The premolars succeed the deciduous molars. Eight premolar teeth are present, with two in each quadrant. The term premolar implies that these will be located immediately anterior to the permanent molars. In the study of human dentition the term bicuspid is often used in place of premolar. This is inaccurate because bicuspid presupposes that a tooth has only two cusps. In human dentition, however, mandibular premolars show variation from one to three in the number of cusps. Thus the use of the term bicuspid is discouraged in favor of the term premolar.
The maxillary first and second premolars and the mandibular first premolars are developed from four lobes just like the anterior teeth. The mandibular second premolars usually develop from five lobes—three buccal and two lingual.
The buccal cusp of a premolar is developed from three labial lobes, as in the anterior teeth. The primary difference in development is that the lingual cusp, which is extremely well formed, develops from the single lingual lobe. In anterior teeth the lingual lobe forms the cingulum of the incisors and canines. In premolars, this single lingual lobe forms an extremely well-developed lingual cusp.
In the case of the three-cusp form (the mandibular second premolar), two lingual lobes are present, each of which forms a separate small lingual cusp. A two-cusp form of the mandibular second premolar also develops from just four lobes. The lingual cusps of the mandibular premolars are small and afunctional when compared with the larger lingual cusps of the maxillary premolars. The premolar crowns and roots are shorter than are those of the canines.
Maxillary first premolars (Fig. 14-1) have buccal and lingual cusps. The buccal cusp is usually 1 mm or more longer than the lingual cusp. These teeth are also the only premolars that normally have two roots, a buccal and lingual, although occasionally, only a single root is evident.
Most maxillary first premolars have two roots and two pulp canals. Even when only one root is present, two pulp canals can usually be found. It is not uncommon for maxillary second premolars to also have two roots; however, usually only one is evident.
A maxillary first premolar is similar in appearance to a maxillary canine. However, the crown is shorter and narrower mesiodistally, and unlike a canine, the mesial and distal contact areas are at about the same level. The mesial and distal marginal ridges are also sharper than are those of a canine.
The tip of the facial cusp is located distally to the midline and separates the occlusal border into a long, straight mesial ridge and a short, convex distal ridge. The mesial ridge may even have a slight indentation at the junction of the mesial and middle lobes. From the contact areas cervically, the distal border is straight, whereas the mesial border is more concave. Two developmental lines on the facial surface mark the coalescence of the developmental lobes. The facial surface of the crown is convex, and an extremely well-developed middle facial lobe is present.
On the mesial surface (Fig. 14-4; see Fig. 14-1, C) of the crown, a groove extends from the mesial marginal ridge cervically. This groove is called the mesial marginal groove. It crosses the mesial marginal ridge and runs from the occlusal third to the middle third of the crown, lingual to the contact area. The mesial surface can also be identified by a mesial developmental depression located cervically to the mesial contact area. The concavity continues cervically from above the contact area across the cervical line, where it joins a deep developmental depression between the roots. The mesial marginal groove is not always present, but the mesial developmental depression usually is quite evident.
The facial outline is convex with the crest of contour located within the cervical third of the crown. The lingual outline is also convex with its crest of contour located within the middle third of the crown. The curvature of the cervical line is greater on the mesial surface than on the distal surface.
From the distal view (Fig. 14-5; see Fig. 14-1, E) a maxillary first premolar is similar to the mesial view, except no groove usually crosses the distal marginal ridge and no developmental depression is present. Some specimens show a distinct distal marginal groove, but the mesial marginal groove is deeper and more obvious. The cervical line is less curved on the distal surface than on the mesial surface. The crown also appears more rounded and smooth. Both the buccal and lingual cusp tips are centered over the root, and this is also true from the mesial view. All maxillary premolars have their cusp tips centered over their root.
The occlusal surface (Fig. 14-6; see Fig. 14-1, C) shows two well-developed cusps. The lingual cusp is more pointed than the facial cusp, but the facial cusp is much larger and longer than the lingual. Each cusp has four ridges emanating from it, and each ridge is named according to its location: facial, lingual, distal, and mesial.
On the facial cusp, the facial ridge descends from the cusp tip cervically onto the facial surface. The mesial and distal ridges descend from the cusp tip to their respective point angles. They are called the mesial and distal cusp ridges.
The lingual cusp ridge extends from the cusp tip lingually to the central area of the occlusal surface. Any ridge that runs from the cusp tip to the central groove of the occlusal surface is called a triangular ridge. Examples are the lingual cusp ridge of the buccal cusp and the buccal cusp ridge of the lingual cusp, which runs from the cusp tip of the lingual cusp to the central groove (Fig. 14-7).
The lingual cusp has four ridges like its counterpart, the buccal cusp. The lingual cusp ridge of the lingual cusp extends onto the lingual surface. The mesial and distal cusp ridges extend from the cusp tip to their respective point angles and fuse into the mesial and distal marginal ridges.
When two triangular ridges join, after traversing the tooth buccolingually, they form a transverse ridge. Thus a transverse ridge exists on the occlusal surface of a maxillary first premolar. It is formed by the union of the two triangular ridges—the lingual cusp ridge of the buccal cusp and the facial cusp ridge of the lingual cusp. See Fig. 14-7 for the transverse ridge formed by the two joining triangular ridges.
From the occlusal aspect, close observation reveals that the crown is wider on the buccal surface than on the lingual surface. Notice also that the buccolingual dimension of the crown is much greater than the mesiodistal dimension.
Thus primary grooves are sharp, deep, and V-shaped. They occur consistently and mark the junction of major anatomic boundaries. All developmental grooves are primary grooves because they occur routinely and are of major importance to anatomic development. Developmental grooves mark the union of what structures?
Secondary grooves are of lesser importance. They differ from primary grooves in that they usually are more shallow and irregular in shape, giving the tooth a more wrinkled appearance. They are not always present.
As a general rule, first premolars and first molars have fewer secondary anatomic features. Second premolars and second molars will have more secondary grooves and pits. Third molars will have even more secondary anatomic grooves, pits, and fissures. Therefore the third molars appear more wrinkled because of the more numerous and shallow anatomic features.
Few secondary grooves are on the occlusal surface of a maxillary first premolar. In most instances the surface is relatively smooth. A well-defined central developmental groove divides the tooth buccolingually. A mesial marginal developmental groove extends from the central developmental groove, across the mesial marginal ridge, and onto the mesial surface of the tooth.
Two developmental grooves connect to the central groove just inside the mesial and distal marginal ridges. These grooves are the mesiobuccal developmental groove and the distobuccal developmental groove. Each can connect at opposite ends of the central developmental groove, at which point they usually end in a deep depression in the occlusal surface called the mesial and distal developmental pits.
The triangular depression that harbors the mesiobuccal developmental groove is called the mesial triangular fossa. Likewise, the depression in which the distobuccal developmental groove lies is called the distal triangular fossa. The terms mesiobuccal developmental groove and mesiobuccal triangular groove are synonymous.
The root of a maxillary first premolar may be either single or bifurcated. The bifurcated root form is far more common, but even in the single root form, two root canals are usually present. The number of pulp horns corresponds to the number of cusps, which in this case is two.
On the single-rooted form, grooves are usually present lengthwise in the middle of the root, giving the appearance of a root trying to divide itself. The mesial root surface has a more highly developed root groove.
The pulp cavity (Fig. 14-8) of the maxillary first premolar has two pulp horns, one for each cusp, and two root canals, one for each root. Sometimes only one undivided root is present. When this occurs, usually two root canals are still evident, although they often combine to form one apical foramen. In some specimens with only one root, only one single root canal is present.
Maxillary second premolars (Fig. 14-9) resemble the maxillary first premolars in both form and function. The crown, however, has a less angular and more rounded appearance. The second premolars also vary from the first in that they usually have only one root. How many roots do the maxillary first premolars have?
Second premolars vary individually more than first premolars. A maxillary second premolar may have a crown that is noticeably smaller cervicoocclusally and mesiodistally. On the other hand, it may be larger in those dimensions and usually is.
From the buccal view (Fig. 14-10; see Fig. 14-9, A), it is evident that the buccal cusp of a second premolar is not as long as that of a first premolar, and it appears less pointed. Also, a second premolar has the same general markings as the first, but they are not as well defined.
The mesial view (Fig. 14-12; see Fig. 14-9, D) shows the difference in cusp length between the maxillary first and second premolars. The buccal cusp of a second premolar is shorter than the buccal cusp of a first premolar, and the lingual cusp is almost as long; thus the buccal and lingual cusps are nearly the same length.
No deep developmental groove crosses the mesial marginal ridge, just as no deep developmental depression is on the mesial surface of the crown; instead, the crown surface is convex. A shallow developmental groove bisects the single root form, giving the appearance of two roots fused into one.
The groove pattern is less distinct than in the first premolar, and the grooves are shorter, shallower, and more irregular. The central developmental groove is also shorter and more irregular, with numerous supplemental grooves radiating from it, giving the occlusal surface a more wrinkled appearance.