Sleuthing Out Hard-to-Find Canals

The increased use of high magnification and accompanying high-intensity illumination via microscopes or high-magnification loupes in endodontics has led to a heightened awareness of the complex nature of the root canal system, and in particular, to a recognition of the existence of often-overlooked extra canals. While the presence of these complexities has been recognized academically for many years,¹ it has taken the relatively recent introduction of enhanced visualization into the clinical setting to drive clinicians as a whole to develop techniques to both find these canals and to clean, shape, and obturate them.

This paper will describe a method for seeking out extra canals and the chairside modification of extra-long, tiny round burs (Figures 1a through 1d) as primary instruments for efficient, deep exploration of these hidden canals.

METHODS OF CANAL EXPLORATION

Figures 1a to 1d. Chairside handmilling to modify the shaft of a 34-mm-long round bur for deep radicular access and troughing procedures: scalloping (1a and 1b), dimensioning (1c), and final dressing (1d).

The use of small instruments to trough in the area of a suspected extra canal is a common method of exploration. The need to be able to visualize the working tip of the instrument during troughing procedures has led to the adaptation of geometrically friendly ultrasonic tips for this purpose. These instruments provide good visibility of the working tip during troughing, however, that is counter-balanced by several disadvantages. These tips create a significant amount of heat, which can damage the delicate periodontal ligament, often only a fraction of a millimeter away.² To dissipate this heat, some advocate the use of water on the tips during operation.³ While water is an effective coolant, its use significantly compromises the otherwise excellent visibility of the working tip and requires the introduction of an evacuation device into the already tight field. Additionally, the abrading capacity of ultrasonic tips is directly related to the intensity of the energy delivered to the tip,⁴ and yet, the higher the energy, the greater the risk of high-energy fracture of the tip itself.

To minimize the risk of fracture, the cutting efficiency must be reduced to the point where the tips are less effective as cutting instruments. This leads to tedious operations when deep exploration is required. With experience, however, many clinicians have found they can determine proper settings for their particular ultrasonic devices such that the incidence of seemingly spontaneous high-energy fractures of these tips is decreased. Nevertheless, considering the relatively high cost per tip (currently $70 to $120 each), even one fractured ultrasonic tip is not an insignificant matter.

Finally, there is a fundamental difference between the very fine dentin dust that ultrasonic tips create and that created by rotating instruments.4 The dust that ultrasonic tips create has a tenacious adherence to surrounding surfaces, creating a pesky visualization problem. By contrast, the coarser dust particles that fluted round burs create are more easily displaced by air blast or small evacuation tips. That said, ultrasonic tips remain an indispensable part of any serious clinicians endodontic instrumentarium, and they are often the ideal finishing devices in deep access cases after bulk structure removal with long, narrow-shafted round burs.

An inexpensive, yet very effective alternative or adjunct to the ultrasonic tip is the modified-shaft, 34-mm-long, tiny round bur. Currently, Brasseler manufactures the Mueller burs, which are round burs 34 mm in length. However, the shaft of the smallest Mueller bur is only ~0.5 mm in diameter, and as such, these bur shafts are excessively flexible during use (Figure 2; access Video Clip 1 at dentistrytoday.com). This is an undesirable feature for troughing and deep-exploration operations, as it compromises the otherwise positive control of a stiff-shafted bur. Imagine, for example, the impracticality of a flexible ultrasonic tip during a troughing procedure or a flexible round bur during carious excavation. Additionally, at a tip diameter of 0.8 mm, the smallest Mueller bur tip is just slightly smaller than the No. 2 round bur (0.9 mm). And very often, round burs of much smaller diameter are required for these delicate, deep operations.

Figure 2. Nonflexible, 34-mm-long, narrow-shafted round bur (left) compared to flexible shaft of the smallest Mueller bur (right).

Figure 3. The Mueller bur (left) and the LN bur (right) have similar shaft diameters, but their respective design features limit their usefulness for deep radicular access procedures. The Mueller is too flexible and lacks small enough tip diameters, and the LN bur, available in 1/2-round tip size only, has an inadequate overall length of only 28 mm.

Figure 4. The 34-mm-long, standard-shaft No. 2 round bur (left) and 30-mm-long, standard-shaft No. 2 round bur (right). The large shaft diameters (2.4 mm) of standard round burs preclude the use of these burs for deep access procedures.

DENTSPLY offers the LN bur, which is a 1/2-round bur with a shaft diameter similar to that of the smallest Mueller burs (~0.55 mm). However, because the narrowed portion of the LN bur shaft is only 8 mm long as compared to the narrowed portion of the Mueller bur, which is 15 mm long, the LN shaft is considerably stiffer. The overall length of the LN bur, however, is only 28 mm (Figure 3), and that renders it far less useful for troughing and deep exploration because of decreased reach and decreased visual vector beyond the head of the handpiece.

Brasseler also manufactures latch-type, 34-mm-long round burs with standard shafts of 2.4-mm diameters. These are 4 mm longer than standard, 30-mm, surgical length, slow-speed round burs (Figure 4), and they are the blanks from which the nonflexible, modified-shaft round burs so useful in troughing and deep exploratory procedures can be created at the chairside. These burs are available in sizes from No. 8 round down to 1/2-round. As manufactured, they have little use in troughing or deep-exploration operations, because the bulky shaft tends to impinge on the walls of the access cavity or the walls of the deep exploratory cavity, unfavorably guiding the tip of the bur, leading to ledging and potential perforation. However, custom-modification of the shaft at chairside down to ~1 mm in diameter preserves adequate shaft stiffness while providing relief from impingement on access cavity walls.

SHAFT-MODIFICATION TECHNIQUE
Custom-Create Deep-Access Instrument At Chairside

These long bur shafts are easily modified at chairside in a handmilling operation, simultaneously holding in one hand the slow-speed handpiece with the selected 34- mm-long round bur in the contrangle, and in the other hand the high-speed handpiece with a straight fissure carbide bur. The long axis of the carbide bur is oriented at about 70 to the long axis of the long, slow-speed bur shaft (Figure 1a), and both handpieces are activated at full speed (access Video Clip 2 at dentistrytoday.com). As a practical matter, having both handpieces connected to the same unit allows the same foot control to simultaneously activate both of them. Stabilizing the operation by palm-grasping both handpieces and placing the backsides of both thumbs together, the carbide bur is then used essentially as a gouge, creating a scalloped shaft from the already-existing tapered portion at the working end of the long, slow-speed bur up to a few millimeters from the point where the shaft emerges from the head of the contrangle handpiece.

The objective is to cut each scallop on the shaft to the same depth, so that the modified portions of the shaft are all approximately 1 mm in diameter (Figure 1b). Then the carbide bur is used in a sweeping motion, starting at the first few scallops near the tip of the bur and working toward the head of the handpiece until the shaft has a uniform diameter of approximately 1 mm (Figure 1c). The carbide bur can be quite aggressive, especially in the scalloping operation, and one might expect to spoil a long bur or two in the learning process, but at their current price of ~$3.30 each, it is an acceptable price to pay for learning to create this useful instrument. Nevertheless, a light touch will prove useful.

It is worth mentioning that a high-speed bull-nosed diamond bur, while less aggressive, is an effective alternative to the carbide bur. It can be used for the entire process, for the second step of reducing the shaft to a uniform diameter after the scalloping procedure with the carbide bur, or for a final dressing of the shaft after dimensioning with the carbide bur (Figure 1d). This entire procedure takes ap-proximately 2 minutes to perform and can be delegated to an assistant or a lab technician to be made in advance. Particularly useful for deepexploration operations are the No. 4, the No. 2, the No. 1, and the 1/2-round burs. As of this writing, such long, stiff, narrow-shaffed, tiny round burs are not commercially available.

GENERAL CONSIDERATIONS FOR DEEP RADICULAR ACCESS

Figures 5a and 5b. Inadequate extent of previous access (5a) compared to extended outline form (5b) required to visualize and gain access to the untreated MB2 canal. Note that in both upper and lower molars, such extension commonly requires invasion of the MB cusp.

Figures 6a to 6c. While the outline form of this extended access is adequate (6a), structure on the mesial wall still impedes visibility of the target area the white line (isthmus) between the MB1 and the suspected MB2 canal. This structure is removed with a high-speed tapered diamond (6b) or a slow-speed round bur until the target area is visible (6c).

Deep radicular access, whether in search of hidden canals and obstructions or for chasing out fractures and other types of canal wall irregularities, relies heavily on prior adequate coronal access. This access will routinely require a degree of exaggeration even beyond the convenience form generally applied to endodontic access cavities to accommodate visibility and deep instrument access to the site (Figures 5a and 5b). If the site is deep in the mesial root of either a maxillary or mandibular molar in search of a second mesial canal, for example, the coronal access will almost certainly require invasion of the MB cusp to provide unimpeded visibility and instrument access to the site. Judicious removal of structure on various aspects of the walls of the coronal access as it transitions into the radicular access will prove indispensable as the process develops. Determining precisely where to remove structure is simply a matter of making a best effort to view the target area using all available direct and reflected viewing options, noting specifically where structure impedes visibility, and then removing enough of that structure to provide an adequate view using either a high-speed tapered diamond or slow-speed round burs (Figures 6a to 6c). This requires a keen working knowledge of the external anatomy of the cervical and radicular structures to avoid perforation.

Figure 7a to 7i. Exploration deep within the root in search of the severely calcified buccal canal (7a) is performed by applying the buccal object rule to paired, multiangled radiographs one straight-on and one angled sharply from the M or the D with a radiopaque marker at incremental increases in exploratory depth (7b to 7g). This technique provides discrete information about the spatial position of deep exploration within the root and reduces the risk of perforation while enhancing the chance of locating a hidden or calcified canal (7h and 7i).

Once beyond the level of the pulpal floor, the work becomes considerably more complex as the 3-dimensional structure of the root cannot actually be seen, but can only be conceptualized based on multiangled radiographs and the operators clinical experience. Judicial troughing in dentin in the area of a suspected canal while progressing apically is perhaps the most common method of exploration for stealth canals. Paired radiographic angles one straight-on and one sharply angled from the mesial or the distal taken at incremental increases in exploratory depth with a radiopaque marker (such as a small portion of cavit placed with an endodontic condenser or a radiopaque liquid deposited by microtip injection at the deepest exploratory point) will provide information as to where the excavation is progressing within the body of the root (Figures 7a to 7i). The straight-on radiograph reveals the mesio-distal location of the marker within the root, and applying the buccal object rule, the sharply angled radiograph gives information about the facio-lingual/palatal location within the root. As a general rule, working into the bulk of root structure is always safest, and that means working away from the furcal aspect of the canal. That said, removal of structure from the furcal wall of the canal or the orifice is sometimes necessary to provide unimpeded visibility and instrument access, but should of course be done with discretion and a steady hand.

TECHNIQUE: DEEP RADICULAR EXPLORATION FOR A HIDDEN CANAl
Use of 34-mm-Long, Nonflexible, Narrow-Shafted, Slow-Speed Round Burs

In this clinical case, the straight-on radiograph and the radiograph angled sharply from the distal (Figures 8a and 8b) revealed an eccentric exit for the previously treated M canal as well as a furcal invagination of the mesial root, both suggesting the existence of an untreated second mesial canal. On access, the 3 previously treated orifices were immediately evident, but there was no anatomic suggestion of a second mesial canal (ie, there was no white line on the chamber floor originating from the treated mesial canal and progressing toward the palatal aspect of the chamber floor).

Figure 8a to 8e. Pretreatment radiographs taken straight-on and angled from the distal both suggested the presence of an untreated MB2 canal (8a and 8b). Deep troughing 7 mm beyond the pulpal floor with narrow-shafted, 34-mm-long, tiny round burs eventually revealed the MB2 canal (8c), which was cleaned, shaped and obturated. The MB1, DB, and P canals were all retreated as well (8d and 8e).

Judicious removal of dentin from the base of the mesial access cavity wall working into the mesio-cervical aspect of the tooth away from the furcal aspect of the floor with a chairside-modified, narrow-shafted, 34-mm-long No. 2 slow-speed round bur eventually revealed the white line of the isthmus between the MB1 canal and the suspected MB2 canal (access Video Clip 3 at dentistrytoday. com). This line was then chased using the modified-shaft Nos. 2 and 1 round burs in a bucco-lingual sweeping motion, again working away from the furcal aspect of the chamber floor, until the groove eventually gave way to orifices at both poles the previously treated MB1 canal orifice and the untreated MB2 canal orifice. In the process, gutta-percha in the previously treated MB1 canal (later removed completely) was removed to approximately the mid-root level. Patency was initially established in the MB2 canal with a No. 06 file (Figure 8c) in the presence of a lubricating and chelating agent (RC-Prep, Premier Dental Products). All 4 canals were cleaned and shaped by the balanced force technique5 using Roane-tipped K-type files (Flex-R files, Miltex), hedstrom files (Miltex), and Gates Glidden burs (Miltex), variously in the presence of a chelating and lubricating agent (RC-Prep), sodium hypo-chlorite, and EDTA (Smear Clear, Sybron). Obturation was via traditional warm vertical condensation technique with nonstandardized gutta-percha points and Kerr Pulp Canal Sealer (Sybron) (Figures 8d and 8e).

DISCUSSION

It cannot be seriously argued today that canal systems are relatively simple, largely tubular elements with an orifice on the chamber floor and a single exit at the root end. All tooth types have well documented morphological outliers, some much more complex than others. Studies have reported that maxillary first and second molars have 2 mesial canals 95.2% of the time;⁶ mandibular molars have 2 distal canals as often as 29% of the time;⁷ maxillary first premolars have 3 canals as often as 6% of the time;8 mandibular first and second premolars have additional canals as often as 26.5% and 13.5% of the time, respectively;⁹ and mandibular lateral incisors have 2 canals 29% of the time.¹⁰ In fact, as these extra canals are more frequently addressed with successful clinical strategies today, these particular anomalies are now coming to be recognized as standard variants, and journals have begun to shift toward publishing case reports showing management of cases now considered to be truly aberrant middle-mesial canals of molars,^11,12 double-rooted and double- or triple-canals of canines,^13-15 and double-rooted and double- or triple-canals of incisors.^16-18

One study of mandibular first premolars reported that the level at which the canals bifurcated from the deep chamber (type IV) occurred at 6 to 9 mm from the cervical level 24% of the time,19 and another study showed that the mesio-buccal root of maxillary first molars had branching complexities of the canal system below the floor (types III, IV, V, and VI) 29.4% of the time.20 The fact that many of these complex canal configurations branch off well beyond the level of the chamber floor points to the need for a variety of practical techniques and instruments to allow the clinician to develop unimpeded visibility and physical access to this area.

CONCLUSION

Figures 9a and 9b. At only 30 mm long, standard surgical length, slow-speed round burs provide an inadequate view corridor beyond the handpiece head. In addition, the 2.4-mm shaft diameter leads to shaft impingement on access cavity walls, unfavorably driving the head of the bur toward ledging and perforation (9a). By contrast, the enhanced view corridor that results with the use of chairside-modified, 34-mm-long, tiny round burs is the result of the favorable geometry created by the increased distance between the handpiece head and coronal structure. The narrow, 1-mm-diameter, modified shaft eliminates impingement on the access cavity walls, significantly reducing ledging and perforation risk (9b).

With the development of high magnification and high-intensity co-axial illumination, the ability to actually see in situ what we have academically known for years about the complex nature of the endodontic system has led to the need for instruments that can safely reach these areas while still allowing adequate visibility sometimes down very narrow view corridors (Figure 9). The introduction of chairside-created, 34-mm-long, narrow-shafted, tiny, slow-speed round burs into the deep access instrumentarium gives dentists the op-portunity to work at previously inaccessible depths with their most common excavation instruments slow-speed round burs in their efforts at sleuthing out hard-to-find canals.

References

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Dr. Munce is a diplomate of the American Board of Endodontics and an assistant professor of graduate endodontics at both Loma Linda University and University of Southern California Schools of Dentistry. Clinical concepts and techniques originated by Dr. Munce have been published in several primary endodontic textbooks, most recently in the 9th edition (2005) of Pathways of the Pulp. He holds a patent on a unique endodontic device, the endodontic Projector, and has been a design consultant for a number of clinical endodontic products. Dr. Munce maintains a private endodontic practice in Santa Barbara, Calif, with an emphasis on endodontic management of teeth with severe coronal-radicular breakdown, canal obstructions, and other complexities. He can be reached at (805) 962-0161 or cjohn.munce@verizon.net.