Endodontic Treatment Advances: Apical Cleaning and Apical Shaping Limits

Dr. Carlos A. Spironelli Ramos

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Figure 1. Examples of modern endodontic devices that have been embraced by endodontists.

INTRODUCTION
A reasonable and prudent general practitioner (GP) must keep up to date with the available advances accepted and embraced by endodontists through research, evidence, and experience (Figure 1). The overwhelming majority of endodontists respect their GP dental colleagues and, although it may seem contradictory at first, wish to see the standard of endodontics performed at as high a level as possible by both sets of clinicians. A survey1 about the major factors concerning the GP’s decision to perform endodontics showed that 9 in 10 GPs say the degree of confidence in their ability to perform the procedure, or the relationship that they have with their patients, are key factors when deciding whether to perform or refer a procedure, and 8 in 10 GPs say the extent to which they enjoy the procedure or the type of equipment/technology needed for it are just as important. It seems reasonable to assume that, since the economy took a downturn in 2008, GPs would want to take on more endodontic cases in their own practices rather than referring out to endodontists, but that isn’t the case. Fewer than half of GPs say the economics of the treatment or their preference for nonendodontic treatments are important factors in their decisions.

Savani et al2 found that many of the 84% of GP respondents in the United States who perform endodontic treatment have widely adopted newer technologies as part of their endodontic practices. Therefore, developments in technology and materials continue to influence the practice of endodontics and have had a considerable impact on the way root canal treatment is practiced by both endodontists and GPs.

To increase endodontic treatment success globally, it is vital to discuss contemporary endodontic procedures, techniques, devices, instruments, and materials that have been adopted by endodontists alongside an intuitive, streamlined technique that can be incorporated by GPs. This article will discuss the importance of correctly determining (1) the apical cleaning limit to clean the canal, and (2) the shaping limit in order to keep the obturation in a biologic level for periapical healing.

Apical Cleaning and Apical Shaping Limits
Endodontic clinical practice is subjected to some paradoxes. One of these discusses the apical extent of root canal filling. Using an orthodox assessment, the result of endodontic treatment is first evaluated by its apical position, despite the general knowledge that the location of the apical foramen does not invariably show at the radiographic apex. However, the majority of clinicians routinely resort to this location to determine the final quality of the treatment, contributing to the importance of correct identification and maintenance of the working length in endodontic treatment (Figure 2). Determining the working length is one of the earliest steps in endodontic therapy and refers to the measurement of the canal length. This makes it possible to identify the necessary references to establish the apical limit of cleaning (debridement) and apical shaping (enlargement) limit—each one in a different position.

Figure 2. The clinical sequence in the right mandibular first molar. (a) A preoperative radiographic image for diagnosis and treatment planning. (b) A radiographic image immediately after filling with extrusion of sealer, indicating a lateral foramen exit of the distal canal, away from the anatomic root apex. (c) A radiographic image 30 days post-treatment, showing resorption of the extruded sealer. An analysis of the follow-up radiograph might lead one to criticize the filling as being “short.”
Figure 3. Apical shaping and cleaning limits (SEM, 40X [JEOL Electron Microscope]).

The apical cleaning working length determines the distance to which the initial small-diameter (8, 10, and 15) instruments may penetrate into the root canal and, consequently, provide the level of depth needed for the tissues, impurities, metabolites, and material(s) to be removed. The file’s action is not to clean the last 1 mm of the canal, but to debride this area, leaving an open space for the irrigants to interact with the canal content by capillary action. This is an important landmark to perform the apical patency, a technique where the apical portion of the canal is maintained free of debris by recapitulation with small-size files through the apical foramen.

The apical shaping limit (Figure 3) identifies the depth that canal shaping and filling procedures may reach during enlargement and obturation. Among other factors, it may positively or negatively affect the healing process.

Determining the Apical Limits
The high resolution of micro-computed tomography (µCT) portrays the apical anatomy in detail and offers accurate 3-D measurements of the location of the foramen without the destruction of the tooth. The µCT studies reveal that the most common form of constriction is the parallel form (meaning “no constriction”). The traditional constriction, with the canal flaring apically and coronally from it, is present in only 10% of the canals,3,4 and its average position is 0.2 mm off the major foramen. These anatomic studies also show that the topography of the canal varies throughout the longitudinal section and that the smallest diameter indicates the narrowest area only in perfectly round canals, which rarely occur naturally.

Figure 4. A representation of the file tip outside the canal and the corresponding image.

Foramen Position Determination Techniques
Several techniques have been scientifically described and assessed to determine the working length; among them are digital tactile sense, radiographic methods, cone beam computed tomography (CBCT) and the electronic method—each used alone or combined.

The methods that use radiographic image interpretations have limitations due to distortions, anatomic interferences, and objects pertinent to endodontic treatment. There are also restrictions with respect to the fact that it is a 2-D image of a 3-D object, making it impossible to visualize the apical foramen (Figure 4), thus leading to subjective interpretation by the operator. Although it is the most widely used working length technique among GPs, the radiographic method has several limitations that tend to diminish its precision and reliability. The final quality of the radiograph is linked to several variables, including (1) the correct positioning of the film in regard to the object being radiographed, (2) the correct angle of the x-ray beam, (3) interference from anatomic structures or the equipment used to isolate the operating field, (4) the exposure time to radiation, and (5) adequate radiographic processing.

Figure 5. A variation of the voltage gradient in the root canal. The ordinate and abscissa provide the potential difference between the poles of the bipolar electrode and the distance of the electrode to the apical foramen, respectively. Positive distances indicate that the tip of the electrode is beyond the foramen, while for the negative distances it is inside the canal.

Studies have shown that electronic apex locators are a reliable tool to accurately locate the apical foramen. Though some authors recommend debriding and shaping the entirety of the canal and shaping and filling to the major apical foramen, this can cause material to extrude past the apex and into the periradicular tissue, as one cannot create an apical stop at the foramen level. The obturation material must be kept in a biologic limit. Apical limit studies demonstrate that filling materials extruding beyond the radiographic apex correlate with a decreased prognosis.5 From these studies, we can determine that it is better to debride until the foramen exit, or the apical cleaning working length, and then create an apical stop and obturate 0.5 to 1 mm short of the foramen, or the apical shaping working length.

The Development of Electronic Measurement
Over the past few decades, the electronic method of determining the apical major foramen has been studied and improved.6,7 Since the first experiments by Sunada,8 the electronic method has shown appreciable technological advances. This method has evolved to the point where we are now able to take electrical conductivity readings in root canals that contain irrigation solution. In recent years, studies assessing the electronic method have indicated that electronic apex locators have found a prominent place in endodontic research.

Figure 6. An appropriate level of irrigant inside the canal facilitates the achievement of a good electronic identification of the foramen position. Figure 7. A 30 .08 Genius File Orifice Shaper (Ultradent Products) should be used in the straight part of the canal in rotary with 300 rpm to preflare the canal prior to the electronic measurement.
Figure 8. Use the right apical tip diameter to ensure that the tip of the file is touching the last millimeters of the apical dentin canal walls.

The early devices showed success rates that were lower than or comparable to radiographic techniques. However, impedance-frequency-dependent, third-generation electronic apex locators are now able to establish the foramen position with an accuracy of ±0.5 mm under different clinical conditions in more than 92% of the cases.9 In addition to being more precise, the electronic method is safer for the patient and more convenient for the clinician because it reduces treatment time, diminishes a patient’s exposure to ionizing radiation, and is easier to use with patients who have difficulty opening their mouths. Because they are less subjective than radiographs, third-generation electronic apex locators also present greater reproducibility of measurements when used correctly.

The apex locator determines the position of the foramen exit by measuring electrical resistance when a direct current is applied, the electric impedance of signals with only one spectral component, or multi-frequency signals between an electrode inserted inside the root canal and another by the lip (Figure 5).10

Although they have different features (eg, the appearance of the equipment, operating interface, marking points on the screen, types of batteries used, and accessories), the third-generation devices that are commercially available operate on essentially the same impedance-frequency-dependent principle.

Precautions During Electronic Measurements to Overcome False or Absent Readings
Some points must be observed during electronic measuring, irrespective of the model used:

  • It is important that the canal contain an irrigation solution, while the pulp chamber should not contain an excess of it (Figure 6).
  • The presence of inflamed pulp in the path of the root canal to be measured makes it difficult to perform electronic measurements. Clinically, these electronic measurements are easier when measuring a root canal with necrotic pulp contents or even in retreatment cases.11 Therefore, a partial pulpectomy is recommended, followed by abundant irrigation with sodium hypochlorite solution, to allow the canal to be measured without interference from the inflamed pulp in the root canal.
  • The endodontic instrument selected to explore the undebrided apical portion of the canal and the associated electronic working length must be 5 mm longer than the temporary working length, which was measured on the initial radiograph. This is due to the need for available space to place the file clip between the rubber stop and the instrument cable.
  • Introduce the file apically by turning it gently in a balanced-force action in the direction of the apical foramen while perceiving the start of movement on the display, which will give the exact speed of penetration of the instrument into the canal.
  • It is important to remove any metal crowns or large amalgam restorations before beginning the procedure, as they can short circuit the electronic device. A rubber dam is a standard of care in endodontic treatment and mandatory for electronic readings.
  • Acquire a reliable initial radiograph, preferably with the parallel technique, using an XCP System locator (the cone is lined up with a ring outside the patient’s mouth while he or she bites down on the radiographic film holder). The temporary working length, measured from a proper initial radiograph, normally differs from zero percent to 15% from the definitive shaping working length.
  • Preflare the canal in rotation with high-tapered instruments—for example, a 30 .08 Genius File Orifice Shaper (Ultradent Products) (Figure 7).
  • The size of the file used for measurement must match the anatomic diameter of the canal (Figure 8). Thin instruments make tip contact at the apical third and electronic reading difficult due to the lack of control over penetration and may give a false positive result. Larger-size instruments will not reach the apical third.
  • The foramen position is shown at 0.0 (Figure 9). This will be the apical cleaning working length. Use the endodontic ruler to determine the apical shaping limit, which is 0.5 to 1.0 mm short of the foramen. The intermediate numbers do not reflect the position of the file in relation to the apical foramen.
  • In some cases, the foramen may be in a position that doesn’t allow the file to reach the exit. In this situation, insert the file to the .5 mm point and take a radiograph to confirm the position of the file in relation to the apex (Figure 10).
  • In cases of an immature open apex, advanced apical resorption, or overdebridement, the foramen shape can be compromised, thus changing the electronic measurement of the root canal. The variation in impedance of the dentinal wall of the apical third will be reduced,12 resulting in apparently shorter readings (Figure 11). The current flow in that location is altered, providing voltage gradient values closer to the values of the apical periodontal ligament. This interferes in the reading of impedance variation.

The Most Common Myths About Apex Locators
I need to send my apex locator back to be serviced because it is not calibrated.

Myth. There is no calibration in an apex locator. The device uses firmware: a computer-based program that has no interaction with the operator. If the device is not working in some cases but working in others, it is usually much more a matter of technique or that the specific case was a challenge in terms of electronic foramen determination.

The canal needs to be dry during the measurement.

Myth. If you are using an old resistance-based apex locator, this is true. The new third-generation devices work better in presence of an electrolytic liquid, so the canal needs to be filled with irrigant. There must be electrical conductivity in the canal environment. The better the signal passage, the easier the measurement. Therefore, the canal should preferably be wet, and the tip of the instrument should be as best suited as possible to the canal walls in the apical third.

The apex locator shows the apical constriction position (or the apex, or the CDJ).

Figure 9. A file tip at different positions and respective electronic measurements (Endo-Eze FIND Apex Locator [Ultradent Products]).
Figure 10. Lateral exit of the foramen, obstructing the file to reach the foramen exit.
Figure 11. In the clinical case, apical resorption resulted in a change of the working length. (a) A pre-op radiograph is shown of a left maxillary central incisor with a periapical lesion and is suggestive of root resorption. (b) The Postoperative radiograph. Note that the shaping limit, calculated by subtracting 1.0 mm from the foramen position determined by the apex locator, promoted the establishment of the apical stop, allowing the filling material to remain at an adequate length.

Myth. The foramen is the position in which the impedance drops during the measurement. The apical constriction is more a histologic landmark (when it is present) and has no influence in the impedance values. The foramen position should be marketed as the cleaning working length. The shaping working length is 0.5 to 1 mm shorter of the foramen.

I cannot use an apex locator in patients with a pacemaker.

True. There is no conclusive evidence showing that using an apex locator in patients with a pacemaker will not affect the pacemaker.

Taking a Radiograph to Check the Electronic Measurement
Although electronic measurements are more accurate than radiographs, it is still important to take a radiograph to verify the accuracy of the method. In this regard, there may be a conflict of objective, but not of procedure. A radiograph must be performed after the electronic working length measurement, even if the basic purpose is not necessarily to confirm the accuracy of the apical limit established by the electronic reading. Radiographs have to be made after electronic working length measurements in order to visualize the direction of the canal(s). The radiographic image of an instrument in a canal allows the clinician to more easily identify details concerning the angle and radius of the curvature, arch length, dilacerations, and thickness of the dentin walls. The radiograph also shows the location of the tip of the instrument in relation to the radiographic apex.

A radiograph with an instrument in situ shows details, in addition to the details shown by the diagnostic image, that allow the identification of situations requiring different technical procedures. This step is invaluable, as it enables a novice operator to evaluate the technical expertise required and will improve his or her confidence in using electronic working length measurements.

CLOSING COMMENTS
It is becoming more common to see general practitioners performing endodontic treatments in their daily clinical practice. Technologies embraced by endodontists over the years are now more accessible to clinicians, and the application of these new techniques, together with the correct assessment of the difficulty of each case, are essential to reach successful cases.


References

  1. American Association of Endodontists GP Referrals Study Final Research Report. Chicago, IL: L.C. Williams & Associates; December 2009.
  2. Savani GM, Sabbah W, Sedgley CM, et al. Current trends in endodontic treatment by general dental practitioners: report of a United States national survey. J Endod. 2014;40:618-624.
  3. ElAyouti A, Hülber-J M, Judenhofer MS, et al. Apical constriction: location and dimensions in molars—a micro-computed tomography study. J Endod. 2014;40:1095-1099.
  4. Schell S, Judenhofer MS, Mannheim JG, et al. Validity of longitudinal sections for determining the apical constriction. Int Endod J. 2017;50:706-712.
  5. Ricucci D. Apical limit of root canal instrumentation and obturation, part 1. Literature review. Int Endod J. 1998;31:384-393.
  6. Custer LE. Exact methods of locating the apical foramen. Journal of the National Dental Association. 1918;5:815-819.
  7. Crane AB. Discussion of nature of methods of making perfect root fillings. Dent Cosmos. 1921;63:1039-1040.
  8. Sunada I. New method for measuring the length of the root canal. J Dent Res. 1962;41:375-387.
  9. Ramos CAS, Bernardinelli N. Influence of the diameter of the apical foramen on the reading accuracy of an electronic apical locator model [in Portuguese]. J Appl Oral Sci Revista FOB. 1994;2:83-90.
  10. Rambo MVH, Brochado VHD, Medeiros MA, et al. New electronic foraminal locator in frequency domain [in Portuguese]. IFMBE Proc. 2004;5:649-652.
  11. Yamaoka M, Yamashita Y, Saito T. Electrical root canal measuring instrument based on a new principle makes measurements possible in wet root canals. Osada Product Information. 1989(6):12.
  12. Iizuka H, Hasegawa K, Takei M, et al. A study on electric method for measuring root canal length. J Nihon Univ Sch Dent. 1987;29:278-286.

Dr. Ramos graduated with a degree in dentistry from the State University of Londrina (1987) in Brazil. He is a specialist and PhD in endodontics and a former endodontics sector coordinator at the State University of Londrina. He has had 3 endodontic textbooks published and wrote more than a dozen chapters for various endodontics books. He accomplished all of this while working part time exclusively as an endodontic specialist at his private office from 1987 to 2012. Since 2012, he has been the Ultradent R&D and clinical affairs endodontic manager. He can be reached via the email address carlos.ramos@ultradent.com.

Disclosure: Dr. Ramos is the clinical affairs endodontic manager for Ultradent Products.

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