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 5 th
Ed. Mosby Yearbook Inc USA 1996
3. Gutmann J L et al: Problem solving in Endodontics 3 rd
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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