EndoSequence: Melding Endodontics With Restorative Dentistry, Part 1

In our more than 75 years of combined experience, we have witnessed many changes in dentistry. The most significant (in our opinion) have been the advent of nonmetallic dentistry (bonding), the introduction of dental implants, and the use of nickel titanium rotary files to shape root canals. But, more importantly, what has become apparent is that dentistry is multi-discipline in nature and must be treated as such. It is unwise and often a mistake to view any particular aspect of dentistry as a lone-standing entity. So it is with endodontics.
Endodontics should be viewed not as a lone-standing entity, but rather as part of an “Endo-Restorative Continuum.” Even if a tooth is well-treated endodontically, its long-term success depends upon the quality of the final restoration. Furthermore, endodontic therapy should be accomplished in such a manner that it facilitates an excellent long-term restoration. As a result of the intimate relationship that exists between endodontics and restorative dentistry, it is imperative that root canal therapy be performed in as conservative a manner as possible.1 Conservative in terms of preserving radicular dentin. If an excessive amount of radicular dentin is lost during the endodontic procedure (particularly in the coronal third), this may lead to increased endodontic failures. It will be demonstrated in this article how endodontics can meld with restorative dentistry, and as well we will introduce an endodontic system (EndoSequence) that will help us attain this goal.

ENDODONTIC FAILURES: FIRST SOME QUESTIONS

A significant number of endodontic cases that fail, do so not as the result of endodontics per se, but actually fail as a result of the restorative effort. An open margin on a crown that allows the continued percolation of oral fluids over time is a clear example. The entire root canal may become contaminated as a result of coronal leakage. The bacteria that caused this failure are not residual in nature, but instead are the result of coronal leakage.
Currently, we are seeing many endodontic preparations that are excessively wide in the coronal third of the radicular dentin.2 As a result, 2 questions that we need to ask ourselves are:

• What is the principle cause of this coronal enlargement, and does it adversely affect the long term prognosis of the tooth?
• Is there a preferred taper for endodontic preparations, and will performing it enhance the prognosis of endodontically treated teeth?

EXCESSIVE CORONAL SHAPING AND ENLARGEMENT

Clearly, the most significant change in endodontics that has resulted in excessive coronal shaping has been the evolution of obturation techniques. As thermoplastic techniques (solid core and warm vertical condensation) became more popular, it was obvious that the larger the opening at the top of the canal, the easier it was to move heated gutta-percha down the canal. That is fine for manipulating gutta-percha, however the real concern is whether this larger coronal shaping has compromised the structural integrity of the tooth.

Figure 1. Silver point case demonstrating the conservation of tooth structure in the coronal third of the root.	Figure 2. Classic hand file case also showing the conservation of radicular tooth structure.

If there are any questions of how excessive coronal enlargement can adversely affect the long-term prognosis of a tooth, think of the following case (Figure 1). This is an x-ray of a silver point case done many years ago. Silver points are not being recommended as a preferred technique, but here is the salient point—think back for a moment—can you remember the last time that you saw a silver point case fail as the result of a root fracture? Most likely the answer is no. When silver cone cases failed, they failed apically and not as the result of structural compromise. The reason is obvious when you examine the x-ray. Notice all the wonderful radicular dentin remaining in the coronal third of the root. The tooth has not been augured out by pizzo reamers, large Gates Gliddens, or large orifice openers. The next image (Figure 2) shows a classic root canal that was completed using hand files (.02 taper). Notice the conservative nature of this case; and when we think of the problems associated with hand filing cases—they too were apical in nature, not coronal.
Excessive coronal enlargement during the endodontic process can and will lead to future complications. Some of the obvious long-term problems associated with excessive coronal enlargement are fractures, coronal leakage, and caries. This is why this issue is so important and needs to be addressed. In fact, the failure of some endodontically treated teeth to stand up long-term has given the implant companies an excellent opportunity to market a “story” that endodontically treated teeth will not last over time. Supposedly, they fracture. Real World Endo (our endodontic education company) takes exception to this “story.” We believe that endodontically treated teeth, when performed in a conservative manner particularly in the coronal third of the root, will have an excellent long-term prognosis and last as long as any successful implant. The truth is that there are indications for both treatment modalities and that endodontics performed as a minimally invasive procedure has an excellent chance of lasting a patient’s lifetime.
In addition to some thermoplastic techniques contributing to the problem of over-enlargement, the recent popularity of carrier-based obturation has also resulted in wider than ideal orifice enlargement. The rationale behind this is again quite simple. The larger the hole at the top of the canal, the less likely it is to strip (denude) the carrier of gutta-percha. This has been one of the challenges associated with carrier-based obturation (stripping the carrier at the orifice during insertion).2 As a result, we often see wider orifices associated with these techniques.
Certainly, one can get good obturation results with carrier-based techniques (as with other methods) if done properly, but this issue of stripping a carrier remains a significant one in endodontics. Therefore, when viewed from a truly endorestorative perspective, we believe the wider coronal shape presently employed by many practitioners for both carrier-based and thermoplastic obturation techniques will adversely affect the long-term prognosis of the tooth. The goal of endodontic therapy should be to satisfy all the biologic requirements of the procedure without compromising 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).

VARIABLE VERSUS CONSTANT TAPER: IS THERE AN IDEAL METHOD?

The second question (concerning taper) must be initially discussed with a comparison of a variable taper technique versus a constant taper preparation. Presently, in endodontics, we have 2 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.

Figure 3. Comparison of variable taper vs. constant taper.

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 instrument, what is sacrificed in its use is the predictability of shape commonly associated with a constant taper preparation (Figure 3). Due to their lack of a constant, reproducible shape, most variable taper methods therefore are associated with thermoplastic obturation. Furthermore, with large variable taper sizes such as .12, .10, and .08 there is the 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 carrier based 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. Obturation should not dictate shape.
A constant taper preparation, on the other hand, is generally much kinder to the coronal dentin of the root canal. 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.3 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, the foremost being conservation of radicular dentin. However, first we must acknowledge a shift in instrument design that has occurred from landed files to nonlanded instruments.
The desire for greater efficiency and predictability of shape associated with nonlanded 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 2 perspectives. The first is that the wider .06 taper preparations are considered easier to obturate if performing warm vertical condensation; and secondly, .06 tapered landed files are 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 nonlanded rotary files. In fact, with nonlanded instruments, cyclic fatigue is the key issue with the result being, .04 taper non landed instruments are more resistant to separation than .06 taper nonlanded 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 nonlanded instruments.4
Additionally, 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.5 So, when it is evaluated from a true endorestorative perspective, the preferred taper should be a constant .04 taper. However, there still may be some clinicians who wish to remain with a .06 taper, and even these practitioners must be aware of certain cases that do much better with a constant .04 taper preparation.

Figure 4. Severely curved molar instrumented with a hybrid technique of EndoSequence rotary files and hand files. (Courtesy of Dr. A. Fleury.)

A second recommendation is that long canals (those greater than 25 mm in length) should also be instrumented with a .04 taper. The rationale for this again is maintaining tooth structure in the coronal third of the canal. For example, quite often a maxillary canine will have a final working length of 27 or 28 mm. The length of the canal itself (orifice to apex) in these cases is often equal to or greater than 16 mm. A .06 taper file has at the top of its cutting portion (D-16) an increase in diameter of .96 mm greater than its apical size (D-1). Therefore, we are, in a sense, getting the full effect of taper in these long teeth. Consequently, this has the potential to be excessive, so these cases are best treated through the use of a constant .04 taper.
Finally, a third recommendation (for when to perform a .04 taper preparation) is a significantly curved canal. These canals stress both the instruments, as well as the clinician. In these cases, a .04 taper instrument generates less flexural stress and works more easily in the canal. The key, in these challenging cases, is to only advance the series of rotary in-struments to the point of increased resistance. At this point, switch over to hand files and use them to create a glide path to the final working length. Once the canal has been enlarged to a size 20, you can follow with .04 taper rotary files. The rotary files will now act as finishing instruments to insure endodontic synchronicity. This “hybrid technique” works very well in both MB canals of maxillary molars and in the mesial root of mandibular molars, where it is not uncommon to have significant curvatures (Figure 4).

Figure 5. Endodontic synchronicity created between the rotary file, endodontic preparation, post, and the eventual restoration.	Figure 6. Premolar case instrumented using EndoSequence constant .04 taper files, and obturated with a matching single cone and bioceramic sealer technique. (Courtesy of Dr. A. Nasseh.)

Figure 7. Mandibular molar demonstrating the consistency of shape achievable with constant taper files. (Courtesy of Dr. A. Nasseh.)

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.6 The key to achieving this endodontic synchronicity, which ultimately results in the conservation of radicular dentin, is the file. In Part II, we will examine the design features of the EndoSequence file and see why it’s unique (Figure 7).

References

Dr. Koch is the Founder and past director of the New Program in postdoctoral endodontics at the Harvard School of Dental Medicine. In addition to having maintained a private practice limited to endodontics, he has written numerous articles on endodontics and lecturers worldwide. He is a co-founder of Real World Endo and can be reached by visiting realworldendo.com.

Dr. Brave is a Diplomate of the American Board of Endodontics and is a member of the College of Diplomates. In endodontic practice for more than 25 years, he has lectured extensively on endodontics and holds several patents, including the VisiFrame. Formerly an associate clinical professor at the University of Pennsylvania, Dr. Brave currently holds a staff position at The Johns Hopkins Hospital. He is a co-founder of Real World Endo and can be reached by visiting realworldendo.com.

Disclosure: Drs. Koch and Brave in association with Brasseler USA are the developers of the EndoSequence system.