10 Top Tips: Access cavities and canal locationAUTHOR:TONY DRUTTMAN

You have to follow some basic rules. You have to know where to look, otherwise a large amount of tooth tissue can be unnecessarily destroyed and the long-term prognosis for the tooth affected (Figure 1).

Figure 1: The floor of the pulp chamber has been considerably damaged, including a perforation while looking for the canals

A good quality preoperative long cone periapical radiograph gives important information about the depth of the pulp chamber, the mesio-distal position of the canal or canals, the canal curvature and whether the canals divide along the way (Figure 2).

Figure 2: Important information can be gleaned from a good quality periapical radiograph

The pulp chamber and the canals reduce in size through life as secondary dentine is laid down, but they also react to trauma and insult, and tertiary or reparative dentine can be laid down to completely or partially obliterate the pulp chamber and canal, especially in the coronal part. This was discussed in the third article of the series on radiography.

The pulp chamber and the root canals should flow one into the other so that instruments can be introduced into the root canals without hitting obstructions on the way. Once entry into the pulp chamber has been achieved, the preliminary shape of the pulp chamber can be created using safe ended Endo-Z burs (available from Dentsply) (Figure 3).

Figure 3: Endo-Z bur

This allows the walls to be shaped without damaging the floor of the pulp chamber. Once the canals have been located and the first instruments placed, a sense of the canal anatomy starts to form and the access design may have to be modified during canal preparation as progressively larger and stiffer rotary instruments are used.

Canal positionsBasic canal positions can be found in any textbook on endodontics so this is not the place to repeat the information. However, it is worth reminding readers how to find an mb2 canal in upper molars. Inability to find the mb2 canal is the most common cause for failure of endodontic treatment in upper molars (Figures 4a and 4b).

Figure 4a: Endodontic lesion above the mb rootFigure 4b: Re-treatment including identification and treatment of the mb2 canalThe incidence varies depending on the papers you read (and how adept the authors are at finding them). The range seems to be about 70-90%, so the answer is always look for the canal.

For those of you that did not see the earlier article on anatomy in the series, I've repeated the schematic diagram to show the relative positions of the canals in an upper molar below (Figure 5) (See the bottom of the page for links to other articles in this series).Figure 5: Schematic of an upper molar showing the relationship of the mb2 canal to the other canals

Draw a line between the palatal canal and the mb1 and then drop a perpendicular line from the db canal to the line between the other two canals. You will usually find the mb2 where they intersect. There are basic rules about canal position which if followed make canal location easier and conserve tooth tissue.

1. The floor of the pulp chamber is always located at the centre of the tooth and at the level of the CEJ - Cementoenamel Junction (Figure 2)2. The walls of the pulp chamber are concentric to the external surfaces of the the tooth at the level of the CEJ (Figure 6)3. The CEJ is the most consistent landmark in locating the position of the pulp chamber4. Except for maxillary molars, the orifices of the canals are equidistant from a line drawn in a mesio-distal direction across the floor of the pulp chamber (Figure 6)5. The pulp chamber floor is always darker than the walls (Figure 6)6. The orifices of the root canals are always located at the junction of the walls and the floor (Figure 6).Figure 6: Position of the pulp chamber and canals in relation to the external outline of the tooth

DifficultiesOften we experience difficulties in being able to recognise the difference between the roof and the floor of the pulp chamber. If the roof is not removed then canals can never be cleaned properly and there will always be a source of bacteria to contaminate the canal system (Figure 7a). It is therefore necessary to examine the floor of the access cavity carefully to determine that any tertiary dentine has been removed (Figure 7b).Figure 7a: The floor of the pulp chamber is covered with reparative or tertiary dentine

Figure 7b: The floor of the pulp chamber is revealedSmall long shank burs are then used to remove pulp stones over the canal entrances and remaining pulp stones in the canals themselves (Figure 8) can be removed with ultrasonic files (Figure 9). It is helpful of course to do this under magnification and with good light, but more about that in the next issue on magnification and illumination.

Figure 8: Pulp stone in the entrances to the canalsFigure 9: Bur selection used in endodontics

Check radiographsOccasionally, canals that are very clear on radiographs can be very difficult to find, often in crowned teeth where the natural shape of the coronal tooth tissue has been lost. In these cases it is important to take check radiographs to ensure that a new canal is not being created heading off towards the periodontal membrane (Figure 10). In heavily restored, damaged or traumatised teeth, the canal may be sclerosed and difficult to locate. The radiograph may well not even show the presence of a canal.

Figure 10: Even though the canal was clearly visible from the pre-operative radiograph, the canal was difficult to locate until a check radiograph had been taken