Some ideas are accepted and pursued more readily than others. As an example, the concept of man flying was not taken seriously until modern times. From Da Vinci’s conceptual illustrations to the Wright brothers’ accomplishments, centuries elapsed. Significant technological advances have produced both the space shuttle, and the hang glider. It took longer to get Da Vinci’s “hang glider” in the air than a rocket. Today’s hang glider works because it has borrowed and modified advanced technology.
Scientific and technological advances in endodontics have allowed the use of simple devices in sophisticated ways. Today we can routinely use paper points to determine the length of a canal.1 This is possible because of modern instrumentation techniques and nickel-titanium files of greater taper.
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| Figure 1. This radiograph shows a central incisor that has had an apicoectomy. There is a retrograde filling that has been placed into the root, but not the canal. | Figure 2. The paper point used to determine the location of the cavosurface of the canal also reveals the angle of the surgically placed bevel and its orientation. |
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| Figure 3. The information gathered from the paper point can be used to customize the gutta-percha cone used for obturation. | Figure 4. The sealer partially obscures the intimate seal developed during obturation. What is noteworthy is the apical control that was developed in this case. Intimate knowledge of this root canal system’s anatomy was gained through interpretation of the paper point. |
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| Figure 5. A complex root canal system treated without working films. The paper point technique was used to determine working lengths. |
While the paper point may seem like Da Vinci’s hang glider when compared with digital radiography or an apex locater, it gives more accurate information about canal length and shape than any other technique available (Figures 1 through 4). The accuracy of paper point measurements also allows for the elimination of working films while improving the technical quality of the endodontics performed (Figure 5).
Achieving technical endodontic excellence is largely dependent upon knowing two variables: (1) at what length the cavosurface of the canal is, and (2) the minimum apical foramen diameter (MAFD) at that length. Knowing these variables allows the dentist to have excellent control of the root canal system. The length to the cavosurface of the canal can be determined to within 1/4 mm accuracy using the paper point technique, a technique that does not require measurement radiographs.
Radiographs and apex locators are used to estimate the working length of canals. When estimates of length and MAFD are made, the quality of endodontic procedures will inherently vary. Accurate knowledge of canal length and MAFD can be used to produce consistent high-quality results. This two-part article will attempt to illustrate an approach that can be used to improve the quality and consistency of endodontic procedures
DEFINITIONS
Before discussing a more accurate way to ascertain the variables required for optimum control of endodontic procedures and outcomes without working films, some definitions are needed.
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| Figure 6. The black line represents the plane of the cavosurface of the root canal. | Figure 7. Another view of the plane of the cavosurface. |
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| Figure 8. Obturation materials need not be placed apical to the plane of the cavosurface. As can be seen in this representation the immune system has easy access to the area apical to the plane of the cavosurface. |
Let the cavosurface of the canal be defined as a plane that bisects the root at a point comprised of the most coronal position of the apical foramen where the internal surface of the canal meets the external surface of the root. This point is now considered to be at the most apical portion of the canal, and no other points on the plane are more coronal to it (Figure 6). It is not important to know if this cavosurface plane bisects cementum or dentin or both. The importance is in knowing where the demarcation of internal canal, and external root, exists (Figures 7 and 8).
Endodontics is successful when an environment is created that will allow the immune system to function to repair injury. We only need to treat what is inside the canal. All points that are located apical to the cavosurface of the canal can be considered to be in contact with the immune system. All points coronal to the plane of the cavosurface are considered to be relatively inaccessible to the immune system, and should be treated with proper endodontic therapy.
Obturation materials only need to be placed inside the canal to its cavosurface margin. By knowing where the endpoint for treatment exists (the cavosurface), better control can be developed in instrumentation and obturation of the root canal system. Improved control goes hand in hand with improved quality in all areas of dentistry.
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| Figure 9. In this idealized situation the minimum apical foramen diameter is between the arrows which is the diameter of the circle. | Figure 10. In a more realistic example, the minimum apical foramen diameter is determined from the two closest points on the cavosurface plane of the canal along the path taken by the files used in instrumentation. |
The minimal apical foramen diameter also needs to be defined. It is the smallest cross-sectional length of the apical foramen at the cavosurface of the canal along the path taken during instrumentation that can be determined with gauging files. In the rare instances where the cavosurface of the canal is at the anatomic and radiographic apex of the root, and the apical foramen is a perfect circle, the MAFD would be the diameter of the apical foramen (Figure 9). In most instances the MAFD will consist of the two closest points on a plane that represent the cavosurface of the canal. There will be many wider areas on this plane, such as an elliptical or irregularly shaped foramen (Figure 10).
How can these determinations of length and MAFD be made without radiographs? With digital radiography, taking a working film—whether a measurement, cone fit, or obturation check film—only takes seconds, and has never been easier. Why choose to abandon these images and the information they contain? How can it be said that better care could be delivered without them?
For me the motive to change the way canal measurements were obtained arose because the results that were achieved using the information obtained from radiographs, the electric apex locator, and tactile sensation were not as consistent as desired. Often the results were beautiful. Sometimes, even with relatively simple teeth like maxillary central incisors, the results were substandard, usually an unsightly overfill, indicating a lack of awareness of the specific anatomy and an inability to control treatment in a simple root canal system. The ability to consistently, reproducibly, and accurately transform the information from radiographs, the apex locator, and tactile sensation into technically excellent endodontics fell short too often, creating anxious moments while waiting to view the postoperative radiographs. Working diligently and only producing an inferior result, coupled with not understanding what precipitated the undesirable outcome, make for a very frustrating endodontic experience. An explanation for these inconsistent results was needed. Endodontic treatment needs to be performed with great consistency in order to ensure a high predictability of results—our patients are counting on this.
OUR TRADITION
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| Figure 11. Length determination from a radiograph can be extremely difficult and unreliable. | Figure 12. A photograph of the radiographic specimen in Figure 11. |
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| Figure 13. A radiograph of the same specimen in Figure11 from a different angle. | Figure 14. A photograph of the specimen in Figure 11 from a different angle. |
Traditionally, radiographs have been used to determine the length of the canal. We know from many studies that this method is inaccurate.2 The difference between the radiographic apex (RA) and the cavosurface of the canal can be significant. Because of the inherent shortcomings with radiographic length determination (Figures 11 through 14) it has been suggested by some authors that working a certain distance short from the RA will result in a satisfactory working length.3-5 The distance to be subtracted from the RA is based on studies where the average distance of the apical foramen from the RA has been measured. Then an average length discrepancy with a standard deviation is determined.
The problem with this approximation technique is that the teeth we treat are not average, but very unique. A working length that is determined based on statistical research may fall within the standard deviation. However, the chances of getting an accurate canal length by using statistical averages and standard deviations are remote. One length does not fit most canals. The estimated length will always either be long or short of the true cavosurface of the root. At best this technique should be considered length approximation, certainly not length determination.
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| Figure 15. Treating this root canal system short of its cavosurface could result in several mm’s of untreated tissue. |
In order to avoid over- instrumentation and overfilling, most of us were taught to work short of the radiographic apex. This philosophy does not address the entire root canal system (Figure 15). Treating short also contributes to canal ledging and blockages, leaving substrate in the canal for bacteria to cause later problems.
Many of us were taught to rely on radiographs and statistics for length “approximation.” Many believe that radiographic measurements are the standard of care, or because of cognitive dissonance have chosen to overlook the shortcomings of this technique.
The standard of care is what a reasonable and prudent practitioner would do. School curricula or textbooks do not dictate the standard of care. The standard of care evolves as advances in the field occur. Endodontic treatment can routinely be performed without working radiographs and consistently produce results that meet, if not exceed, the standard of care.
INCORPORATING NEWER TECHNOLOGY WITH LONGSTANDING TRADITIONS
A new level of accuracy in length determination over radiographs has been achieved with the electronic apex locator (EAL). The EAL is free of the problems that visual interpretation of two-dimensional radiographs present. With the EAL there is no chance for our eyes to misinterpret the information that the EAL presents. Unfortunately, the EAL is not 100% accurate, and may be perceived as difficult to use. Even though the EAL is not infallible, it is more accurate than radiographs.6
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| Figure 16. The “Endo Q” for apex locator training. |
For those of us who find it difficult to use an EAL there is a new product, the “Endo Q” (Acadental) (Figure 16), that will dramatically decrease the learning curve for mastering this device. I have found that when using the “endo Q” in the hands-on courses I teach in my office, the participants experience a dramatically accelerated learning curve.
When the EAL indicates that the apex has been reached, it means that the very tip of the file is protruding through the foramen. Repeatedly taking larger instruments (greater than a 15 or 20 K file, depending on the curvature of the canal and the hardness of the dentin) past this length will lead to transportation of the apical foramen, commonly known as zipping the apex. The EAL does not accurately reveal where the canal ends and the extra-radicular structures (the PDL, bone, granulation tissue, cyst) begin. It does not accurately reveal where to terminate treatment of the RCS.
With the EAL a similar problem that radiographs present recurs—to what distance from the EAL reading should we work? Again, studies have been done that give information in terms of average length discrepancies between the EAL reading and direct observation.7 An average distance with a standard deviation is determined, and adjustments to working length are made accordingly. Again, the problem is in using averages and applying them to unique situations. Most of the time the length selected will be incorrect, either too short or too long.
An example of a situation where using averages is inappropriate would be when a patient is treated with IV sedation, when titration of medications is required for safety and effect. The response to medications follows a bell curve. Individuals will either be hyper responders, hypo responders, or somewhere in between. Unfortunately, preoperatively it is not known how an individual will respond. Giving an average dose of an anxiolytic agent to a hyper responder can result in a dangerous situation. The same dose given to a hypo responder will be ineffective. Titration is imperative in this situation for maximum safety and effect.
In endodontics there also exists a bell curve with canal lengths. Use of radiographs or the EAL combined with measurement adjustments based on average length discrepancy studies inherently results in working lengths that are excessive or insufficient, but seldom just right. Unfortunately, with these measurement techniques we don’t know which canals will be treated long, short, or just right until the final film is viewed.
Our patients expect us to treat their unique anatomy. We don’t usually place crowns based on averages. We make a unique crown for each individually prepared tooth, and we must use the same approach in endodontic treatment.
CONTINUING THE EVOLUTION OF OUR TECHNIQUE
What if there was a way to “titrate” working length for each individual canal or foramen? No more underfilling or overfilling unless desired. Such a method exists. It allows determination of the distance to the cavosurface of every unique canal in each root. This measurement can be performed to within tolerances of 0.25 mm, consistently, without radiographs. It allows us to “see” the cavosurface of the canal.
The beauty of this method is that once the distance to the cavosurface of the root is known, the MAFD at that length can be determined. Once length is determined (which means we have negotiated the canal to its terminus), the MAFD at that length can also be determined, then the two variables needed to successfully complete the endodontic equation are known. With this knowledge comes control of instrumentation and obturation that allows the doctor to determine where to terminate treatment. The results designed are the results produced. We can truly become masters of our domain.
This measurement technique is performed with paper points. In addition to extremely accurate and consistent length information, paper points can sometimes give three-dimensional information regarding the location and slope of the apical foramen.
This three-dimensional information can be extremely valuable in developing control over the most apical extent of the canal. There is no need to worry about overextension of gutta-percha. There is more concern that fitting the gutta-percha short of the ideal termination point will result in obturation that remains short of the ideal termination point. With good apical control the gutta- percha cone will not advance even under the pressure developed by vertical condensation forces.
When we are lacking length or minimal apical foramen diameter information we cannot attain the same degree of control. In these situations we need a little bit of luck to help us pull it off. Gifted clinicians can pull it off routinely, intuitively knowing and feeling the variables. But for those of us who might need to take a few practice swings before going on the golf course, or maybe even need a lesson now and then, it is nice to have all the information we could possibly need. Wouldn’t you like to know the exact yardage for your golf shot, the exact slope and speed of the green? Tiger Woods has a feel for it. Most others would like all the help they can get.
THE POINT
The concept behind paper points being used to provide accurate length information comes from the idea that when the contents of the root canal system are removed the canal should be dry. The extraradicular (or more accurately extracanalular) environment is living and hydrated. There is the PDL, granulation tissue, pus, blood, bone, or some other hydrated tissue containing fluid that exists beyond the cavosurface of the canal.
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| Figure 17. A paper point will stay dry in a canal if it is short of the cavosurface of the canal. | Figure 18. When the paper point is overextended, capillary action may allow excessive fluid to collect on the point. |
If a paper point is placed into a dried canal and removed short of the apical foramen, it should be retrieved dry (Figure 17). If a paper point is placed into a dried canal and taken past the cavosurface of the canal it will be retrieved with fluid (blood, pus, serous fluid, or mucus) on that portion of the point that extended through the cavosurface of the canal. Because of capillary action the wet portion will be extended some distance further along the point than the portion that was directly in contact with the fluid (Figure 18). The length of paper point affected by this capillary action is dependent on the viscosity of the fluid present beyond the canal and the absorbency of the paper point. We don’t need to know this information in order to get accurate length information from paper points.
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| Figure 19. The apical extent to which the paper point can be placed, and remain dry is recorded as the length to the cavosurface of the canal. | Figure 20. As soon as the paper point is extended past the cavosurface of the canal it will be discolored. |
The technique for paper point measurement can be simple. Into a dried, patent canal place a paper point. A trial paper point is placed 1/2 mm short of the EAL length. If the point comes out dry advance it until it picks up some fluid. Note the length of the point that is dry. Now, another point is taken just short of this length, removed and observed. For this example assume that the point comes out dry (Figure 19). Re-introduce and advance the point until the very tip of the point has the slightest bit of fluid on it (Figure 20). The point should not remain in contact long enough for any capillary action to have taken place. Record the maximum length that the point can be placed into the canal and remain dry as the length of the canal. This is the cavosurface of the canal
Now that an accurate canal length is known, the MAFD at this length can be determined. By taking K files to the paper point length without rotating them, but with apical pressure, a file that will not go long but will bind at or just short of paper point length will be found. This file represents the MAFD.
Of course there are some subtleties to the technique. In teaching this technique for the past few years I have also become aware of the common problems and misconceptions that doctors new to the technique have in common. In part 2 of this article I will address several of these issues and offer solutions.
CONCLUSION
It is my hope that this article will facilitate quality endodontic treatment by taking advantage of the developments that have taken place in the last decade. The paper point technique would not be a consistently reproducible technique without advances in instruments that have taken place, notably the introduction of greater taper files.
Acknowledgement
The author would like to thank Steven Barney (EKTAL@aol.com) for his graphics, and Dr. Gary Carr for his mentoring.
References
1. Buchanan LS. Paradigm shifts in cleaning and shaping. J California Dental Assoc. 1991;23:24-34.
2. Tamse A, Kaffe I, Fishel D. Zygomatic arch interference with correct radiographic diagnosis in maxillary molar endodontics. Oral Surg. 1980;50:563-565.
3. Ingle JI. Endodontic instruments and instrumentation. Dent Clin North Am. 1957;11:805-822.
4. Weine F. Endodontic Therapy. 3rd ed. St Louis, Mo: C.V. Mosby; 1982.
5. Cohen S, Burns RC. Pathways of the Pulp. 7th ed. St Louis, Mo: C.V. Mosby; 1998.
6. Pratten D, McDonald NJ. Comparison of radiographic and electronic working lengths. J Endodont. 1996;22:173-176.
7. ElAyouti A, Weiger R, Lost C. The ability of roor zx apex locator to reduce the frequency of overestimated radiographic working length. J Endodont. 2002;28:116-119.
Dr. Rosenberg is a diplomate of the American Board of Endodontics and a fellow of the American College of Dentists. He teaches a course on conventional endodontics in Vero Beach, Fla, where he maintains a private practice limited to endodontics. Dr. Rosenberg is also developing the protocol for teaching a hands-on re-treatment course in conjunction with Dental Education Laboratories. For additional information, Dr. Rosenberg can be reached at (772) 234-3333 or through Dental Education Laboratories at (800) 528-1590.
