|SECTION VI||TERMINOLOGY USED TO DESCRIBE THE MORPHOLOGY OF A TOOTH|
Teeth are made up of many rounded elevations, ridges, depressions, and grooves. Specific tooth structures that occur with some frequency on teeth within a class have been assigned specific names. To identify the following anatomic structures, reference will be made to representative drawings of various teeth seen in figures throughout this book.
1. Elevations: Pointed Cusps and Linear Ridges
a. Cusps: Cusp Names and Numbers
A cusp is a pyramidal elevation with a peak called a cusp tip. Cusps are located on the occlusal surfaces of molars and premolars and on the incisal surfaces of canines. Canines have one cusp, premolars normally have two or three cusps, and most molars have from three to five cusps. On teeth with multiple cusps, each cusp is named according to its location on the tooth. For example, on a two-cusped premolar, the two cusps are named after the surface adjacent to each cusp: a buccal cusp and a lingual cusp. Three-cusped premolars have one buccal and two lingual cusps, and the two lingual cusps are named after the adjacent line angles, that is, mesiolingual cusp and distolingual cusp. A four-cusped molar has four cusps named after the adjacent line angles: mesiobuccal, distobuccal, mesiolingual, and distolingual. A three-cusped maxillary molar has two buccal cusps (mesiobuccal and distobuccal) and one lingual cusp. On a five-cusped molar, the three buccal cusps are called mesiobuccal, distobuccal, and the smallest distal cusp. Refer to Figure 1-21 for examples of cusp names on posterior teeth with two, three, four, and five cusps.
b. Cusp Ridges
Many cusps can be thought of as having four cusp ridges (linear prominences of enamel) converging toward the cusp tip. These four ridges form the shape of a four-sided pyramid with rounded surfaces. If you draw a line along the greatest linear bulge of each of these four ridges, the lines would intersect at the cusp tip indicated by the “X” on Figure 1-22. On this example of a buccal cusp on a premolar, three of the ridges are named after the circumferential tooth surface they extend toward: the more subtle buccal ridge extends onto the buccal surface, the mesial cusp ridge extends from the cusp tip toward the mesial surface, and the distal cusp ridge extends from the cusp tip toward the distal surface. The fourth ridge extends from the cusp tip toward the faciolingual middle of the tooth and is called a triangular ridge.
The buccal ridges that run cervico-occlusally on the buccal surfaces of premolars or molars are often the least distinct of the four ridges that emanate from the cusp tip, although they may be more prominent on some types of teeth (Fig. 1-23). Lingual cusps do not normally have prominent lingual ridges running cervico-occlusally from the cusp tips.
The mesial and distal cusp ridges are also known as cusp slopes or cusp arms. They are most evident when viewing teeth from the facial or lingual aspect where they can be seen as inclined ridges that converge toward the cusp tip to form an angle (seen in green on a buccal cusp of a premolar and on a buccal cusp of a molar in Fig. 1-24). For some teeth, the sharpness or bluntness of a cusp angle can be an defining trait. These ridges are more difficult to discern when viewing teeth from the occlusal, denoted in green on the two cusps of a premolar in Figure 1-25.
Triangular ridges are located on the major cusps of posterior teeth. Each triangular ridge extends from a cusp tip toward the depression (sulcus) near the middle of the occlusal surface faciolingually, most easily identified when viewing a proximal surface as on Figure 1-26, but also evident when viewing the occlusal surface as on Figure 1-25. When a triangular ridge from a buccal cusp joins with a triangular ridge from a lingual cusp, these two ridges together form a longer ridge called a transverse ridge. A transverse ridge crosses the occlusal surface of posterior teeth in a more or less buccolingual direction, running between the buccal and lingual cusps on a premolar (seen from an occlusal view and a proximal view in Figs. 1-25 and 1-26) or connecting the buccal and lingual cusps that are lined up across from one another on a molar (seen on a mandibular molar in Fig. 1-27 and on the two-cusped premolar). An oblique ridge is found only on maxillary molars. It crosses the occlusal surface obliquely (diagonally) and is made up of one ridge on the mesiolingual cusp joining with the triangular ridge of the distobuccal cusp (seen in Fig. 1-27 on the maxillary molar). According to Ash,1 the ridge of the mesiolingual cusp that forms the lingual half of the oblique ridge is the distal cusp ridge of the mesiolingual cusp.
The single cusp of many canines may also have four ridges emanating from its cusp tip (Fig. 1-22): a mesial cusp ridge and a distal cusp ridge, a labial ridge similar to a buccal ridge running cervicoincisally from the cusp tip, and sometimes a fourth ridge called a lingual ridge that extends lingually toward the cervical bulge (cingulum). These ridges can be prominent on maxillary canines (Fig. 1-28A and B).
c. Marginal Ridges and Cingulum
On the lingual of all anterior teeth, a cingulum [SING gyoo lum] (plural cingula) is the prominence or bulge in the cervical third of the lingual surface of the crown (incisors and canines) (seen on the lingual view in Fig. 1-29 and seen as a prominence in the cervical third of the crown on the proximal view in Fig. 1-30). On anterior teeth, mesial and distal marginal ridges form the mesial and distal borders of the lingual surface, and these ridges converge toward a rounded elevation or bulge in the cervical third called a cingulum, as seen on an incisor in Figures 1-29 and 1-30. When distinguishing a mesial from a distal marginal ridge on anterior teeth, it can be useful to remember that the mesial marginal ridge is normally longer than the distal. When determining which marginal ridge is longer, think of the length of a marginal ridge as extending from the incisoproximal line angle to its junction with the cingulum (as on Fig. 1-29 where the mesial marginal ridge appears slightly longer than the distal marginal ridge).
On posterior teeth, marginal ridges form the mesial and distal borders of the occlusal surface. The mesial marginal ridge on a premolar is shaded red in Figure 1-31.
d. Occlusal Table Outline versus Crown Outline
When viewing posterior teeth from the occlusal view, it is important to distinguish the entire crown outline of the tooth from the occlusal table of that tooth. The occlusal crown outline is the outer outline of the entire tooth crown from the occlusal view, whereas the occlusal table is the occlusal surface that is bounded by the continuous cusp ridges and marginal ridges. On the premolar in Figure 1-31, the occlusal table is bounded by a mesial marginal ridge joined with the mesial and distal cusp ridges of the buccal cusp, then the distal marginal ridge, and the cusp ridges of the lingual cusp.
This would be a good time to refer to Figure 1-32 and perform the learning exercise to test your knowledge of cusp ridges.
The diagram in this Figure 1-32 the ridges seen from the occlusal view that bound the occlusal table of a two-cusped premolar. Name each ridge next to its corresponding number. (Note that ridges labeled 1, 3, 4, 5, 6, and 7 form a continuous outline on the occlusal surface. The area inside of this line is called the occlusal table.)