Throughout this chapter, specific statistics are referenced with superscript letters like this (data^A). The statistics are then listed with the referencing letters at the end of this chapter.

A. THE SHAPE OF PULP CAVITIES AND CONFIGURATION OF PULP CANALS

The pulp cavity is the cavity in the inner portion of the tooth containing the nerves and blood supply to the tooth. It is divided into the pulp chamber (more coronal) and the root canals (in the roots).

1. Pulp Chamber and Pulp Horns

The pulp chamber is the most occlusal or incisal portion of the pulp cavity. There is one pulp chamber in each tooth. It may be located partly in the crown of anterior teeth, but in posterior teeth, it is mostly in the cervical part of the root. Its walls are the innermost surface of the dentin. Each pulp chamber has a roof at its incisal or occlusal border often with projections called pulp horns, and the pulp chambers of multirooted teeth have a floor at the cervical portion with an opening (orifice) for each root canal (Fig. 8-1). The number of pulp horns found within each cusped tooth (molars, premolars, and canines) is normally one horn per sizeable cusp, and in young incisors, it is three (one horn in each of the three facial lobes, which is the same as one lobe per mamelon). An exception is one type of maxillary lateral incisor (called a peg lateral with an incisal edge that somewhat resembles one cusp) that has only one pulp horn. Refer to Table 8-1 for a summary of the number of pulp horns related to the number of cusps normally found within different tooth types.

2. Root Canals (Pulp Canals)

Root canals (pulp canals) are the portions of the pulp cavity located within the root(s) of a tooth. Root canals connect to the pulp chamber through canal orifices on the floor of the pulp chamber, and pulp canals open to the outside of the tooth through openings called apical foramina (singular foramen), most commonly located at or near the root apex (Fig. 8-1). The shape and number of root canals in any one root have been divided into four major anatomic configurations or types (Fig. 8-2). The type I configuration has one canal, whereas types II, III, and IV have either two canals or one canal that is split into two for part of the root. The four canal types are defined as follows:

Type I—one canal extends from the pulp chamber to the apex.

Type II—two separate canals leave the pulp chamber, but they join short of the apex to form one canal apically and one apical foramen.

Type III—two separate canals leave the pulp chamber and remain separate, exiting the root apically as two separate apical foramina.

Type IV—one canal leaves the pulp chamber but divides in the apical third of the root into two separate canals with two separate apical foramina.

Accessory (or lateral) canals also occur, located most commonly in the apical third of the root (Fig. 8-3A and B) and, in maxillary and mandibular molars, are common in the furcation area.^A

B. SHAPE OF PULP CAVITIES IN SOUND YOUNG TEETH

LEARNING EXERCISE

Section extracted teeth to expose the pulp cavity: the size, shape, and variations of pulp cavities are best studied by the interesting operation of grinding off one side of an extracted tooth. Extracted teeth should always be sterilized as described in the introduction of this text and kept moist. Wearing a mask and gloves, you can use a dental lathe equipped with a fine-grained abrasive wheel about 3 inches in diameter and 3/8 inch thick to remove any part of the tooth. Simply decide which surface is to be removed, hold the tooth securely in your fingers, and apply this surface firmly to the flat surface of the abrasive wheel. Operating the lathe at a fairly high speed is less apt to flip the specimen from your fingers than operating it at a low speed. If you can devise an arrangement by which a small stream of water is run onto the surface of the wheel as the tooth is ground, you will eliminate flying tooth dust and the bad odor of hot tooth tissue. If such an arrangement is not feasible, keep the tooth moist by frequently dipping the surface being ground in water or by dripping water onto the wheel with a medicine dropper. Look often at the tooth surface you are cutting, and adjust your applied pressure to attain the plane in which you wish the tooth to be cut. A high-speed dental handpiece and bur will greatly facilitate your exploration of the insides of teeth.

As you examine different sides of each kind of tooth, notice how the external contours of the pulp chamber are similar to the external morphology of the tooth. On incisors and canines, you can remove either the facial or lingual side from some teeth to view the mesiodistal plane (as seen in Fig. 8-4A and E) and remove the mesial or distal side from others to view the faciolingual plane (as seen in Fig. 8-4B–D). On premolars and molars, the removal of either the mesial or distal side will expose the outline of the roof of the pulp chamber where pulp horns can be seen extending beneath the cusps (as seen in premolars in Fig. 8-4C and D). When the buccal or lingual sides are removed to the level of the buccal and lingual cusp tips, pulp cavities can be seen in a mesiodistal plane (as seen in Fig. 8-4E), the view similar to that seen on a dental radiograph. Finally, on molars, the removal of the occlusal surface will reveal the openings (orifices) to the root canals on the floor of the pulp chamber (as seen later in the diagram in Fig. 8-9 and the close-up view in Fig. 8-13).

1. Pulp Shape in Anterior Teeth (Incisors and Canines)

a. Pulp Chamber and Pulp Horns of Anterior Teeth

When an incisor is cut mesiodistally and viewed from the facial (or lingual) (similar to the view on dental radiographs), the pulp chambers are broad and may appear as three pulp horns. Only two horns can be seen in the maxillary central incisors in Figure 8-5. However, the incisal border of the pulp wall (roof of the chamber) of a young tooth may show the configuration of three mamelons, that is, has developed with three pulp horns: located mesially, centrally, and distally. (Recall, however, that there is an unusual peg lateral incisor that only has one pulp horn.) Knowing the number and location of these pulp horns becomes important when the tooth is fractured or badly decayed and must be prepared for an incisal restoration. When an anterior tooth is cut labiolingually and viewed from the proximal, the pulp chambers taper to a point toward the incisal edge (Fig. 8-6). In maxillary and mandibular canines, the incisal wall or roof of the pulp chamber is often less pointed, having only one pulp horn (Fig. 8-7).

b. Root Canal(s) of Anterior Teeth

Recall that all anterior teeth are most likely to have one root. The number of root canals in each type of anterior tooth is also most frequently one. Maxillary central incisors, lateral incisors, and canines almost always have one canal (type I), whereas mandibular anterior teeth, although most likely to have one canal, may have two canals (one facial and one lingual) with the frequency varying depending on the study cited.^1–3,B The mandibular canine is the anterior tooth most likely to have two roots (though still uncommon), one facial and one lingual, and this configuration would have two root canals, one in each root.

2. Pulp Shape in Premolars

a. Pulp Chambers and Pulp Horns in Premolars

When premolars are cut mesiodistally and viewed from the facial (or lingual) similar to the view on dental radiographs (Fig. 8-8A), the occlusal border or roof of the pulp chamber is curved beneath the cusp similar to the curvature of the occlusal surface. When cut buccolingually and viewed from the proximal, the pulp chamber often has the general outline of the tooth surface, sometimes including a constriction near or apical to the cervix (seen in Fig. 8-8C). The pulp horns on the roof are visible beneath each cusp, and their relative lengths are similar to the relative heights of the cusps. Thus, the buccal horns are longer than the lingual horns.

In general, premolars have one pulp horn per sizeable cusp. Therefore, the premolars that are the two-cusp type most often have two pulp horns (Fig. 8-4C), but mandibular second premolars that are the three-cusp type have three pulp horns, and the mandibular first premolars that have a functionless lingual cusp may have only one pulp horn (Fig. 8-4D), similar to a canine.

b. Root Canal(s) and Orifices of Premolars

Maxillary first premolars most often have two roots (one buccal and one lingual) and two canals (one in each root as seen in Fig. 8-8B and C). Even maxillary first premolars with a single root almost always have two canals. The average incidence of two canals, one in the buccal root and one in the lingual root, is 90%,^C although there is a small incidence of three roots.^D The dentist must know the location of each canal opening on the pulp chamber floor in order to remove diseased pulpal tissue from the entire pulp cavity. The buccal canal orifice in the maxillary first premolar (viewed through the prepared access opening and the roof of the pulp chamber removed in Fig. 8-9) is located just lingual to the buccal cusp tip. The lingual canal orifice is located just lingual to the central groove.

Maxillary second premolars most often have one root and one canal, but two canals are frequently present.^E When there is one canal, its orifice on the pulp chamber floor is located in the exact center of the tooth (Fig. 8-9). If the orifice is located toward the buccal or the lingual, it probably means that there are two canals in the root.

Mandibular first and second premolars most frequently have one root and one root canal (type I) (Fig. 8-10), but mandibular first premolars may have two canals, which are type IV.^F The single canal orifice is located on the floor of the pulp chamber just buccal to the center of the occlusal surface (Fig. 8-9).

3. Pulp Shape in Molars

a. Pulp Chambers and Pulp Horns in Molars

The pulp chamber of maxillary first and second molars is broader buccolingually than mesiodistally (like the crown shape) and is often constricted near the floor of the chamber (seen in Fig. 8-11A and B). On mandibular first and second molars, the chamber is broader mesiodistally than buccolingually (like the crown shape). This difference in shape of pulp chambers for maxillary versus mandibular molars can be appreciated by studying the openings used to access the pulp chambers for molars in Figure 8-9. As in all cusped teeth, molars have one pulp horn per functional cusp, and they are located in the roof of the pulp chamber well beneath each cusp. Therefore, if we consider the cusps of Carabelli to be functionless, all four-cusp types of molars have four pulp horns, three-cusp maxillary molars have three pulp horns, and the mandibular first molar with five cusps is the only type of molar to have five pulp horns. Three pulp horns are visible under the three buccal cusps in Figure 8-12A. The pulp chamber is normally deep to, or some distance from, the occlusal surface, actually located within the cervical part of the root trunk (Fig. 8-12). Surprisingly, the dentist does not often penetrate the pulp chamber on a maxillary molar until the drill reaches the level of the gum line. One exception might be the long pulp horn of the longest mesiolingual cusp of the maxillary molars (Fig. 8-11A). The floor of the pulp chamber is considerably apical to the cervical line; it is located in the root trunk. The pulp floor has multiple openings (orifices), one for each root canal. The floor is level or flat in young teeth. It may become convex in older teeth with the deposition of additional dentin over time.

b. Root Canal(s) and Orifices of Molars

Maxillary first molars most frequently have three roots (mesiobuccal, distobuccal, and palatal), but four canals: one each in the distobuccal and palatal root and two in the mesiobuccal root. In the palatal root, a single canal is larger and more easily accessible from the floor of the pulp chamber than for the other two roots,^G but this root and its canal often curve toward the buccal in the apical third, requiring skillful procedures to clean and treat it. On maxillary first molars, there are therefore four orifices on the floor of the pulp chamber: one for each canal (Fig. 8-13). Maxillary second molars, like maxillary first molars, most frequently have three roots and four canals. The mesiobuccal root usually has two canals.^H The distobuccal and palatal roots each have one canal. The location of the orifices in the maxillary second molar is similar to the maxillary first molar, except that they are closer together (Fig. 8-9).

Both mandibular first and second molars most frequently have two roots (mesial and distal) and three canals. The wider mesial roots most often have two canals: mesiobuccal and mesiolingual.^I The narrower distal roots most often have only one canal.^J The roof of the pulp chamber is often at the same level as the cervical border of the enamel, with only the pulp horns extending into the anatomic crown (Fig. 8-12). Most of the pulp chamber is located within the root trunk. Location of the orifices of mandibular molars is shown in Figure 8-9.^K

Maxillary third molars usually have three root canals, and mandibular third molars usually have two. However, they do vary considerably in root form. Third molars are 9 to 11 years younger biologically than first molars, completing their development later in life than do first and second molars. Therefore, on radiographs (x-ray films), their pulp chambers and root canals are generally larger than in the other molars in the same mouth, especially for persons between the ages of 15 and 35 years.

Refer to Table 8-2 for a summary of the number of root canals related to the number of roots normally found within different tooth types.

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