Correction of Ledged Canals With Ultrasonic Tips

Nonsurgical endodontic therapy can be challenged by any of the following iatrogenic errors: perforations, blocked canals, ledged canals, and broken files. This article will focus on the proper management of ledged canals.
Ledged canals can be formed during the biomechanical preparation of the canal system, mostly in curved canals. There are 2 major causes for the creation of ledged canals1: inadequate extension of the access opening to allow straight access to the apical part of the root canal2-6, and using a noncurved stainless steel instrument that is too large for a curved canal.2-5,7,8
Other contributory factors involved in ledge formation include the following: tooth type, canal location, working length, the master apical file size, the clinician’s level of expertise and experience in endodontics, and root canal curvature. One study revealed that the maxillary left second molar had the highest incidence of ledging, and the degree of the canal curvature also had an influence on ledge formation.9 Another study showed that experienced endodontists had significantly fewer ledged canals than less experienced operators, and that there was a higher incidence of ledge formation in retreatment cases.4 In addition, the type of canal was found to have an effect on the incidence of ledging. The mesiobuccal (MB), mesiolingual (ML) and distobuccal (DB) canals of molars exhibited a significantly higher ledge incidence rate than the distal and palatal root canals. This study also shared the same conclusion as mentioned earlier that canal curvature was the most significant variable affecting the incidence of ledging.4
Histologically, complete calcification of the apical third of the canal system does not occur. In reality, this clinical perception is due to the clinician’s lack of ability to establish patency, and/or a lack of patience to do so. The incidence of ledge formation, when using flexible files (such as Ni-Ti files), is reduced when compared to conventional stainless steel K-type hand files.6,10 The introduction of Nickel Titanium (Ni-Ti) alloy to the endodontic world by Walia6 in 1988 revolutionized the way the canal system is shaped, however the improper use of Ni-Ti rotary instruments has resulted in procedural mishaps.11 Despite the advancement of modern Ni-Ti files, ledge formation during root canal therapy is still inevitable in certain cases. Once a canal is ledged, the endodontic treatment becomes difficult to complete, often resulting in compromises.
There are many techniques available to assist in negotiating a ledged canal, and usually these involve prebending the tip of K files. The size 10 prebent file is the key instrument used in identifying the original canal.8 Prebending of an appropriate file plays a vital role in the negotiation of a ledged canal. This article will present 2 case reports in which ledged canals were bypassed using a modified conventional method with a BUC 1A ultrasonic tip (Obtura Spartan), a CPR-8 ultrasonic tip (Obtura Spartan), and 2 prototype endodontic explorers with No. 10/.08 tapered tips—one with a diamond-coated tip, and the other with a smooth-surfaced tip.

A PROCEDURE FOR BYPASSING LEDGED CANALS

Figures 1a to 1c. Procedures for bypassing a ledged canal using a modified conventional method: (a) Initial canal with a ledge. (b) Standard K file will not easily follow the original canal path and it has a tendency to drop into the ledged canal. (c) Exploration of the original canal with Prototype explorer.	Figures 1d to 1f. Procedures for bypassing a ledged canal using a modified conventional method: (d) Reorientation of the canal orifice (dotted line). Highlighted portions in yellow show the extension of the canal orifice to get access to the original canal. (e) Enlargement of the original canal opening with BUC 1A. (f) Negotiation of the original canal with a pre-bent K file.

Figures 1g to 1i. Procedures for bypassing a ledged canal using a modified conventional method: (g) Shaping of the original canal with a GT file. (h) Flaring the original canal to the full working length. (i) Ledged canal was successfully bypassed with the complete shape of the original canal.

A ledged canal usually makes it difficult to negotiate the original canal due to the straight access to it (Figures 1a and 1b). A ledge is most often created in a curved root when the access to the original canal is off-centered and not straight. Prototype endodontic explorers are very helpful in locating both the ledged canal and the original canal, due to the small diameters and a tapered tip (Figure 1c). (Note: The tips of these explorers should be prebent to fit the curve of the original canal.) The explorer with the smooth-surfaced tip should be used first to locate the original canal. Once the original canal is located, it needs to be placed deep enough into the canal for a glide path. Then, the other explorer with the diamond-coated tip should be used next with a push-pull motion in order to enlarge the canal orifice.
The procedure for negotiation of a ledged canal commences with reorientation of the canal orifice (Figure 1d). The canal orifice should be enlarged toward the wall opposite the ledge to facilitate the exploration of the original canal to its apical foramen (Figure 1e). The BUC 1A is very efficient when it comes to creating straight access to the canal orifice because of its long length and the diamond-coated tip. Its tip can also be prebent if necessary. Other BUC tips can be used, depending on the location of the ledge and the amount of dentin sacrificed for straight access.
The CPR-8 is also long and thin enough to visually confirm its position in the canal under the microscope, and it is also useful to prebend the tip for the access opening of the original canal. It can easily be fitted into the canal, following the pathway created by the 2 prototype explorers. The CPR-8 is made of titanium alloy and is less aggressive in cutting dentin than the BUC-1A, since it is thinner and more flexible. The CPR-8 tip should be used when the amount of remaining dentin is limited or enlargement of the canal orifice requires judicious dentin removal.
Enlargement of the canal orifice should continue until the original canal is discernible under the microscope. Then, the roof of the canal opening should be removed with either of the ultrasonic tips in a continuous “picking” motion to create a pathway that facilitates file insertion into the canal. Once the canal opening is enlarged apical to the ledge, the prototype explorer with the smooth-surfaced tip should be gently slid in and out of the canal with very short push-pull movements, ensuring that the path is smooth. This process is very important to confirm the pathway to the original canal.
The next step is to use a No. 10 K file prebent 2 mm from the tip to negotiate the canal (Figure 1f). The insertion of the curved tip should be smooth by this stage. If there is resistance to negotiation with the file, the process of canal enlargement should be repeated with the ultrasonic tips and the explorers. If the No. 10 K file can be placed to the full-working length, this should be followed by a pre-bent No. 15 K file—if possible up to a No. 20 K file. This is because a Greater Taper (GT) Ni-Ti hand file (DENTSPLY/Tulsa Dental) with a tip diameter of 0.2 mm is going to be used next (Figure 1g). GT files can bypass or even remove ledges, since they have 3 to 6 times the tapers of ISO K files. If needed, the tip of the GT file can also be pre-bent to the curve of the original canal, beyond the Ni-Ti shape memory (Figure 1g). GT files should be used in a “pecking” motion as the canal is initially negotiated. These files are used sequentially to create a glide path with a crown-down technique. Once the GT file reaches full-working length without resistance, preparation of the original canal should be completed (Figure 1h). One should proceed to obturation if the ledged canal stays too narrow for a GT file to drop into it (Figure 1i).

CASE 1

A 38-year-old male was referred to an endodontist for treatment of his mandibular left second molar. The patient was in good general health with no significant past or present illness. His chief complaint was intermittent pain to chewing, ever since the tooth had been treated 3 years previously. Clinical examination revealed that his tooth was slightly sensitive to percussion, periodontal probing was within normal limits, and there was no mobility.
In the preoperative radiograph, both the mesial and the distal canals appeared to be perforated, and obturation material was extruding from the main canal system (Figures 2a). Axially-sliced CT images revealed a C-shaped root with a mesial canal and a distal canal.
Retreatment (Figures 2b, 2c, 2d and 3a to 3d) was commenced. The metal crown was removed and gutta-percha root fillings were noted in both canals. Removal of the fillings was accomplished by rotary Ni-Ti instrumentation without a solvent. The prototype explorer (smooth-surfaced tip) was then inserted in the mesial and distal canals to explore the canal space. Both the original canals and the ledged canals were visually confirmed under a microscope with the prototype endodontic explorer connected to an apex locator in order to detect any perforations. The ledged canals, in both the mesial and the distal canals, were found to be perforated. The perforated canals were filled with light-cured composite resin immediately after the canals were cleaned and dried. The access opening, in the area of the mesial and distal canal orifices, was at an angle to the longitudinal axis of the root. Straightline access to the mesial and distal canals was created by extending the access preparation with a diamond bur (SybronEndo) and a BUC 1A ultrasonic tip. The ledges were then bypassed as described in the procedural section above.

Radiographs were taken with No. 10 K files inserted in the mesial and distal canals to confirm the negotiation of the original canal. The canals were also cleaned and shaped with GT files. The canals were then obturated with .08 taper gutta-percha points (Sybron Endo) and Pulp Canal Sealer (SybronEndo) using a System B heat source (SybronEndo) and the Obtura system (Obtura Spartan) for backfill. The postoperative radiograph showed successful obturation of both canals and the pa-tient was returned to the referring dentist for the final restoration.

CASE 2

A healthy 42-year-old male presented to a general dentist with a chief complaint of cold sensitivity in his mandibular left first molar. Non-surgical root canal treatment was initiated. Unfortunately, because of a severe midroot dilaceration of the mesial root, the MB canal had been ledged and this had resulted in a midroot perforation. The patient was subsequently referred to an endodontist for treatment of the ledged canal and to complete the endodontic procedure.
The patient had no significant medical history. Clinical examination revealed that the tooth was slightly sensitive to percussion and periodontal probing was within normal limits. The preoperative radiograph (Figure 4a) showed a core build-up material in the coronal portion of the mesial root and it appeared that no canal had been previously negotiated. Subjacent to the buildup, the previously placed obturation material could be seen deviating from the main canal system resulting in a mid-root perforation. The canal system was devoid of obturation material beyond the ledge, and there was no evidence of periapical radiolucency.

Treatment (Figures 4b, 4c, 4d and 5a to 5f) was begun by first gaining access, followed by removal of the filling material from the MB canal. A radiograph was taken with an exploration file in place. This demonstrated a straight path in the MB canal contrary to the curve of the mesial root. The ledged canal was immediately visible under the microscope and the bypass procedure was commenced with the enlargement of the access opening. The original canal was soon located and was found to be almost on the distal side of the ledged canal. The ledge in this tooth was then by-passed in the same way as previously described in this article. All other canals were then located, negotiated, cleaned, and shaped with GT files.
Radiographs were taken to confirm canal negotiation with a .06 taper GT file (positioned to the working length) in the MB and distal canals. The canals were then obturated with .08 and .10 taper gutta-percha points and Pulp Canal Sealer (SybronEndo) using a System B heat source (Sybron-Endo) and the Obtura system for backfill (Obtura Spartan).
The postoperative radiograph showed excellent obturation with the ledge successfully bypassed in the MB canal. The patient returned to the referring dentist for the final restoration.

DISCUSSION

The use of an endodontic explorer is important for 3 reasons: it can be used to locate the original canal opening with the aid of a microscope; it can remove dentin around the canal and enlarge the opening with a tactile sensation; and since it is very thin, the whole process can be observed under a microscope. Once one is able to insert the explorer into the original canal, which usually identifies the pathway to the opening of the canal, the ultrasonic tip can also be placed into the canal due to the similar design of those tips.
Both of the ultrasonic tips played a very important role in removing or bypassing a ledged canal, making the whole process easier. These ultrasonic tips convey direct feelings to the handpiece, as well as allowing for high visibility while in the canal. Also, compared to thin K files, they can be more easily negotiated into a constricted canal. This is because a K file is too flexible to penetrate through the narrow opening of the canal and usually bends back on the canal wall. In the cases described above, unnecessary removal of healthy dentin was avoided due to the use of the small, sharp ultrasonic tips. An operating microscope was also essential for the proper correction of ledge canals. A microscope provides illumination and magnification for a dark root canal in situations when only a 1-mm stroke of an instrument can make a big difference in the outcome of the case. Negotiation of a ledged canal is just like removal of a broken file from a root canal—an attempt to correct a ledged canal could end up in iatrogenic accidents such as perforation and/or root fracture. Wilcox et al12, showed that canal enlargement of 40% to 50% of the root width increased susceptibility to vertical fracture. It requires the ability to perform delicate manipulations within a small area.
The prognosis of these cases depends on when the ledge was created. If the ledge was created early in the process of instrumentation, there would be plenty of debris and bacteria left in the remaining canal beyond the ledge. As a result, periapical surgery should be considered because the prognosis is usually considered to be poor. If a ledge formed late in the process, then the apical portion of the canal would already be disinfected sufficiently to allow periapical healing to occur. However, in most cases it would be difficult to determine the true extent of how well the canal was disinfected at the time the ledge was created, especially if it is short of working length. Therefore, it is important to be able to negotiate the ledged canal without further damage to the tooth structure. Furthermore, it is recommended that disinfection of the canal, from the ledge to the foramen, be routinely attempted.
It is obvious from the postoperative radiographs (Figures 2d and 4d) that the use of GT files had smoothed out the ledges and facilitated the glide path when used to the canal terminus during the instrumentation procedure. This made the obturation process easy. It should be noted that it is usually more difficult to bypass a ledge with a large file than a small file in order to reduce the ledge formation. One single GT file contains the equivalent of up to 13 ISO-tapered files, and it is large enough to remove or reduce the ledge with Ni-Ti flexibility.

CONCLUSION

The sequential process described in this article provides a safe and predictable method for correcting ledged root canals. Every instrument used in the cases presented herein played an essential role in achieving a predictable outcome. Overall, the results obtained demonstrated successful management and correction of the ledged canals while preserving canal anatomy. Iatrogenic perforation and further weakening of the tooth structure were also avoided.
Despite our ability to successfully manage ledged canals, when done with the proper materials and techniques, it would be ideal if the canals were not ledged in the first place. Once ledge formation has occurred, it takes a lot more time to reorient it and to complete the endodontic treatment required.

References

Dr. Terauchi is one of the few endodontists practicing in Japan at the CT and Microendodontic Center. He is well known for his expertise in removing fractured endodontic files from root canals. He received the Presidential Awards from the Japan Endodontic Association in 2001, and in 2008. He can be reached at yterauchi@intellident.or.jp.