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The advent of rotary nickel-titanium (RNT) instruments in the early 1990s made possible improved and more efficient endodontic canal preparation with the caveat that instrument separation, canal blockage, and transportation remain problematic in some complex cases.
Reducing iatrogenic outcomes, is, in large measure, a combination of realizing iatrogenic risk before treatment and taking steps to avoid such outcomes. Two such steps in optimizing endodontic cleansing and shaping procedures include canal negotiation (ideally, achievement of patency) and glide path creation. This article was written primarily for the general practitioner to discuss clinical strategies in initial canal negotiation and glide path preparation with the end goal of minimizing iatrogenic risk and creating an ideal final prepared shape. The endodontist is encouraged to compare his/her chosen techniques with those discussed here.
Describing every anatomy where canal negotiation is challenging is beyond the scope of this article. While radiographs give important clues to the expected difficulty of negotiation and achievement of patency, radiographic appearance can be difficult to interpret. Radiographs showing calcification may yield easily negotiable canals and sometimes vice versa. In short, the clinician often does not know what he/she will encounter until treatment is underway. Cone beam technology is invaluable for providing the “third dimension” to information not included in two-dimensional images. While not infallible, cone beam images give strong clues to possible negotiation challenges and are the best available predictor of case complexity (Figures 1A and 1B).
A glide path is not necessary in an open and easily negotiable canal whose minor constriction (MC) is approximately the equivalent of a #15 hand file. The challenge clinically, especially for endodontists, is that few canals fit this description. It is difficult to know, as mentioned above, from the initial radiographs, what the initial canal dimensions are. As a result, assurance of patency and glide path preparation are essential before RNT insertion.
The preparatory and foundational steps to canal negotiation and glide path creation are ideally performed under a surgical operating microscope (Global Surgical and Zeiss). They include straightline access, cervical dentinal triangle removal, and removal of restrictive dentin in the coronal and middle thirds.
Irrespective of whether the case is vital or nonvital, the above steps should take place in the presence of copious irrigation. Abundant and bleeding pulp (vital cases on younger patients) should be removed from the pulp chamber in the presence of a chelating agent, File-Eze® (Ultradent) or ProLube® (Dentsply Tulsa Dental Specialties), and flushed with sodium hypochlorite. In the most general sense, such vital teeth will be easier to negotiate, but propulsion of canal debris apically can easily block a canal. Nonvital cases, conversely, while not having the same blockage risk from tissue as vital ones, will generally be more calcified.
Vital or nonvital, the pulp chamber and orifices should be entirely prepared before moving into the middle third. Clinically, this means to fully uncover the MB2 canal, remove isthmus tissue with ultrasonics prior to attempting apical canal negotiation, and remove the cervical dentinal triangle. Moving apically before comprehensively preparing and cleaning the chamber increases the risk of debris propulsion and subsequent iatrogenic outcomes.
As a starting place, the clinician should conceptually link apical negotiation with every step taken coronally. If the access is not straightline, if restrictive dentin is not removed, etc., it is difficult, if not impossible, to achieve apical patency or use RNT efficiently. Optimal access and orifice management gives the clinician the best possible visual and tactile control over the apical third.
Prior to initiating treatment, it has value for the clinician to estimate the working length (EWL) from the initial radiographs. An EWL allows the clinician to interpret tactile sensations as the file moves apically.
Once the aforementioned issues have been considered, hand files are pre-curved for apical negotiation. An Endo-Bender® pliers (SybronEndo) is a valuable instrument with which to curve files. A curved hand file traverses a curved canal more easily than a straight one. Which hand files are used for initial negotiation is a matter of personal preference. I recommend stiffer instruments for the initial negotiation of the canal, irrespective of the anatomy. Popular options include D Finders (Mani, Inc), C+ files (Dentsply) among others (Figures 2A and 2B). These files are inherently stiff. A stiffer file is more desirable for this application than a flexible one. Flexible files tend to buckle easily when used in calcified canals. If the canal is easily negotiable with stiff hand files, then the clinician can easily switch to a more flexible variety, Flexile Files (Mani, Inc) and Lexicon® FlexSSK® Files (Dentsply Tulsa Dental Specialties) as desired.
The stiff pre-curved hand files referenced above are used to attempt to reach the apical terminus in the presence of copious irrigation. Once reached, ideally the clinician feels a corresponding tactile “pop” as the file passes through the MC. If a tactile “pop” is felt at or near the EWL, the clinician can use an electric apex locator to determine the true working length (TWL) and/or take a film. If a pop is not felt, an apex locator or film can determine the relative position of the file in the canal if the clinician is unsure of its apical penetration.
How easily the apical terminus of a root is reached is often a function of the type of cases a clinician treats. Calcified or previously accessed cases will obviously be more challenging. Previous access, especially by a different clinician than the one finishing the case, requires special caution. A ledge or blockage may already be present, and using force in this negotiation (or any negotiation) to bypass the trouble spot can exacerbate an already problematic iatrogenic issue.
Anatomical variations are the rule, not the exception. The clinician must keep in mind that the canal may be patent, even though multiple introductions of the negotiating files do not easily allow patency to be established. For example, if the apical foramen is at right angles to the primary canal or splits into multiple delta canals, finding a canal path may or may not be possible. The challenge is that the clinician is interpreting tactile feedback (feeling the path) via a hand instrument and unable to see the path. There is no substitute for experience in this endeavor. In general, the longer the root, the more curved and calcified, the greater the challenge to negotiate. Negotiation in these clinical situations requires time, must be done with visual and tactile control, and with great care taken to interpret the tactile feedback.
Hand files are advanced slowly and with intention. If the canal feels spongy to file introduction, generally, additional length can be gained in the canal. Alternatively, if the file feels as though it hits a brick wall, the chances of apical advancement are less likely. Caution and clinical judgment are advised.
A pre-curved file may take multiple insertions to find the true apical path to the MC. Once apical patency has been achieved, especially in a canal that was difficult to negotiate, the clinician should leave the file in place, at length, to the MC and either reciprocate it (as described below) or manually use a 1-3 mm amplitude watch-winding motion to begin creation of the glide path. Alternatively, if apical patency was obtained easily, the file can be removed, and the glide path prepared by whatever means desired.
In any event, taking RNT into the apical third without assurance of patency and a glide path presents, in the author’s view, an unnecessary risk for instrument fracture along with other iatrogenic outcomes.
It is an open question whether it is safer and more effective to prepare a glide path after canal negotiation using hand files or with RNT navigating files (glide path creators). While systems differ, Pre Shaper (Specialized Endo), PathFile™ (Dentsply), regardless of the product a clinician uses, the basic instrument design is a relatively small tip size on a minimally tapered RNT file. The techniques for using these systems have been detailed elsewhere (West, September 2010, January 2011, March 2011) and the descriptive PDF found at the Specialized Endo website. When used appropriately, these are valid methods for glide path creation.
It is a matter of personal preference whether a hand glide path (reciprocated or not) is superior to a RNT-generated one. There are pros and cons to each method and certain anatomies that, in the opinion of the author, favor one technique over the other. Said differently, it is not that either of these methods is inherently safer than the other, rather the safety and efficiency is in the hands of the clinician and how the devices are used. The above notwithstanding, when preparing a glide path, it is very unlikely that stainless steel hand files will separate in complex multiplanar curvatures unless they are grossly abused. Alternatively, after canal negotiation, when RNT glide path files are used in such complex anatomy, separation is more of a risk especially when the canal has not been first explored with hand file files to their terminus.
Once a hand K-file is taken to the terminus of the canal, one safe alternative to hand filing the glide path is the use of a reciprocating handpiece. Reciprocation is provided by a reciprocating handpiece attachment, W&H Synea Endo Cursor contra angle handpiece – WA-62 LT and WA-62 A (MounceEndo), with or without fiber optics or the M4 (SybronEndo) [Figure 3]. These instruments turn a hand file alternating between clockwise and counterclockwise but not in full rotation. The W&H attachments above, for example, have a 4:1 gear reduction and fit onto an E-type coupling; reciprocation takes place at 900 rpm. This motion, a clockwise, counterclockwise movement (lacking full rotation) is the same concept as employed in the WaveOne® technique (Dentsply Tulsa Dental Specialties) system for canal enlargement.
Reciprocation of hand files is very safe. It is recommended the canal be flooded with irrigant, and the file placed to the MC before reciprocation to enlarge the glide path.
It is a simple matter to take a small and calcified canal from a #8 hand file equivalent to a #15 hand file equivalent, saving time and hand fatigue relative to solely manual enlargement. When the canal has been negotiated to its terminus, the type of hand file used for reciprocation changes to a safe-ended variety, for example, the SEC-O files K type (Mani, Inc).
If, for example, an MB2 canal has been difficult to negotiate, once a #8 hand file reaches the MC, the W&H Synea is placed over the file (while the file is at length) and activated for 15 to 30 seconds using a 1 to 3 mm vertical amplitude motion. After reciprocation, the #8 will spin freely in the canal. The canal is irrigated, and a #10 is inserted. The #10 is reciprocated in the same manner.
Successively larger hand files can be reciprocated until the clinician has prepared a glide path before the introduction of either a system like WaveOne, which uses reciprocation for the entire canal shaping or one that uses full rotation like the MounceFile in Controlled Memory© (MounceEndo) or some other RNT system (Figure 4).
In complex, three-dimensional anatomy, once a glide path has been created, it may have value to use smaller tapers and tip sizes first before embarking on a strictly crown down approach. Specifically, the clinician might consider using a .04/20 or similar instrument to TWL rather than solely employing a crown down approach. The MounceFile is available in .01/20, .02/20, .03/20 and .04/20 dimensions. Any of these MounceFile sizes in either the CM© or standard nickel-titanium variety, case and clinician dependent, can accentuate the glide path before bulk shaping. This action reduces the torque placed on subsequent RNT files. CM© technology allows the RNT file to retain its curve (CM thermomechanical treatment removes shape memory from nickel-titanium) as it negotiates the curvature reducing canal transportation1-3 (Figures 5-6).
Clinical techniques for canal negotiation, glide path creation, and the blending of the glide path with RNT shaping have been presented. Emphasis has been placed on straightline access, removal of restrictive dentin, patient and methodical negotiation with stiff hand files, and a sequential enlargement (from glide path to final shape) with copious irrigation and possible recapitulation. I welcome your feedback.
Rich Mounce, DDS, is in full-time endodontic practice in Rapid City, South Dakota. He is the owner of MounceEndo, LLC, an endodontic supply company specializing in bulk purchases of rotary nickel-titanium and stainless steel hand files.
1. Shen Y, Qian W, Abtin H, Gao Y, Haapasalo M. Fatigue testing of controlled memory wire nickel-titanium rotary instruments. J Endod. 2011;37:997– 1001.
2. Shen Y, Qian W, Abtin H, Gao Y, Haapasalo M. Effect of environment on fatigue failure of controlled memory wire nickel-titanium rotary instruments. J Endod. 2012 Mar;38(3):376-80.
3. Shen Y, Zhou HM, Zheng YF, Campbell L, Peng B, Haapasalo M. Metallurgical characterization of controlled memory wire nickel-titanium rotary instruments. J Endod. 2011 Nov;37(11):1566-71.
4. West, JD. The endodontic glidepath: secret to rotary safety. Dent Today. 2010 Sep;29(9):86, 88, 90-3.
5. West, JD. Manual versus mechanical endodontic glidepath. Dent Today. 2011 Jan;30(1):136, 138, 140.
6. West, JD. Glidepath implementation: return to the beginning. Dent Today. 2011 Apr;30(4):90, 92-7.
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Research has shown that irrigants are more effective when they are electro-mechanically activated.
Research has shown that irrigants are more effective when they are electro-mechanically activated.
Acoustic streaming and cavitation have been proven to significantly enhance cleaning of difficult anatomy. Studies have shown that low frequency (Sonic) oscillation (160-190Hz) was not sufficient to create acoustic streaming or cavitation within the canal space.
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