| SECTION VI | TERMINOLOGY USED TO DESCRIBE THE MORPHOLOGY OF A TOOTH |
A. MORPHOLOGY OF AN ANATOMIC CROWN
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.
FIGURE 1-21. Cusp names on teeth having two, three, four, and five cusps, viewed from the occlusal and buccal views. Notice that the cusps are named after the adjacent surface or line angle EXCEPT on five-cusped mandibular first molars with three buccal cusps. On five-cusped molars, the two larger buccal cusps are named mesiobuccal and distobuccal cusps, as on the four-cusped molar, but the smallest cusp is called the distal cusp.
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.
FIGURE 1-22. The four-cusped ridges of the buccal cusp of a two-cusped premolar have a somewhat pyramidal design (actually, a pyramid with rounded sides called a gothic pyramid). The cusp ridges are numbered 1 to 4 and converge at the cusp tip (at the “X”). Ridge 1 is the mesial cusp ridge of the buccal cusp; ridge 2 is the distal cusp ridge of the buccal cusp; ridge 3 is the more subtle buccal ridge of the buccal cusp; and 4 is the triangular ridge of the buccal cusp. (Courtesy of Drs. Richard W. Huffman and Ruth Paulson.) Cusp ridges on the single cusp of a maxillary canine also have a mesial cusp ridge labeled 1 and distal cusp ridge labeled 2, the same as on the premolar, but 5 is the labial ridge of the canine cusp (similar to a buccal ridge on a premolar), and 6 is the lingual cusp 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.
FIGURE 1-23. A rounded buccal ridge can be seen on the buccal surface of many premolars.
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.
FIGURE 1-24. Cusp ridges (cusp slopes) are labeled on the buccal cusp of a mandibular premolar and on the mesiobuccal cusp of a four-cusped mandibular molar.
FIGURE 1-25. On this two-cusped maxillary premolar, the mesial and distal cusp ridges of the buccal and lingual cusps are shaded green. The triangular ridges of the buccal and lingual cusps are shaded blue and together are called a transverse ridge. The buccal ridge of the buccal cusp is shaded green.
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.
FIGURE 1-26. When seen from the mesial view on this maxillary two-cusped premolar, the outlines of two triangular ridges form a “V” shape and join at the depth of the occlusal sulcus to form one transverse ridge.
FIGURE 1-27. Three posterior teeth show transverse and oblique ridges. A. Two triangular ridges on a two-cusped premolar form one transverse ridge. B. Two pairs of triangular ridges on a mandibular molar join to form two transverse ridges. C. One pair of triangular ridges on a maxillary molar is aligned buccolingually and forms one transverse ridge in blue, and another pair of ridges is aligned obliquely (diagonally) to form an oblique ridge in purple.
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).
FIGURE 1-28. A. On this maxillary canine, a labial ridge can be seen running from the cusp tip cervically along the labial surface. B. On this maxillary canine, a prominent lingual ridge is visible running from the cusp tip cervically to the cingulum.
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).
FIGURE 1-29. The mesial and distal marginal ridges are shaded red, and the cingulum is shaded green on the lingual surface of a maxillary incisor. If you think of the length of a marginal ridge as running from the proximal incisal line angles to the cingulum, you can see that this mesial marginal ridge is slightly longer than the distal marginal ridge.
FIGURE 1-30. This maxillary canine demonstrates that the cingulum bulge in green is located in the cervical third of the lingual surface. One visible marginal ridge is shaded red.
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.
FIGURE 1-31. Occlusal view of a two-cusped premolar shows the difference between the occlusal outline of the tooth (the black outline surrounding the entire tooth from this view) and the smaller red occlusal table (or occlusal chewing surface) bounded by six ridges: the two cusp ridges of the buccal cusp (green), the two cusp ridges of the lingual cusp (green), and the two marginal ridges (red).
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.
FIGURE 1-32. Identify the ridges numbered on this maxillary premolar.
LEARNING EXERCISE
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.)
Stay updated, free dental videos. Join our Telegram channel
VIDEdental - Online dental courses
