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Pecora et al¹⁰⁹

The maxillary first premolar is prone to mesiodistal root fractures and fractures at the base of the cusps, particularly the buccal cusp. If a fracture is suspected, all restorations should be removed and the coronal anatomy inspected with fiberoptic light and magnification. Full occlusal coverage is required after root canal therapy to prevent cuspal and crown and root fracture.

The access preparation for the maxillary first premolar is oval or slot shaped (Fig. 7-91, A to C). It also is wide buccolingually, narrow mesiodistally, and centered mesiodistally between the cusp tips. In fact, the mesiodistal width should correspond to the mesiodistal width of the pulp chamber. The buccal

extension typically is two thirds to three fourths up the buccal cusp incline. The palatal extension is approximately halfway up the palatal cusp incline. The buccal and palatal walls funnel directly into the orifices. Because of the mesial concavity of the root, the clinician must take care not to overextend the preparation in that direction, as this could result in perforation.

Variation

When three canals are present, the external outline form becomes triangular with the base on the buccal aspect. The mesiobuccal and distobuccal corners of the triangle should be positioned directly over the corresponding canal orifices (Fig. 7-91, D).

Fig. 7-90

Maxillary first premolar. Development and anatomic data. Average time of eruption: 10 to 11 years.

Average age of calcification: 12 to 13 years. Average length: 20.6 mm. Root curvature (most common to least common): buccal lingual, straight, buccal; palatal straight, buccal, distal; single root-straight, distal, buccal.

Fig. 7-91

Fig. 7-92

Lateral bony lesion associated with a filled lateral canal.

Fig. 7-93

Two canals that have fused and then redivided.

Fig. 7-94

Three canals.

Maxillary Second Premolar

premolar. A buccal and a palatal pulp horn are present; the buccal pulp horn is larger. A single root is oval and wider buccolingually than mesiodis-tally. The canal(s) remain oval from the pulp chamber floor and taper rapidly to the apex.

The roots of the maxillary second premolar are approximately as long as those of the first premolar, and apical curvature is common, particularly with large maxillary sinus cavities. The proximity of this tooth to the sinus can lead to drainage of a periradicular abscess into the sinus and exposure of the sinus during apical root surgery.

Like the maxillary first premolar, the second premolar is prone to mesiodistal root fractures and fractures at the base of the cusps, usually the buccal cusp. If a fracture is suspected, all restorations in the tooth should be removed and the coronal anatomy inspected with a fiberoptic light and

magnification. Full occlusal coverage is required after root canal therapy to prevent cusp and crown root fracture.

Table 7-12 Studies of Apical Canal Configurations for the Maxillary Second Premolar AUTHORS

28

permit straight line access to these canals than the first premolar with two roots and diverging canals. If only one canal is present, the buccolingual extension is less and corresponds to the width between the buccal and palatal pulp horns (Fig. 7-96). If three canals are present, the external access outline form is the same triangular shape illustrated for the maxillary first premolar (see Fig. 7-91, D).

Fig. 7-95

Average time of eruption: 10 to 12 years. Average age of calcification: 12 to 14 years. Average length:

21.5 mm. Root curvature (most common to least common): distal, bayonet, buccal, straight.

Fig. 7-96

Access cavity for a maxillary second premolar as viewed through the DOM. (×5.1 magnification with cervical fiberoptic transillumination.)

Fig. 7-97

Second premolar with three canals and a large lateral canal.

Fig. 7-98

Single canal that has split into three canals.

Maxillary First Molar

The maxillary first molar is the largest tooth in volume and one of the most complex in root and canal anatomy. The pulp chamber is widest in the buccolingual dimension, and four pulp horns are present (mesiobuccal, mesiopalatal, distobuccal, and distopalatal). The pulp chamber’s cervical outline form has a rhomboid shape, sometimes with rounded corners. The mesiobuccal angle is an acute angle; the distobuccal angle is an obtuse angle; and the palatal angles are basically right angles. The palatal canal orifice is centered palatally; the distobuccal orifice is near the obtuse angle of the pulp chamber floor;

and the main mesiobuccal canal orifice (MB-1) is buccal and mesial to the distobuccal orifice and is positioned within the acute angle of the pulp chamber. The second mesiobuccal canal orifice (MB-2) is located palatal and mesial to the MB-1. A line drawn to connect the three main canal orifices (MB orifice, distobuccal [DB] orifice, and palatal [P] orifice) forms a triangle, known as the molar triangle.

Table 7-13 Studies of Apical Canal Configurations for the Palatal Root of the Maxillary First Molar AUTHORS

ONE CANAL (%)

Pineda and Kuttler¹¹⁷ 100 100

Acosta Vigouraux and Trugeda Bosaans¹ 100

66.7

Case report

* Percentage of cases in which one canal divided to form two.

† Percentage of cases or case report in which two canals joined to form one.

Table 7-14 Studies of Apical Canal Configurations for the Distobuccal Root of the Maxillary First Molar

3.6

Caliskan et al²¹ 98.4

1.6

Acousta Vigouraux and Trugeda Bosaans¹ 100

Case report

* Percentage of cases in which two canals joined to form one.

The three individual roots of the maxillary first molar (i.e., mesiobuccal root, distobuccal root, and palatal root) form a tripod. The palatal root is the longest, has the largest diameter, and generally offers the easiest access. It can contain one, two, or three root canals (Table 7-13). The palatal root often curves buccally at the apical one third, which may not be obvious on a standard periapical radiograph.

From its orifice the palatal canal is flat, ribbonlike, and wider in a mesiodistal direction. The distobuccal root is conical and may have one or two canals (Table 7-14). From its orifice, the canal(s) first is oval and then becomes round as it approaches the apical third of the root. The mesiobuccal root has generated more research and clinical investigation than any other root in the mouth. It may have one, two, or three root canals (Table 7-15). A single mesiobuccal canal is oval and wider buccolingually; two or three

second molars.⁵² The clinician must always keep in mind that the location of the MB-2 canal varies greatly; this canal generally is located mesial to or directly on a line between the MB-1 and palatal orifices, within 3.5 mm palatally and 2 mm mesially of the MB-1 orifice (Fig. 7-101). These authors found that not all MB-2 orifices lead to a true canal. A true MB-2 orifice was present in only 84% of molars in which a second orifice was identified (Fig. 7-102).¹⁴⁴

Negotiation of the MB-2 canal often is difficult; a ledge of dentin covers its orifice, the orifice has a mesiobuccal inclination on the pulp floor, and the canal’s pathway often takes one or two abrupt curves in the coronal part of the root. Most of these obstructions can be eliminated by troughing or

countersinking with ultrasonic tips mesially and apically along the mesiobuccal pulpal groove (Fig. 7-103). This procedure causes the canal, when present, to shift mesially, meaning that the access wall must be moved farther mesially. Troughing may need to be 0.5 to 3 mm deep. Care must be taken to avoid furcal wall perforation of this root. Apical to the troughing level the canal may be straight or may curve sharply to the distobuccal, buccal, or palatal.

Because the maxillary first molar almost always has four canals, the access cavity has a rhomboid shape, with the corners corresponding to the four orifices (MB-1, MB-2, DB, and P) (Fig. 7-103). One study demonstrated that the access cavity should not extend into the mesial marginal ridge.¹⁸² Distally, the preparation can invade the mesial portion of the oblique ridge, but it should not penetrate through the ridge. The buccal wall should be parallel to a line connecting the MB-1 and DB orifices and not to the buccal surface of the tooth.

Table 7-15 Studies of Apical Canal Configurations for the Mesiobuccal Root of the Maxillary First Molar