According to Franklin Weine,1 two canals of the same root can present one of the following types of configurations (Fig. 1):
– Two separate and distinct canals from chamber to apex (type III) (Figs. 2A & B)
– Two separate and distinct canals from chamber to apex, with communications at any levels (Figs. 3A & B)
– Two canals leaving the chamber and merging to form a single canal short of the apex (type II) (Figs. 4A, B, C & D).
In the first configuration the two existing canals could be cleaned, shaped and packed separately, as if they were in different roots.
In the second configuration, which is impossible to diagnose early and should be always expected, great care should be taken during the obturation procedure, in order to obtain a perfect apical seal.
In the third configuration, which is easy to diagnose early, great care must be used during the cleaning and shaping2 and the packing3 procedures.
CLEANING AND SHAPING OF CONFLUENT ROOT CANALS
Cleaning and shaping two canals in the same root which merge to a common foramen require particular precautions to prevent tearing of the apical foramen or pointless weakening of the root, including stripping.4
It is futile to reach the same foramen arising from a different canal and thus from a different direction, because this risks tearing the foramen.
It is equally useless to enlarge the apical portion of the canal common to the two canals of the same root: this could lead to stripping of the canal wall or fracture of an endodontic instrument.
Therefore, it is important to estabilish as soon as possible if the two canals existing in the same root have a common apex or if they are independet. The easiest way to do this is to prepare one canal at a time. As soon as you are ready to check the working length of the second canal (after early coronal enlargement5), insert a gutta-percha cone in the first prepared canal and the file in the second (Fig. 5).
The confluence is detected by the impression left by a small file on a gutta-percha cone inserted into the prepared canal (Fig.6). You can now determine the position of the confluence (distance from the apical foramen) and adjust the working length of the instruments in the second canal, as well as the flare to be given to it. The conical form that is developed in the second canal which has been diagnosed to be confluent with the first one, will therefore begin from the confluence itself rather than from the apical foramen.6
The mesial root of a lower molar will serve as an example to illustrate the operative sequence:
1. Clean and shape the mesiolingual canal, which has a more rectilinear course. It is more difficult to cause stripping in this canal, since it is more centered within the root;
2. When the mesiobuccal canal is ready to receive a small size instrument to determine the working length, introduce a gutta-percha cone in the already prepared mesiolingual canal and then the file (e.g. n 10 K-file) in the mesiobuccal canal to measure its length;
3. The file is worked with short movements and it is possible to check whether the cone present in the other canal is being displaced;
4. The file is withdrawn from the mesiobuccal canal;
5. The gutta-percha cone is withdrawn from the mesiolingual canal and carefully examined, preferably with a magnifying lens, to search for any grooves, scratches or folds left by the file (Figs. 7A to D);
6. Once the presence of the confluence and its distance from the apical foramen has been confirmed, start the cleaning and shaping of the mesiobuccal canal, measuring the working length from the point of confluence (Figs. 8A &B). The preparation of this canal is therefore shorter and less marked than the canal already prepared.
The same sequence is suggested in the mesiobuccal root of an upper molar (Fig. 9), once the confluence of the two canals to a single foramen has been diagnosed. It is extremely important to perform a moderate enlargement of the mesiopalatal canal, given the natural buccopalatal thinning of this root that is very often present (Fig. 10). This is an important consideration to prevent the high risk of stripping.
OBTURATION OF COMMUNICATING AND CONFLUENT ROOT CANALS
When you perform the obturation of the two canals that lie in the same root, and when there is a suspicion of presence of a communication between canals of two different roots (Figs. 11A & B), you must obturate the two canals simultaneously. This prevents obturation material introduced into the first canal during the compaction procedure from passing into the second canal through the natural communication, hindering proper filling of the latter.
The approach is different in the following instances:
– Two canals merging to a single foramen;
– Two canals with independent apical foramina, which could communicate at any level.
In the case of two canals merging to a single foramen, the endodontist very often recognizes the convergence only when the radiograph of the cone fit is obtained. Individually, the cones advance unhindered to the desired maximal depth, but when they are placed simultaneously in the various canals, they advance only alternately in the mesial root of a lower molar, for example, if the buccal goes to length, the lingual remains short and vice versa.
This is clear proof that the two canals merge together, and only one of the two cones can be engaged at the apical foramen. The endodontist must decide which of the two is preferable to advance to the apex and this would be the one that has better “tug-back” and is found in the more easily obturated canal. The other point is then shortened so that it touches the other cone.
As previously mentioned, nevertheless, it is preferable to diagnose the confluence of two canals at a common apex as early as possible, to prevent unnecessary over-instrumentation and transportation of the apical foramen, which could occur as a result of cleaning and shaping it twice from two different directions. The early diagnose of a common apex will also prevent the risk of instrument fracture.
At the time of obturation, when using the vertical compaction of warm gutta-percha technique, each of the two cones must be inserted in its canal (Simultaneous Introduction) (Figs. 12A & B). But the compaction must proceed only at the expense of the cone that has been positioned at the foramen (Alternate Compaction) (Figs. 12C & D). For the sake of convenience, this will be called the “first canal.” Only after the successful obturation of the apical third of this canal has been confirmed radiographically (the gutta-percha has moved apically and the obturation appears well compacted), can the operator proceed with the compaction of the second cone of gutta-percha that will be pushed, heated, and compacted against the gutta-percha in the first canal (Figs. 12E & F). If, instead of using this important trick, you perform simultaneous downpacking in the two canals, short apical obturation may occur (because the gutta-percha of the first canal was not pushed sufficiently apically). It may then be impossible to advance this mass of gutta-percha further apically because the cone in the second canal has already been compacted against the first one (Fig. 13).
This technique of downpacking must be used in every case of two merging canals of a single root and, more specifically in the mesiobuccal root of upper molars, after you have ascertained the presence of the mesiopalatal canal and its convergence in the mesiobuccal canal, and in the mesial canals of lower molars.
In the case of canals that communicate half-way along their length (obviously, this communication cannot be diagnosed, but must always be suspected in two canals of the same root) but that flow into independent foramina, you must simultaneously perform not only the introduction of the two cones (Simultaneous Introduction as in the preceding case), but also their compaction (Simultaneous Compaction) (Figs. 14A-14C). This serves to prevent obstruction, with material that flows into one of the canals and to have simultaneously good apical control of the obturation in the two distinct foramina. The communication will be filled more or less in equal parts by the gutta-percha of the two cones. Packing one canal at the time could lead in an inadequate seal of the second canal, because the material flowing back from the first one will impede the correct introduction and downpacking of the second gutta-percha cone.
This technique of obturation must be applied any time you suspect communications, even among roots that appear to be independent on the radiograph.
As previously indicated, it is extremely important to know early whether the two canals of the same root are independent or if there is a convergence of the two canals into a common foramen, since this could influence your approach. For example, the mesiopalatal canal of an upper molar sharing an apex with the mesiobuccal canal can and must be enlarged less with less risks of weakening of the root.
There are several methods available to aid this early diagnosis. The easiest and safest way is to try this little trick: place the gutta-percha cone in the canal that has just been prepared, and introduce a small instrument (n 10 K-file) into the other canal that you wish to determine whether its apical foramen is independent or not. If the foramen is shared, the file, working in the canal, will leave its impression on the gutta-percha cone. In this way, it is possible to determine whether there is a confluence and at how many millimeters from the foramen it is located.
In this author’s opinion, the introduction of two instruments into two canals with the aim of demonstrating a possible confluence is not free of risks or false indicators. If the canals are narrow, you can easily fracture one of the two instruments. If a canal has already been prepared and can receive a larger instrument, the small instrument introduced into the other canal can pass between the spirals of the preceding instrument and the dentinal walls, arriving equally at the foramen and conceal the tactile sensation of the confluence.
Another very safe and precise method requires the use of an electronic apex locator (E.A.L.). After the first canal has been prepared, the operator checks the working length of the second canal with the use of the E.A.L. Then repeat the operation leaving the last apical file inserted in the first canal at the foramen. If the working length of the second canal this time appears to be shorter by several millimeters, this would indicate that the second canal is sharing the foramen with the first one and the communication is at the same distance from the common foramen.
An awareness of the presence of two canals in a single root is very important today, when using rotary nickel titanium instruments (Fig. 15). The main disadvantage of rotary instrumentation is the risk of instrument fracture. It is well known that the introduction of the rotary instrument in the common part of two canals joining together is one of the main causes of fracture. It is therefore crucial to fully understand and show the maximum respect of this canal configuration in order not to transport the foramen, not to strip the root and not to fracture an endodontic instrument. Furthermore, this increased awareness of the anatomy of the root canal system will help us fill our canals with a perfect apical seal, thus guaranteeing long term success.
Dr. Castellucci is an active member of the American Association of Endodontists, European Society of Endodontology and the Italian Society of Restorative Dentistry.
Oral Health welcomes this original article.
1.Weine F.S. Endodontic Therapy, 3rd ed. St. Louis: The C.V. Mosby Company, 1982; 207-55.
2.Castellucci A. Endodonzia, Bologna, ed Martina, 1993; 371.
3.Castellucci A. Endodonzia, Bologna, ed Martina, 1993; 492-6.
4.Berutti E. Il rispetto dell’apice dentario nella strumentazione dei canali confluenti. G It Endo 1990; 1: 6-21.
5.Ruddle J.C. Endodontic Canal Preparation: Breakthrough Cleaning and Shaping Strategies, Dentistry Today, February, 1994.
6.Castellucci A., Becciani R. I canali comunicanti e i canali confluenti: considerazioni cliniche e suggerimenti pratici. G It Endod 1994; 3: 90-6.