics has been, and continues to be, all about the preservation of the natural dentition. In the clinical practice of endodontics, once proper straight line access has been created, the next task is the cleaning and shaping of the root canal system. However, the real challenge is to accomplish this in a consistent and predictable manner.

The first question we must ask ourselves is, "Why do we shape a root canal?" The answer is clear. We shape a root canal 1) to allow our irrigation agent (sodium hypochlorite) to work in the most effective manner possible and 2) to expedite obturation.

The next question is the issue of taper and a comparison must be made between a variable taper preparation and a constant taper preparation. Presently, in endodontics, we have two methods of root canal instrumentation: variable taper or constant taper. Both of these methods can do a good job of cleaning the root canal system. However, there are some profound differences. A variable taper method uses rotary files that have the same common apical tip size but vary in their tapers. For example, one may use a variable taper technique that employs a .25 apical tip size and utilizes variable tapers in the range of .12, .10, .08, .06, .04 and .02. While a variable taper sequence tends to promote disengagement of the instruments, what is sacrificed is the predictability of shape commonly associated with a constant taper preparation. Due to their lack of a constant, reproducible shape, most variable taper methods are associated with thermoplastic obturation. Furthermore, with large variable taper sizes such as .12, .10 and .08 there is the serious concern of coring out the orifice and removing an excessive amount of tooth structure in the critical coronal third of the canal. Consequently, while a variable taper technique may facilitate the insertion of carrierbased obturation methods, or thermoplastic techniques, extreme caution must be used not to remove excessive tooth structure and thereby compromise the long-term retention of the tooth. This is a very key point.

A constant taper preparation, on the other hand, is generally more conservative in the coronal third of the radicular dentin. (Fig. 1) But it must be noted that a constant taper preparation can also remove excessive coronal tooth structure if too large a size and taper is utilized. A constant taper preparation routinely employs a series of files with a constant taper such as a .04 or .06 and with varying apical tip sizes. Because of the constant taper, the net result of using a series of constant taper files is a more conservative, reproducible shape. A key factor in the use of constant taper preparations is that the consistency of shape facilitates the primary cone fit, which expedites the overall obturation process. This is one of the principle reasons why constant taper preparations are the choice of most endodontic specialists. But a question still remains: Is there such a thing as an ideal taper?

This question can be addressed in the following manner. As endodontic shaping has evolved, we have come to the conclusion that the preferred taper (in most cases) is a constant .04 taper preparation, rather than a .06 taper. There are multiple reasons for this (foremost being the conservation of radicular dentin) but first we must acknowledge a shift in rotary instrument design that has occurred, from landed to non-landed instruments. The desire for greater efficiency and predictability of shape associated with non-landed instruments (such as EndoSequence) has generated this change. But it was not always this way.

When rotary files were first introduced, the original constant taper files had lands and a .04 taper. This quickly evolved into an additional series of .06 tapers. As the acceptance of rotary instrumentation by endodontists grew, we saw an increased use of .06 tapers. This seemingly made sense from two perspectives. The first was that the wider .06 taper preparations were considered easier to obturate if performing warm vertical condensation and secondly, the original .06 tapered landed files were considered stronger and less susceptible to separation than .04 taper landed instruments. However, this concept of a .06 taper file being less susceptible to separation does not apply to non-landed rotary files. In fact, with non-landed instruments, cyclic fatigue is the key issue with the result being, .04 taper non-landed instruments are more resistant to separation than .06 taper non landed files. So, in addition to conserving tooth structure, a move to a constant .04 taper preparation also makes sense (as a safety factor) if using non-landed instruments.

Furthermore, recent studies have again demonstrated that .04 taper shapes are more than adequate to allow irrigation agents to generate a thorough cleansing of the root canal. So, when it is evaluated from a true endo-restorative perspective, the preferred taper should be a constant .04 taper.

Once the effects of taper are understood, the relationship between endodontics and restorative dentistry becomes obvious. In fact, since we are talking about the same space, it is a continuum; an Endo-Restorative Continuum. The real challenge is to find a technique that melds the two disciplines together. We believe that the EndoSequence System (Brasseler USA, Savannah, Georgia) meets this challenge by creating precision shaping that allows for predictable obturation through a matching system of laser verified gutta percha cones and a revolutionary bioceramic sealer. The significance of this is that as a result of the matching laser verified cones, endodontic synchronicity is now established between a machined preparation and the master cone. The key to achieving this endodontic synchronicity, which ultimately results in the conservation of radicular dentin, is the constant taper preparation created by the EndoSequence System (Fig. 2).

The EndoSequence rotary file by Real World Endo and Brasseler USA has a creative design and characteristics that enables clinicians to truly machine (or mill) preparations.

In fact, this technique has also been designed to generate predictability in obturation through a matching system of laser verified bioceramic coated cones. The significance of this predictability is that synchronicity is now created between a machined preparation and the master cone.

As previously mentioned, the key to achieving endodontic synchronicity, (the matching of the master cone to the preparation), is the file. There are numerous features that readily distinguish the EndoSequence file from previous generations of rotary files. In fact, this file has been considered the first fourth generation rotary file.

The first circumferential nickel titanium rotary file (first generation) introduced into North America was the Lightspeed System. This file, although similar to the stainless steel Canal Master, changed the way clinicians thought about performing endodontic procedures. The Lightspeed System created the option of using nickel titanium rotary instrumentation in addition to (or in place of ) stainless steel hand files. This was a seminal event in the history of endodontic preparations.

The next generation of rotary files (second generation) were files that had radial lands. Examples of these files are the Profile, GT, Quantec and K3. While design differences do exist between these files (such as pitch and helical angles), the overwhelming similarity is the existence of radial lands.

Although the radial lands were helpful at centering, they reduced flexibility and significantly increased the torque requirements of the respective files. Radial lands create an increase in working torque requirements. This is a result of their decreased cutting efficiency, along with an increase in lateral resistance (drag). However, in our opinion, the Profile, in particular, changed the course of endodontic shaping.

The third generation of rotary files consists of the ProTaper and the RaCe. Both of these files are considered third generation due to their lack of radial lands and their individual attempts at addressing increased cutting efficiency. To their credit, these files were products of "thinking out of the box."

The ProTaper, with its modified triangular design, addressed the need for increased cutting efficiency through the use of a progressive taper. The ProTaper has, in fact, achieved improved cutting efficiency. Unfortunately, the lack of a centering device remains a limitation.

The RaCe file on the other hand addressed the need to reduce overall working torque by incorporating an alternating spiral design. However, the RaCe file, although effective at reducing torque, also does not have a definitive centering device.

The introduction of fourth generation circumferential rotary files begins with EndoSequence. When we examine the EndoSequence file, it becomes apparent that this file is a product of both evolution and revolution. Let's further examine its design features and performance.

Before we discuss the proper techniques associated with the instrument, it would be useful if the design features of the EndoSequence file were discussed, along with the rationale behind their creation. The first thing we must realize is that the EndoSequence file is actually a true reamer, not a file. This means that the cross section is actually an equilateral triangle rather than a modified square or triangle. Additionally, the file is made in such a manner that there is a series of alternating contact points along the shaft (Fig. 3). These contact points, which appear somewhat wave-like in appearance, allow the file to stay centered in the canal. Also, as a result of the alternating contact points, the file is never totally loaded. So a really significant feature of this instrument is that the EndoSequence file is the first mechanized reamer that stays centered in a canal. Additionally, the alternating contact points dramatically reduce the torque requirements of the instrument and at the same time promote the disengagement of the flutes (Fig. 4). Reduced torque and easy disengagement of the flutes are what make a file easy to work with. This is actually what a variable taper technique strives for in its use of multiple tapers.

However, think what is occurring with the EndoSequence file. We're getting all the benefits of a variable taper technique (such as reduced torque and ease of use) but all of this is occurring on a constant taper blank. The constant taper allows us to create predictable, reproducible shapes that, when combined with laser verified matching paper points and cones, allows us to create endodontic synchronicity (Fig. 5). So, in essence, what we are truly getting with the EndoSequence file is the best of both worlds. We're getting the ease of use normally associated with variable taper techniques and we're getting the predictability of shape associated with constant taper preparations. This is a very important concept and is the foundation upon which the entire system is built.

Another design feature that is often overlooked, or misunderstood by dentists, is the depth of chip space associated with an instrument. The EndoSequence file has a deep chip space (being a true reamer) and consequently, this makes the instrument a very efficient tool. In addition, the electropolishing adds to its cutting efficiency (Fig. 6). Consequently, because of this increased efficiency (and it is dramatic), you only need to be in the canal two to three seconds with the EndoSequence file. Working longer than two to three seconds with the instrument in contact with dentin will result in an instrument that has a full chip space. This is not good. Once a file is full, it is no longer working efficiently and, therefore, will require additional torque. This is why you should clean the EndoSequence file after every use of two to three seconds.

Now that the key design features and their rationale have been discussed, the question that remains is: "How do we use the EndoSequence file?" The answer is there are two techniques: basic and advanced.

The Basic Technique

The first technique is the basic, or rhythm technique, and this works very well, especially for those dentists just beginning with rotary instrumentation. The technique itself is very simple.

The motion of the file is based on a rhythm technique. It's one back, two back, three and out. One back, two back, three and out. The most important thing to remember is that with each stroke you are not taking a large bite out of the canal and, in fact, are not advancing to any predetermined length. (We are letting the canal dictate the depth of penetration.) Simply, one back, two back, three and out. You may also think of this as advancing just one millimeter at a time. The important thing to remember is to never force the file.

In terms of the technique itself, the biggest mistake clinicians make at the beginning is the backstroke. The tendency is to want to pull it back too far, and too quickly, or to take it completely out of the canal. This is wrong. The whole idea of the backstroke is to slowly disengage the flutes of the file. It should be no more than one-and-a-half to two millimeters.

Once you know how to use the file, the next question is, "In what order do I use the files?" The sequencing of instruments associated with the basic technique is as follows. We begin with the largest file in the selected package and we do two series of three engagements (one back, two back, three and out... clean the file... one back, two back, three and out... next instrument). We continue to work the instrument's crown down until we have a rotary file that reaches our working length with resistance. First rotary file to reach the working length with resistance completes the preparation. [Editor's Note: For a more detailed explanation of this technique, please view the videos on Dentaltown.com or on realworldendo.com]

The Advanced Technique

The advanced EndoSequence technique is based on the concept of sharing the task of cutting dentin among several files, while emphasizing the fact that each file only removes a small amount of dentin in a crown down fashion. This minimal cutting and the use of several files in a predetermined sequence (along with this file's triangular cross section) creates minimal torque on each file. The sequence of files used in this technique is referred to as a "cycle." A series of files is laid out in sequence in this cycle and the first file is followed by the second, third, and so on until the last file is used at the end of the cycle. Once the end of the cycle is reached, the file sequence is repeated from the beginning for a second cycle. Multiple cycles are used until the desired file reaches the apex. The final file is the Master Apical File, which will be fitted with the corresponding EndoSequence cone and obturated. The EndoSequence files in each cycle consist of sizes 40/.04 through 20/.04 in descending order of size. Therefore, each cycle consists of a total of five .04 constant taper EndoSequence files.

After straight line access and identification of the canal orifice, check patency with a size 10 hand file, thereafter a size 20 or 25/.06 EndoSequence file can be used lightly as a orifice shaper. After the use of the orifice opener (and the confirmation of a patent canal with hand files), the cycle begins. A size 40/.04 taper file is introduced with a very light touch. Heavy engagement will be encountered very quickly due to this file's large size. This means that the file is only used once with a single gentle motion (enough to engage the flutes and grab a chunk of dentin with its tip). The next file in the cycle, (35 /.04) is now introduced. It's important that each file is used in a gentle, one stroke motion with minimal engagement. Each file gently shaves off a thin layer of dentin from the canal walls and is not forced at any time. After each use, the next successive file (smaller tip) is used in this crown down fashion until either the end of the cycle is reached (size 20/.04) or a larger size file already reaches the apex. If a size 20/.04 is used and the apex is still not reached, a new cycle should begin (back to size 40/.04). If the apex is reached before the end of the first cycle (e.g., size 25/.04 reaches apex before size 20/.04), and if that size is too small for a Master Apical File, then the cycle begins from the beginning, skipping smaller sizes (it would be redundant to take a smaller size file to the apex when a larger size has already reached the apex). It's also better to start from the beginning of the cycle rather than moving back up the cycle one file. Often, larger size files may reach the apex if a crown down method is utilized. What size Master Apical File should be used for a given canal is a decision that cannot be made without considering the root shape, curvature, internal diameter of the canal versus the external root diameter, root anatomy and other related factors. No universal number is available for this decision. The only guiding principle is that the apical diameter should be the minimum size that allows for complete cleaning, disinfection and obturation of the root canal space without compromising the structural integrity of the root. [Editor's Note: For a more detailed explanation of this technique, please view the videos on Dentaltown.com, realworldendo.com or nasseh.net]

What is quite unique about this technique is that it also addresses the question of cyclic fatigue. By using the files in the manner described, he significantly reduces the cyclic fatigue of each instrument. This is critical because cyclic fatigue, unlike excessive torque forces, cannot be seen. Therefore, this is a technique that not only facilitates the treatment of difficult cases, it is also a method that will significantly reduce the potential for file separation.

In this article, we have addressed some of the issues associated with endodontic instrumentation and have also introduced the concept of a tapered canal being matched with laser verified paper points and cones (endodontic synchronicity). Endodontic instrumentation should be accomplished in such a manner that we can effectively clean and debride the root canal system without removing excessive root structure. The goal of endodontic therapy should be to satisfy all the biologic requirements of the procedure without comprising the long-term prognosis of the tooth. When performing a root canal, it is neither acceptable nor necessary to structurally compromise a tooth while achieving these goals (cleaning, shaping and obturation).

Author Bios
Dr. Ali Nasseh received his Doctor in Dental Surgery (DDS) degree from Northwestern University Dental School (Chicago, IL) in 1994 and completed his postdoctoral endodontic training at Harvard School of Dental Medicine in 1997, where he also received a Masters in Medical Sciences (MMSc) degree in the area of bone physiology. He has been a clinical instructor and lecturer in the post-doctoral endodontic program at Harvard School of Dental Medicine since 1997 and the alumni editor of Harvard Dental Bulletin. Dr. Nasseh is the president and Chief Executive Officer of Real World Endo (RealWorldEndo.com). He is the endodontic advisor to several educational groups and study clubs and is editor to several peer reviewed journals and periodicals. He has published numerous articles and lectures extensively, nationally and internationally, in surgical and non-surgical endodontic topics. Dr. Nasseh is in private practice (MSEndo.com) in downtown Boston.

Dr. Kenneth Koch received both his DMD and certificate in endodontics from the University of Pennsylvania School of Dental Medicine. He is the founder and past director of the new program in postdoctoral endodontics at the Harvard School of Dental Medicine. Prior to his endodontic career, Dr. Koch spent 10 years in the Air Force and held, among various positions, that of Chief of Prosthodontics at Osan AFB and Chief of Prosthodontics at McGuire AFB. In addition to having maintained a private practice, limited to endodontics, Dr. Koch has lectured extensively in both the United States and abroad. He is also the author of numerous articles on endodontics. Dr. Koch is a co-founder of Real World Endo.
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