|SECTION II||CLASS TRAITS THAT APPLY TO MOST PREMOLARS|
1. Most Anterior Teeth and Premolars Develop from Four Lobes
Like all anterior teeth, the facial surfaces of all premolars develop from three facial lobes, usually evidenced by three prominences: a central buccal ridge (Appendix 5a) and a mesial and distal portion separated by two shallow, vertical depressions (Fig. 4-4). The prominent buccal ridge on the maxillary first premolar is similar to the pronounced labial ridge on the maxillary canine. Also, the lingual surfaces of most premolars (like all anterior teeth) develop from one lingual lobe. In premolars, this lobe forms one lingual cusp; in anterior teeth, it forms the cingulum (recall Fig. 1-62). An EXCEPTION is a common variation of the mandibular second premolar, the three-cusp type, which has one buccal and two lingual cusps. The facial cusp still develops from three facial lobes, but the two lingual cusps form from two lingual lobes, one lobe for each lingual cusp. Due to this variation of the mandibular second premolar with three cusps, the term bicuspid (referring to two cusps) is hardly appropriate for this group of teeth. See Table 4-1 for a summary of the number of lobes forming each type of premolar.
|TABLE 4-1||Guideline for Determining the Number of Lobes for Premolarsa|
aNumber of lobes = 3 facial lobes form the facial cusp + 1 lobe per each lingual cusp.
2. Crown Shape Similarities to Anterior Teeth
a. Crowns Taper Narrower toward the Cervical
From the facial, all premolar and anterior crowns are narrower in the cervical third than more occlusally or incisally (Appendix 5m). This is because the widest proximal crests (height) of contour (or contact areas) are located in the occlusal or the middle thirds on posterior teeth (similar to the location on anterior teeth in the incisal to middle thirds).
b. Crowns Taper Narrower to the Lingual
When viewed from the occlusal or incisal, there is less bulk in the lingual half than in the facial half (Appendix 5r). Said another way, premolar and anterior crowns are narrower mesiodistally in the lingual half than in the facial half, EXCEPT some three-cusped mandibular second premolars (with two lingual cusps) where the lingual half is equal to, or wider than, the buccal half (seen later in Fig. 4-33).
c. Distal Contacts Are More Cervical Than Are Mesial Contacts
When viewed from the facial, both mesial and distal sides of the crown are convex around the contact areas. As on most anterior teeth, the distal contacts of most premolars are more cervical than are the mesial contacts (Appendix 5e). Two EXCEPTIONS are the mandibular central incisor where the mesial and distal contacts are at the same level due to the crown symmetry and the mandibular first premolar, where mesial contacts are more cervical than are distal contacts.
3. Cervical Line Curvature
As on all anterior teeth, cervical lines of all premolars, when viewed from the proximal, curve toward the biting surfaces (occlusal or incisal) (Appendix 5o), and the amount of curvature is slightly greater on the mesial than on the distal surface. However, the amount of curvature is less on posterior teeth than on anterior teeth. When viewed from the facial or lingual, cervical lines of all anterior teeth and premolars are curved toward the apex (Appendix 5n).
4. Root Shape
Like on all anterior teeth, the root surfaces of premolars with one root are convex on the facial and on the lingual and taper apically (Appendix 5q). Also, these roots taper toward the lingual, resulting in a narrower lingual bulk of the root mesiodistally compared to its buccal bulk (or, in the case of the two-rooted maxillary premolar, the lingual root is less bulky than the buccal root). When the apical third of the root is bent, it is most often bent distally (Appendix 5p). Notice the similarities in root and crown taper and cervical line curvature on incisors, canines, and premolars when the incisal/occlusal third has been removed in Figure 4-5.
1. Tooth Surface Terminology
Compared to the anterior teeth, the facial surfaces of the posterior teeth can be called buccal (resting against the cheeks) instead of labial, although this surface on all teeth could be called the facial surface. Further, posterior teeth (premolars and molars) have occlusal surfaces instead of incisal ridges. These occlusal surfaces have multiple cusps, ridges, fossae, and grooves.
2. Premolars Have Cusps versus Incisal Edges
Unlike anterior teeth with incisal edges (or cusp ridges on canines) and a lingual cingulum, premolars have one buccal cusp, and most have one lingual cusp (Appendix 5b). The EXCEPTION is the more common form of mandibular second premolars, which has two lingual cusps.A
3. Crown and Root Length
Premolar crowns in both arches are on average shorter than crowns of anterior teeth,B but first premolar crowns are slightly longer than second premolars. In other words, there is a gradation in size in each quadrant from the longer canine crown to the shorter first premolar and then the even shorter second premolar crown.
Roots on maxillary and mandibular premolars are considerably shorter than on canines in the same arch.C Complete data on premolar crown and root dimensions can be found in Tables 4-5A and 4-5B at the end of this chapter.
|TABLE 4-5A||Size of Maxillary Premolars (Millimeters) (Measured by Dr. Woelfel and His Dental Hygiene Students, 1974–1979)|