Another Look at Root Canal Obturation

Many ingenious techniques have been devised for what should be a simple process of filling a root canal space. Some techniques require average skills, whereas others demand a high level of skill. There are now new techniques that focus on simplification and ease of use, a welcomed change if they are effective.

How best to obturate a canal remains elusive. Even after 100 years, we are still searching for the “right way.” Need the “right way” be complicated? General practitioners, faced with a variety of tasks (not just root canals), do not need a technique that makes canal obturation a complex procedure.

Obturation and instrumentation are so closely related that it is almost impossible to discuss one without the other. Without three-dimensional instrumentation, a three-dimensional obturation is not attainable. Understanding the dependence of one procedure upon the other, let’s begin with root canal instrumentation.

BACK TO BASICS: THE IMPORTANCE OF CLEANING

To emphasize the importance of cleaning, an earlier truism stated that a canal obturated with “sterile bird droppings” would be successful if the canal had been well cleaned. We agree—partially. Surely, a clean canal is the most important goal of instrumentation, but we prefer a better filling material for long-term success.

Some dentists believe that a clean canal is not that important for success. They think that when obturating material surrounds debris, it prevents noxious elements from invading the periradicular tissues (PT) and causing problems. Again, we agree—partially. It is true for certain locations, as we will see later.

Let’s take another look at root canal obturation and consider what obturation can or cannot do when instrumentation results in less than “perfect” root canals.

What happens when debris can be isolated? For this article debris is defined as bacteria and their toxins, tissue and tissue breakdown products, and predentin and dentin. Isolation is defined as the total encapsulation of debris by obturating material, rendering it unable to affect the PT.

Debris in a cul de sac, isthmus, or fin has just two paths available for causing problems. The noxious elements (let’s say bacteria) can reach the PT via the main canal or dentinal tubules. Obturating material traps bacteria on the canal side, and cementum traps them at the other end of the tubule. A good obturation can help us in this situation by isolating the debris and preventing future problems (Figures 1a and 1b). Were this not the case, we would have a much higher failure rate. These areas can be almost impossible to clean with current instrumentation and irrigation techniques.

Lateral canals and apical deltas, unlike cul de sacs, isthmuses, or fins, are open-ended on one side. Debris located there cannot be isolated from the PT by the obturating material.

For vital cases debris is not a factor. But what happens in nonvital cases where debris in lateral canals and apical deltas cannot be isolated from the PT with obturating material? If the main canal is not cleaned well, we would expect failure. If the main canal is well cleaned and obturated, debris in lateral canals and deltas is blocked on the canal side. On the PT side, defense cells will invade the areas, destroy the bacteria, and resorb the necrotic tissue. Healing will occur—just as elsewhere in the body. Areas surrounding roots of teeth are no different! Please note that we are talking about areas in close proximity to an abundant blood supply, not areas remote from a blood supply.

Critics of the body’s ability to heal argue that lateral canals and deltas are too small in size for the body’s defenses to be effective. At first glance this appears logical, but loses strength when considering the relative size of defense cells (very, very small). Besides, why would there not be room for defense cells when there was room for blood vessels, nerves, and connective tissue? Unfortunately, some have tried to make the periapex a uniquely mystical zone.

In conclusion, a good obturation (and, of course, a good cleaning) helps us in this situation by closing the door on one side (canal side) of lateral canals and deltas. Then, as normally happens, the body attacks the problem and heals from the other side where the body’s defenses are present.

What Obturation Can Do When Debris Remains in the Main Canal

As with cul de sacs, isthmuses, and fins, debris in the main canal that is isolated by obturating material is unlikely to cause failure if all else is done correctly. However, debris remaining close to the apical foramen cannot be isolated from the PT by obturating material (same as with lateral canals and deltas). Does this mean that to ensure success, instrumenting to the foramen (or slightly beyond) in order to remove all bacteria and necrotic tissue is required?  Not if the main canal is well cleaned and obturated to about 1 mm short of the foramen, the approximate location of the apical constriction (Figure 2). As with lateral canals and deltas, this closes the door on the canal side. Defense cells from the PT will invade the unclean area, destroy the bacteria, and resorb the necrotic tissue. The space then fills in with healthy tissue.

However, if the main canal is instrumented apically with instruments smaller than the existing canal diameter,1 then the canal is not well cleaned. Remaining debris will make adequate obturation impossible. The door is not closed on the canal side. Instead, debris and obturating material coexist, and there is no isolation—a situation totally different from the well-cleaned and obturated canal mentioned above.

If it is not blocked or isolated, bacteria can multiply and invade the PT. Figures 3a and 3b and 4a, 4b, and 4c show cross-sections of clinical cases that failed because the canals were underprepared apically. In both cases adequate obturation was impossible because debris already occupied the space. This unrelenting insult to the PT was just too much for the body to handle, and resulted in a no-win situation. Failure could have been avoided had the dentist been able to remove the debris by instrumenting the canal to the required larger size (diameter).

Figure 2. Photomicrograph of the apical area of a decalcified human specimen, X25. The apical foramen is not located at the apex because the canal deviates to the side (very common). The size (diameter) of the apical constriction is much smaller in size than the foramen. Sufficient space exists within the apical few millimeters of the root canal for the body’s defenses to function as elsewhere in the body.

We explained earlier, from a biological perspective, why it is not necessary to instrument to (or through) the apical foramen for healing to occur. There are also physical reasons, but a review of apical anatomy,2,3 the basis for our views, indicates: (1) the root canal ends at the apical foramen, not at the apex (Figure 2); (2) the apical constriction or CDJ (not always present) is located about 0.7 to 0.9 mm from the foramen; (3) the average diameter of the constriction is about an ISO No. 30; (4) from the constriction to the foramen the canal gets larger; (5) from the constriction to the foramen the canal is lined with cementum (not dentin), and the tissue is periodontal ligament (not pulp); and (6) the diameter of the apical foramen is about an ISO No. 60 (considerably larger than the apical constriction).

We are inclined to believe that the apical foramen is small—it isn’t. The impression comes from incorrect terminology. Frequently, the word “constriction” (average size = ISO No. 30) is used when “foramen” (average size = ISO No. 60) is meant. This problem with terminology is also seen when “apex” is used, but “foramen” is actually meant, or vice versa. The two are not interchangeable. Unfortunately, this confusion leads many to believe that most canals can be properly cleaned with small instruments (ISO size 30 or smaller). We will see later that this is not at all what research tells us.

Why We Do Not Instrument to the Apical Foramen

The authors do not instrument to the apical foramen because of the apical anatomy (Figure 2) and the known behavior of root canal instruments. If the intent is to clean the canal to the foramen, then, on average, a size 60 instrument would be required. This size instrument would destroy the natural apical constriction (average ISO size 30). Instrumenting to the foramen would result in instrumenting the periodontal ligament and cementum, not the pulp and dentin. Instrumenting to the foramen will very likely transport the apical 1 to 3 mm of the canal and change the shape of the foramen from round to oval. Oval shapes are almost impossible to seal with round gutta-percha cones.

Transporting the canal and ovaling the foramen occurs because the canal usually curves to one side of the root as it approaches the apical foramen. Gutierrez and Aguayo4 found that in 140 human specimens, most of the root canals deviated to one side and ended short of the apices. They also reported that multiple foramina was a common finding. Others have reported similar findings.2,3 A large size instrument taken to the foramen is much too rigid, and we all know that means trouble in curves.

Figures 5a and 5b. SEM photomicrographs showing the (a) apex of a maxillary first molar and (b) apex of a mandibular central incisor, both with multiple foramina. The presence of multiple foramina is a common occurrence.

Whether or not to instrument to the foramen brings forth some questions: (1) Can we always instrument to the apical foramen? Dummer et al3 found that in 270 specimens, 6% of the canals were completely blocked; (2) With multiple foramina could we negotiate all of them even if we wanted to? (Figures 5a and 5b); and (3) When a main canal divides into an apical delta, can root canal instruments even fit in the much smaller branches?

Knowing the obstacles noted previously cautions us to use restraint when claiming to always instrument to the apical foramen (foramina). How is it physically possible? And where is the science showing that instrumenting to the foramen is the right thing to do? Our endodontic forefathers debated this issue thoroughly and decided that limiting instrumentation to (not through) the apical constriction is best. We strongly agree. There is still no strong evidence to suggest they were wrong.

What About the Dentinal Tubules?

Another area of interest is the role infected dentinal tubules play in determining success and failure. Knowing that infected teeth harbor bacteria in the predentin and tubules, it may not be acceptable to just “touch” the surface of canal walls with instruments and irrigating solutions. Canals need to be enlarged enough to remove the predentin and the most heavily contaminated area of the tubules. This can only be accomplished with instruments. Expecting irrigation alone to clean a canal would be like throwing away our toothbrushes and relying on vigorous swishing with mouthwash to clean our teeth.

The problem with worrying about tubules is not knowing how deeply the bacteria have penetrated. Even if we did, it may not be possible to remove them. We should accept this reality and not be overly concerned. With effective instrumentation the heavily contaminated dentin would be removed. Any remaining bacteria would be isolated by obturating material (canal side) and cementum (other side). It is worth noting that the mere presence of bacteria does not necessarily indicate infection.

A Word About “Puffs”

How do apical and lateral “puffs” fit in the obturation success/failure discussion? We think that too much importance is given to the presence of extra-radicular obturating materials, or apical and lateral puffs. Seeing them in radiographs does not mean that main canals, lateral canals, or apical deltas have been well cleaned and obturated. Sealer can easily bypass debris with the skillful use of hydraulic forces. Puffs only indicate the presence of filling material, nothing more.

Figures 6a and 6b. (a) Photograph showing gutta-percha beyond the apical foramen. Note that the foramen has been instrumented to an oval shape and the foramen is not sealed. (b) Cross-section of the same tooth approximately 1 mm from the foramen. Note transportation of the canal and remaining debris—the true cause of failure and not the excess gutta-percha.

Figures 7a and 7b. (a) Photograph of another tooth also showing gutta-percha beyond the apical foramen. Note that in this case the foramen appears sealed by the gutta-percha. (b) Cross-section of the same tooth approximately 1 mm from the foramen. Note that the canal is round and well cleaned and obturated. In this case, failure from the excess gutta-percha would not be expected.

Will a puff cause failure? As with any foreign body, puffs will delay healing, but they should not cause failure if everything else is done correctly. This also applies to overfilling with gutta-percha (Figures 6a and 6b and 7a and 7b). Failure occurs because a canal is not cleaned and sealed—not because of the presence of puffs and/or gutta-percha. We often wonder why some give so much importance to apical and lateral puffs.

Figure 8. Cross-sectional photograph showing a lateral canal containing vital tissue. The value of forcing filling material into areas with vital tissue is biologically questionable.

Another point about puffs is considering their value in vital cases. It seems groundless—and perhaps a source of pain—to deliberately push filling material into lateral canals (Figure 8), apical deltas, and periodontal ligaments. We believe the endodontic community should reconsider the use of internal forces (lateral or vertical) as a necessary component of obturation. There has to be a better way. Because an internal force is by far the easiest way to fracture a root, we believe that eliminating spreaders, pluggers, and all devices that can produce excessive internal forces would be a giant leap forward.

Why Posttreatment Pain?

Most agree that posttreatment pain arising from a necrotic case is understandable. We are not happy when it occurs, but we accept it. What is hard to understand and accept is why moderate to severe posttreatment pain sometimes follows treatment of vital cases. What is going on?

Mishandling vital apical tissues is what is going on. Preparing canals with instruments too small for the existing canal diameter will lacerate the pulp instead of removing it. Tearing tissue can cause pain. Fortunately, the pain disappears and healing is usually uneventful, if everything else is done correctly. Yet, pain is not something patients should endure, especially if it can be minimized with correct instrumentation.

The “Perfect” Root Canal Treatment

Being aware of the complexity of apical anatomy should make it easier to accept as fact the impossibility of always doing “perfect” root canals. Knowing this, we have to accept the important role the body plays in the healing process. Yet, we should strive to do the best we can, not knowing the healing potential of patients. Only after doing our best can we ask the body to take care of the rest.

Success and Failure

It is common to hear, “my technique works great, I almost never have a failure.” In Wildey’s practice, on average, about 35% of the cases involve re-treatment of referred failures. Other endodontists report similar percentages.

Unfortunately, rarely are dentists made aware of their failures. Not being told otherwise, they believe their treatment is a success. We have come to realize that most re-treatment referrals are not from those who performed the treatment. They come from other dentists, after patients make the change because they are frustrated by the continuous discomfort of the treated tooth. If we were all required to report endodontic failures to a central data bank, we would be surprised with the high failure rate. The fact is that many failures are extracted. Who is counting these failures?

Given that no data bank exists, how can we determine an accurate success/failure rate? The only way is with good research. One such study5 involved single-appointment root canal therapy. The results showed that teeth obturated with negative cultures had a success rate of 94%, whereas for those with positive cultures at the time of obturation it was only 67%. We should not accept, nor do statistics support, the claim that endodontics is over 95% successful.

Unfortunately, we tend to think that a positive culture is a tube with growth, without understanding the true significance. A positive culture is really saying: doctor, the canal is still dirty. You should clean it better before obturating. Research sends a clear message: for greater success we need cleaner canals.

Taking Responsibility

Most dentists take seriously the responsibility for providing quality care. Taking responsibility means not using a technique just because it’s simple or fast or “comfortable” to use. Taking responsibility certainly does not mean adopting the most popular technique just because that’s what the others are doing or a popular lecturer suggests it. Be aware that some lecturers have a financial interest in the products that they suggest you use. Certainly, this is not necessarily bad—just be aware that it does happen. Caution is in order: be wary of the “latest and greatest!”

Taking responsibility does mean finding out what instruments do the best job of cleaning canals as determined by independent researchers and good science. Beware—some recent developments have not been fully studied and lack sufficient clinical evidence to support clinical success. Before making a change ask for the research—then make an informed decision. And finally, taking responsibility also means a willingness to devote the time, effort, and money necessary to learn and implement appropriate techniques in your practice.

CONCLUSION

This article has tried to explain why it is unrealistic to discuss obturation without also addressing instrumentation. It has attempted to dispel the myth that a good obturation is the most important part of root canal therapy, replacing the myth with a self-evident truth that an obturation can only be as good as the instrumentation. It has tried to stress that a three-dimensional obturation must be preceded by three-dimensional instrumentation. And finally, we strongly urge giving due importance to a correct working width (instrument size) in addition to a correct working length. Doing so will go a long way toward achieving the ultimate goal of instrumentation—a well-cleaned and obturated root canal system.F

Acknowledgment

The authors wish to thank Mr. Steven S. Senia, BE, MBA, for his contributions to this manuscript.

References

1. Senia ES. Canal diameter: the forgotten dimension. Dent Today. 2001;20:58-62.

2. Kuttler Y. Microscopic investigation of root apexes. J Am Dent Assoc. 1955;50:544-552.

3. Dummer PMH, McGinn JH, Rees DG. The position and topography of the apical canal constriction and apical foramen. Int Endod J. 1984;17:192-198.

4. Gutierrez JH, Aguayo P. Apical foraminal openings in human teeth. Oral Surg Oral Med Oral Pathol. 1995;79:769-777.

5. Sjogren U, Fidor D, Persson S, et al. Influence of infection at the time of root filling on the outcome of endodontic treatment of teeth with apical periodontitis. Int Endod J. 1997;30:297-306.

Dr. Wildey served four years as a general dentist in the United States Air Force and is presently in an endodontic practice in the Dallas/Ft. Worth area in Texas. He is the primary author of two articles published in a major national dental journal.

Disclosure: Dr. Wildey is the co-inventor of the LightSpeed root canal instrument and SimpliFill obturation systems.

Dr. Senia is a diplomate of the American Board of Endodontics, a former Journal of Endodontics Editorial Board member, and a consultant for NASA’s space program. He has lectured throughout the world and published extensively. He can be contacted at DrSteveSenia@aol.com.

Disclosure: Dr. Senia is the co-inventer of LightSpeed and a member of LightSpeed Endodontics Board of Directors.