Gaining access to root canals

                                    Endoseries 13, Access Cavity Preparation MOLARS

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The access cavity preparations for endodontic therapy are designed for efficiently uncovering the roof of the pulp chamber and providing direct access to the apical foramina by way of the root canals.

As part of the access preparation, the unsupported cusps of posterior teeth must be reduced by trimming until a definite clearance in occlusal or lateral movement is obtained. This decreases the chance for cuspal fracture beneath the gingival or bony attachment, which is so difficult to repair, or vertical fracture of the root, which is hopeless.

In teeth where the external and internal forms have been affected by physiologic, pathologic or iatrogenic causes, special care needs to be taken during access cavity preparation.

 

1. Altered tooth position. Before starting the access cavity preparation, assess the tooth both buccally and mesially. Tilt may be due to malocclusion, which is usually in the lingual or buccal direction. Remember, these tilts will not be apparent in the radiographs.

 

2. Altered external crown form may be because of large restorations or full crowns. If possible, remove the crown before preparing the access cavity. Even the X-ray taken with crown in position will not give you information on the size of the pulp chamber, extent of calcification and the approximate depth at which to look for the canal orifices.

 

3. Altered internal crown form is due to calcification and resorption. Aging, caries, abrasion, restoration, and trauma can induce calcification leading to narrowing of the pulp chamber.

 

Even when an X- ray shows obliteration of space, assume that there is sufficient space for lodgment of bacteria and other irritants, that can cause inflammation in the surrounding area of the tooth.

In rare cases of resorption in the pulp chamber, the difficulty is not in entering the pulp chamber, but to locate the root canal orifices.

When proximal or gingival tooth destruction is present, affected areas should be excavated and restored, with either a temporary seal or a permanent restorative material. Then the normal access cavity is prepared through the occlusal surface.

After you have spent sufficient time studying the position and angulation of the tooth, noting the reference points such as central grooves and cusp tips, and studying the radiograph critically, proceed with the actual cutting. The bur should always be kept in line with the long axis of the tooth while preparing the access cavity. So, if there is a tilt of the tooth, hold the bur also tilted so that it is in line with the long axis of the tooth.

Failure to remove the entire roof of the pulp chamber is a common problem that precludes locating the canal systems in posterior teeth. Adhere to the following guidelines to ensure complete removal of the roof.

1. Measure the size and depth of the pulp chamber space on the radiograph by holding the mounted bur in the hand piece next to the image of the crown on the radiograph.

2. Place a safe ended bur adjacent to the overhanging roof and cut laterally to remove the overlying dentin and to flare the walls of the access opening occlusally.

3. Use an explorer to evaluate the removal of the roof or dentin overhangs.

4. Visually inspect the chamber to ensure an unobstructed entry into the canal systems.

 

Maxillary first molar

Maxillary first molar has three roots, one palatal and two buccal. The palatal root is the longest, with largest diameter. The orifice of the palatal canal, located below the mesiopalatal cusp, is large and funnel shaped and usually easy to locate.

There is a high incidence of two canals in the mesiobuccal root and it is safer to assume so, unless proved otherwise.

Distobuccal root has generally only one canal. Presence of four canal orifices dictates an access cavity that is quadrilateral in shape, with rounded corners. Fig 1

Holding the bur in line with the long axis of the tooth, start the access cavity preparation with a fissure bur held in the middle of the central groove. After you feel the drop of the bur into the pulp chamber, continue the preparation with a safe tipped bur in the direction of the mesiopalatal cusp. Locate the larger palatal canal first because it is easier to find and will act as a reference point for locating the orifices of the smaller buccal canals.

Keep the bur in contact with the floor of pulp chamber and move mesiobuccally to the center of the mesiobuccal cusp. The mesiobuccal canal lies beneath the cusp tip. Now move the bur distally and slightly palatally to locate the distobuccal canal orifice.

First molars have three or four canals. The mesiobuccal root has the highest frequency of occurrence of a second canal compared to all the roots of the human dentition. The second canal, referred to as mesiolingual canal, mesiopalatal canal or second mesiobuccal canal (MB2) is clinically detectable in about 77- 85% of the time.( fig 2)

The orifice of the mesiolingual canal is generally found at a distance of about 1.5- 2 mm lingual to the mesiobuccal orifice. It may be obscured by the presence of a dentinal rounded growth, or shelf- like projection over the orifice. Remove the projection by a brushing like motion of a round bur run on slow speed. While attempting to locate mesiolingual canal, remember, the cutting should be done at the expense of the mesial wall. If you direct the bur towards the trifurcation, it would invariably lead to perforation.

 

Maxillary second molar

Maxillary second molars have three or four canals, but don’t be surprised if you come across a tooth with two canals, one palatal and one buccal.

The shape of the access cavity is quadrilateral, but can be ovoid, since the canal orifices are closely situated.

Prepare the access cavity in the same way as for first molar. But you don’t have to cut as far buccally. The mesiobuccal and distobuccal orifices are usually located close to each other. The distobuccal orifice may sometimes be found almost directly palatal to the mesiobuccal orifice, or at a considerable distance distopalatal to the mesiobuccal opening.( fig 3and 4 )

The incidence of mesiolingual canal is not as high as the first molar (only 40- 78%). Nevertheless, you have to make an attempt to locate it.

Maxillary third molar may dictate root canal treatment because of its strategic importance. The canal orifices are situated still closer than in second molar, sometimes it may have only one canal.

Mandibular first molars are the most frequently treated teeth; they also have the credit of having the most complicated canals.

The shape of the access cavity is trapezoidal with rounded corners. The mandibular first molar has usually two roots, but rarely, three roots may be present. The distal root can have one or two canals. Mesial root has generally two canals, mesiobuccal and mesiolingual, and rarely, there may be a third canal named middle mesial or intermediary mesial canal. ( fig 5 and 6)

The distal canals are usually larger. Orifices of the mesial canals are situated below the respective cusp tips. The access opening can be confined to the mesial two thirds of the crown.

Start the preparation in the central pit with a tapered fissure bur. Switch to a safe tipped bur on penetrating the pulp chamber roof. Try to locate the larger distal canal first. Then move the bur in the mesiolingual direction to find the mesiolingual orifice. Since this may be found almost directly mesial to the distal orifice, don’t cut too far in the mesiolingual direction. After locating the mesiolingual orifice, move the bur buccally to find the mesiobuccal orifice, which is usually found below the mesiobuccal cusp tip.

If the distal canal orifice is not in the middle, or small, search for a second distal canal. Remember that it is the distolingual canal, which is frequently left untreated.

Mandibular second molar can have one to four canals. It has usually two roots or rarely three roots. Occasionally, the roots are fused, which will appear as a single root in the X-ray. Be on your guard when you see such a case, because this single rooted tooth can have more complex root canal anatomy than two-rooted tooth. It may have three canals, lateral canals, transverse anastomoses and C- shaped canal.

 

The access cavity is trapezoidal with rounded corners. An important difference from the first molar is that the orifices of the mesial canals are not always corresponding to their cusp tips in the second molars. They may be quite close to each other, at a distance of about a millimeter. In some cases, the mesiolingual and mesiobuccal canals may share a common orifice.

When you locate one mesial canal just mesial to the distal canal, search for the second one in the immediate vicinity.

 

C- shaped canals. Sometimes, the root canal orifices may not be placed in their normal locations, but you may find a thin strip of groove running in a wide ‘C’ shape along the pulpal floor periphery. It may run the whole length of the root like a curtain and exit at or near the root apex as a single foramen. It may also divide in the depth of the canal into two or three canals and exit separately. ( fig 7)

Mandibular third molar may have one, two, or three canals.

Ref:

1. Fabra-Campus H: J Endodont 11; 568-572, 1985

2. Franklin S Weine: Endodontic therapy 5th Ed. Mosby Yearbook Inc USA 1996

3. Gutmann J L et al: Problem solving in Endodontics 3rd Ed. Mosby St. Louis 1997

4. Jacob Daniel: Advanced Endodontics for clinicians. J&J Publishers, Bangalore 1998

                           5. Kulild JC, Peters DD: J Endodont 16(7); 311-317, 1990

 

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