Endodontic access
By Bradford R. Johnson, DDS, MHPE
Featured in General Dentistry, November/December 2009
Pg. 570-577
Posted on Friday, November 06, 2009
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Access preparation may be the single most important mechanical phase of root canal therapy. This article reviews the essential components of good access design and research intended to help guide treatment planning and clinical treatment, with an emphasis on using available information to develop a three-dimensional mental image of the tooth prior to endodontic access, allowing for safe and efficient endodontic therapy. Guidelines for developing an ideal access form, locating small and calcified canals, and avoiding iatrogenic mishaps are presented. Specific tools for efficient access preparation are identified and explained; in addition, case selection and indications for possible referral to a specialist are discussed.
Received: May 14, 2009
Accepted: July 14, 2009
In the rush to begin treatment, it is possible to overlook the need for proper pretreatment evaluation and planning for an ideal endodontic access. This omission is unfortunate, since spending a little additional time perfecting the access preparation can result in significant time savings during subsequent phases of root canal therapy. Careful pretreatment evaluation also can help to avoid most iatrogenic mishaps and may suggest when a case needs to be referred to a specialist prior to initiating treatment.
The ideal endodontic access preparation should unroof the pulp chamber completely, conserve tooth structure as much as possible, provide straight-line access to at least the coronal half of each canal, and avoid iatrogenic mishaps such as perforation.
Access preparation has been described as the single most important mechanical phase of root canal therapy.1 Inability to locate all canals is a major cause of treatment failure.2 For purposes of defining the limits of this article, endodontic access includes initial entry into the coronal pulp chamber, shaping the access preparation, and locating all root canal orifices.
Principles of ideal access form: Developing a three-dimensional mental image
Although standard dental radiographs offer two-dimensional views of a three-dimensional object, the use of multiple angled radiographs and the addition of a bitewing radiograph can provide valuable information about the three-dimensional aspects of the root canal space.2,3 Cone beam computed tomography (CBCT) can provide even more information but currently is not commonly used as part of a routine preoperative examination. The use of CBCT for endodontic diagnosis and treatment planning will almost certainly become more routine in the near future.4,5
Even though each tooth is unique, several characteristics have been identified that are surprisingly common to all teeth. Krasner and Rankow have identified three general rules for determining the relationship of the pulp chamber to the clinical crown: the law of centrality (the floor of the pulp chamber is always located in the center of the tooth at the level of the cementoenamel junction (CEJ)), the law of concentricity (the walls of the pulp chamber are always concentric to the external surface of the tooth at the level of the CEJ) (Fig. 1), and the law of the CEJ (it is the most consistent, repeatable landmark for locating the position of the pulp chamber) (Fig. 2).6
Understanding these three basic rules will allow clinicians to develop a three-dimensional mental image of the pulp space for any tooth prior to initiating access. It may be helpful to visualize the external anatomy of the tooth at the level of the CEJ; that is, the occlusal outline form should follow the shape of the tooth at the CEJ (Fig. 1) and not necessarily the shape of the occlusal table. The cross-sectional outline of a tooth at the CEJ provides valuable and consistent information regarding the location and shape of the pulp chamber, even when the teeth are broken down or heavily restored.
The floor of the pulp chamber is always darker than the surrounding dentin and canal orifices are located either at the floor/wall junction or where two of the pulp chamber walls meet.6 Reparative dentin typically is lighter than the darker pulp floor and often obscures the canal orifices (Fig. 3). With increasing age and multiple restorative procedures, the vertical dimension of the pulp chamber of posterior teeth decreases from 2.0–3.0 mm (or more) to 0.5 mm or less, reducing the target area and therefore decreasing the dentist’s ability to feel the drill bur “drop” into the pulp chamber. A bitewing radiograph is probably the most useful image for determining the location of the floor of the pulp chamber, since this type of radiograph provides a clearer view of the relationship between the pulp chamber and external tooth anatomy than a standard periapical radiograph.
As a general rule, the roof of the pulp chamber can be found at the level of the CEJ in mandibular and maxillary molars, even when extensive pulp space calcification is present.7 This finding may be somewhat surprising, since many dental textbook illustrations show the pulp chamber extending well into the clinical crown of maxillary and mandibular molars. Although pulp horns may extend above the CEJ, the roof of the molar pulp chamber is found at the same level as the CEJ more than 95% of the time; by comparison, premolars and incisors often demonstrate a pulp chamber coronal to the CEJ.7
Understanding this anatomical relationship allows dentists to measure bur length prior to treatment and thus decrease the possibility of a furcal floor perforation. Vertical penetration to the level of the CEJ is relatively safe, as long as the bur is properly directed toward the center of the pulp chamber. If the pulp chamber has not been located at this level, stop and take at least two radiographs (a standard bitewing and one or more angled periapical images) before proceeding (Fig. 4). To properly visualize the access preparation in the area of the CEJ, it often is necessary to occlude the access temporarily and remove the rubber dam prior to taking a radiograph.
After the pulp chamber has been located and unroofed completely, all canals should be located. Although each tooth should be considered unique, anatomical studies can be used as a general guide for determining the expected number and location of canals for each type of tooth (see the table). If root fusion lines are present, orifices will be found at the end of these lines.6 It is common for reparative dentin to obscure the canal orifices either partially or completely. Reparative dentin is lighter in color than the pulp chamber floor and can be removed with low-speed burs, ultrasonics, or a small spoon excavator.
With the exception of maxillary molars, the internal anatomy of most teeth is remarkably symmetrical. For example, an imaginary line drawn through the middle of a mandibular molar in a mesiodistal direction (Fig. 5) can be a great aid in locating undiscovered canals. If a canal has been found that is not located along the midline, one can expect to find another canal on the opposite side of the midline and approximately equidistant from the midline. Mandibular second molars occasionally have only one canal in the mesial root; in such instances, the mesial canal will be similar in size to the distal canal and will be centered in the root in the facio-lingual dimension.
Tools for access
Most access preparations can be performed with a relatively basic selection of tools and instruments. The benefits of enhanced magnification and illumination cannot be overemphasized. Magnifying loupes and a high-intensity light source should be the minimum standard for endodontic access, even with teeth that appear uncomplicated. Many endodontists would not be comfortable searching for a receded pulp chamber or calcified canal without the benefit of a dental operating microscope. The combination of increased magnification and light in the access preparation allows dentists to identify subtle differences in dentin color and texture, reducing the risk of perforation and increasing the probability that all canals will be located.8-11
Before accessing the pulp chamber, all caries should be removed completely to help reduce the potential for microbial contamination of the root canal space in otherwise uninfected teeth and to help the dentist evaluate the restorability of the tooth. Removing the existing restoration will improve visibility and prevent accidental dislodgment of restorative materials into the canal space during root canal therapy (Fig. 6). Removing the entire restoration may compromise the dentist’s ability to isolate the operative field; in these cases, it is preferable to leave the existing restoration (assuming there is no visible caries) unless it is feasible to remove it and place an interim build-up restoration that would allow for aseptic isolation.
Occlusal reduction is recommended for most posterior teeth (those without existing full occlusal coverage restorations) and reduces the risk of fracture prior to full-coverage restoration. Occlusal reduction also creates a reliable, flat reference point for canal length determination and may decrease the incidence of postoperative pain.12
With most teeth, the access preparation is started at the center of the occlusal table and directed toward the most prominent pulp horn and/or largest canal. One notable exception is the maxillary molar, where the pulp chamber is located mesial to the center of the occlusal surface (Fig. 3). It is worth noting that the occlusal anatomy of teeth with full-coverage restorations does not always reproduce the natural anatomy of the tooth accurately. To ascertain position in the pulp chamber, it is necessary to examine these teeth carefully after initial penetration of the pulp space.
Although the bur selected for initial access is largely a matter of personal preference, one relatively safe strategy involves initially penetrating the pulp chamber with a small round or straight carbide fissure bur, followed by expansion and refinement of the access with a safe-ended tapered fissure bur. Several burs with longer, tapered cutting surfaces have been designed especially for endodontic access. These burs are efficient but it should be noted that they are not safe-ended and therefore pose a greater risk of creating a perforation during access. Diamond-coated burs (used with copious water coolant spray) are recommended for access through porcelain restorations.
Access preparation is improved by the use of special ultrasonic instruments to refine the access preparation, to remove pulp stones and calcifications, and to trough for hidden canals.13 Piezoelectric ultrasonic units are most appropriate for endodontic applications. When searching for calcified canals, the relatively small size of an ultrasonic tip (compared to a low-speed bur and handpiece head) allows for enhanced visualization and control. Diamond-coated tips cut dentin more efficiently than either stainless steel or zirconium nitride-coated tips but also may be more prone to breakage.14 Special long-shank low-speed burs have been designed to assist dentists in troughing for calcified and hidden canals.
The DG-16 endodontic explorer is used most commonly for locating canals. The CK-17 (CK Dental) is a slightly thinner and stiffer version of the standard endodontic explorer and may be useful for locating small or calcified canals.
Special cases and common access errors: Prevention and management
Inclination of teeth
Understanding the natural inclination of a tooth in the alveolus is key to preventing lateral perforations during access. For example, due to the facial inclination of maxillary incisors, the most common perforation location is the facial surface; Figure 7 offers an example of such a perforation. When accessing a maxillary incisor with an intact clinical crown, it is highly unlikely that the dentist will perforate the palatal surface. As proper mesial-distal positioning also is critical, the bur should be directed to the palatal and carefully advanced apically. Mandibular incisors demonstrate a similar facial inclination and share a similar risk of perforation, although the narrow mesial-distal width means that the most common perforation site for these teeth is mesial or distal.
It is not always obvious that mandibular molars are typically inclined to the lingual; as a result, perforation on the lingual is most common if the access is prepared perpendicular to the occlusal plane. Conversely, the crown of the mandibular premolar is inclined lingually but the root is relatively vertical in the alveolus; as a result, following the inclination of the crown can lead to perforation on the facial surface. Proper pretreatment evaluation of the tooth’s position and inclination can help to prevent a perforation during access. Figure 8 is an example of an inclined tooth that also serves as a long-span fixed bridge abutment and could be expected to present a high level of difficulty for safe endodontic access.
Unusual root anatomy
Variations in root canal anatomy are common and can make treatment more complex. Fortunately, most variations can be identified by using multiple angled radiographs in the pretreatment evaluation. For example, approximately 5% of mandibular second molars have a C-shaped canal (Fig. 9) and can present challenges during access, instrumentation, and obturation.15-17 Even though preoperative radiographs may not clearly demonstrate the true canal anatomy, the appearance of atypical anatomy on the radiograph should be obvious and a decision to treat or refer can be made before initiating root canal therapy.
The use of the dental microscope has allowed clinicians to notice canal configurations that previously had been considered extremely rare, such as the detection of a middle mesial canal (in addition to the expected mesiobuccal and mesiolingual canals) in the mesial root of a mandibular first molar (Fig. 10). The prevalence of a third canal in this root may be as high as 10–15%.18 Approximately 2–5% of maxillary first premolars have three canals (two buccal and one palatal).2
As a general rule, the presence of three canals should be suspected when the mid-root mesiodistal width of the tooth is greater than the mesiodistal width of the crown.19 The common presence of a second canal in the mesiobuccal root of maxillary molars is now common knowledge, although locating and negotiating the second canal can be challenging. The presence of a separate distolingual root in mandibular first molars is relatively uncommon but such roots can present a treatment challenge since they typically are small and often curve toward the facial (Fig. 11).
These examples are not intended to be all-inclusive; rather, they are intended to suggest that complex anatomy can be expected whenever the radiographic appearance of the root seems unusual.
When to perform initial access without the rubber dam
Use of the rubber dam for root canal therapy is the established standard of care.20,21 If it is not possible to place the rubber dam, the tooth is not a candidate for root canal therapy. However, when the canal space appears calcified, when the tooth is inclined or rotated, or when access must be performed through an existing crown or bridge abutment (Fig. 8), it will be easier to visualize the external crown and root anatomy (and therefore safer to initiate access) without the rubber dam in place. The rubber dam must be placed as soon as the presumed pulp chamber is located, prior to the insertion of endodontic files or irrigating solutions.
Accessing the wrong tooth is one of the more embarrassing and least defensible errors in dentistry. When adjacent teeth are similar in appearance (for example, mandibular incisors, maxillary premolars, adjacent teeth with full porcelain coverage restorations), the author recommends starting the access prior to placing the rubber dam.
When to remove a crown prior to access
Although endodontic access through an existing crown is not uncommon, removing the crown can provide significant advantages. First, it may not be possible to maintain adequate isolation if extensive caries is present at the crown margins. Removal of the crown followed by caries excavation and restorative build-up will improve isolation and also help to establish restorability of the tooth. Second, searching for a heavily calcified pulp space through an existing crown increases the risk for perforation, especially if the crown covers a rotated or inclined tooth. Finally, removing the crown should provide greater visibility of the tooth form, allowing for a more conservative access preparation and for the preservation of additional tooth structure.
Limited access
Patients with small mouths or limited opening can present a significant challenge, especially when attempting root canal therapy on posterior teeth. Bite blocks may be helpful but not all patients tolerate them equally well. High-speed handpieces with a smaller head (such as those typically used in pediatric dentistry) may allow for easier access. Since nearly all posterior teeth will require full occlusal coverage restoration after the completion of root canal therapy, performing occlusal reduction prior to access will increase the vertical working space. Additional reduction or even complete removal of the mesiobuccal cusp of maxillary and mandibular molars will improve access to the pulp chamber and root canal space.
Locating calcified canals
A thorough understanding of the consistent rules of root canal anatomy will help the clinician know where to locate canals. When the entire pulp space appears to be obliterated, it is often useful to search for the largest canal first (for example, the palatal canals of maxillary molars and the distal canals of mandibular molars). Locating one canal in a multi-rooted tooth often provides useful information regarding the depth and location of other canals.
Various burs and ultrasonic tips have been designed specifically for performing the deep troughing necessary to locate calcified canals. Special dyes and stains (for example, methylene blue) may be useful for identifying canal orifices and for highlighting cracks or fracture lines. Since sodium hypochlorite reacts with and dissolves pulp tissue, placing full-strength sodium hypochlorite in the pulp chamber and looking for small bubbles can help dentists to identify the location of calcified canals that still contain a small amount of pulp tissue but might not be visible otherwise.
These techniques require the additional magnification and illumination provided by a dental microscope. Figures 12 and 13 present two teeth with heavily calcified canals. It would be very difficult to safely and predictably treat teeth such as these without the benefit of a microscope.
Mandibular incisors
Mandibular incisors present a special challenge due to their narrow mesiodistal width and the challenge in achieving straight-line access via a lingual access approach.20 Figure 14 illustrates an initial access approach on a mandibular central incisor with a near-perforation. Fortunately, the clinician removed the rubber dam and obtained a radiograph to determine the location of the access preparation in relation to the canal. With this information, the access was redirected and root canal therapy was completed without iatrogenic mishap.
The routine access approach of choice for all incisors has traditionally been through the lingual aspect of the tooth, primarily for esthetic reasons. However, an anatomical study of mandibular incisors demonstrates that ideal straight-line access can be achieved only by performing an access that includes the incisal edge and, in many cases, extends to the facial surface.22 Extending the access to the facial provides better straight-line access to the lingual aspect of the root and is particularly important to assist in the location and treatment of frequently missed lingual canals in mandibular incisors.
Approximately 30% of mandibular incisors have two canals exiting from the pulp chamber, although more than 95% of these bifid canals merge and exit through one apical foramen.20,23-25 All mandibular incisors have greater faciolingual width than mesiodistal width; however, teeth with a relatively higher faciolingual:mesiodistal ratio are more likely to have two canals.26
Perforation during access
Even the most careful clinician can create a perforation during endodontic access (Fig. 15). The prognosis depends most directly on the size and location of the perforation and the dentist’s ability to prevent microbial contamination. An electronic apex locator can be extremely useful for identifying a small perforation (that is, the size of a small endodontic file) and managing it more predictably. Prevention of microbial contamination requires early identification of a perforation and sealing the area with an appropriate material. Currently, mineral trioxide aggregate (MTA, Dentsply Tulsa) is the material of choice for perforation repair.27,28 If the clinician has no training for managing this particular type of complication, referral to an endodontist is indicated.
Summary
Careful clinical and radiographic evaluation prior to treatment is the essential first step toward providing safe and efficient endodontic therapy. Specific rules and suggestions can assist clinicians in creating an ideal endodontic access form, locating all canals, and decreasing the risk of an iatrogenic mishap.
Disclaimer
The author has no competing interests or relationship with any of the manufacturers cited in this article.
Author information
Dr. Johnson is a Diplomate of the American Board of Endodontics and an associate professor and director of Postdoctoral Endodontics, Department of Endodontics, University of Illinois at Chicago College of Dentistry.
References
1. Walton RE, Torabainejad M. Principles and practice of endodontics, ed. 3. Philadelphia: W.B. Saunders Company;2002.
2. Cantatore G, Beruti E, Castellucci A. Missed anatomy: Frequency and clinical impact. Endo Topics 2009;15(1):3-31.
3. Slowey RR. Root canal anatomy. Road map to successful endodontics. Dent Clin North Am 1979;23(4):555-573.
4. Cotton TP, Geisler TM, Holden DT, Schwartz SA, Schindler WG. Endodontic applications of cone-beam volumetric tomography. J Endod 2007; 33(9):1121-1132.
5. Estrela C, Bueno MR, Leles CR, Azevedo B, Azevedo JR. Accuracy of cone beam computed tomography and panoramic and periapical 6. Krasner P, Rankow HJ. Anatomy of the pulp-chamber floor. J Endod 2004;30(1):5-16.
7. Deutsch AS, Musikant BL. Morphological measurements of anatomic landmarks in human maxillary and mandibular molar pulp chambers. J Endod 2004;30(6):388-390.
8. Buhrley LJ, Barrows MJ, BeGole EA, Wenckus CS. Effect of magnification on locating the MB2 canal in maxillary molars. J Endod 2002;28(4):324-327.
9. Rampado ME, Tjaderhane L, Friedman S, Hamstra SJ. The benefit of the operating microscope for access cavity preparation by undergraduate students. J Endod 2004;30(12):863-867.
10. Baldassari-Cruz LA, Lilly JP, Rivera EM. The influence of dental operating microscope in locating the mesiolingual canal orifice. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2002;93(2):190-194.
11. Schwarze T, Baethge C, Stecher T, Geurtsen W. Identification of second canals in the mesiobuccal root of maxillary first and second molars using magnifying loupes or an operating micro- 12. Rosenberg PA, Babick PJ, Schertzer L, Leung A. The effect of occlusal reduction on pain after endodontic instrumentation. J Endod 1998;24(7):492-496.
13. Plotino G, Pameijer CH, Grande NM, Somma F. Ultrasonics in endodontics: A review of the literature. J Endod 2007;33(2):81-95.
14. Lin YH, Mickel AK, Jones JJ, Montagnese TA, Gonzalez AF. Evaluation of cutting efficiency of ultrasonic tips used in orthograde endodontic treatment. J Endod 2006;32(4):359-361.
15. Melton DC, Krell KV, Fuller MW. Anatomical and histological features of C-shaped canals in mandibular second molars. J Endod 1991;17(8):384-388.
16. Fan W, Fan B, Gutmann JL, Cheung GS. Identification of C-shaped canal in mandibular second molars. Part I: Radiographic and anatomical features revealed by intraradicular contrast medium. J Endod 2007;33(7):806-810.
17. Rahimi S, Shahi S, Lotfi M, Zand V, Abdolrahimi M, Es’haghi R. Root canal configuration and the prevalence of C-shaped canals in mandibular second molars in an Iranian population. J Oral Sci 2008;50(1):9-13.
18. Navarro LF, Luzi A, Garcia AA, Garcia AH. Third canal in the mesial root of permanent mandibular first molars: Review of the literature and presentation of 3 clinical reports and 2 in vitro studies. Med Oral Patol Oral Cir Bucal 2007; 12(8):E605-E609.
19. Sieraski SM, Taylor GN, Kohn RA. Identification and endodontic management of three-canalled maxillary premolars. J Endod 1989;15(1):29-32.
20. Cohen S, Hargreaves KM. Pathways of the pulp, ed. 9. St. Louis: Mosby Elsevier;2006.
21. Ingle JI, Bakland LK, Craig J, Baumgartner CJ, eds. Ingle’s endodontics, ed. 6. Hamilton, Ontario: BC Decker, Inc.;2008.
22. Mauger MJ, Waite RM, Alexander JB, Schindler WG. Ideal endodontic access in mandibular incisors. J Endod 1999;25(3):206-207.
23. Vertucci FJ. Root canal anatomy of the human permanent teeth. Oral Surg Oral Med Oral Pathol 1984;58(5):589-599.
24. Vertucci FJ. Root canal anatomy of the mandibular anterior teeth. J Am Dent Assoc 1974;89(2): 369-371.
25. Mauger MJ, Schindler WG, Walker WA 3rd. An evaluation of canal morphology at different levels of root resection in mandibular incisors. J Endod 1998;24(9):607-609.
26. Warren EM, Laws AJ. The relationship between crown size and the incidence of bifid root canals in mandibular incisor teeth. Oral Surg Oral Med Oral Pathol 1981;52(4):425-429.
27. Pace R, Giuliani V, Pagavino G. Mineral trioxide aggregate as repair material for furcal perforation: Case series. J Endod 2008;34(9):1130-1133.
28. Main C, Mirzayan N, Shabahang S, Torabinejad M. Repair of root perforations using mineral trioxide aggregate: A long-term study. J Endod 2004;30(2):80-83.
Manufacturers
CK Dental, Orange, CA; 800.675.2537, www.ckdental.net
Dentsply Tulsa, Tulsa, OK; 800.662.1202, www.tulsadental.com
radiography for detection of apical periodontitis. J Endod 2008;34(3):273-279.
scope. Aust Endod J 2002;28(2):57-60.
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General Dentistry, November/December 2009 , Volume 57 , Issue 6
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