Written by John West, DDS, MSD Wednesday, 31 May 2006 19:00
Endodontics can be considered a game. The dictionary defines the word game as a "competitive activity with rules, involving some form of skill." The degree and consistency of endodontic success depend on the vigilance and resolve with which the endodontic game is played. This 2-part article is divided into six different and distinctive rules: The Game, Access, and Glide Path in Part 1, and Cleaning, Shaping, and Obturation in Part 2. While these are not the only rules, they are the essential guidelines for creating masterful, efficient, energizing, and enjoyable endodontics.
|Figure 1a. Nature's anatomy has no straight lines, and each creation is one-of-a-kind. Tree branches are always curved.||
Figure 1b. India ink infusion of dog's canine.
Figure 1c. Fingerprints, like DNA, are unique in the universe.
Figure 1d. Tree stump on a beach in Oregon.
Figure 1e. Maxillary molar. (Courtesy of toothatlas.com.)
Figure 1f. Maxillary first and second molars cleaned, shaped, and packed.
The game of endodontics is simple. Any tooth that is endodontically involved can be predictably saved if the root canal systems portals of exit (foramina) can be sealed either nonsurgically or surgically, if the periodontal condition is healthy or can be made healthy, and the tooth is restorable.1 The simple but profound fact of Mother Nature's rules is that all root canal systems are unique and different. Just like no two DNAs are the same, no two fingerprints are the same, and no two trees are the same, etc, no two root canal systems are the same. Nature only makes one-of-a-kind.
Secondly, nature does not make straight lines (Figures 1a to 1f). What this means to the endodontic clinician in the 21st century is that no instrument or technique fits all. In fact, all root canal systems must be treated separately and uniquely, and all root canals curve to varying degrees from access to radicular terminus.
|Figure 2a. Access. The secret to seeing in endodontics is the microscope.||
Figure 2b. Simplified access kit by Dentsply Maillefer instruments. New X-Gates Glidden features all Gates geometries in one instrument.
|Figure 2c. Pretreatment line diagram from mesial view of anterior tooth.||Figure 2d. Typical access cavity with triangle Nos. 1 and 2 still present.|
|Figure 2e. Labial enamel triangle removed. Triangle No. 1.||Figure 2f. Lingual dentin triangle removed. Triangle No. 2.|
Figure 2g. Resulting straight-line access when triangles No. 1 and No. 2 are removed.
|Figure 2h. Removal of triangle No. 2 allows discovery of lingual canal.|
|Figure 2i. Triangles also exist in posterior teeth as seen in this line drawings distal canal.||Figure 2j. Without removal of distal triangle, the angle of incidence and the angle of access are dissimilar. No straight-line access exists. The dentinal triangle must be removed.|
The first step toward successful cleaning and shaping is the proper access cavity, and it must be thoroughly evaluated for straight-line access before beginning glide path preparation (Figures 2a to 2j).2 It has been said that you can't treat what you can't see. Seeing canals in appropriately prepared access cavities requires magnification and illumination.3 This can be achieved through headlamps and lights or microscopes. The advantage of the microscope is that the coaxial light is present in addition to the magnification and the instrument is independently positioned. The clinician is not tethered to a light, which means the microscope can be used readily without the cumbersome attachment to the doctor's head.
In addition, do dentists see and believe, or believe and then see? In order to believe that canals are present, it is first appropriate to review some of the facts about canal statistics. The easiest way to know the number of canals that are present in particular teeth is to review the literature and assume that these numbers of canals are present unless proven otherwise4 (Table5).
Knowing the average "number of canals," while useful as a general roadmap, does not accurately describe the labyrinth of root canal system complexity, which consists of webs, fins, interconnections, cul-de-sacs, and rivers of anatomic deltas. (See Brown and Herbransons toothatlas.com.)
The basic armamentarium philosophy for access preparation is that fewer burs is better. As in all efficient endodontics, less is more the fewer instruments dentists need, the more efficient they are. Basically, a No. 2 or No. 4 round diamond is utilized for the access cavity in teeth with porcelain, and a No. 2 or No. 4 round bur is utilized for teeth without porcelain coverage. The access can be refined using a beaver bur or a tapered diamond. Occlusal reduction is important, particularly in teeth that will require cuspal protection such as an onlay or crown. A second occlusal reduction advantage is that occlusal right angles will result and act as file position references instead of using the slopes of cusps. Third, the occlusal relief may prevent periradicular percussion discomfort after treatment because there will be space for slight periradicular inflammation. Typically, the inflammation after cleaning, shaping, and obturation is subclinical, but it can occur on occasion, and here an ounce of prevention is clearly worth a pound of cure. It is important "to pave your way with words" and suggest to patients that teeth may feel sore for several days after endodontic treatment.
Basic technique for endodontic access is to penetrate and flare repeatedly until the chamber roof has been reached. The chamber roof is then peeled away with the round bur. This is followed by refining the walls with a tapered diamond. A slight taper within the access itself is prepared in order to create a "positive seat" for the placement of a temporary restoration between multiple-visit treatments. Ultrasonics are also extremely useful in refining access cavity walls and removing calcifications from the pulp chamber and particularly the canal orifi.
In addition to these general guidelines, each group of teeth has common errors that need to be avoided during access cavity preparation.
Common Errors in Access Preparation of Maxillary and Mandibular Anterior Teeth:
(1) Facial perforation caused by failure to remove triangle No. 1, which is essentially in enamel (Figure 2e).
(2) Failure to extend lingual access in order to remove triangle No. 2 (Figure 2f). Removal of triangle No. 1 and triangle No. 2 enables straight-line access into the root canal system. Many clinicians will fail to remove these triangles, while at the same time removing too much tooth structure mesially and distally. It is important to make a thoughtful access versus a large access. A proper design of the access cavity is a very slight taper of the mesial and distal walls. Many clinicians overprepare this area and inadvertently remove mesial and distal (or potential) ferrule structure. The ferrule is important in all teeth, but particularly important in mandibular anterior teeth, where the mesio-distal dimension is already very narrow. Many dentists miss the second lingual canal of mandibular teeth because of failure to remove triangle No. 2 (Figure 2h).
Common Errors in Access Preparation of Maxillary Pre-molar Teeth:
(1) Underextending the facial and lingual preparation and therefore failing to create straight-line access for multiple-canal teeth.
(2) Overextension of mesial and distal walls.
(3) Perforation of the mesial because of failure to recognize the distal-axial inclination of these teeth. The maxillary first premolar is one of the most commonly perforated teeth in the mesial direction.
(4) Failure to align the access cavity with a full-coverage restoration. Careful examination of the root structure is key in order to observe any rotation or difference in angulation of the prepared crown and the root itself.
(5) Failure to discover a third canal (6% of the time) in the maxillary first premolar.
(6) Failure to discover a second canal (24% of the time) in the maxillary second premolar.
Common Errors in Access Preparation of Mandibular Premolar Teeth:
(1) Similar mesial perforation error as a maxillary premolar, since many of the mandibular second premolars have a distal tilt.
(2) Failure to extend buccal access sufficiently for straight-line access.
(3) Entire access outline too far lingual.
(4) Overextension in a mesio-distal direction.
Common Errors in Access Preparation of Maxillary Molar Teeth:
(1) Underextended preparation; particularly toward the mesial in order to discover the mesio-buccal No. 2.
(2) Furcal perforation when there is a narrow occlusal-gingival pulp chamber and it has been inadvertently passed by the clinician's access penetration/flare bur. It is important to take an accurate bite-wing in order to see the pulp chamber or to see that no pulp chamber exists at all! Where there is full coverage, this information may be unavailable, since the restorative material masks the chamber on the radiograph. In this instance, extreme caution must be taken. It is important to observe the mesio-buccal prominence either before or after rubber dam placement. It is useful to evaluate this, however, before rubber dam placement. Many attempts at finding the mesio-buccal No. 1 have resulted in perforation in the chamber floor because the clinician's access was not far enough in the mesio-buccal direction.
Common Errors in Access Preparation of Mandibular Molar Teeth:
(1) Outline form is too far lingual.
(2) Buccal extent of occlusal outline form is too far lingual.
(3) Perforation of the furca, again due to the narrow pulp chamber that has been inadvertently passed by the access bur or masked by full coverage. The clinician is waiting to fall into the chamber and instead falls into the furca.
In summary, the key to successful access cavities in anterior teeth is removal of labial triangle No. 1 and lingual triangle No. 2. Triangle No. 1 is essentially enamel, and triangle No. 2 is essentially dentin. Triangle No. 1 can be removed with the same access round bur used in the original penetration and flaring, and refined with a tapered diamond. Triangle No. 2 can be removed with a thin, tapered diamond, a Gates Glidden drill, or a ProTaper SX file (DENTSPLY Tulsa Dental). Posterior teeth or multicanal teeth also have chamber triangles that are in dentin (Figures 2i and 2j). These can be carefully removed with a No. 1 or No. 2 Gates Glidden drill or a ProTaper SX file. It is important to have previously followed the coronal portion of the canal with small endodontic files before the entrance of the SX file in order to guide its tip. Canals calcify in a crown-down direction and may not become patent for several millimeters apically. The entrance to these canals will often present as a white dot. The canal itself also will reside in the discolored portion of the dentin. Ultra-sonic instruments and Mueller burs are helpful in following the calcified dentin until the canal physically begins.
|Figure 3a. Glide path is the key. Glide path is finished when a No. 10 or 15 endodontic file easily follows the canal to the radiographic terminus.||Figure 3b. ProTaper S1 (Dentsply Tulsa Dental) shaping file predictably follows the glide path and carves restrictive dentin away from the coronal one third of the canal. Note the preserved surface position of the apical portal of exit. Careful glide path management prevents apical transportation, especially when the clinician follows the directions for use of a particular rotary system.|
|Figure 3c. Successful 3-dimensional obturation of the root canal system sealing not only the cleaned and shaped apical portal of exit but also potentially significant multiple apical lateral canals.|
The endodontic glide path may be defined as a smooth preparation from the chamber orifice to the root canal system's terminal constriction (Figures 3a to 3c).6 The smallest glide path that is appropriate for rotary shaping instruments is a No. 10 file, but most endodontic teachers would suggest that a loose No. 15 file is the minimal instrument of choice. The difficulty with the ISO (International Standards Organization) standard, however, is the inappropriate increase in D1 diameters from file size Nos. 10, 15, and 20. D1 is the diameter size in one-hundredths of a millimeter, 1 mm from the tip of the instrument. Typically, blades extend to the most coronal part of the cutting edge at D16, which is 16 mm from the tip of the instrument. A tip diameter of a No. 10 file is 0.10 mm, while a tip diameter of a No. 15 file is 0.15 mm. The No. 15 file is a 50% increase in D1 diameter! A No. 20 file is 33.3% wider in diameter than a No. 15 file. Because of such large jumps, the No. 15 file and then the No. 20 file do not easily follow the path of the previous instrument; they are too wide in apical D1 diameter. In a curved canal and in a structure such as dentin, this means that it is easy for the dentist to make a shelf in the dentin walls outside portion of the curve. This breakdown happens because all instruments have a memory to straighten, and when they straighten combined with apical pressure, a shelf or gouge in the outside dentinal wall can result.
Three instruments are aimed at minimizing the sizing problem: Golden mediums (DENTSPLY Maillefer), the ProFile Series 29 files (DENTSPLY Tulsa Dental), and the ProFinder Series (DENTSPLY Maillefer). Each of these systems is useful in that the progressively wider diameter instruments have enough space to accommodate them and, therefore, minimize the risk of damaging the outside wall of a curved canal. It is very difficult to remove these shelves, ledges, or gouges because the angle of access is so different than the angle of incidence of the shelf itself. In other words, the damaged point is deep in the canal and away from clinical control. After all, the handle of the instrument and the dentist's fingers are outside the access cavity, and direct removal of the shelf is impossible.
In order to prevent shelf creation, clinicians must progress to the next instrument only when the previous one fits loosely. Extreme delicacy is required. The dentist must show restraint in pushing or pressing the instrument in order to have it advance down the canal. Forcing, even slightly, is a recipe for instant disaster. It is important to precurve the files so that they follow the existing path without creating a false path. Shelf correction can be predictably accomplished by precurving an F1 (DENTSPLY Tulsa Dental) finishing file using a pair of orthodontic bird-beak pliers. The apically curved F1 is slid past the shelf manually, rotated, and removed. Not only is the shelf eliminated, but the apical preparation is often finished at the same time.
There are four separate and distinct manual motions in developing a glide path for rotary shaping. All of them should be used in a slurry of sodium hypochlorite in order to carry away any dentin mud that the files produce and also to digest detached pulp.
Four Manual Motions:
(1) Follow the root canal anatomy is always present, and the dentist's job is to follow that path. This can be achieved by first selecting an instrument that is small enough to fit and follow the existing canal. Second, the last few millimeters of the instrument needs to be curved using a pair of metal cotton pliers. The clinician should hold the handle of the file delicately with a minimal amount of contact in order to maximize the tactile sense. The idea is not to find the canal, but rather simply to follow it. This would be analogous to a child, for example, sliding down a curved slide. The first instrument to the radiographic terminus is the critical moment of endodontic preparation, and the successful operator understands that he or she wants to flow with the canal instead of fighting the canal. It is natural to want to push, because that would be a normal human reaction when reaching an impasse. Restraint must rule here, however, and the payoff will be further advancement toward the end of the canal with each following motion.
(2) Smooth once the first instrument has reached the terminus and is verified by the apex locator and/or radiograph, the next motion is to reproduce the existing path using the smoothing motion. This motion involves making in-and-out, short, vertical amplitude motions of 0.5 to 1.0 mm until the file is loose and can easily slide from orifice to radiographic terminus. This may take 2 or 3 vertical strokes, or it may take several dozen or more.
(3) Balanced Force the balanced force technique, which was originally described by Dr. Jim Roane, is simple to understand but challenging for doctors to perform.7 Quite simply, however, the motion involves turning the handle of the file in a clockwise direction so that the dentin is engaged and then, while applying gentle apical pressure, turning the handle in a counterclockwise direction so dentin can actually be cut off the walls and deposited into the flutes of the file. The file is then turned clockwise, and during the clockwise turning is removed from the canal. On close inspection and under magnification, small amounts of dentin can be observed in the flutes.
Remember that endodontic shaping is not a big job it is a little job. It is, however, a smart job. It is important to trust that sufficient dentin is being removed to allow the next pass of the same instrument to go deeper toward the canal terminus. The distinguishing step of this motion is that apical pressing must be done during the counterclockwise turn, or the file will simply unscrew without removing any dentin. In review, the motion is first to turn clockwise, and without any apical pressure the file will advance apically. Then turn the file handle counterclockwise with slight apical pressure. Lastly, turn the file handle clockwise and remove the file. Several of these clockwise-counterclockwise-clockwise balanced force motions can be done in the same pass. If the instrument advances deeper easily, then 2 or 3 or even 4 of these combined clockwise-counterclockwise-clockwise motions are recommended. The balanced force motion easily removes restrictive dentin in the coronal portion of the root canal.
(4) Envelope of Motion the envelope of motion is an engineering term to describe removal of restrictive dentin as the precurved instrument is removed from the canal itself in a clockwise direction. The envelope of motion is the only motion that carves shape on the outstroke of the file. The technique is to precurve a file such as a No. 15 file and then follow the instrument into the root canal system. Short of maximum resistance, make a clockwise rotation while removing the instrument. With each envelope of motion, the instrument creates a space reservoir that will allow the tapered file to advance deeper toward the end of the root canal system.8
If at any point an instrument does not follow to the terminus, there are 4 possible reasons or a combination of reasons and solutions:
(1) The apical extent of the canal is clogged with dentin mud or dense collagen. The solution is to precurve the last millimeter of the instrument, follow to the blockage, touch the blockage, and envelope the instrument back out. This is repeated multiple times until the coronal portion of the blockage is disrupted. Each pass guarantees apical movement, although this may be a very small and imperceptible advancement. Each time, however, the tip of the instrument will disrupt the densely packed dentin mud in the coronal portion of the block until the instrument can easily pass into the softer apical portion of the dentin mud.
(2) The tip of the instrument is curved in a different path than the canal itself. The solution is to follow with multiple different arcs and lengths of arcs of the apical extent of the instrument until the instrument follows to the terminus. This solution can only be achieved through random re-curvatures of the instrument in combination with patience and delicate finesse.
(3) The tip of the instrument is too wide. The solution is to make another pass with a smaller-diameter instrument. The smallest instrument that exists in the ISO System is a size 06. The D1 millimeter diameter is 0.06 mm. The apical tolerance of machined instruments is +/- 0.02 mm, so there can be variations within the instruments themselves. Some 06 instruments are smaller than others!
(4) The shaft of the instrument is too wide for the canal. The tip fits loosely, but the dentist perceives that the tip is tight. The only way to really discover this solution is to follow with the same instrument, and using the balanced force or envelope of motion, restrictive dentin is removed, and the same instrument then advances deeper within the root canal system.
It is advised that the dentist slow down during this part of the canal preparation. As he or she slows down, the canal preparation will speed up because fewer mistakes will be made, such as blocks, ledges, or transportations.9
In Part 2 of this article, the final 3 rules of cleaning, shaping, and obturation will be discussed.
1. West JD, Roane JB. Cleaning and shaping the root canal system. In: Cohen S, Burns RC, eds. Pathways of the Pulp. 7th ed. St Louis, Mo: Mosby; 1998:204.
2. Levin HJ. Access cavities. Dent Clin North Am. Nov 1967:701-710.
3. West JD. The role of the microscope in 21st century endodontics: visions of a new frontier. Dent Today. Dec 2000;19:62-69.
4. Ingle J, Himel VT, Harwish CE, et al. Endodontic access preparation. In: Ingle JI, Bakland LK, eds. Endodontics. 5th ed. London, England: BC Decker, Inc. 2002:405-570.
5. Stropko JJ. Canal morphology of maxillary molars: clinical observations of canal configurations. J Endod. 1999;25:446-450.
6. West JD. Finishing: the essence of exceptional endodontics. Dent Today. Mar 2001;20:36-41.
7. West JD, Roane JB. Cleaning and shaping the root canal system. In: Cohen S, Burns RC, eds. Pathways of the Pulp. 7th ed. St Louis, Mo: Mosby; 1998:244-248.
8. Ruddle CJ. Shaping for success... everything old is new again. Dent Today. 2006;25(4):120-127.
9. West JD. Perforations, blocks, ledges, and transportations: overcoming barriers to endodontic finishing. Dent Today. Jan 2005;24:68-73.
Disclosure: Dr. West maintains a royalty position with ProTaper Rotary Files, DENTSPLY/Tulsa.
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