Written by David B. Rosenberg, DDS Friday, 28 February 2003 19:00
In part 1 of this 2-part article on paper point measurements, the rationale behind the technique was presented. As with all new techniques, there is a learning curve. In order to speed the learning process and minimize the frustrating experiences associated with incorporating new procedures, this article presents an algorithm that allows the paper point technique to work consistently. Subtleties that need to be recognized are also reviewed.
Paper point length measurements are extremely accurate. In order to determine an accurate paper point working length, the canal needs to be patent. Patency is defined in Stedman’s Medical Dictionary as the state of being freely opened or exposed.1
|Figure 1. Electronic apex locators distinguish between pulp and periodontal ligament. The author finds that when the electronic apex locator reads that it is at the apex, the file is actually slightly protruding beyond the cavosurface of the tooth.|
In developing patency, the minimum distance that an instrument must negotiate to be at or slightly beyond the apical cavosurface of the canal must be determined. The electronic apex locator (EAL) is what gives us this information. When the EAL reads that we are at the apex, the tip of the file is actually protruding slightly beyond the apical cavosurface of the canal (Figure 1).
The EAL is describing where we don’t need to obturate beyond. It also tells where a file must go to in order to confirm patency. Rarely is there a need to instrument beyond EAL length. Obturation past the EAL length into the PDL, bone, sinus, or granuloma is not required for resolution of endodontic disease.
|Figure 2. In this diagram, the file is binding along most of its length. When the surface area of the file in contact with the canal is excessive, the amount of force required advancing the file increases to the point where separation of the file is possible.||Figure 3. A larger diameter file or greater tapered file binds short of the apical progress of the previous file. Balanced forces can now be used to remove this restrictive dentin.|
|Figure 4. In a relatively straight but occluded canal, removing restrictive coronal dentin with balanced forces and recapitulation with smaller files will allow apical progress to proceed to the canal cavosurface.|
To begin the process of obtaining patency, a 15 K file needs to be negotiated to EAL length. It is understood that the file will be slightly overextended. There are many ways to get this initial 15 K file to EAL length. Simply start by attaching a 10 K file to the EAL after the file is placed in a canal filled with Glyde (DENTSPLY Maillefer). Use balanced forces to take the 10 K file to EAL length. If the 10 K file doesn’t advance to EAL length, analyze why not. Is there a curve impeding progress, or is the canal full of restrictive dentin? In an occluded canal, when one file fails to advance when using balanced forces, using a smaller file will result in apical progress. Recapitulation using balanced forces will eventually allow a 15 K file to attain EAL length (Figures 2 through 4).
|Figure 5. In a canal with moderate curvature, a small, straight, flexible file may negotiate the curve with ease.||Figure 6. In the same canal, a slightly larger straight file, which is not as flexible as a smaller file, will often not negotiate the curve.|
|Figure 7. If the larger straight file is attempted to be forced past the curve, the most likely result will be a ledge and a poor result.||Figure 8. The endobender (Sybron Dental Specialities) can be used to precurve files made of stainless steel or NiTi. Precurved files can be used with the endo dance to negotiate curved canals. In the “endo dance,” the precurved file is advanced until apical progress stops. It is then withdrawn 2 mm, rotated 10°, and reinserted. If the file advances beyond its initial depth, it has started to negotiate the curved canal. If the file does not advance, the process of withdrawing, rotating, and reinserting is continued until the canal can be negotiated.|
|Figure 9. In this case, a NiTi GT file was precurved and, with balanced forces, was advanced to the cavosurface of this canal exhibiting multiple planes of curvature.|
When a curve or ledge is impeding apical progress, using balanced forces with smaller files usually will not allow the file to advance. Sometimes smaller files will advance around the curve or ledge with balanced forces, and then the next larger size file will not progress. The danger here is that greater force is applied to this file, forcing it along an iatrogenic path, creating a ledge or exaggerating an existing one. If the canal is curved or ledged, do the “endo dance” with a file precurved using the Endobender (Sybron Dental Specialities) (Figures 5 through 9).
Another technique for getting to EAL length is to first open the coronal two thirds of the canal with rotary instruments, to remove coronal interferences and ease the progress of the initial K files to length.
Whatever method is employed, the goal remains the same: to establish a pathway that will lead to patency.
Using the EAL, the maximum length for the canal has been determined. A 15 K file has been taken to this length. Now patency can be developed.
To believe that once a 15 or 20 K file goes to length the case is near completion or that the canal is patent is wishful thinking. Being able to place a stainless steel file through the apical foramen of the tooth is not the same as having a clean and patent canal that is ready to obturate. The stainless steel file has the rigidity and sharpness required to pierce through pulp and debris, perhaps giving a false sense of patency in an underprepared canal.
Rotary instruments are very efficient for the process of canal enlargement and debris removal, which are required to attain patency. Personal preference dictates using ProTaper (DENTSPLY Maillefer) and GT files (Dental Educational Laboratories) for this. Generally, a ProTaper S1 followed by a ProTaper S2 are each taken 0.5 mm short of EAL length. Next, a GT rotary file that is appropriate for the particular canal is taken to the same length.2 These rotary instruments are taken 0.5 mm short of EAL length because experience has shown that the EAL reading is usually between 0.5 and 1 mm long. Overinstrumentation can be avoided this way. If it turns out that 0.5 mm short is too short, we can always increase our working length.
Debris removal along with creating a patent canal with a shape that can easily be obturated with control are our desired objectives. An indication that debris removal is progressing toward the maximum that our instruments can achieve comes from the flutes of our GT rotary instruments. When the flutes are inspected after having been taken 0.5 mm short of EAL length and are clean, this is a promising sign that progress towards patency is occurring. If the flutes are covered with debris, it can be inferred that there is still more debris in the canal. This debris will interfere with attaining and maintaining patency. The GT file is used until the flutes come out clean after placement to 0.5 mm short of EAL length in a wet canal. At this point, the file is not working as a cutting instrument, but in a debris removal capacity.
|Figure 10. This file is typical of what can be removed from the most apical portion of the canal after instrumentation and irrigation are thought to be complete. This debris tends to settle in the apex of the canal and needs to be removed.|
When the GTs come out clean, it is time to check the most apical portion of the canal for debris. Settling of debris at the apex can occur because of the pumping action of repeatedly inserting files into the canal. To remove this debris, a 15 K file is inserted to EAL length, rotated 90º, and then removed. The apical last few millimeters of the file will probably be filled with debris (Figure 10). The 15 K file is recapitulated until its flutes are clean. The canal is now ready to be checked for patency.
|Figure 11. In a curved canal that is properly prepared, a gutta-percha cone should slide effortlessly through the apical foramen.|
The definition for patency is that the canal is freely open. When a gutta-percha cone that corresponds to the taper of the largest greater taper instrument taken to 0.5 mm short of EAL length is placed in a freely open canal, it should pass through this space unimpeded. Ideally, the cone will be overextended effortlessly (Figure 11). The cone should be vertically pumped a few millimeters several times while in the canal to agitate the irrigants and aid in debris removal. The cone is then withdrawn and inspected. If the cone is retrieved without kinks after being placed long, the canal is patent at this time.
|Figure 12. When a ledge is present, cone fit is difficult. The cone does not advance past the ledge, but buckles at the ledge.|
If the tip of the cone is wrinkled, there are several possible explanations. If the apex is blocked or a ledge is present, the cone will begin to buckle at the level of the interference (Figure 12). To check this, grasp the cone 0.5 mm short of EAL length and insert and withdraw it. If the cone is wrinkled, somewhere in the canal is an impediment to patency, and the canal preparation needs refinement.
|Figure 13. When the apex is not patent, the gutta-percha buckles at the point of contact with the apical impediment.|
Another reason for the cone to be wrinkled is that after passing through the foramen, it encountered tissue that deflected the cone. This cone will have its overextended tip wrinkled, but the portion of the cone that was within the canal should be smooth (Figure 13). To check, take another cone and cut it back just short of where the wrinkles begin. This cone should still be able to be placed long, but do not place it as far as before. If the cone now goes long, but comes back smooth, the canal is patent.
|Figure 14. A patency file is used to remove debris from the apex. It should not enlarge the canal, but should be large enough to effectively engage and remove apical debris.|
Once patency is achieved, we can determine the size of a patency file used to maintain patency. The maximum size of this patency file is related to the minimum apical foramen diameter (MAFD). The larger the MAFD, the larger the patency file can be, and vice versa. The patency file ideally should not engage the canal wall along its entire length. Most often a 15 K file is used to maintain patency (Figure 14).
LENGTH AT LAST
Once the canal is patent, the actual length from a reference point to the apical cavosurface of the canal can be determined. Paper points are used to make this measurement to within 0.25 mm accuracy.
|Figure 15. In a dry canal, the paper point should remain dry if it is short of the cavosurface of the casnal.||Figure 16. When the paper point is taken to the cavorsurface of the canal, it will absorb whatever fluid exists at this level.|
The canal is dried with suction and paper points. A paper point of lesser taper than that to which the canal has been prepared is inserted into the dried canal to EAL length. If the tip of the paper point is wet, a new paper point is placed 2 mm short of EAL length. If this point is dry, advance the point until it is just moist. Ideally, when a new paper point is placed 0.25 mm short of this length, the point will remain dry. Advancing the point another 0.25 mm will result in the point again being moist (Figures 15 and 16). This wet/dry interface coincides with the location of the cavosurface of the canal. Where the paper point is dry, it is inside the canal; where the paper point is wet, it is outside the canal. In order for the measurement to be reliable, it should be reproduced at least 2 times for each canal.
Unfortunately, paper points do more than just dry the canal. They can also act as a plunger, advancing debris towards the apical foramen. If a paper point length determination is attempted in a canal where debris can still be removed with GT files, success is unlikely. This debris can become packed into the apex and desiccated through contact with paper points. This can be deduced as happening when a wet/dry interface cannot be obtained. In this situation, the paper point always comes back dry (Figure 17). This desiccated debris causes a loss of patency. It is easily corrected by placing a patency file to EAL length, rotating it 90º, and thereby removing the debris that was preventing the paper point from contacting the extraradicular tissue (Figure 10). This apical clearing should be done in the presence of EDTA, followed by irrigation with NaOCl. The EDTA will remove the inorganic component of the smear layer that comes from debris in the canal and open up the dentinal tubules. The NaOCl will remove the organic portion of the smear layer. Given enough time to work, the NaOCl will remove the organic matter in the tubules.
|Figure 17. Paper points can desiccate, drive, and compact debris into the apical foramen. Paper point length measurements will not work in this environment. Patency must be established in order for accurate paper point length determination.||Figure 18. The 30 K file binds at the cavosurface of the canal and represents the MAFD. The 20 K file is smaller than the MAFD and can be taken through the apical foramen.|
The environment in which the canal is being treated allows for debris that is suspended in solution to find its way to the apex, causing a temporary loss of patency. It is easy to correct this problem if we are aware of it. Once the canal is patent, paper points can be used for accurate length determination. Once an accurate length measurement is attained, half of the information needed to provide consistent high-quality endodontic care is known. The other half of the information, the minimum apical foramen diameter (MAFD) at this length, can now be determined.
To determine MAFD, 0.02 taper standardized K files are used to gauge the MAFD at working length. The files are inserted without rotation to paper point length, and then apical pressure of about 7 pounds is applied. If the K file goes long, it is smaller than the MAFD. The K file that binds in the canal at paper point length closely represents the MAFD. If one file goes long and the next file is short of paper point length, the larger file can be advanced to paper point length and then represent the MAFD (Figure 18).
|Figures 19 and 20. Complex anatomy can be addressed using the techniques described in these articles. No working films were required to achieve these results.|
Once the canal length and MAFD are known, it is possible to develop the apical control needed for consistent and reproducible quality endodontic care. Creating an apical capture zone can further enhance apical control. When the variables of length and diameter are known and controlled, obturation becomes more predictable and embarrassing outcomes can be eliminated. Fear of overfilling by using too much pressure during obturation is eliminated (Figures 19 and 20).
In this 2-part series, a process for attaining extremely accurate canal length and foramen diameter measurements has been described. With all new techniques, there is a learning curve, which can be frustrating. Having taught this technique for several years and seeing the more consistent results achieved by my course participants has made me believe that this technique can be used successfully by anyone who takes the time to learn it.
1. Stedman’s Medical Dictionary. 25th ed. Baltimore, Md: Williams and Wilkins; 1990.
2. Buchanan LS. The standardized-taper root canal preparation, part 2: GT file selection and safe handpiece driven file use. Dent Today. 1999; 18(10):68-76.
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.
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