Management of the Coronal Third: Progressively and Passively

For many, entry into and instrumentation of the coronal third of root canal systems has almost been taken for granted or considered “simple,” especially with the advent of rotary Ni-Ti (RNT) orifice openers. Fortunately, while this is true on many occasions, it is not uncommon to have a coronal third that challenges even the most talented clinician. Especially in these problematic cases (which will be defined below), diligence in canal location, coronal third negotiation, and instrumentation can make all the difference between perforations, canal blockage, non-negotiability, frustration, and wasted time (amongst other problems), and achieving an efficient and desired result. Said differently, at times, the coronal third can be much more complex than anticipated, especially radiographically.

CRITICAL FACTORS

What factors are critical for creating excellence in orifice and coronal third management and initial enlargement of root canal systems?

First, it is suggested that the clinician take 3 angles of radiographs (buccal, mesial, and distal) to assess carefully the position of the orifice and the angle at which the canal exits the pulpal floor. Caution is advised. Metal crowns obscure some chamber floors and coronal thirds, and there may be no visual clues as to cervical anatomy. In these cases, it may be anyone’s guess as to what type of cervical canal anatomy is present. While many canals are large enough that they can be easily located and exit relatively vertically from the chamber floor, many do not. It is not uncommon to find narrow canals that exit at an acute angle from the pulpal floor, and if this is not appreciated, placement of a Gates Glidden drill or RNT orifice opener into the narrow diameter of such a canal lumen can rapidly cause chips to be generated that will occlude the canal. In such an event, subsequent negotiation, even with small K-files, can be difficult, and even impossible in extreme cases. Leaving such significant uncleaned and unfilled space (if the clinician becomes blocked out entirely in the coronal third before canal instrumentation commences) severely diminishes the prognosis, as the entire contents of the canal will become nonvital (if they were not already), risking long-term failure. Such a blockage can be very challenging in terms of achieving patency, even if the canal should later prove negotiable with a concerted effort.

The given angle of exit from the pulpal floor and the size of the canal at the orifice level make all the difference in terms of how the canal will be negotiated initially. If the canal is wide open and easily negotiable, it is acceptable to place a RNT orifice opener into the canal as the initial instrument, especially if copious irrigation is carried out after each pass of the instrument. Upper anterior teeth, lower molar distal roots, and the palatal roots of upper molars that are not calcified in any way are often easily negotiable with orifice openers and generally do not risk blockage. If an orifice opener does not progress apically for whatever reason, it should never be forced. Alternatively as a precaution, if the clinician wishes to establish patency with small K-files before using any orifice opener, especially in the coronal and middle thirds and irrespective of the orifice and canal size, it is certainly acceptable to do so (Figures 1a through 2b).

If the clinician doubts that the orifice openers will easily negotiate the coronal third after careful examination of the anatomy, the most efficient course of action is to use new 6 or 8 K-files (21 mm ideally) that are gently inserted both progressively and passively (at this stage in the process) to a depth no greater than the junction of the middle and apical third. The purpose of this examination with small K-files is to assess the ease of negotiation of the canal in the coronal and middle thirds, canal curvature, and canal diameter. Ideally, all of these K-files will be gently curved by hand before insertion. The tactile information derived from the file can be extensive in this negotiation and exploration. If the K-file comes out severely curved from the canal in the coronal and/or middle thirds, or the file will not easily progress apically, it can be assumed that the anatomy present is complex, potentially problematic, and will risk an iatrogenic event if not managed correctly. Caution is advised, as roots possess multiplanar curvatures, and even if a canal may not look curved in a mesial to distal direction radiographically, it may be very curved in a buccal to lingual dimension that will not be easily visible, if at all, unless the tooth is rotated.

It is noteworthy that management of the coronal third begins with straight-line access that is large enough to permit unrestricted access into any canal that is essential for subsequent negotiation of the canal system. Access that is too small will almost certainly diminish the control possible over the orifice and will put the clinical case at risk for subsequent iatrogenic events. Ideally, coronal access preparation, once completed, will be large enough that a file can stand vertically in the canal without touching the adjacent wall of the opening.

AFTER CORONAL ACCESS

After coronal access and before attempted canal location, negotiation, and initial instrumentation, it is essential for the clinician to flush the chamber with irrigant, ideally 5.25% sodium hypochlorite, to remove any chips and pulp tissue. Alternatively, 2% chlorhexidine (Vista Dental) can be used for this flushing. Chlorhexidine and sodium hypochlorite should never be used in tandem because of the precipitate that will form when they are combined. Water or a liquid EDTA should be used to rinse one of these 2 solutions before the other is introduced. Water can be utilized to clear the pulp chamber of debris, as described above, before canal location and negotiation. However, if the desired goal is to eliminate bacteria from the coronal access to set the stage for future canal disinfection (and sterilization) to the greatest degree possible, an antibacterial solution is ideal for this initial flushing as well as during canal irrigation.

It is noteworthy that if the tooth is vital, it is optimal for the clinician to have a viscous EDTA gel placed into the chamber before instrumentation below the orifice commences (eg, FileEze [Ultradent], Glide [DENTSPLY Tulsa Dental], or RC Prep [Premier Dental Products]). This will help emulsify the vital tags of pulp tissue, avoid their apical compaction (and hopefully prevent formation of dense blockages of pulp tissue in the form of dentin mud), and facilitate their flushing. Such an EDTA gel can of course be used in nonvital cases as indicated and if desired. Once instrumentation begins, this viscous EDTA gel should ideally be utilized until the bulk of the pulp is removed from the tooth and/or negotiation of the apical third is ready to be undertaken. At this point, the clinician can then switch to chlorhexidine, sodium hypochlorite, or both (flushing between solutions as previously noted) for irrigation of the apical third.

Figure 3. The cervical dentinal triangle, which places unnecessary torsional and cyclic fatigue forces on files, reduces visibility and predisposes a tooth to iatrogenic events if not removed.	Figure 4. After removal of the cervical dentinal triangle, true straight-line access is achieved from the cusp tip to the initial curvature of the canal.

Figure 5. The Line Angle burs from SybronEndo, which can help create straight-line access, act as orifice openers and remove the cervical dentinal triangle.

Removal of the cervical dentinal triangle (CDT) is essential. If the CDT is left in place, it will necessarily place both a torsional force and cyclic fatigue force on files. Iatrogenic problems of all types can result from leaving this easily removed obstacle. Removal is very straightforward if done under a surgical microscope with a fine diamond. Alternatively, placing a very subtle amount of pressure on an orifice opener brushed against the CDT laterally is usually adequate for its removal. The CDT is very often present in lower and upper molars cervically, amongst other possible anatomic variations. In addition, the LAAxxess line angle burs from SybronEndo can be used to create straight line access  as well as define line angles in dentin, and can be used as orifice openers in an easily negotiated coronal third (Figures 3 to 5).

In cases where the clinician has chosen to explore the coronal and middle thirds with K-files before the introduction of orifice openers, once the 6 to 8 K-file has negotiated the coronal and middle thirds and can spin freely, the clinician should then instrument up to a 15 K-file to a level (at this stage in the process) no greater than the junction of the middle and apical third. Achievement of a minimal diameter and glide path in the coronal two thirds is vital in order to set the stage for the subsequent rotary files that will follow in these levels of the canal. The clinician is encouraged to stay out of the apical third at this stage until the coronal and middle thirds are fully instrumented.

The apical third, in the authors’ empirical opinion, should be negotiated and instrumented last (crown-down instrumentation). In a coronally narrow and potentially curved canal, negotiating the coronal and middle thirds, creating a glide path (instrumenting the canal to a minimum 15 hand file), and then using orifice openers gives the greatest safety margin to prevent blockade, instrument separation, and canal transportation, amongst many possible iatrogenic outcomes of mismanagement.

Once the clinician has instrumented the coronal and possibly the middle third to a size 15 K-file (assuming an orifice opener will not initially traverse these thirds of the canal), the clinician can begin using orifice openers of a proper taper into the canal passively in order to begin initial enlargement of the canal. The authors are advocates of the K3 shaper files (SybronEndo) for RNT orifice openers for their cutting efficiency, fracture resistance, and durability as well as the fact that they can be used for a wide variety of canal preparation functions (as an orifice opener, a master apical file, and a canal-shaping file). In other words, their use is not limited to just the coronal third. The K3 Shapers come in 3 tapers—0.12, 0.10, and 0.08—with a fixed, 25-tip size that allows them to be useful in a wide range of clinical anatomy. If the clinician desires, the K3 Shapers can be rotated at 900 rpm (higher than the manufacturer’s recommendations) for maximum efficiency, especially if they are introduced gently, minimizing engagement of the file against canal walls (ideally 1 to 2 mm and no more than 4 to 6 mm per insertion).

The Shapers are used crown-down in that they are generally used from the larger tapers to the smaller. In other words, the 0.12 is generally used first, especially in larger canals, followed by the 0.10, and then 0.08. Alternatively, if the canal will only accept a 0.08-tapered shaper initially, a 0.12 and then 0.10 can follow it. In any event, the space created by the larger Shaper will make introduction of the next smaller taper much more fluid.

It is valid to ask, “How big should the orifice preparation be?” In other words, “To what size should the orifice be prepared?” Inherent in the answer are 2 important considerations. First, whatever the size of the orifice opener used, it should never risk perforation, especially on the distal side of the mesial roots of lower molars and the distal side of the mesial-buccal roots of upper molars. These lateral root surfaces are often thin, even if radiographically they do not appear at risk of strip perforation. Aggressive or injudicious use of any system of orifice openers in this delicate region invites strip perforation. Secondly, the orifice diameter needs to be large enough that the clinician has command over the coronal third. Such command would imply that files can easily be inserted into the orifice (drop in), the orifice can be easily visualized, and the orifice doesn’t restrict the insertion of irrigating needles in any way. In essence, if possible, the access that begins at the cusp tip (assuming there is a cusp tip) is vertically straight without transporting the canal to the point of the initial canal curvature that is often in the middle third of many roots (Figures 6a to 6c).

CONCLUSION

Management of the coronal third is a vital link in the endodontic continuum in that if the access and management of the coronal (and middle) third are ideal, then the stage is set for excellence in apical third shaping. Conversely, if the coronal third shape is too small and/or transports the canal or orifice location, risk of ledging, perforation, canal transportation, and iatrogenic events of all types (including instrument fracture) rises dramatically. All of the referenced problems are clinically relevant. If the coronal third is mismanaged, the clinician is at risk for leaving uncleaned and unfilled space, and potentiates failure. Alternatively, excellent management of the coronal third can lead to optimal control of instruments in the apical third and ideal irrigation of this often-challenging space.

Dr. Mounce is in private endodontic practice in Portland, Ore. He is the author of a comprehensive DVD on cleansing, shaping, and packing the root canal system for the general practitioner. The material is also available as audio CDs and as a Web cast pay-per-view. He can be reached at comfort@mounceendo.com.

Dr. Glassman is the author of numerous publications and is renowned as an international lecturer on endodontics. He is a fellow and endodontic examiner for the Royal College of Dentists of Canada, a fellow of the Academy of Dentistry Inter-national, and a fellow of the International Academy For Dental-Facial Esthetics. He is also the endodontic editor for Oral Health dental journal and provides an interactive hands-on participation endodontic program, teaching state-of-the-art endodontic technique and technology. Past president of the George Hare Endodontic Study Club, he maintains a private practice, Endodontic Specialists, in Toronto, Ontario. He can be reached by visiting his Web site at rootcanals.ca.